European Geothermal Update 1989 - IRETHERM

European Geothennal Update

This 4th International Seminar is organised jointly by the Commission of
the European Communities, Brussels, Belgium and Ente Nazionale per
l'Energia Elettrica (ENEL), Italy.

Commission of the European Communities
European
Geothermal Update
Proceedings of the Fourth International Seminar
on the Results of EC Geothermal Energy Research and Demonstratior,
Florence, 27 - 30 April 1989
Edited by
K. LOUWRIER, E. STAROSTE and J. D. GARNISH
Dirtctoratt-G/mtral for Scitnct. Rtstarch and Dtvtlopmtnt.
Commission of tht Europtan Communitits. Brussels. Btlgium
and
V. KARKOULIAS
Dirtctoratt-Gtntral for Entrgy,
Commission of tht Europtan Communitits. Brusstls. Btlgium
KLUWER ACADEMIC PUBLI~~o;.------
DORDRECHT I BOSTON I LONDO .-- a_!~_"'~_~JTI r'),~ "'. ... •• ' 1'1 0\ .. L IJ
c
Il,C!J8

Library of Coogress Cataloging in Publication Data
International Seminar on the Results of EC Geother.al Energy Research
and Demonstration (4th: 1989 : Florence. Italy)
European geothermal update: proceedings of the Fourth
International Seminar on the Results of EC Geothermal Energy
Research and Demonstration. Florence. 27-30 April 1989 I edited by
K. Louwrler ... let al.1.
p. cm.
At head of title: Commission of the European Com.unltles.
Includes Indexes.
ISBN 0-7923-0198-6
1. Geothermal resources--Europe--Congresses. 2. Geothermal
englneerlng--Europe--Congresses. I. Louwrler. K .• 1933-
II. Commission of the European Co.munlties. III. Title.
GB1199.8.E85157 1989
553.7--dc19 89-2432
ISBN 0-7923-0198-6
Organisation of the seminar by
Commission of the European Communities
Directorate-General for Science, Research and Development and
Directorate-General for Energy, Brussels, Belgium
Publicatioo arrangemenu by
Commission of the European Communities
Directorate-General Telecommunications, Information Industries and Innovation, Scientific and
Technical Communications Service, Luxembourg
EUR 12038
@ 1989 ECSC, EEC, EAEC, Brussels and Luxembourg
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PREFACE
Following the three previous seminars held in Brussels (December
1977), Strasbourg (March 1980) and Munich (November 1983) the Commission
of the European Communities is organising, this time in cooperation with
ENEL, its Fourth International Seminar on Geothermal Energy at the
Conference Centre of Florence from 27 to 30 April 1989.
Not only will this seminar deal with formal presentations of EC
contractors in the field of research and development, but it will also
address the trends and accomplishments noticed in the follow-up
demonstration programme.
This Fourth International Seminar, reported in this volume, has as
its aim to present to the scientific world the results of four years
coordinated work in the field of geothermal energy, in a European
Community that consists now of 12 Member States.
This proceedings volume has the following objectives
to present an evaluation of the results of the third Community
geothermal R&D programme (1985-1988) by means of reports describing
the individual research programmes;
to present the state of geothermal demonstration in nearly all the
Member States of the Community.
It is hoped that these proceedings will reach a large group of
readers, leading to an important dissemination of the results of this
Community sponsored research.
The results should also form the base for the selection of topics
that should be pursued in the fourth R&D programme on geothermal energy.

Preface
V
PAIlT I ~ USEAICB AlID DEVELOPIIEIIT
Introductioa
Geothermal research in the European Community
R. LOUWRIER, E. STAROSTE, J.D. GARNISH, Directorate­
General for Science, Research and Development, Commission
of the European Communities
3
8eaaioa 1
&calilla. corrosion aDd reserYOir .oclelliq
Laboratory studies and field tests for the definition of new
materials and components for uses in geothermal well drilling
and completion
G. CULIVICCHI, ENEL (Italian Electricity Board),National
Geothermal Unit, Italy 10
Optimizing the composition of casing cementing materials in
high-enthalpy geothermal wells
D. DEGOUY, M. HARTIN, Institut Fran~ais du Patrole,
France 20
On-site material damage evaluation for low enthalpy geothermal
venture based on saline cretaceous formation water
H. TAS, J. DRESSELAERS. P. DlRVEN, S.C.R./C.E.N., Belgium 36
Instrumental method for counting sulphate-reducing bacteria in
geothermal vater
F. COLIN, M.J. JOURDAIN, Institut de Recherches
Hydrologiques, Nancy, France
Ths behaviour of .. tallic .. terials in a lov-enthalpy
geothermal enviroDllent - The Paris BaSin, France
J.L. HONEGGER, H. TRAINEAU, Institut Hixte de Recherches
Glothermiques, Orlaans, France
Calcium carbonate scale foraation and prevention
E.R. GIANNlHARAS, A.G. XYLA, P.G. ROUTSOUROS, University
of Patras, Depart .. nt of Cheaistry, Greece
46
56
63
vii

viii
A study of scaling due to high enthalpy geothermal fluids
N. ANDRITSOS; A. MOUZA, A.J. KARABELAS, Chemical
Engineering Department, University of Thessaloniki,
Greece 74
Sulfide deposition and well clogging in the Dogger aquifer of
Paris Basin (France)
C. FOUILLAC, A. CRIAUD, J.L. HONEGGER, I. CZERNICHOWSKI­
LAURIOL, A. MENJOZ, BRGM - Institut Mixte de Recherches
G~othermiques, Orl~ans, France 84
Deep exploration of second geothermal reservoir in Viterbo
area (Latium)
C. GARELLI, AGIP S.p.A., S. Donato Milanese, Italy 94
Deep exploration in the Torre Alfina geothermal field (Italy):
the test hole Alfina 15
G. BUONASORTE, A. FIORDELISI, E. PANDELI, ENEL (Italian
Electricity Board), National Geothermal Unit, Italy 98
Characterization and modelling of low enthalpy geothermal
reservoirs - Example of the Paris basin
J. ROJAS, M. BRACH, A. CRIAUD, C. FOUILLAC, J.C. MARTIN,
A. MENJOZ, Institut Mixte de Recherches G~othermiques
(BRGM/AFME), Orl~ans, France 109
Modeling in the Mofete field
A. GUIDI, AGIP S.p.A., G. ANTONELLI, DAL S.P.A.,
S. Donato Milanese, Italy 119
Technical handbook for the planning of district heating
systems fed by geothermal sources
C. PIEMONTE, A. PIATTI, Energy Department, Polytechnic of
Milan; E. SZEGO, Research and Development Division,
TECHINT S.p.A., Italy 129
Session 2
Bot dry rock and related studies
Characterisation of the Rosemanowes HDR geothermal reservoir
using an extended circulation programme
R. PARKER, Camborne School of Mines, United Kingdom
Cost modelling of HDR systems modelling methods and interim
results
R. HARRISON, I. COULSON, P. DOHERTY, Faculty of
Technology, Sunderland Polytechnic; S. MlNETT,
N.D. MORTIMER, School of Urban Studies, Sheffield City
Polytechnic, United Kingdom
Hot dry rock geothermal energy cost modelling: drilling and
stimulation results
N.D. MORTIMER, S.T. MlNETT, Sheffield City Polytechnic,
United Kingdom
141
154
170

ix
Experimental inveatigation on forced fluid flov through a
granite rock .. aa
F. H. CORNET, Department of Geophyaica, Stanford
Univeraity, United Kingdom (On Sabbatical leave from
Inatitut de Phyaique du Globe de Paria, France) 189
Apparatu. to provide an i .. ge of the vall of a borehole during
a hydraulic fracturing experiment
J. HOSNIER, Laboratoire de GAophyaique AppliquEe (LGA),
CNRS, OrlAana; F. CORNET, Inatitut de Phyaique du Globe,
Paria (IPGP), France 205
Rock .tre •• orientation. from borehole breakouta
N.R. BRERETON, C.J. EVANS, Britiah Geological Survey,
Keyworth, United Kingdom 213
Strea. meaaurement. by hydraulic fracturing in BRGH
D. BILLAUX, D. BURLET, Bureau de Recherchea Glologiquea
et Hiniirea, France 232
Stability of deep geothermic exchanger under theraalhydraulic-mechanical
.ollicitationa
G. BERTHOHIEU, P. JOUANNA, Civil Engineering Laboratory,
Hontpellier, France 243
In-.itu atrea.e. evaluated from meaaurementa on core a .. plea
P.J. PERREAU, O. HEUGAS, F.J. SANTARELLI, Inat1tut
Fran~ai. du PAtrole, C.F.P. TOTAL, SNEA(P), France 253
Stimulation of vella Latera 10 and Latera 4
A. BARELLI, G. CAPPETTI, ENEL (Italian Electricity
Board), National Geothermal Unit, Italy 271
The European geothermal project at Soultz-aoua-Forlt.
O. KAPPELMEYER, Geothermik Conault GmbH, Hannover,
Federal Republic of Germany; A. GERARD, BRGH-AFHE,
OrlAan., France
283
Economic modeling of HDR
O. KAPPELHEYER, K. SMOLKA, Geothermik Con.ult GmbH,
Hannover, Federal Republic of Germany 345
Hydrogeothermic .tudie. on hot dry rock technology
R. SCHELLSCHHIDT, R. SCHULZ, , Geological Survey of Lover
Saxony, Federal Republic of Germany 351
Granite-water interaction. in relation to hot dry rock
geothermal develop.ent
W.H. EDMUNDS, Briti.h Geological Survey, Wallingford,
Oxon; J.N. ANDREWS, N. HUSSAIN, University of Bath;
D.P.F. DARBYSHIRE, NERC Isotope Geology Centre, London;
D. SAVAGE, British Geological Survey, Keyworth,
Nottingham.hire; T.J. SHEPHERD, British Geological
Survey, London, United Kingdom 363

x
Behaviour of trace elements in water-rock interactions
G. MICRARD, H. PAUWELS, P. ZUDDAS, Laboratoire de
G~ochimie des Eaux, Universit6 de Paris; S. DUJON, 375
M. LAGACHE, Laboratoire de G6ologie, ENS, France
Study of reactions between feldspathic rocks and heat exchange
fluids
E. ALTHAUS, E. TIRTADINATA, Mineralogisches Institut,
Universit~t Karlsruhe, Federal Republic of Germany 385
Dissolution of feldspars: solid state chemistry of
hydrothermally treated sanidine
D.A. GOOSSENS, J.G. PHILIPPAERTS, R.H. GIJBELS,
Department of Chemistry, University of Antwerp, Belgium;
A.P. PIJPERS, DSH Research BV, Geleen, The Netherlands;
E. ALTHAUS, Mineralogical Institute, University of
Karlsruhe, Federal' Republic of Germany 395
Helium isotope systematics in crustal fluids from W. Germany
and adjacent areas
E. GRIESSHABER, R.K. O'NIONS, E.R. OXBURGH, Department of
Earth SCiences, University of Cambridge, United Kingdom 407
Experiments on reinjection of geothermal brines in the deep
Triassic sandstones
A. BOISDET, J.P. CAUTRU, I. CZERNICHOWSKI-LAURIOL,
J.C. FOUCHER, C. FOUILLAC, J.L. HONEGGER, J.C. HARTIN,
Institut Mixte de Recherches G6othermiques, BRGM/AFHE,
Orl6ans, France 419
Study of the variations in permeability and of fine particle
migrations in unconsolidated sandstones submitted to saline
circulations
J. BAUDRACCO, Universit6 Paul Sabatier, Laboratoire de
Min6ralogie, Toulouse, France 429
Variation in the permeability and cation exchange kinetics in
a clayey sandstone s~bmitted to percolation of different
saline solutions
M. AOUBOUAZZAi J.,BAUDRACCO, Universit6 Paul Sabatier,
Laboratoire de Min~ralogie, Toulouse, France 439
Space and time evolution of the geochemical processes arising
from geothermal injection in an aquifer
A. COUDRAIN-RIBSTEIN, P. MERY, A. VINSOT, Ecole Nationale
Sup6rieure des Mines de Paris, France 444
Testing geophysical exploration techniques on the island of
M110s (Greece)
J. WOHLENBERG, Rheinisch-Westf~lische Technische
Hochschule, Aachen, Federal Republic of, Germany 452
Seismic reconnaissance of the upper crust in the volcanic zone
of Olot (NE of Spain)
J. GALLART, Dep. Geologica Dinamica, Universitat de
Barcelona, Spain; A. HIRN, Institut de Physique du Globe,
Paris, France 464

lIi
Hicroseismic and seismotectonic study of the island of Lesbos
N.D. DELIBASIS, N.S. VOULGARIS, Geology Department,
University of Athens, Greece 474
Atlas of geothermal resources in the European Community,
Austria and Switzerland
R. HAENEL, Department of Geophysics, Geological Survey of
Lower Saxony, Federal Republic of Germany 482
Geothermal resources and reserves: updating of temperature
data base
R. SCHULZ, K.H. WERNER, Geological Survey of Lower
Saxony, Federal Republic of Germany 490
Exploration and evaluation of geothermal resources in the
Central Graben area, The Netherlands
J.P. HEEDERIK, TNO Institute of Applied Geoscience;
R.H. VIERHOUT, Comprimo BV, The Netherlands 500
Assessment of the low enthalpy geothermal resources of the Po
Valley Plain, Italy
G. GHEZZI, R. GHEZZI, H.P. MARCHETTI, GE.T.AS.s.r.l.,
Piaa, Italy 510
Thermometry and hydrogeochemistry of the southern border of
the south Pyrenean foreland Basin
H. FERNANDEZ, A. FREIXAS, X. BOSCH, X. BERASTEGUI, Servei
Geologic de Catalunya, Generalitat de Catalunya,
Barcelona; E. BANDA, Institut Jaume Almera, Barcelona,
Spain 522
Hydrothermal activity related to recent explosive volcanisa on
the island of Kos, Greece - An assessment of the geothermal
potential of the Volcania area
J.H. BARDINTZEFF, R. BROUSSE, Laboratoire de
patrographie-Volcanologie, Universit' Paris-Sud, France;
P. DALABAKIS, I.G.H.B., Athens, Greece; H. TRAINEAU,
BRCH. IMaG, Orlaans, France 532
Study St Cugat geothermal resource in fracture granites to
heat greenhouses
J. KUNIZ, ENHER, Spain 541
Preliminary results from temperature, heat flow and heat
production studies in Ireland
K.J. BARTON. A. BROCK, A.D. SIDES. Applied Geophysics
Unit, University College Galway. Ireland 551
An investigation of low enthalpy geothermal resources in
Ireland
P.H. BRUECK, F.X. HURPHY, Depart.ent of Geology.
University College, Cork, Ireland 560

xii
Development of a two-phase flow turbine for geothermal
application
F. DEHAINE, R. GROSSIN, R. SCHLEGEL, Soci~t~ Bertin &
Cie, Plaisir, France
Design of two-phase flow lines for geothermal applications:
the slug flow regime
P. ANDREUSSI, A. MINERVINI, G. NARDINI, A. PAGLIANTI,
University of Pisa, Italy
569
576
PART II -
DEMONSTRATION PROJECTS
Introduction
Energy demonstration programme of the European Communities -
Geothermal energy
G. GERINI, V. KARKOULIAS, I. GALANIS, Commission of the
European Communities, Directorate-general for Energy, New
and Renewable Energies Unit, Geothermal Sector, Brussels,
Belgium 589
Session 4: Overview of geother.al resource development and
de.onatration
Overview of the geothermal demonstration activities in
Catalunya
A. MITJA, J. ESTEVE, JJ. ESCOBAR, Direcci6 General
d'Energia, Generalitat de Catalunya; J.F. ALBERT-BELTRAN,
Tecnologia y Recursos de la Tierra (TRT S.A.) Madrid,
Spain 595
Geothermal demonstration project in Denmark
A. MAHLER, Dansk Olie og Naturgas A/S, Denmark 604
Geothermal demonstration activities in Belgium
L. SUN FAN, N. VANDENBERGHE, Belgian Geological Survey,
Brussels, Belgium 612
Overview of geothermal demonstration activities in Germany
R. SCHULZ, Geological Survey of Lower Saxony, Federal
Republic of Germany 623
Overview of high enthalpy projects in Italy
G. ALLEGRINI, ENEL, National Geothermal Unit, Italy 630
Overview of low enthalpy projects in Italy
G. SOHHARUGA, G. VERDIANI, Valorizzazione Risorse
Geotermiche, Milan, Italy 638

"iii
Progress on geothermal research & system implementation in the
United Kingdom
W.S. ATKINS & Partners, Surrey, United Kingdom 648
Hilos demonstration project
E.E. DELL IOU , Public Power Corporation, Direction of
Alternative Energy Forms, Athens, Greece 652
Geothermal energy in Greece - Potential and exploitation
Sp. KYRITSIS, Center for renewable energy sources, Koropi
Attikis, Greece 661
Bilan de la fili~re gAothermique en France - Actions de
dAmonstration
J. LEHALE, Agence Fran~aise pour la Hattrise de
l'Energie, France 662
Se.sioa S - Technology. ob.tacle. and actiona to pro.ote
develo~Dt of geotber.al energy
Technical problems to exploit geothermal energy (high and low
enthalpy)
R. CORSI, STEAM s.r.l. Energy and Environmental System
Exploration et exploitation en gAothermie dans les pays de la
CommunautA europAenne - Quelques aspects de la lAgislation et
des r~gles administratives
J. BARBIER, Service d'information sur l'Energie, BRGH,
OrlAans, France
Economie de la gAothermie dans la CEE
Ch. BOISSAVY, Expert aupr~s de la Direction gAnArale de
l'Energie, Commission des CommunautAs europAennes
Obstacles and recommendations to promote geothermal energy
development
R. CARELLA, AGIP S.p.A., Hilan, Italy
677
692
698
715
IDdex of author.
727

PART I -
RESEARCH AND DEVELOPMENT

3
GEOTHERJW, RESEARCH III DIE EUROJ>EAII COIUMIITY
K Louwrier, E Staroste, J D Garnish
DG XII, Commission of the European Communities
Rue de la Loi 200, B-1049 Brussels, Belgium
SUlll!l8rv
The European Commission has been supporting geothermal research and
development in the "ember States since 1975. During that period it
has encouraged and stimulated programmes of resource assessment and
exploration in every country, to the extent that it was possible
last year to publish an Atlas of Geothermal Resources with data from
all countries expressed on a common basis. Its biggest single
achievement has been the extent to which continuing co-operation has
been established between research teams in the different "ember
States. Research in many areas has now been completed, with the
work moving into the demonstration stage, and the Commission's
future research programme envisages a much more selective approach
with attention focused on the problems of Hot Dry Rock and deep
geology.
INTRODUCTION
Since 1975, DG XII of the European commission has supported three
programmes of research into geothermal energy. The Third programme, which
came to an end last December, involved for the first time the new "ember
States of Greece, Spain and Portugal.
The Commission's aim for the research programmes is to help and
encourage "ember States to establish their geothermal potential and to
assist in its development.
pROGRESS DURING THE IHIRD R&D pROGRAMME
At present we can look back at important results and advances in all
five topics of the Third programme:
• In an earlier programme a subsurface temperature atlas of the
European Community was established. An extension of this study
resulted in the publication recently of the Atlas of Geothermal
Resources of the Community, Austria and SWitzerland.
• A co-ordinated geophysical study of the island of "ilos has been
completed, with the twin aim of improving interpretation techniques
and adding to the understanding of the reservoir study beneath the
volcanic island.
• Corrosion and scaling have been studied extensively, with
interesting results.
• A better understanding has been obtained of the water-rock
interactions that may occur in hot dry rock reservoirs and in high
and low enthalpy reservoirs.

4
• A feasibility study has been carried out on a possible site for a
European Hot Dry Rock project in France, with German, French and
British groups co-operating in the work.
Geothermal Resources
It is obvious that as complete as possible an understanding of the
characteristics of a geothermal reservoir will contribute considerably to
the success of a geothermal project. A basic requirement for the more
widespread uptake of the technology is therefore that an assessment of the
geothermal resources and reserves should be available.
Following the publication of the Temperature Atlas (Hanel 1980), the
Commission took on the much greater task of compiling a similar Resources
Atlas for the Community. It is based on results obtained by teams in each
country, mostly with support from EC research contracts, and has used a
uniform set of conventions and assumptions to allow the data from the
different countries to be presented for the first time in a directly
comparable form. It has also been possible to obtain similar
contributions from Austria and SWitzerland, thereby presenting a complete
survey of Western Europe.
The Atlas was published last year (Hanel and Staroste, 1988). It
presents the main characteristics of each reservoir investigated,
including depth, thickness, geological cross-section, temperature, water
pressure (if known), the distribution of boreholes, the effective porosity
and permeability, the transmissibility and the salinity. Syntheses of
these detailed data allow the presentation of either the geothermal
resources (expressed in joules per square metre) or a more generalised
indication of "potential geothermal areas", i.e. areas which appear
qualitatively suitable for geothermal exploitation but for which not all
the necessary quantitative data are yet available. These maps, on a
national or regional scale, are preceded by a set of maps on a European
scale demonstrating the relationship between geological structures and
geothermal reservoirs, showing updated data on heat flow densities and
temperatures at 1000 m and 2000 m depth and outlining the geothermal
resource areas investigated. The maps are accompanied by a detailed text
which also includes information on the present status of geothermal usage
in each country and reviews the geothermal potential worldwide.
It must be stressed that this edition of the Atlas presents only the
geothermal resources of investigated reservoirs in promising areas. No
assessment can yet be made outside these areas, but this does not of
course preclude the existence of further resources. This comment applies
particularly to those countries which have only recently been able to
participate in EC programmes or which have only just begun to develop lowor
high-enthalpy geothermal activities.
Testing geophysical exploration methods
It has become evident that standard geophysical exploration
techniques require adaptation in order to give maximum benefit in
geothermal exploration, and an EC-supported collaborative study has
produced valuable results that will be presented in this conference. The
first study was performed on the Travale geothermal anomaly and was part
of the second research programme. The present programme concentrated on
Milos and eight teams of various EC countries supported by the Commission
co-operated in this study. The results are being submitted for
publication in a Special Issue of Geothermics.

5
Studies on water-rock interaction
Water plays an important part in geothermal energy. It is the heat
transfer medium and is therefore in constant contact with the rock, not
only in HOR systems (where the water is supplied by the operator) but in
all natural systems as well. It may not be immediately obvious that water
reacts chemically with rock - after all, most man-made structures for
retaining water, such as sea walls, dams and canals, are made of rock -
but reactions do in fact occur. Particularly at higher temperature or at
lower pH, these can happen relatively fast; it was a surprise to find
that, in the laboratory, effects are detectable in a matter of hours and,
in the field experiments, changes in fluid chemistry were seen within one
or two days. An understanding of water-rock interactions is essential for
the proper exploitation and management of a geothermal system.
Geothermal reservoirs are found in many different source rocks, and
the reactions important in natural systems, especially those at higher
temperatures, have been studied for years. The EC's role of co-ordinating
research has proved especially effective in this area by bringing together
the relevant specialists to tackle a slightly different challenge, the
interaction between rock and water in HOR reservoirs. Heat extraction
from "artificial" reservoirs could be achieved in almost as many rock
types as are found in natural reservoirs but, to date, most HOR
experiments have been carried out in granites. Consequently, this is the
material on which attention has been focused. The results to date have
been summarised in reports of two contractors' meetings (Louwrier and
Garnish, 1987; 1988) and will be part of presentations during this
conference.
Scaling and corrosion
Scaling - the deposit of solid materials from a solution - in a
production pipeline, in a reinjection well or in a pump can have serious
consequences for the economy of a geothermal operation. The most common
scales found in European systems are calcite, silica and sulphides.
In some situations, calcite scaling can be avoided, primarily by
maintaining a high partial pressure of carbon dioxide in contact with the
geothermal fluid. This is not always possible, however, and an
alternative approach has been tested successfully with EC support in the
Italian geothermal field of Latera. This involves injection downhole of a
few ppm of an inhibitor, usually a phosphate compound, which interferes
with the process of crystallisation and allows incipient scale to be
flushed away by the flowing fluid.
Sulphide scaling causes problems in some high enthalpy systems, as in
the power plant of "ilos (Andritsos and Karabelas, 1988) and sulphide
deposits have also caused trouble in some of the submersible pumps used in
the low-enthalpy developments in the Paris Basin. Both systems are being
studied and the findings are presented in this volume. Despite the
differences in the geothermal fluids, it seems that the underlying
mechanisms are similar and that the two studies are complementary.
One area which has not yet been the subject of EC-supported research
is silica deposition. This is a serious problem in high-enthalpy
developments worldwide, from which a number of lessons can be learned, and
its resolution will be important for the development of high-enthalpy
brines in Italy. It is the Commission's intention to investigate this
problem further within the next year or two.

6
Hot dry rock
The fifth major area of EC research involves HDR technology, the
development of which in Europe has been contemporaneous with the EC R&D
programmes. Work has been concentrated in Germany, France and the UK and
the progress during the first two programmes has been widely reported;
see, for example, Garnish (1987) and Kappelmeyer and Rummel (1987).
By the end of 1985, Europe possessed teams with experience of working
at shallow sites (25O-S00 m) in all three countries, plus a major field
experiment at depths of 2000-2600 m in the UK. Like the earlier American
experiments, these had shown that hydraulic fracturing was capable of
producing heat transfer surfaces in granite, the UK work having been on a
sufficiently large scale to generate a full-size reservoir. Even that is
only a rock mechanics experiment, however, the operating depth having been
chosen as shallow enough to keep rock temperatures below 100°C and thereby
simplify the experimental process.
The next stage in Europe, therefore, was the search· for a site
suitable for a full-depth pilot project, "full depth" being defined as
deep enough to produce temperatures of commercial value in that particular
location. This work has been started under the Third EC Programme as a
fully-integrated collaborative project between France and Germany, the aim
of which is to develop a sound understanding of the geology of a site at
Soultz-sous-For~ts, just north of Strasbourg in the Upper Rhine Valley and
close to the border between the countries. This region has long been
known to possess a marked thermal anomaly, with temperatures around 110°C
at the top of the granite basement beneath 1000-lS00 m of sedimentary
cover. The current project, with which the UK team is also beginning to
collaborate at the same time as continuing with its own field experiment,
has entailed drilling an exploratory borehole to 2000 m (600 m into the
granite) where the temperature was found to be around 140°C. Geophysical
and hydraulic tests have been used to characterize the granite and assess
its suitability as the host rock for a pilot reservoir.
A full scale pilot project will be very expensive (SO-100 KECU) and
is likely to be affordable only as a joint European project, with at least
two or three Kember States sharing the cost with the Commission. An
important activity over the next year or so will be the definition of a _
single European HDR project, its participants and organisation, and the
selection and preparation of a suitable site.
PLANS FOR THE FOURTH pROGRAMME
Kuch has been achieved in the last 13 years, the most important
aspect of which has been the degree of cohesion and co-operation that has
been established within the European geothermal community. On the
technical side, the Temperature Atlas of the European Communities has been
followed by the Atlas of European Geothermal Resources, in which it has
been possible to include Austria and SWitzerland as well as the Kember
States. Several deep holes have been drilled in hostile environments,
mainly in Italy but also in Greece, and valuable information about
drilling techniques has been obtained. Taken together, these achievements
have completed the exploration task of the research programme and most
work on natural resources has moved into the demonstration phase.
The Fourth Programme will concentrate on those aspects that still
need attention. These include

1
• corrosion and scaling in low and high enthalpy wells;
• Hot Dry Rock studies, including the definition of a European pilot
project and the drilling of at least the first well;
• the handling of high enthalpy, high salinity brines;
• exploration in a few selected areas;
• deep geology.
This last item is a new departure. An immense programme could be
supported under such a title, but the scope of the work will in fact be
quite modest because of the limited availability of funds. The studies
will all be energy-oriented. They will supplement the HDR studies in
particular but will also investigate the formation of hydrocarbon
reservoirs, with particular reference to the influence of deep structural
events on such formation. The topic will also allow the Commission to
support additional studies in some of the deep research holes now being
planned or drilled in several of the "ember States.
REFERENCES
Andritsos N, Karabelas A J, 1988 - Deposition of colloidal lead sulphide
in a pipe - Int~rnational Conf~renc~ on Fouling and Cleaning of
Proc~ss Plant - Oxford, 25-29 July
Garnish J (ed), 1987 - Proceedings of the First EEC/US Workshop on
Geothermal Hot Dry Rock Technology - Geothermics (Special Issue),
~ (4), 323-461
Hanel R (ed), 1980 - Atlas of Subsurfac~ T~~ratures in the European
Co..unity - Th. Schafer Druckerei GmbH (Hannover)
Hanel R, Staroste E (eds), 1988 - Atlas of Geothermal R~sourc~s in the
European Coa.unity, Austria and 5Witz~rland - Verlag Th. Schafer
(Hannover)
Louwrier K, Garnish J D (eds), 1986 - Contractors' H~ting and Workshop on
Geoch€>mistry - Antwerp (5 November) - EUR Report 11362 EN, (publ.
1988)
Louwrier K, Garnish J D (eds), 1987 - Contractors' H~ting and Workshop on
Geochemistry - Toulouse (24-25 November) - EUR Report (in press)

ScalI .... corroel00 aad reae~lr .odell 1 ...
Laboratory etudies and field tests for the definition of new
materiale and components for uses in geothermal well drilling
and completion
Optimizing the composition of casing cementing materials in
high-enthalpy geothermal wells
On-site material damage evaluation for low enthalpy geothermal
venture based on saline cretaceous formation water
Instrumental method for counting sulphate-reducing bacteria
in geothermal water
The behaviour of metallic materials in a low-enthalpy geothermal
environment - The Paris Basin. France
Calcium carbonate scale formation and prevention
A etudy of ecaling due to high enthalpy geothermal fluids
Sulfide depoeition and well clogging in the Dogger aquifer of
Parie Baein (France)
Deep exploration of second geothermal reservoir in Viterbo
ana (Latium)
Deep exploration in the Torre Alfina geothermal field
(Italy): the test hole Alfina 15
Characterization and modelling of low enthalpy geothermal reeervoire
- Example of the Paris basin
Modelling in the Mofete field
Technical handbook for the planning of district heating
eyet .. e fed by geothermal sources

10
Contract nO EN3G-0083-1
LABORATORY STUDIES AND FIELD TESTS FOR THE DEFINITION OF,NEW MATERIALS
AND COMPONENTS FOR USES IN GEOTHERMAL WELL DRILLING AND COMPLETION
G. Culivicchi
ENEL (Italian Electricity Board)/National Geothermal Unit
Summary
The necessity of having adequate materials to handle hard drilling and
well operation conditions has directed experimentation towards innovative
steels for this type of activity.
A series of materials with high matrix fineness are in the experimental
stage for drill pipes, such as: AISI 4137 H, API G 105 and UNI 30
NiCrMo 12 (all remelted under slag).
For the materials for utilizations such as reinjection well casings,
tubes and couplings of the following materials have been industrially
prepared: AISI 4130, AISI 420, UNI X2 CrNiMo N 22 5, UNI Xl Ni, CrMoCu
31 27 4.
Experimentation will be performed on these materials both in the
laboratory and directly in wells.
O. FOREWORD
The growing energy needs of the industrial world and the high
incidence of energy in the various industrial processes are factors that
contribute to making the exploitation of very deep reservoirs competitive
from the economic standpoint as well.
One factor that prevents, or tends to slow, development in this
production sector, at least in the immediate future, is that the resistance
of the present materials to mechanical stresses and corrosion is not always
sufficient.
The aggressiveness of the fluids that are encountered at such depths
is primarily due to the presence of high concentrations of hydrogen
sulphide (H 2 S), carbon dioxide (C0 2
), chlorides, and 'other chemioal species
in lower concentrations.
It thus appears that priority should be given to the assessment of new
materials for geothermal applications, to be used in drill pipes or casings
for reinjection wells.

II
1. TYPES OF FAILURE
1.1 Drill pipes
The occurrence of a failure during the drilling represents a critical
event for the life of the well, since it involves all the pipes present in
the drillstring. The most frequent failures regard the thread of the
female end of drill collars (Table I), while failures on Hev; Wates and
drill pipes are more rare. The consequences of a failure, by contrast, are
much greater when they are due to drill pipes, as they are located in the
highest zone of the well, and therefore with greater probabilities of
falling and being damaged, making for difficult retrieval of the drillstring
below.
1.1.1 Types of breakage
~r!l! £o!l!r!: Almost all breaks occur at the bottom of the groove of the
last engaged female thread. The failure is triggered in most cases by a
stress corrosion phenomenon and continues for about 70-80% of the section
through a corrosion fatigue mechanism until the final break (Fig. 1).
Secondary stress corrosion cracks are often present at the bottom of
the threads adjacent to the failure, and are branched and typical of the
formation mechanism (Fig. 2).
Other cases are triggered by mechanical stresses, mainly of the cyclic
type, and subsequent advance through fatigue.
~r!l! p'ip'e!: Nearly all the cases analyzed in the last three years displayed
a trigger mechanism that was due to the formation of cracks on the
inside surface in correspondence to the pipe body adjacent to the upset
zone, and subsequent mechanical advance (Fig. 3).
These cracks are located in a particularly corroded zone at the bottom
of craters or pits (Fig. 4). The cracks are straight and filled with
corrosion products in which sulphur and chloride are always present. The
morphology is typical of a corrosion fatigue process (Fig. 5).
!:!e~i_W~t~S: The failure has been located, in the cases analyzed, in
proximity to the weld between the body and the joint base.
The trigger mechanism in this case is due to the formation of cracks
triggered by phenomena of stress corrosion in proximity to geometric or
structural discontinuities in the zone adjacent to the weld (Fig. 6).
£r£s!-£v~r!: The failure morphology is similar to the one encountered in
drill collars, with a prevalence of trigger mechanisms due to corrosion
fatigue.
2. TEST ENVIRONMENTS
2.1 Drill pipl's
The drilling fluid is normally composed of bentonite mud stabilized
and treated to have a pH of about 10. In these conditions the fluid is not
corrosive.
If large fractures are encountered during the drilling, river water
condi tioned to pH '1.11 wi th caustic soda is used. In such cases the fluid
is partially absorbed in the fracture as it rises in the well, while the
remainder mixes with the steam and gas from the fracture.

12
As base for the investigation in progress it was decided to take a
fluid derived from river water mixed with endogenous steam.
The analysis of the reconstructed fluid and its composition is contained
in Table II.
2.2 Reinjection casing
The reinjected fluids are composed of waste water taken from the
cooling towers. This water is derived from the condensation of endogenous
steam with subsequent air saturation, which involves the oxidation of
hydrogen sulphide, principally, and colloidal sulphur and sulphates.
For the characterization of these fluids, the waters present in the
coolants of a few power plants were analyzed. On the basis of this analysis,
a composition of a water to carry out the corrosion tests on was
formulated (Table II).
3. TESTS ON MATERIALS FOR DRILL PIPES
The materials examined were AISI 4137 H ESR, API G 105 ESR and UNI 30
NiCrMo 12, whose compositions and mechanical characteristics are shown in
Tables III and IV.
The electrochemical tests run at both 20°C and 80°C always indicate an
active behaviour (generalized corrosion) for all the materials.
For the evaluation of the susceptibility to stress corrosion, tests
were conducted by the exposure of U-bend test pieces in an autoclave at
200°C for 720 h. In no case was there formation of cracks.
The SSRT tests, conducted under the same conditions, evidence the
presence of stress cracking induced by the H 2
S.
4. TESTS ON MATERIALS FOR CASINGS
The materials examined were API C90, UNI X2 CrNiMoN 225 and UNI Xl
NiCrMoCu 31274.
Of each material, 3 tubes and 3 couplings were made. These will. be
lowered into a reinjection well for a field test.
The compositions and mechanical characteristics are given in Tables
III and IV.
The electrochemical tests on materials for tubing, performed in an
autoclave at 80°C, indicate that the C90 steel is always active in such
conditions.
The martensitic stainless steel 13 Cr displays the formation of passive
metal, but the passive range is very limited and localized corrosion
phenomena are evidenced.
The austeni tic-ferri tic and superausteni tic stainless steels, on the
other hand, display a wide range of passivity.
SSRT tests were conducted in air (as reference) and in the environment
at the velocity of deformation: ~o = 2.228 x.l

13
chemical study.
The absence of stress corrosion cracking is documented by the macrographic
aspect and by the appearance under the SEM of the fracture surface,
which is completely ductile.
The 13 Cr steel, on the contrary, showed an extensibility in area of
t.La "'5 mm and an extensibility in aggressive solution of t.LS '" 2 mm. This
sharp drop in ductility indicates the presence of stress corrosion.
The duplex and superaustenitic steels display extensibilities in air
and in the environment which are comparable to each other, and hence did
not undergo stress cracking.
Bibliography
Bruno R., G. Culivicchi, A. Lannaioli, and V. Scolari (1982). Corrosion
phenomena in drill pipes used in deep geothermal boreholes. Presented at
International Conference on Geothermal Energy, Florence, Italy, May
11-14.
Culivicchi G., G. C. Palmerini, and V. Scolari (1985). Behaviour of materials
in geothermal environments. Geothermics, 14, 73-90.
ENEL (1987). Breakages evaluation on drillings materials. Internal Report,
unpublished.
CIllPOS I T 1111
( p~ )
pi! ..
• •
NH. H 80 3 3
SO.
-
- - CO 2
H 2
S
C1 He° 3
S
(e) (e)
IJIIlllN6 S.S 20
130 700 200
15 - - 9 1
RE lUCY 1111 6.7 sa
200 700 533
20 150 (-) g 1
(e)
P.rtl.1 p,.. ... ,.. la bar
(-) s.t .... t.d
T1bh I.

14
TYPE OF I NIT I All 011 ~ CRACK
DRILLING STRING FA I LURES %
CIllPONENTS % Fatigue
S.C.C. Corrollon "echanlcal
DRILL
PIPE
HEVI
IlATES
DRILL
COLLARS
Total 27 - 73 27
-
Body 80 92 8
Box 10 - - -
Pin 10 - - -
Total 15 67 22 11
Body 12 - - -
Box 76 67 22 11
Pin 12 - - -
Total 38 52 5 43
Body - - - -
Box 96 50 5 45
Pin 4 100 - -
CROSSOVER Total 20 11 56 33
AND
STAB STRING
Box 100 11 56 33
Pin - - - -
Tabl. U. Drilling COIIponllllt Failures

IS
IlATERIALS
IlECIlAIICAL
PROPERTIES
Rp 0.2 Rn A Z Hardn .. a
( NI • .') ( ./.') ( ~ ) ( ~ )
0
AISI 4137 N 900 10211 15.7 58.2 301118
R
I AISI 4137 H ESR 853 971 21.8 &!I.1 297 III
L
L
API 8 1re 730 812 27.5
p
I API 8 105 ESR 730 812 27.5
p
E UMI 31* ICrllo12 ESR 864 1142 21.8 114.8
T
U
8
AISI 4130
UMI X1J:r13
1141
101
752
8re
28
23
20 lilt
20 IiIC
I
I UMI X2CrlilloI225 1158 1003 11.5 411 33.3 lilt
6
UMI X1IICrllaClI31274 1178 804 20.3 52 25.11 NRC
Tabll III. lIecwlcal prop.rll ... f
t .. t.d •• t.rl.lI at 20 It

0\
IIATERIALS
CHEHICAL COIIPOSITION ~
C S P SI lin Cr HI 110 Cu Sn N Al Y
0
AISI ~137 H 0.38 0.015 0.016 0.25 0.90 1.05 0.21 0.0~5
R
I AISI ~137 H ESR 0.37 0.002 0.007 0.28 0.90
L
1.06 0.11 0.20 0.70 0.009 0.015
L
API G 10S 0.31 0.007 0.22 0.22 1.38 0.35 0.08 0.11 0.18 0.007 0.08
P
I
P
API G 10S ESR 0.32 0.002 0.18 0.23 1.29 0.33 0.08 0.12 o.n 0.010 0.08
E
UNI 30NICrll012 ESR 0.33 0.001 0.009 0.18 0.60 0.79 2.96 0.~6 0.09
T
AISI ~130 0.29 0.004 0.011 0.26 0.70 0.88 0.09 0.21 0.15 0.013 n.d. 0.026 n.d.
U
B UN I X13Cr13 0.21 0.004 0.12 0.35 0.32 13.23 0.10 0.01 0.04 0.003 0.080
I
N
UN I X2CrN IlIoN225 0.018 O.OOS 0.028 O.~O
G
1.62 22.5 5.15 2.7~ 0.18 0.008 0.18 0.0~7 0.088
UNI X1NICrlloCu312H 0.16 0.009 0.016 1.07 1.80 25.81 33.00 3.~5 1.0~ 0.006 0.10 0.023 0.136
Tabl. IV. Chelleal eOlpo,lllon of ll,ted laterlal,

17
1/2 X
F I g. 1
DR ILL COLLAR S" BREAKA6E
200 X
FIg. 2
STRESS CORROSIIJI CRACXIII6 MN(J(IIA IJI IlRIU COlLAR
THREAD. REF. F 16. 1

18
1/4X
Fig. 3 CRACK ON CONSTANT TH ICKNESS 5' DR I LL PI PE
5 X
Fig. 4 OUTSIDE SURFACE 5' DRILL PIPE. REF. FIG. 3

19
100 X
Fig. 5
lON61TllliHAl SECTION. CORROSION FATIG~ CRACKS STARTS
FRaI PITS. REF. F 16. 3
5 X
Fig. 6
UJiGlTlIllMAl SECTION III FRICTION VELD AREA

20
EEC contract nO EN3G -
0042 - F (CD)
OPTIMIZING THE COMPOSITION OF
CASING CEMENTING MATERIALS IN
HIGH-ENTHALPY GEOTHERMAL WELLS
D. Degouy and M. Martin
Institut Fran~ais du Pet role
SUMMARY
After cementing materials have been pumped into the casing-formation
annulus, they must maintain suitable characteristics during the entire
lifetime of the well .. This study concerns the long-term behavior of some
actual or potential cementing compositions after they have been cured and
hardened under bottomhole conditions. The study is still in progress,
and only the data now available are given.
Bottomhole conditions are simulated by specially designed laboratory
-:quipment, including high-pressure (up to 50 MPa) and high-temperature
(150-350 0 C) cells capable of withstanding long-term exposure to
aggressive fluids. Several types of materials are considered, including
API well cements, experimental hydraulic cements with ~ specified
composition, and ari organic phenol-formol-furan resin. Behavior during
aging is characterized by compressive strength and water
permeability. Scanner tomographic analyses were performed on some
specimens, giving a cartographic representation of density and porosity.
Class G cement-silica systems which are characterized by a
2:3 silica-cement ratio still maintain suitable performances at 345°C,
at least for short-time aging. However, above 150°C, tomographic
analyses reveal a heterogeneity of the specimens due to slurry particle
settling and probably to differences in the local structure of the
cement. Class J cement does not seem to have better performances than
class G cement-silica systems.
A steam environment is unfavorable for a class G cement-silica system
and for class J cement. Indeed, permeability increases very fast between
three and six months after cementing. Under the same conditions, cements
with aluminous slag are less damaged and their water permeability is only
slightly modified.
The first tests with the phenol-formol-furan resin seem to be
hopeful.
1. INTRODUCTION
The tightness of the casing-formation annulus is a safety condition
of prime importance for both oil and geothermal. wells. It prevents
formation fluid channeling from one formation to another or to the
surface of the well. This tightness can be achieved if the following
conditions are satisfied: (1) complete mud displacement by cement slurry
while cementing, (2) cementing material impermeability after curing and
hardening, and (3) good cement bonding quality with both casing and
formation. Furthermore, during the life of the well, a loss of tightness
may occur, requiring remedial cementing jobs, which are always very

21
expensive and often fail.
The laboratory study described in this paper concerns cementing
materials. It alms to select long-lasting materials capable of
withstanding severe bottomhole. conditions, i.e. high-temperature
(150-350·C), aggressive formation fluids, and high-enthalpy geothermal
wells.
The following two types of materials are considered: hydraulic
cements, and organic resins.
The study is still ongoing. At the present stage, the results
cone-ern mainly cementing-material behavior after curing and while aging
under different bottomhole conditions.
2. LITERATURE DATA
Numerous laboratory results have already been published on the
behavior of cements designed for high-temperature oil wells or geothermal
wells. They mostly focus on systems consisting of class G or H cement
(according to the American Petroleum Institute specifications) and silica
or on API class J cements (Eilers, 1974, 1980; Gallus, 1978, 1979;
Kukacka, 1977, 1981; Nelson, 1981). For class G or H cement-s il ica
systems, the effect of silica grain size has been discussed (Eilers,
1979) .
Few other water-b inder compos i tions have been invest igated, such as
lime-alumina-silica-water (Kalousek, 1976; Roy, 1979) or lime-magnesiasilica-water
(Roy, 1979). More recently, the conclusions of research on
cements for thermal enhanced oil recovery wells have led to re·commending
the use of high alumina-content cements (Nelson, 1986). As a general
ru le, for cement-base mater I als, I t appears that formulat ions must be
carefully selected In keeping with bottomhole conditions (temperature,
formation fluids, thermal cycling).
Some other research has been done on the applicability of organic
resins, organosiloxane-base resins or resin-cement systems for geothermal
environments (Kukacka, 1977; Zeldin, 1980). But the field use of
organosiloxane-base resins cannot be envisaged because of their very high
e-ost. Concerning the organic resin group, the application of a
furfuryi-alcohol base resin has been proposed for a geothermal well
having a static bottomhole temperature of 227·C (Pettitt, 1979).
Some design criteria have been defined for evaluating the quality of
geothermal-well cements. The following values have been proposed for the
material's e-haracteristlcs after curing (Kukacka, 1981):
- compressive strength> 6.9 MPa
- water permeability < 0.1 mD
- bond strength to steel casing> 69 kPa
stability: no significant reduction in compressive strength or increase
In permeability after prolonged exposure at 400·C to 25% brine
solutions, flashing brine or dry steam.
3. TEST PROCEDURE AND EQUIPMENT
The evolution of the cementing materials studied is determined during
aging under bottomhole conditions.
3.1. Procedure
Materials were tested with cylindrical specimens having a 2.5 cm
diameter and 5 cm length. The characteristics were measured with set
cement, after aging during varying lengths of time under bottomhole
conditions, and after returning to atmospheric pressure and

22
temperature. They consisted of compressive strength (Rc) and water
permeability (k)
Moreover, tomographic analyses were made for some specimens by a
medical scanner. Axial cross-sections and perpendicular sagittal
cross-sections were investigated, thus glvlng a cartographic
representation of the whole sample. The cross-sections were 3 mm
thick. This method enabled the local variations of density to be
determined. Moreover, an evaluation of local porosity could be obtained
by subtracting the image of a dry specimen slice from the image of the
same slice when it is water saturated*.
3.2. Experimental parameters
The different test conditions are given in Table 1.
The behavior of the materials was determined by experiments with both
slow temperature variations (less than l°C/min) and very fast ones
(thermal shock).
3.3. High pressure and high temperature curing and aging apparatus
They included units including a curing and aging cell, a heating and
pressurizing device, and aging cells placed either in an oven or in a
high-pressure high-temperature fluid circuit. The main characteristics
of the laboratory apparatus are given in Table 2. Figure 1 illustrates
the 55-specimen capacity aging unit that was put into service quite
recently.
3.4. Nota
A great number of specimens are required for each formulation in
order to follow long-lasting aging by ,means of compressive-strength
measurements. Owing ,to the 'restricted capacity of the high-pressure
high-temperature equipment, a compromise had to be found between the
number of formulations studied and the aging time. Moreover,
p~rmeabili ty measurements (nondestructive testing) were performed more
frequently than compressive-strength measurements (destructive testing).
4. FORMULATIONS
They consisted of hydraulic cements (type G Portland well cement and
type J well cement) and a phenol-formol-furan thermosetting resin.
5. BEHAVIOR OF CLASS G CEMENT-SILICA SYSTEMS
All the cements used in this study were provided by the same
manufacturer. Forty parts of silica were mixed with 60 parts of dry
cement (weight parts).
* The scanner analyses were performed in the IFP Reservoir Engineering
Research Department.

23
5.1. Effect of silica grain size
As shown in Table 3, the finest grain-size silica has the following
effects: (1) the water permeability is reduced; but if the surrounding
fluid is salt water, at at least 290·C, this permeability increases
faster during aging; (2) if the surrounding fluid is fresh water, the
compressive strength tends to increase; (3) if the surrounding fluid is
steam, the compressive strength decreases.
5.2. Short-tena behavior
S.2.1 Mechanical characteristics and water permeability
Table 4 gives the characteristics after curing and 1 month of aging
under different simulated bottomhole conditions.
Depending on the bottomhole conditions, the di fferences in behavior
can be very great. Compared wi th the material cured and maintained in
fresh water at 1S0·C, the following variations can be noted:
- 1S0·C, salt water: Rc decreasing
- 200·C, steam: Rc decreasing
k increasing
- 200·C, fresh wat.er
_ 200.C, salt water } Rc decreasing
- 28S·C ~ T , 34S·C, fresh water R decreasing
- 28S·C , T ~ 34S·C, salt water } kCincreasing
Surrounding steam is particularly unfavorable.
When the temperature is higher than 200·C, the compressive-strength
values do not depend on the type of surrounding water (fresh or salt).
S.2.2. Density and porosit.y
Scanner tomographic analyses were undertaken for materials cured in
fresh water at different. temperatures. As shown in Table S and Figure 2,
they reveal some variations in both density (d) and porosity (p) between
the upper part (cross-section a) and the lower part (cross-section b) of
the cy li nder •
The average densi ty and pOI' os i ty of the sagittal cross-section (s)
are respectively representative of the average density (dm) and porosity
(pm) of the whole specimen.
It can be seen that: (1) at temperatures higher than 1S0·C,
extensive particle settling occurs before curing; average density
variations of up to 8% are observed between section a1 and section
a2; (2) the specimen's porosity increases with curing temperature;
(3) porosity variations between the lower and upper parts of the
cylinders may be very great (ratio of up to 2); they may be related to
rement particle settling and probably to local structural differences of
the hardening cement.
5.3. .iddle and long-tena behavior
Available results are given in Table 6. The unfavorable short-term
effect of a surrounding steam flow increases with time.
5.4. Thenaal shock effect
Table 7 compares the permeability to water for specimens aged under
the same conditions of temperature, pressure and surrounding fluids, with
half of them having been subjected to thermal shocks. The specimens were
either plain cement cylinders or cement cylinders kept inside the
sidewall of the curing mold (with strong bonding).

24
The thermal shocks were produced by very fast cooling from 150°C to
20°C.
No significant variation in water permeability is noticeable.
6 • BEHAVIOR OF CLASS J CEMENT
The results are shown in Table 8. At 150°C, the characteristics are
greatly modified during the first months. Between three and six months,
the compressive strength increases and the water permeability
decreases; this latter characteristic becomes very low.
A comparison with the behavior of class G-silica systems shows that:
(1) the surrounding steam flow is also unfavorable and rapidly leads to a
deteriorated material, and (2) between 285 and 350°C, in fresh water,
compressive strengths have the same level, but water permeabilities are
higher (about 2 to 5 times).
7. BEHAVIOR OF EXPERIMENTAL CEMENTS
Experimental cements were composed of clinker, aluminous slag and
silica. They were different by the slag-alumina content and the silica
grain size. The main trends of behavior according to compos i tion are
summarized in Table 9. The silica grain size does not seem to have any
primordial effect.
Table 10 compares the characteristi cs of the experimental cements
(called Exp 1, Exp 2, ... , Exp 7) and the characteristics of class G
cement-silica systems (already mentioned in Tables 3 and 4). It is
particularly noticeable that cement·s containing aluminous slag maintain
very low permeability even when surrounded by a steam phase flowing at
200°C.
8. STUDY OF PHENOL-FORMOL-FURAN RESIN
The choi ce of this type of res in was guided by the properties
required for field applications, i.e. nontoxic and nonflammable material
that can possibly be implemented with standard cementing-job equipment.
Table 11 shows the characteristics obtained for pure resin at
temperatures of around 300°C.
Fillers were selected. They consisted of mineral powders, that are
thermally stable and compatible with the resin. Their concentration must
be adjusted, depending on the grain size, in order to obtain a correct
slurry rheology and to prevent particle settling between setting inside
the hole and curing.
9. CONCLUSIONS
At the present stage of the study, the following results must be
underlined:
1. A surrounding steam phase flow is quite unfavorable for the hydraulic
cement. At 200°C if this is the case, aluminous slag cements
advantageously keep permeabili ty low, at least for medium-duration
aging.
2. Class J cement does not seem to result in better performances than
class G cement-silica systems. However, long-term behavior still has
to be determined at temperatures higher than 200°C.
3. The results of the first tests with phenol-formol-furan resin are
rather hopeful.
Of course, the conclusions will be completed later on for each
material in keeping with the results obtained for longer aging.

25
REFERENCES
Eilers, L.H., and R.L. Root (1974). Long-term effects of high temperature
on strength retrogression of cements. 49th Ann. Fall Mtg of S.P.E. of
A.I.M.E., Paper S.P.E. 5083.
Eilers, L.H., and E.B. Nelson (1979). Effect of silica particle size on
degradation of silica stabilized Portland cement. S.P.E. of A.I.M.E.
Int. Symposium on Ollfield and Geothermal Chemistry, Paper
S.P.E. 7875.
Gallus, J.P., D.E. Pyle, and L.T. Watters (1978). Performance of oil-well
cementing compositions in geothermal wells. 53rd Ann. Fall Techn.
Conf. of S.P.E. of A.I.M.E., Paper S.P.E. 7591.
Gallus, J.P., D.E. Pyle and L.K. Moran (1979). Physical and chemical
properties of cement exposed to geothermal dry steam. S.P.E. of
A.I.M.E. Int. Symposium on Ollfield and Geothermal Chemistry, Paper
S.P.E. 7876.
Kalousek, G.L., and S.Y. Chow (1976). Research on cements for geothermal
and deep oil wells. 51st Ann. Fall Techn. Conf. of S.P.E. of
A.I.M.E., Paper S.P.E. 5940.
Kukacka, L.E. (1977). The applicability of concrete polymer materials for
use in geothermal environments. S.P.E. of A.I.M.E. Int. Symposium on
Oilfield and Geothermal Chemistry, Paper S.P.E. 6611.
Kukacka, L.E. (1981). Current
development. Int. Conf. on
Technology, Albuquerque.
status of
Geothermal
geothermal well cement
Drilling and Completion
Martin, M. (1986). Mat~riaux de cimentation de faible densit~ pour puits
g~othermiques. Contrat CEE - EGB - 1 - 008 - F (D), Final Report.
Needham, P.B., and others (1980). Chemical analysis of brines from four
Imperial Valley, C.A., geothermal wells. S.P.E. Journal, 20, 2,
105-112.
Nelson, E.B., L.H. Eilers and L.B. Spangle (1981).
development of cement systems for geothermal wells.
Mtg of S.P.E. of A.I.M.E., Paper S.P.E. 10217.
Evaluation and
56th Ann. Fall
Nelson, E.B. (1986). Improved cement slurry designed for thermal EOR
wells. 011 and Gas Journal, 84, 49, 39-44.
Pettitt, R.A. (1977). Completion of hot dry rock geothermal well systems.
54th Ann. Fall Techn. Conf. of S.P.E. of A.I.M.E., Paper S.P,E. 8627.
Roy, D.M., and others (1979). Potential new high temperature cements for
geothermal wells. S.P.E. of A.I.M.E. Int. Symposium on Oilfield and
Geothermal Chemistry, Paper S.P.E. 7877.
Zeldin, A.N., and L.E. Kukacka
well-completion materials
Laboratory, Doc. B.N.L. 51287.
(1980). Polymer cement geothermal
Final Report. Brookhaven National

26
Table 1
Simulated bottomhole conditions
Ref. T("c) P(MPa) Fluid
A 150 1.5 fresh water
B 150 1.5 salt water*
C 200 1.45 dry steam
D 200 20 fresh water
E 200 20 saltwater
F 285 ~T~ 350 40 fresh water
G 285 ~T ~ 345 40 saltwater
• The pH of the salt water in the fonnation was 5.2. The salt content was 206 gIL (with CI- =
125 gIL, Na+ = 51 gIL, Ca++ = 20 gIL), which is close to the published chemical analyses of
brines from Imperial Valley geothermal wells (Needham, 1980).

Table 2
Laboratory equipment for studying cementing materials under simulated bonomhole conditions
Test conditions
State of the Equipment Capacity Equipment
cement studied type Tmax Pmax fluid Circulation Number (specimen) origin
fC) (MPa)
350 50 Water no 2 6
Curing and
Curing and/or hardened 350 50 Brine no
aging units
1 6 Designed and manufactured
material evolution 350 50 Water yes
Brine
1 5 by IFP
under high pressure (Martin, 1986)
Aging units
350 50 Water yes 1 II
Brine
500 50 Water no 1 55 Designed and manufactured
by IFP (Fig. I)
180 2 Water no 10 6 Marketed
Hardened material Brine equipment
evolution, under AgingceUs
medium pressure 200 2 Steam yes 2 100 Designed and
manufactured by IFP
(Martin, 1986)

28
TabIe3
Silica grain size effect
Simulated bottomhole conditions Silica· Age: 1 month Age: 3 months Age: 6 months
Ref P T Fluid Dso Content k.1Q3 R., k.1Q3 Rc k.l03 R.,
(MPa ("C) ijun) (part) (mD) (MPa) (mD) (MPa) (mD) (MPa)
A 1.5 150 Fresh water 50 40 2 49

29
Table 4
Class G cement - silica systems. Shon-tenn behavior
Composition (weight pans) - Cement = 600, Silica = 400, Water = 424.
Age = 1 month.
Simulated booomhole conditions
Ref. P (MPa) T("C) Fluid
(MPa)
A 1.5 150 Fresh water 59
8 1.5 150 Salt water 40
C 1.45 200 Steam 23
0 20 200 Fresh water 46
E 20 200 Salt water 47
40 285 Fresh water 38
Rc
k.lQ3
(mD)
2
1
7
2
3
Rc k
~ (k)A
1 1
0.7
0.4 4
0.8 1
0.8
0.65
F 40 330
40 345
" 36
..
34
40
0.6 20
0.55
G
40 285
40 330
Salt water 38
..
33
Table 5
Tomographic analysis results
20
40
0.65 10
0.55 20
.----.
1---+-+-- 12
Class G cement - silica system. (Oven dried specimens)
Simulated bottomhole
conditions It. 1 Q3 dua d(s) d(a.) d(aV Pm
P (MPa)
T("c)
(mD) (%)
12 60 0.9 1.59 1.58 1.58 1.61 33.6
1.5 150 0.2 1.60 1.61 1.60 1.62 29.7
25 175 0.4 1.52 1.50 1.47 1.60 42.0
25 216 10 1.44 1.40 1.40 1.52 48.0
40 290 50 1.44 1.43 1.42 1.53 49.1
40 358 200 1.40 1.40 1.40 1.51 50.9
1---+--4-II
p(s) p(a.) p(a2)
(%) (%) (%)
36.1 36.7 30.1
29.5 31.2 26.1
45.6 54.0 27.5
49.5 56.2 35.6
49.8 59.4 33.9
51.9 59.4 39.2

30
Table 6
Medium and long-tenn behavior
Composition (weight pans) - Cement = 600, Silica = 400, Water = 424
Simulated bottom hole conditions
Age I 2
Ref. P (MPa) T(°C) Auid (month)
A 1.5 150 Fresh water R., (MPa) 59
k.I03(mD 2
B 1.5 150 Salt water R.,(MPa) 40 40
k.I03(mD

31
Table 8
Class J cement behavior
Composition (weight pans) - Cement = 1000, Water = 440
Simulated bollomhole conditions
Age I 2 3 6

32
Table 9
Experimental cements.
Main behavior trends versus cement composition
Sim\!lated bonomhole
conditions
Behavior
Ref. P (MPa) T("q
A 1.5 150
Fluid
Fresh water
Very similar, whatever the composition
may be : good stability
C 1.45 200
Steam
Better stability for formulations containing
aluminous slag
D' 40 200
E' 40 200
285
F 40 to
350
Fresh water
Salt water
Fresh water
f
Very similar, whatever the composition may
be (insofar as it contains gypsum)
When the cement contains low alumina slag,
at T > 300°C, higher Rc with fewer
variations versus T
G 40 290
Salt water
When the cement contains aluminous slag,
less sensibility to salt (at least for a shon
time)
Table II
Rc (MPa)
T(oq
k.lQ3 (mD)
Behavior of pure resin
Simulated bottomhole conditions
P = 40 MPa - 285°C < T < 345°C
Fluid = fresh water
Age = 1 month
285 330
1
63.5 57.5
345
1
63

33
Table 10
Comparison between experimental cements and class G cement - silica systems
Simulated bottomhole conditions
Comparison with G - Si~ systems
Ref. P T Fluid Material T Age R., k.lQ3
(MPa) ("C) fC) months (MPa) (mD)
A 1.5 150 Fresh water G+Si~ 150 12 60 2
I.a. Exp2
.. ..
39 I
2.a. Exp7
.. ..
39 I
2.b. Exp4 46.5 1.5
C 1.45 200 Steam G+Si~ 200 6 28 500
I.b. Expl
.. ..
25 0.7
I.a. Exp3
.. ..
21 I
D 20 200 G +Si0 2 200 I 46 2
Fresh water
D' 40 200 2.b. Exp4
.. ..
26.5 7
I.a. Exp3
.. ..
25.5 I
E 20 200 G + Si0 2 200 I 47 3
Salt water
E' 40 200 2.b. Exp4
.. ..
22.5 3
I.a. Exp3
.. ..
26.5 I
F 40 285 Fresh water G + Si0 2 285 I 38 30
to I.a. Exp2
..
33.5 60
350 2.a. Exp5
..
33 15
2.b. Exp4
..
45.5 5
2.b. Exp4 350 3 44 20
2.a. Exp7
.. ..
25.5 60
G 40 290 Salt water G + Si0 2 290 I 38 30
I.a. Exp2 " " 31 30
2.b. Exp4 "
..
26 15
I. Formulation with aluminous slag
2.
a.
" with low alumina content slag
with coarse silica (D > lOO~)
b. with both coarse and fine silica
(D> 100 ~m : 300 parts ; 1 < D < 10 ~m : 100 parts)

34
Fig.1. Horizontal ageing unit
(Capacity : 55 specimens)
Fig.2. Specimen holder during.
its placement into the cell.

35
Fig.3. Cartographic representation of density by means of a medical scanner
Parameters : curing temperature en and pressure (P)
T' : 80°C - P : 21 MPa
T' : 1 ~C - P ; 1.6 MPa
T' : 216°C - P : 40 MPa

36
EEC contract nO EN3G - 0039 - B(GDf)
On-site material damage evaluation for low enthalpy geothermal venture
based on saline cretaceous formation water
H. Tas, J. Dresse1aers and P. Dirven
S.C.K./C.E.N. - BELGIUM
Summary
The Maastrichtiaan aquifer, situated at the top of a cretacious
formation, extends over a relatively large area in the north of
Belgium. A first well situated in the urban centre of Turnhout
(depth: 800 m, temperature at well head: 37°C) was taken in
operation at the end of 1985 for feeding a thermal powder station.
In order to evaluate the corrosion and deposition damage, which may
occur in this geothermal water, a water characterization and
materials test station has been connected to the geothermal circuit,
which has been operated during a first relatively short period (2
months) and has now been taken in operation for a relatively long
period (twelve months) (Fig. 1).
This report describes the water chemistry evolution during the
two-month test campaign; gives results of the instantaneous corrosion
rate measurements and presents detailed analyses of the exposed
material coupons. Correlations are proposed between the amounts of
deposits observed and the construction materials of the system.
Finally, the results of bacteriological analyses are given and the
performance of some important components of the geothermal station is
described.
1. Introduction
A yearly thermal energy of 650,000 kWh is produced by the geothermal
power station and is fed to the air conditioning system of a swimming
poo 1, a theatre ha 11 and meeti ng rooms, to the san i tary water of the
swimming pool and to the swimming pool by using part of the geothermal
water as direct feedwater.
A submerged pump installed at a depth of 75 m brings the geothermal
water to the surface with a temperature of 37°C.
The geothermal water is 1 ed to a set of three pressuri zed storage tanks
with a capac i ty of 800 1 each before be i ng di stri buted to the users. A
fraction of the hot geothermal water flow is fed into the high temperature
test section of the test station. The geothermal water, cooled down to
about 19°C by means of a heat pump, is fed into the low temperature
section of the test station. . 3
The test station was constructed for a flow of approximately 5 m per
hour and compri ses di fferent on-1 i ne water characteri zati on un i ts. four
material coupon test sections, water, gas and bacteriological sampling

37
lines and electrochemical corrosion rate sensors.
The measurement units are installed before the material test sections,
allowing continuous measurement of some important characteristics of the
water such as temperature, pH, conducti vi ty and oxygen content. The
material test sections consist of two parts : one part with a high water
flow rate and an other part with a low flow rate.
2. Water chemistry evolution
The parameter evolution (degree of acidity, temperature, pressure,
conductivity) proved to be constant over the entire two-month test campa
i gn. I t can therefore be concl uded that the water qual i ty of the
geothermal well is stable.
3. Deposit formation
Depos i ti on products found in the hi gh-temperature test secti on and
the low-temperature test section are very different. Deposits observed in
the high-temperature test section are very thin and black. The specimens
of the low-temperature test section are covered with a thick brownish
deposition layer (Fig. 2).
Based on the wei ght di fference between as-exposed and cleaned coupons,
deposition rates could be established and related to the flow rate
and water temperature in the test sections. Table I gives the mean values
of the weight changes as measured for some of the most representative test
specimens. In general, a better adhesion of the deposition products onto
the synthetic materials as compared to the metallic materials is apparent.
At the higher flow rates the amount of deposits is diminished by a factor
of two. In the low temperature test sections the amount of deposition
products is much higher than in the high temperature test sections. This
effect is to be related to the higher amount of precipitated matter in the
low temperature water flow (table II). After test completion all specimens
were covered with a thin deposition layer. However, the adhesion of
thi slayer to the surface is very low, as there is no reaction between
these deposits and the exposed coupon materials.
Table I : Mean weight change values (mg/cm 2 )
Representative specimens
Test Section
HT-HF HT-LF LT-HF LT-LF
No corroded specimens 0.15 0.25 1.2 3.5
Synthetic materials 0.5 0.8 1.5 8.5
HT High Temperature LT : Low Temperature
HF High Flow rate LF : Low Flow rate
As a control of the amount of precipitated matter in the water flow
as a function of time, filters of 0.45 1.1 were introduced into the test
sections at regular time intervals. Because the flow rate of the geothermal
water across the filters diminished as a function of exposure time,
the val ues represented in 3ab leI I are mean values normal i zed for a
geothermal water flow of 1 m. The same controls are planned for the one
year test run.

38
Table II Mean value of preci~itated matter as collected on a 0.45 ~
filter out of a 1 m geothermal water flow (mg).
Sampling number Cold test section Hot test section
1 38.5 13.4
2 43.5 10.8
3 36.8 13.8
4 33.7 13.0
5 28.1 12.6
6 27.3 8.5
We can concl ude that the amount of preci pi tated matter di d not change
duri ng the test run except for a small decrease at the end of the test
period. However, the fact that the amount of precipitated matter in the
co 1 d 1 eg is hi gher by a factor of three as compared to the hot 1 eg,
demonstrates that extensive precipitation did occur in the cold geothermal
circuit.
Typical chemical compositions of the deposits collected on both filters
are given in table III.
Table III : Chemical compositions of the deposits collected on the filters
(wt.%)
Fe Mg Sr Ca Cr Zn Cu Na Sal.
Cold test 21.8 0.4 0.24 1.0 0.07 0.79 0.26 5.8 (Org.Pr.+H 2
O)
section
Hot test 7.3 0.35 0.18 1. 55 1.03 2.4 17.3 1.4 (Org.Pr.+H 2
O)
section
Silicon was found in both cases but could not be determined quantitative~y
by the analysis technique used. The high quantity of iron found on the
filters of the cold test section indicates that important precipitation of
iron compounds in the cold geothermal circuit took place. The higher
quanti ty of sodi urn may be caused by a lower so 1 ubi 1 i ty of some sodi urn
compounds at lower temperature. The origin of the relatively high copper
and zi nc concentrati ons on the fil ters inserted in the hi gh temperaure
test sections in not evident. Further analyses will be performed during
the one-year test period in an attempt to elucidate these observations.
4. Coupon analyses
The corrosion rate of the exposed materials is measured for two
temperatures and two flow rates. All weight changes were determined both
with and without removal of· the deposits. Chemical cleaning procedures
were used as described in the ASTM-standards.
In general, no clear relation could be found between the flow rate and the
exposure temperature on the one hand and the observed corrosion rates on
the other hand for most of the tested materials. This has to be ascribed
to the good resistance of most of the materials to the geothermal water
environment.

39
The corrosion results are discussed for each of the different
material families or materials applicable for particular components of the
geothermal station. Optical metallography, SEM and EDAX analyses were
carried out in more detail for those specimens, which were found to show
particular phenomena.
a) Construction materials for heat exchangers (titanium - Hastelloy C -
AISI 304 - Incoloy 800).
The corrosion rates of all candidate construction materials for heat
exchangers are very low, i.e. the materials tested appear to have a
very good resistance against the corrosivity of the geothermal water
(Fig. 3).
b) Hastelloy Band C
A pronounced difference of corrosion rates was observed for Hastelloy
Band Hastelloy C. The chemical composition of these materials differs
by their chromium and molybdenum contents. The higher chromium content
of Hastelloy C results in much lower corrosion rates than for Hastelloy
B. Chlorine, sodium and sulphur are probably the prime corrosion
promotors found in the corrosion layer.
At lower flow rates the corrosion rate of Hastelloy B coupons
diminishes by about thirty per cent.
c) Casin and roduction tubin materials
All ca ing and tubing materials except "API C-75" are corroded at a
mean r te of 100 ~/year in the high temperature test section and 250
~/year in the low temperature test section (Fig. 4). Influence of the
chemical composition (Table IV) on the corrosion rate is hardly
apparent; however, the casing material "API C-75" has a much better
corrosion resistance in the cold test section possibly due to its
chromium content.
Table IV : Chemical composition of the exposed casing and tubing
materials (wt.'X.)
C Mn P S Si Mo Cr V Ni M
API-N-80 0.40 1.10 0.022 0.22 0.20 0.24
API-C-75 0.40 1.50 0.015 0.23 0.22 0.21 0.03 0.03
API-J-55 0.40 1.00 0.023 0.025 0.30
API-St-37 0.037 0.25 0.05 0.015 0.003
Fig. 4 shows the correlation between temperature and the observed
corrosion rates. The corrosion rate of the casing materials is
clearly lower at the higher exposure temperature. This unexpected
phenomenon can be explained by the higher oxygen content in the
cooled geothermal water leg.
d) Aluminium alloys
All aluminium alloys suffer important corrosion damage of about + 40
~/year. Both temperature and the flow rate tend to increase thecorrosion
rate. Metallograpic examinations reveal well developed
corrosion layers at the surface of the tested alloys. Microprobe
analysis identifies sufphur and chlorine as corrosion promoting
elements.

40
5. Performance of construction materials used during 2 years in the
geothermal station
- ~~:£Q~!~Q_£~rQQ~_~!~~l_£~~i~g_!~Q~
A general view of the examined tube shows deposition products on the
outside as well as on the inside of the tube. The chemical composition of
the tube as such is given in table V. Sulphur and chlorine are present in
the deposition layer. However, the zinc layer, although affected by
ch I ori ne and suI phur, sti 11 protects the carbon steel cas i ng tube even
after two years of exposure.
- Y~lY~_Qf_!~~_g~Q!~~r~~l_~!~!iQ~ (Nickel-aluminium bronze)
The valve functioned in the hot leg of the geothermal station at a
location, which is comparable with the hot test section of the test
station. Its chemical composition is given in table IV. Although little
scaling developed, sticking of the valve nevertheless occurred by
corrosion at its outer surface.
Microprobe analysis shows that copper and aluminium react with chlorine
and that sulphur reacts with aluminium. The thickness of the reaction
layer reaches about 200 1.1. Corrosion pits with a depth of 300 IJ were
detected also.
- ~~11_2!QQ~£!iQ~_!~Q~
The geothermal well was equipped with a flexible well tube lWellmaster).
The "Wellmaster" tube was cut into rings and mounted on specimen holders
of the test sections. No deformation or damage of the rings was
established. Only a thin layer of deposition products was observed.
Table V : Chemical composition of construction materials examined (wt.%)
Ni Mn Si C N Fe Cu Zn Al Mn
Casing tube
Valve
Well tube
(We11master)
0.015 0.25 0.05 0.037 0.003 Bal. 0.02
4 81.3 9
composite construction with a thermoplastic elastomer
as primary component.
1.2
6. Bacteriological investigations
At the head of the geothermal well a sampling device was installed
for bacteriologically contamination-free water sampling.
Results of the bacteria population an~lyses :
- Aerobic bacteria : 2 x 10 Iml
- Anaerobic bacteria: 300 - 800 collml
- The presence of sulfate reducing bacteria could be
demonstrated in a qualitative way only.
Relations between the presence of bacteria and the observed corrosion
rates will be established.

41
7. Gas analysis
A system for gas sampling without aeration was installed in the hot
and cold legs of the test facility, consisting of an equilibration vessel
and a volume measuring flask. Sampling bulks of 50 ml volume could be
connected to it for regular off-line gas analysis. The mean composition
of the gas was found to be
< 0,1' 95,5' 0,55' 1,5' 2,5'
A str1k1ng feature 1s the h1gh concentration of N 2
.
8. "Corrator" measurements
Interpretati on of the fl uctuat1 ons of the corrator measurements is
poss1ble 1n terms of oxygen content and flow rate variations. The regular
fluctuat10ns of the flow rate in the cold test section are the result of
the per10d1c operat10n of the heat pump for higher heat production. The
oxygen content (0.01 PPM) in the cold leg of the test station follows the
fluctuat10ns of the working period of the heat pump. During the periodic
1 nact 1 v1 ty of the heat pump the oxygen content decreases to zero. The
very h1 gh oxygen peaks correspond wi th short operati on interrupti ons of
the geothermal stat10n. The oxygen content of the hot circuit (-6 PPB) is
somewhat more stable but also increases drastically at every operation
1 nterrupt1 on of the geothermal well. The corros i on rates estab 1 i shed by
the automat1c corrosion measurements are in good agreement with the
results of the coupon analyses.
- Sta1nless steel AISI 316
The corrosion rate of stainless steel does not appear to depend upon the
flow rate 1n the test sect10ns. After 500 hours a corrosion rate of 7
~/year 1s measured 1n the hot test section. This corrosion rate decreases
aga1n probably due to a passivation layer formed on the sta1nless steel
probes. No corros10n rate higher than 1.3 lA/year can be establ ished in
the cold test sect10n and only small fluctuations are evident alternating
w1th zero read1ngs.
- Casing mater1al - St 37 (Figs. 5 and 6)
The corros10n rate (! 300 lA/year) 1n the cold test section 1s obviously
h1gher than 1n the h1gh temperature test sect10n (+ 100 lA/year). This
phenomenon may be due do the h1gher oxygen content in-the low temperature
test sect10n. It is remarkable that the corrosion rate in both test
sect10ns 1ncreases dur1ng a temporary 1nactiv1ty of the geothermal
stat10n (th1s phenomenon 1s 1ndicated by points A, B, C 1n Figs. 5 and 6).
In the cold test section the fluctuat10ns of the corros10n rate follow the
fluctuations of the flow rate. In the hot test section however, such
dependency could not be established because the flow rate 1n this se€tion
was stable.
- Galvan1zed steel
In both test sections a decrease of the corrosion rate to a stable low
value of about 25 lA/year gradually established. In the cold test section
a small 1nfluence of the flow rate fluctuations was observed. The
temporary high corros10n rate observed in the high temperature test
sect10n mAY be due to a defect 1n the coated zinc layer which subsequently
healed by the cathod1c protection capability of the zinc.

42
9. Preliminary conclusions
- Higher corrosion rates were observed in the cold leg of the geothermal
circuit probably because of a higher oxygen content.
- In general alloyed steels and synthetic materials were little or not
affected after the two-month exposure to the geothermal water flow.
- The water quality of the geothermal well during the two-month test
period remained constant, except for occasional oxygen excursions at
operation interruptions.
- Oxygen, chlorine and sulphur were identified as the main agents causing
enhanced corrosion of carbon steel, aluminium, Hastelloy Band nickelaluminium
bronze.
- Interruption of the operation of the geothermal station gave rise to
enhanced corrosion rates.
- Deposition rates depend on the material the deposit forms on.
Synthetic materials retain larger amounts of deposition products than
metallic materials.
- No major chemical interaction between the deposits and the exposed
material coupons could be observed. At higher flow rates lower
amounts of deposited material were found.
- The amount of deposited material was higher in the cold leg of the
geothermal circuit than in the hot leg of the geothermal circuit.
References
- Vandenberghe, N. and J. Bouckaert (1980). Geological Aspects of the
Potential to Exploit Geothermal Energy in the North of Belgium.
Aardkundige Dienst van Belgie, Professional Paper 1980/1 No. 168.
Tas, H., J. Dresselaers a~d P. Dirven (1988). On-Site Material
Performance Test in Hypersaline Geothermal Water as found in the North
of Belgium. Contribution to the International Conference on Applied
Geothermal Energy and Thermal Energy Storage (JIGASTOCK 88).
Versailles, October 18-21.
Louwrier, K.P. and A.J. Van Riemsdijk (1981). Corrosion problems in Low
Enthalphy geothermal systems. International Conference on Geothermal
Energy, Florence, Italy, May 11-14.

43
Fi g. 1
Implantation of the tpst station in the geothermal po\~e r
station.
Fig . 2
A. Deposit on the specimens of the high temperature test
section.
B. Deposit on the specimens of the low temperature test
section.

44
Fig3
Materials for heat exchangers.
o.a
0.11
>.
--
:::I.
'-'
Q)
'"'

45
FIG 5 - INSTANTANEOUS COl. RATE OF CASING MAT.(STJ7) IN THE COLD TEST SECT.
_500
~
A
.. -0
u
_ 400
...
....
c
.. lOO
..
-0
en
0
:; 200
u
100
a
141 747 141 151 1051 I22J IJ27 14JO
TIME (H)
FIG I - INSTAITANEOUS COR. RATE OF CASING MAT.
(STJ7) II THE HOT TEST SECTION
-
_700
~
C
5100
u
...
....
c
-~50a
_ 400
0
en
:;: loa
...
0
A
u
200
loa
a
141 747 141 151 1051 122J
TIME (H)
lU7 1430
I

46
E.E.C. Contract No EN 36-D037-F (C.D)
INSTRUMENTAL METHOD FOR COUNTING SULPHATE-REDUCING
BACTERIA IN GEOTHERMAL WATER
F. COLIN and M.J'. JOURDAIN
Institut de Recherches Hydrologiques, N'ancy, France.
Summary
Development of an instrumental method for the detection
and semi-quantification of sulphate-reducing
bacteria in geothermal water, based on the
following principle I measurement of the culture
duration in the exponential phase of growth required
for reaching a predetermined metabolite concentration
(sulphides) detected by electrochemical
way :
- definition of the culture medium and of the
detection system,
- validation on pure and mixed cultures (water from
the natural environment),
solving cell sterilization problems and maintaining
the long-dated efficiency of the e~rochemical
detection system,
The obtained method presents the advantage of
giving a response time of only 24 to 48 hours,
instead of 8 to 30 days with a classical analysis.
The conception of an automatic apparatus in line
can be considered.
1. INTRODUCTION.
Sulphate-reducing bacteria constitute the most frequent
form of bacterial corrosion in geothermal waters,
which are generally characterized by a high salinity and a
high gulphate concentration.
In previous works (Colin, 1985), we clearly established,
in the course of laboratory tests, the relation
between the corrosive activity and the importance of the
applied population of sUlphate-reducing bacteria.
A simple count of these organisms nevertheless does
not suffice for the quantitative appreciation of the bacterial
corrosion risk, which depends not only on the bacterial
count, but also on their activity connected to their physiological
condition. The classical methods of bacterial counting
(A.S.T.M., 1965) moreover present the drawback of a long
response time up to 30 days,' which rnakes them unattractive
for the management and' control of preventive or curative
measures for controlling bacterial contamination and its
effects (use of biocides).

47
These drawbacks j usti fied the need for the
research described her., the objective of which is the development
of an instrumental method for the detection and semiquantification
of such bacteria, with a response time much
shorter than that of conventional analytical methods. The
goal which we enrleav~ur to reach is of a few days, and in
any case less than one week.
The general principle of the studied method is as
follows :
A water sample is injected into a sterile culture
medium with a special formulation for obtaining an optimum
growth of bacteria. The present microorganisms multiply according
to an exponential law, by modifying the characteristics
of the culture medium, at the same time by substra,te removal
and generation of metabolites. The amplitude of the modifications
of the medium after a fixed period or, on the contrary,
the time required for reaching a predetermined condition,
can be quantitatively related to the initial count of the
microbiological population.
This principle was successfully applied to coliform
bacteria (Bernard and al., 1987), by using the detection of
the production of molecular hydrogen during the growth of
these bacteria on a specific culture medium. The success
obtained in the study of this method, leading to the marketing
of a patented instrument (Socea Balency, 1985) with an industrial
vocation and a response time of a few hours, led us to
proceed analogically in the definition of our research programme.
In the present account, we shall report the followed
approach, the encountered difficulties and the obtained performances.
As the application for a patent is considered,
certain informations with a technological character (nature
of the indicator electrode and of the culture medium) will
be intentionally very limited here.
2. THE FOLLOWED APPROAC~.
A first phase of the research consisted to define
the couple culture medium/detection system of metabolite
production, giving the maximum performances in terms of response
time, sensitivity of the detection system, and specificity
to sulphate-reducing bacteria.
Various culture media were thus defined on the basis
of literature informations, and notably the compilations by
Postgate (1979) and Herbert & Gilbert (1984), and thereafter
were empirically adjusted to the required specific conditions.
This led to the preliminary selection of 4 media used for the
tests of instrumental detection.
As to the detection of metabolites, we decided to
study in priority potentiometric systems, because of their
extreme simplic!ty. The problem is then to detect, by means

48
of a sensitive electrode coupled with a reference electrode,
the production of sulphide or hydrogen sulphide. This phenomenon
must be accompanied by a sudden shift of the electrode
potential, which can be easily identified. We have this way
tested electrodes pertaining to one of following classes :
- noble metal electrodes, the potential variation of which
shall be determined by the redox potential of the medium,
- electrodes of a metal corrodible by the sulphides produced
by the bacterial growth.
Four electrodes were thus selected and studied by
using the above four culture media, in the course of discontinuous
tests effected in laboratory cells, and enabling to
draw a parallel between the initial count of the medium,
determined by the laboratory reference method (ASTM Standard
o 993-58, technique of the most probable number MPN), and
the test results, i.e.: initial potential of the electrode,
final potential, importance of the potential shift observed
during the test, and finally the time required for eventually
reaching a quite distinct potential shift.
At the end of the tests, a final selection of the
couple culture medium/detection electrode was effected.
The second part ~f the research related to the
solution of various technological problems that are of
maj or importance for the automat i sat i on of the
method :
- definition of a working cycle of an automatic cell with
discontinuous measurement : filling, measure, emptying,
restoring the starting conditions of electrode, sterilization,
- development of a technique which secures at the same time
the roles of cell sterilization and restoring the starting
condi tion of the eLectrode by using reagents with a non rema-
- nent character,
- search and adjustment of a reference electrode unaffected
by the above processes and which cannot be fouLed or aLtered
with time by the environmental conditions of the measuring
cells (insensitivity to sulphides).
These problems were solved, and the solutions were
confirmed by laboratory investigations, which reproduced
successions of cycles similar to those of an automatic device.
The third part of the research related more specially
to the evaluation of the measuring performances obtained
in various cases :
- on pure cultures of sUlphate-reducing bacteria in aqueous
medium,
- on (non-surface) waters from natural environment, corresponding
to mixed cultures of sulphate-reducing bacteria and
common germs, in the presence of complex and poorly defined
substrata and pollutants,
- on geothermal waters containing sulphate-reducing bacteria
(in progress at the writing date of the present account).

49
3. OBTAINED RESULTS.
3.1. Selection and evaluation of the couple culture
medium/detection system.
The following fundamental selections were effected
in function of the results obtained in the course of this
phase I
- selection of a culture medium deprived of iron and specially
optimized in function of the detection system,
- selection of a measuring electrode of the type ·metal
corrodible by sulphides·.
The principle of the measurement is then such, that
the formation of metabolites (sulphides) is detected by the
corrosivity of this species against the utilized metal. The
method thus reproduces the corrosion process itself and,
when sulphate-reducing bacteria may exist under various more
or less active forms (sporulating forms for instance), the
results will be expressed as bacteria-equivalent counts with
the same corrosivity as those utilized for the calibration.
- provisory selection of a detection method with a simple
measurement of the potential shift of the above metal electrode,
determined by means of a classical Ag/AgCl-reference
electrode.
In such conditions, calibrations were effected at
35°C with a pure strain of Desulfovibrio desulfuricans
(NCIB 8 372) for various options according to the above
selections.
The obtained potential records show that I
- during incubation, the bacterial growth is indicated by a
distinct shift of potential from a very stable base line,
- this potential .hift is the more rapid as the count of
sulphate-reducing bacteria in the studied water is high.
It is reached after a period of 30 to 40 hours according
to the initial count (from 10 to 104 germ. per 100 ml).
Thanks to a special treatment of the electrode metal, it
was possible to divide by 2 these response times, but with
an unacceptable lack of reproducibility.
- for the first tests, the amplitude of the potential shift
is about 170 millivolts, an easily measurable value. A
further optimization of the characteristics and of the
nature of the indicator electrode enabled to reach potential
shifts of more than 400 mv.
- the course of the potential shift with time depends very
little on the count of the studied sample ; this contribute.
to the reproducibility of the method and allows not
to wait until the end of the potential shift, the duration
of which is comprised between 2 and 3 hours.
3.2. Definition of disinfection solutions and solution
of the problem of bringiDQ back the indicator
electrode to its initial condition.

50
The searched solution consists to simultaneously
solve both these problems, to whieh are added constraints of
the absence of any infLuence on the reference eLectrode.
Our work related to the composition of a solution
for filling the measuring cell after the operations of removal
of the polluted medium and rinsing at the end of the measuring
period, during the cycle. Following solutions were tested :
: 0,1 and 1 N nitric acid solution,
- potassium phta1ate solution in acidic medium,
- another solution definitively retained at the closure of
tests.
In the first two cases, the desired result is obtained
during the first cycles, and then difficulties appear
after 5 cycles, under the form of nfa1se1y positive type n
results, which seem to indicate that the system was incompletely
sterilized. Some observations enabled us to localize this
problem at the level of the frit of the reference electrode.
In the third case, the duration of the performances
of the measuring device was obse~ved for a number, amounting
up to 20, of successive cycles applying only easily automatisable
elementary actions. This should give a wo~ing autonomy
longer than one month for a fully automatic apparatus. The
optimization of the technique consisted to compare the actions
of several doses of sterilizing reagent for various contact
periods. The best results are obtained for a concentration of
10 mg/1 during a contact time of 15 minutes.
3.3. Solution of the long-dated stability problem
of the reference electrode.
This problem relates to the sensitivity of this e1ec~
trode to sulphides formed during the measuring phase. It may
be solved by means of a conception of the latter, preventing
the access of sulphides at the level of the respohsive part.
Comparative tests concerned various types of reference
electrodes :
a) classical Ag/AgC1 reference electrode,
b) Ag/AgC1 electrode with liquid electrolytic bridge,
c) Ag/AgC1 electrode with gel type electrolytic bridge,
d) Electrodes of same type as the above, but protected by a
ion-exchange membrane,
e) Electrode with ion-axchange membrane, such as described by
Onioiu & a1. (1987), and conceived for working in extreme
conditions of pH-value or in the presence of oxidants or
reducing substances,
f) Electrode selected at the closure of tests.
The number of successive cycles without any problem
was limited (between 3 and 10) in the case of above (a, b, c)
options ; the interposition of ion-exchange membranes (d, e)
lowers the sensitivity of detec~ion, and only the option f)
enabled to reach the fixed minimum performances (20 successive
cycles) with potential improvement outlook.

SI
3.4. Experimental evaluation of performances.
3.4.1. In pure culture: see above under 3.1
and the reference curves of Figs. ~ to 5.
3.4.2. In mixed culture and complex medium
{surface water}.
Th~ measuring technique was applied to samples collected
at 5 points of the natural environment, corresponding to
rivers with v~rious pollution degrees and salinities (400 to
2,000 mg/l), and in the course of 2 sampling campaigns (May
and June, 1987) ; every individual measurement was effected
in triplicate.
As an example, the following table (page 7) groups
the results obtained during the first experimental campaign.
The examin~tion of the various results obtained for
counting sulphate-reducing bacteria of natural origin with the
experimental electrochemical method applied to the various
types of river water containing an heterotrophic flora, shows
a good reproducibility of the method ; with measurements effected
in triplicate, we have obtained a variation coefficient
lower than 5 % in 4 cases among 5, the last case being that of
an extremely contaminated water.
On the other hand, we observe a good agreement between
the measured value (time elapsed up to the appearance of the
potential shift) and the value forecast by the calibration curve,
and this, whichever the considered water type may be :
water with low or high mineralization and sulphate content,
containing more or less suspended matter and organic substances.
The deviations do not exceed 8 %, which is quite acceptable
for a method with an objective of detection and semi-quantification.
It should be remarked that the deviations are most
frequently positive ; this means that the method leads to a
slight systematic underevaluation of the count .f sulphatereducing
bacteria.
The deviations may eventually be attributed to the
non-universal character of a calibration curve established
with a pure culture of sulphate-reducing bacteria. This hypothesis
was tested by registering, for each water, its own
calibration curve (measurements made on increasing dilutions
of a aame initial sample). and comparing it with the curve
drawn for the pure culture.

52
Table I Results obtained on the 1st s~mple series.
Rivers 'Moselle Fensch Moselle Bist Rosselle
Sampling site Uckange Floran- Manom Creutz Petitege
wald Rosselle
Initial seeding, 24 46 46 240 4,600
count of sulphate
reducing bacteria
per 100 ml with
the laboratory
reference method
(MPN technique)
-_.
Potential shift 465 470 470 470 480
(absolute value 470 480 450 460 470
in mV) 470 470 480 460 470
Time elapsed un- 37 36 36 32 24
til appearance of 38 36 37 31 23
potential shift 35 35 34 31 21
(hours)
Average 36,6 35.6 35.6 31.3 22,6
Standard deviation
1.5 0.6 1 .5 0.6 1.5
Variation coefficient
in "
4.1 1 .7 4.2 1.9 6.6
Time forecast by 35 33 33 31 24
the calibration
curve determined
for pure culture
(hours)
Variation rela- + 1.6 + 2.6 + 2.6 + 0.3 - 1,4
tive to calibra- i.e. i.e. i.e. i.e. i.e.
tion curve (hrs)
7.9 " 7.9 " 7.9 " 1.0 % 5.8 "
The good linearity of the calibration surves is confirmed.
It nevertheless appears that the slope of the calibration
curve depends on the nature of the concerned water.
The calibration curve drawn with a pure culture
presents a medium slope relative to that of the other curves,
and can therefore be utilized as a first approach (semi-quantification)
•
The steepest slope is apparently obtained with the
most mineralized water. It is nevertheless to be noted that
this behaviour integrates the action not only of the water

53
Figures 1 to 5
Calibration curves of sulphate-reducing
bacteria for the various studied river
waters.
Bec~1_ ... It.~cluctr1c .. l100 al
10
-.- -.en. pure
___ 11 ... 11. (Uk ..... )
~-.en. pure
---'enac:h
(Flannae )
F1aure 2
II 12 24 311
II 12 24 36
B.c~~r1 .. au1r.to-r6dutr1c •• /100 .1
10~
10 4
~ -.:he pure
__ ..... 11. (-.c.)
lOS
10 4
~-.en. pure
--Btat
(er.u~d)
10 3
'1aure 3
10 3
Fiaure 4
10 2
10 2
101
101
II 12 24 36 48 Ta.pa (heuretI)
II 12 24 36
48 T .... (heuretI)
~1 .. IIUltato-r6c1uctr1c •• Il00 .1
~.ucbe II'"
__ ..... 11. (P.tite R .... U.)
II 12 24 36 T .... (heurea)

54
mineralization, but also of the nature of sulphate-reducing
bacteria and their growth potential in the environment where
they live.
The disparity of the calibration curves causes
important deviations as to the response time of the method,
i.e. to the time required for detecting only 1 sulphatereducing
bacterium present in the examined sample (100 ml).
This period varies approximately between 40 and 55 hours
according to the studied water. The best solution is consequently
that corresponding to the establishment of a calibration
curve particular to the site by which one desires to
observe sulphate-reducing bacteria with time. In the reverse
case, an error of a power of 10 may easily be made in the
count ; this is not necessarily a drawback if the method
is used for detection or semi-quantification objectives.
3.4.3. Characterization of geothermal waters.
As these tests were not yet concluded when the
present account was written, we shall give only the first
conclusions drawn from tests conducted on water from the
boring of Nancy Thermal (water with a high chloride and a
low sulphate and bicarbonate content) and on water from
Chatelguyon (high content in chloride and bicarbonate). The
necessity of resorting to a calibration specific for every
water type, and thus for every site, is largely confirmed.
In the case of strongly mineralized waters, adaptations of
the culture medium are not to be excluded.
4. CONCLUSIONS.
The works effected within the limits of the research
enabled to define and develop an original method of instrumental
detection and semi-quantification of the counts on sulphate-reducing
bacteria in water. It seems then possible to
shorten the duration of measure (from about 10 to 30 days
with the conventional methods of bacteriological analysis)
to a period of about 24 to 48 hours.
Such a technique should not replace the laboratory
methods which will still constitute the reference, but should
be applied in operational objectives for the quality control
and the management of eventual biocide treatments for geothermal
waters and installations.
Technological solutions were studied in the laboratory
in order to solve the chief problems which oppose to the
automatisation of the method, and specially :
- the efficient disinfection of measuring cells with nonremanent
reagents,
- the stability with time of the electrochemical detection
device.
The obtained solutions may still be largely perfected,
but they already correspond to the acceptable minimum

55
for the conception of an automatic apparatus for following in
line the contamination of water by sulphate-reducing bacteria.
We hope that the last works conducted within the
Limits of th~ contract wiLL enabLe us to vaLidat~ the method for
different geothermal waters and will justify the development
of a specific automatic instrument.
LITERATURE.
A.S.T.M. (1965). standard methods of test for sulphatereducing
bacteria in industrial water and water formed
deposits. Standard 0 993-58 (1965).
Bernard, C., G. Boissonnade, Y. Regnat, F. Colin, M.J. Jourdain,
J. Bouchinet (1987). Automativ detection of coliform
bacteria for industrial control of drinking water
quality. Water Research, 21, 1089-1099.
Colin, F. (1985). Mise au point de methodes de l'activit6
microbiologique et corrosive des bacteries sulfatoreductrices.
Commission des Communautes Euro 'ennes, Cirection Gonerale
Science, Recherche et 0 veloppement. Rapport EUR 9701 FR.
Herbert, B.N. and P.O. Gilbert (1984). Isolation and growth
of sulfate-reducing bacteria, in Microbiological methods
for environmental biotechnology. Academic Press London.
Oniciu, L., O.A. Lowy, I.A. Silbert, C.E. Florea (1987).
A new multipurpose reference electrode. Analusis 15,
197-199.
Postgate, J.R. (1979). The sul~ata-reducing
Cambridge University Press, Cambridge.
bacteria.
SOCEA-BALENCY (SOBEA) (1985). Applica~ion
No. 85.11.583.
for a French patent

56
EEC contract nOEN3G-0038-F
THE BEHAVIOUR OF METALLIC MATERIALS IN A LOW-ENTHALPY
GEOTHERMAL ENVIRONMENT - THE PARIS BASIN, FRANCE
J.L. HONEGGER and H. TRAINEAU
Institut Mixte de Recherches Geothermiques, Orleans, France
ABSTRACT
Corrosion affects low-enthalpy geothermal exploitations. This study aims to
characterise and quantify the various types of corrosion that affect the
equipme"nt in geothermal loops, with a view to optimising the choice of
materials. Several parallel approaches have been taken - studies on metal
test pieces in situ directly connected- to: the pipes, development of logging
tools to measure casing corrosion, and bacteriological and electrochemical
studies in the laboratory.
For the investigations on metal samples, a test pilot was set up and
connected in parallel to two installations - Melleray in the Loiret department
and Coulommiers in the Seine et Marne department. Tests lasting from 9 to
79 days were carried out under controlled conditions, taking into account of
flow rates, pressure, temperature and the presence of bacteria. Results
backed up observations of corrosion made during exploitation, i.e. a tendancy
towards generalised corrosion of carbon steel and pitting of stainless steels.
In addition to the numerous results obtained from the various materials
tested, more general conclusions could be drawn from the two measurement
programmes:
• The rate of corrosion (measured on non-passivable metals), is roughly the
same on both test sites.
Flowrates play an important role.
• The weak development of biofilm limits the role of bacteria in corrosion,
this is thus a zone of the Dogger with a low scaling potential.
• The range of electrochemical equilibrium potentials of the material
immersed in the two fluids is highly negative.
Others more specific means of measuring geothermal corrosion have been
developed during the programme. In particular, the precision of the
mechanical, electromagnetic and sonic logging tools has been improved for
geometrical measurements during the tests carried out in real or controlled
conditions (inner and outer diameters, "thickness of metal).
INTRODUCTION
The design itself of geothermal installations, with casings of the order of
1800 m deep, cemented to the rock, prevents direct analysis of the behaviour of
this material. Even for the most accessible equipment (surface installations,
immersed pumps), direct analysis is not possible due to exploitation conditions,
deposition and electrical currents, but more specific and complex evaluations can
be made.

57
So, this project aimed to develop different ways of evaluating corrosion and
to test their suitability for the specific geothermal environment. An important part
of this study was concerned with conventional corrosion evaluation on test pieces.
In order to do this, a corrosion pilot plant connected to geothermal exploitations
has been designed to operate automatically.
In addition to these qualitative aspects of corrosion, we have improved
methocis of measuring the behaviour of metals in exploitation. The logging tools
have been tested in geothermal conditions, with and without deposits. And a new
concept of corrosion analysis has been developed, based on a correlation of well
head iron concentration and flow rate.
CORROSION MEASUREMENTS ON TEST PIECES
Presentation of tests
To measure the rate of corrosion, the conventional technique of corrosion
tests on metal test-pieces was used for this study (Traineau H. et al., 1986;
Honegger J .L. et al., 1988). The metal specimens were placed either in a test pilot
connected in parallel with two installations exploiting different geothermal
reservoirs {Melle ray (Loiret) in the Trias; Coulommiers (Seine et Marne) in the
Dogger}, or placed directly in pipes that were in use. This study specifically
identifies the corrosivity of the fluids with respect to different materials. The
main characteristics of these fluids are: Melleray (T = noc; P = 5 bars; total
mineralization • 3Z gil), and Coulommiers (T = 83°C; P = 8 bars; total
mineralization • 3Z gil).
A range of different metallic materials has been selected to undergo
corrosion testss carbon and low-alloy steels, cast iron, stainless steels and various
alloys (table 1). These are materials currently used in the Paris Basin geothermal
installations luch as the API K55 carbon steel which constitutes the tubings and
casings of many geothermal wells, or the AISI 316L stainless steel which is used in
different parts of the surface equipment.
We have also selected other materials which are not used at present but could
represent alternative solutions to the problems of corrosion in a geothermal
environment.
Results and discussion
Tests lasting from 9 to 79 days were carried out under controlled conditions
taking into account flowrates, pressure, temperature and the presence of bacteria.
The results supported observations made about these materials in actual working
conditions, i.e. a high tendency towards general corrosion of carbon steel, pitting
and crevice corrosion of stainless steel, and the high resistance of titanium.
API K55 carbon steel and APS Z410w-alloy steel, Ni-resist type 1 and DZ cast
iron, MONEL K500 Ni alloy, and copper are principally affected by general
corrosion whose rate has been determined by weight loss measurements (table Z). If
we compare carbon steel and low-alloy steel, we see that the addition of alloying
elements tends to reduce the corrosion rate, but promote the development of
localized corrosion (pitting). The cast iron shows nearly as much material loss as
the carbon steel, while the Ni alloy and Cu alloy are much less affected.
Stainless steels undergo localized corrosion (pitting, crevices, cracks,
intergranular corrosion), more or less severe depending on the type of steel. It
seems that geothermal fluids are particularly agressive towards the most of the
stainless steels tested here, such as the 316L type often used in installations. Only
the high-alloy steels such as SANICRO Z8 (Z7 Cr - 31 Ni - 3,5 Mo) and Z90 Mo
(Z9 Cr - 4 M~Ti) show a good resistance to localized corrosion without, however,
being totally passive (table 3). This agrees with the conclusions of previous work
which had demonstrated the beneficial effect of alloying elements such as
Chromium, Molybdenum and Nickel on the resistance of stainless steel to localized

PRINCIPAL ALLOYING ELEMENTS ( in we1lht I!ercent
A.F.N.O.R
TYPE DESIGNATION SPECIFICATION C Hn S1 Ni Cr No Cu Ti Al Fe
Carbon steel API K55 0.35 1.24 0.34 O.OS 0.15 0.02
Low-alloy steel APS 24 10 CA 16 0.10 0.4 0.4 4 0.')
Csst iron Ni resist I 2.S O.S 2.7 16.6 2.05 6
Ni resist D2 2.S O.S 2.S5 1').7 1.5
Ferritic ISO HoT Z6 CDT IS-{)2 0.017 0.25 IS.3 I.S5 0.27
·stainless s. 2')0 Ho Z2 CDT 2,)-{)4 0.021 0.34 2S.5 3.76 0.61 0.06
AUStenitic AISI J\6L Z2 CND 17-12 0.017 11.1 17.5 2.12
stainless s. SANICRO 2S ZI NCDII 31-27

.59
c~ion in a chloride environment.
Titanium is the only material whlch shows DO corrosion.
The effects of corrosion are similar in the two geothermal sites. These are no
lignificant differences in the nature and intensity of the types of corrosion which
affect the different materials tested. The only exception to this rule is the 316L
type Itainless Iteel which seeml more sensitive to crevice corrosion and pitting on
the Melleray lite, while it is more sensitive to pitting on the Coulommiers site.
The corrosion productl which develop on the surface of materials which
undergo general corrosion consist mainly of metallic sulfides. Their composition
indicates that the metallic ion comes from the attacked steel, whereas sulfur is
provided by the fluid. These are, for example:
- API1t55 I Mackinawite {(Fe, Ni)q Sal, Pyrrhotite (Fel _ x S), etc ...
- Cuproalumlnium I Bornite (Cus Fe S~, Chalcopyrite (Cu Fe SZ), etc .. .
- Uooel1t500 I Chalcopyrite (Cu Fe SZY, Heazlewoodite (Ni3 SZ), etc .. .
In the case of materials affected by general corrosion, the corrosion rate
decreasel with time (Testl lasting from 9 to 79 days). This highlights a possible
protective role of the corrosion products (mainly sulfides) which form at the
lurface. This oheervation, which is valid for test pieces, should not be applied
directly to the real conditions, where the continuity of the deposited layer along
the tubing is le .. certain, and where the hydrodynamic conditions are different
(flow rate much higher than in the corrosion test plant). This is clearly shown by
the different corrosion rates obtained for the API K55 carbon steel test pieces
placed within the reacton of the corrosion plant (150 to 300 ~m/year) or within the
pipel that were in use at the Coulommiers installation (400 ~m/year).
During these tests, the test pieces were placed in two reactors, one of which
had a Iystem of O.Z ~m fUten intended to stop the particles and bacteria contained
in the geothermal fluid. The test pieces placed within the reactor with fUtered
water generally presented a higher degree of corrosion (in terms of material loss by
general corrosion, or depth of attack by localized corrosion), than the ones placed
within the non-fUtered reactor. In the case of Coulommiers, we could suggest that
the fUtration of hydrocarbons contained in the fluid of this geothermal well
prevented the formation of a protective fUm of hydrocarbons on the surface of the
test pieces, and thUi indirectly promoted the corrosion.
This result indicates also that the thin biofUm analysed have little weight in
the corrosion proce .. of theses installations.
CORROSION MEASUREMENTS MADE ON THE WELLS AND FLUIDS
This approach usel measurementl of corrosion, incorporating its effects on
the Iteels and fluids during exploitation.

60
Table 2 - Coupon material loss results (pm) for materials
affected by general corrosion in geothermal fluids
MATERIAL LOSS
\l1li)
TYPE
Carbon steel
Low-alloy steel
Cast iron
Cu Alloy
Ni Alloy
DESIGNATION HE.l.L1!RA Y PI.ANT COULOKHI I!RS PI.ANT
9 days 44 days 73 days 24 days 79 days
API K55 7-13 20-22 34-44 17-21 37-48
APS 24 28-32
Ni-resist type 1 4 6-8 7 3-8 8-14
Ni-resist type D2 4-9 8-11 14 2-8 15-24
Cupr,oallllllinilllll 2-5 3-6 4-6 2-5
HONEL K500 2-3 4 1-2 1-2
Table 3 - Pitting and crevice corrosion results (greatest depth in ~m)
on coupons of stainless steels affected by localised corrosion
Hl!LLI!RAY PI.ANT
COULOMHI I!RS PI.ANT
A.F.N.O.R 13 days 79 Days
TYPE MATERIAL SPECIFICATION Pittiq Crevice Pittiq Crevice
Ferritic 180 HoT Z6 CDT 18-02 150 380 300 330
290 Ho Z2 CDT 29-04 60 30 60 140
Austenitic AISI 116L Z2 CND 17-12 50 300 260 80
SANICRO 28 Zl NCoo 31-27 200
SAF 22-05 Z5 CND 21-08 50 10 130
Austeno- URAIIUS 35 N Z5,CN 23-04 450 50
ferritic URAIIUS 45 N Z5 CND 22-06 40 60
URAIIUS 52N Z5 CNoo 26-06 160 80
Hartensi tic SOLEIL A7 Z40 C 14 180 20
SOLEIL D5 Z50 CD 15 180 30
Precipitationhardeniq
17-4-Ph Z6 crru 11-04 460

61
Measurement by corrosion logs
Improved knowledge of corrosion logging tools currently enables their use to
be optimised as regards utilisation conditions (Sainson S. and Honegger J.L., 1986).
............ .IUlio
M ... lllre ......
lloa1dual
&Jw.r....... 01
c ... a.
I ..&eraal
di ...... r
.. l
Kalin.
I ...... I ....... t:aleraal
-
d ....&er •• O' Itou ....... ...-
Kalla. .... ....
r.TT U' n:s Yt:S NO NU n:s Yt:S
n:T II ~Il Nil n:s YI!S" n:s n:s
III1TV NU'" NU Yt:S YES NO NO
AI~C !'o() NU n:s n:s NO ~O
• The E.T.T.D. is the only tool which simultaneously evaluates internal and
external metal losses (corrosion sensu stricto)
•• Only in the case of very slight scaling (the resonance method needs very
accura te cali bra tion of the tool)
••• Only in the case of a perforation
E.T.T.D. - electromagnetic tool
C.E.T.D. - sonic tool with 9 transceivers
B.H.T.V. - sonic tool with a rapid-rotation transceiver
M.F.C. - mechanical tool with sensors
Table" - Selection of corl'OSion logging tools according to type of
measurement to be made
The choice of tools was made and their sensitivity evaluated on the basis of
tests either in real, characteristic conditions (wells with and without scaling,
ruptured casing), or in test wells where various typical casings were demounted and
logged (Sainson S. and Honegger J.L., 1988).
Measurements made on fluids
The analysis of fluids collected at the surface, in particular for dissolved iron
content, can give informations on corrosion of the production casing. The dissolved
iron at wellhead originates from the initial concentration in the reservoir and a
large contribution from corrosion. In wells where there is little scaling, a linear
relation was established between the concentration of iron in the fluid at wellhead
and the reciprocal value of the flow rate (Ouzounian G. et al., 1987). A preliminary
model, based on simple assumptions (uniform corl'OSion of casing, concentrations of
iron identical in the various production levels), led to results on corrosion rates
very similar to those measured at the surface (for example, 0.4 mm/y at Creteil)
and Iron concentrations in the reservoir compatible with thermodynamic
equilibrium between fluid and pyrite ({Fe) < 0.1 ppm).
In addition, assessment of the initial properties of the fluid, particUlarly
redox potential (Criaud A. et al., 1988), enables the nature of the corrosion
products to be forecast.
CONCLUSION
The results of this study have improved our knowledge of the behaviour of
ma terials thanks to:
• The qualification and quantification of the various types of corrosion, in
order to define necessary treatment (corrosion inhibitors (Fouillac C.,
1987), cathodic protection) •
• Optimisation of the choice of logging tools to measure in situ corrosion.

62
• Development of various electrochemical measurement techniques (on site
and in the laboratory), and bacteriological measuring techniques (influence
of the biofUm).
• A technique for indirect measurement of casing corrosion, based on the
inverse correlation of dissolved iron concentration in the fluid and the
production flowrate, active in wells with little scaling. A similar technique
based on hydrogen production by corrosion is currently being studied.
ACKNOWLEDGEMENTS
This work. was carried out in the framework of the Commission of European
Communities contracts EN3G-0033-F and EN3G-0038-F, and received financial
support from the AFME and BRGM.
BIBLIOGRAPHY
CRIAUD A., FOUILLAC C. and MARTY B. - Low enthalpy geothermal fluids from
the Paris Basin: Z -Oxidation reduction state and consequences for the
prediction of corrosion and sulphide scaling. Geothermics (in press).
FOUILLAC C. (1987) - Les traitements par additifs chimiques des phenomenes de
corrosion et de depOt. Experience de l'exploitation du Dogger du Bassin
Parisien (Treatment of corrosion and scaling by addition of chemicals.
Example of the Dogger of the Paris Basin). Geothermie Actualites, v. 4,
n03, p. 35-38.
HONEGGER J.L., TRAINEAU H.,CRIAUD A. and SAINSON J. (1988)·­
Comportement des matEiriaux metalliques en milieu geothermal
(Behaviour of metals in a geothermal environment), contract EN3G
0038 F (CD) - CEe final report;
OUZOUNIAN G., CASTAGNE S., FOUILLAC C. and CRIAUD A. (1987) - L'origine
du fer· dans les eaux geothermales exploitees au Dogger (Bassin
Parisien) (The origin of iron in the geothermal water of the Dogger of
the Paris Basin). Geothermie Actualites, v. 4, n03, p. 31-34.
SAINSON S. and HONEGGER J .L. (1986) - Suivi de revolution des cuvelages par
diagraphies (Monitoring by logs of the evolution of casings). Geothermie
Actualites, v. 4, n 0 4, p. 46-50.
SAINSON S. and HONEGGER J.L. (1988) - Les diagrapbies de cuvelage -
Experimentation sur puits test (Casing logs - Experiments on a test
well). BRGM report 88 SGN 03Z IRG.
TRAINEAU H., HONEGGER J.L. and LOMBART R. (1986) - Premiers resultats d'un
pilote de corrosion sur les eaux geothermales du Trias. (Initial results of
a pilot test on corrosion on the geothermal water of the Trias). CEC
Workshop DG XII: Corrosion and scaling in geothermal systems,
Orleans, 17/18 Nov. 1986, 10 p.

63
EEC Contract No. EN36-0036-GR
CALCIUM CARBONATE SCALE FORMATION AND PREVENTION
E.K.GIANNIMARAS, A.G.rlLA
and P.G.KOUTSOUKOS
University of Patras, Department of Chemistry, Physical Chemistry
Laboratory and the Research Institute of Chemical Engineering
and Chemical Processes at High Temperatures, P.O. Box 1239,
University Campus, 26 110 Patras, GREECE
Summary
The precipitation of calcium carbonate in supersaturated solutions
has been investigated over a range of pH between B.OO
and 9.50 and at various temperatures between 25-BOoC. The investigation
involved both stable and unstable supersaturated solutions.
The experimental conditions in the latter case were
selected so that spontaneous precipitation occured following
the lapse of well defined, highly reproducible induction periods.
At all temperatures it was found that vaterite precipitated,
being kinetically favoured. The presence of phosphonate
compounds both retarded the calcium carbonate precipitaion and
stabilized vaterite formation. At higher temperatures, the
transformation of the initially forming vaterite into aragonite
has also been observed.
The crystallization of calcium carbonate in the stable
supersaturated solutions was investigated by seeded growth
experiments, using calcite seed crystals. In all cases calcite
was the only phase forming as predicted by thermodynamics, following
a surface controlled spiral growth mechanism, with an
apparent growth order of 2.
The presence of additives such as oxalate favoured the stabilization
of the unstable calcium carbonate monohydrate, which
was rapidly converted into calcite. The presence of inorganic
orthophosphate retarded the precipitation by adsorption onto
the calcite surface which resulted in surface charge reversal.
Finally, the inert electrolyte effect appeared to influence
the rates of formation of calcium carbonate without affecting
the second order kinetics of the seeded growth of calcite. In
concentrated electrolyte media, it has been shown that it is
possible to apply the constant supersaturation approach.

1. INTRODUCTION
The existence of hot water at various depths is a very
important alternative source for energy production. Equipment
such as metallic tubes, turbines, separators, e.t.c are routinely
used for handling the hot water. Contact however of the
hot water with this equipment results in scale formation, i.e.
formation of layers of sparingly soluble salts that cause severe
problems to the management of the geothermal energy
ranging from reduced thermal conductivity to complete clogging
which in many cases forces the energy production plants to
interrupt their operation.
In a large number of geothermal power plants and depending
on the water chemistry of the waters involved, calcim carbonate
is a frequently encountered form of scale (Cowan and Weintritt,
1976; Benoit, 1988). The problem of calcium carbonate scale
formation is more pronounced at elevated temperatures due to
the inverse solubility of this salt.
The problems associated with the undesirable formation of
calcium carbonate have encouraged research in the last decades
in the area of prevention of the formation of this salt. The
use of acids in order to maintain an unfavourable pH for
calcium carbonate deposition most probably accelerates corrosion.
The use of ion exchangers on the other hand is a rather
expensive solution requiring frequent maintenance. Attention
has been focused on the use of water soluble water additives
which at very low concentrations may retard or completely
prevent the formation of calcium carbonate. Unfortunately, up
to present there is no unique inhibitor working effectively for
any kind of insoluble deposit. In order to effectively control
therefore the formation of a particular scale it is necessary
to understand the mechanism of its formation and its exact
chemical nature. This is true. especially for minerals that
present a number of polymorphs varying in stability. The
polymorph to be formed is decided not only by thermodynamic
but in some cases by kinetic factors.
Several studies have been done in batch solutions at low
supersaturations employing the seeded growth approach (Reddy
and Nancollas, 1976; Kazmierczak and Nancollas, 1982).These
studies have the advantage of high reproducibility and precise
control of the experimental parameters but so far they
have been limited to low supersaturaitons and in low ionic
strength media. Attempts to extrapolate the results of such
studies to the conditions encountered in geothermal brines
are highly questionable. Studies on the other hand of the
calcium carbonate precipitation at high supersaturations done
by direct mixing of the components, suffer in general from
poor reproducibility and control of the experimental parameters
which is mainly due to the rapid desupersaturation
followng the spontaneous precipitation. Thus this type of
experiments, in which the experimental conditions change rapidly
are not suitable for the identification of the initially
forming polymorph which is determining the precipiation kinetics.
In the present work we aimed at clarifying first the nature

65
of the calcium carbonate polymorph forming and then at investigating
the mechanism of its formation, under conditions of
spontaneous precipitation. Moreover the effect of organophosphorus
compounds on the spontaneous precipitation of calcium
carbonate formed was investigated. The experimental work was
done under plethostatic conditions,approach that makes it
possible to look at the precipitation process at its very
beginning in a highly reproducible, carefully controlled manner
(Tomson and Nancollas, 1978: Meyer, 1979). Experiments at low
supersaturations were also done under plethostatic conditions
by using the seeded growth technique. The seed crystals were
well characterized calcite seed crystals. Such experiments
were used for the thorough investigation of small ionic
additives with respect to their effect on the precipitation
of calcium carbonate and for the comparison of these results
those obtained from spontaneous precipitation. Finally with
experiments at plethostatic conditions were extended to artificial
seawater, an aqueous solution of high ionic strength
approaching in several cases that of geothemal brines.
2. EXPERIMENTAL
I. Spontaneous precipitation experiments: The nucleation of
calcium carbonate was investigated in unstable supersaturated
solutions prepared in a thermostatted (± 0.1 0 C) double walled
vessel, volume totaling 0.5 dm3 by direct mixing equal volumes
of calcium nitrate and sodium bicarbonate solutions made
up from stock solutions prepared from A.R. solids (Merck) and
standardized. Calcium nitrate stock solutions were standardized
by atomic absorption spectroscopy (Varian 1200) and by
photometric titrations using murexide indicator at 504 nm (Pye
Unicarn SP 406 spectrophotometer). Fresh sodium bicarbonate
solutions were prepared for each experiment. Following the
mixing of the solutions the solution pH was adjusted by the
addition of standard potassium hydroxide. Since experiments
were done at high pH and by minimizing the volume above the
solution, the partial pressure of C02 could be considered constant.
Following pH adjustment, the supersaturated solutions
were constant as indicated by the stability of the solution
pH. Calcium carbonate precipitation, would be accompanied by
the release of protons in solution, which could easily be
measured by a pair of glass/saturated calomel electrodes
standardized before and after each experiment by NBS standard
buffer solutions. Following the lapse of an induction period,
the precipitation of calcium carbonate started, accompanied by
a decrease in pH. A decrease in pH as small as 0.005 pH units,
triggered the addition of titrants from the two, mechanically
coupled burettes of an appropriately modified pH-stat (Metrohm
615 dosigraph 614 impulsomat and 632 pH meter). The titrants,
calcium nitrate and~ sodium carbonate and sodium bicarbonate
were of such concentrations as to ensure constant solution
supersaturation. (Giannimaras and Koutsoukos, 1988). The ratio
of total calcium, CaT: [Na2C03] = 1:1, had the stoichiometry
of the precipitating CaC03. It should be noted that the
addition of Ca(N03)2 and Na2C03 titrants, necessitated the
addition of inert electrolyte in the working solution, in

66
order to avoid changes in the solution ionic strength.
Samples were withdrawn from time to time and were analyzed
for calcium by atomic absorption spectroscopy or by spectrophotometric
titrations.
The rates of precipitation were obtained directly from the
traces of the titrants added as a function of time, from the
recorder of the titrator. In these traces, the y axis was converted
into moles of calcium carbonate forming and the rates
at time t=O were calculated by the tangent at this point. As
the reaction proceeded, the rate increased due to increase of
the number of active growth sites resulting from the simultaneous
nucleation and crystal growth processes. In all cases,
the reproducibility of the induction periods, preceding the
onset of precipitation was 5% and that of the initial rates
between" 4-10%.
It should be noted that a number" of the spontaneous preci-"
pitation experiments were done at constant pH. In this case
only one burette was used for the additon of the potassium
hydroxide required for pH maintenance. Kinetics were followed
by the potassium hydroxide consumption and by analysis of the
samples for calcium.
II. Seeded growth experiments: The same procedure, as already
described, was followed for the preparation of the supersaturated
solutions and of the titrant solutions. In this case however,
the initiation of the crystallization process was
effected by the introduction of well characterized calcite
seed crystals in the supersaturated solutions. The calcite
crystals were prepared from the slow addition of 2 dm3 of
0.2 M calcium nitrate solution into 2 dm3 of 0.2 M sodium bicarbonate
solution, over a period of 4 hours, at pH 8.50. The
precipitate was filtered and resuspended in triply distilled
water, where it was kept under stirring for one week. Next it
was filtered, washed thoroughly with cold, distilled water and
it was dried at 105 0 C overnight. The powder x-ray" diffractogram
and infrared spectrum confirmed that the solid was calcite,
exc!.usively. The specific surface area of the solid,
measured by a multiple point B.E.T. nitrogen adsorption method
(Perkin" Elmer Sorptometer 214 DJ was found to be 3.26 m2g-l .
In the seeded growth experiment~, the supersaturated solutions
employed were stable for time periods up to one week.
Following pH adjustment, a time period of" two hours was
allowed for the verification of the solution stability, as
verified from the constant pH value. Next, a quantity of the
calcite seed crystals was added and the reaction started
immediately without any induction period. It was found that
doubling" tripling or halving the amount of seed crystals,
double, tripled or reduced to one half respectively the rate
of crystallization, pointing out to crystal growth on certain
active growth sites on the surface of the seed crystals introduced.
Varying the rate of stirring in the crystallization
solution did not affect the rate of crystallization, pointing
out to absence of secondary nucleation.
The process of crystallization was followed by the constant
supersaturation approach already described. It should
be noted, that in the case of additives study, a quantity of

67
the additive investigated was added in the solution, in order
to avoid dilution effects.
III. Crystallization of calcite in high ionic strength media:
Artificial seawater was prepared according to the methodology
given in the literature (Kester et ale 1967). The high salinity
aqueous solution was used to make up the working supersaturated
solutions by the addition of the required quantities
of calcium nitrate and sodium bicarbonate. Moreover the pH
was adjusted to 8.50 by the addition of dilut~ potassium hydroxide
solution. The titrant solutions were made as previously
described with the difference that in this case artificial
seawater was used as a solvent, thus avoiding dilution of the
working solution. The precipitation process was initiated by
the addition of calcite seed crystals. The formation of the
calcite on the added seed crystals started following the lapse
of well defined induction periods inversely proportional to
the degree of solution supersaturation.
IV. Electrokinetic measurements: The electrophoretic mobilities
of the calcite particles were measured in suspension 0.1
or 0.01 mol dm- 3 in potassium nitrate both without and with
additives on their surface. The measurements were done in a
Rank, MK II microelectrophoresis apparatus equipped with videocamera
and monitor (Phillips), using a four electrode, capicallary
cylindrical cell, The velocities of at least 20 particles
(in both directions of the electric field) in each of the two
stationary layers were measured. The difference in the velocities
measured at the two stationary layers did not exceed 7%.
The calcite particles, prior to measurements of their electrophoretic
mobility, were equilibrated with the potassium nitrate
medium for 12 hours.
3. RESULT AND DISCUSSION
In order to estimate solution supersaturation with respect
to any calcium carbonate polymorph, the solution speciation
must be calculated. The calculations were done taking into
account all acid-base and ion-pair equilibria (Nancollas,1966),
mass balances and the electroneutrality conditions, making
successive approximations for the ionic strength. For the estimate
of the activity coefficients, the Davies equation was
used (Davies, 1962). The driving force for the formation of
each calcium carbonate polymorph, ~Gx' is the change in Gibbs
free energy for going from the supersaturated solution to
equilibrium. ~Gx is given by eq.(21:
(ca 2 +) (C0 2 3 -)
~G = - RT In = _ RT Inn
x 2 KO 2 x
s,x
In eq.2, parentheses denote activities of theions enclosed
and K~ x is the thermodynamic solubility product of the polymorph'
considered. R is the gas constant and T the absolute
temperature. ax is the saturation ratio or relative supersaturation
with respect to polymorph x. The factor t in eq.2
stems from the number of ions (mean activity coefficient) of
(1)

68
the salt involved (for CaC03) v = v+ + v_ = 2. Seeded growth
experiments were conducted at concentrations ensuring stable
supersaturated solutions, while spontaneous precipitation
experiments were done in the labile area. The solution conditions
in the latter case, were such that calcium carbonate
precipitated following the lapse of easily measurable induction
periods. It should be noted that the induction periods or the
mea sued rates of precipitation of calcium carbonate were not
influenced by the rate of solution stirring or by filtration
of the solutions prior to their mixing through 0.1 ~ membrane
filters. The solution supersaturation was kept constant
throughout the precipitation constant on it may be seen in
Table 1. One of the major advantages of the constant supersaturation
methodology is the accuracy of measurement of the
rates of precipitation and the identification of the phase
forming, by growing amounts, sufficient to be characterized
by physicochemical methods at the initial solution conditions.
A plot of the measured rates as a function of the solution
supersaturation according to the phenomenological equation
(Kazmierczak et aI, 1982).
R = k(IP - K O )n
(2 )
s,x
yielded an apparent order for the precipitation reaction of
n = 15, assuming that calcite was forming. This value was
significantly higher as compared with n=2 found in the case
of seeded growth experiments. The kinetics plot from seeded
growth experiments yielded an apparent order of n = 2, thus
confirming findings from experiments at comparable conditions
(Reddy, 1978, Kazmierczak et al. 1981). Careful examination by
powder x-ray diffraction and by infra-red spectroscopy, of the
solid phase forming by spontaneous precipitation showed that
vaterite was the initially forming polymorph, subsequently
converting into the thermodynamically more stable calcite.
Application of the constant supersaturation approach made it
possible for the detection of vaterite even at 25 0 C at solution
conditions in which it was believed up to present
that the phase forming is calcite (Kitano, 1962). It should
be noted that earlier studies, done at constant pH, failed
to show the existence of any phase other than calcite (Nielsen,
1981) probably due to the rapid change in solution supersaturation
during the course of reaction. Substitution in the
rate expression of eqn.(2) of the solubility product of vaterite
yielded satisfactory fit over the temperature range
between 25-80 0 C (Xyla and Koutsoukos, 1988). From linear logarithmic
plots an apparent order of n = 4 was calculated. It
is obvious therefore, that both the nature of the phase forming
and the kinetics of crystallization in the metastable zone are
different than those in the labile area. It is not possible
therefore to extrapolate results obtained at low supersaturations
in order to predict the behaviour of the system at high
supersaturations. Vaterite formations may be clearly seen,
along with calcite crystals in the scanning electron micrographs
in Fig.1. It is interesting to note that the presence
of organophosphorus compounds in solution which caused re-

69
tardation both in the induction times and in the rates of
precipitation of the spontaneously forming vaterite, stabilized
vaterite, which could be easily detected even after staying
for 24 hours in suspension in the mother liquor. The
effect of 1,1 Hydroxyethylideno 1-1 diphosphonic acid (EHDP)
on the spontaneous precipitation of calcium carbonate is shown
in Fig.2. It is also interesting to note that at elevated
temperatures (>40 0 C) aragonite, apparently coming from the
conversion of vaterite was seen as an intermediate phase coexisting
with vaterite and calcite.
Seeded growth experiments are of particular interest,since
they sucessfully model systems in which the supersaturation
with respect to the crystallizing salt is low and situations
in which crystal growth takes place on already existing substrates.
The advantage of high reproducibility of the seeded
growth experiments, when combined with the constant composition
approach may yield valuable information, not only on
precise kinetics, especially in the presence of additives, but
also on the nature of the crystalline phase forming. Kinetics
analysis revealed that crystal growth took place on the
calcite seed crystals introduced, exclusively. The presence
of inorganic orthophosphate in solution caused significant
retardation in the rate of precipitaion. Phosphate was adsorbed
on the active growth sites of calcite as it was confirmed
from adsorption experiments (Giannimaras and Koutsoukos,
1987). As a result of the adsorption the surface charge on
the calcite particles was drastically changed as it may be
seen from the curves of the dependence of the electrokinetic
charge on solution pH, shown in Fig.3. It may be seen that
phosphate adsorption resulted in charge reversal over the
entire pH range. It should be noted that the relative amount
of phosphate adsorbed did not affect significantly the
charge on the calcite particles.
Of particular interest was found to be the presence of
oxalate ions in the supersaturated solutions. The presence
of oxalate drastically reduced the rate of calcite crystallization,
even though
solution supersaturation was not significantly
affected due to additional ion pairing in solution.
The presence of oxalate anions in solution however, altered
the apparent order of reaction from n = 2 to n = 4 pointing
out to a different mechanism. Examination of the solid phase
formed revealed the existence of calcium carbonate monohydrate.
This phase, growing onto calcite was identified both
by x-ray and infrared spectroscopy. Calcium carbonate monohydrate
has never been observed at such low degrees of supersaturation
since it is very unstable(Hull and Turnbull, 1973,
Brooks et al., 19501. It seems therefore that the presence of
oxalate ions has stabilizing effect on this kinetically favoured
phase.
The effect of metal ions such as zn2+, Cd2+ and Fe3+ has
been investigated. In the case of Fe3+ the results are rather
complicated, since at alkaline pH, the formation of iron hydroxide
is likely, although such formations could not be
detected by spectroscopic methods. Both Zn2+ and Cd2+ had a
strong retarding effect as it may be seen in Fig.4. Spectroscopic
and kinetics analyses suggest that these ions retard

70
by adsorption and subsequent blocking of the active growth
sites on the surface of calcite. Moreover, it should be noted
that the nature of the inert electrolytes in which the formation
of calcite takes place exerts a marked effect on the
crystallization of calcite. The rates of calcium carbonate
crystal growth showed the order: NaN03 NaCl KCI04>KCl>KN03'
These differences may be due to either incorporation of ions
into the growing calcite or to significant changes in the
interface of calcite/water due to the different electrolyte.
Finally, it should be noted that the constant solution
composition approach was applied to the growth of calcite in
seawater medium. Aside from the presence of various ions,
which influence the rate of calcite precipitation, the solution
is of high ionic strength. The estimate of the solution supersaturation
was made by adopting the Pitzer formalism for the
activity coefficients (Pytkowicz, 1969). Analysis of samples
during the crystallization experiments showed excellent constancy
of solution supersaturation, as it may be seen in
Table 1. These, results, suggest that it is possible to study
the crystallization of sparingly soluble salts in high ionic
strength media under plethostatic conditions.
Table 1: Calcite crystallization on calcite seed crystals at
plethostatic conditions, in synthetic seawater. pH 8.50, 25 0 C.
Total Calcium, Cat = Total carbonate, Ct.
Sample Time cat % growth
~ min /x10
-2
mol dm-3
w.r.t. original seed
1 0 1,16 0,0
2 26 1,18 23,2
3 48 1,20 50,1
4 72 1,17 83,2
5 125 1,18 164,1
6 184 1,20 244,5
References
Benoit, W.R.(1988). Carbonate scaling characteristin Dixie
Valley, Nevada geothermal wellbores. In Proceedings of deposition
of solids in geothermal sytems. Peykjavik, Iceland.
Brooks, R., L.M.Clark and E.F.Thurston (1950).Calcium carbonate
and its hydrates. Phil.Trans.Royl Soc. London Ser.A. 243,
145-167.
Cowan, J.C., and D.J.Weintritt (1976). Calcium carbonate scale
deposits. In Water formed scale deposits. Gulf Publishing Co.,
Houston, Texas. pp 110-111.
Davies, C.W. (1962). Ion Association. Butterworths, London
pp. 112-114.
Giannimaras, E.K. and P.G.Koutsoukos (1987). The crystallization
of calcite in the presence of orthophosphate. Journal of
Colloid and Interface Science, 116~.423-430.

11
Hull, H. and A.G.Turnbull (1973).A thermochemical study of monohydrocalcite.
Geochim. Cosmo Acta, 37, 685-694.
Kazmierczak, T.F., E.Schuttringer, B.Tomazic and G.H.Nancollas
(1981). Controlled composition studies of calcium carbonate
and sulfate crystal growth. Croat.Chem.Acta, 54, 277-287.
Kazmierczak, T.F., M.B.Tomson and G.H.Nancollas (1982). Crystal
Growth of calcium carbonate. A controlled composition kinetic
study. J.Phys.Chem., ~, 103-107.
Kester, D.R., I.W.Duedall, D.N.Connors and R.M.Pytkowitz(1967).
Preparation of artificial seawater. Limnol.Oceanogr., !l,
176-184.
Kitano, Y.(1962). A study of the polymorphic formation of
calcium carbonate in thermal springs with an emphasis on the
effect of temperature. Bull.Chem.Soc.Japan, ~, 1980-1985.
Meyer, H.J. (1979). Wachstumsgeschwindigkeit von calcit ans
wasserigen losungen. Journal of Crystal Growth, 47, 21-28.
Nancollas, G.H.(1966). Interactions in el~ctrolyte solutions.
Elsevier, Amsterdam. pp. 62-70.
Pytkowicz, R.M. and D.R.Kester (1969). Harned's rule behavior
of NaCl-Na2S04 solutions explained by ion association model.
Amer.J.Science, 267, 217-229.
Reddy, M.M. (1978). Kinetic inhibition of calcium carbonate
formation by wastewater constituents. In A.J.Rubin(Ed.),
The Chemistry of Wastewater Technology. University Press, Ann
Arbor MI. pp. 31-58.
Reddy, M.M. and G.H.Nancollas (1976). The crystallization of
calcium carbonate.IV the effect of magnesium, strontium and
sulfate ions. J.Crystal growth, ~, 33-38.
Tomson, M.B. and G.H.Nancollas (1978). Mineralization kinetics:
A constant composition approach. Science, 200, 1059-1060.
Xyla A.G. and P.G.Koutsoukos (1988). Spontaneous precipitation
of calcium carbonate in aqueous solutions at sustained supersaturation.
In Proceedings, JICASTOCK' 88 conference, Vol.2.
A.F.M.E., Versailles, France. pp. 693-697.

72
Figure 1. Scanning electron micrograph of spontaneously precipitating
vaterite transforming into the thermodynamically stable
calcite; pH 8.50, 25 0 C.
:!:
• I
0
...... "
E
::s
u
c
U
c
-0
I-
30
25
20
15~------~--------~------~------~
o 10 20
Time / min
30 40
Figure 2: Effect of EHDP concentration on the spontaneous prec~p~tation
of calcium carbonate. Total calcium, Cat = Total carbonate,
Ct = 3.2 x 10-2M, pH 8.50, 25 0 c. (0 ):blank; (t::.): 0.8 x 10-7M;
( ~):1.5 x 10-7 ; (CJ ):3 x 10-7 ; ('V ):5 x 10-7 .

73
\·0
o.o1-------.i-r---~8·-:::0---=9_0~--lo_~0----f--.......... -
\·0
Figure 3. Electrokinetic charge of calcite seed crystals; 25 0 C.
( 0 ): bare calcite; (A) :with phosphate adsorbed; (0 ): with
oxalate adsorbed.
Cd- 10-7'M
3·0 2·0 1·0
-o
c
o
ti
::::II
'i
a:: 2
~
O~ ____ ~ ________________ ~~ ________________ ~
1·0 1'5 2·0
Znx \0-', M
Figure 4. Effect of Zn2 + ( 0 ) and Cd 2 + ( A ) on the rates of
calcite crystal growth.

74
EEC contract No EN3G-0040-GR
A STUDY OF SCALING DUE TO HIGH ENTHALPY GEOTHERMAL FLlJIDS
N. ANDRITSOS, A. MOUZA and A.I. KARABELAS
Chemical Process Engineering Research Institute, P.O. Box 19517 and
Chemical Engineering Department, University of Thessaloniki
GR 540 06 Thessaloniki, Greece
S ummarv
Analyses of samples from the Milos 2MWe Geothermal Plant have been
used to characterize the ·scale forming in various parts of the plant.
Heavy metal sulfides are the dominant compounds of scale close to the
point of primary fluid flashing, whereas silicates and silica tend to
dominate at the other end of the plant (point of fluid reinjection). Laboratory
experiments with a typical sulfide (PbS) show a very strong
effect of pH and concentration on the rate of scale formation. Similar
results are obtained in silica polymerization experiments. Data on the
effect of other factors on PbS scale formation and on silica polymerization
are summarized. A practical implication of this work is that pH
control of geothermal fluids may provide an effective means of combating
the scaling problem.
INTRODUCTION
The formation of hard and tenacious scale in pipes and in equipment of
geothermal plants constitutes the main obstacle in the economic exploitation
of many geothermal fields. The scaling problem is encountered in almost
every geothermal installation, but it can be more acute in plants handling
high enthalpy brines.
Scale formation can be controlled, at least partially, with careful design
of the plant and choice of appropriate operating conditions, with
chemical modification of the geothermal fluid, and finally with periodic
shut-downs of the plant and removal of the scale with mechanical or chemical
means.
The main objective of the work presented here is to document the nature
and the composition of the scale encountered in a fairly typical high
enthalpy field, such as the Milos geothermal field, and to understand the
mechanism of scale formation. Moreover, it is expected that a combination
of field measurements and of scale characterization with carefully controlled
laboratory experiments (performed with the most abundant scale constituents,
i.e. heavy-metal sulfides and amorphous silica), will help evaluate
possible ways of scale prevention. In this paper we report on the following
aspects of the work carried out so far:
(1) Chemical analyses of geothermal fluids of the Milos Power Plant,
chemical and morphological analysis of various scale samples, and evaluation
of parameters affecting scale deposition.
(2) Experimental deposition studies of lead sulfide in a pipe at ambient
conditions. These experiments provide invaluable information on the mech-

75
anisms responsible for the PbS deposition and on the factors which influence
scaling.
(3) Experiments on silica polymerization kinetics which is of great
significance in the formation of silicious deposits.
Related work is also in progress on two-phase flow, which is important
In clarifying the mechanism of scaling and in providing valuable information
for the design and operation of geothermal plants. Results of this
work have been summarized in progress reports to EEC and will not be included
here due to space limitations.
SCALE FORMATION - GENERAL CHARACTERISTICS
The geothermal fluid under the reservoir conditions of high pressure
and temperature is usually saturated in slightly soluble salts. The main
cause of scale formation is the "flashing" of the fluid, that is, both the
continuous pressure drop along the pipe due to flow and the sudden pressure
reduction of the fluid at a certain point (henceforth referred to as "the main
flashing point"), which is dictated by the operating conditions. Flashing
results in the simultaneous reduction of the brine mass, in the drop of temperature
and finally in the pH increase of the brine, due to the release of
CO 2 and H 2 S. Consequently, many compounds in the brine become supersaturated
and tend to precipitate and to deposit onto the pipe wall.
The scale composition is usually very complex and depends heavily on
the brine composition, the preuure and temperature of the system, and the
distance from the main flashing point. Low or medium enthalpy fluids
(T

76
-Heavy metal sulfides dominate in the VIcinity of the main flashing
point while the percentage of silica tends to increase towards the reinjection
well.
-There are indications of a "crystallization fouling" type of mechanism
in the deposition of heavy metal sulfides.
-Primary flashing at -24 bar resul ts in significant reduction of scale
formation, compared with that for primary flashing at -8 bar at which
silica supersaturation is much greater. As expected, the main constituent
of scale when flashing at 8 bar is amorphous silica.
Additional analyses and observations of scale samples from the Milos
Plant are included in a recent report by Andritsos (1988). Most of the samples
analyzed were collected between the hot water collecting tank and the
sampling point No 2, as illustrated in Figure I, covering a pipeline length
of -200 m.
The main crystalline phases, identified by X-ray Diffracti'on in most of
the samples, are heavy metal sulfides (PbS, a-and ~-ZnS and Fe O . 9 SS). Additionally,
most of the samples show a broad band at about 4 A, indicative
of amorphous silica. The chemical analysis of the samples is presented in
Table I.
Special attention must be paid to samples No 4 and No 8. The former
comes from a vertical pipe section, 18 m downstream of the control valve.
It consists of several layers, but with two very distinct bands. The upper
band (sample No 4A), having a rippled appearance and a thickness of -7
mm, is a moderately hard and dark-grey material with obvious crystalline
grains. Comparison of its composition with that of other samples has revealed
that this band was formed during plant operation at 24 bar. Its main
crystalline compounds are FeO.9SS, ~- and a-ZnS and PbS. The -30 mm
thick lower band is a light colored, brittle material, which consists of
amorphous Si0 2 , crystalline Si0 2 , NaCl, KCI and FeS2. The scale deposited
on the turbine blades (sample No 8) consists primarily of CaS0 4 , FeS 2
and Fe 3 0 4 . Small quantities of NaCI and Cr0 2 were also identified. The
presence of Cr and Ni in this sample supports the idea that both these elements
as well as the iron are corrosion/erosion products of the turbine material.
LABORATORY STUDIES OF PbS DEPOSITION
Lead sulfide can comprise a significant part of the scale deposited on
metal surfaces in most geothermal plants, provided that lead ions exist in
the brine even at low concentration. The objective of th'is part of the project
is to determine and evaluate the parameters which influence PbS deposition,
under conditions that· partly imitate the sudden supersaturation of
the brine at the main flashing point. Initial deposition rates are calculated
from the measured mass of PbS deposited on specially designed coupons.
The lead sulfide sol is formed by mixing two liquid streams containing sulfide
and lead ions in stoichiometric ratio and in various concentrations. Additional
information on the deposition pattern is obtained by scanning electron
micrographs of the deposit-covered surfaces of small interchangeable
plugs.
Details on the experimental techniques and the effects of pH and flow
velocity can be found in Andritsos and Karabelas (1988a), while the effects
of time, of PbS concentration and of the substrate are reported in Andritsos
and Karabelas (1988b). Results of typical runs for a certain concentration
and various pH values are shown in Figure 2.
Figure 3 illustrates the effect' of flow rate on deposition, showing a
linear dependence of deposited mass on flow rate. This is indicative of a
"diffusion-controlled" type of mechanism. Additional evidence in support of

n
this mechanism is that the measured maximum deposition rate for a certain
concentration is very close to the computed diffusion rate of the elementary
PbS crystal unit and comparable to the diffusion rate of the lead ion.
The influence of PbS concentration on deposition is clearly shown in
Figure 4b, where initial deposition rates for three PbS concentrations are
presented. The maximum deposition rates are obtained in the region of pH
values where a dramatic change of the sol absorbance takes place (shown in
Figure 4a), indicating a complete dissolution of PbS. It is important to
note that the maximum deposition rates appear to be proportional to the
concentration. Direct crystallization on the pipe surface seems to be the
dominant mechanism of the deposition for pH values lower than that of the
maximum deposition rate, while at higher pH particle deposition and particle
agglomeration also occur. As is well known, with increasing particle
size (due to agglomeration) the deposition rate tends to decrease and this is
indeed observed in the experiments. Moreover, some very loosely bound agglomerates
are observed to deposit on the pipe surface at pH values higher
than that of maximum deposition.
EXPERIMENTS ON SILICA POLYMERIZATION KINETICS
As is well known silica precipitation and scaling depends on the polymerization
rate of monomer Si0 2 or H 4 Si0 4 . Factors affecting the rate of
polymerization are the pH, degree of monomer supersaturation, salinity,
temperature and foreign particles or seeds (Makrides, Turner and Slaughter,
1980). An experimental set-up was constructed to carry out silica polymerization
tests at elevated temperatures and pressures, i.e. at conditions close
to those occurring in geothermal plants. The reaction pressure in this setup
can be adjusted in the range 1 to 10 bar by means of high pressure nitrogen
gas, while the maximum working temperature is ISO C.
Experiments are performed to study the effect of initial concentration,
pH and temperature. Some results are shown in Figure S. As expected the
rate of monomer disappearance is quite strongly dependent upon the initial
monomer concentration, which influences the degree of supersaturation. On
the other hand, the temperature does not seem to have a strong influence on
the net rate of silica polymerization since two opposing effects are taking
place. Indeed, a temperature rise tends to enhance the mobility of molecules
thus increasing the intrinsic reaction rate, but simultaneously the
Si0 2 solubility increases thus reducing the degree of supersaturation.
Preliminary experiments have also been performed to study the effect
of seeding the solution with colloidal particles. Figure Sb shows that seed­
Ing with colloidal spherical silica particles of diameter -20 nm (Ludox TM
by DuPont) causes a rather drastic reduction of the so-called "induction period"
of polymerization, effectively increasing the rate of silica precipitation.
This Is important from the practical standpoint since geothermal
fluids almost always contain suspended solid particles from the reservoir or
rapidly forming heavy metal sulfide grains.
There is a strong pH effect on silica polymerization, as is also shown
in Figure Sc, which can be explained as follows. The effect of pH on silica
solubility is not significant for pH values smaller than 8.S (e.g. Her,
1979). However, the pH affects the rate of polymerization reaction which
seems to be catalyzed by the OH- ions. The maximum polymerization rates
are observed in the pH range S to 8, but a reduction occurs below pH-S
with the minimum reaction rate observed at pH

78
CONCLUS IONS AND COMMENTS
The main conclusions which are drawn from this work so far are as follows:
(I) The PbS deposition at ambient conditions is strongly dependent on
pH and total concentration. The same trend regarding pH and concentration
is obtained from the silica polymerization experiments.
(2) For a fixed PbS concentration, appreciable deposition occurs in a
limited range of pH values of about two pH units. The maximum deposition
rate is observed at the pH value of complete dissolution of PbS.
A practical implication of this work, provided that the same deposition
features and trends in the results hold at high temperatures and high salinity
fluids, is as follows: To prevent sulfide scale formation it seems necessary
to reduce the pH by about one unit below the pH value at which practically
all the scale forming heavy metal ions are in solution. This is
demonstrated in Table II in which the concentration of heavy metals in the
Milos geothermal brine is compared with the solubility of the corresponding
sulfides, at pH values 5.3 (the brine pH) and 4.3. The solubilities are
calculated according to Helgeson (1969) for a fluid with 2M salinity, at
230°C. The pH reduction is expected to have a benefic{aL effect on siLica
deposition as weLL.
A second implication is that by increasing the pH of the brine, and
possibly adding a coagulant, the sulfides will tend to agglomerate thus facilitating
their separation by mechanical means. It is very likely that silica
will behave similarly in the same pH range (5.5-7.5). Obviously mo're
work is necessary to firm up the above conclusions and to arrive at economically
acceptable solutions of the scaling problem.
REFERENCES
Andritsos, N., and A.J. Karabelas (1988a). Deposition of colloidal lead sulfide
in a pipe. International Conference on Fouling and Cleanin g of Process
Plants, 25-29 July, Oxford.
Andritsos, N., and A.J. Karabelas (1988b). Laboratory studies of PbS scale
formation in steel pipes. International Workshop on Deposition of Solids
in Geothermal Systemll, 16-19 August, Reykjavik.
Andritsos, N. (1988). Observations and analysis of scale samples from Milos
Geothermal Plant. Report to Geothermal Section. Greek Public Power
Corporatjon. CPERI, Thessaloniki (in Greek).
Helgeson, H.G. (1969). Thermodynamics of hydrothermal systems at elevated
temperatures and pressures. Am. Jour. Scj., 2..6.l, 729-804.
Owen, L.B., and D.E. Michels (1984). Geothermal Engineering Reference
Manua!. Department of Energy, Rep No DOE/SF/11520-Tl.
Iler, R.K. (1979). The Chemistry of Silica. John Wiley and Sons Inc .. N.
York.
Karabelas, A.J., N. Andritsos, A. Mouza, M. Mitrakas, F. Vrouzi, and K.
Christanis (1988). Characteristics of scales from the Milos Geothermal
Plant. International Workshop on Deposit jon of Soljds in Geothermal
SYStems, 16-19 August, Reykjavik.
Makrides, A.C., M. Turner, and J. Slaughter (1980). Condensation. of silica
from supersaturation silica acid solutions. 1. Colloid Interf Science 1.l..
345-367.
ACKNOWLEDGEMENT Grateful acknowledgement is made of the financial
support by the Commission of European Communities (under contract No
3N3G-0040-GR), the Public Power Corporation, and the General Secretariat
for Research & Technology of Greece.

79
To Atm
Sep.r.tor
H.1l
Sep.r.tor
Brine
Collecllng
T.nk
WIlli M-2
Collecllng
T.nk
Brine
Reinjection
Pump
~ cP cP (j)
~~c==r~~
Semple Pipe Nol S.mple Pipe No2 Sample Pipe No3
Well M-l
Figure 1. Flow diagram of tbe Milos 2MWe pilot plant showing the sampling
points

80
6
5
........
c
§. 4
0
~
e 3
'-"
c
.g
'Vl 2
0
0.
8
1
0 pH= 1.43
pH= 1.65
•
•
A pH= 1.93 0
pH=2.30 0 Ii!
D~ • •
~.
/
~,
/ _A_A'A A Ii
.f- AA-A
• • •
.~ .-.-.-
~''-iI-
0
0 50 100 150 200
TIme (min)
•
250
Figure 2. Effect of pH on deposition (0= 26.5 ppm, Re= 6000).
i "ab
100
80
e 60
'-"
8
- *
c
40
:~
'"
fr
0 20
•
o
A
Re= 3000
Re= 6000
Re=9000
Re=I2000
5 10 15 20
Time (hr)
25
Figure 3. Effect of flow rate on deposition (pH=3.0, C=3.5 ppm)

81
S~--------------------------------~
• C=3.S ppm
C C= 11 ppm
4
• C=26.S ppm
3
2
.-
(a)
O+-----~~~~-r--~~--~~~~~~
o
2 3 4 S 6
pH
--
I
8 .....,
~
* £
g!
C.
~
IS
10
S
0
0
r. • c •
• •
I
;.'c
f \ \
pg, •.•
_ ....... '- c .......
2 3
C= 3.Sppm
C= 11 ppm
C=26.S ppm
4 S
6
(b)
pH
Figure 4. Effect of pH on sol absorbance (a) and on deposition rate (b).

82
1200
1000 • • • •
•
• •
T = 150 C
P = B bar
......
e BOO
a.
.9- • • •
~
c (a)
[] c
Q 600 [] [] []
[] []
II) c c c
400
I~
Co= 600 pp~1
Co=1000 ppm
200
0 30 60 90 120 150 1BO
1200
1000
c
...... •
e BOO • •
[] [] []
Time (min)
[]
c
Co = 1000 ppm
T = 100 C
P = B bar
a. • []
.9- (b)
~ c
0 []
i7.i 600 []
•
c
• • • • • •
400 [] no seedin
• seeding
200
0 30 60 90 120 150 1BO
1200
][][][][][]c
[][][]
Time (min)
1000
• c
Cc
......
e BOO
•
Co = 1200 ppm
T = 45 C
[][]
a. [][]
.9- •
[]
~
Q 600
II)
• II.
I~
[][]
400
••• pH = 5.7
•••••••
pH = 4.~1
200
0 60 120 180 240 300
Time (min)
(c)
Figure 5
Effect of various factors on Silica polymerization:
(a) initial monomer concentration, (b) seeding, (c) pH.

83
TABLE I: Chemical Analy.1I or Scale Samples
Composition (wt ~)
SAMPLE
Component 1 2 3 4A 4B 6 7 8 9
Na 1.0 0.8 1.8 1.1 4.7 2.3 1.7 1.0 0.1
K 0.3 0.3 0.6 0.4 1.2 0.6 0.6 0.3 0.0
M& 0.0 0.2 0.2 0.2 0.1 0.3 0.4 0.0 0.0
ca 0.2 0.1 0.2 0.1 0.9 0.1 0.2 7.5 0.0
Mn 0.1 0.6 0.6 0.5 0.3 0.8 1.0 0.1 0.1
Pb 1.2 3.7 1.1 3.0 0.3 3.0 1.5 0.2 7.2
Fe 18.5 30.0 35.8 39.6 6.9 27.1 23.6 29.8 14.8
Zn 4.7 13.5 3.1 7.8 0.1 13.0 7.3 0.2 7.8
cu 0.2 0.6 0.8 0.3 0.1 0.1 0.3 0.1 0.3
N1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0
Cr 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.0
AI 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1
SI 24.4 6.4 8.6 4.6 32.3 7.2 14.8 5.0 20.3
a 1.9 1.2 2.9 2.2 9.2 3.6 2.7 1.4 0.1
S04- 0.2 0.3 4.0 1.0 0.4 0.5 0.5 16.9 0.3
TOTAL 52.7 57.7 59.7 60.8 56.5 58.6 53.6 64.3 51.1
TABLE IIjComparllon between the .olubllities of heavy metal
.uUldes and the concentration or the correspondlnc heavy metal.
In the brine.
Metal
Concentration Solublllty, ppm w+
In brine
(ppm) pH .... 3 pH-5.3
Pb 1.8 7.3 0.75
Zn 3.3 39.0 4.10
Fe 20.0 17.5 1.80

84
EEC contract EN3G-0033-F
SULFIDE DEPOSITION AND WELL CLOGGING IN THE
DOGGER AQUIFER OF PARIS BASIN (FRANCE)
C. FoUILLAC, A. CRIAUD, J.L. HONEGGER
I. CZERNICHoWSKI-LAURIoL and A. MENJoZ
BRGM - Institut Mixte de Recherches Geothermiques
Summary
The programme began following the first signs of loss of hydrodynamic
properties in certain wells exploiting the Dogger. As a result of
measurements carried out on wells characterized by high contents of
dissolved sulfide, analyses of samples have enabled material balances
to be drawn up, and have shown the direct relation between iron
sulfide scaling and the start of clogging. The sulfide scale is
present from the base of the production casing, and consists mainly
of mackinawite, pyrite and pyrrhotite. Isotopic studies of dissolved
sulphur species have shown that the unequal distribution of sulfides
in the fluid results from the varying degree of bacterial reduction
of sulphates within the reservoir on a regional scale. The knowledge
of the chemical conditions in the reservoir was increased as a result
of further analyses. These results were used as the input parameters
of a thermodynamic model (TPDEGAZ) simulating chemical changes in the
fluid (pH, saturation with respect to minerals deposited), caused by
variations in physical parameters (temperature, pressure, degassing),
during its migration to the surface. The main result of the modelling
was to show the major role played by corrosion in the formation of
scale. A description of the overall mechanism including fluid
chemistry of the fluid phase and evolution of corrosion and scaling
products is given.
1. INTRODUCTION
Partial clogging of certain geothermal wells in the Dogger was
noticed in 1985. This was signalled by increases in reinjection pressure,
a decrease in production, and by the need to clean out the heat exchangers
at periodical intervals, varying according to the site.
Further investigations into the chemical composition of the fluids of
the Dogger showed a marked variation in dissolved sulfide; in particular,
they showed that the zones of maximum sulfide concentration coincided with
the location of the sites at which clogging had occurred (fig. 1).
The aim of the study was to evaluate the relationship between the two
phenomena, by carrying out diagnoses of the clogging at the sites
analysing fluid and solid samples, and creating a model which accounts for
the scaling processes.
2. MINERALOGY AND CHEMISTRY OF THE SCALE DEPOSIT
Workover operations for mechanical cleaning of the casing from surface
to the reservoir have provided most of the material for the detailed
studies presented here. Samples from different depths in a great number

of WId In nil rev'n1.-d hI! presence of lerg 8J1l0Un a of Hon sulfIde, wlt
m r.kjnnwitll he mool abundant, and pyri e and pyrrhotlte the other
Qth production and InJOC ion wella, the presence 0
111 \' to pyri e and pyrrho i a tends to decrease wi ttl
d p h. Iron hydro d G or chlorid -hydro ide aider e and calcite are
rv'd in mlnor 9IT10unt I and troil1 I marcaaite end grelglte ~re also
r por d . Cr , Mn, Ni, Cu , As and Mo ara also presenl in lh~ iron ~ulrid~
pha , bu th Y do no orm true aulfide mInerals.
Th various iron aulfide phaaes alternste 1n distinct luyel"!$
id nt and hydroXldcs bein9 also n 1m8 ely sssoc aled with ttltHI', The
o 1 thi knoss of t h scale can reach seversl centilJ1elere, wh.ile the
ub-lay r of cliff r n porosity are sometimes less lhan 0.2 ~ in thicknCtit;
.
Oi nolo ion-prcclpi tion end deh 'dration patterns have b~e l l
l"~coqnil d in ::lomo sll1pl's but the chronology ot depositIon seefll5 to be
r th,:r complex 1n moat of he observed 8sO'oples. Though the mixed
-hloridc-hydro)(id ph lS S wallknown corrosion product. it is not
cl r wh ·th 'r h d posi 10n occurs primsrily . or ofter enrly
pl'eC:ipltnlion of Iron RLd firl • Clustere of grein of pyr te, fOackiriOWile
und pyrrhoti~e c mcnl d by carbona e slso occur. TheBe observ Hons show
th tho ord r nd conditions of precipltetion of the scale componenls
ur v ry com!,,1
------
-.. '
......
......... --,
0A
'" .
,
I
'--___.o.._..... r-
lqurr l - 013 rlbution of d ssolved sulfide 1n lh~ 009ger fluids .
Th au fide deposition occurs 1n the northern nrt.

86
3. ORIGIN OF THE SCALE COMPONENTS
Since these a-re the main components of the scale, we shall
concentrate on the dissolved iron and sulfide; the other species
involved,- such as chloride or carbonate, undoubtedly originating from the
deep reservoir.
3.1 Sulfide
Isotopic studies of dissolved sulfates and sulfides have been carried
out in order to understand the distribution of dissolved sulfide displayed
in fig. 1, Fouillac and coworkers (in prep.). The results show a
distribution of the634S of the sulfates according to geographic zones that
also corresponds to variations in the geochemistry of the fluids, in
particular the sulfide content.
Samples taken in the seine-St-Denis area and in the areas north and
west of Paris show high 634S values ranging from +26.3 to +48.~. This is
reflecting the effect of bacterial reduction in a confined part of the
basin. Modelling this reduction made it possible to calculate a value of
1.038± 0.003 for the coefficient of fractionation between the sulfates and
sulfides. This value is in agreement with the ones commonly found for
sulfato-reducing bacteria. In the other zones, south of Paris and in the
Val de Marne, the 6 34S values for the suI fates are more uni form, ranging
between +22.4 and +26.6ti, despite the existence of bacterial activity.
This is most probably due to a higher rate of flow of the waters,
entailing renewal of the sulfate stock.
This detailed isotopic study shows that :
Sulfides are produced by b~cterial reduction of sulphates.
The efficiency of reduction varies within the reservoir, and is
controlled by two parameters: the temperature and confinement of the
fluids. These two effects are summarized in fig. 2.
A large proportion of the dissolved sulfide formed has been extracted
from the liquid phase, mainly by precipitation of pyrite.
The results are fully consistent with a Rayleigh distillation model.
This suggests that the duration of bacterial reduction is much greater
than the period of exploitation of the Dogger.
3.2 Iron
The concentration of dissolved iron in the fluid within each
sub-aquifer of the Dogger reservoir is controlled by the solubility of
pyrite. Mineralogical studies in cores confirm the presence of this
mineral, which is the only iron sulfide phase encountered. Very low
concentrations of dissolved iron (less than 0.1 ppm) are thus expected to
be derived from that source in the fluid that enters the casing downhole.
Nonetheless the corrosion of the mild steel composing the casing is
responsible for a very large input of iron into the fluid, as has been
proved by observations, chemistry and measurements. Ouzounian and others
(1987) showed variations in iron concentration as a function of well
discharge, and presented arguments that this is related to the occurrence
of corrosion reactions. Moreover Sainson and others, (1987) evaluated the
thickness of corroded material by in situ measurements in a number of
boreholes.

87
• •
• •
. -\ .
•
• •
•
·ft • •••
-tt
• •
-30 • •
•
0
• 0
90 8
0
0 •
0 6 o .0
0 6 0
, •
·ro+---------_r--------~----------~--------_r----------
40 50 60 70 80
Figure 2 - Variation Of6 34 S(S04) versus temperature
4. MECHANISM OF SULFIDE DEPOSITION
4.1 Thermodynamic basis
The basic reactions to consider in describing the processes
corrosion: Fe ~ Fe++ + 2 e-
2Fe++ + Cl- + 3 H2O -= Fe2(OH)3Cl + 3H+
deposition: (l+x)Fe2+ + HS-
-= Fel+xS + H+
or Fe++ + HS- ~ FeS + H+
or Fe++ + 2HS- ~ FeS2 + 2H+ + 2eor
3Fe++ + 4HS- - Fe3S4 + 4H+ + 2e-
---
evolution: FeS + HS- ~ FeS2 + H+ + 2e-
3FeS + HS- - Fe3S4 + H+ + e-
-
2FeS + Fes2 - Fe3S4
• -
are:
(1)
(2)
Fel+xS stands for mackinawite, FeS for pyrrhotite, and FeS2 for
pyrite. Half redox reactions are likely counterbalanced by the formation
of atomic hydrogen (IMRG, unpublished data).
Equilibrium is achieved between the solution and any mineral if the
product of its ionic activity lAP equals the solubility product K:
(Fe++) (HS-)
_ (Fe++) (HS-)2
ex.: K4(T) = (H+) KS(T) -(H+)2 (e-)2 .
Precipitation may occur when the lAP becomes larger than the
solubility product. Because of kinetic considerations, a metastable phase
such as amorphous iron sulfide or mackinawite (Fe9Se) should precipitate
first.
Several reasons can account for the initiation of the precipitation
of the sulfide minerals: mixing of different fluids from separate
producing levels in the aquifer - increase in dissolved Fe due to
corrosion - increaae in pH due to degassing - drop in temperature and
pressure - increase of HS- due to bacterial reduction of sulfate.
(3)
(4)
(s)
(6)
(7 )
(e)
(9)

88
4.2 Mixing effect
For a given well, at least two layers should be considered, but up to
ten distinct producing zones were evidenced in some places (Rojas and
others, 1987). The changes of temperature between the shallower and deeper
levels are in the range 2° to 4°C. Slight differences in major chemistry
have been indirectly demonstrated (Criaud and others, 1988a). It is
thought that the more reactive dissolved species such as iron and sulfide
could show greater variations. It has also been observed that the
concentrations of sulfide and iron vary with time, at wellheads, the
relative variations reach 30% for both species. The contribution of
different producing layers could explain these variations, together with
corrosion and scaling reactions (Ouzounian and others, 1987).
A simplified fTlOdel is represented in Figure 3. As a first approach we
assume that pH, ionic strength and temperature are constant for the two
mixing components A and B. Within each layer, the activities of dissolved
iron and bisulfide lie on the solubility curve of pyrite. Any mixing would
result in a point on the straight line AB, which becomes oversaturated
with respect to pyrite, and thus precipitation of mackinawite, for kinetic
reasons, is expected to occur. These particles can then grow, be
transported by the fluid or fix on the casing where the second step,
evolution, occurs.
pyrite
---r-----------------~-~~~~
r 18
r
I
Fe
Figure 3 - Simplified mixing model involving two fluids A and B.
4.3 Evolution of a single fluid
The use of programs such as TPDEGAZ (Czernichowski, 1986) enables
simulation of the variations in temperature and pressure in the system.
These are due to heat exchange or microdegassing induced by immersed
pumps. It has been shown by Czernichowski (1988), that for typical site
of the Dogger, corrosion is the main factor inducing the precipitation of
mackinawite. Changes of pressure, and the subsequent slight rise in pH,
can hardly explain the observed massive iron sulfide deposits. Some input
of iron is required, and values are in agreement with corrosion rates
measured with classical methods on coupons (weight, or polarization
resistance) or wireline logging techniques (Honegger and others, 1988).
4.4 Scale growth and evolution
If the presence of mackinawite is consistent with thermodynamic and
kinetic considerations in the early stages of deposition, several
questions come to mind concerning the other suI fide minerals. Do they
result from evolution of the mackinawite, either by ageing or under the
r
r

89
influence of very local conditions of pH, salinity or redox potential
(Criaud and others, 1988b)?
The scale can thicken, or evolve through its external interface with
the geotherlllal solution or its internal interface with the pipe. After
the forlllation of a sulfide fillll on the wall of the well, sorre iron Il1Ust
be added to the systelll since the concentrations of sulfide in the fluid
are far in excess and do not need renewing. The IlIechaniSll1s can be:
* Evolution: Reactions (7), (8) and (9) define a IlIechaniSll1 by which the
initial IlIackinawite could evolve, at the fluid-scale boundary, to other
sulfides. This accounts for evolution but not for growth.
* Corrosion: The corrosion of IlIaterials in an environrrent of sulfur
species (502, polysulfides, elelllental sulfur, hydrogen sulfide,
thiosulfates) is always associated with the deposition of a scale
cOll1posed of a null1ber of layers of IlIetal sulfides. The requisite
conditions include aqueous solution, gas IlIixtures, and telT1peratures in
the range 10 0 to 10000C. In IlIost cases, the environment cannot provide
the source of iron (or other IlIetal) for the forlllation of the scale, so
that corrosion is unalllbiguously responsible for the scale. Carbon steels
develop pyrite and pyrrhotite scales (Ikeda and others, 1985). Elliasson
and others (1983) found that the sulfide fillll at least slowed down the
corrosion in the geotherlllal wells of Iceland (and presumably the growth
of scale), but experilllents and theoretical investigations confirlll th~t
the sulfide strata have variable perllleability and density, and that they
allow the diffusion of IlIetallic cations frOll1 the IlIaterial to the
solution. The diffusion controls the progress of the reaction and the
kinetics scale growth (Orchard and others, 1985; SIlIeltzer and others,
1985). In contrast, the IlIobility of 5 in the scale is reduced
(Gilewicz-Wolter, 1985). The thickness of the corroded IlIater ial and of
the scale are generally in the sallie range. Thus, experience in general
indicates that the propagation of the scaling reaction is related to the
corrosion, with iron (or other IlIetal) diffusing frOll1 the casing and
sulfide transported by the fluid.
* Migration: SOllie particles, forllled in the fluid (e.g. by IlIixing of
waters) or at the external interface, can also be carried by the flow and
settle on the scale after sOllie distance, during which they probably
undergo sOllie Il1Odification, such as clustering. Nonetheless, the little
all10unt of particles and their slllall size show that is not the IlIajor
process.
4.5 Overall IlIecanislll of sulfide scaling
NUlllerous local or global phenOll1ena IlIust be considered to explain
the iron sulfide scale which is observed in the geotherlllal wells of the
Paris Basin. They are schelllatically sUll1marized on figure 4.
At an early stage along the path of the fluid, a very slllall
contribution can be IlIade by the initial pyrite transported by the rluid
within the reservoir. Before any contact with the casing, the IlIixing of
several fluids with differing concentrations of iron and sulfide will
lead to the forlllation of IlIackinawite. These particles will settle on the
walls in the well soon after they are forllled, or after having grown
and/or evolved to another sulfide IlIineral.
General corrosion, by increasing the iron content of the fluid in
the presence of a large alllount of dissolved sulfide is responsible for
the rapid deposition of a IlIetastable IlIackinawite fillll. The iron
hydroxide-chloride phase is typical of corrosion reactions. It appears
either directly on the steel surface, or as a thin fillll on IlIackinawite.
In both cases, a local enrichll1ent of the concentration of chloride forllls

90
as 8 result of adsorption onto the mate-rial 01' sulfide. Thi:; f;Jvors the
formation of a chloride-iron aqu~ ou s complex and the deposition of this
corrosion product. Islets 0 chloride can again attack this weak layer
and make it thinner, unt I reaching th~ "internal" i ron suI fide, or th~
steel i tsslf. Because of thelr variable porosity, the iron sulfide strat~
allow the diffusion 0 th ron from the material to the external fluid,
and subsequent growth of the scale . Modelling the cheff1istry of the fluid
w· thin the well also demonstrates that continuous addition of 'ron is
required to rssult in deposltion along the whole length of the casing
(Czernichowski, 1988). Special conditions of conFinement, high chloride
content, reduced potential and iron and suI fide gradients are created
beneath the scale. Intense corrosion (uniform, pitting, bacterial) will
follow, with possible precipitation of pyrite pyrrhotite and othel"
sulfides. Locally, pH increase is likely as a result of reactions (2) to
(8), explaining tha depOSition of iron carbonate intimately associated
with the sulfides snd hydroxide-chloride phases.
TO' ~ ,..-_ __ ~~~~ ...... , ,N t",UI~
,-_ _ _ ,""1 .. 1 ..... ir. ''''''11 ....
Poll'''., .r
1I~I'"'bt
fI",14
,.,t
OIHw~IfI1.1
Ikt~iJh'''' ,/WCN .
\lU'"
DIKI .. ~ 1 1t1 oc" .. II,
Vndfol nw- .co'"'
__ ..... ,cQHI.CI
_ ,. . ~.r . 1cf.i ,tII. ,
... , t\AIII_'
== lulll6t 1'.0'"
-(Ff"'j, "d. · .. l
_~$~'I"O'WI ' ,
~f:lr~'lvCI~
~ AO"t C'foOl.·C lr ..... n
Figure 4 - Mecanism of sulfide scal ng end corrosion
S. EXAMPLE OF MASS BALANCE
Other arguments suggesting t hat corros on i8 t he main source of
iron for scale growth can be Found n mass balance calculation on iron.
Such a calculat on was made on the La Courn8uv Nord geothermal site,

91
Precise fll()nitoring of the various phases of IIIechanical cleaning, with
interll'ediate hydraulic IIIeaSUrBll'ents, s8ll'pling of scale deposits at
different depths, and lateral core-drilling of the reservoir have enabled
tt~ type and amount of the scale to be assessed.
Well-logging techniques (CCE report EN3G-0038F) provided further
inforlllation on the residual scaling left in the well after cleaning, and
in particular, on the quantities of steel rellloved by corrosion. The main
results of the hydraulic IIIeasurelllents show that III0St of the scaling
present in the well lines the whole length of the casing, and this causes
63~ of the increase in pressure. The rest is related to the partial
clogging of the bottOlll of the reinjection well. For the IIIass balance
assesslllent,it has been necessary to IIIake certain assUll'ptions. In
particular, it is illlpoasible to ascertain accurately the density and
water content of the scale. A well-log to show corrosion in the
reinjection well was also lacking. Nevertheless, the table 1 below enablea
the order of II'8gnitude of the iron taken frolll the corroded tubing to
be cOlllpared with that of the iron estilllated for all the scale.
Production well Injection well
Thickness Iron Thickness Iron
Iml tonnes 1m' tonnes
Total
Iron
(tonnes)
Evaluation of
corrosion by 1.5-2.7 11.9 1.5(3) 9.6 21.5
electrOlllagnetic
logging
(1 )Scale left
after cleaning 1.5 2.4 1.5 4.2 6.6
(2)Scale recovered
by well / 2.1 / 5.9 8.0
cleaning
(1) density 4,000 kg/III3, 75~ iron sulphide, of which 53~ iron (estilllated
by ll'echanical logging)
(2) density 1,750 kg/III3, water content 35~, 75~ iron sulphides, of
which 53~ iron
( 3) assulllption.
Table 1 - Iron balance (doublet of La Courneuve Nord)
This balance shows that corrosion is sufficient to provide iron in
such alllounts as to explain the quantity of scale present. It IIIUSt be
assLWlled that the excess iron has been reinjected into the forlllation in
the forlll of very fine particles, since the cores drilled frOlll the wall of
the well have revealed only very slight clogging, and this within only a
1-2 CIII radius in the wall-rock.
6. CONCLUSIONS
The geotherlllal wells tapping the Dogger are mainly affected by
corrosion, which is directly responsible for the scale observed. The
other causes alone would have only IIIinor effects.
The corrosion rates of carbon steels in such fluids are increased

92
because of the chloride content. The thin iron sulfide film cannot
stabilize and provide an efficient barrier against corrosion, because the
chloride concentrations are quite high.
The solutions to be applied to cases such as the Paris Basin must
attack both corrosion and scaling because these are different aspects of
the same problem. Corrosion should be limited, or suppressed, from the
casing shoe of the production well to the casing shoe of the injection
well. Injection of corrosion inhibitors at the base of the production
well requires : the i ilJlprovment of engineerin.g techniques.
BIBLIOGRAPHY
CRIAUD, A., and FOUILLAC, C. (1988c). Sulfide scaling in low enthalpy
geothermal environlJlents: a review. Proceedings of the 1st Workshop on
Deposition of solids in Geoth. System. Geothermics, in press.
CRIAUD, A., FOUASSIER, P., FOUILLAC, C. and BRACH, M. (1988a). Natural
flow and vertical heterogeneities in a sedimentary geothermal reservoir
(Paris Basin, France): geochemical investigations. Proceedings of the
13th Workshop on Geothermal Reservoir Engineering, Stanford, in press.
CRIAUD, A., FOUILLAC, C. and MARTY, B. Low enthalpy geothermal fluids
from the Paris sedimentary basin - 2: Oxidation reduction state and
consequence for the prediction of corrosion and sulfide scaling, Geothermics
(in press). ---­
CZERNICHOWSKI-LAURIOL, I. (1986). Degassing of geothermal fluids: a
geochemical model. Geothermal Resources Council Transactions, 10,
113-118.
CZERNICHOWSKI-LAURIOL, I. (1988). Modelisation de l'evolution de la
chilJlie des fluides geothermaux lors de leur exploitation par forages.
Doctorate Thesis of the Poly technical National Institute of Lorraine,
DoculJlent du BRGM nO 159, 196 p.
ELLIASSON, E.T., and EINARSSON, A. (1983). Corrosion of mild steel in
water from Icelandic low temperature geothermal systems. Proceedings of
the Int. Symp·. on Solving Corrosion and' Scaling Problems in Geothermal
Systems, San Francisco, 64-71.
FOUILLAC,I C., FOUILLAC, A.M., and CRIAUD, A. Sulfur and oxygen isotopes
of dissolved sulfur species in the Dogger Geothermal aquifer of the Paris
Basin. Submitted to Applied GeochelJlistry.
GILEWICZ-WOLTER, J. (1985). Application of autoradiography in studying
the matter transport in scales formed on metals in oxygen and sulfur
atmospheres. Isotopenpraxis, 21(10), 365-367.
HONEGGER, J.L., CZERNICKOWSKI-LAURIOL, I., CRIAUD, A., MENJOZ, A.,
SAINSON, S. and GUEZENNEC, J. (1988). Detailed study of sulfide scaling
at La Courneuve Nord, a geothermal exploitation of the Paris Basin,
France. Geothermics (in press).
IKEDA., MUKAI, s. and UEDA, M. (1985). CO~ corrosion behavior of carbon
and Cr steels. The Sumitomo Search, 31, 91-~02.
ORCHARD, J.P. and YOUNG, D.J. (1986):"" Gas phase composition on the iron
sulfide scaling reaction. J. Electrochem. Soc., Solid State Science and
Technology, 133(8), 1734-1741.
OUZOUNIAN, G., CASTAGNE, 5., FOUILLAC, C. et CRIAUD, A. (1987). L'origine
du fer dans les eaux geothermales exploitees au Dogger (Bassin parisien).
Geothermie Actualites, 4(3), 31-34.
ROJAS, J., MENJOZ, A., -MARTIN, .J.C., CRIAUD, A. and FOUILLAC, C. (1987).
Development and exploitation of low enthalpy geotherlJlal system: example
of the Dogger in the Paris Basin, France. Proceedings of the 12th
Workshop on Geothermal Reservoir Engineering, Stanford, in press.

93
SAINSON, S. et HONEGGER, J. L. (19B7). Interpretation de la diagraphie
~lectromagnetique ETTD •• BRGM report nO B7 SGN 6Bl IRG.
SMELTZER, W.W., NARITA, T. and ERELFAIE, F .A. (19B5). High temperature
sulfidation properties of Fe-26,6Cr and Fe-2B,7Mn alloys in H2S-H2 atmospheres.
Mat. Sci. Monogr., 2B(A), 165-167.
ACKNOWLEDGEMENTS
This work was carried out in the framework of the COlmlission of
European Communities contracts EN3G-0033-F and EN3G-003B-F, and received
financial support from the AFME and BRGM, B. GAYET drafted the figures
and J. RABIAN typed the manuscript. The authors are indebted to M. BRACH
and E. PROUST, who collected the samples during tedious workover
operations.

94
EKC Contract nO KN3G-Q014-1
DEEP EXPLORATION OF SECOND GEOTHERMAL RESERVOIR IN VITERBO AREA (LATIUM)
C. Garelli (AGIP S.p.A.)
AGIP S.p.A. - I - 20097 S. Donato Milanese
Summary
The VetraLLa 1 weLL wiLL be driLLed beginning of 1989 in an area near
Viterbo (Latium) in the "M. Cimini" prospecting permit.
The purpose of the deep exploration is to verify the existence of a second
reservoir under" the Mesozoic limestone tested by ENEL during 1971.
We can advance two hypotheses:
A) a tectonic repetition of the Mesozoic carbonatic reservoir separated by
impermeable flysch terrains.
B) Presence of metamorphic phylladic basement directly underlying the first
carbonatic sequence (with the possibility of fractured reservoir zones).
Deep exploration of second geothermal reservoir in Viterbo area (Latium)
The "M. Cimini" Prospecting Permit was granted to the Joint Venture AGIP­
ENEL in 1979. Activities therein by the Joint Venture consisted in detailed
geological surveys, geophysical surveys (geoelectrical prospecting,
land magnetometry, aerial magnetometry, reflection and refraction seismics),
geochemical surveys and gradient wells. In the central southern part of
the Permit area - on the eastern edge of the Vi co caldera - a deep well
(Cimino 1) was drilled by the joint-venture through volcanics, shales and
limestone formations, down to a final depth of 3.000 meters.
The presence of active thermal springs (temperature 5O-60 0 C) and travertine
deposits in the western point of the area indicate thermal activity
and, in· the past, this has stimulated the undertaking of geothermal exploration."
This search led to the drilling of the wells: Zitelle, Bagnaccio
(TERNI, 1953), VICO I, VICO 2 (ENEL, 1971-1973) (Fig. 1).
The maximum depth reached was 1,100 meters with the VI CO 1 well. The maximum
measured temperature was 78°C at the Bagnaccio well. The geothermal
reservoir explored by these wells is the same one which feeds the thermal
springs and is located in limestone formations at a depth on the order of
hundreds of meters.
From the geological and structural points of view, the western area is
characterized by a buried ridge, called the "M. Razzano ridge", which is
located west of the city of Viterbo. This structure is recognized as being
a "high", also confirmed by the study carried out on the outcroppings,
and is very clearly evidenced by the gravimetric survey. The "M. Razzano
ridge" has a NNW-SSE orientation and cuts through the whole western side
of the Permit area.
The outlying areas having analogous geological and structural characteristics
are also undergoing geothermal exploration by AGIP and KNEL ("LATE-
RAn and "CESANO" Prospecting Permits). This exploration has evidenced

9S
structural repetitions in several wells and has led to the hypothesis
that tectonic repeti tions of the stratigraphic series also exist in the
Mount Razzano Ridge area (fig. 2) •
Therefore it wsa decided to test the hypothesis that on the ridge a lower
section of the Mesozoic carbonstes, present owing to repeti tion of the
beds by faulting and intercalated with thick layers of impermeable sedimenta,
constitutes a separate deep geothermal system unconnected with the
upper one and containing fluids at temperatures exceeding 200-250 o C. An
al ternati ve possibility is that under the Mesozoic carbons tic section
lies a meta.orphic unit (basically impermeable) which, especially in Mt.
Amiata (South Tuscany), contains a productive fractured reservoir.
The deep Vetralla 1 exploration well will be drilled at the expected finsl
depth of 3,600 meters.
The geographic coordinates for the wells are:
42°16'53" N - 0°21'13" W
The well will be located near Valle Cesate (elev. 405 m), in the township
of Vetralla.
The expected profile is the following (fig. 3):
Hypothesis A) 0 170 II Volcanics
170 500 m Flysch formation
500 - 1800 m Mesozoic carbonatic series
1800 3200 m Flysch formation
3200 3600 m Mesozoic carbonatic series
Hypothesis B) 0 170 m Volcanics
170 500 m Flysch formation
500 1800 m Mesozoic carbonatic series
1800 3600 m Metamorfic phyllites
Drilling on the location will start early 1989.

N
J)
I
II • Cl£fP ItruS
.... ~ ~ SfCT'OI
'-., '" J.SOrHfi:lMS It: J
Fig. 1: ~a p of e6timated tempe~ature at the top of the L i me~ tooe reservoir
A
u .
QOOLOGICAL C~£C'TIOH
I71.l
~
1:9
'UORlt
fl ' ~'
Mttnralt "."",fll
lIeo .Olt.. IU
o rulrl •., ""1
III \ »L1 Id Tarrs
Fig. 2
CIIII'" IOlC"-, CS
o lAt' rLII1
CiJ t IM 1M)

HYPOTES1S A
o I.' O . • • I? Vole 1c:a
---­
~---
1000
Jite0020 c
1.1 stone
HYPOTESIS B
- Pot.en 1 1 productive zone~
Pig. 3 ; VBTJW.1.A 1 W'ELL

98
EEC Contract No. EN34-0l9-1
DEEP EXPLORATION IN THE TORRE ALFINA GEOTHERMAL FIELD (ITALY):
THE TEST HOLE ALFINA 15
EN~L
G. BUONASORTE,. A. FIORDELISI and E. PANDELI
(Italian Electricity Board), National Geothermal Unit
Summary
The well drilling was performed by ENELINational Geothermal Unit in
1987 with a MASS 6000 E rig. The goal consisted of a presumed second
deeper reservoir within a depth of 4000 m. Although the objective was
not achieved, test hole Alfina 15 supplied new information that, going
far beyond the local interest, makes a fundamental contribution to the
stratigraphic, structural and thermal knowledge of the whole region.
Beneath the "Ligurid" cover, the well crossed three "Tuscan" tectonic
units overlying, from 3545 m, an "Umbrian" type succession. The
drilling then continued to 4826 m, beyond the envisaged depth, with
the goal of intercepting the impermeable regional "basement", controlling
the temperature trend and acquiring additional tectonic and
stratigraphic information. For the first time, the Mesozoic carbonate
formations (the main regional geothermal reservoir) were' crossed for
such a great thickness (approximately 3700 m), above a presumed deeper
impermeable "basement". The tectonic thickening of the permeable
rocks' constitutes a single reservoir with extremely low' thermal gradients
(0.2-0.3 °C/10 m) and temperatures ranging between 140 and
210°C. Despite stimulation the well proved to be unproductive due to
low local permeability. From a drilling technology point of view the
well did not present any difficulties despite the great depth never
before reached in geothermal wells in Italy.
1. INTRODUCTION
The Torre Alfina geothermal field is located north of Bolsena Lake in
the northern sector of the Volsini Volcanic Complex and corresponds to a
structural high of Tuscan Upper Trias to Eocene prevalently carbonate
formations underlying the "cover" made up of clayey Ligurid sequences and
volcanic terrains (Cataldi and Rendina, 1974; Dallan and others, 1977;
Buonasorte and others, 1988). The deepest previous well at tQe summit of
the structure (Alfina 2) attained a depth of 1040 m, crossing 360 m·or carbonate
reservoir. The weLL reacli~d its rnaxir,lUllI terilperature

99
The fluid is cornposed of CO 2
-saturated water; the carbon dioxide is
concentrated in a thin cap in the central, highest part of the structure.
The objective of the well Alfina IS, drilled with the financial contribution
of the EEC, was to check for the existence of a second reservoir
separated from the first one by poorly permeable formations inside which
the geothermal gradient was suspected to reach values at least equal to the
regional one delimiting the thermal anomaly (approx. loo°C/km).
Attempts were made to define the stratigraphic-structural trend deeper
than the known one using seismic reflection prospecting, as all the other
geophysical prospectings performed (electrical, gravimetric, etc.) did not
have resolution beneath the first reservoir.
Unfortunately, even the seismic prospecting did not yield satisfactory
resul ts on account of the volcanic cover formations, which excessively
attenuate the reflected signals.
2. GEOLOGICAL AND GEOTHERMAL FEATURES OF THE TORRE ALFINA AREA
The Alfina area is located on the southern, N-S trending extension of
the Monte Cetona horst made up of "Tuscan" and "Ligurid" sequences (Fig.
1). It is bounded to the east and west by tensional faults of considerable
throw (Buonasorte and others, 1988). Other transverse faults created a
structural high which corresponds to the Alfina geothermal field.
The products of the Volsini Volcanic Complex have an alkaline-potassic
character and belong to the "Roman Comagmatic Province". The thickness of
the volcanites, according to drilling data, varies from a minimum of a few
metres (northwards) to a maximum of 250 m (southwards).
The post-orogenic Pliocene sediments (clays, sands, conglomerates)
fill the Radicofani and Chiana - Paglia graben at the borders of the Monte
Cetona ridge. Only the southernmost wells encountered these sediments,
which reached a maximum thickness of 350 m.
North of the Torre Alfina area the II Ligurid" formations outcrop widely
(Costantini and others, 1977). They are composed of two Units: the "Santa
Fiora Unit" (Lower Cretaceous - Eocene), made up of shales, marls, limestones
and sandstones, and the "Ophiolitiferous Unit", composed of shales,
limestones and quartzose sandstones, incorporating masses of ophiolites
(Lower Cretaceous).
The thickness of the Ligurids ranges from 500 m to about 1700 m. Together
with the Plio-Quaternary terrains, they represent the "cover" of the
geothermal reservoir.
The "reservoir" consists of the underlying, predominantly carbonate
formations of the Tuscan sequence (Upper Trias to Paleogene).
Nine wells were drilled in the Torre Alfina field, seven of which are
productive and the remainder dry.
The reservoir was crossed for a maximum thickness of about 800 m. Its
maximum elevation was found at about 500-600 m from ground level. The
fluid, composed of CO.-saturated water, has temperatures ranging from 120
to 150°C and a total salinity of approx. 6 gIl (predominantly sodiumchloride
and subordinately calcium-bicarbonate type). The uppermost part
of the Alfina structure contains a gas cap (almost entirely CO.) which has

100
LEGEND
2
3
6
~ 7
8
"Q
10
•
11
/' 12
0 ,
rIG.l. - GBOLOGICAL AND GBOPllYSICAL Sn«TllBS18 HAP I 1) vulcult.. (Quat.) I
a) clay.,su4. u4 conglolllerate (Plioo.) I 3) Liquri4. (cratac.-lIoc.), .)
Mt.Cetona TUsoan sequenoe (u.Tria.-o.Oliqoc.)I S) kt •• Cervarola-~.lterona
Onit (Boo.-X.Kioo.)1 6)Ulllbriu Sequenoe (V.Tria.-V,Mioo.), 7) R.ai4ual
gravimetrio po.itive anolllalie., 8) Regional 1.ogal oontour line., ')
struotural High. of tbe ra.isti ve .ubstratuml 10) I.oqra4iant oontaur lina
= l'C/10., 11) .el1., 12) Tr.aoe ot geologioal oro ••-.aotion.

101
a pressure of about 40 atm.
The temperature at the top of the reservoir is practically constant;
furthermore, temperature remains quite uniform within the explored parts of
the reservoir, indlcating the presence of a strong convective circulation.
3. THE ALFINA 15 DEEP TEST HOLE
3.1. Objectlves and characteristics
The test hole is located near the top of the above-mentioned structural
high; its planned depth was 3500-4000 m.
The aim of the test hole was to drill beyond the first reservoir and
to investigate a possible deeper second reservoir lying within a depth of
4000 m, constituted by the Mesozoic Carbonate Series or metamorphic basement.
It was suspected to be separated from the first one by the formations
of the Ligurid Formations entrapped in the tectonic overthrusts.
Instead, the well revealed at this depth only the presence of the
first reservoir constituted by a pile of carbonate tectonic Units, with
hydrogeologic continuity.
In order to obtain more knowledge of the structures at depth of regional
importance, drilling was continued with the goals of ascertaining
the existence of an impermeable basement, verifying the temperature trend
at depth and acquiring further information on the stratigraphy and tectonics
of the deepest formations.
3.2. Notes on drilling
The drilling was performed by ENEL/National Geothermal Unit with a
MASS 6000 E rig. The well construction began on 16 April 1987 and ended on
7 February 1988. Owing to the great planned depth, the well was drilled
wi th large rock bi ts and the cas ings lowered were: 32" with shoe at 22 m,
24 1/2" wi th shoe at 137 m, 18 5/8" with shoe at 508 m, 13 3/8" with shoe
at 1320 m and 9 5/8" with shoe at 3950 m (Fig. 2); drilling then proceeded
wi th an 8 3/8" RB to well bottom (4826 m).
Measurements to check the slope and direction of the well were made to
the depth of 3731 m, both to facilitate the drilling work and the interpretation
of the data, and to keep the test hole from shifting too far from
the vertical by correcting any excessive spontaneous deviations. The maximum
total displacement was about 181 m, recorded at the measured depth of
3731 m, corresponding to the vertical depth of 3723.6 m.
Losses of circulation and modest absorption found in the well are
shown in Fig. 2.
After a gas lift production test the well was shut in.
The great depth, never before reached in geothermal wells in Italy,
did not present any difficulties from the drilling technology point of
view.
3.3. Stratigraphy
The stratigraphy of the sequences crossed (Fig. 2) by test hole A15
was reconstructed on the basis of examination of the lithology, petroaraphy,
the micropaleontological contents of the cuttings (sampled every 5

~
~i
&l E-~I~\lde : Q#'30·
. ,
~
, 2
14
au,,: _-_ a
~
'v I:
~ LoDCl\u

103
metres) and the cores recovered during the drilling. From the top downwards,
the following Units were recognized:
yo!c!n!t!s (0-125 m). Represented in large part by tephritic-leucitic lavas
and by a basal layer of .! argillified pyroclasti tes (with volcanic and
sedimentary elements), both of them deriving from the Quaternary apparatuses
of the Monti Volsini ("LS" and ''7'1'' in Buonasorte and others, 1988).
!:iiu!:i~s. Made up of two Units: an upper one (125-430 m), prevalently
shaly ("Ophiolitiferous Unit" - Lower Cretaceous) and the lower one (430-
1050 m), shaly-marly ("Santa Fiora Unit" - Cretaceous/Paleocene), in which
layers of limestones and sandstones are intercalated.
:T~s£B!!"_U!!i.!s. There are three "Tuscan" type Units: I) 1050-2245 m; II)
2245-3505 m; III) 3505-3545 m. The top two are composed of formations
ranging in age between the Middle-Upper Eocene ("Scisti Policromi Group")
and the Upper Trias ("Calcari e Marne a Rhaetavicula contorta" or "Anidriti
di Burano"); the geometrically lowest one is represented only by the
Cretaceous-Eocene terrains of the "Scisti Policromi Group". Some characteristics
common to the "Tuscan" Units: 1) presence in the "Scisti Policromi
Group" of Eocene sedimentary breccias and conglomerates with Mesozoic
and Lower Tertiary "Tuscan"/"Umbrian" type elements; 2) fair lithologicalmicropaleontological
affinities between many of the Mesozoic formations and
the corresponding ones of the "Umbrian" sequence (example: layers such as
the Liassic "Rosa a Crinoidi" and the Upper Dogger-Ti tonian "Calcari
Diasprini"; 3) local presence of tectonic surfaces (with associated cataclastic
layers) which tend to laminate or increase the thickness of the
formations (example: see in Figs. 2-3 the considerable difference in thickness
of the "Calcari Selciferi" and the "Calcare Massiccio" in the first
two Units).
:U~b!:i!n: !egu!n£e (3545-4826 m). Represented by formations from the Upper
Eocene/Oligocene ("Scaglia Cinerea") to the Upper Trias ("Anidri ti di
Burano") • Worth emphasizing are: 1) the condensation of the Cretaceous­
Eocene "Scsglia Rossa" (tot. 35 m) and, particularly, of the formations
between the Lower Cretaceous ("Marne a Fucoidi") and the Middle-Upper Lias
("Rosso Ammonitico"), compressed into 5-10 m; 2) the presence of a characteristic
grey-pink layer rich in Saccocoma wi thin the "Calcari ad Aptici"
(Malm-Titonian); 3) the lack of the Middle Lias "Corniola" (unconformity?);
4) the lack of the Miocene formations ("Bisciaro" and "Marnoso Arenacea')
due to probable tectonic elision. This sequence, owing to the presence of
extensive condensations and likely sedimentary hiatuses, probably indicates
Fig. 2. PROFILE OF TEST HOLE A15. Volcanites (Quaternary): 1) tephriticleucitic
lavas, 2) pyroclastites; Ligurids (Cretaceous-Paleocene): 3) Ophiolitiferous
Unit, 4) Santa Fiora Unit; "Tuscan" Units (Upper Trias - Mid/
Upper Eocene): 5) "Scisti Policromi", 6) "Diaspri e Calcari Diasprini", 7)
"Marne a Posidonia", 8) "Calcari Selciferi", 9) "Calcare Massiccio", 10)
"Cslcari e Marne a Rhaetavicula contorta"; 11) "Anidri ti di Burano";
"Umbrian" sequence (Upper Trias-Oligocene): 12) "Scaglia Cinerea"; 13)
"Scaglia Rossa"; 14) condensed formations; 15) "Calcare Massiccio"; 16)
"Calcar! e Marne a Rhaetavicula C."; 17) "Anidriti di Burano".

104
an intrabasinal "high" series originated due to "block faulting" phenomena
which occurred during the Jurassic in the Umbrian-Marches domain (Centamore
and others, 1969; Colacicchi and others, 1970).
3.4. Well loggings
3.4.1. !e!l=b£t!o~ !h~r~o~e!rl
Eight well-bottom thermometric measurements were made, 6 with a TP 92
probe (*) and 2 with a Kuster device.
The maximum extrapolated temperature was 207°C at the depth of 4557 m.
The undisturbed temperature profile (Fig. 2) shows that the thermal gradient,
inside the potential regional reservoir, ranges from 0.15 to 0.45
°C/10 m. Such very low gradient values are closely connected with a convective
heat flow regime.
3.4.2. Qe£P!!y~i~a! !o&s
In accordance with the aim of the test hole, various geophysical logs
were scheduled in order to evidence and characterize physically differentiated
horizons, as well as to supply useful stratigraphic correlation and
calibration elements for the surface prospectings. In particular, resistivi
ty, gamma ray, sonic and density logs were made between 1325 and
3958 m.
For the purpose of reconstructing probable structural orientations,
therefore, a dipmeter log (SHDT) was also made over significant intervals,
between 2200 and 3700 m.
Figure 2 summarizes with schematic diagrams the results of the qualitative
interpretation of the geophysical logs made.
In agreement with the essentially carbonate nature of the whole investigated
interval, the variations of density and sonic velocity are minimal,
although in very high value ranges. They in fact oscillate between 2.75-
2.95 g/cm3 and 5.3-6.1 kmlsec respectively.
The variations are more marked for the resistivity curve (100-10,OOL
n.m) and the gamma ray curve (5-60 API units).
In particular, higher values of the natural radioactive content and
lower values of the resistivity are characteristic of layers with predominant
pelitic lithofacies (e.g. "Scisti Policromi").
Vice versa, "Calcari Selciferi", "Calcare Massiccio" and "Anidriti di
Burano" are characterized by low values of natural radioactive content and
higher values of resistivity and velocity. In particular, the "Anidriti di
Burano" is distinguished unequivocally by higher density values and peaks
of resistivity up to over 20,000 n.m.
Porosi ty has been computed from the density log using "Chart Por-5".
In agreement with the carbonate nature of the formations in question, the
porosi ty is practically nil except in correspondence to a few very thin
layers interpreted as probable fractured horizons.
(*) TP 92: Temperature and pressure measuring probe with power supply,
transmission and recording at the surface.

lOS
The table below reports, for every physical parameter recorded, the
average values characterizing the single formations investigated.
TECTONIC FORMATION G.R. RESISTIVITY VELOCITY DENSITY POROSITY fro FDC
UNIT (API units) (oh•••) (k.fsec) (9fc.3) (')
1st TUSCAN CALCARE MASSICCIO 5 1000 6 2.75
o *
UNIT CALCARE RHAET. CONT. 20 300 6 2.75 o (30) ( )
---------------------------------------------------------------------------------------------
2nd TUSCAN
0-10
UNIT
1
SCISTI POLICROMI
MARNE A POSIDONIA
CALCAR I SELCIFERI
CALCARE MASSICCIO
CALCARE RHAET. CONT •
ANIDRITI 01 8URANO
100 100 5.3 2.75
55 100 5 2.75
12 1000 6.1 2.75
1000 6.1 2.75
30 800 6 2.75
13 20000 6.1 2.95
0
0
0 (10)
3rd TUSCAN
UNIT
SCISTI POLICROMI
35 500 5.5 2.75
o (40)
UM8RIAN
SEQUENCE
CALCARE MASSICCIO
3000 6 2.75
o (30)
(*) The value. in parentheses refer to the probably fractured horizons.
Figure 2 also supplies a schematic representation of the SHDT logs
only, where dip angle and direction of the layers were detected with better
defini tion.
No variation of inclination can be detected, owing to the indeterminateness
of the data, at the passage between the first and second Tuscan
Uni ts. Wi thin the latter, the angle of inclination is better defined in
correspondence to the "Marne a Posidonia" and "Calcari Selciferi" formations;
it varies in a restricted range of values of approximately 10°. For
these formations the direction can also be approximated with fair definition,
to N200E.
Even the passage between the second and third Tuscan Units, due to the
poor data definition, does not show evident variations of the inclination
angle. Both the bed of the "Anidri ti di Burano" and the top of the underlying
"Scisti Policromi" formations are characterized by an identical
variabili ty of this angle (20-40°). Only the bottom part of the "Scisti
Policromi" is better defined, displaying strata inclined '1.35° dipping
towards the south (S200W).
3.5. Production test
The aim of the test was the production and subsequent chemico-physical
characterization of the fluid (CO.) encountered during the drilling from
4600 m to 4826 m (final wellbottom).
During the penetration from 4600 m to the final depth, there were gas

106
manifestations which, from a field analysis (gas chromatography), proved to
be prevalently CO 2
, Other gases such as H 2
S and hydrocarbons turned out to
be almost totally absent.
The negative outcome of the test prevented obtaining data that could
be used to establish the nature of the fluids present in the rocks affected
by the bore-hole.
Unfortunately, the inferred productive zones, linked to gas manifestations
and verified losses of circulation, would have been in any case of
negligible industrial utilization.
4. FINAL REMARKS
Although the geothermal objective (the deeper reservoir) was not
reached, the test hole Alfina 15 supplied new information which makes a
fundamental contribution to the stratigraphic, structural and thermal
knowledge of the whole region.
From a structural and stratigraphic point of view (Figs. 2 and 3),
beneath the "Ligurids" the well crossed three "Tuscan" Units, made up of
formations not younger than the Eocene, which are tectonically superposed
on an "Umbrian" type sequence. The latter represents the first evidence of
an Umbrian substratum in the areas west of the Chiana - Paglia graben.
The effects of the Oligo-Miocene compressive tectonics, such as the
piling up of the above-said tectonic units, and the Pliocene tensional
faults, which border the structural high of Torre Alfina, are shown in the
cross section of Fig. 3.
For the first time the Mesozoic carbonate formations, which constitute
the regional reservoir, were crossed for such a great thickness (approximately
3700 m), displaying tectonic doublings above a presumed deeper metamorphic
"basement".
Regionally the Meso-Cenozoic sequence of Monte Cetona belongs to the
most external ones of the whole Tuscan Domain (see Buonasorte and others,
1988). The test hole A15, however, showed the presence of other underlying
Triassic-Eocene sequences, which are thus more eastern, with transition
characteristics to the "Umbrian" series. These sequences could be considered
to be the original stratigraphic base, unknown until today, of the
easternmost "Tuscan" terrigenous sequences ("Monte Cervarola - Monte Falterona
Unit" of the Eocene-Middle Miocene) which, during the Tortonian, were
detached from their substratum and partially overthrusted eastward onto the
"Umbrian" sequence.
From a thermal point of view the test hole evidenced that the geothermal
anomaly of the Torre Alfina field, and more generally the regional one
of the Monti Volsini, decreases towards NE. The deepest thermometric measurements
recorded a temperature of 207·C at 4557 m.
The well did not reach the formations of an impermeable basement where
a deep regional geothermal gradient could be measured in conductive heat
flow conditions.
The very low thermal gradients existing in the whole sequence of
carbonate rocks testi fy to the existence of a geothermal reservoir over
3500 m thick with temperatures ranging between 140·C and about 210·C. Dur-

600
•
mO
A5 "1 415
2000
fIG.3 - GEOLOGICAL CROSS-SECTIONS. In the sect i on A the
ctu 1 structural Be Inq 1s shown; n B the Upper Torton ao
one 1s hypothesized (trae n' g.1). l}Volcan tes ( uat . );
2,post-Orogen1c Sediments (PI oc.) ; 3}L~gur ' ds (Creta­
Eoc.) : ~TUSCAN· UNITS: 4)~SCi st POlicrom· (U,Creta-Eoc . );
S,C lcareou8 and Ca eareou - 5 iceous rormat lono (L.Lias ~
~.Cret); 6 ) · Calcarl a Rha thavicula C,· (U.Tr as);
7, -An1drltl d Burano· (U . T las) ; ·UHBRIA~· S£OUE NCE :
S) ·sc gl i a Clnerea - and · Scaql1a Rossa· (U . Creta-Ol q~c.);
IC Ie T ous and Calcareus- Sl11ceou5 Formations (l.lldS­
~ . Cr t); O)·Cd car Rhaethavicula C.· (U.~rla s );
ll) -Anldr d Burano (U.Trias); 12)Over hrusts;
3)T no ond fau ts .

108
ing the drilling only modest circulation losses were encountered. Despite
stimulation, the well was unproductive due to low local permeability.
Partly because of the modest existing temperatures, the well did not
present any particular difficulties from the drilling technology standpoint
despite the great depth, never before reached in geothermal wells in Italy.
Fortunately the gas cap present in the highest part of the reservoir,
which could have entailed difficulties for the realization of the well, was
not crossed. In addition, no significant gas inflow was encountered during
the drilling.
References
Cataldi, R., and M. Rendina (1974). Recent discovery of a new geothermal
field in Italy: Alfina. Geothermics,~, 106-116.
Dallan Nardi, L., G. Pieretti, and M. Rendina (1977). Stratigrafia dei
terreni perforati dai sondaggi ENEL nell'area geotermica di Torre Alfina.
Boll. Soc. Geol. It., 96, 403-422.
Buonasorte, G., R. Cataldi, A. Ceccarelli, A. Costantini, S. D'Offizi, A.
Lazzarotto, A. Ridolfi, P. Baldi, A. Barelli, G. Bertini, R. Bertrami, A.
Calamai, G. M. Cameli, R. Corsi, C. D'Aquino, A. Fiordelisi, C. Ghezzo,
and F. Lovari. (1988). Ricerca ed esplorazione nell'area geotermica di
Torre Alfina (Lazio-Umbria). Boll Soc. Geol. It., 107, in press.
Costantini A., A. Lazzarotto, and M. Micheluccini (1977). Le Formazioni
Liguri dell'area a Sud del Monte Cetona (Toscana meridionale). Atti Soc.
Tosc. Sc. Nat., Mem., Ser. A., 84, 25-60.
Centamore E., U. Chiocchini, G. Deiana, A. Micarelli, and U. Pieruccini
(1969). Considerazioni preliminari su alcune Serie Mesozoiche dell'Appennino
Umbro-Marchigiano. Mem. Soc. Geol. It., ~, 237-263.
Colacicchi R., L. Passeri, and G. Pialli (1970). Nuovi dati suI Giurese
Umbro-Marchigiano ed ipotesi per un suo inquadramento regionale. Mem.
Soc. Geol. It., ~, 839-874.

109
CEC contract n° EN3G-oO~6-F
CHARACTERIZATION AND MODELLING OF LOW ENTHALPY GEOTHERMAL RESERVOIRS
EXAMPLE OF THE PARIS BASIN
J. ROJAS, M. BRACH, A. CRIAUD, C. FOUILLAC, J.C. MARTIN, A. HENJOZ
Institut Mixte de Recherches ~othermiques (BRGM/AFHE)
BP 6009 - F 45060 Orleans Cedex 02
Summary
The detailed study of the Dogger reservoir in the Paris basin is a
multi-field research including specific and various topics: geology,
sedimentology, geochemistry, hydrodynamic and thermal modelling.
Based on previous investigations from petroleum exploration the
analysis is focused on the numerous data collected on the 110 deep
geothermal wells. The characterisation of this important resource
required a progressive approach in the different techniques: knowledge
of parameter distribution, understanding of reservoir state,
analysis of reservoir behaviour under exploitation conditions with
doublets. The different analyses of the reservoir parameters converge
towards the identification of an heterogeneous structure with a great
number of productive layers and an important regional variability.
The origin of fluids is more complex; chemical parameters favor the
hypothesis of a common parent for these waters, with various evolutions
depending on regional trend and local velocities.
1. INTRODUCTION
The sedimentary beds of the Paris basin host numerous aquifers.
Several of these deep aquifers offer good geothermal energy possibilities,
but the main interest was concentrated on the Jurassic limestones of the
Dogger. This limestone is better known because it was the subject of
intensive drilling for petroleum exploration. Thanks to this drilling and
also geothermal energy research this formation was found to be a very
large aquifer. At present it constitutes the principal geothermal objective
in France and alone provides 90 % of the geothermal resource in the
country. The reserves in place are estimated to the equivalent of 600
millions Toe.
For several years, the detailed study of the Dogger reservoir has
benefitted from joint financing from the BRGH, the AFHE and, since
1986, from CEC (DGXII).
The current sedimentological, geochemical, hydrodynamic and thermal
study involves compiling and assessing all available data on geothermal
boreholes; i.e. 110 wells drilled between 1976 and 1986.
The method used, which can be applied in other geothermal fields,
consists of a detailed local analysis by well (vertical structure of the
reservoir), and a regional analysis by geostatistical treatment of parameters
characterising the geothermal potential and its conditions of
exploitation. The synthetic model of the reservoir can then be used for
resource simulation programs. The area chosen for compiling the data (fig.l)
about 100 km by 100 km, includes the various subareas previously

110
studied. It coincides with the centre of the basin, the greatest
density and approximately with the limits of resource exploitation
ductivity, temperature).
urban
(pro-
160.0,-----------------------~------------~--~~--------------------_,
170.0
UO.O
..
~
Co
... •
150.0
~ 1-40.0
~
..
~
• 130.0
c
" k
g 120.0
.!!
t o •
~
" 110.0
100.0
'0.0
60.0
x a.oaraphic coord'h.~ •• (km L&mb.r~)
o
Figure 1 - Location of the 106 geothermal wells and regional distribution
of reservoir temperature obtained from the geostatistical analysis.
2. SEDIMENTOLOGICAL STUDY
The sedimentological analysis
(1) characterize the sedimentary
criteria and study the relations
different productive layers.
aimed to:
environments and their distinguishing
between the various facies and the

III
(2) draw up vertical well sections including all these components in order
to establish correlations between wells as a base on which to reconstitute
the paleogeography of the reservoir.
The work was carried out in successive stages comprising:
(1) detailed sedimentological analysis of a cored drillhole so as to
define the facies. (depositional) environments. petrology and diagenetic
phenomena affecting the reservoir.
(2) well logging characterization of facies cut by this drillhole enabling
facies in other boreholes to be identified by morphological study of
curves (electrofacies).
At the same time as the logs were gone through. cuttings from some
boreholes were examined section by section so as to: (i) check a logging
hypothesis. (ii) characterise the porosity from cuttings impregneted with
coloured resin and mounted on thin sections.
2.1 Lithostratigraphy
The Dogger of the Paris basin corresponds to a predominantly limestone
litho-stratigraphic assemblage located between the marl at the top
of the Lias and the lower Callovian. During the Bathonian. a large carbonate
platform was built up and its formation accelerated with subsidence
of the basin. Oolitic reef bars formed on the basin borders and progressed
towards the centre leaving behind them a protected internal shelf zone.
This zone. in which lagoon-type facies (Comblanchian) were deposited came
progressively to occupy the center of the basin at the end of Bathonian
(fig. 2). This north-west striking carbonate body is bounded on the west
by a marly belt which in turn limits the oolitic reef installed on the
Armorican margin. As an example. the description of the typical stratigraphic
succession encountered in one of the geothermal wells is given in
figure 3.
2.2 Facies - Reservoir characteristic relations
Study of the porosity in the limestones showed that their reservoir
characteristics depend on:
- the depositional environments and their hydrodynamic energy.
- diagenetic changes which can affect them.
The porosity is clearly of two types:
(1) macroporosity. intergranular (pore radius> 7.5 microns) typical
grainstone of reef facies
(2) microporosity. inter or intragranular (pore radius < 7.5 microns)
typical of gravelly-bioclastic facies of the external shelf.
The values obtained by interpretation of porosity logs do not convey
the degree of productivity of the rock. In fact. at equal porosity values.
the limestone from the serie of "Alternances" does not have the same
productivity as the "White oolite" (fig. 3). The origin of this difference
is probably due to the different diagenetic evolution in these ancient
reservoirs: thus rocks belonging to the "Alternances" are blocked with a
deposit of cement reducing the intergranular space while, at the same
time. the micropore network is increased by reduction of the macropores
and by dissolution of the matrix. On the other hand. the limestones belon­
,ing to the "White Oolite" facies and the Comblanchian. have kept their
original porosity and in places it is even increased by fracturing-solution.
3. GEOCHEMICAL STUDY
Almost each production well has been sampled for complete analysis of
the fluid (water. ,as. isotopes). either soon after drilling. or later

112
lIIIIlJ Marl and arQlllocAoua IIm •• tonA
[Ill Logoon fO I.,
.. Oolitic: IIm •• tone: moln r ••• r'Vo,lr
Figure 2 - Block d agram of paleoAeosraphy of he Do&&er ~t th nd
of the Bathonian.
"'" 0.. .,
0
8!~
eIO
. ~
~
...
.;; ~
~ "
~
iIIot
;U
e
..a
°
.n .!~
..,
--
-....
!IOO IlOOO IlOOO
ec»
om
/j\O
11"
LOQs Pio4u~9 Lilholoqy
L("I! 14 : Mudllont
liilh W : Wockulont
GAMMA RbY SO!\IC P : Pocblont
API Unils Porosily iI 'Yo 'I. of yidd
_ 1\ 1:0\ _ 0 QJO ' I~
G : Glcilslont
~- , _, _ ,_,_,- ~
1-'_-
I- I;;F MOIfslloi1
~- ,~
• -
- - - - IIII,,~O! ..". 01
.- - btl9f .,,,flle
· - - !ft>(.' !Orl( IMI
- - - --
g,d bdQ! 910'
vtiy bootlosl,c
· . . 1m~IIO/Ie
- - ~It'
WloPI
.· - ·. ~
~
~ ...
· • ~ S3\ r
• I' I '~ - 1M
-
--
- -
•
I
• c:>
:E1:
IGr.!;
005lk SOlid, IGI
I I • [ I~
.,
-
· . - Biodoslic imtllO/lt IPI
- -
): 4't Ooli1ic 1im,,'on.IGI~
- , - - Ar~mU.\ 1M!
- - l.Jmes lont
-
om
I ~I 'I' [Ir
-· ~ ~
~
~
-
. . -
~
ttl"!,,
·
I ~~ fUonl ~V I
-
..,
- -- - B .. cI4'1~ !moslone
- - - end IPliwl
- AlQlo«o.\ inI

113
during normal working of the doublet. or both. from 1980 to 1988. Data
for fluids from injection wells could be obtained only from the production
tests and drill reports. but with a lesser quality. The sampling methods
and analytical procedures have been described in Criaud and others (1986),
Harty and others (1988), Fouillac and others (in prep). Host of the
results were published in the former papers of Fouillac and others (1986).
The aims of the geochemical study were
(1) to acquire complete data for the Dogger fluids. understand their
origins and general chemistry within the reservoir
(2) to improve by chemical constraints the hydrodynamic model describing
the aquifer (see paragraph 4.3): residence times of the fluids. recharge
zones
(3) to test the use of chemical and isotopic geothermometers in a sedimentary
environment.
Not all data are available or are fully interpreted at the present
time. We focus here on the origins of the mineralization and its distribution
in the basin • and give some conclusions regarding the circulation
paths. The other points will be approached briefly.
T'C
80
70
--- .---­
• •
60
40
*
•
• Seine Sc Deni •
o Val. Marne
• Mcaua-Coulommien
C Fontainebleau
A Wcst of Po ...
~ Mel ...
'If Dc.uni.
* Crcil
20+--------.---------r--------~---------
o 20 30 TOS gIl
Figure 4 - Correlation between salinity and temperature.
3.1 General characteristics
The salinity ranges from 6.5 to 35 gIl. Na and Cl are the main
constituents. with C02. N2 and CH4 as dominant gases. The TDS content is
not correlated with the temperature at basin scale (fig 1) but local
trends can be distinguished. Rather complex mixing patterns are confirmed
by the detailed chemistry. though a common parent for all fluids within
the reservoir was early recognized • The very constant CI/Br ratio (-420)
is best explained by the evolution (evaporation) of an initial seawater
(CI/Br-660). This paleoseawater would have undergone various diagenetic

114
processes keeping the Cl/Br ratio unmodified (Fig 5). The ratios between
soluble elements (Cl,B, S04) and reactive ones (Ca, Mg) together with the
isotopic mesurements and gas patterns has led us to separate the following
zones:
(1) The fluids from the center of the basin (Coulommiers) combine the
highest temperatures and TDS content, they may be the closest remainder of
the initial paleocomponent, despite its later evolution Meaux and
Coulommiers display relatively high S04/Cl (unmodified by bacterial
reduction) and B/Cl, but low Mg/Cl and high Ca/Cl. Deuterium and 180 of
water fall under the meteoric water line but are not among the more
isotopically evolved fluids. They are methane rich and low in nitrogen.
(2) The Seine St Denis fluids are characterized by their variable but low
S04/Cl, resulting from intense bacterial reduction of sulfate, which was
favored by the moderate temperatures and confining. They are enriched in
nitrogen and low in hydrocarbons.
(3) The group of Fontainebleau is remarkable for its inverse correlation
between temperature and salinity (Fig.4). The low mineralization and the
isotopes give evidence for an actual recharge zone in the south (Morvan
basement). The S04/Cl and B/CI ratios are very high relative to the
other groups.
(4) The Val de Marne fluids are similar to the first group for Ca/Cl and
Mg/Cl ratios, but lower in salinity. More, the deuterium and 180 clearly
indicate that an external (hot and saline) component is responsible for
the observed mixing patterns. The dilute end member is likely derived
from the group 3. The presence of hydrocarbons is linked to the occurence
of oil-bearing zones.
(5) In the western zone, the chemistry (low TDS but inversely related to
temperature, low sulfate because of reduction, Ca/Cl and Mg/Cl ratios
comparable to group 1) may result from the mixing of a cold and evolved
component (which also explains the trend observed inside the group 1) and
a relatively dilute fluid, derived from 3 and flowing from south-east
along the geological marly boundary (see paragraph 4.3).
(6) ~ith the exception of Melun, intermediate between 3 and 4, the other
wells cannot so far be interpreted relative to this circulation sketch,
because of their "isolated" location on the border of the studied area
(Creil, Beauvais, Epernay). ~
.. Bcslide 1985
• This study
0,
O+---------.--------.---------r--------,---------,--
o OJ 0,2 0,3 0,4 CI M 0,5
Figure 5 Correlation between Cl and Br in geothermal fluids.

lIS
Following these considerations, the geochemical constraints to the
flow model are
-a recharge zone south east
-a connection south of Paris with an unknown fluid
-variable directions of flow (SE to NW in the south, but E to W in
Val de Harne)
-a more confined zone to the North of Paris
3.2 Vertical heterogeneities
The fluid collected at wellhead originates from various sub-layers
(see paragraph 4.1) , which are thought to produce for a single well
slightly different waters. The samples collected at various depths could
not clearly demonstrate this point for different reasons: unfavorable
conditions for bottomhole sampling and/or analytical techniques unsufficiently
precise to distinguish the fluids. Later, it was observed that
the composition of reactive components vary with time and when the exploitation
conditions (mainly the pumping rate) are changed.
Analytical improvements for the determination of Cl, S04, Ca, Mg, Na
and K has made possible detection of tiny variations of these components
(Criaud and others, 1988). We have established that the hypothesis of the
regional flow is not consistent with the extent of chemical changes
observed in Fontainebleau within two years. The detailed study of Cr~teil
(Val de Harne), collected at different discharge rates, confirms that the
evolution of the concentrations in Ca, Mg, Cl and S04 are due to the
variable contribution of the different strata to the total flow.
3.3 Origin of the fluids
The inert gas trends are best explained by the occurrence of mixing
between a paleocomponent, possibly an evolved seawater, and a recent
recharge in the South. This is in agreement with the conclusions presented
above • Helium dating gives ages for the fluids consistent with the
geology of the host formation, but not with the present hydrodynamics and
flow velocities. This discrepancy can be overcome if an exotic flux of
helium is assumed, maybe caused by the migration of fluids from deeper
aquifers. The earlier results for carbon 14 dating have been reconsidered
and a new method for l4C sampling has been recently developed; the
preliminary results indicate that the percentage of modern carbon in the
fluids is extremely low, but can be measured in some wells. Because of
the complex chemistry of carbonates in such an aquifer, more interpretation
is actually needed to obtain informations on flow velocities with
this method.
4, CHARACTERIZATION AND MODELLING OF THE RESERVOIR
A detailed knowledge of the hydrodynamic and thermic parameters of
the reservoir is essential in understanding its natural state and to ~ry
to explain the anomalies encountered during exploitation of the geothermal
resource (scaling, interferences, variations).
Work carried out was concentrated in three principal and complementary
areas:
(1) Collecting and analysing data gathered from the boreholes for all
operations in Paris region (110 wells on the Do~er reservoir, fig. 1). The
parameters chosen enabled a database on the boreholes to be set up.
(2) The main parameters neccessary for simulation calculations were
entered in regional databases with the help of geostatistics to estimate
each of their regional distributions within the area of study.
(3) The estimated parameters were then used to simulate the hydrodynamic

116
behaviour of the area by a heterogeneous model (estimation of natural flow,
interferences in certain sensitive zones, analysis of coherence with
the distribution of coupled parameters).
P"ODUCTlDN WELL • AULNAY-SDU5-BOJS-ROV GAYl SURFACE COOIltD. X I 1I11a88 y. 138258 (LAM8£JtT J ,
FLQWIltATE (Z) ~. TRANSM1SS)V)TY W.
ZD 40 aa 80 .v'SL 10.0
FAelH
FAC.
UM.
P£RlCAlllL.JTY an PQlltDSlTY tI>
1.0 10.0 10 ZD
IIIIID
CALLOVIAN
CAL.LE NACRE
1831
rcp OF' IlATHDNJAN L.JM€STDNI
4.0
CDMIlLANCHI
0.&
3 •• ZOo
4.5
1.0
I
11.11
I.
•
DDLITE
3.8
1l.2 187 7.5
4.0 0.8 14.0
0.4
IlL TEflNANCES
1.5 18.0
~-L __ ~~ __ ~~17~~-L~~UU __ ~L-______ ~
Figure 6 - typical vertical structure of productive layers in Dogger
(Compilation of flowmeter data and stratigraphy).
4.1 Vertical structure of the reservoir
Taking all the wells into account confirms the stratified
of the reservoir, identified by the numerous and thin (1 m
productive layers, without individual correlation between
(fig.6).
structure
or less)
the wells
In order to achieve a composite model of the reservoir, with respect
to its vertical sequence, the various productive layers were grouped
according to their facio logical classes into three sub-groups.
For each well, the features of the different levels and those of the
three equivalents were included in the database. This detailed information
is very useful during rehabilitation operations, to assess or locate
causes of variations of production and injection due to scaling for
example.
4.2 Regional distribution of parameters by geostatistics
An estimation of the regional distribution of the different parameters
was made on the one hand for a general aquifer (all levels), and on
the other for each of the three aquifer-equivalents. Four main types of
result were obtained from this analysis:
- the structure of parameters distribution (variogram model): homogeneous
or heterogeneous, regional drift, range of correlation,
- mapping of the estimated parameters and of their standard deviation
according to the variogram model,

117
the regionalized database in the form of a set of parameters
attributed to the nodes of a regular (Lambert) grid,
- finally, for the three selected facies, interpretation in terms of
their probability of occurence ~es ~ exUrtjng composite map of the Paris
region, based on geological data, to be improved and updated (fig.2).
-1100.0
-1200.0
-1300.0
-1400.0
::; -1500.0
•
oS
0
1-1600 . 0
q,
f -1100.0 i 8
.!
; -1100. a
-1900.0
AVERAGE LINEAR LAW
'OR THe PARII BASIN
IG.odIen, I 0.03l .C/'")
A'/£RAGE L..INEAR LAW FOR
THE 110 GEOTHERMAl. WEllS
(C.odIen' I 0.041 -C/m)
Figure 7
Correlation of temperature with
depth inside Dogger reservoir.
The measured reservoir temperatures
deviate from the average
linear laws.
Abnormal thermal gradients can
be iden~ified and positioned in
specific zones of the wells location
map (fig.l) according to
classification symbols.
The main part of the thermal ano
maly in south of Paris (triangles)
can be explained with a
stream tube model issuing from
the deepest zone of the basin.
An agreement is found with a local
velocity of 3 m/y and a heat
flux from Trias increased by 7 %
in this area.
-2000.0 6 Hor rEUS
o NORJIAL rEUS
-2100. 0 0 COLD rEUS
-2200. 0 +-~""""'~~-'-~T"""'.......,.~--r-~"'-~r-"'-+
45. 0 SO.O 55. 0 60. 0 65.0 JO.O J5. 0 10. 0 IS.O 90.0
mlPDATUU (oC)
4.3 Estimation of regional flow
The first approach was by analysis of the observed piezometry, using
downhole pressure and the classical pseudo-potentiometric concept (equivalent
fresh water head). Heterogeneous modelling based on the data of
chapter 4.2 is consistent with the previous mapping. The regional Darcy
velocity is rather low (0,4 m/y) and oriented towards north-west. The
calibration was done by analysis of the coherence of this preliminary flow
path with the distribution of the coupled parameters (temperature and
chemistry). Using a stream tube model the thermal anomaly in the south of
Paris can be explained if reservoir topography is integrated and using a
higher local velocity (factor 10). On the other hand, to be in agreement
with the geochemical signature of the fluid in the eastern region, the
general and homegeneous flow path must be deformed. These results show the
importance of density effects and topography on both the geometry of the
stream tubes and on the local velocity inside each of them, in order to
obtain a coherent synthetic model.

118
5. CONCLUSIONS
Including all the available geothermal data, the study of the characterization
of the Dogger confirms the heterogeneous structure (vertical
and lateral) of the reservoir. Except for pressure, the fluid state
variables are not systematically correlated with depth, as one can expect
in a sedimentary basin. Thermal anomalies suggest the perturbation of the
geothermal flux by local or regional fluid velocity. In turn, the geometry
of the flow path, and the location of the recharge areas are ·highly
dependent on the geochemical constraints.
A main result of this multi-field research is the importance of the
coupled information coming from the individual and analytical techniques.
Each result on a given topic must be consistent with the associated
phenomena ; this is specially true for hydrodynamics associated with both
chemical and thermal fields, a calibration condition for the synthetic
model. Up to now, some discrepancies remain, new investigations are
required to specify the or.igin of fluid salinity, the age of water and the
flow path perturbations induced by density effects coupled with topography.
6. ACKNOWLEDGEMENTS
The authors gratefully acknowledge support of this work by
B.R.G.H. and C.E.C (DG XII) under contract No EN3G-0046-F.
A.F.H.E.,
7. REFERENCES
Bastide, J. P. (1985). Etude g~ochimique de la nappe du Dogger du bassin
parisien. 3rd cycle thesis, University of Paris VII.
Criaud, A., Fouillac, C., and Brach, H. (1988). Chemical evolution of the
fluids from the Dogger aquifer. Workshop on geochemistry, CEC contract.
meeting, Antwerp.
Criaud, A., Fouassier, P., Fouillac, C., and Brach, H. (1988). Natural
flow and vertical heterogeneities in a sedimentary geothermal reservoir.
Proceedings of the 13th Workshop on Geothermal Reservoir Engineering,
Stanford, California.
Fouillac, C., Fouillac, A. H., Criaud, A., lundt, F., and Rojas, J.(1986)
Isotopic studies of oxygen, hydrogen and sulfur in the Dogger aquifer
from Paris basin. Proceedings of the 5th Intern. Symposium on Water Rock
Interaction, Reykjavik, pp 210-205.
Hartin, J. C., Henjoz, A., and Rojas, J. (1988). Caracteristiques
hydrodynamiques et thermiques du reservoir du Dogger du basin parisien.
Enerstock88, Versailles, France.
Harty, B., Criaud, A., and Fouillac, C. (1988). Low enthalpy geothermal
fluids from the Paris sedimentary basin. 1: Characteristics and origin
of gases. Geothermics, 17(4).
Rojas, J., Henjoz. A., Hartin, J. C., Criaud, A., and FouiHac,. C. (1987).
Development and exploitation of low enthalpy geothermal systems: Example
of the Dogger in Paris basin, France. Twelfth Workshop on Geothermal
Reservoir Engineering, Stanford, California.

119
EEC contract nO EN3G-0035-F (S)
MODELING IN THE MOFET! FIELD
A. GUIDI (AGIP S.p.A.) - G. ANTONELLI (DAL S.p.A.)
AGIP S.p.A. - I - 20097 S. Donato Milanese
Abstract
The simulation of the dynamic behaviour of the field confirmed the
result of previous simulations which indicated the installable
electrical power, as 7.5 and 15 MW, respectively using the existed
wells and with the maximum development of the field.
The above mentioned values are referred to a 25 year economic life of
the field and in the present work this hypothesis is always
maintained.
The resulting powers are, in the two situations, 9.5 and 20 MW
respectively with a slight decrease of power with time. The improvement
obtained using this model is probably due to the possibility of
keeping into account the presence of C02 ; this gas has a noticeable
effect on the fluid upflow in the well, by reducing the hydrostatic
head in the borehole and thus permitting the extraction of geothermal
fluid with lower pressures st the depth of the productive layers.
Moreover the simulation ascertained the absolute necessity of fluid
reinjection, in order to supplement the insufficient natural
recharge.
In the 9.5 MW scheme, using the existing wells, the flow rate remains
constsnt while the power generated decreases from 9.61 to 9.34 MW at
the end of the 25 year life of the field.
For the maximum development of the field, it will be necessary to
drill seven new successful wells, four as productors and three as
reinjector.
The total flow rate varies from 203.0 Kg/s to 185.6 Kg/s while the
power decreases from the initial 22.76 MW to 19.66 MW at the end of
25 years.
During this study all exploration, drilling and production test data
were considered. In particular the long term test resulta ware very
important because interference data gave reliable information on the
average characteristics of the geothermal reservoir. Nevertheless the
documentation showed the necessity of carrying out new teats for a
better knowlegde of the characteristics of some wells, especially
Mofete-2.
1. INTRODUCTION
In this report is presented an evaluation of the geothermal potential
of the Phlegrean Fields using a 3D model called CHARGR.
For thia reason the mining and petrophyaical data from the drilling and
teata done in the wells of the Permit "L8&o di Patria" was used.

120
2. GEOLOGICAL CHARACTERS OF THE AREA
The volcanic area of the Phlegrean Fields is located
position inside the "Conca
Naples.
Campana" graben and covers 70
in a
Km2
central
west of
Phlegrean Fields are constituted by a collection of eruptive monogenic
centres, mainly pyroclastic. The main volcano-tectonic structure is a
caldera with a 13 Km diameter generated by the paroxysmic eruption of the
Ignimbrite Campana, 35.000 years ago (Rosi and others, 1983).
This eruptive event suggested the division of the precaldera activity in
two phases i the former began in a marine environment, the latter occurred
inside the same caldera with another subdivision in three phases.
From a magmatological point of view, the volcanic products of this area
belong to the Roman Province. Its petrographic composition varies from
trachybasalts to latites, trachytes, alkali-trachytes and peralkali-phono­
Ii tic trachytes.
The most abundant products ejected are trachytes with sanidine (dominant
phase) clinopyroxene, sporadic Na-plagioclase, biotite and sporadic titanomagnetite
(AGIP internal report, 1985).
In accordance with these data, the presence of a shallow magmatic chamber
which should represent the main heat source of the geothermal field was
assumed.
The existence of an active hydrothermal system is demonstrated by the
numerous natural fumaroles in the Pozzuoli area and in the gulf.
3. MODELING OF THE FIELD
Characteristic of the computer programme
The simulation of the Mofete field was performed using the CHARGR
programme written by J.~. Pritchett of S-CUBED Company.
This programme is capable of simulating geothermal fields in six different
geometries, considering the presence in the fluid of free or dissolved
gases and precipitated or dissolved salts.
However, these last possibilities require the presence of the respective
data libraries in our case two libraries, ~ATSM and C02H20 were
available, the former considering the presence of pure water and the latter
of pure water and C02.
Due to the existence of high concentrations of C02 (2-3 % in weight) in the
fluids of the Mofete reservoir, the C02H20 library had to be used, though
this choice implied more complications and an increase in computing time.
Both the libraries consider that the fluid can exist in all the aggregation
states. The main features of the above mentioned code are the following:
CHARGR is a dynamic model, which uses the finite differences method
in fully implIcit form, thus reducing numerical convergence
problems.
As CHARGR code is based on the finite differences method, six
different geometries are provided, three of which are monodimensional
(slab, radial and spherical), two bidimensional (2D
Cartesian and cylindrical axisymmetrical), and the last one 3D
cartesian tridimensional.
In the geothermal fluid all the three aggregation states can be
present (solid, liquid and gaseous) i the presence of a solid phase
in the fluid could be represented, for example, by undissolved
solid particles, carried by the fluid itself.
Besides the water, other chemical species (gases and salts) can be
considered in the fluid, without particular limitations, provided
that the proper library exists.

121
- The geothermal reservoir is considered as an heterogeneous and
anisotropic porous medium that could change its characteriatics in
different zones and, with regard to permeability, also in different
directiona.
The programme also considers that the rock matrix could modifY its
dimensions in consequence of pore pressure and temperature variations. This
means that the local porosity changes depend on these two quantities.
All the main characteristics of the rock can be calculated using the
classical formulae present in the literature or through specific functions
defined by the user. The boundary conditions, together with sources and
sinks inside the field volume, can be defined in a very detailed way.
The description of the wells is very elaborate, especially in 3D geometry
where each well crosses many zones of the reservoir. The code can divide
automatically the total flow rate of each production or reinjection well
between every crossed block. Furthermore, it is possible to calculate the
pressure drop caused by the local effect close to the hole.
As Mofete field has characteristics variable with depth, it was
necessary to adopt a 3D Cartesian geometry. Consequently, the geometrical
subdivision was realized by right parallelepiped shaped blocks. Regarding
ieometry, it has been defined using exploration and drilling results. The
reservoir total depth was assumed equal to 2500 metres b.s.l., close to the
maximum drilling depth (2700 metres) ; the horizontsl section is represented
by a square matrix, whose elements had a 350 metres long side.
The parallelepiped was vertically divided into twelve layers with
different thickness corresponding to the different characteristics of the
reaervoir as a function of depth. The upper base is chosen at sea level.
The field boundaries were settled as impermeable barriera around the
field, oriented to the cardinal points. The shallowest layer in the field
(from 0 to -250 metres below sea level), made by pumiceous tuff, was
included in the model even though, due to its low permeability, fluid can
move very slowly inside respect to the other zones of the field.
Chemico-physical characteristics of fluid and embedding rocks
The water of the Mofete area has a chloride alkaline chemistry. The
dissolved salts are mainly sodium and potassium chlorides, though great
quantities of calcium and silica are present.
Furthermore, boric acid, sulphides, bicarbonates, lithium, strontium
and arsenic, even if in small quantities, demonstrate intense hydrothermal
circulation.
The most common gas is carbon dioxide (2-3 % in fluid weight) which
represents 93-94 % by volume of the non condensibles, while the remaining
fraction is composed of methane with traces of hydrogen sulphide and
hydroaen.
The comparison between fluids in Mofete area suggests a common
oriain, even if the fluids produced in the shallower layers are probably
conditioned by mixina with superficial cold aquifer.
The pH value in the Mofete fluids waa determined by laboratory
analyaea and varies between 4.68 (shallow layers of Mofete-3d and
Mofete-9d) and 5.42 (Mofete-2).
The Mofete-2 fluid presents some important chemical differences if
compared with the fluida of the other wells in the same area, but it is
neceasary to emphasize that it ia in different conditions (two phases) and
the paraaeneaes of the layers are different.
Therefore, waters of Phlearean Fielda springs can be considered as a
mixture of three kind of water a : meteoric, marine and aeothermal: the
la.t one represents the chemical evolution of the others fro. reactions

122
with the embedding rocks (Antrodicchia and others, 1985).
About the origin of the fluids produced by the geothermal wells it is
possible to conclude that Mofete field has a marine recharge in the deepest
layers j then the composition of this fluid varies by reactions with rock.
The following lithotypes have been considered in this model :
Lithotype N° 1 - Pumiceous tuff
Lithotype N° 2 Chaotic tuffites
Lithotype N° 3 - Alterated lavas
Lithotype N° 4 Clay silt complex
Lithotype N° 5 - Thermometamorphic rocks
The permeability values of these lithotypes is quite variable, even
within the same lithotype, due to the micro-macrofracturing conditions
which are a characteristic of the Mofete field.
Initial thermodynamic state
Unlike what has been done previously, when the available data were
poor and only the temperature and pressure profiles of Mofete-2 could be
used, the present modeling allowed advantage to be taken of the information
obtained from the subsequent drillings and tests of all the wells.
Therefore, three main characteristic temperature profiles have been
singled out j these profiles have been associated with the three thermal
zones into which the field section was subdivided. Each zone corresponds in
space to a cylindrical solid with irregular cross section stretching out
vertically through all the layers of the field.
Starting from this initial situation, the geothermal system was
simulated in undisturbed conditions for about 1000 years, in order to
obtain a quasi-stable thermodynamic state. This state was used for setting
the initial conditions of the field before the first drilling.
As regards the pressure profiles, the following procedure was
initially followed :
Assumption of a common pressure value (119.9 bar) in the middle
point of layer 8 at 1350 metres depth.
Reconstruction of the pressure profiles in the three different
zones, on the basis of the hydrostatic pressure. These pressures
are computed using the saturated water density at the temperatures
defined by the above mentioned profiles.
About C02 concentration, for which no detailed information versus
depth was available, an arbitrary profile was assumed with only the
constraint of an average concentration of 2.5 % in weight on the total
geothermal fluid in the field.
Boundary condition
As ascertained in the previous studies, geological results, even if
they show a marine origin of the fluid, indicate that at present the
connection with the sea is rather difficult. Therefore, we have assumed
that the natural recharge cannot supply a SUbstantial part of the mass of
fluid extracted during the commercial exploitation of the field.
Moreover, the natural springs in this area, even though numerous, are
very small in terms of total mass and energy j this fact confirms that the
marine recharge should be insignificant.
On the basis of this consideration, it was preferred to assume a
condition of total impermeability at the boundaries for the mass flux,
while the energy exchange with" the rocks surrounding the field was taken
into account, even if limited to a process of pure conduction.
Therefore, during the simulation, no natural recharge was considered
in the field j in order to balance this effect, which reduces in part the

In
power, the presence of the superficial layer (tram 0 to -250 a. b.s.l.) was
maintained. This layer is generally ignored in the geotheraal field
simulation due to its low permeability.
In spite of that, this layer, characterized by a high porosity
(~O.35), repreaents a non-negligible reserve of fluid and so contributes
to maintaining a hiah pressure in the underlying productive layers
because of its low temperatures, it is poasible, in this way, to simulate a
meteoric recharge.
In conclusion, instead of introducing a deep recharge, very uncertain
in location and in quantity, it was preferred to choose a scheme without
external mass recharge, but well enough dimensioned in volume and fluid
capacity.
About boundary temperature conditions, which regulate the heat
exchange to and from outside, it was supposed that every boundary face is
in contact with rocks having the same temperature of the block to which the
face belongs. In this way the heat flux towards outside begins when the
field boundary blocks start modifYing their temperature. For the upper face
of the field, a 15°C temperature is assumed corresponding to an average
value in the Mofete area.
4. CALIBRATION OF THE MODEL
Achievement of the static conditions
In order to reach static conditions, a preliminary simulation of the
ateady natural state of the reservoir for a period of 1000 years was
carried out. With this computation, a readjustment of the thermodynamic
state for each block of the field was pursued, in order to reach a quasi
steady situation, in which the field was presumed to be before the beginning
of drilling. It has been possible to observe that the presence of C02
above the solubility limit in water causes wide variation in the fluid
average density this is related to the existence of a gaseous phase
containin& also saturated steam in pressure and temperature conditions
where the fluid would remain liquid if it were composed only of H2O and
salts.
The temperature profiles, during the 1000 years period, underwent a
quite limited evolution, as could be expected, maintaining, within the
experimental error limit, the temperature profiles obtained tra. testa : on
the contrary, the other thermodynamic quantities showed more sianificent
variations.
As reaults of this firet part of the work, the initial ther.odynamic
conditions of the field before drilling were obtained: due to practical
reasons, the simulation was limited to 1064 years, even though the system
continued ita natural evolution very slowly.
Location of the Morete wells in the computer model
The next step consisted in siting the wells in the adopted
geometrical acheme according to the drilling and short testa data.
CHARGR programme needs detailed inforaation about the rock characteristics
around the well (permeability and skin) for each block crossed by
the well itself where open intervals are present. The reason for this
dependa on the fact that the programme, besides solving the .ain equation
of the geothermal field, carries out a computation of the local draw-down
around the well : with the aid of an auxiliary model the values of pressure
and internal energy at the inner edge of the well allow a check to be aade
of the possibility of fluid upflow in the well.
Furthermore, the thickness in aetres of the production Bone inside each

124
block is requested, because the actual open interval does not coincide
necessarily with the vertical dimension of the block.
5. SIMULATION OF THE FIELD EXPLOITATION
General criteria
The configurations analysed consider not only the present situation
of drilled wells, but also schemes in which a certain number of new additional
wells are foreseen in order to exploit all the field potential. The
programme does not permit the power that 'is intended to be extracted from
each well or from the whole field to be supplied as an input parameter : in
fsct for a production well is possible to supply the designed mass flow
only, but the internal energy of the fluid remains dependent on the field
evolution without a direct control of the user. For each simulation the
following hypothesis were assumed
- Steam flow referred to 8 bar
Specific output : 2.2 (Kg/s)/MW
- Economic life of the field : 25 years
About production wells, the value of the extracted mass flow was
assigned in accordance with the results of testing, while the corresponding
internal energy of the fluid was computed by the programme, assuming the
value determined by the composition of the fluid and by the temperatures of
the blocks where the productive layers have been localized on the basis of
the previous investigations. The considered configurations refer to three
possible exploitation schemes :
1) Three production and two reinjection wells, all of them existent at
present.
2) Five production wells, all existent at present, without reinjection.
3) Six production and six reinjection wells, seven of which (four producing
and three reinjecting) to be drilled in future.
Simulation N° 1 (9.6 MW initial)
This simulation considers the utilization of Mofete-1, Mofete-2 and
Mofete-7d as production wells and Mofete-3d and Mofete-8d as reinjection
wells. The reasons for this choice are the following :
a) Mofete-1 and Mofete-7d showed a good productivity characteristic.
b) Mofete-5 and Averno-1 are not useful as production or reinjection wells.
c) Considering the extracted flow rate and enthalpy, two reinjection wells
at least are necessary. Therefore, Mofete-8d and Mofete-3d, due to their
good reinjectivity characteristic and their hydraulic connection, were
selected for reinjection.
d) Mofete-2, in spite of the problems caused by the characteristics of its
fluid, has high enthalpy and so was included in the production wells.
Initial data
Production Total flow rate Fluid enthalphy Steam flow rate Power
wells (kg/s) (kJ/Kg) (Kg/s) (MW)
Mofete-1 35.0 1100 6.5 2.95
Mofete-2 18.6 ,1460 6.7 3.05
Mofete-7d 48.0 1060 8.0 3.61
TOT A L 101.6 21.2 9.61

125
Reinjection Total flow rate Fluid enthalpy Fluid temperature
wells (Kg/s) (kJ/Kg) (DC)
Mofete-3d 40.1 715 169
Mofete-8d 40.3 715 169
TOT A L 80.4
Final data after 25 ,lears
Production Total flow rate Fluid enthalphy Steam flow rate Power
wells (kg/s) (kJ/Kg) (Kg/s) (MW)
Mofete-1 35.0 1077 6.1 2.77
Mofete-2 18.6 1404 6.2 2.82
Mofete-7d 48.0 1072 8.2 3.75
TOT A L 101.6 20.5 9.34
Reinjection Total flow rate Fluid enthalpy Fluid temperature
wells (Kg/s) (kJ/Kg) (DC)
Mofete-3d 40.1 715 169
Mofete-8d 40.3 715 169
TOT A L 80.4
Simulation N° 2 (MW initial power)
This simulation is based on the utilization aa producers of all the
existing positive wells of the field, namely Mofete-1, Mofete-2, Mofete-3d,
Mofete-7d and Mofete-8d. This scheme was considered not in view of a
possible realization, but in order to verify the field behaviour and the
limits of its exploitation without reinjection. In spite of the previous
consideration the possibility that also Mofete-3d and Mofete-8d could be
used as producers, was taken into account.
Initial data
Production Total flow rate Fluid enthalphy Steam flow rate Power
wells (kg/s) (kJ/Kg) (Kg/s) (MW)
Mofete-1 35.0 1100 6.5 2.95
Mofete-2 18.6 1460 6.7 3.05
Mofete-3d 14.7 1085 2.6 1.19
Mofete-7d 48.0 1060 8.0 3.61
Mofete-8d 28.1 1095 5.1 2.33
TOT A L 144.4 28.9 13.13

126
Final data a:fter 25 ;rears
Production Total :flow rate Fluid enthalphy Steam :flow rate Power
wells (kg/s) (kJ/Kg) (Kg/s) (MW)
Mo:fete-1 (.)
Mo:fete-2 18.6 1410 6.3 2.85
Mo:fete-3d 14.7 1070 2.5 1.14
Mo:fete-7d 48.0 1055 7.8 3.56
Mo:fete-8d (... )
TOT A L 81.3 16.6 7.55
( .. ) Cemented a:fter 12 years
( .... ) Cemented a:fter about 14 months
Flow rate and power cannot be maintained constant during the li:fe o:f
the project, but they decrease at 50 % o:f the initial values as a
consequence o:f Mo:fete-1 and Mo:fete-8d being cementing o:f:f. This simulation
demonstrated that, without reinjection or strong natural recharge, to
exploit the :field adopting this con:figuration is technically and
economically inconvenient.
Simulation N° 3 (22.7 MW initial power)
This simulation considers the utilization as producers o:f the best
existing wells, namely Mo:fete-1 and Mo:fete-7d, and :foresees the drilling o:f
:four new production wells.
For reinjection o:f the waste water it is planned to use Mo:fete-2, Mo:fete-3d
and Mo:fete-8d, with the addition o:f three new wells to be drilled in
:future: This scheme represents a reasonable con:figuration o:f :field
development :for its optimum exploitation.
The new wells were located in the central part o:f the :field, where temperatures
are higher ; the production layers were assumed by comparison with
the producing ones o:f the closest existing wells.
Initial data
Production Total :flow rate Fluid enthalphy Steam :flow rate Power
wells (kg/s) (kJ/Kg) (Kg/s) (MW)
Mo:fete-1 35.0 1100 6.5 2.95
Mo:fete-7d 48.0 1060 8.0 3.61
Mo:fete-X 30.0 1420 10.2 4.66
Mo:fete-Y 30.0 1425 10.3 4.69
Mo:fete-Z 30.0 1100 5.6 2.53
Mo:fete-T 30.0 1370 9.5 4.32
TOT A L 203.0 50.1 22.76

127
Reinjection
wells
Total flow rate
(Kg/s)
Fluid enthalpy
(kJ/Kg)
Fluid temperature
(DC)
Mofete-2
Mofete-3d
Mofete-8d
Mofete-RX
Mofete-RY
Mofete-RZ
19.6
40.1
28.3
21.6
21.6
21.6
715
715
169
169
TOT A L
152.8
Final data
after 25 lears
Production
wells
Total flow rate
(kg/s)
Fluid enthalphy
(kJ/Kg)
Steam
flow rate
(Kg/s)
Power
(MW)
Mofete-1
Mofete-7d
Mofete-X
Mofete-Y
Mofete-Z
Mofete-T
29.4
48.0
30.0
30.0
18.2
30.0
1075
1050
1355
1325
1200
1270
5.1 2.31
7.7 3.51
9.3 4.22
8.9 4.02
4.3 ~.94
8.0 3.66
TOT A L
185.6
43.3 19.66
Reinjection
wells
Total flow rate
(Kg/s)
Fluid enthalpy
(kJ/Kg)
Fluid temperature
(DC)
Mofete-2
Mofete-3d
Mofete-8d
Mofete-RX
Mofete-RY
Mofete-RZ
TOT A L
19.6 715
40.1 715
28.3
21.4
21.4
21.4
152.2
169
169
In this aimulation, in spite of the presence of reinjection, Mofete-1
and Mofete-7d do not succeed in maintaining the initial flow rate, because
the pressure decrease close to the well reduces the allowable upflow.
6. FINAL EVALUATION OF THE RESEARCH
For the translation of the conceptual model and of all the information
obtained from test ina into a computational scheae, the CHARGR
proaramme was adopted ; on the whole, this code showed a good level of
reliability, even if sometimes there have been some difficulties due to the
scarce flexibility of the code. However, one of the DOst important
limitations of this proaramme consists in the assumption of the same
temperature for the rock and the fluid. This fact improves the effect of
reinjection, favouring hest recovery froa the rock in quantities areater
than in the actual conditions.

128
Suggested developments in the Mofete field
As regards the evaluation of the potential of the Mofete field and
its possible developments, on the basis of the simulation carried out and
on the hypothesis of a 25 year economic life, the following conclusions
have been reached :
- The maximum exploitable power from the field in optimum conditions
is about 20 MW. This hypothesis requires the drilling of seven new
successful wells, three of which are for production and four for
reinjection. The greater difficulty in performing this job consists
in finding suitable areas to locate the new wells.
- The simulation of strategy for the maximum exploitation of the
field using the existing wells gave very discouraging results for
the cementation of two of five production wells. The power
decreased from 13 MW to 7.5 MW at the end of the 25 years period.
- In the hypothesis of avoiding new drillings, the exploitable power
is 9.5 MW, with a little decrease (0.3 MW) during the life of the
field. This power was obtained by means of three production and two
reinjection wells.
The simulations carried out demonstrate that the exploitation of
the Mofete field cannot be realized without reinjection of the
waste water; this necessity results from the ascertained poor
natural recharge and complies with the environmental protection
caution and with the Italian Law.
- In every case it is advisable to carry out further tests in order
to define better the characteristics of Mofete-2, whose fluid
presents a complex chemistry, with a tendency to precipitation of
silica when the temperature falls below l80 G C.
- The presence of C02 contributes to the life of the field, favouring
the up flow of the geothermal fluid by reducing the hydrostatic
head. This fact could suggest the hypothesis of partially reinjected
the C02 extracted, together with the waste water, in order not
to reduce too much the C02 concentration in the reservoir.
In conclusion, a precautionary evaluation suggest setting the
exploitable powers at about 9.5 MW and 20 MW in the case of utilizing the
existing wells and in the maximum field development scheme respectively.
REFERENCES
1. AA. VV. (1987). Studio di fattibilit~ Mofete. AGIP-DAL Intesa.
2. CHELINI, W., ANTRODICCHIA, E., BALDUCCI, S., GAMUCCI, A. (1987). Aggiornamento
geologico geochimico del campo geotermico di Mofete-P.R. Lago di
Patria. AGIP-Esge.
3. GIANNONE, G. (1983). Studio del campo geotermico di Mofete (Parte
l)-Pozzo Mofete-2. Simulazione delle prove di falloff e pit test.
AGIP-Prav.
4. PRITCHETT, J.W. (1980). Numerical Techniques Employed in the MUSHRM and
CHARGR Geothermal Reservoir Simulators. Systems, Science and Software
Report. SSS-R-81-4681.
5. PRITCHETT, J.W. (1981). User's Guide to the CHARGR Fully-Implicit, Multi
Phase, Compositional Multi-Dimensional Geothermal Reservoir Simulator
System. Systems, Science and Software Report.
6. PRITCHETT, J.W. (1982). A Computer Program for Calculating Flow in a
Producting Geothermal Well.· Systems, Science and Software Report.
SSS-R-82-5660.
7. SCHMIDT, E. (1982). Properties of Water and Steam in SI Units (0-80 G C,
0-1000 bar). Sringer Verlag & Oldenbourg Ed.

129
EEC COntract nO EN3G-0070-I
C. PIEMONTE and A. PIATT!
Energy Department - Polytechnic of Milan (Italy)
E. SZEGO
Research and Development Division - TECHINT S.p.A - Milan (Italy)
The handbook develops the technical description and critical
analysis of different possible conf igurations fer ~ energy production
station connected to district heating systems and supplied by
low-enthalpy geothermal energy.
These configurations are firstly examined as far as thermodynamics
and thermal balances are concerned; then, the simulation of the
behaviour of these systems is carried out by means of the "GEOTERM"
computer program, leading to the technical and economic analysis of
96 cases and showing the effects of variation in the following
basic data:
* maximum thermal power of the system
* flow-rate and temperature of the geothermal fluid
* cost of the geothermal well
* operating temperature of the district heating network
* structure and flow chart of the production stations.
This paper shows the results of the thermod~amic analysis of
different possible arrangements of heat pumps, presenting
significant diagrams relevant to the energy efficiency indices
assumed for the comparison.
1. INTRalUC'l'IOR
The heat production units in a thermal station fed by geothermal
energy for the supply of a district heating network may be the
following:
a. primary heat exchangers between the geothermal fluid and the
district heating water;
b. heat pumps;
c. heat recovery units of the heat pumps' prime movers;
d. boilers.
In the following part of the text, the units indicated under a., b~
and c. will be defined "base load units", while the units indicated
under d. will be called "peak load units".
Peak load boilers are absolutely necessary when the supply temperature
of the district heating network is so high that it cannot be
obtained by means of base load units only; otherwise, the decision of
installing peak load boilers and the choice of the thermal power of
these units depend on an economic opt~zation calculation.
In this paper, three basic flow systems including heat pumps and
peak load boilers will be defined and analysed. The differences between
the three systems are due to:

130
* inclusion or otherwise of a primary heat exchanger between the
geothermal circuit and the district heating network;
* direct or indirect connection between the geothermal fluid and the
evaporator.
The three basic system are the following:
* HPO: heat pump only, with direct connection between the evaporator of
the heat pump and the geothermal fluid;
* HPA-DE: heat pump assisted, with direct evaporator; this scheme can be
applied only when the geothermal fluid well-head temperature is higher
than the district heating return temperature;
* HPA-IE: heat pump assisted, with indirect evaporator: the evaporator
is located in a primary position on the district heating network
return branch in order to extract heat from the district heating
water, to reduce its inlet temperature into the heat exchanger and
to increase the heat extracted from the geothermal fluid. The
geothermal energy is transferred exclusively by a single route across
the primary heat exchanger. Indirect evaporator schemes are used when
the geothermal fluid is particularly fouling or corrosive and allow
the reduction of the heat pump evaporator cost. The basic HPA-IE
system may deal with the two following variants:
.- partial by-pass of the distribution network water flow to the heat
pump evaporator/primary heat exchanger branch;
.- parallel connection between the heat pump condenser and the
evaporator/primary heat exchanger branch.
In all cases the heat pump ( s) may be both electrically or eng ina
driven, with recovery of the waste heat from the prime mover.
2. ENERGY KFFICIENCY OF TBK SYSTEMS
The three basic flow systems defined in the previous paragraph are
compared on the basis of their energy efficiency which is represented by
means of two ratios:
* ratio Pg/Pmc between the thermal power Pg extracted from the
geothermal source and the mechanical power Pmc required for driving
the heat pump compressor
* ratio Pb/Px between the thermal power Pb supplied by the base load
uni ts and the Px power input of the heat pump mover; the Px power is
therefore different according to the type of heat pump mover:
.- for electrically driven heat pumps the Px power is represented by
the electrical input Pe of the motor
for heat pumps driven by heat and power engines the Px power is
represented by the thermal power Pf of the fuel burned by the
engine.
The energy efficiency ratios calculated for the three examined
systems may be expressed in the following terms (see the "Glossary
of Symbols" in section 5) :
* HPO system:
Pg/Pnc COP- 1
Pb/Pe COP * Ee
Pb/Pf COP * Em + Et
* HPA ~ DE system:
Pg/Pmc (COP-I) * (Tgwh-Tgd) / (Tdr+Taphe-Tgd)
Pb/Pe (COP + (COP-I) * (Tgwh-Tdr.-Taphe) / (Tdr+Taphe-Tgd» * Ee
Pb/Pf (COP + (COP-I) * (Tgwh-Tdr-Taphe) / (Tdr+Taphe-Tgd»*Em + Et
* HPA ~ IE system:
Pg/Pmc < = (COP-I) * (Tgwh - Tgd + Taphe) / (Tdr - Tgd + Taphe)

131
Pb/Pe (= COP * Ee * (1+ (Tgwh-Tgd+Taphe)*(COP-l)/(Tdr-Tgd+Taphe»
Pb/Pf (= COP * Em * (1+ (Tgwh-Tgd+Taphe)*(COP-l)/(Tdr-Tgd+Taphe»
+ Et
3. calPARISON BE'1'1IKEN ENERGY EFFICIEHCY RA'fIOS FOR '!HE DIFP'KRER'l
FLOII SYSTEII BASED ON '!HE O'l'ILlZATION OF HKA'1' PUMPS.
3.1. Pg/PIDc ratlo
The fundamental difference is between the HPO and the HPA system.
Referring to the expression in section 2., the Pg/Pmc ratio for HPA
system is higher than the same ratio for the HPO system because:
Tgwh - Tgd > Tdr + Taphe - Tgd
3.2. Pb/p:.. ratio
The fundamental difference is between electrically driven heat
pumps (Px - Pe • electrical input of the motor) and engine driven heat
pumps (Px a Pf • thermal power of the burned fuel).
For the HPA system, the Pb/Px ratio for the two types of movers can
be expressed in the following terms:
* electrically driven heat pumps:
Pb/Px • Pb/Pe • K * Ee
* engine driven heat pumps:
Pb/Px • Pb/Pf • K * Em + Et
where:
K • COP + (COP-I) * (Tgwh - Tdr - Taphe) / (Tdr + Taphe - Tgd)
For both the considered types of movers, therefore, the Pb/Px ratio
can be expressed as a function of the same variables, as follows:
Pb/Px • f (COP, Tgwh, Tgd, Tdr, Taphe)
The primary heat exchagger temperature approach Taphe may usually
vary in the range l C - 5 C; its variations do not affect the Pb/Px
ratio to any great extent, so that for the following investigation a lOC
value will be assumed as constant for the Taphe parameter.
As for the HPA-DE system the COP depends upon the district heating
water outlet temperature from the condenser Tdl and the geothermal fluid
discharge temperature Tgd, finally the Pb/Px ratio can be expressed in
the following terms:
Pb/Px • f (Tgwh, Tgd, Tdl, Tdr)
This ratio, therefore, comes out as function of the inlet and
outlet temperatures of the geothermal fluid and district heating water
in the system "primary heat exchanger + heat pump".
The trend of such a relationship has been calculated for different
combinations of said temperatures and is presented in form of
diagrane.
The diagrams relating to the Pb/Px trend versus the district
heating temperatures, assuming the geothermal fluid temperatures
(different combinations are exemined), are presented in Figs. I and
II.
The diagrem& relating to engine driven heat pumps are calculated
according to the following assumed efficiencies:
8m • 0.l2 Bt • 0.48
Blectrically driven heat pumps are examined fran two different
standpoints:
a) only the district heating system is considered, so that the heat
pump motors 'power input is the electrical power Pe; in this case a
0.95 electrical efficiency Be is assumed;
b) the overall energetic conversion is considered, including in the
eystem the conventional power plant supplying the electrical power

132
Pe for the heat pumps 'motors, assuming a 37,4% electrical efficiency
(corresponding to a 2300 kcal/kWh specific thermal consumption) for
this plant, so that the overall electrical efficiency to be
considered is
Ee = 0.95 * 0.374 = 0.3553
This latter standpoint allows a comparison relating to the real
overall exploitation of the primary fuel for the two ~onsidered types of
movers.
A preliminary consideration relating to the analysis of the
diagrams is that the examined ratios are expressed as a function of
the district heating water outlet temperature from the heat pump
condenser Td3; when comparing diagrams relating to the two types of
movers, therefore, it must be remembered that:
* a comparison assuming the same Td3 temperature for the two types of
movers involves that the system "primary heat exchanger + heat pump
condenser" supplies the same thermal power for both of them, so that
the thermal power Pb supplied by the base load units is higher for the
engine driven heat pump case;
* a comparison assuming the same Pb power for the two types of movers
implies that the Td3 temperature is lower for the engine driven heat
pump case, so that the corresponding Pb/Px ratio is higher; this
latter temperature is about 3 0 C .- Soc higher than Td3, so that, when
comparing the Pb/Px ratio on the basis of the same thermal power
supplied by the base load units Pb (i.e. the Td3 temperature for the
electrically driven heat pump must be equal to the Td5 temperature for
the engine driven heat pump), the Pb/Px ratio relating to engine
driven heat pumps comes out 3%-4% higher.
The analysis of the diagrams presented in Figs. I and II leads
to the following considerations:
* the Pb/Px ratio for electrically driven heat pumps, considering the
district heating system only, is the highest of the compared cases,
because of the highest mechanical efficiency of the heat pump mover;
* when considering the overall energy conversion of the fuel burned in a
conventional power plant to the shaft power of the electric motor
driving the heat pump, the corresponding Pb/Px ratio is lower than the
one relating to the engine driven heat pump, because of the waste heat.
recovered from the engine in the latter solution;
* the difference between the two cases discussed in the latter point
varies in the opposite way to the temperature difference (Tgwh - Tdr),
because an increase in this temperature difference improves the
thermal power released by the geothermal fluid through the primary
heat exchanger and, consequently, the share of the Pb thermal power
due to the heat pump becomes less important; the difference between
the two cases also increases with the geothermal fluid discharge
temperature Tgd, because an increase of the Tgd temperature causes a
reduction of the thermal power through the heat pump evaporator and,
as a consequence, the same effect as described above; in addition, the
heat pump COP is higher and this leads to a reduction of the
mechanical power absorbed by the compressor and, consequently, of the
waste heat recovered from the engine.
3.3. Comparison between the BFA-DE system and the BFA-IX system.
The comparison between the two systems is developed referring to
the Pg/Pmc ratio.
Said ratio, for the two considered systems, is expressed as

133
follows:
• HPA-DE system:
Pg/Pmc - (COP-I) * (Tgwh - Tgd) / (Tdr + Tgd - Taphe)
• HPA-IE system:
Pg/Pmc < ~ (COP-I) * (Tgwh - Tgd + Taphe) / (Tdr - Tgd + Taphe)
This latter formula expresses the upper limit for the Pg/Pmc ratio
relevant to the HPA-IE system; this upper limit corresponds to the
minimum allowed ratio of the district heating water flow to the
geothermal fluid flow; the energy efficiency ratio Pg/Pmc for the HPA-IE
system is a monotonic decreasing function of the above flow ratio.
This paragraph examines the comparison between the Pg/Pmc ratio
relating to the HPA-DE system and the above-mentioned upper limit of the
ratio relating toothe HPA-IE system.
Assuming a 3 C temperature approach for the primary heat exchanger
(Taphe) and expressing the heat pump COP in terms of the outlet
temperatures from evaporator and condenser, the Pg/Pmc ratio, both for
the direct and for the indirect evaporator, can be indicated in the
following terms:
Pg/Pmc - f (Td3, Tdr, Tgwh, Tgd)
For tge HPA~DE system, the geothermal fluid discharge temperature
is about 3 C - S C higher than the evaporation temperature of the heat
pump cycle; the difference between these two temperatures is represented
by the temperature approach of the evaporator; therefore, in or'ber to
avoid freezing in the evaporator, a minimum temperature of about S C is
allowed for the geothermal fluid at the outlet of the evaporator.
For the HPA-IE system, for the same reason as described above, a
minimum temperature of about SoC is allowed for the district heating
water at the outlet of the heat pump evaporator, so that the minimum
corresponding discharge temperature for the geothermal fluid (at the
outlet of the primary heat exchanger) is about aOc.
Therefore, a comparison relating to the Pg/Pmc ratio for the HPA-DE
and HPA-IE systems, can be carried out assuming the same values for the
Td3, Tdr and Tgwh temperatures, whereas, with regard to the geothermal
fluid discharge temperature Tgd, it must be remembered that:
• when the thermal power obtainable by cooling the geothermal fluid is
not completely exploited, so thaS the heat pump cycle evaporation
temperature is much higher than 0 C, the same discharge temperature
Tgd of the geothermal fluid can be assumed for the two compared heat
pump systems; this leads to a higher COP for the HPA-DE system,
because the heat pump cycle evaporation temperature, for this system,
is closer to Tgd; at the same time, however, the temperature ratio
which appears in the Pg/Pmc ratio expression is higher for the HPA-IE
system, so that the two effects just about counterbalance;
* when the thermal power obtainable by cooling the geothermal fluid is
completelYoexploited, with a heat pump cycle evaporation temperature
close to 0 C, the geothermal fluid discharge temperature Tgd admitted
for the HPA-DE ~stem is lower thanothe one characterizing the HPA-IE
system (about S C for HPA-DE and a C for HPA-IE); consequently, the
thermal power extracted from the geothermal fluid (Pg) for the HPA-DE
system, is higher; on the contrary, the Pg/Pmc ratio for the HPA-IE
system is higher, because:
- the term (COP-I) is equal for the two systems, as the heat pump
cycle evaporation temperature is the same one;
- the term (Tgwh - Tgd) for the HPA-DE system is equivalent to the
term (Tgwh - Tgd + Taphe) for the HPA-IE system, because of the

134
different value admitted for the Tgd temperature;
the term (Tdr - Tgd + Taphe) for the HPA-DE system is higher than
the corresponding one for the HPA-IE system, because of the lower
value admitted for the Tgd temperature
In this situation, therefore, the HPA-DE system is characterized by
a higher thermal power Pg extracted from the geothermal fluid and a
lower Pg/Pmc ratio.
For this comparison, the same Tgd temperature is assumed for the
two heat pump systems and the investigation deals with the following two
values for Tgd:
* TgI = aOe (which is the minimum allowed value for the HPA-IE system),
* Tgd = 20 0 e
The general conclusions about the Pg/Pmc ratio (relative to both
the considered heat pump systems) are the following:
* Pg/Pmc is a monotonic increasing function of the geothermal fluid
inlet and outlet temperatures in the considered system (Tgwh, Tgd);
* Pg/Pmc is a monotonic decreasing function of the district heating
water inlet and outlet temperatures in the considered system (Td3,
Tdr) .
The differences between the Pg/Pmc ratios of the two examined heat
pump systems may be shown clearly by presenting their Pg/Pmc ratio trend
in the same diagram; Fig. III includes three diagrams showing this
direct comparison, relating to different combinations of the four above
temperatures.
As a general feature, the Pg/Pmc ratio of the HPA-DE system is
higher than the one for the HPA-IE system, i.e. the increase in COP for
the HPA-DE system, due to the higher evaporation temperature of the heat
pump cycle, prevails. over the effect due to the existence of an
additional Taphe term in the numerator of the formula giving the Pg/Pmc
ratio for the HPA-IE system.
The analysis of the diagrams in Fig. III (a) shows that a
variation in the district heating return temperature does not affect the
difference between the Pg/Pmc ratios relating to the two heat pump
systems: in fact, the relative trend of the two diagramg to a 40 0 e and
ssoe district heating water return temperature is the same.
The comparison between the diagrams in ~) reveals that an
increase in the geothermal fluid discharge temperature causes a
reduction of the differences between the Pg/Pmc ratios of the two
examined systems; this is due to the following combined effects:
* as the Pg/Pmc ratio formula, includes the factor (COP-I) as numerator,
for both the examined system~ the COP improvement due to an
increase of the Tgd temperature makes the relative advantage for the
HPA-DE system less important;
* the Pg/Pmc ratio formula for the HPA- IE scheme includes, as the
numerator, an additional term represented by the primary heat
exchanger temperature approach Taphe; as an increase of the Tgd
temperature causes a reduction of the numerator of the Pg/Pmc formula,
the influence of the constant term Taphe (a 3 0 e constant value has
been assumed for this analysis) becomes more noticeable.
The above-mentioned diagram (b) shows that, with a 20 0 e Tgd, the
difference between the Pg/Pmc ratios relating to the two considered heat
pump systems is negligible.
A comparison between the diagrams of Fig. III (e) shows
that an increase of the geothermal fluid well head temperature causes an
increase of the difference between the Pg/Pmc ratio of the two examined

135
systems; in fact, a Tgwh increase also causes a corresponding increase
in the numerator of the Pg/Pmc ratio formula, so that the influence of
the constant term Taphe becomes less noticeable.
.. • CXJtICLDSIalS
The HPO system energy efficiency is lower than in the case of the
HPA systems, and this conclusion is quite expected.
As far as the type of heat pump mover is concerned, systems based
on heat and power engines as heat pump movers are characterized by the
highest overall energy efficiency; the choice of the mover in real
applications, however, should usually be carried out on the basis of
economic evaluations.
With regard to the comparison between the HPA-DE and HPA-IE systems,
the analysis described in Par. 3.3. shows that if the flows and
temperatures of the district heating water and geothermal fluid are the
same for the two systems, they can reach a substantially equal
energy efficiency.
The HPA-IE system should apply when, considering all the possible
operating conditions, a heat pump cycle "temperature stretch" could
resul t which would be too low for normal operation : in this case, in
fact, a by-pass of the district heating water over the primary heat
exchanger + heat pump evaporator branch allows a reduction of the
distr ict heating water outlet temperature from the evaporator, and so
maintairEthe "temperature stretch" of the cycle at its lowest practical
value.
5. GLOSSARY OF SYJmOLS
COP
Ee
Em
Et
Tgwh
Tgd
Tdr
Taphe
..
Coefficient of Performance of heat pumps
Electrical efficiency of heat pump motors
Mechanical efficiency of heat pump prime movers
Thermal efficiency of heat pump prime movers
Geothermal fluid well-head temperature
Geothermal fluid discharge temperature
District heating water return temperature
Primary heat exchanger temperature approach.
6. RKFERKMCRS
Angelino G., E. Macchi and E. Sacchi (1982) - Energy and economic
comparison of conventional district heating vs. centralized heat
pump system. V I.D.H.C. (International District Heating Conference)
- Kiev.
Aureill. M. Geothermal heating. Report EUR 7801 H, 1982 - E.C.
Contract 584-78-7-EGF.
Harrison R. and N.D. Mortimer (1985) Handbook on the economics of
low enthalpy geothermal energy developments. E.C. Contract
EG/A2/05/UK H.
Piatti A., C. Piemonte C. and E. Szego (1987) GEOTEL: a computer
program for simulating the behaviour and optimizing the operation
of a geothermal district heating system. 23rd UNICHAL Congress -
Berlin.
Piemonta C. at al. (1987) Metodologie di valutazione della
fattibilitl di progetti per usi diretti dell'energia geotermica
2nd C. N. R. - PFB2 Congress on "Geothermal energy" - Ferrara.

136
A) Electrically driven beat pump. considering the
district beating system only
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B) Electrically driven beat pump. considerina the overall energetic
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the thermal power supplied by base load un! ts
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137
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A) Electrically driven beat pump. considering the
district beating system only
... ...
..•
a) Ilectric.lly driven beet p~p, conelderlna the over.ll sneraetic
converalon rro. the ru.l burned 1n _ conventloMl power station to
the theraal power eupplied by bu. load unite
~
l
....
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T., _ .-oc
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_tar outlet t ..... r.ture f~ the beat ~ c:ondanaar (Td3).
for 41ff.rant ~1nat1ona of:
• !'9d • vaothemal fluid 41acbar9a t ..... r.ture
• Nr • 41atr1ct t-t1nv _tar ratlU1l t...,.r.ture
Gaothemal flu1d .. U-haa4 t...,.r.ture • 90°C

138
•
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T.,. _ .oe
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•
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Fig_ III -
D1rec:t c:aq>ar18CX1 of the P9/PE ratio for tha BPA-DE and
HPA-IE .ch_., for different combination. of:
• Tgwh • geotharmal fluid wall h .. d temperatura
• T9d • geothermal fluid di.charge temperatura
• Tdr • di.trict h .. ting _ter return temperatura

ae.dOD 2
Hoc dry rock .... relaced .bldie.
Characterisation of the Rosemanoves HDR geothermal reservoir
u.ing an extended circulation programme
Cost modelling of HDR systems modelling methods and interim
results
Experimental investigation on forced fluid flov through a granite
rock mass
Apparatus to provide an image of the vall of a borehole during
a hydraulic fracturing experiment
Rock stress orientations from borehole breakouts
Stress measurements by hydraulic fracturing in BRGM
Stability of deep geothermic exchanger under thermalhydraulic-mechanical
sollicitations
In-situ stresses evaluated from measurements on core samples
Stimulation of wells Latera 10 and Latera 4
The European geothermal project at Soultz-sous-For@ts
Economic modeling of HDR
Hydrogeothermic studies on hot dry rock technology

141
EEC CONTRACT NO EN3G-0003-UK(N)
EN3G-0099-UK
CHARACTERISATION OF THE ROSEMANOWES HDR GEOTHERMAL RESERVOIR
USING AN EXTENDED CIRCULATION PROGRAMME
ROGER PARKER
Director of the Camborne School of Mines
Hot Dry Rock Geothermal Project
SUlllDllry
The continuous hot dry rock reservoir circulation programme carried
out by the Camborne School of Mines in the period 1985-1988 at
Rosemanowes Quarry in Cornwall is reported. The programme can be divided
into three stages: a gradual step-wise increase in injection flow rate to
a level of 35 lIs; the use of a downhole pump to lower the pressure in
the main production well to simulate the sub-hydrostatic conditions which
are expected in a 6 km deep production well; and a long period of
constant injection flow rate (21.5 lIs), during which a flow path
characterisation experiment was carried out.
Conclusions are drawn concerning the characteristics and performance
of the reservoir.
1 INTRODUCTION
In Phase 28 of the Camborne School of Mines Hot Dry Rock Geothermal
Energy Project, a third well (RHI5) was drilled to a depth of 2.6 km, in
a system already containing two wells 2 km deep (RHll and RHI2). A new
reservoir system was stimulated using a viscous gel (CSM, 1988) and
circulation of this reservoir began in August 1985, with RH12 as the
injection well and RH15 and RHll as major and minor production wells
respectively.
The aim of this extended circulation programme, which occupied part
of Phase 28 and the who le of Phase 2C, was to measure the nature and
behaviour of this reservoir system, and to confirm predictions of its
long term operational performance. It was also intended to improve the
understanding of reservoir creation and development, with a view to t~e
eventual construction of a prototype commercial system at greater depth.
The Phase 2C reservoir circulation programme (1986-1988) can be
divided into three stages:
I A gradual increase in the injection flow rate, using periods of
up to six weeks at each flow rate step increase, to allow
approximately steady state conditions to be achieved at each
step. This was a continuation of the Phase 28 reservoir
circulation programme, 1985-1986.
II The use of a downhole pump in the main production well (RHI5) to
lower the pressure in this well, thus silOOlating the

142
sub-hydrostatic conditions which are expected in a 6 km-deep
production well.
III A long period during which the injection flow rate was kept
constant at 21.5 lis. During this period the hydraulic, thermal
and chemical behaviour of the reservoir were monitored, and a
flow path characterisation experiment was carried out. Inert
tracer was injected at selected positions in the open hole
section of the injection well (RH12), and drillpipe was used in
RH15 to extract water from different points in the open hole
section of the production well. The tracer concentration in
these extractions was monitored continuously.
Figure 1 gives a complete picture of circulation of the reservoir
since its development by viscous gel stimulation in July 1985. The
injection flow rate recorded for RH12 is shown in this diagram, together
with the production flow rates recorded for RH15 and RH11. It wi 11 be
seen that flow from RH11 has continued throughout the period to account
for a small (and fairly constant) proportion of the water injected. This
three-year period of continuous circulation is by far the longest
circulation of a HDR geothermal energy reservoir, and has provided an
important and extensive set of data on which the work of characterisation
of the reservoir has been based.
Examination of Figure 1 shows that water losses have remained at a
fairly constant proportion of the water injected (20% overall), but there
are two important exceptions to this. At high injection flow rates (and
consequently at high injection pressures), water losses have increased.
Since the maintenance of a constant injection flow rate (RH12) in the
latter part of Phase 2C, the production flow rate has dropped steadily in
RH15, but has remained almost constant in RH11.
The analysis of hydraulic data is complicated by the flow from RH11.
This can be considered as a contribution to production flow (and
therefore enhancing water recovery), or as a component of the water
losses from the RH12-RH15 reservoir. The connection between RH12 and
RH11 was developed in Phase 2A, and was seen to be of a diffusive nature,
and to have high impedance compared with the RH12-RH15 connection
developed in Phase 2B. Where diagnostic techniques are being used to
characterise the main Phase 2B/2C reservoir, the flow from RHll shoull!!
therefore be counted as a water loss, and impedance should be measured as
the ratio of the pressure drop between RH12 and RH15, to the RH15
production flow rate.
2 RESERVOIR CIRCULATION STAGE I: INCREASING INJECTION FLOW RATES
The Phase 2B circulation programme was reported in the Final Report
of Phase 2B (CSM, 1988). Stage I of the Phase 2C reservoir circulation
programme was reported in the Technical Progress Review covering Phase 2C
up to September 1987 (CSM, 1987b). This stage was a continuation of the
Phase 2B programme of invest i gat i ng the characteri sti cs and performance
of the reservoir at increasing levels of injection flow rate. The flow
rate was held constant at each flow rate step, for a period of about six
weeks, to allow approximately steady state conditions to be achieved, so
that tracer tests and production logs could be run. It must be
remembered that at the beginning of Phase 2C, the programme aimed to
raise the injection flow rate to the highest possible level, which it was
thought would be limited to about 451/s by the power supply to the
pumps. The review of the technology carried out in 1987 (CSM, 1987a)
suggested that the Rosemanowes reservoir cou 1 d not susta i n an inject ion
flow rate of 35 lis without an increase in water loss and excessive

143
microseismicity. The original Phase 2C circulation progranme was then
perceived as being founded on an exaggerated idea of the size and
hydraulic capacity of the reservoir, and of its relationship to the size
of reservoir which would be required to support a prototype of a
conmercial HDR system.
Figure 1 shows the continuation of the Phase 2B progranme, during
the first stage of Phase 2C, using injection flow rate steps of 21, 24,
29 and 35 lis. In Figure 2, the injection pressures measured at
different injection flow rates are a continuation of the trend shown in
Phase 2B. The relationship between injection pressure and both the net
water loss and the number of microseismic events recorded per day is
shown in Figure 3. The minimum in situ horizontal earth stress (in
excess of hydrostatic stress) at 2 km is about 10 MPa. The data in
Figure 3 indicate that once this pressure is exceeded, water losses and
the incidence of microseismic events (indicating reservoir growth)
increase significantly. An injection pressure of 10 MPa in Figure 2
implies an injection flow rate of about 24 lis. These performance
characteristics indicate the upper limits for circulation of the
Rosemanowes reservoir in its present condition. The impedance of the
RHI2-RHI5 reservoir would be 0.6 MPa/kg/s at this upper limit of
satisfactory circulation. Figure 4 shows the decrease of steady state
impedance with increasing RH15 production flow rate. At an injection
flow rate of 24 lis, and impedance of 0.6 MPa/kg/s, the production flow
rate from RH15 would be 16.5 lis, implying a water recovery of 69% in
RHI5. (To facilitate comparison with total water recovery values, a flow
of 2.5 lis from RHII would give a total recovery of 79%). It is possible
to run the reservoir at an impedance as low as 0.5 MPa/kg/s, but this
requires higher flow rates and higher injection pressures, implying the
risk of reservoir growth and increased water losses.
The need in a conmercial system for a steady build up of reservoir
pressure (using steps in the increasing flow rate schedule) is a matter
for debate. The period of six weeks per step could be cut to about a
week if there was no need for a full range of diagnostic testing at each
step.
Production logging in Stage I showed very little change in the flow
profile for RH12, but the logs run in RH15 showed some redistribution of
flow between the flowing zones. The thermal drawdown occurring in the
reservoir during this stage was continued throughout Phase 2C. The most
important observat ion was the steady fa 11 in temperature shown by the
three main flowing zones in RH15 (Figure 5).
The temperature of the small amount of water entering Zone 4 (up to
about 5% of the total flow), did not vary significantly, and the flow
path characterisation experiments (Stage III, below) have demonstrated
that water entering this zone has a long residence time.
Tracer tests were run using the inert tracer sodium fluorescein
(NaFl) to characterise the reservoir, (CSM, 1987c). It would seem from
these tests that the volume of the major production flow paths, and
possibly the volume of the low impedance flow path connections, increased
with increase in injection flow rate. The value of these parameters did
not change with time at constant flow rate in Stage III of the Phase 2C
circulation, so that the change in flow path volume appears to be
associated with injection flow rate, and not time. This volume increase
may be a result of the inflation of the reservoir flow paths as injection
pressure increases. Marked changes in flow path distribution such as
those which apparently occurred during the downhole pump test (Stage II,

144
below), may have more significance than these flow path volume changes
for the characterisation of the reservoir.
In the later months of Stage I of the circulation programme, the
injection flow rate reached 35 lIs, at a wellhead pressure of 11.1 MPa.
Excessive microseismic activity and water losses were observed, and it
became clear that sustained high flow rate circulation would be of
limited value because the size of the reservoir was small in comparison
with that required for a commercial-scale system. The decision was taken
in July 1987 to lower the injection flow rate to 21-22 lIs, a level at
wh i ch an overa 11 water recovery of 80% had been ach i eved in
November 1986, at a pressure of 9.5 MPa, sufficiently below the effective
minimum stress at 2 km depth (10 MPa) to minimise water losses and
seismic activity.
3 RESERVOIR CIRCULATION STAGE II: DOWNHOLE PUMP TEST
This test was carried out from August 1987, to the end of
October 1987. The objective of the test was to simulate the
sub-hydrostatic conditions of the production well which are expected in
the 6 km-deep system, resulting from the long hot column of water in the
production well which would have a lower density than the normal
hydrostatic column. The influence of this effective pressure reduction
on the system impedance, heat product ion and the inc i dence of
microseismic events was to be investigated. As explained in the previous
section, it became necessary to lower the injection flow rate to avoid
microseismicity before the start of the downhole pump test, and it was
therefore not possible to examine the effect of the test on
microseismicity. During August and the first three weeks of September,
the injection flow rate was kept at 21.7 lIs, with a drop of 4.5 MPa in
the production well hydrostatic pressure caused by the downhole pump. In
the last week of September, the injection flow rate was increased to
25 lIs and maintained at this rate for the remainder of the test. During
the second week in October, the downhole pump was put on full production
and this caused the RH15 wellhead pressure to fall to 4.6 MPa drawdown.
The drawdown was then set at 2.2 MPa for the final part of the test,
(Table 1).
An inter-well interference test carried out at the start of th~
downhole pump test showed higher permeability close to the production
well bore , compared with the main reservoir region. This indicates that
the fractures intersecting the wellbore are not the cause of high
reservoir impedance.
Production flow rate declined gradually each time the circulation
conditions were set, throughout the downhole pump test, probably due to
increased effect i ve stress on the flowing joints. Recovery to pre-test
conditions took about six weeks after the pump was removed. There was no
change in flow distribution in RH12, and 10g9in~ of RH15 before and after
the test showed a slight redistribution (2%) from Zones 1 and 3 to
Zone 2. (Zones refer to Figure 5 above). The therma 1 drawdown rate
remained unchanged at about 1 D C per month during the test.
Five inert tracer tests were run during the downhole pump test and a
further two were run afterwards. The residence time distribution curves
were very consistent and showed no flow path volume changes, but a change
in flow characteristics (ie distribution between different flow paths)
compared with those from tracer tests in Stage I. Tracer curves obtained
after the completion of the test showed similar characteristics, thus
indicating a permanent change in reservoir flow characteristics. It is
interesting to note that similar tracer curves were obtained before the

145
period of major reservoir oscillations in January 1986. Examination of
the tracer curves produced inmediately before the downhole pump test
shows a slight indication of a change to the Stage II type, but it seems
clear that lowering the product ion well pressure encouraged a permanent
rearrangement of flow characteristics of the reservoir.
TABLE 1 STEADY STATE CONDITIONS (DOWNHOLE PUMP TEST)
00\1£ 8A1J1i 14 SEP 6 OCT 13 OCT Z3 OCT 11 IIJII 13 DEC
RH12
We Ilhead pressure (lIPa) 9.5 9.5 9.8 9.8 9.8 9.9 9.4
Flow rate (1/1) 21.7 21.7 25.2 25.3 25.4 25.4 22.0
RH15
Wellhead pres lure (lIPa) 0.2 -4.5 -4.4 -4.6 -2.2 0.2 0.2
Flow rate (1/1) 15.2 16.4 17.9 18.3 16.8 15.5 14.9
T~rature (DC) 57.4 59.0 58.5 58.7 58.4 55.6 53.7
RHll
We Ilhead preslure (lIPa) 0.2 0.2 0.2 0.2 0.2 0.2 0.2
Flow rate (l/s) 2.4" 2.1 2.3 2.3 2.4 2.5 2.3
Tlql8rature (DC) 52.8 SO.6 51.8 51.8 52.2 52.9 51.8
lql8dance (lIPa/1/I) 0.61 0.82 0.79 0.79 0.72 0.63 0.61
Overall recovery (') 81.1 85.3 80.2 81.4 75.6 70.9 78.2
RHI5/RHI2 recovery 70.0 75.6 71.0 72.3 66.4 61.0 67.7
• not steady state
The most important result of the downhole pump test was the
observation that the 4.5 MPa drop in production well pressure only
produced a drop of 0.5 MPa at the injection well. The impedance
therefore rose from 0.61 to 0.82 MPa/kg/s, even though the production
flow rate rose slightly, from 15.2 to 16.4 lIs at RHI5. RHll flow seemed
to be unaffected by the test.
The increased pressure drop was apparently necessary to overcome the
increased resistance to flow through the joints, resulting from the
'pinching in' effect caused by the increased effective stress acting on
the joints as a result of the subhydrostatic conditions at the production
wellbore. Proppants placed in the joints near to RH15 might reduce this
effect, but they would have to renetrate the rock mass beyond the
inmediate vicinity of RH15 because t has been demonstrated that there is
high permeability where the joints intersect the wellbore.
There were no detectab le therma 1 changes so, if hotter water
were drawn from the far field, it was cooled again by the rock in the
reservoir nearer the production well. As the downhole pump had very
1 ittle effect on the injection pressure, it can be argued that it would
have had little effect on the tendency to produce microseismic events at
higher injection flow rates and higher injection pressures. No

146
geochemical changes were detected which could have been caused directly
by the downhole pumping.
4 RESERVOIR CIRCULATION STAGE III: STEADY INJECTION FLOW RATE AND
FLOW PATH CHARACTERISATION
The downhole pump was removed from RH15 on 28 October, 1987, and
circulation was held at an injection flow rate of 25 lis until
18 November, to allow logging and tracer testing to be used to examine
the reservoir characteristics. The injection flow rate was then lowered
to 22 lis, although this gradually declined to 21.7 lis by the end of
February, 1988, when it was fixed at 21.5 lis for the duration of the
flow path mapping experiment, (Figure 1). .
The need to obtain more information about the water flow paths in
the reservoir became greater when it was realised early in 1987 that the
level of thermal drawdown had become serious (approximately I D C per
month). Apart from understanding whether or how a short circuit (or
preferential flow path which might consist of a cluster of closely-spaced
flowing joints) had occurred, any attempt to block a short circuit would
need its existence and location to be established.
The flow path characterisation experiment began on 10 March 1988,
with fluorescein tracer samples being injected at three injection points
in the open hole section of RHI2, and sampling of water flowing from
three zones in the open hole section of RHI5. The latter sanipl ing was
achieved by running open-ended 4 inch diameter drillpipe from the surface
to the top of the flowing zone in RHI5. Samples extracted above flowing
Zone 4 in RH15 (See Figure 5) before injection of tracer in RH12 showed
high fluorescein content, indicating that the fluid entering this zone
had a long residence time. This is confirmed by temperature logs which
show no thermal drawdown in this flow zone.
The injection points in RH12 were at 1790 m, 1980 m and 2170 m
(above zones which previous logging had shown to take a significant
proportion of flow out of the well). The sampling points in RH15 were at
2218 m, 2409 m and 2464 m, above flowing Zones 1, 2 and 3 respectively
(Figure 5). Each injection point was used to produce a separate
residence time distribution curve (RTD) at each of the three sampl ing
points. Thus, nine successfu 1 tracer runs were carried out, and the
experiment ended in the last week of July 1988. Conclusions here are
on ly preliminary, and the experiment wi 11 be reported fu lly at a later
date.
Initial evaluation of the data shows that the feature described as a
'short circuit' is a connection from the bottom of RH12 (below 2170 m) to
Zone 1 (2409-2225 m) in RHI5. There appears to be more than one pathway
involved, and more than 66% of the total production flow in RH15 is
obtained from this flow leaving the bottom of RHI2. It is interesting to
note that this conclusion is consistent with the analysis of microseismic
data recorded during the stimulation in Phase 2A, which shows an
al ignment of seismic events connecting the same zones. It is possible
that the structure causing the 'short circuit' in the Phase 2B/2C
reservoir was established in the large-scale hydraulic stimulation in
Phase 2A.
Examination of the hydraulic data during the flow path
characterisation period of constant injection flow rate indicates a
steady rise in injection pressure, and a steady fall in production flow
rate. Although the flow rate in RH15 dropped in the preceding period
(1 December 1987-mid-February 1988), this was accompanied by a drop in
injection flow rate, keeping constant water recovery and a steady RH12

147
wellhead pressure of 9.4 MPa. During the flow path characterisation, the
flow rate in RH15 declined at 0.18 lis/month and the RH12 wellhead
pressure rose from 9.45 to 9.8 MPa (0.07 Mfa/month). Thus the RH12-RH15
impedance rose from 0.65 to 0.72 MPa/kg/s, and water recovery in RH15
decreased from 67.4 to 63.2%. Flow in RHll was unaffected during this
period and remained at 2.3 lis.
One reason for th i sri se in impedance cou 1 d be that in order to
prevent comp 1 icat ions ari sing from tracer conta ined in product ion water
returning in closed loop circulation to the injection well, the entire
flow path characterisation experiment was carried out on open loop
circulation. To achieve this, the injection water was taken direct from
a nearby stream, although the back-up water storage reservoir was used on
a few occasions. The water from the stream contains fine particulate
material and samples have been taken on a regular basis to assess the
amount, particle size distribution and mineralogical composition of
material in the injection and production water.
The amount of particulate material retained by the reservoir during
circulation with fresh water appears from rough calculations using sample
data to be great enough to suggest that impedance may be increasing due
to this effect.
The one big difference between particulate material entering the
reservoir and that leaving the reservoir is the very high proportion of
organic matter in particulate material injected into RH12 (estimated as
up to 75%) and the complete absence of similar material in production
water. This suggests that either the organic matter is remaining in RH12
or is being broken down within the reservoir. Organic matter is able to
form very stable mats capable of trapping inorganic particles. If
positioned at flow entries into the reservoir, such mats would have the
potential to interrupt or impede the flow into the reservoir.
Since the end of the flow path characterisation experiment,
closed-loop circulation conditions have resulted in a levell ing-out of
reservoir impedance, thus confirming this interpretation of the cause of
the hydrau 1 i c drawdown as the retent ion of part icu 1 ate matter in the
reservoir.
5 CONCLUSIONS
The 1987 review of the technology (CSM, 1987a) considered the design
of a prototype commercial system, and concluded that the current
Rosemanowes HDR reservoir was very much smaller than the size which would
be necessary for the prototype. The evidence in Phase 2C strongly
confirms this conclusion. The upper limits of controlled circulation
flow rates in the current reservoir have been established, and valuable
experience and data have been gained concerning the onset of
microseismicity at high injection pressures and flow rates. The tendency
for reservoir growth, water losses and microseismicity to increase when
production flow rates exceed one fifth of the production flow rate
required of a prototype indicates that the reservoir is far too small.
Therma 1 mode 11 i ng, mi crosei smic ana lys i s and the pre limi nary resu lts of
the flow path characterisation experiment all indicate the presence of a
'short circuit'. Further work is necessary to establish the nature and
location of this short circuit, which may be a combination of a number of
preferential flow paths.
The optimum impedance of the Rosemanowes reservoir is higher
(0.6 MPa/kg/sec) than the target for a prototype, but if the suggestion
is correct that the prototype reservoir should be created by stimulation
of a number of segments in parallel, the prototype reservoir should have

148
the low impedance required of it (0.1 MPa/kg/sec). The size and number
of stimulation operations necessary will be the subject of the conceptual
design of the prototype in Phase 3A (1988-1990), aided by the experience
of such operations worldwide, and by the enhanced modelling capability
which has begun to be developed by the Project in Phase 2C.
6 ACKNOWLEDGEMENTS
The first year (Phase 28) of the progranune described was funded
jointly by the UK Department of Energy and by the Conunission of the
European Conununities. The second and third years (Phase 2C) were
entirely funded by the UK Department of Energy. The Camborne School of
Mines is grateful for this financial support. The author thanks both
sponsors for permission to publish this paper, and is grateful to the
staff of the Project who are responsible for producing the results which
are reported.
7 REFERENCES
Camborne School of Mines Geothermal Energy Project, (1987a). Current
status of HDR techno logy with reference to further development in
South West England. Report 2C-2, 198 pp. (ETSU Report G137-P11).
Camborne School of Mines Geothermal Energy Project, (1987b). Phase 2C
technical progress review, October 1986 - September 1987.
Report 2C-4, 103 pp. (ETSU Report G137-P12).
Camborne School of Mines Geothermal Energy Project, (1987c). Phase 28
tracer results. Report 28-40, 52 pp. (ETSU Report G137-P10).
Camborne School of Mines Geothermal Energy Project, (1988). Phase 28
final report. Report 28-45. (To be published by Pergamon Press Ltd
on behalf of the Conunission of the European Conununities).

-f~
.. 0
-----PHASE 28 ----_·.... 1·0------___ PHASE 2C -------_ -l
INCREASING
INJECTION FLOWRATE
STAGE!
OOWNHOLE
PUMP TEST
·1· ·1
STAGE n
FLOW PATH
CHARACTERISATION
I •
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FIGURE
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I STAGEm
t ':" 'J-~:'.....
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_ _._._._.,.,..._,_._--_. .......,
o 2000 1000 GOOO 0000 10000 12000 11000 16000 1 BOOO 20000 22000 21000
Tlmp 51ncp OS-AUG-19BS 00:00:00 (hours)
'A'S'O'N'olJ'F'M'A'M'J'J'A'S'O'N'OIJ'F'M'A'M'J'J'A'S'O'N'OIJ'F'M'A'M'J'
1985 1988 1987 1988
FLOWRATE SINCE START OF CIRCULATION
I I:
:'.
PHI2
" ....... . PHIS
RHII

150
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INJECTION FLOW RATE (l/s)
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PHASE 28
PHASE 2C
10
FIGURE 2 COMPARISON OF PHASE 26 AND PHASE 2C
RESERVOIR CHARACTERISTICS

:!! 50 11
Ci)
c::
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III
15 0 PHASE 2B VATER LOSS
PHASE 2C VATER LOSS
• 12
+ SEISMIC EVENT RATE
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o 5 10 15 20 25
RH15 PRODUCTION FLOWRATE (lIs)
FIGURE 4 STEADY STATE IMPEDANCE vs FLOWRATE

IS3
,';0 TEMP T 15725 3-AlJG-1988 10

154
EEC Contract nO EN3G-0090-UK(H)
COST MODELLING OF HDR SYSTEMS
MODELLING METHODS AND INTERIM RESULTS
R. Harrison, I. Coulson and P. Doherty
Faculty of Technology, Sunderland Polytechnic
S. Minett and N.D. Mortimer
School of Urban Studies, Sheffield City Polytechnic
Summary
Approaches to the cost modelling of Hot Dry Rock geothermal energy
systems are discussed and some initial interim results are
presented. Although these results are not definite at this stage,
they do indicate important features of the techno-economic aspects
of the system. Thus the size and the spatial arrangement of the
reservoir emerge as important issues, together with thermal
gradient and depth.
Introduction
A cost model of 'Hot Dry Rock' (HDR) geothermal systems is being
developed under contract to the UK Department of Energy and the
Commission of the European Communities. This paper describes the
completed first phase of this study.
During this phase, the basic structure of the model has been
defined and modelling approaches which are appropriate to the level of
knowledge of different areas of the system have been identified. The
development of the cost model is now in progress and the main features
of the modelling approach are described in this paper. An interim model
has also been brought together in order to obtain some indications of
sensitivities and so assist with the modelling process. This is
described here and some interim results are presented.
The ultimate objective of this study is to develop a full
engineering cost model of electricity-producing HDR systems which
includes all surface and sub-surface systems and components.
Once developed the aims of the model are to:
- estimate capital and operating costs of HDR systems and their
development over time.
- investigate the sensitivity of unit electricity costs to changes in
parameters defining the HDR reservoir and also those defining the
size and design of the power station.
- assist in the assessment of European HDR resources, as a function of
cost and location.
The costs of drilling deep wells and stimulating HDR reservoirs in
crystalline rocks are the major components of system costs. However,
very few deep wells have been drilled in crystalline rocks so that data

ISS
on rates of penetration and bit life are sparse. The sequence of
operations in re.ervoir stt.ulations is uncertain. As a result, cost
estimating is difficult and requires very careful study.
The optimization of HDi syste .. is another problem of great range
and complexity. In any resource 'setting' there are a number of
optimization issues with the following being the main ones.
- How deep .hould the system be?
- How large a re.ervoir should be developed?
- How many wells should be used and what flow should be circulated?
- What size and type of power station should be used?
Many solutions are possible and it is to be expected, in general,
that different optimal .olutions will be obtained in different
'settings'. There is also an important corollary in that re.ults of
studies, or of experience, obtained in one 'setting' may not be easily
transferable to another.
These considerations have major implications for the cost modelling
approach and the model structure, and the nature of these optimization
issues largely determines the main input variables. In addition, the
model (or group of models) must cover a range of different power plant
types.
UK 'settings' have low thermal gradients (20 to 40·C/km) whereas in
the Community as a whole gradients cover a wider range; this is an
important variable.
Technical uncertainties are also important in determining model
atructure through the input variables. In HDi systems the main technical
uncertainties relate to the size, the costs and the configuration of the
reservoir and to ita phy.ical performance. Thus many reservoir
parameters are input variables of the model.
The study is divided into two parts. Drilling costs are being
modelled by N.D. Hortimer at Sheffield City Polytechnic (as
aub-contractors to Sunderland). The overall model, which will
incorporate the drilling model as a component, is being developed by
Sunderland. This presentation is concerned with the overall model and
makes only brief reference, as appropriate, to the drilling studies.
Hodel Structure and Approaches
The overall model atructure has been defined and this is shown in
Figure 1.
The model dividea naturally into two main modules (although the
atructure of the computer program is more complex than this). The
aub-aurface module includes wells and reservoir and there will be a
number of optiona to cover different gea.etries and types of behaviour.
The surface module includes the pumpa, the water supply, the fluid
gathering syste., the power station and grid inter-connections. There
will be a number of alternatives here to cover different power station
types.
In each module the calculations will be broken down into two
aequential stagea:
- Physical calculations. These cover the perforaance of the syste.,
the calculation of paraaitic loads and the calculation of quanti tie.
relating to the ai.e of the ca.ponents e.g. turbine power, cooling
tower theraal load. The.e specify the plant and equi~nt.
Costing calculation.. These follow the physical calculations and
... ign costa to the components based upon design criteria input by
the u.er and .i.e parameters obtained from the physical
calculations.

156
The forms of the sub-models comprising each module depend upon the
level to which the engineering science of the component or sub-system is
understood. Computing requirements can also be a restriction and must be
taken into account. Thus the engineering science of the nature and
behaviour of the reservoir is not well understood and it is necessary,
initially, to base the modelling of this sub-system on simple theories
which describe
- the relationship between average reservoir temperature and overall
structure and geometry;
- the relationship between reservoir volume, circulation flow and
thermal drawdown;
- system impedance;
- the relationship between reservoir costs and volume and number of
stimulation operations.
As far as possible the current thinking of reservoir mode11ers
working in the UK programme is being incorporated as it becomes expressed
in a form which is suitable for this level of modelling.
The engineering science of the power plants, on the other hand, is
well understood and it is possible to carry out detailed thermodynamic
performance calculations on the cycles if data or fluid properties can be
obtained or can be calculated using some equation of state. In order to
assess the feasibility of carrying out these calculations as part of the
model, separate computer programmes have been prepared (as "Pascal"
codes) which analyse the two-stage-f1ash and the tri1atera1-wet-vapour
cycle (using n-pentane as the working fluid) (Attewe11, 1988).
The analysis of organic Rankine cycle options is more difficult.
The number of options is very large; many fluids have been suggested,
mixtures of fluids are possible and boiler pressure also alters the
overall performance of the cycle. It is possible to carry out detailed
thermodynamic calculations and search the results for the optima.
However, this is a major undertaking and it is more appropriate to
construct an outline model based upon the loss of 'availability' in the
machines and at the heat exchanger and the condenser. This latter method
can be adopted for all of the cycle options which will be offered in the
model by choosing appropriate factors which represent the loss of
availability. This is the approach which is being used in the model.
Cost Methods and Data
The major cost component in the sub-surface systems is the cost of
drilling. The basic costing methods established by previous modelling
studies have been extensively validated now and form a basis for these
current studies. However, the results are dependent upon basic drilling
data, such as rates of penetration, which are relevant to the formations.
The costs of stimulations are also likely to be a major item and may
dominate the sub-surface costs in shallow systems. The problem here lies
more with the technical definition of the operation involved and the
quantities of materials to be used.
The surface plant is a minor element in the costs of deep systema
but takes on more significance in shallower systems. The costing of
surface plant represents a problem as the information which is available
from other studies is often sparse. Attempts are being made to collect a
body of consistent information so that 'reliable plant costs can be
obtained.

IS7
Interim Model
At this interm.diat •• tage of the modelling. a preliminary complete
model ha. been brought together to produce re.ult. to a •• i.t in the
proc ••• of model dev.lopment.
Th •• y.tem studi.d i. a. follow.:­
- On. doublet
- El.ctricity producing.
Th. main d.parture. from the previous UK a •• e.sment by Shock (1986)
are:-
- Drilling co.t. are ba.ed upon d.tailed .tudie. by Sheffi.ld City
Polyt.chnic draving upon a range of data on deep drilling in
granit •••
- Th. co.t. of .timulation u.ing vi.cou. gel. i. included.
- R ••• rvoir g.om.try i. ba •• d upon multiple .timulation. di.tributed
up the vell.. Thi. aff.ct. both co.t. and .y.tem performance.
- R.s.rvoir imp.danc. varie. a •• n inver.e function of differential
pr ••• ur.. Thi •• eem. to refl.ct the b.haviour of the int.rmediate
d.pth .y.tem curr.ntly b.ing te.ted by CSM in the UK.
- Pow.r pl.nt p.rformanc. i. e.timated on the a •• umption of dual flash
.t.am. and ba •• d upon the d.tailed calculations mentioned above.
- T.mperatur. dr.vdown in the reservoir is relat.d to the flow through
it.
A .ummary of the main a.sumptions is as follows. These ar.
preliminary .s.umption. at this stagei many of them are under sctiv •
• tudy and ar. lik.ly to chang. or to represent only one among several
option. in the final mod.l.
Reservoir-Size, Geometry and Temperature
It i •• ssum.d that the r.servoir is composed of s.parate zones vhich
have b •• n produced by multiple stimulation .vents along the inclined
•• ction of the vell. Th ••• zones ar. cylinders vith vertical axes and
vith the following dim.n.ions:
h.ight • 400 m
radius • 220 m 6
volume • 60 x 10 m 3
Th. mod.l c.n .ccommod.t. betwe.n 1 .nd 5 stimulations.
Th. .rrang.m.nt for five .timulations in • vell vhich is deviat.d at
.n .ngl. of 30· to the v.rtic.l is .hown in Figur. 2. The r.s.rvoir
.xt.nd. ov.r • v.rtic.l distanc. of 2 ta. It is r.cognis.d that this ia
.n unf.vour.bl. g.om.try vhich h.s • significant .ff.ct on the average
r ••• rvoir t.mp.r.tur.. How.ver. it has be.n used at this stag. in order
to •••••• this .ff.ct. Alt.rnativ. res.rvoir geometries are being
.xamin.d .nd a numb.r of optiona vill b. offered in the final model.
Th •• ff.ctiv. r ••• rvoir temp.r.tur. is calculated by first taking
the .v.rag. of the t.mp.r.tur.s at the top and bottom of the r.s.rvoir.­
Thi. i. th.n incr •••• d by 2.5% to t.k •• ccount of the .nhancemant due to
high.r flow. through the lov.r zon.s. In the final model this
approximation vill b. r.plac.d by • more pr.cis. calculation of the vay
flow. divide b.tw •• n the •• p.rat. zon.s.

158
Reservoir Impedance
It has been assumed that reservoir impedance falls as differential
pressure is increased.
IS - B - APs
where P s - differential pressure acros!lthe reservoir (MFa)
IS - reservoir impedance (MFa.kg s)
A and B are constants which characterise the impedance.
The dependance of pressure upon flow m f
is then given by:­
P - 5
m f
B
(1 + m f
A)
Two different impedance characteristics have b~fn used - see Figu!f 3.
These give impedances at 7.5 MFa of 0.1 MFa.kg sand 0.01 MFa.kg s
respectively.
The characteristics of the impedance is an important issue and a
number of approaches are being developed for inclusion in the full model.
Wellhead Temperature
The effect of heat loss from the fluid to the cooler rock
surrounding the well bore is taken into account. This is a transient
effect and quickly stabilises to a temperature drop of a few degrees
which is effectively constant over the lifetime of the resource. This
effect varies with the flow. Increasing flow reduces the temperature
drop and increases wellhead temperature.
Temperature Drawdown
It is assumed that effective reservoir temperature draws down with
time following the error function approach. This is calibrated by
choosing an effective reservoir are!lwhich gives 5% temperature drawdown
after 25 years at a flow of 75 kg.s for 5 stimulations.
Power Plant and Net Energy
Double flash steam power plant has been assumed. operating at a
condenser temperature of 37.8°C. The performance is corrected for
parasitic power losses in the cooling towers - see Figure 4. Aa the
wellhead temperature draws down the power output of the plant falls. The
model assumes that the output always follows the relationship in
Figure 4. Thus the performance is not penalised specially because the
power plant is operating off the design conditions. Some detailed
calculations have indicated that this is valid for flash steam plant in
the vicinity of the design conditions. However. it is not knoWn whether
or not this behaviour is valid for wellhead temperatures which may be 20%
or more from design values.
Net energy (HE) is calculated in each year by calculating the power
output of the plant as described above and subtracting the power
consumption of the well pumps. The pump power does not vary over the
life of the scheme.

159
Costa
-----Coata ara estimated in the following categories.
Capital Costs
- Costs 01 vells and stimulations (these have been calculated by
Sheffield and are shown in Figure 5).
- Coats of injection pump.
- Costs of aurface plant (these are based upon plants costs in Shock
(1986) - aee Figure 6».
- Engineering etc.
Interest During Construction (5%)
- It is assumed that drilling and stimulations take two years and that
one year's interest is paid on the entire sum before the scheme
begins to produce pover.
Operating Costs
- General maintenance
It is assumed that the annual plant maintensnce amounts to 2% of
capital and that the sub-surface system requires no general
maintenance.
- Major maintensnce and renevals
The surface system requires no major renevals. The sub-surface
aystems require a major vorkover after 10 years.
Discounted Unit Costs
The discounted unit coats (DUC) are calculated as follows.
PWC
DUC -
n
~ NE
~ (I + l)j
j-1
PWC - present vorth of all costs listed above.
NE - net energy produced in year j
j
r - discount rate
Results and Observations
In these runs it has been necessary to limit the cases to vellhead
temperatures of 300·C or less. Thus at a thermal gradient of 80°C/km no
results are shown for depths greater than 4 km because of this
reatriction.
The results are shown in Figurea 7 to 14.
A number of observations can be made from these results.
- The limitation of a maximum vell head temperature of 300·C means
that only shallow systems can be modelled in the high gradiant
cases.
- For a one stimulation reservoir (Figure 7) unit coata are high for
all of the depths and gradients shown. indicating that. although the
cost of the scheme vould be low for 1 stimulation. the amount of
heat extracted from the reservoir is insufficient to compensate for
the cost of the scheme and consequently the discounted unit costs
are high. As the reservoir volume is increased from one
stimulation. the cost of the scheme increases but the amount of heat
extracted from the reservoir also increases. This causes the unit
cost for all of the aeven different gradient cases to fall.

160
However. the results indicate that there is an optimum reservoir
volume beyond which the unit cost for the various cases start to
rise. Figures 7 to 13 suggest an optimum number of stimulations of 2
or 3. These are not firm conclusion at this stage because many
aspects of the model. which yet need to be developed. may change the
position of the optimum.
- The effect of increasing the number of stimulations is more
pronounced at low thermal gradients than it is at high thermal
gradients. With thermal gradients of 70 and 80°C/km it may be
possible to obtain unit cost of between 2 & 3 p/kWh with three
stimulations.
- Figure 14 shows the effect of flow variation for the 3 km and
80°C/km case. Low flows (25 kg/s) restrict the outputs from larger
reservoirs and. as the number of stimulations is increased. all this
does is add to cost so unit costs rise. Large flows (125 kg/s)
'saturate' the small reservoirs, drawing them down quickly but
giving high parasitic losses and water costs. As the number of
stimulations is increased more electricity is produced. parasitic
loads fall and unit costs fall.
Clearly unit costs are lower in the higher gradient settings. but
not radically so. Thus in Figure 10 costs lie between 2.5 and 4p per kWh
for thermal gradients between 80°C/km and 40°C/km and depths between
4 and 8 km.
At shallower depths with the higher gradient cases a number of
effects limit cost reductions:
- Drilling costs are falling but stimulation costs. at about £2 x 10 6
per stimulation. are a large fixed element.
- Reservoirs extend over large vertical distances so that for the same
bottom hole temperature the top of the reservoir and hence the
average temperature is lower in the high gradient case compared with
the low gradient case.
- Buoyancy drives are smaller in the shallower systems and hence
parasitic loads are greater.
As yet no firm conclusions can be made regarding any of these
issues. There are a number of difficult areas in the model which need to
be examined further before this will be possible.
References
Shock. R. (1986). An economic assessment of Hot Dry Rocks as an energy
source for the UK. ETSU R34.
Attewell. B. (1988). Part load performance of flash steam power plant.
Sunderland Polytechnic. Energy Workshop. Working paper.

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162
RESERVOIR GEOMETRY
~
- VERTICAL CYLINDERS
- VOLUME/CYLINDER = 60 M m 3
- HEIGHT/CYLINDER = 400 m
- RADIUS/CYLINDER = 220 m
DEPTH FROM SURFACE
I
I
I
Figure 2: Schematic of reservoir geometry

163
0.20
"
'"" 0.18
01
0.16
~
0
Cl 0.14
~
Q)
0.12
------~--------~~.
0
t::
0.10
0
'b
Q)
Q.. 0.08
.§
0.06
.!;:
0
~ 0.04
Q)
(/)
0.02
Q)
Q::
~~.
• I
•
0.00
0 2 345 6 7 8 9 10 11 12 13 14 15
Differential Pressure (MPa)
o 0.1 MPa/(Kg/s) 0 0.01 MPa/(Kg/s)
Figure 3: Reservoir impedance v differential press impedance stated at
7.5 MFa
350.-----------------------------------------~
300
o (/)
E -g
Q) 0
:5 ~
o 0
111 .c
01 ....
250
200
150
100
50
100 150 200
Tgi (C)
+ We = Wgross - Wfp x Wet
250 300
o Wnet = We- Wet
·_ ,-... .. :J7.c_
--..
Figure 4: The variation of Wnet, We and Wet with Tgi for the dual flash
.te.. • y.tem, a. reported by Khalifa & Rhodes
350

164
>.
;!::
30r-------------------------------------______ -.
27
24
21
18
15
12
9
6
3
OT------.-------,------r------.------~----~
3 4 5 6 7 8
Depth (km)
+1 x2030405
Figure 5: Drilling & stimulation cost for 1,2,3,4 & 5 stimulations
4000.-----------------------------------------------.
u 3600
0
n..
0
u
~
3200
~ 2800
...........
w
c 2400
.....
2000
c
.S2
n..
...... 1600
0
.....
III
1200
0
u
800
0
;!::
400
n..
0
u
0
100
Dot. .. ropart.. by Shack

165
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....... 14
~
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~
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0
u
- 8
'-c:
::>
'tl 6
~
c:
::l 4
0
IJ
,~
(:)
2
0
2
,-,."
Figure
7:
.3 4 5 6 7 8
Well Depth (km)
Discounted unit cost v well depth - Reservoir impedance -
0.1 MFa/(Kg/s) at 7.5 MFa differential pressure
9
16
....... 14
~
~ 12
~
-I/)
10
0
u
- 8
'-c:
::>
'tl 6
~
c:
::l 4
0
IJ
,~
(:)
2
0
,-
2
Figure
.3 4 5 6 7
Well Depth (km)
8
9
8:
Discounted unit cost v well depth - Reservoir impedance
0.1 MFa/(Rg/s) at 7.5 MFa differential pressure

166
16
14
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.......
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(.)
10
....... 8
'-
:§
'\) 6
~
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:J
0
4
(,)
,~
c:::,
2
0
2
3 4 5 6 7
Well Depth (km)
8
9
.s SUmulatlon.
Figure
9:
Discounted unit cost v well depth - Reservoir impedance
0.1 MFa/(Kg/s) at 7.5 MFa differential pressure
16
'"'"
~
~
~
.......
11)
0
(.)
14
12
10
~ 8
:§
'\) 6
~
c:
:J 4
0
(,)
,~
c:::,
2
0
2
f SUmulatlon,
Figure
10:
3 4 5 6 7
Well Depth (km)
Discounted unit cost v well depth - Reservoir impedance -
0.1 MFa/(Kg/s) at 7.5 MFa differential pressure
8
9

167
16
-14
~
~ 12
~
.....
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c
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10
..... 8
'- :§
"t) 6
~
c:
::;,
c
\J
4
,~
Cl
2
0
2 3 4 5 6 7
Well Depth (km)
8 9
Figure 11: Discounted unit cost v well depth - Reservoir impedance
0.1 MFa/Kg/s} at 7.5 MFa differential pressure
SYMBOLS FOR FIGS 7 - 11
Thel'lNll gr8dlent I
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20
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169
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170
E.E.C. Contract No. EN3G -
0090-UK (H)
HOT DRY ROCK GEOTHERMAL ENERGY COST MODELLING:
DRILLING AND STIMULATION RESULTS
N.D. Mortimer And S.T. Minett
Sheffield City Polytechnic,
United Kingdom.
SUMMARY
The cost of creating a reservoir, by drilling and stimulation,
likely to form a significant fraction of the total cost of
constructing any proposed Hot Dry Rock (H.D.R.) geothermal energ
system. The cost of these operations can be estimated using a
cost model that has been developed to assist the management of
H.D.R. geothermal energy research and development programmes.
This paper describes the basic features of the drilling and
stimulation cost model, explains the essential modelling
assumptions and presents the latest results derived from the cos
model.
INTRODUCTION
Work began on the development of a H.D.R. geothermal energy
system cost model in the United Kingdom in October 1986.
Modelling of the cost of reservoir creation, by drilling and
stimulation, has been conducted at Sheffield City Polytechnic,
whilst modelling of the cost of surface plant and the performanc
of the complete 8.D.R. geothermal energy system has been
undertaken at Sunderland Polytechnic. The model, which is
formulated as a spreadsheet programme written on SMART software
for an Olivetti M24 personal computer, can either be run as an
integrated package or as individual components. Although
preliminary versions of the cost model have been completed,
development will continue until June 1990. This work is funded
by the United Kingdom Department of Energy, through the Energy
Technology Support Unit, and by the Commission of the European
Communities.

171
Figure 1
The Hot Dry Rock Geothermal Energy Cost Hodel
HAIH MODEL
HAifl TITL.E
SPREADSHEET
DRILUHG AND STIHULA traM
COST MODEL
~
~ ~
WELL. DATA
IIOR D01
SPREADSHEET
~ ~
RESERVOIR
G~ETRY
SPREADSHEET
'"
I
I
DATA TRAIISFER
SPREADSHEET
!
TlI1E EUl1£NT
SPREADSHEET
~
!
COSTifiG
SPREADSHEET
~
OTHER
1
SPREADSHEETS
FOR HAIN HODEL
~
RESERVOIR
STIHULATIOII
SPREADSHEET
~
1
~ ~
KlDEL
OUTPUT
SPREADSHEETS
'"
ADOlTIOIIAL
COSTS
SPREADSHEET
~
I

172
An essential aim of any cost model is to provide a convenient
method of predicting the costs of any project under a chosen set
of circumstances. This assists project management by offering a
means of conducting design optimisation studies and performing
sensitivity analysis. Cost models for energy systems are also
appropriate for resource analysis since they can be used to
classify the energy available in economic terms or cost bands.
The cost model for H.D.R. geothermal energy systems has been
designed with these applications in mind. To achieve this, the
model must be relatively flexible and be able to accommodate a
suitable range of diversity. In particular, the final version of
the drilling and stimulation cost model will be able to account
for the affect of varying;
- depth
- geothermal gradient
- geology, in terms of subsequent rates of
penetration and bit life
- borehole breakout
- thickness of sedimentary cover
well design, including configuration,
angle of deviation and casing programme
- reservoir characteristics, such as volume,
shape and number of stimulated zones.
Other factors that will be taken into consideration include the
type of logging and coring programmes selected and the general
drilling market conditions which affect the cost of drilling
supplies and services.
MODEL FEATURES
In order to present an easy-to-use standard format, the model has
been developed as a spreadsheet pro9ramme. This is based on
SMART software which runs on an Olivetti M24 personal computer.
Individual components of the model are written as separate
spreadsheets. The relationship between the drilling and
stimulation cost spreadsheets and the rest of the model is

173
Figure 2 Example or Well Data and Reservoir Geometry
Drilling Conditions:
Rate or penetration = 5.07 m hr- 1
Bit lire = 15.99 hr
KOP I 3925m -
KOPI4325m -
ABP:4807m -
ABP:5207m -
TVDI5600m -
TVD:600Om -
Reservoir consisting or
3 stimulated zones, each
comprising or rull
horizontal discs, with
individual volumes or
6 x loT m 3 , separated by
50 m along the wells
916m
~ PRODUCTION WELL
X'
400m
~ INJECTION WELL

174
demonstrated schematically in Figure 1. When the model is run as
an integrated programme, the user can enter certain design
parameters for the particular system under consideration by means
of a number of spreadsheets which include the well data and
reservoir geometry spreadsheet. These common input spreadsheets
contain the basic design parameters required for estimating
drilling and stimulation costs. In many instances, these
parameters can consist of either user-defined or default values.
An indication of the type of information needed is given in
Figure 2 which also summarises a set of design parameter values
that form the basis of an example used throughout this paper.
The details of this example are described later.
The cost of drilling and stimulation depend, fundamentally, on
the time taken to complete these operations.
estimation is a central feature of the model.
Consequently, time
In the latest
version of the model, eighteen separate time elements are
specified and these are summarised in Table 1.
The drilling time
elements are calculated in the time element spreadsheet and the
stimulation time element is derived from the reservoir
stimulation spreadsheet.
Costs are divided into twenty six
separate groups and these are summarised in Table 2.
drilling cost groups are obtained from the costing spreadsheet
and all stimulation cost estimating takes place in the reservoir
I
stimulation spreadsheet. The cost estimating procedure in either
of these spreadsheets consists of combining the design
parameters, the estimated time elements and other, subsequently
derived technical information with appropriate price and charge
data obtained by means of a regular survey of drilling supply and
service companies.
The
Such surveys have been conducted for a number
of years and sufficient data has been collected to reflect the
substantial fluctuations that have occurred in the drilling
market in recent times.
In addition to drilling and stimulation costs, other sub-surface
system capital and operating costs can be calculated by means of
the additional costs spreadsheet. These include the costs of
initial studies, casing surveys and periodic reservoir testing,

175
Table 1
List of Time Elements
Element
Time (hours)
Rotating 2417 + 384
Reaming 283 + 52
Coring
Tripping 1640 + 398
Connecting 215 + 23
Bottom Hole Assembly 138 + 59
Circulating 71 + 25
Logging 908 + 246
Casing 319 + 91
-
Cementing 306 + 102
Waiting on Cement 75
-+ 65
Drilling on Cement 64 + 41
Mishap 407 + 208
Rig Maintenance 37 + 14
Well Testing 73 + 5
Wellhead Installation 55 + 19
Stimulation 1680 + 168
Miscellaneous 557 + 98
-
-
-
-
-
-
-
-
-
-
-
Total 9244 + 693
-
6 km doublet with 3 stimulations and drilling conditions similar
to Roeemanowes, Cornwall, United Kingdom.
-
-
-
-

176
Table 2
List of Cost Groups
Group Cost (.£:1988)
Civil Engineering 226,000 + 339,000
Rig Moves 92,000 + 24,000
Payment to Drilling Contractors 1,922,000 + 226,000
Directional Drilling 320,000 + 32,000
Surveying
Downhole Motors
Stabilisers and Reamers
ROCK Bits
Drilling Muds
Casing
Wellhead
Cementing
Christmas Tree
Logging
Coring
Testing
Fuel and Lubricants
Water Supply
Supervision
Transport
Abnormal Drillstring Wear
Inspection
Fishing Tools and Services
Labour
Stimulation
Miscellaneous
66,000 + 5,000
428,000 + 49,000
952,000 + 108,000
831,000 + 121,000
792,000 + 280,000
1,038,000 + 56,000
108,000 + 38,000
301,000 + 90,000
432,000 + 153,000
289,000 + 20,000
84,000 + 10,000
194,000 + 21,000
37,000 + 4,000
450,000 + 32,009
112,000 + 8,000
233,000 + 51,000
168,000 + 36,000
280,000 + 155,000
75,000 + 5,000
6,004,000 + 901,000
884,000 + 160,000
Total
16,318,000 + 1,086,000
6 km doublet with 3 stimulations and drilling conditions similar
to Rosemanowes, Cornwall, United Kingdom.

In
seismic activity monitoring system construction and operation,
and eventual decommissioning of the injection and production
wells. Although the costs of these items are relatively small in
comparison with drilling and stimulation costs, they are
included to ensure that the model completely describes the
system under consideration. The final part of the model consists
of gathering together the results of all the spreadsheets and
presenting them in the model output spreadsheets.
A BASIC EXAMPLE
The essential assumptions incorporated into the model can be best
described by considering some illustrative results. For this, a
basic example must be selected. The design parameters of this
example have already been given in Figure 2. This example refers
to an injection well and a production well, forming a commercial
H.D.R. doublet system, drilled to a total vertical depth of 6
kilometres in granite with similar drilling characteristics to
those found at Rosemanowes in Cornwall in the United Kingdom.
There is no sedimentary cover and it is assumed that the
phenom~mnon of borehole breakout is not encountered. The
reservoir is created between the inclined sections of the wells
which are drilled at an angle of 30 0 from the vertical for 1300
metres and separated from each other by a vertical distance of
400 metres. The reservoir is formed by three stimulated zones
which give a total reservoir volume of 1.8 x 10 8 cubic metres.
The wells are completed with 7 inch (0.1778 metre) liners in 8.5
inch (0.2159 metre) holes. The model is formulated so that these
and other design parameters can be changed within practical
limits by the user.
ASSUMPTIONS AND RESULTS: TIME ELEMENTS
Subsequent estimates for the time elements of this example of a
doublet system are given in Table 1. It will be seen that these
estimates are presented in the form of averages with associated
standard deviations which have been generated by a single
propagation of errors procedure. This assumes, of course, that

178
all the values used in the calculations follow normal probabilit
distributions. The same assumption and procedure is adopted in
the current version of the model for evaluating the accuracy of
the cost group estimates. Future versions of the model may
incorporate more sophisticated methods of assessing the accuracy
of results.
A further illustration of the estimated time elements given in
Table 1 is provided by Figure 3.
This emphasises the conclusion
that the majority of the time involved in creating this H.D.R.
doublet system is accounted for by just three elementsT rotating
time, tripping time and stimulation time.
The rotating time is
the time taken by the drilling bit to actually penetrate the
rock.
The tripping time is the time involved in replacing a
drilling bit after it has reached the end of its useful life. A
defined here, the tripping time consists of the time required to
remove and disconnect the drill string from which the drilling
TRIPPING: 68 days
18%
ROTATING:
26%
STIMULATION: 70 days
18'j;
OTHER: 146 days
38%
Total Time = 385 days
6 km. doublet with 3 stimulations drilled under similar conditions to those
at Rosemanowes, Cornwall, United Kingdom.
Figure 3
Breakdown of Time Elements

179
bit is suspended in the hole, and re-connect and lower the
drill string with a new drilling bit attached so that drilling
can recommence. Both the rotating time and the tripping time
depend on the rate of penetration during drilling. The tripping
time also depends on the life of the drilling bit. Assuming best
drilling practice, both the rate of penetration and the bit life
tend to be functions of the drilling characteristics of the rock
formations encountered.
Measurements of the rate of penetration and the bit life can be
obtained from the bit records of drilling reports. These are
very important sources of information for drilling cost
modelling. Previous work involving the analysis of drilling
reports from a variety of sedimentary rock regions has indicated
that rates of penetration decline with increasing depth until
compact sedimentary or basement rock is encountered, whereupon
relatively constant rates of penetration are observed (Ref. 1).
Consequently, at relatively shallow depths, up to about 3
kilometres, in sedimentary rock regions, the rotating time
increases in a non-linear manner with depth. In fact, this
variation can often be described by one or more exponential
functions. This has a fundamental influence on the variation of
total drilling costs with depth since the rotating time affects
the total drilling time which, subsequently, governs the payments
to drilling contractors. This is an important cost group, as
will be explained later.
The tripping time also increases in a non-linear manner with
depth. Analysis of bit records from drilling reports indicates
that, as a first approximation, the bit life can often be treated
as a constant. Even if the rate of penetration is also constant,
the depths from which trips are made accumulates as an arithmetic
series. If the rate of penetration is declining exponentially,
as likely in uncompacted sedimentary rocks, the non-linear
increase in tripping time with depth is reinforced further.
This enhances the effect of rotating time on the variation of
total drilling costs with depth. The combined effect of both the
rate of penetration and bit life through the rotating and

180
tripping times is very influential on drilling costs because
other factors are either less signifcant or vary linearly with
depth. Hence, because the majority of drilling occurs in
sedimentary rocks, for hydrocarbon exploration and production,
the statistically-derived variation of drilling costs with depth
is normally expressed as some form of exponential equation.
However, this is not wholly appropriate for drilling in hard
rocks, such as granite.
Analysis of bit records for the few deep boreholes that have been
drilled in hard rocks clearly demonstrates that, provided the
drilling engineer maintains constant operating conditions, the
rate of penetration is fixed and does not vary significantly with
depth (Refs. 2 and 3). This is illustrated in Figure 4 which
6000
5000
f 4000
~
~
... 3000
2000
...
.....
....•
....
I Cambome; ~Hl I
2 Cambome; ~H12
J Cambome; ~H15
, Fenton Hill; ClT-2
5 Fenton Hill; EE-l
6 Fenton Hill; EE-2
7 Fenton Hill; EE-J
I Silj an; Oravberg-l
9 Kola; SO-J
10 Soultz-aous-P"orets; OPICI
1000
o 2
J 5
6 7
8 9 10 II 12
TOTAL VERTICAL DEPTH (kII)
Figure 4
Variation of Rotating Times with Depth for Hard Rock Drilling

181
shows the variations of the rotating time with depth for the
three Rosemanowes boreholes, the four Fenton Hill boreholes in
New Mexico, U.S.A., the Gravberg borehole in Sweden, the Kola
borehole, U.S.S.R and the Soultz-sous-Forets borehole in France.
Since rotating times are plotted against vertical depth in Figure
4, it was necessary to make appropriate adjustments to data from
any deviated sections of these boreholes. Apart from this, the
data were not otherwise adjusted. The derived values of constant
rates of penetration for each borehole are summarised in Table 3.
It should be noted that these rates of penetration only apply to
those sections of the boreholes which were drilled in hard rock.
Constant rates of penetration were not observed in the overlying
volcanic and sedimentary cover of the Fenton Hill boreholes (0 to
730 metres) and the Soultz-sous-Forets borehole (0 to 1400
metres), respectively. In addition, a marked change in the rate
of penetration occurs at 4 kilometres in the Gravberg borehole.
This was due to a change in operating conditions, namely an
increase in mud weight, which was necessary to control the effect
of borehole breakout at this depth.
Analysis of the bit records for these nine hard rock boreholes
also suggests that there is no consistent variation of bit life
with depth. For modelling purposes, the characteristic bit life
in a given rock formation can be treated as fixed and the derived
values for the boreholes considered are also summarised in Table
3. The assumption of a constant rate of penetration and a
constant bit life in the model means that the variation of the
simulated total drilling time with depth only departs slightly
from a linear trend as depth increases. This is shown in Figure
5 which gives results for single vertical wells drilled in
granite with similar properties to that observed at Rosemanowes.
In this example, it is assumed that no borehole breakout is
encountered and increasing temperature which depth has no
influence on drilling times. Future versions of the model will
enable these assumptions to be altered. Similarily, the effect
of changes in the assumption of a constant rate of penetration
will be accommodated in future versions of the model. However,
at present, the assumed constant rate of penetration ultimately
results in an almost linear variation of drilling costs with
depths, as will be seen shortly.

182
5000
III
to
;:I
0
.c
r.J
::e::
H
f-<
Cl
:z;
H
...J
...J
H
I:t:
Q
...J
.0:
f-<
0
f-<
4000
3000
2000
1000
t/
f/f/
o 3 4 5 6 7 8
TOTAL VERTICAL DEPTH (kmJ
Single vertical wells in drilling conditions similar to those at Rosemanowes,
_Cornwall, United Kingdom.
Figure 5 Variation of Simulated Total Drilling Time with Depth
ASSUMPTIONS AND RESULTS: COST GROUPS
Estimates of the cost groups for the basic example of a H.D.R.
doublet system adopted here were shown previously in Table 2. A
further illustration of these results is given in Figure 6 which
emphasises that the majority of costs ~e accounted for by just
five groups: stimulation costs, payments to drilling contractors,
the cost of stablisers and reamers, casing costs and the cost of
drilling mud. All these costs are calculated using quotations
obtained from the 1988 drilling supply and service company survey
which reflects the relatively low drilling market that existed at
that time. In the present version of the model, some cost groups

183
PAYMENTS TO DRILLING
CONTRACTORS: £1.922m
12%
STIMULATION:
37%
STABILISERS AND
REAMERS: £0. 952m
6%
ROCK BITS: £0.831m
5% "
DRILLING" MUDS: £0.792m
5%
OTHER: £4. 179m
29%
Total Cost = £16.318m
6 km. doublet with 3 stimulations drilled under similar conditions to those
at Rosemanowes. Co mwall. Un i ted K ingdOlll.
Figure 6 Breakdown or Cost Groups
are based on a fairly detailed job specification. An example of
this is the casing costs which are calculated by combining
information on casing diameters, grades, unit weights and lengths
from the chosen well design and geometry with appropriate quoted
prices. Other cost groups, such asthe cost of stabilisers and
reamers, are estimated by means of simple algorithms derived from
suitably adjusted costings from actual boreholes and independent
prognoses (Refs. 4 and 5). As an alternative to these two types
of modelling method, all remaining cost groups are calculated
using a combination of both approaches. For example, payments to
drilling contractors are based on the detailed simulation of
drilling time, described earlier, and a relatively simple

1M
30
~
~
~ 20
~
~
~
a
u
~
z
~
~
~
~
~
c
~
< 10
b
~
I 1
L
~
I
•
o
Figure 7
r 2 3 5 6 7 8
TOTAL VERTICAL DEPTH (km)
Comparison of the Variation of Total Drilling Costs with Depth
expression relating drilling rig day rate charges to depth. This
expression is derived from quotations from drilling companies.
In future versions of the model, drilling rig requirements will
be specified in more detail than just depth capability and rig
day rates will reflect whether the rig is hired or owned as part
of an integrated H.D.R. geothermal energy operation.
Another cost group which will be evaluated in more detail in
future versions of the model is the cost of drilling muds. At
present, an algorithmic approach is adopted which only accounts
simply for increasing temperature with depth. This incorporates
the assumption that, if the bottom hole temperature rises above
about 120 0 (at a depth of approximately 3.5 kilometres at
Rosemanowes), special lubricant must be used and mud cooling

18S
equipment must be installed. In future versions of the model,
the mud programme will be specified more thoroughly so that the
effects of increasing temperature and borehole breakout can be
simulated more reliably. The assumptions for the drilling mud
costs incorporated in the current model result in a discontinuity
in the variation of total drilling costs with depth.
This is
demonstrated in Figure 7 which shows the total cost of drilling,
without stimulation, for H.D.R. doublet systems at Rosmanowes.
The costs are given in 1985 £ sterling values, reflecting a
relatively high drilling market situation, to enable direct
comparison with the more frequently quoted drilling cost curve
for H.D.R. doublet systems (Ref. 6). This curve is adapted from
u.s. statistics on deep drilling in sedimentary basins. It can
be seen that this curve is, in fact, a relatively slowly varying
function incorporating an exponential term.
Figure 7 indicates
that this curve does not differ substantially from the nearly
linear trend generated by the model.
The latest estimates of the costs of both drilling and
stimulation are presented in Figure 8. These costs are given in
1988 £ sterling values and results are provided for a range of
depths between 3 and 8 kilometres and for up to five stimulations
in the reservoir region. As indicated earlier the cost of
stimulations makes a very important contribution to the total
cost of creating a H.D.R. doublet system. The costs included in
Figure 8 are based on an initial stimulation programme which was
first considered for preliminary costing purposes in July 1988
(Ref. 7). As shown in Figure 9, the main direct costs of the
stimulation programme are the cost of the stimulation materials,
including gel, and the cost of perforating the liner in the
reservoir region with explosive shots. Although the gel which
was specified for this prelimenary attempt at costing
stimulations was suitable for the pressures likely to be
experienced, it was known that the gel would not be able to
withstand the high temperatures encountered in deep H.D.R.
doublet systems. Consequently, the present stimulation costs
only act as an initial guide and more realistic stimulation
programmes will have to be designed, cos ted and incorporated into
later versions of the model.

186
30
(l)
(l)
0'
.,.. E
~
0
()
...J
...:
t;
E-<
25
20
15
10
N~be, ----.-- of Stimulat!oM .-----.:::::=--::. ----.- ----.
3.--·----·--·--·:----·
-
I ----:-------=.---------.
=-----.-----
5 .---:----.
4 .-------.--
.----.
2 .----.----.-----.
o
4 DEPTH (km)
::3~~~--~~-------;~T~O~TA~L~VE~RT~I~C~A:L~~~~~-------
7 8
Figure 8 _ Variation of
5 6
__ ~~~~~~D~r~i~l~l~i~n~g~a~n~d~S~t: timulation
_ __ ~~~~C~o~s~t~s~w~i~t~~~plt Depth
GEL: £3.79Dm
78"
3 stimulati on zones,
7
each with a volum
Total Direct e of 6 x 10
Costs. £4.89601
Total Costs (including extra rig time) £6.00401
OTHER: £0.55Sm
11"
PERFORATING:
11"
£0.54&
Figure 9 Breakdown of the Direct Costs of Stirn ulation

187
Table 3
Rates of Penetration and Bit Lives for Hard Rock
Drilling
Site
Rate of Penetration
(metres per hour)
Bit Life
(hours)
Rosemanowes, U.K.
5.1 + 1.2
16.0 +
3.5
Fenton Hill, U.S.A.(~0.7
km)
2.8 + 0.8
20.9 +
9.3
Siljan, Sweden (~4.0
km)
3.2 + 0.9
30.2 +
21.0
Soultz-sous-Forets,
1.6 + 0.6
21.3 +
11.4
France (~1.4
km)
MODEL DEVELOPMENT
In order to assist the advance of H.D.R. geothermal energy
research and subsequent project management, the drilling and
stimulation cost model will continue to be developed and refined.
Particular attention will be given to the implications of:
- increasing bottom hole temperatures
- borehole breakout
- different drilling mud programmes
- technological drilling improvements
- integrated drilling supply and service arrangements
- alternative stimulation programmes
This should enable the model to be applied to a wide range of
H.D.R. geothermal energy resource sites. It is hoped that such
further work can incorporate the progress in knowledge achieved
by the various research teams engaged in H.D.R. and related areas
of study, such as the H.D.R. geothermal energy projects at the
Camborne School of Hines, Urach and Soultz-sous-Forets, and the
K.T.8. project in West Germany. The continued assistance of
commercial drilling supply and service companies will also prove
to be influential.

188
References
1. "Penetration Rate Analysis and Prediction" by N.D. Mortimer
and C. Nunn, Energy Workshop Paper No. 5.2, Sunderland
Polytechnic, May 1983.
2. "Analys i s of Bi t Records for Various Boreholes in Hard Rock'
by S.T. Minett and N.D. Mortimer, Report No. SCP1/5,
Sheffield City Polytechnic, August 1987.
3. "Comparison of Rotating Times for Deep Wells" by N.D.
Mortimer and S.T. Minett, Proceedings of the International
Symposium on Deep Drilling in Crystalline Bedrock, Mora and
Orsa, Sweden, 7th - 10th September 1987.
4. "Preliminary Report on the Construction of a Deep Prototype
H.D.R. Geothermal System in Cornwall" Camborne School of
Mines, H.D.R. Geothermal Energy Project, 1985.
5. "H.D.R. Drilling costs: Revised Calculations and Estimates
Accuracy" by S.T. Minett and N.D. Mortimer, Report No.
SCP1/2, Sheffield City Polytechnic, March 1987.
6. "An Economic Assessment of Hot Dry Rocks as an Energy Soure
in the U.K." by R.A.W. Shock, E.T.S.U. Report R34, April
1986.
7. "Interim Model Results: Drilling and Stimulation" by S.T.
Minett and N.D. Mortimer, Report No. SCP1/10, Sheffield Cit
Polytechnic, July 1988.

189
EEC contract nO EN3G-0051-F
EXPERIMENTAL INVESTIGATION ON FORCED FLUID FLOW THROUGH A GRANITE
ROCK MASS
F. H. Comet
Department o/Geophysics; Stanford University; On sabbatical leavefrom
InstilUl de Physique du Globe de Paris, France
Sumrnm
An in-situ experiment on forced fluid percolation through a granitic rock
mass has been carried out between two 800 m deep boreholes, 100 m apan. The
purpose of this experiment was to define a reliable method for extracting heat from
hot but relatively impervious fonnations. Fust. the well III-8 was percussion drilled
and then used to 1) characterize the natural fracture network by various logging
methods, 2) detennine the regional stress field and 3) perfonn small scale-injection
tests in order to detennine the main flow direction at depth by location of induced
microseismic activity. Then the second well, I11-9, was drilled. Some more work
on reconnaissance of the natural fracture network was conducted and a first
circulation system was developed between the two wells, with III-9 as injection
well. For this rust circulation test the injection rate was 30 m 3 Jh with a 8.2 MPa
well-head pressure; the production rate was only 10.8 m3Jh . Next the downhole
circulation system was improved progressively, mainly due to various sand
injections in the production well. Finally hydrothennal characteristics of this
system were evaluated through a 66 days circulation experiment at flow rates
ranging from 20 m3Jh to 75 m3Jh and well head presures ranging from 7.7 MPa to
12.6 MPa. The corresponding production rates ranged from 82.8 % to 44.1 % of
injection rate whilst the corresponding hydraulic impedance of the system ranged
from 1.9 to 1.3 GPa/m 3 /s. For a 27 m 3 Jh injection rate, the efficient heat exchange
area was evaluated to be of the order of 50 000 m2. An imponant conclusion is that
most of the flow occurs -in a limited number of hydraulically condu~tive zones.
I. INIROPUCUON
A research program on methods for extracting heat from impervious rock formations
has been underway from 1984 to 1988 in the Le Mayet de Montagne granite massif, some
25 km S.E. from Vichy, in central France. The goal of the project was to develop between
two 800 m deep boreholes a heat exchanger with an efficient heat exchange area larger than
200 000 m2, with a hydraulic impedance less or equal to 1 Gpalm3/s. Water, injected
through the rust well at flow rates of the order of 60 m3Jh, was to be produced back at
ground surface through the second well, with losses less or equal to 10% of injection rate.
These figures were chosen for they are approaching those required by a system
designed for space heating, except for the rock temperature. Indeed, the depth was chosen
so that the experiment could be carried out in a stress environment representative of deeper
conditions but shallow enough for providing the best possible control on the experiment
and for keeping the testing procedure as flexible as possible in order to investigate various
system geometries without incurring too large costs for mobilizing equipment in the wells.
Because all previous large scale testing (Whenen et al., 1987; Pine, 1988) had failed
to achieve satisfactory hydraulic conditions for their respective system, it was felt that some
more work on reservoir development was needed before running tests in real temperature
environments. As in the Cornwall project (Batchelor, 1980), the concept for developing the
heat exchanger was to stimulate preexisting fractures by injecting some fluid under pressure
so as to relieve the effective normal stress supported by these fractures and induce some
shear motion. These shear displacements were supposed to increase significantly the
hydraulic conductivity of the fractures because of the dilatancy effect associated with them

190
(Barton et al. 1985). However, while in the Cornwall project, pressure was applied in the
whole well, in the Le Mayet de Montagne experiment, inflatable straddle packers were used
to stimulate selected impervious fractures in order to avoid loosing fluid in hydraulicaully
conductive zones.
The program encompassed three phases:
1. drilling of a first well (ill-8) for reconnaissance of the natural fracture
network, for direct in-situ stress measurements and for running small scale injection tests
designed to induce some microseismic activity;
2. drilling of a second hole ( ID-9) at a location and with a geometry
coherent with both the local stress field and the location of induced microseismicity;
development of a heat exchanger between the two wells;
3. optimization of the heat exchanger and characterization of its hydrothermal
properties.
Drilling of the wells was conducted with a downhole air percussion hammer which
provided fast drilling rates: 16 m/h down to 4OOm, 11 m/h at the bottom of the wells
(780 m for ID-8 and 840 m for ID-9). Because some significant deviation from the vertical
direction was observed in the first well ( ID-8 is inclined by 8 0 with respect to the vertical
direction between 700 m and 780 m), some special attention was given to the drill string
assembly when drilling the second well ( ID-9). This resulted in a maximum deviation from
verticality smaller than 3 0 at any depth, down to the bottom of this well. The downhole air
percussion technique was chosen because of its low cost and because it was felt that coring
would not be helpful for identifying the fracture properties relevant to the project.
2 RECONNAISSANCE OF THE ROCK MASS
The reconnaissance of the rock mass has already been discussed by Comet et al.
(1987) and Comet (1987) and will be only briefly recalled here. It encompassed the
following:
- mapping of natural fractures in local quarries and on nearby outcrops of
the granite in order to obtain a statistical description of the natural fracture network
(Thomas, 1988);
- analysis of natural fractures intersecting the boreholes by various logging
techniques;
- determination of the regional stress field;
- determination of the main flow direction in the rock mass, at depth.
Reconnaissance of hydraulically conductive fractures
Natural fractures intersecting the boreholes were identified by:
- inflow of water observed while drilling with the downhole air-percussion
technique;
- analysis of the cuttings oollected every meter of drilling (Couturie and Binon,
1986);
- electrical logs including Sclumberger's Laterolog and dipmeter logs as well as
an imaging log (Mosnier, 1981);
- full wave form acoustic logs run by Sclumberger (BHC) and by Elf Aquitaine
with the EVA tool (Mathieu and Arditi, 1986);
- thermal logs run after the wells had been at rest for one month (Jolivet, 1988);
- analysis of the borehole water geochemistry for ID-8 (Bidaux, 1987);
- spinner logs and thermal logs run when the wells were either under
injection conditions (30 m3/h flow rate) or production conditions because of injection in the
other well.
It was observed that thermal logs, and more precisely thermal gradient logs, were
very sensitive for identifying water producing fractures. Dip and azimuth of fractures were
obtained with Mosnier's electrical log which yields a complete electrical map of the
borehole wall so that fractures, which are more electrically conductive than the rock matrix,
appear as eliptical features, in a manner somewhat similar to pictures obtained with the
borehole televiewer tool (although with somewhat less resolution).

191
Use of geophysical logs for identifying the hydraulic conductivity of fractures
proved much less successful. For example, a comparison of results obtained on the one
hand with electrical logs and with P, S and Stoneley wave attenuation logs computed from
full wave fonn acoustic logs and, on the other hand, with spinner logs or thermal logs run
when the wells were either under injection or production conditions, showed that only S
waves and Stoneley wave attenuation logs were of some help for identifying hydraulically
conductive fractures. But neither of these two logs, taken alone, could identify all
production zones, nor were they able to identify which of the hydraulically conductive
fractures were the most conductive. This has been attributed to the fact that these logs, like
all geophysical logs, sample the fractures only at their intersection with the wells and are
not dependent on the hydraulic interconnectivity of these fractures. However this
interconnectivity is one of the key hydraulic characteristics of the natural fracture network
which needs to be identified. With this respect, only spinner logs and thermal logs appear
to yield relevant information. It will be shown in later parts of this paper that analysis of
flow rate related logs in terms of hydraulic impedance must take into account the pressure
in the well because of the non-linear response of the fractures when this pressure reaches a
critical level. Present results have outlined the fact that only a limited amount of fractures is
relevant when forced fluid percolation is of concern. It is not clear at this point whether the
statistical analysis of fractures, conducted on surface observations, could be of any help
for this problem. It seems that a deterministic approach should be followed but the
difficulty is to identify the few hydraulically significant fractures away from the wells. It
will be shown in the third section of this paper how induced micro seismic activity has been
utilized with this respect
Regional stress determination
The regional stress field has been determined with the H.T.P.F. method proposed
by Comet and Valette (1984). Except for the two tests run around 340 m, results from 18
tests run between 50 m and 730 m were found coherent (Cornet and Julien, 1988) with the
stress field described by equation (1):
T=S+zA (1)
where T is the regional stress field at depth Z, with a principal direction assumed to be in
the vertical direction, and S and A are two second order tensors defined as follows:
Eigen values of S defmed by S I = 5.1 MPa; S2 = 0.2 MPa; S3 = 0 (in the vertical
direction); orientation of SI eigen vector N-1560 E (in the horizontal plane);
Al • 0.0226 MPa/m ; A2 = 0.0084 MPa/m ; A3 = 0.0264 MPa/m (in the vertical
direction); orientation of Al eigen vector with respect to SI eigen vector: +1040
Variation of principal stresses with depth are presented in figure 1.
30 Figure La Principal Stress Magnitudes
--- T1 (in Mega Pascal)
---+-- T2
-0-- Tv
20
10
O+-~ __ ~~~ __ ~~~~~~~,-~d~e~p~th~(m~)~
o 200 400 600 800 1000

Figure 1.b T1 Orientation
(positive to the East)
depth (m)
400
600 800
1000
Determination of main flow direction at depth
In order to define the location of the second borehole, it was considered necessary
to identify the main direction of flow in the rock mass when injection would proceed in the
lower portion of the well. For this purpose, a small scale injection test was run. 800 m3 of
water were injected at a 1.31 m 3 /min flow rate between the bottom of the well and an
inflatable packer set at 643 m. This flow rate was enough to trigger some induced
micro seismic events which were located with a network of 15 three component stations,
after calibration shots had helped identify the P wave and S wave velocity fields (falebi and
Comet, 1987). These seismic stations had a linear response in the 10 Hz - 2000 Hz
frequency range. Only 12 events were clearly identified. Nine of them were in a subhorizontal
zone located at a depth of about 560 m; three events were located around 750 m.
Because of this poor definition of the flow direction at depth, the second borehole location
was chosen mostly on the basis of the horisontal principal stress direction.
The two boreholes are 103 m apart at ground surface and are aligned with the
N 1350 E direction. At the 750 m depth, the wells are 93 m apart and are aligned with the
N 160+ 5 0 E direction. At this depth the maximum horizontal principal stress is oriented
N 140 +8 oE.
3 DEVELOPMENT OF THE HEAT EXCHANGER
Initial reservoir development with 111-9 as injection well
The various logs conducted in the second well revealed that the granite
encountered between 650 m and 780 m was very homogeneous, with only a few fractures,
as opposed to the first well which intersected, in the same depth range, a very densely
fractured and altered granite. Accordingly, it was decided to use the first borehole ( m-8)
as production well and the second borehole ( ill-9) as injection well after some hydraulic
stimulations to help connect ill-9 with the natural fracture network.
Three stimulations were run at 720 m, 730 m, and 645 m in ill-9. In all cases,
100 m3 of gel of low viscosity (70 centipoises) were injected, at a 1.3 m3/min flow rate,
followed by an injection of 100 to 300 m3 of water at the same flow rate. As mentionned
above, these injections were run through an inflatable straddle packer set on a preexisting
fracture of poor hydraulic conductivity. Only in the last case was some induced seismicity
observed. In none of these cases did we observe any surface defonnation on the 5 stations
tiltmeter network set up around the wells. The sensitivity of the tiltmeters was equal to
5 .10-8 radians

193
A short circulation test (70 h total duration) was run. Water was injected at a
30 m3/h flow rate and 8.2 MPa well head pressure in III-9, between the bottom of the well
and an inflatable packer set at 600 m. A tubing was anchored in III-8 on an inflatable
packer set at 279 m so as to limit water losses in the upper part of the well. Production flow
rate was 10.8 m 3 /h after 70 hours of pumping, and was still rising very slowly (0.6 m 3 /h
increase in production flow rate in 19 hours). During this test, 31 microseismic events were
observed, 20 of which could be located (Cornet and Julien, 1988). 12 were located in the
950 m - 800 m depth range, 5 between 800 m and 600 m and 3 between 600 m and 400 m.
After 70 hours, the production well was shut off and at the same time injection flow
rate was increased to 80 m 3 /h in order to raise the interstitial pressure in the rock mass so as
to try to induce some microseismic activity along the main flow paths. This injection
process lasted 3 hours and another 26 microseismic events were recorded out of which 4
were located between 900 m and 750 m, 3 between 750 m and 600 m, and 13 between
600 m and 400 m.Thus the microseismic activity location does not seem to be the same
when water circulates between the wells and when the production well is shut off. This
observation has been confIrmed during subsequent circulation tests. Location of all events
recorded during the complete experiment is shown in fIgure 2. It can be observed that
microseismic activity is not evenly distributed with depth. The focal mechanisms of these
events are coherent with double couple sources. In the 400 - 600 m depth range the focal
mechanisms are evenly distributed between nonna! and reverse faulting type mechanisms.
Below 750 m most of the mechanisms correspond to nonna! faulting. This is coherent with
the stress variation with depth determined with the H.T.P.F. method. Inversion of the
mechanisms from events of the 750 m - 950 m depth range suggests a rotation of principal
stress directions during injection tests (Cornet and Julien, 1988).
Circulation experiment with 01-9 as production well
Because of the location of induced seismicity observed during the initial reservoir
development (main flow in the N 160 0 E direction) and because of the N 140 0 E maximum
horizontal principal stress direction, it was considered worth trying to interchange the wells
functions so that III-8 became the injection well and III-9 became the production well.
A new hydraulic stimulation was conducted at 758 m in III-9, following the same
procedure as that of the previous stimulations except for the injection of 2 tons of sand at the
end of the test. Then a circulation was started with a 30 m 3 /h flow rate injected between the
bottom of III-8 and an inflatable packer set at 710 m. A second inflatable packer was set at
550 m so as to limit short circuits through the annulus of the well. Accordingly, water could
flow to the surface through the annulus of I11-8 only in that portion of the well above 550 m.
A packer was set at 600 m in III-9 so that the flow drained by this borehole below 600 m was
flowing through a tubing while that drained above 600 m was flowing through the annulus
space.
It soon appeared that a major fracture was draining most of the injected water back to
ground surface in the immediate vicinity of injection well :
injected flow rate through III-8, below 710 m: Qj = 30 m3/h
well-head injection pressure:
9 MPa
flow rate drained by III-9 below 600 m:
5.3 m 3 /h = 17.7 % ofQj
flow rate drained by III-9 above 600 m: 0.1 m 3 /h
flow rate drained by III-8 above 550 m:
9.8 m3/h = 32.7 % of Qi
During this test. no induced microseismic activity was observed. In order to obtain a
better definition of the fracture causing the short-circuit observed around III-8, flow rate was
increased so as to reach a well head pressure larger than 10 MPa. After pumping for 48 h at a
60 m 3 /h flow rate with a wellhead pressure equal to 11 MPa,9 microseismic events were
observed. Their location (fig 3) indicates fairly clearly that the short circuit was caused by a
subvertical plane striking to the north, with the deeper events to the north and the more
shallow events to the South. This main fracture is considered to be that identified at 472m
during the inital thermal logs and during preliminary injection tests run at this depth in 1984
(Talebi and Cornet, 1987).

194
LE MAVET
N
• E
••
7
•
•
a)
Figure 2. Location of induced microseismic activity observed during the innitial reservoir
development. a) projection in horizontal plane; the circle is centered on the top opf III-8; its
radius is 100 m; black dots are seismic stations. b) projection in a vertical plane passing thru
the head of ill-8 and striking to the East
b)
LE nAVET
[STI~
•
~ ..
•
ISO
m.-II
0m'lI
0
4000 Z ,-.I

:I
195
(i)
o o
Figure 3. Location of induced seismicity observed during the circulation test in which ill-S
was the injection well. Projection in a vertical plane striking Nonh. marks on axes are every
100 m; axes are centered on top of ill-S, black dots are seismic stations
Spinner logs and thermal logs were run in well ill-9 in order to identify the water
producing fractures. These logs revealed (fig. 4) that the most productive fracture was that
mjected with sand but that no water was flowing through the other stimulated zones. In addition,
water was observed flowing from the preexisting altered zones (646 m and 7S1 m)
identified with previous logging. During the 60 m 3 Jh injection test. after 24 h of pumping, a
4 MPa back pressure was applied on ill-9. Production rate, which had reached 7.6 m31h,
decreased to 6.3 m 3 /h. This has been taken as a demonstration that flow is occurring in an
"open " system.
It was decided to return to the previous well configuration for the circulation test,
after developping a hydraulic fracture propped with sand at the bottom of ill-S.
Optimization of the downhole heat-exchanger
In order to improve the hydraulic connectivity between ill-S and ill-9, an attempt was
made to develop in m-s a true hydraulic fracture propped with sand Indeed, a true hydraulic
fracture would develop normal to the minimum principal stress direction, i.e. in the
N 140 0 E direction and therefore would intersect the zone in which induced microseismic
activity had been observed during the innitial development stage .
100 m3 of gel with low viscosity (70 cp) were first injected at a 1.6 m3/min flow rate,
in order to stimulate preexisting fractures by shear (11.2 MPa well-head pressure). Then.70
m3 of high viscosity gel were injected at a I.S m 3 /min flow rate (maximum capacity available
on site) and 12 MPa wellhead pressure., so that 7 tons of sand were injected in the rock mass.
During this injection no seismic events were recorded, nor was any surface tilt observed, i.e.
tilt larger than S 10. 7 radians.

196
Figure 4 Thermal log run
in ill-9 when injection was
proceeding at a 30 m3Jh
flow rate in I1I-S. a)
thermal gradient (in degree
Celsius per meter ); b)
absolute tempe-rature (in
degree Celsius); vertical
axis is depth (in meters)
1+ +-
~~
I
I
~-~
. ~
-L-.:..
I
I
.. I :-r-r--;-
a)
I
-i-l-I- . -IH-
14
-I-~ 1-.
I+t= 1-. t~
b)
.1-.
, I
I-i- - H+
h- -!-,-.~.
+t+
The subsequent circulation test, which lasted about 21 days, yielded the following results,
once steady state conditions had been reached :
injection flow rate in uncased ill-9, below 600m : Qj = 30 m3Jh
injection wellhead pressure:
9.2 MPa
production flow rate in ill-S, below 710 m: 1O.S m3Jh = 36 % Qj
production flow rate in ill-S, above 560 m: 6.5 m3Jh = 22 % Qj
total recovery rate:
17.3 m3Jh = 57.7 % Qj
A total of 30 microseismic events were recorded during this circulation test, most of
them below 750 m. During a chemical tracer test (iodine), the first arrival of tracer was
noticed below 710 m, in ill-S, 7 hours after injection, whilst the peak concentration was
observed, for that part of the flow coming below 710 m, 27 hours after injection. For that
part of the flow coming above 560 m, the first tracer arrival was noticed 20 hours after
injection and the peak concentration 30 hours after injection.
During the hydraulic stimulation of the bottom section of ill-8, the wellhead injection
pressure always remained smaller than 12 MPa, a value too small for initiating a true
hydraulic fracture. Accordingly it was considered that the hydraulic connection between the
two wells would be improved if a true hydraulic fracture, propped with sand, could be
developed. In order to reach this goal, a new hydraulic fracturing experiment was
undertaken. 200 m 3 of high viscosity gel were injected at an average flow rate of 4.4 m3Jmin,
with an initial wellhead pressure equal to 24.5 MPa, i.e. a value large enough to ensure

197
development of a hUe hydraulic fracture. However the wellhead pressure dropped
progressively to 17.5 MPa after injecting 70 m3 of gel. A total of 40 tons of sand were
mjected by the end of this work. The downhole shut-in pressure observed at the end of
pumping was equal to 16.5 MPa, i.e. larger than the minimum principal stress evaluated for
this depth range with the R T .P.F. method. Thus, at this point it is not clear wether a hUe
hydraulic fracture has been developed or whether only preexisting fractures have been
stimulated. During all this process, no microseismic event was recorded nor was any
significant surface tilt observed. After completing this hydraulic fracture, the tubing was
removed from III-8 and then anchored on an inflatable packer set at 402 m so that it was
possible to separate the flow coming into ill-8 between the bottom of the weU (which was at
764 m because of the deposit of sand in the lower portion of the weU) and 402 m, from that
coming into the borehole above 402 m (flow through the annulus).
4. TIlE 66 DAYS CIRCID.ATION lEST
The purpose of this circulation test was to obtain the hydro-thermal characteristics of
the heat exchanger that had been developed during the preceding phases. For one month,
tests were conducted to determine the response of the system to various injection rates. Then
injection proceeded at a constant 27 m3/h flow rate for 17 days so that steady state conditions
could be reached for running a chemical tracer teSL The final part of the circulation test was
designed for observing the break-through of a cold front in the production well. However
this last part of the testing was not successful.
Testing the hydraulic behaviour or the system
The main testing phases and the corresponding hydraulic characteristics are
presented in table 1.
Table 1. Hydraulic characteristics of the main testing phases. Qj is the injection flow rate; Pi
is the well-head injection pressure; Qa is the flow rate in the annulus of I11-9; Qp is the
production flow rate in the tubing of m-8.
Phase duration ill-9 ill-8 Hydraulic
Qj Pi Qa Qp impedance
days m 3 /h MPa m3/h m3/h %ofQj GPa/m3/s
I 6.96 30.5 +1. 9.5 0.22 16.7 54.7 2.05
2 2.96 43.3 +2. 10.1 0.6 21.4 49.4 1.70
3 4 59.4 + 0.6 11.3 1.35 29.2 49.1 1.40
4- 1.46 76.2 +4.5 12.6 33.6 44.1 1.35
S 2.69 60. +1. 10.5 29.4 49.0 1.29
6 5.83 20.7 + 0.5 7.7 0.8 16.0 77.3 1.73
9 17.4 26.8 + 0.5 8.3 0.92 18.3 68.3 1.64--
11 4.1 30.0 + 0.5 9.2 0.65 18.7 62.3 1.73
12 2.75 45.0 + 0.5 10.2 1.15 24.5 54.4 1.50
13 4.2 19.2 + 0.3 8.3 0.75 15.9 82.8 1.88
• This injection phase was stopped before steady state conditions were reached. By
comparison with results from other phases, it can be considered that the production rate
would not have been larger than 34.5 m3/h .
.. Because of technical difficulties with the pressure transducer caused by lightning, this
pressure value is only approximate and the corresponding impedance value is doubtful.

Figure 5. Injection (upper curve) and production (lower curve) flow rates versus time for the 66 days circulation test.
Flow rates (vertical axis) are expressed in m 3 /h, time (horizontal axis) is expressed in days.
10
00
a
CD a ~------~--------~--------'---------1---------'--------~---------r--------'---------r---'
..
~
i \
a
~
~ ~------~----~~~-~~lL-~.~:~~--4---------+--------4---------+---------r--------+----------t---i
~--------~!~--~:Ll--~~--.~I~I~I--~ ~ ~ ~ ~ -r~~---Tl---t---;
; ________ ____.____ ________ __.______ ________
i:~.: tit:::
.. .:1: ::
..-, ii \ '
~I .r I;.~ ~ i-lkL n ~\ ... -.".-"-"'--....~---"'! fl: .. ~::!: ~ ~ ... I
l .. .,- i\ 3 '"i!- t:.:. ,..;.
I~~ ~~ N i"oyoo. --1: ~ j 1 : j~ ~
~ ~----;-,",.,I-. = § :: \r~--~Lr. \ I ~ ... ~ ~ ..JI'I-'
If\~:..l
..""--- ~ I ~"".::"::_.::: - I It I:;, ... E' '.•
= :1'::"'/ I.!i' II '~l.::::
~ ~ I ~ ~ ..••. i:.:·: g
:: r .""--="""--_1 \
0
0 • 00 7.00 14.00 21.00 28.00 35.00 42.00 49.00 56.00 63.00
II U ._'--1...--..L.I

199
In this table only the rate of flow occurring in the tubing set in the production well is
shown. The flow occuring in the annulus of this well, i.e. the flow collected above 402 m,
varied between 2 % and 3 % of the injection flow rate. This result shows that in the previous
circulation test most of the flow collected in the annulus of the production well was coming
through fractures intersected between S50 m (depth of the upper packer for this test) and
40201.
An overall view of injection and production flow rates for the total duration of this
circulation experiment is shown on figure S. Results from table 1 show a clear decrease of
hydraulic impedance with flow rate. Further, the hydraulic impedance for the 30 m 3 /h
injection rate IS smaller at the end of testing (Phase 11) than before running the high flow rate
circulation tests (phase 1). The same result holds for the 4S m31h injection rate (Phases 2 and
12). A similar observation is made with respect to water losses: while the recovery factor is
only 54.7 % for phase I, it is equal to 62.3 % for phase 11 and while it is 49.4 % for phase
2 it is 54.4 % for phase 12. Thus, clearly, the high flow rate circulation phases have
improved the hydraulic characteristics of the system.
For the complete duration of this circulation test the micro seismic activity was
continuously monitored. A total of 46 microseismic events were detected, with an automated
acquisition technique based on the signal to noise ratio measured at six stations. It can be
observed that their location is nol evenly distributed with depth but that they define several
distinct zones (fig. 6).
II
__ __ ____ ~~ ~ ~ ~~--~~--~.----r---~--~
•• • • I
•
Figure 6 Location of induced seismicity observed during the 66 days circulation test
Vertical projection in a plane striking to the East; black dots are seismic stations; axes are
centered at the top of m-8, they are marlced every 100 01.
No activity was observed during phase 1 while, for similar injection conditions, some
activity had been recorded prior to the injection of the 40 tons of sand. 18 events were
recorded during phase 2 but only one during phase 12 which occured at the same flow rate,
near the end of the circulation experiment (see table 1). 16 events were recorded during phase
3 and another 9 during phase 4, but only 1 during phase S and then none until phase 12.
Accordingly, it seems that this activity has been triggered by the increase in interstitial
pressure associated with the larger flow rates. Focal mechanisms for all these events have
been determined. All but one are coherent with a double couple source, i.e. they correspond
to shear motions. A comparison of the variation in hydraulic impedance and water losses for
the system with the periods of microseismic activity suggests that the shear events which
were induced by the high fluid pressure were also associated with some joint dilatancy so that
the hydraulic conductivity of the fractures was increased. This is coherent with the results
described by Pine (1988) for the Cornwall project

200
However no significant surface tilts were observed, except perhaps during the high
flow rates tests. This suggests that if some fracture opening did take place, this opening must
have remained fairly small since it did not yield any measurable deformation at ground
surface.
Spinner logs were run in m-9 at the end of phase 5, when the well-head pressure
was equal to 11.3 MPa, for a 60 m3Jh injection flow rate, and during phase 9, when the
wellhead presure was 8.6 MPa for a 27 m3Jh injection rate . Interpretation of these. logs in
terms of flow rate absorbed by the various hydraulically conductive zones shows that, during
phase 5, some flow occurred in the stimulated fractures at 721 m and 730 m but that no
detectable flow occured in the sand propped fracture at 758 m. However, during phase 9,
when the well-head pressure was lower, the contrary was observed. This clearly indicates
that the flow path in fractured rock masses, which depends on the relative hydraulic
conductivity of the fractures, is strongly dependent on the interstitial fluid pressure. By
comparison of these results with the ground surface tilt observations, it is concluded that the
opening of fractures observed at the wellbore had only a limited extention away from the
injection well. Indeed a calculation based on elasticity theory showed that tilts of the order of
10-6 radians should have been observed at ground surface, for the water pressure observed at
the well head. if one assumes a fracture extension larger than 20 m.
Twenty four thermal logs were run in the production well during the circulation experiment.They
outline eleven water producing zones, three of which were not observed on the
previous flow rate logs (spinner logs or thermal logs). The new producing zones (713 m ,
661m and 649 m) are considered to have been stimulated during the two sand injections. The
flow rate for each of the water producing zones can be determined if the initial temperature
proflle of the rock mass is known (Bruel, 1988). Results are presented in figure 7 in which
each column corresponds to a thermal log. In each column, each producing zone is
represented by a segment the length of which is proportional to the flow rate. For figure 7.a,
the flow rate is expressed in absolute value whilst in figure 7.b flow rates are normalized
with respect to the total production rate measured at ground surface.
A comparison of these results with the location of induced microseismic activity
suggests that flow occurred in the rock mass along three main zones. A first one, fairly
diffuse, is found around 800 m and accommodates about 40 % of the flow. It is drained by
the fractures encountered below 764 m and at 754 m, 713 m and probably 674 m. This last
value is suggested by the fact that some sand was deposited above the packer set at 710 m
when injection occured below this packer. Another 40 % of the flow occurs along the main
fracture intersected by III-8 at 472 m. This fracture was the cause of the short-circuits
observed when injection was conducted in this well. It is drained by III-8 through its
intersection at 472 m and through a set of fractures at 440 m which were developed during
the initial phase of reconnaissance of the rock mass. The fmal 20 % of the production flow
are drained through a set of fractures around 630 m.
This description of main flow zones may appear fairly speculative; tt is confirmed by
the results from chemical tracer tests.
Determination or the efficient heat exchange area
The efficient heat exchange area was to be determined first by chemical tracing,
secondly by direct observation of the decrease in production water temperature.
Unfortunately, the difference between the temperature of injected water and that of the rock
mass was fairly small (15 0 C) and no decrease in temperature had been observed in the
production well by the end of the circulation test
Chemical tracing was run successfully (Chupeau et al, 1988), when steady state
hydraulic conditions had been reached with the 27 m3Jh injection rate (phase 9). Various
chemical tracers were used: 30 moles of iodine, two moles respectively of uranium, thorium,
lanthanum, samarium, cerium and dysprosium. Variations of tracer concentration with time
as observed at the production well head (later referred to as concentration curves) are shown
in figure 8. It can be observed that the best results were obtained with dysprosium and
iodine, whereas thorium and lanthanum arrivals could not be detected.

20 1
7.3. Flow rates
of tile various producing rone$
0
.....
• 784m
134m
m 713m
674m
0 661 m
• 849m
!I 844m
639m
[J 632m
0 411 m
•

202
250
- D .... SPROSIUM ppD
- SAMARIUM ppb
-- URANlUW ppb
-- lODE ppb/l0
.D
a.
a.
5 10 15 20 25 .30
Figure 8 Variation with time of tracer concentration in the water produced at the top of
ill-8. Time is shown in hours after injection at the top ofill-9
Results from both iodine and dysprosium show a break-through time of 3 hours, i.e.
a much shorter time than that observed during the first tracer test conducted before the
injection of the 40 tons of sand. The peak concentration is observed between 6.5 hours and
10 hours after injection of tracer.
Preliminary interpretation of these concentration curves, based on a curve fitting
procedure (Goblet, 1988), has shown that they are coherent with flow through a multiple
fracture system. A first fracture is found to have accommodated a 4.1 m 3 /h flow rate; its
total volume is equal to 38 m 3 and its mean hydraulic thickness is found to be 2.4 mm so
that its efficient heat exchange area is equal to about 32 ()()() m 2 . Because this fracture
corresponds to the first tracer arrival, it has been associated with the deep microseismic
zone. Indeed the first tracer test had shown that tracer was coming first below 710 m and
only later reached the well at 472 m (see previous section).
After substraction from the actual concentration curve of the theoretical curve
calculated with the above parameters, the characteristics of the second main fracture can be
identified. This second fracture is found to accommodate a 3.5 m 3 /h flow rate; its total
volume is 68 m 3 and its hydraulic thickness is found equal to 10 mm so that the efficient
heat exchange area is about 14 ()()() m 2 . Results from the first tracer test suggest that this
second fracture might be associated with that intersected at 472 m in ill-8.
The remainder of the flow is assumed to occur in a third fracture so that the efficient
heat exchange area can be considered to be larger than 50 ()()() m 2 • However these results
must be compared with those derived from the therma1logs interpretation. It clearly appears
that the flow rates computed for the various fractures from the chemical tracer test are
underestimated; this may be attributed partly to losses of tracer by other means than fluid
losses. However if the flow rates are larger than the values chosen for the above
interpretation then the resulting hydraulic aperture of the fractures is found to be larger so
that the efficient heat exchange area is smaller than the computed values. Accordingly the
efficient heat exchange area of the system is certainly not larger than 50 ()()() m 2 •
These results are only preliminary; the concentration curves suggest however that
flow did occur through a limited number of fractures, a result which is coherent with the
other observations (location of microseismic activity and identification of main flowing

203
zones in the boreholes). For the larger flow rates, the correlative increase in interstitial
pressure should have resulted in larger hydraulic apertures of the fractures so that the
resulting efficient heat exchange area was probably smaller than that estimated here above.
S CONO,USION
When comparing the results obtained in this experiment with the goal set initially
for our research program, it appears that the hydraulic impedance is not too far off the
initial goal but that some improvements are still required with respect to both water losses
and efficient heat exchange area.
The main conclusion is that the forced fluid percolation (flow rates larger than
20 m3 }h) in this granite rock: mass is mostly controlled by a few hydraulically conductive
zones. The difficulty is to determine their exact geometry away from the wells. Oearly, the
injection of sand in these hydraulically conductive zones, in the vicinity of the production
well, has significantly improved the water recovery factor.
It is considered that the original objectives set to the project could have been reached
had the boreholes uncased sections, used for injecting and for producing the water, been
longer so that the number of hydraulically conductive zones intersected by the wells was
larger. In such conditions, for the same well head injection flow rate, the flow rate within
each of these zones would have been smaller and as a consequence, if our results can be
extrapolated, the corresponding recovery factor for each of the zones would have been
larger. This, in addition, would have resulted in a larger heat exchange area.
For normal heat flux regions, these results suggest that the Hot Dry Rock:
technology can probably be developed successfully for producing heat for space heating.
However present results are still at least one order of magnitude off the values required for
the production of electricity, unless very deep boreholes (8 to 10 ()()() m) are considered.
But then, for such deep conditions, extrapolation of our results is not valid.
Aknowledgements
This project was financed by the European Economic Community (DG-XII;
Geothermal Energy Program), by Agence Fran~aise pour la Maitrise de l'Energie and by
Centre National de la Recherche Scientifique ( Programme Interdisciplinaire de Recherche
sur l'Energie et les Matieres Premieres and Institut National des Sciences de l'Univers)
from France. This paper is only a short summary of the work conducted by the many tearns
which participated in this project. I wish to thank: them all very sincerely. However I bear
the entire responsibility for the views expressed in this paper. My very sincere thanks to
Mark Zoback for his helpful suggestions concerning this paper.
Bibliography
Banon, N., S. Bandis and K. Bakhtar (1985). Strenght, deformation and conductivity
coupling of rock: joints; Int Jour. Rock Mech. & Geomech. Abs.,vol 22,nb 3, p 121-
14
Batchelor A. S. (1984). Hot Dry Rock: Geothermal Exploitation in the United Kingdom;
Modem Geology; vol. 9, nb. I, pp 1-43; Gordon and Breach Science Publishers.
Bidaux P. (1987). Contribution a l'etude des circulations profondes en milieu fissure peu
permeable, identifications a partir de mesures hydrochimiques Ie long de forages;
PhD. Thesis in Geology, Montpellier University, France. .
Brnel D. (1988). Interpretation des essais hydrauthermiques; in 'Pro jet Mayet de
Montagne - Etude in-situ de la percolation forcee d'eau en milieu granitique' edited by
F.H. Comet, 1988··.
Chupeau J., S. Bigot, P. Toulhoat and M. Treuil (1988). Etude hydrodynarnique, a l'aide
de traceurs, du doublet geothennique du Mayet de Montagne; in 'Projet Mayet de
Montagne - Etude in-situ de la percolation forcee d'eau en milieu granitique' edited by
F.H. Comet, 1988··.
Couturie J.P., M. Binon and F. Carmier (1984). Rapport geologique sur Ie forage IINAG
III-8; in 'Identification et caracterisation des proprietes hydrauliques des fractures
recoupees par un forage par methodes geophysiques' edited by F.H. Cornet, 1987.

204
Couturie J.P. and M. Binon (1986). Rapport geologique sur Ie forage INAG ill-9; in
'Identification et caracterisation des proprietes hydrauliques des fractures recoupees par
un forage par methodes geophysiques' edited by F.H. Cornet, 1987*
Comet F.H.* (1987). Identification et Caracterisation des proprietes hydrauliques des
fractures recoupees par un forage par methodes geophysiques; Report for Agence
Francaise pour la Maitrise de l'Energie (Programme Geothermie Profonde Generalisee)
and for European Economic Communities (00 XII - Geothermal Energy, Contract nb
EN3G - 0051-F(CB».
Comet FR.** (1988). Projet Mayet de Montagne - Etude in-situ de la percolation forcee
d'eau en milieu granitique; final report for Europ. Econ. Communities (DG XII -
Geothermal Energy) contract nb EN3G-0051-F(CB) and for Agence Francaise pour la
Maitrise de l'Energie (Programme Geothermie Profonde Generalisee)
Comet F.H. and B. Valette (1984). In situ stress determination from hydraulic injection
test data; Jou. Geophys. Res., vol. 89, nb B13, pp 11527 - 11537.
Comet F.H., J. Jolivet and J. Mosnier (1987). Reconnaissance par methodes
geophysiques des fractures recoupees par un forage; 6th Internat. Congress on Rock
Mechanics, Montreal, Theme 1; Balkema publisher.
Comet F.H. and Ph. Julien (1988). Stress determination from hydraulic test data and focal
mechanisms of induced seismicity; Int. Jou. Rock Mech. Min. Sc. & Geomech. abst.,
in press.
Goblet P. (1988). Caracterisation de la geometrie d'un ecoulement en fracture au moyen
d'un essai de tracage; in 'Projet Mayet de Montagne - Etude in-situ de la percolation
forcee d'eau en milieu granitique'*, edited by F.H. Cornet*
Jolivet J. (1986). Etude thermique du forage INAG III-9; in 'Identification et
caracterisation des proprietes hydrauliques des fractures recoupees par un forage, par
methodes geophysiques' edited by FR. Cornet*
Mathieu F. and P. Ardity (1986). Rapport interne Elf Aquitaine, Departement sismique de
puits, Paris-La defense.
Mosnier J. (1982). Detection electrique des fractures natureUes ou artificieUes dans un
forage; Annal. Geophys., vol 38, pp 537-540.
Pine R. J. (1987). Ph.D. Thesis, Camborne Scool of Mines, Cornwall, United Kingdom.
Talebi S. and F.H. Comet (1987). Analysis of the microseismicity induced by a fluid
injection in a granitic rock mass; Geoph. Res. Letters, vol 14, nb 3, pp 227-230.
Thomas A. (1988). Analyse de la fractuiation natureUe; 'Projet Mayet de Montagne-Etude
in-situ de la percolation forcee d'eau en milieu granitique' edited by F.H. Comet **
Whetten J., B. Denis, D. Dreesen, L. House, H. Murphy, B. Robinson and M. Smith
(1986). The U.S. Hot Dry Rock Project; EEC I US Workshop on Hot Dry Rock;
Brussels, 1. Garnish editor, published in a special issue of Geothermics, 1987.

EEC contract N° EN3G-0053-P
APPARATUS TO PROVIDE All IMAGE OP THE VALL OP A BOREHOLE DURING A
HYDRAULIC PB.ACTURING EXPERIMENT
J. MOSNIER* and P. CORNET**
* Laboratoire de G60physique Appliqu6e (LGA) , CRRS, Or16ans, Prance
** Institut de Physique du Globe, Paris (IPGP), Prance
Abatract
The apparatus described here is intended for atudying fractures
produced artificially by hydraulic overpressure in a borehole, during
the actual procesl of fracturing.
To . achieva this, the instrument measures, twice per second, the
electrical conductance of the wall of the compression chamber at 160
different points, and presents the results in the form of an image
obtained in real time.
The appearance of a fracture is in most instances accompanied by a
local increase in conductance, enabling the fracture to be detected
and its dip and strike to be determined.
The first tests, carried out at Mayet de Montagne, Prance, were
successful. However, since the artificially produced fractures are
very small, the signals have to be summarily processed in order to
obtain images of acceptable quality.
The instrument can also demonstrate the existence of natural
fractures in the wall of a borehole during a conventional logging
operation.
1. INTRODUCTION
The apparatul deacribed below wal initially intended for studying
fracture I produced intentionally in boreholes in order to create
Irtificial heat exchangers with a view to exploiting the energy in hot dry
rockl.
A IIcond application that hal appeared more recently ia that of
determining in situ the Itresl tensor within a terrain through the
mea.urement of the fracture. produced by hydraulic overpressure (Haimson,
1978 I Hickman and Zobach, 1983 I Cornet and Valette, 1984). The
parameterl measured are on the one hand the preslures (of breakdown,
Ihut-in, reopening, etc ... ), and on the other hand the parameters
characteri.ing the geometry of the fracture (dip and .trike).
The latter have up to now been measured after the fracturing
operation, which necellitated lowering into the well either a conventional
logging tool (eg, borehole televiewer or microscanner) or a special packer
Inabling a print to be taken of the fracture.
It i. obviously much preferable to be able to determine the
Ilometrical parameterl of the fracture in real time at the moment of
fracturing, and not afterward., which avoids in particular the necessity
of using two different equipment.. It is on the development of such an
inltrument that the LGA and IPGP have been working, with the financial
lupport of the EEC, on the basil of previous LGA work on the detection of
natural fracture I in boreholes.

206
2. PRINCIPLE
In its initial form, the tool developed by the Laboratoire de
G~ophysique Appliqu~e (LGA) is a kind of azimuthal laterolog.
An alternating electric voltage is applied between a number of
electrodes arranged in a ring in the center of the sonde, and a distant
receiving electrode. The currents emitted by each of the electrodes
constituting the ring, effectively proportional to the conductance of that
part of the wall opposite the electrode and varying according to whether
the electrode is opposite a fracture (conductive) or sound rock
(resistant), are measured successively.
By moving the sonde vertically in the well a series of conductance
values is obtained that can be displayed on a graphic screen. For this
purpose a number of squares, equal to the number of electrodes, is
disposed along a line, and to each square is allocated a level of grey
(selected from among 64 possibilities) proportional to the intensity of
the current emitted by the corresponding electrode. Below the first line
is placed a second, obtained from a second series of intensity values, and
so on, until all the data have been used. The result is a kind of image of
the well - represented as being split vertically along a generatrix and
unrolled in which the fractures appear in black against a light
background or vice versa (Figure 1). The resolution depends upon the
number of electrodes in the ring, the rate of movement of the sonde in the
well and the ratio of the diameter of the sonde to that of the well. In
practice it is of the order of a few centimetres in depth and 50_100 in
azimuth.
The equipment developed for the EEC operates on the same principle
but is built differently for, in addition to detecting pre-existing
fractures during the logging phase as a normal tool, it will also enable
a fracture produced artifically by hydraulic overpressure to be seen
during the process of fracturing and no longer only after it has appeared.
Obviously, during this operation, the sonde is motionless, and a
conventional tool will provide the image of only a narrow horizontal band
opposite the electrodes. In order to analyse a greater height of wall a
choice must be made between :
- providing a single group of electrodes with the possibility of
moving up and down, within the limits of the compression chamber, or,
- using several groups of fixed electrodes distributed throughout the
height of the compression chamber.
The second solution, which is simpler and allows more rapid analysis, is
that which has been selected by the LGA.
3. DESCRIPTION OF THE INSTRUMENT (Figure 2)
The first prototype built on this basis, whose construction has just
been completed, comprises two independent parts :
I) An assemblage of mechanical parts whose purpose is to create the
conditions required for fracturing, comprising :
a - Two inflatable packers (length 1.9 m, diam. 140 mm), closing off
a compression chamber 1.3 m high.
b - A "distributor" directing the water pumped from the surface,
through a flexible, reinforced rubber pipe, either to the packers or to
the compression chamber, according to the traction on the cable supporting
the sonde (this part has not yet functioned satisfactorily, tests carried
out so far having been with two separate pipes).
c - Sensors measuring the hydraulic pressure in the packers and in
the compression chamber.
II) Electronic circuits to provide an "electrical image" of the wall,
visible at the surface. These comprise I
a An injector for establishing a difference of alternating
potential (f - 6000 Hz), the ·level of which can be regulated from the
surface, between the measurement electrodes and the armour of the bearer
cable acting as receiver electrode.

- 160 meaeurement .lectrodee dietributed in 10 ringl one above the
other and occupying the larger part of the comprellion chamber.
c - 11 focusing electrodes separating the groups of measurement
Ilectrodes, whoa. function ia to ensure that the currenta penetrate more
or le.s radially into the formation rock. The bare parts of the sonde on
either side of the packers alao contribute to focusing the currents.
d - 160 feed back circuits for maintaining the potential of the
measurement electrodes at the same level as that of the body of the sonde.
I - A mUltiplexer connecting the outlets of the 160 amplifiers to the
inlet of the variable-gain measurement amplifier.
f - A pha.. detector, followed by an integrator, transforming the
6000 Hz alternating current into virtually direct current sent to the
lurface (0

208
fracturing of the sound rock. During this operation the variation of
pressure in the compression chamber(which should remain slightly lower
than that in the packers) is recorded continuously. A sudden drop in
pressure, from a value Pb, indicates the appearance of a fracture, which
can then be propagated by continuing to pump slowly. If pumping is stopped
the pressure in the chamber will fall to a value P s ' known as the shut-in
pressure.
At the same time as giving the pressure, the LGA/IPGP equipment
provides an image of the compression chamber (reduced to 10 lines at 10 cm
vertical spacing). It should in principle be possible to follow the
evolution of the wall, observe the appearance of the fracture and
determine its geometrical characteristics. In particular, if the fracture
is subvertical, the direction of the minimum principal horizontal stress
( 0h) can be deduced.
In the simplest case, provided that the elastic strength ° T is
known, determination of the shut-in pressure will enable calculation
of ° h,. of breakdown pressure P ,
stress 0H'
b
and of the maximum horizontaL
Other information can be obtained by measuring the reopening pressure
Pr of a natural or previously artificially created fracture, the
instantaneous shut-in pressure (ISIP) or the long waiting time shut-in
pressure, or by determining other parameters. It is important in all cases
to be able to associate a particular hydraulic operation with any change
in aspect of the fracture, whence the interest of a tool that can provide
an image of the wall in real time.
5. TESTS
~umber of tests of the sonde described above have been carried out
in borehole INAG 111-9, at Mayet de Montagne, at depths between 200 and
450 m. The tests are not yet complete, and certain modifications will
still have to be made to the tool, but nevertheless some interesting
results ha.ve already been obtained.
- As far as the detection of natural fractures is concerned, the new
instrument provides images entirely comparable lITith that of Figure 1,
obtained with a conventional detector. Only the angular resolution is
slightly less satisfactory since the number of electrodes is reduced to 16
instead of 24.
- Operating in a fixed position opposite a sound wall that is to be
fractured, the instrument behaves somewhat differently. In effect, the
experiment has shown that fractures obtained by hydraulic overpressure are
much smaller than natural fractures.
In order to be able to show these artificially stimulated fractures
it is necessary to increase substantially the sensitivity of the
measurement circuit. This causes interference to appear, the amplitude of
which remained negligible while only natural fractures were being sought.
This could be due either to electrical currents of industrial frequency
(50 Hz) or to currents circulating in the fluid in the well. The result is
often a confused image similar to that shown on Figure 3a.
When a small artificial fracture is produced, this barely appears
(Figure 3b) or sometimes does not appear at all, either because it is
drowned by background noise, or because it is produced outside of the part
of the chamber 'seen" by tho sonde.
The results can be appreciably improved in the following manner :
Most of the disturbing factors are either stationary in time or
variable with a period different from the sampling period. By calculating
the average of a certain number of images (eg, 10) the time-dependent
signals are considerably attenuated •. Then, the difference, point by point,
between two succes~ive images ~ffected at intervals of a few seconds are
calculated. If the images are both obtained before the process of
artificial fracturing, the backgrour.d is almost entirely cancelled (Figure
4a) . If the same operation is effected bet~Teen an image obtained

209
immediately before and one obtained immediatly after the fracturing.
Figure 4b ie obtained. on which. thie time. a fracture ie quite clearly
shown.
The case under con.ideration is a subvertical fracture at a depth of
339 m in an experimental borehole. Other artificial fractures have been
identified in the same way. The method is equally applicable to the
reopening of exieting fractures. though leu efficiently since it shows
only the variability in permeability of the fracture and not its
existence. For this reason it is essential to improve still further the
quality of the basic image by reducing the relative importance of the
interference. Research into thie is in progrese.
6. CONCLUSIONS
A f.w months from the end of the contract signed with the EEC. it can
be eaid that the objectivee fixed by the LGA and the IPGP have been
achieved. The instrument that hae been developed is effectively capable of
giving an image. in real time. either of the vall of the borehole during
conventional logging or of that of the compression chamber during an
artificial fracturing experiment. Natural fractures are readily detected.
The much emaller fracturee obtained by hydraulic overpressure require
relatively simple proceBling of the lignah. The dip and strike of the
fractures can in all cases be determined without difficulty either in real
time or later from recorded eignale.
The existing prototype can still be improved. mainly in respect of
development of an efficient pressure distributor. reduction of electrical
interference and the adaptation of the instrument to taking measurements
in eedimentary terrain or at high temperatures. None of the problems
existing at present appears insurmountable. but the development of a fully
operational apparatu. may require a certain time.
BIBLIOGRAPHY
Haimson. B.C. (1978). The hydrofracturing etresa measuring method and
recent field reeults. Int. J. Rock Mech. Min. Sci .• 1S. 167-178.
Hickman. S.H .• and M.D. Zoback (1983). The interpretation of hydraulic
fracturing pressure-time data for in situ stress determination.
Proceedings of the Works chop on Hydraulic Fracturing Stress
Heasurements. 44-S4. US National Committee on Rock Mechanics. National
Academy Pr •••• Wa.hington D.C.
Cornet. P .• and B. Valetta (1984). In situ stresa determination from
hydraulic injection test data. J. Geophys. Ree .• 89. 11.S27-11.S37.

210
cable
It--- hiBh pressur pipe
.....1~tr---- connection head
_+ ___ water distributor
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0132.0) II
""",,",""" __ ~,ea surente nt e i ectrode
~,""" ____ focusing electr ode
1 O\~e'r packer:
Otl) .73 ..
electronic case
Figur e 1
"Electric image"
of fractures in a
borehole
01l~.44 m
Figure 2
Schematic structure
of the orobe

211
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213
EEC contract no ENlG-0054-UK
ROCK STRESS ORIENTATIONS FROM BOREHOLE BREAKOtn'S
N.R. BRERETON and C.J. EVANS
British Geological Survey, Keyworth
Summary
Until recently very little in situ stress data has been available
from the UK crust. Literature reviews list only a limited number
of measurements which include information on the direction of
maximum horizontal compression. Unequal horizontal crustal
stresses imposed on vertical boreholes will often cause localised
rock spalling along the direction of least compressive stress.
The resultant elongations of the borehole cross section, called
breakouts, can be identified from four arm caliper recorda
collected routinely with conventional geophysical dipmeter logs.
Eighty one boreholes in the United Kingdom have been analysed for
breakouts using criteria for identification of breakouts unique to
this paper. The majority of borehole breakouts in southern
Britain are aliqned NE-SW, indicating a maximum horizontal stress
in a NW-SE direction. There are insufficient data to confidently
assess a dominant borehole breakout direction in northern Britain,
although the four boreholes analysed indicate possible variations
in the crustal stress field.
1. INTRODUCTION
Stresses in the ·Earth I s crust are speculated to arise from many
sources, plate driving forces, temperature changes (cooling and
heating), qravitational loading by volcanic and sedimentary processes,
and from poorly understood processes of mid-plate tectonics (Solomon et
41. 1980). The maqnitude and direction of the principal stresses in
the brittle crust influence the propagation of faults, the opening and
closing of fractures, and pore water circulation.
The databank of stress measurements in the earth I s crust remains
inadequate because methods of measurement are in general expensive.
Research has demonstrated that stress measurements cannot be reliably
extrapolated any distance from the measurement location (Hyett et al.
1986). Therefore, our knowledge of regional or local stress levels
r_ins poor due to a lack of measurements both of a qualitative and
quantitative nature.
Theoretical and laboratory based studies carried out in recent
years have shown that unequal horizontal crustal stresses imposed on
near vertical boreholes will often cause localised rock spalling along
the direction of least compressive stress (Zoback et al. 1985, Haimaon
and Herrick 1986). The resultant elongations of the borehole

214
cross-section have been referred to as breakouts. The measurement of
the azimuthal frequency of breakouts, using data from existing
boreholes, presents a unique opportunity to expand the regional stress
direction data base and to study the distribution and mechanisms of
stress fields in more detail than has been possible hitherto.
Borehole breakouts can be identified from the four arm caliper
records collected routinely with conventional geophysical dipmeter
logs. The majority of boreholes drilled for hydrocarbon exploration
purposes are logged for dipmeter and so the number of boreholes
potentially available for breakout analysis in the UK is large
(Brereton and Evans 1987).
2. A Review of Stress Measurements in Britain and NW Europe
until comparatively recently, very few stress measurements had
been made in Britain and knowledge of the state of stress in British
rocks was limited. Klein and Brown (1983) carried out a comprehensive
review and found only six sets of measurements which included complete
information on the state of stress (Fig. 1). Even so, the authors cast
doubt on the reliability of many of these data.
until recently the only high quality horizontal stress direction
and magnitude determinations came from work at RoseDlanowes Quarry in
the Carnmenellis Granite, Cornwall (Batchelor and Pine 1986). Sixteen
hydraulic fracture tests were attempted from near-surface to a maximum
depth of 2550 metres. Although the tests were successful in achieving
• breakdowns', only in one case was a hydraulic fracture identified.
Therefore, although the minimum stress results are considered to be
reliable, the maximum stress and the orientation of the stress fields
remains questionable. A series of overcoring tests in South crofty
Mine, 10 km from Rosemanowes, plus hydrofracture testing and overcoring
(Cooling and Hudson 1986 ) from Carwynnen, also in the Carmenellis
Granite, are in good agreement with the work of Batchelor and Pine
(1986) and confirm a regional NW-SE maximum stress orientation and a
very high ratio of maximum to minimum stress in the granite.
Cox (1983) was the first to publish breakout results from western
Europe when he included data from three boreholes in the North Sea.
This was followed by BIUmling et al. (1983) who reported results from
the Urach 3 crystalline rock geothermal borehole in southern Germany.
European breakout analyses have subsequently been reported for a
further twelve boreholes in Northern Germany and Holland (Draxler and
Edwards 1984) and for five additional boreholes in SWitzerland (Becker
et al. 1984 and 1987).
Klein and Barr (1986) extended the data set for Britain by
presenting borehole breakout azimuth plots for eleven boreholes from
the North Sea, two offshore to the north of Scotland and a further.six
from onshore Britain (mostly in the East Midlands). The boreholes
penetrated a variety of generally near horizontally-bedded sedimentary
rocks including sandstones, siltstones and mudstones. Borehole
penetration ranged in depth from 240 m to 3970 m. Stress magnitude
data have recently been further augmented by three hydrofracturing
measurements at Morley Quarry and Rempstone in the East Midlands and at
Wray in northern England (Evans 1987). The results from these
hydrofracturing tests are in broad ·agreement with those reported for
the Cornish granite.

21S
'l'he .,re recent .tres. measurements, alonq with compilations of
Briti.h and _.tern European data from in .itu .tres. measurements,
earthquake focal plane .olutions and geoloqical .tress indicators
(Klein and Barr 1986), all provide evidence to suqgest a generalised
NW-SE IIIIlXiIllUlll .tre.. direction for IlUCh of North west Europe and the
Briti.h Isle •• It ha. been argued (Bott and I(ueznir 1984; and lUein
and Barr 1986) that this predominant orientation i. largely deterained
by plate tectonic boundary force ••
Compari.on of Briti.h stre •• _qnitudes with EurOpean results i.
difficult becau.e of a lack of deep data throuqhout Europe. H~ver, a
qlobal review and theoretical analysis of the stress pattern in the
upper crust (Rummel 1986) cited the Cornish results a. an extreme case
of hiqh deviatoric .tres.. It i. thus not surpri.inq,perhaps, that all
three fault plane mechanism .olutions available from the United Kinqdom
deaon.trat. strike Blip .,tion within the middle-upper cru.t (0-20
km). Th.s •• arthquakes. and recent stres.s aeasurements (Evants 1987).
confira a high .h.ar stress regime for Britain.
3. The Br.akout Hypoth.sis
Babcock (1978) introduc.d the t.ra br.akouts followinq a study in
which h. .xamin.d the r.lation.hip bet_.n bor.hol. .longations and in
.itu .tr••• r.qime.. Althouqh Babcock (1978) was somewhat sceptical of
the relation.hip be~en the orientation of br.akouts and in situ
.tr•••,
Bell and Gouqh ( 1979 ) arqued that boreholes drilled throuqh
rock. bearinq unequal horizontal .tre.... could concentrate the
.tr..... .0 a. to produce sub-surface br.akouts parall.l to the minilllUlll
.tr... dir.ction. In a .ubeequent paper, Gouqh and Bell ( 1981 )
.uqqeated that br.akout. fir.t propaqate in the borehole very soon
aft.r the pa.sage of the drill bit and continue to develop with time
both alonq the axis of the bor.hole and into the fo~tion. In a
.erie. of .ub.equent .tudie. by Bell and Gouqh and th.ir co-workers, it
became incr.a.inqly cl.ar that breakouts are caus.d by localised
compr ••• ive .hear fracturinq of the bor.hole walls in a direction
to that of the maxiIIIUIII horizontal principal stress (Podrouzalt
no~l
and B.ll 1985).
'l'he application of br.akout .tudie. has .xpand.d rapidly coverinq
topic. frOll! tectonic. of the oceanic crust (N~k et al 1984) to
geothe~l en.rqy (Plumb and HiclaDan 1985). A compr.hensi v. test of the
breakout hypoth •• i. in a .inqle USGS - Schlumberger borehole experiment
cSemon.trat.d .ucce•• fully that dipmeter-deriv.d bor.hole .longations
can be u •• d to infer dir.ction. of miniIIIUIII horizontal in .itu stress
and that the.e dir.ction. are in excell.nt aqr.ement with those
determin.d ind.pendently frOll! borehole televiewer and hydraulic
fracturinq t •• t. (Plumb and HiclaDan 1985, HiclaDan et al. 1985, and
Zoback et al. 1985). Plumb and Cox (1987) corroborated the findinqa of
the USGS - Schlumberger experiment by analysinq borehole elongations
(breakout.) for 47 borehole. in ea.tern North America. 'l'he str.ss
dir.ction. they determined _r. in aqr.ement with published
interpr.tations f~ hydraulic fracture te.tinq and .arthquake fault
plane .olutions. 'l'hey found that breakouts are &ystematically aliqned
perpendicular to the direction of hydraulic fracture., NB-.triltinq
natural fracture. and centreline. fractur.. (near-vertical, tensile
fracture. which fora durinq corinq). 'l'hey further concluded that the
unifora br.akout pattern i. not caused by borehol.. inter.ectinq a •• t

216
of NW-striking natural fractures but is caused by stress-induced
borehole failure,so confirming the work of Hickman et al. (1985).
4. Experimental and Theoretical Studies
With the advent of the borehole televiewer it is possible to
determine not only the depth and azimuth of the breakouts but also
their span and shape. This has led to a quest to develop theoretical
relationshj,ps between breakout dimensions and "principal stress
magni tudes.
Bell and Gough (1979) presented a theoretical hypothesis based
upon the Kirsch (1898) equations to support their thesis that breakouts
are caused by the symmetrical spalling of regions of the borehole wall
where the stress concentrations exceed the shear strength of the rock.
Gough and Bell ( 1981) and Bell and Gough (1982) predicted that the
localized stress concentrations cause a region of failure, triangular
in cross section and enclosed by flat conjugate shear planes.
Extension of the theoretical analysis of breakout formation mechanisms
(Zoback et aI, 1985) has shown that many breakouts can be rather broad
and flat-bottomed, unlike the triangular or "dog-ear" shapes predicted
by the work of Gough and Bell (1981).
Paillet and Kim (1987) found that breakouts appear to have little
effect on refracted compressional and shear waveforms, and that
velocities calculated from the waveform arrivals also showed no obvious
changes. On the basis of their observations, they concluded that the
effects of spalling are confined to the immediate vicinity of the
borehole wall and do not extend to depths equivalent to a significant
portion of one acoustic wavelength (about 25cm). The validity of these
observations is however debatable, since the exact path of the seismic
wave is unknown with regard to the restricted zone of fracturing caused
by the breakouts.
Zoback et al. (1985) used a plane strain model to calculate the
radial, tangentia"l and shear stress. These were then compared with the
streases required to cause shear failure using the linear Mohr-Coulomb
failure criterion. Near the borehole, the stress concentration results
in markedly curved potential shear failure surfaces brought about by
rotation of the azimuths of the principal horizontal stresses near the
free surface of the cylindrical borehole (Fig. 2). Their theory was
able to predict the general shapes of observed breakouts and
demonstrated that, as the stress ratio increases, the depth, though not
necessarily the width, of the breakouts will increase. It was also
demonstrated that the breakout shape is strongly influenced by the
difference between the fluid pressure in the borehole and that in the
formation. A high fluid pressure in the borehole diminishes
substantially the size of the breakouts while a lower pressure promotes
breakout development. This influence is due to the change in normal
stress on failure planes near the borehole wall. The cohesive strength
of the rock was also shown to be important, no breakouts developing in
rocks with high cohesive strength under certain conditions of
differential stress and frictional sliding (Zoback et al. 1985).
However, for rocks with low cohesive strength the breakouts would be so
large as to nearly extend around the borehole. This latter situation
could appear to be similar to that often described as a washout (Cox
1983).

217
an. of the limitatiolUl of the theory described by Zoback .t ale
(1985) 18 that it could only accOllllllOdat. the formation of breakouts in
initially cylindrical boreholes (Oetournay and Roegiers 1986).
Although the th.ory explailUl satisfactorily the broad flat-bott~d
br.akout., it i. not able to .xplain the deeper V-shaped breakouts
which are v.ry lik.ly influ.nc.d by anelastic deformation during
failure and time-dependent .ffect. r.lat.d to sub-critical fracture
growth.
If the maximum to minimum str.ss ratio is greater than three it is
po.tulat.d by B1Umling .t ale (1983) that compressional stress at the
bor.hol. wall in the maximum str.ss dir.ction is reduc.d and may .ven
become t.nsil.; th.r.for., the drilling of a circular borehole may
caus. t.n.il. fracturing parallel to the maximum stress direction in
addition to br.akout .longations parall.l to the minimum stress
dir.ction. Thi. mechanism of tensile failure is a possible explanation
for the c.ntr.-lin. fracturing obs.rved in cor •• (Plumb and Cox 1987).
Furth.rmor., B1Umling .t ale (1983) suggest.d that the determination of
the horizontal compr ••• ion axis from breakouts, in conjunction with
fault plane .01utiolUl, provid •• a means not only of determining the
int.rnal friction of the rock at depth but also of defining the activ.
fault plan ••
During laboratory t.st. Haimson and Herrick (1986) observed four
.tage. of br.akout. Firstly, short v.rtical hairline cracks appeared
which r •• ult.d from grain splitting. Th. hair lin. cracks occurred at
the po.ition wh.r. the minimum str.ss axis intersects the borehole
wall, but the crack. thems.lv.s were aligned roughly with the maximum
.tr••• axi.. S.condly, the segment defin.d by the hairline cracks
remain.d approximat.ly con.tant, but as the magnitude of the maximum
.tr••• incr.a •• d the crack. t.nded to link up to form long winding
crack.. Thirdly, at high.r valu.s of maximum stress magnitude,thin
.lab. of rock began to .pall away from the bor.hole wall while others
were d.tach.d from behind. Finally, a maximum .tr.ss magnitude was
reach.d at which .palling ceas.d and the br.akout appeared to
.tabiliz••
Th. experiment. confirmed that,wh.n the stat. of in situ
stres. i. the only controlling factor, br.akouts conc.ntrat. along two
diametrically oppos.d arc. of the bor.hol. cro ••- •• ction and the
centr.. of the arc. are align.d with the dir.ction of minimum str.ss.
Confirmation of the third .palling .tage i. known from the r.covery of
two .andaton. casinga, from a d.pth of about 3000 metr ••, which
depicted both the original concav. bor.hol. wall and a typical convex
compr ••• ive .h.ar failure .urfac. (Xl.in and Barr 1986).
The most important r.sult from the experimental work of Haimson
and H.rrick ( 1986) i. the demonstration of the cl.ar correlation
betwe.n breakout dimen.iOlUl and in .itu .tr••• magnitude. Theor.tical
curve., d.ri v.d from the work of Zoback .t ale (1985), which relate the
depth and .pan of the br.akouts to the minimum str.s. magnitude, show
good qualitative agr.ement with the experimental r.sults. However, the
quantitative agr.ement is not .0 good,implying that r.finements to the
theoretical model are .till n •• d.d.
5. Br.akout R.cognition Crit.ria
The four ana dipmet.r mea.ur •• the .trik. and dip of bedding
plane. by cro ••-correlation of el.ctrical a!cror •• i.tivity trac ••
recorded by each of four orthoqonal pada pr •••• d aqailUlt the bor.hol.

218
wall. One of the pads is magnetically oriented (usually referred to as
pad 1) and the two opposite pairs of pads (pads 1-3 and 2-4) provide
two independent caliper measurements of the borehole diameter. The
tool is also equipped to measure the angle of deviation from vertical
of the borehole. As the dipmeter tool is winched up on the wire line it
rotates and when one pair of the caliper arms encounters an elongation,
the pads tend to "lock" into the elongation and rotation ceases.
Interpretations of characteristic dipmeter caliper logs from
out-of-gauge boreholes are described in terms of breakouts, washouts or
key seats (Fig. 3). Plumb and Hickman (1985) extended criteria devised
by Fordjor et a1. (1983) as follows:
1) the tool rotation stops in the zone of elongation,
2) the difference between the two calipers is greater than O. 6cm
( O. 24 inches),
3) the smaller of the caliper readings is close to bit size, or if
the smaller caliper reading is greater than bit size it should
exhibit less variation than the larger caliper,
4) the elongation zone length is greater than 30cm (11.81 inches),
5) the direction of elongation should not consistently coincide with
the azimuth of the high side of the borehole when the hole
deviates from vertical.
Conditions 2 and 4 were introduced in recognition of the
limitations imposed by the physical size of the dipmeter tool. On the
other hand condition 5 takes account of the fact that as a borehole
becomes increasingly deviated from the vertical, drill-pipe wear of the
formation causes preferential elongation (key seating) which, combined
with the weight of the tool, inhibits tool rotation.
6. Borehole Breakouts in the United Kingdom and Methods of Analysis
The study has concentrated on two aspects, a critical assessment
of the breakout recognition criteria and methods of analysis, and the
analysis of data from as many UK onshore boreholes as resources
allowed. A statistical approach is adopted where data are analysed in
detail from each borehole, without examining the possible relationships
between stratigraphy, lithology, fracturing, faulting or other
information which may be available for each borehole or each region.
It is felt that the available borehole breakout recognition
criteria are somewhat subjective and that their application would
result in the rejection of valid data. For example, discontinuous
breakouts of the type depicted by Pai11et and Kim (1987) from
televiewer records would be unlikely to cause the dipmeter tool to
cease rotation. Similarly, poorly developed breakouts in rocks of high
cohesive strength could not only inhibit the cessation of tool rotation
but could also be rejected on the grounds that the difference between
the two calipers is not great enough.
To minimise the likelihood of asymmetric elongations due to drill
pipe wear (key seats) being confused with breakouts, a borehole
deviation upper limit of 10° was set for all the boreholes analysed. A
sensitivity analysis was carried out on several boreholes which
deviated from less than 1° to greater than 30° and it was found that
the 10° limit was well within the range where the effects of the
deviation could no longer be detected. This value of 10° is in
concurrence with that set by Klein and Barr (1986), although Plumb and
Cox (1987) set a limit of only 5°.

219
In a borehole with a predominant elongation in a particular
dir.ction, the tool rotation will result in the pad 1-3 caliper
becoming proqres.ively larger and then smaller, while the pad 2-4
caliper will bec~ correspondingly .maller and then larqer. Thus, the
differ.nce between the two calipers, referred to _ the caliper
.cc.ntricity, will o.cillate between positive and neqative values.
Since only the magnitude of the eccentricity is of interest, a computer
proqrua i. ..t to as.ign the larger of the two caliper values to
caliper 1 irrespective of whether it is actually pad 1-3 or pad 2-4
caliper. Similarly, a. the tool rotates frOlll O· to 180· the caliper
value. measured by the pad 1-3 caliper will mimic the values measured
by the .ame caliper a. the tool rotate. frOlll 180· to 360·. Since the
borehole IllU8t be assumed to be syaaetrical, it is only necessary to
consider azimuth values frOlll O· to 180· and all values frOlll 180· to
360· can be a.signed to correspond to O· to 180·.
Th. Hobqoblin borehole drilled near Stoke-on-Trent by British Coal
is us.d a. an example of the concepta developed _ part of the
analytical methoo.. In this borehole there are clear differences between
the ov.rlain caliper log. which may be attributed to breakouts (Fig.
4) • The azimuth of pad 1 data for Hobqoblin borehole shove a main
trend between about 45· and 6S 0 (Fig. Sa). A eross plot of eccentricity
again.t a.imuth .how. that the highest eccentricity values occur mainly
between 40· and 80· in both borehole. (Fig.Sb).
An additional method of presenting the data is to produce a cross
•• ctional view of the bor.hole by -stacking- all the caliper
information with depth onto one diaqrua (Fig. Sc). The same 40· to 80·
a.imuthal data grouping can be seen along with a clearly defined
orthoqonal qrouping which exhibits much .maller caliper values. The
lIO.t illustrative presentation of azimuthal frequency is the rose
diaqrua (Fig. 6a). Thi. is e •• entially an alternative to the frequency
hi.toqrua but it allows a much clearer visual presentation of the
a.imuthal di.tribution of the data. Of the varioua methods of data
pr •• entation the rose diaqrua was chosen _ the III08t suitable for the
final presentation of r •• ult ••
During the initial .taqe. of developing analysis concepts it was
r.cogni •• d that cla •• ical breakout occurrences (Fig. 3) usually exhibit
a .harp ri •• in the .ccentricity value at the onset of the breakout
interval, followed by en equally .harp fall at the .nd of the breakout
int.rval. However, in .c.e borehole. there can be long depth intervals
wh.r. there i. a continuous and .ignificant caliper difference but with
no cl.ar pr.f.r.ntial orientation. Thi. behaviour can be seen to a
limit.d deqr •• between 743 • and 750 m in the Hobqoblin borehole (Fig.
e). A .... ur. of the rat. of change of the eccentricity with depth,
r.f.rr.d to .. the caliper qradient, .hould be able to differentiate
between th... diff.rent type. of eccentricity occurrence by filtering
out .uch d.pth interval. and highliqhting the cla.sical type of
br.akout occurr.nce ( Fig. e).
Th. ro •• diaqram present.d in Fig. 6a doe. not take account of the
"gnitude of .ith.r eccentricity or gradi.nt but .imply repre.ents the
nuaber of caliper 1 (i••• larqer caliper) values within each azimuthal
cla.. int.rval. There are •• veral alqorithma which can be adopt.d to
utili •• the .cc:entricity/gradient .. qn1tude.. on. _thod i. to apply a
cut-off function to ~ve all the low eccentricity/gradient data
(c.f. Plumb and Hickaan 1985). Th. aa1n weakn ... of the _thod is that

220
a subjective decision is needed to identify which cut-off function to
apply. The preferred method of data treatment is to weight each
azimuthal data point according to the magnitude of the eccentricity or
gradient. The simple approach is to choose a linear weighting function
whereby the sum of the eccentricity/gradient values within each
azimuthal class interval is plotted (Figs. 6b and 6c). Comparative
studies show that this method produces a similar result to that
achieved by the cut-off function, without recourse to some arbitrary
choice of cut-off value.
The linear weighting function gives equal prominence to both the
very large number of low eccentricity/gradient values and to the much
smaller number of higher values. For some boreholes, where there are
very few high eccentricity/gradient breakouts, it is sometimes
difficult to decide upon a prominent breakout direction and also
difficult for the computer to locate a maximum breakout trend
unequivocally. To find the orientation of minimum stIess, the computer
program locates the maximum breakout trend and computes the mean
azimuthal angle within a :25 0
window about the trend.
Although the mean breakout orientations have been derived from
linearly weighted data, this approach, as applied here, is not
statistically rigorous in that the eccentricity/gradient values are not
nOr3alised to dimensionless quantities. Therefore, the minimum stress
orientations derived from the eccentricity and gradient data, and more
particularly their associated errors, are not strictly comparable.
Fitting a Gaussian distribution to the data sets would enable a
normalised weighting function to be devised based upon the standard
deviation about a dimensionless mean. Direct comparability could then
be established between the eccentricity and gradient results and
between the caliper and resistivity results.
Despite these reservations the mean orientations derived from the
eccentricity data and from the gradient are usually very close. Where
there is a significant difference it is substantial, and often there
are two orientations which are orthogonal, or nearly so.
7. Stress Orientations from Resistivity Anomalies
Zoback et ale (1985) noted that stress concentrations near the
borehole result in curved potential shear failure surfaces and that
breakouts occur when the surfaces spall away from the borehole wall
(Fig. 2). Microfractures associated with borehole failure result in a
localised enhanced formation porosity which is preferentially invaded
by drilling fluid (shaded areas on Fig. 3). The localised invasion
produces anomalously low electrical microresistivity readings on the
dipmeter pads as they traverse the minimum stress orientation. The
resistivity information recorded by the dipmeter pads can therefore be
used to determine minimum stress orientations in much the same way as
the caliper differences. Such an azimuthal correlation between caliper
eccentricity and resistivity anomalies confirmed breakout orientations
in the Auburn borehole (Plumb and Hickman 1985).
At any given depth, differences are to be expected between the
resistivity pads by virtue of the strike and dip of bedding features.
Ideally, the effects of the strike and dip should be taken into account
before the stress-induced effects are evaluated. However, this would
significantly increase the complexity of the computing so a simple
averaging is used instead. Thus, the resistivity values recorded by

221
each opposite pair of pads are averaged to give resistivity 1 (pads 1
and 3) and resistivity 2 (pads 2 and 4). The relative values of
resistivity 1 and resistivity 2 are directly analoqoua to caliper 1 and
caliper 2 and the subsequent data analysis can therefore be treated in
the same way. A direct comparison of the eccantricity and gradient
results for caliper and resistivity 1098 shows a broad correspondence
(Fig. 4). When the resistivity data are used to plot linear weighted
eccentricity and gradient rose diagrams (Fi98. 6d and 6e), the overall
characteristics and the mean derived orientations of minimum stress are
again very similar to those exhibited by the caliper data.
8. Results and Discussion
Insufficient resistivity data have been analysed to draw any firm
conclusion.However, from the nine boreholes with resistivity data there
is often, though not always, a good agreement between the
caliper-derived and resistivity-derived orientations. The deviation
about the mean for the resistivity data _y sometimes be higher than
for the caliper data, but this _y be because the effects of dip and
strike have not been accounted for.
The dominant eccentricity and gradient directions for all
boreholes studied are shown at their appropriate geographical locations
(Figs. 7a and 7b respectively). on each of the figures is an inset
representing the dominant direction of the sUDBation of all breakout
orientation data presented. The results presented in the figures have
not been no~lised in terms of depth or the number of data values used
in the analysis, i.e. results from a 300 • borehole will appear to be
of equal significance to those from a 3 ka borehole.
The _jority of the boreholes analysed in southern England and the
Midlands present a coherent picture of ainimum in situ stress with an
average orientation of 54-/234- * 11. This is in close agreement with
previous stress studies (Xlein and Brown 1983, and !G.ein and Barr 1986)
and aleo with geological indicators (Bevan and Hancock 1986). The
_jority of breakouts in one borehole exhibit a unimodal orientation,
but a significant number are bimodal and exhibit an orthogonal
distribution while others have a uniDlOdal trend which is orthogonal to
the average orientation. orthogonal trends are particularly applicable
to the four boreholes analysed from Scotland.
Bimodal orthogonal distributions have been observed in many North
American boreholes. Pluab and Hickman (1985) suggested that orthogonal
distributions _y be due to a transition fro. elastic to plastic
defo~tion and argued that a borehole drilled into rocks that deformed
plastically will tend to have the largest diameter aligned with the
~ stress direction rather than the ainimum stress direction.
Plastic def~tion has been identified in this study. High
drillincraud pressures, which result from using dense additives to
stabilise boreholes drilled in partially consolidated rocks, not only
ainiaise spalling in the ain1mua stress direction (ZOback et ale 1985)
but _y initiate localised hydraulic fracturing in the maximum stress
direction, which subsequently cave and increase the borehole diameter
(Bell " Babcock 1986). A8 an alternative mechanis., Bell and Babcock
(1986) aleo proposed that preferential aud-cake build up in breakout.
could result in an eccentricity aligned with the aawimum stress
direction. In addition, B1Ua1ing et ale (1983) postulated that tensile
fracturing in the aaxiaua stress direction _s dependant upon the

222
maximum to minimum horizontal stress ratio. Whatever the mechanism it
seems that breakout orientations which correlate orthogonally with the
regional average breakout orientation may be aligned parallel to the
maximum in situ stress direction.
The Sutherland borehole in northern Scotland (northernmost data
point Fig. 7a and b) exhibits a clear unimodal breakout orientation
which is almost orthogonal to the regional trend and to the minimum
stress orientation which would be expected from geological indicators.
Clearly, a more detailed study of the available data is needed in such
cases before it can be said whether this orientation represents the
maximum stress direction derived from tensile fractures or a regional
variation in minimum stress direction derived from breakouts.
Hyett et a1. (1986) stated that the residual stresses, locked into
the rock by previous tectonic events, are responsible for a significant
component of the scatter in results experienced during most in situ
stress measurement programmes. Pai11ett and Kim (1987) observed a
systematic drift in breakout direction shown by a televiewer record
which could indicate a real rotation in stress orientation or, as they
felt to be more likely, represent the effects of ferro-magnetic
minerals in the borehole wall on the orienting device. Plumb and
Hickman (1985) attributed anomalous orientations in the Auburn borehole
to drill pipe wear but they also affirmed the value of the resistivity
anomalies to help distinguish between stress-induced and anomalous
elongations. Systematic variations in the orientations of anomalous
breakout trends could, on the other hand, lend support to the idea of
stress decoup1ing at geological discontinuities (Becker et a1. 1987,
Haimson 1982).
To help interpret the regional significance of stress orientations
in western Canada, breakout orientations were used to construct a
horizontal stress trajectory map from which stress field rotations were
inferred (Bell and Babcock 1986). Stress rotations were then
correlated with subsurface basement topography. The application of
similar interpretational techniques to the UK and to western Europe
could help elucidate some of the detail of finer stress-field
orientation. No attempt has been made to advance geological
interpretations based upon the results presented here since this
requires a wider ranging integration of information pertaining to deep
geological and geophysical structures.
9. Conclusions
It is clear from previous research that the drilling of a borehole
in pre-stressed rock produces a concentration of stress near the free
surface of the borehole wall. This in turn may induce shear fractures
with associated spa11ing of material in a direction parallel to the
minimum stress orientation, a borehole breakout. The fracturing of the
rock also results in anomalously low formation resistivity values which
can also be used to determine the minimum stress orientation.
Analytical methods are developed which limit the need for
subjective judgements. However, the level of detail that can be
achieved during an analysis is such that a range of results is derived
for each borehole. Additional refinements of the analytical techniques
(i.e. depth, lithological correlation) are proposed which will allow
for a closer comparison of results derived from within and between
individual boreholes.

223
In the united Kingdom, breakout orientations have been analysed
for eighty one onshore boreholes and regional distributions of inferred
minimum .tre.. orientations have been plotted. The overall picture is
one of an average breakout orientation of 54 ° /234· :t 11·, whJ.ch is
.imilar to the previous few measurements of minimum stress directions
in Britain and _stern Europe. In selected areas the results can be a
little more variable. In the Midlands there is a bimodal trend at
71°/251· :t 10° and at approximately 45°/225°. In southern England the
orientation is at 52°/232° :t 10°, while two sub regions of southern
England are 45°/225. :t go (Hampshire Basin) and 61°/241° :t SO (wytch
Farm). Individual boreholes exhibit variations about the average
trends, some of which are orthogonal and could be explained as
indicating the maximum stress orientation. There are also indications
of stress rotationa or decoupling horizons; h~ver, the fine details
encompassed within the results have not yet been fully exploited.
The re.earch demonstrates that borehole breakouts can be used
successfully to construct coherent distributions of regional stress
orientation.. The raw information suitable for breakout analysis has
been, and continues to be, widely collected for other purposes in the
hydrocarbon and minerals industries. It is clear therefore, that the
potential to evaluate detailed regional and local stress systems is
considerable.
10. Acknowledgements
The authors gratefully acknowledge the Commission of the European
Communities (Contract EN3G. 0054. UlC (H» for funding this research.
Thank. are given to Mr T J Fowkes, Mr C P Royles and Mr A D Evans who
helped with the analysis. We also wish to express our gratitude to
Briti.h Coal, Briti.h Petroleum, Car less Exploration, Taylor Woodrow
Energy, and Ultramar Exploration for providing data.

224
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(B7), 6223-6234.
Plumb, R. A. and Cox, J. W., 1987. Stress distributions in eastern
North America determined to 4.5 ~ from borehole elongation
_asur_ents. Journal of Geophysical Research, 92, (B6) ,
4805-4816.
Plumb, R. A. and HiciaDan, S. B., 1985. Stress induced borehole
elongation 1 A comparison between the four arm dipmeter and the
borehole televiewer in the Auburn Geothermal well. Journal of
Geophysical Research, 90, 5513- 5521.
Podrouzek, A. J. and Bell, J. S., 1985. Stress orientations from well
bore breakouts on the Scotian Shelf, eastern Canada. current
Research, Part B, Geological survey of Canada, Paper 85-1B, 59-62.

226
Rummel, F., 1986. Stresses and tectonics of the Upper Continenta~
Crust; a review. Proceedings of the International. Symposium on
Rock Stress and Rock Stress Measurements. stoc)cholm, Edi tor
stephansson, 0., 177-186.
solomon, S. C., Richardson, R. M. and Bergman, E. A., 1980. Teotonio
stres'S! mo~s and magnitudes. Journal of Geophysical Research,
85, 8 11, 6086- 6092.
Sprin~, J. E. and Thorpe, R. K., 1985. Borebole elongation versus in
situ suess o:cientation. Proe. conf. on Updating Surfaoe
Samplings of Soil and Rocks and their In Situ Testing, Santa
Barbara. Jan. 1982, 425-432. Pabl o New York ; Engi neering
Foundation , 1985.
Zoback, M. D. Moos, D., Mast in, L., and Anderson R. N., 198S. well bor e
breakouts and in s1tu stress. Journal of Geophysica~ Researcb ,
90, (B7),
Figure 1 :
Directions of UJaX.imwn borizontal OOIIIpression in the Brit ish
Isl es (after Klein and Brown, 1983 ) .

227
Ud
!b
CV-orr
(el
(dl
+
(b)
I~ 1-3 I
~2-4--
Figure 2 :
a) Orientation of potential shear
failure aurfaces adjacent to a
borehole wall
b) Areas in which failure is expected
(after Zoback et al. 1985)
Figure 3:
Examples of dipmeter caliper logs
and common interpretations of the
borehole geometry (after Plumb and
Hickman. 1985)

o
PIAl 360 5
OEVl 10 5
C2 12 o FCI 240
Cl 12 0 ECC 5 -25 GRAD 25 0 FC2 240 0 ECCR 120 -300 GRAR 300
690
700
710
i 720
E
i 730
740
750
760
770
780JL----~~~~~~=-~~~~--~--~~~--~~i5~~~;;~~--~~;i
KEY TO LOGS:- PIAl-Pad 1 Azimuth (donad linal OEVl-Oaviation Cl-Caliper 1 C2-Caliper 2 (donad final ECC-Calipar accentricity
GRAD -Caliper gradiant FCl- Pad 1 resistivity FC2 -Pad 2 resistivity (donad linal ECCR -Rasistivity eccentricity GRAR -Rasistivity gradiant
Figure 4:
Section of caliper and resistivity logs from Hobgoblin
borehole with associated derived eccentricity and gradient
functions.

"'lOU ty
---------,
y
A.
A
20 40 eo 10 100 120 l ~a lea lea
AZIMUTH '1.0 t
CALtPfR
lCCOmllCtTy
B
AZI WUTlUllQ 1
c
-2
/'
/
I
I
\
",, -
AZIMUTH 'AD 1
HOBGOBUN BOREHOLE
D"pth "'"g from 500 to 1 300 metra
Number 0 1 til pOil'lt - 643
r1qure Sa!
Sbl
SCI
Frequency 4i8tribution of P Ad tho
CrotUlplo of P 4 1 lUi :thv r.us lip6C .. cc ntrlclty .
ero.. a ctJ..oo of bar.bol . t c Upar data ,
(o.ah.d cir-cl • repr •• nt 6 , . 8 4.nd 12 . 5 inch tor
bo~llol ..)

230
HOBGOBLIN BOREHOLE
Depth range from 500 to 1300 metres
Number of data points: 9B43
Figure Ga: Rose diagram of caliper 1 direction (where caliper is
larger caliper).
Gb: Rose diagram of linearly weighted caliper eccentricity.
Gc: Rose diagram of linearly weighted caliper gradient.
Gd: Rose diagram of linearly weighted resistivity eooentricity.
Ge: Rose diagram of linearly weighted resistivity gradient.

Figure 7: Dominant breakout orientations for the United Kingdom
derived from a) eccentricity data b) gradient data.
(Inset shows the result of the sUllllllation of data from all
the boreholes) .

232
EEC contract DO EN3G-0056-F(CD)
STRESS MEASUREMENTS BY HYDRAULIC FRACTURING IN BRCH
D. Billaux and D. Burlet
Bureau de Recherches Geologiques et Minieres
Summary
During the last two years, BRGH has deveLoped a hydrauLic fracturing
unit and carried out in-situ stress measurements using this
unit, a wireline operated straddle packer system. Technological
developments performed by BRGH on the equipment were aimed at
improving its reliability and its precision. The stress measurement
campaigns are reviewed with particuLar emphasis on the use of interpretation
methods. Two theoretical works are also presented.
Firstly, a way of incorporating the anisotropy of the rock in the
interpretation has been investigated. Secondly, a special fracture
element, capable of reproducing hydro-mechanical coupling, has been
deveLoped: for incorporation in a finite element code. This should
allow the test interpretation to consider the whole pressure response
curve, instead of a few characteristic points.
1 - INTRODUCTION
Hydraulic fracturing was introduced about 20 years ago by Haimson
and Fairhurst (1969) as a stress measuring technique. This method, now
widely in use, consists of creating and propagating a fracture by pressurizing,
at constant fLowrate, an intact portion of a borehoLe seaLed off
using a straddle packer. Characteristic points of the pressure response
curve then yield the magnitudes of the horizontal principal stresses.
The fracture strike gives the orientation of the principal stresses, and
the vertical stress is taken as the weight of the overburden. This method
rests on several simplifying assumptions, and in particular on the
hypothesis that the fracture extends in the plane perpendicular to the
direction of the minimum principal stress 0h. However, when the borehole
is not orthogonal to Ob (Daneshy, 1973), or when the rock mass is
anisotropic, this assumption is not valid. Consequently, Cornet and
Valette (1984) proposed to test natural joints with various orientations.
The normal stress acting on each of these planes can be derived from the
pressure response curve. Knowing their orientations, a uniform or
linearly varying stress field is fitted to the data, using a
inversion technique.
numerical
Since 1986, BRGM has undertaken a research programme on
measurements supported by the CEC, the French Agency for Energy
stress
(AFNE)
and the French Ministry for Industry. The aims of this project are
threefold :
-development of an operational testing system,
-realisation of a number of stress measurement
French territory,
campaigns on

233
-improvement of the interpretation methodology, by adapting it in
particular to anisotropic media.
The testing system is first described.
campaigns already carried out are reviewed,
The five stress measurement
and difficulties in their
interpretation are pointed out.
A way of incorporating the anisotropy of the rock in the interpretation
is described. Finally, a special fracture element, capable of
reproducing hydro-mechanical coupling in a finite element code, is
presented.
2 - 'I'm STRESS KP.ASUREHElfT SYSTEM
After studying the various hydraulic stress measurement systems
available, BRGH bought in 1987 a "Wire line Perfrac System" built by the
Hesy Gmbh Company (Bochum,FRG), and designed after the work of
Prof. F. Rummel (Rummel, Baumgartner and Alheid, 1983).
Several modifications of the system were performed. The original
packers were replaced by packers closer to the ones used by the oil
industry. Because of the mounting of the sleeve on the axis of the packer,
as soon as the injection chamber is hydraulically sealed off from the
rest of the borehole,pressure variations in the chamber are integrally
transmitted to the packer. This means that only a very small pressure
(about 2HPa) needs to be imposed in the packers before the injections,
the packer then "following" whatever pressure history is imposed in the
chamber.
A hydraulic distribution system connects the high pressure hose to
either the packers (high position) or the injection chamber (low position)
• The passage from high to low position of this distribution system
is controlled by the tension on the cable and the weight of the probe. A
2S kg mass has been added at the top of the system to help this movement.
A pressure transducer was added in the probe to enable simultaneous
control of the pressures in the packers and in the injection chamber.
This helps in detectlng any anomalous behaviour of the distribution system,
or any leak in the hydraulic circuit.
An automatic digital and analogic data acquisition system has been
added to the probe. It records downhole and surface injection pressures,
packer pressure, and injection flowrate.
Following the loss of the probe in a borehole, a new probe with
several added safety features has been built: 1) the probe itself can be
decoupled from the packer system by breaking a shear pin; 2) a motordriven
valve has been added and can be used to deflate the packers if the
distribution system is blocked; and 3) an overshot has been designed on
the top of the loading rod to enable recovery of the probe by a drilling
rig.
l - STRESS KP.ASUREHElfT CAHPAIRS
Five measurement campaigns have been carried out until now in the
french territory. We were not really able to choose the geographic and
geologic location of the boreholes, but had to use holes drilled generally
for mining exploration, with technical specifications which were
generally far from ideal for our purposes. The locations of the
five boreholes are shown on Figure 1. Figure 2 gives the results of the
fi ve measurement camr>a i gns.

234
Cezallier campain
This borehole is located 60 km south of Clermont Ferrand, and is
drilled through 1400 m of a gneissic formation named "Saint Alyre orthogneiss"
• Eight tests were performed between the depths of 345 m and
410 m. Six of the tests used preexisting fractures, and two tests effectively
created fractures in intact rock portions.
The interpretation of the two "true" hydraulic fracturings using the
classical method on one hand, and the interpretation of the eight tests
taken together by the HTPF method on the other hand gave comparable
results. The fitting of a linearly varying stress tensor to the data by
the latter method is satisfactory :
*the difference between the normal stress measured on each fracture
and the normal stress back calculated from the solution never goes
above 0.5 MPa ;
*the uncertainty on the modulus and the orientation of the maximum
horizontal stress are 1 MPa and 7" respectively.
The vertical stress gradient obtained (0.0235 MPa/m) is lower than
would be expected from measuring the density of the overlying materials
(0.0265 MPa/m). This discrepancy can be explained by the topography
around the site. There is a 450 m difference in elevation between the
site and a valley situated at a 10 km distance to the East, whereas the
deepest test depth is 410 m.
Salau campain
Twenty tests were performed at depths varying from 100 m to 500 m, in
a metamorphised Devonian limestone. The interpretation of the data
obtained using an inverse method gave no significant result. Two
geometric factors may explain why the hypotheses of the HTPF method are
not verified : the borehole is in the bottom of a steep valley, and the
inclination of the borehole goes up to 18" from vertical (Figure 3).
Because of this geometry, the borehole axis is unlikely to be a principal
axis of the stress tensor. In this case, artificial fractures are likely
to rotate away from the borehole. We plan to use a Monte-Carlo type
method to try to assess the most probable stress tensor variation with
depth, based on tests on natural fractures, and on only the fast flowrate
reopening pressures (pressures resulting from.. activation onLy in
the vicinity of the borehole) for artificial fractures.
Aix en Provence campain
Twelve tests were performed at depths ranging from 1035 m to 1065 m,
in a vertical borehole in sub-horizontal layers of more or less compact
limestone interbedded with one centimeter to several meter thick layers
of lignite. Interpreting the complete set of results using an inverse
method proved unsatisfactory (Burlet, Cornet and Feuga, 1988). The
stress tensor does not vary linearly with depth.
The classical theory was used to interpret only the "true" hydraulic
fractures. This shows sudden variations of the modulus and orientation
of the principal stresses over short depth ranges. Such an heterogeneity
may be due to the local influence of the material properties. We hope
that a laboratory determination of the mechanical characteristics of the
various layers, now under way, will confirm this hypothesis.

235
Le Grais campain
This borehole was drilled for ~n1ng exploration in an intrusive
rhyolite body, which cuts late Precambrian schists. The density of
fractures cutting the borehole proved to be very high (from 5 to
40 fractures per meter). Nineteen tests were performed, between the
depths 184 m and 285 m, in the least fractured parts of the borehole.
From the values of the shut-in pressures obtained,we can deduce that
horizontal stresses are much higher than the weight of the overburden.
The intrusive rhyolite is characterised by an intense hydrothermal
activity. In such an environment, weathering can produce swelling
minerals. These swelling clay minerals are likely to induce local
abnormally high horizontal stresses, thereby explaining our untypical
results.
Guerting campain
This 800 m long vertical borehole is situated in the Lorraine coal
basin. It cuts a geological series starting in the upper Buntsandstein
and finishing in Carboniferous schists and sandstones. Only seven tests
could be performed, all of them true hydraulic fracturings, at depths
ranging from 690 m to 720 m,before the probe was lost in the borehole.
Only five fracture prints could then be done.
The characteristic pressures we obtained suggest that the local
stress tensor varies linearly, accordingly with the hypotheses of the
HTPF method. But because we possess only five complete tests,
convergence of an inverse method is difficult. Also, because all the
fractures tested have similar orientations, probably close to the
direction of the maximum horizontal stress, the modulus of this maximum
stress cannot be inferred correctly from only the closing pressures.
Therefore, the data were interpreted using the classical theory. which
also takes into account the fast flowrate reopening pressures.
4 - 11I!ORETICAL D!V!LOPM!RTS
The interpretation of the fast flow reopening pressures is limited
until now to vertical artificial fractures in an isotropic rock, the
borehole being a principal stress direction. In order to extend their
use to inclined boreholes, to anisotropic rock, and to inclined (natural)
fractures, we need to know the stress concentration due to the borehole
in such cases.
An analytical solution was derived by Amadei (198)), after the work
of Lekhnitskii (196)), to the problem of the determination of the stress
tensor in the vicinity of a cylindrical opening of infinite length, the
axis of which is at any angle with the principal directions of the
"regional" stress tensor, in an elastic and homogeneous material with any
orthotropic anisotropy. A computer code was developped to compute the
variations of the stresses around the opening based on this analytical
solution. Figure 4 shows an example output of this programme, in a
relatively simple case. Plotted are the variations around the borehole,
at several distances from its wall, of the normal stress on radial planes
0... The principal axes of anisotropy, and of the regional stress
tensor, are the (X,Y,Z) axes, the X axis being the maximum stress
direction and the Y axis the minimum Young's modulus direction.

236
This programme was used to model laboratory hydraulic fracturing
experiments in cubes of a mylonite, with an anisotropic strength ratio of
2.45, and a Young's moduLus ratio of 1.17. The computed distribution of
the stresses at the wall of the borehole was found to be consistent with
the direction in which the fracture was effectively created, despite the
crudeness of the assumptions (infinite hole, and sample supposed infinite
although the sample size is 18 cm, and the hole is only 9 cm long).
In order to widen the range of methods of interpretation for hydraulic
fracturing tests, a variational formulation of flow in a deformable
rock mass has been developped (Modaressi and Aubry, 1989).
Particular attention was given to the continuity conditions between
the fracture and rock matrix. A finite element discretization method was
used to provide the means of developing a practical solution technique.
General two or three dimensional quasi-static initial boundary value
problems of coupled stress and fluid flow in deformable fractured porous
media can be handled by this method. The mechanical behaviour of the rock
matrix could be non-linear and the fluid flow in the fracture is not
constrained to classical laminar flow.
To demonstrate the capacity of the proposed model a hypothetical
example of injection in a horizontal fracture in a porous medium was
chosen and some results of the analysis are presented in figure 5. The
excess pore pressure has been chosen as the unknown so that the initial
state of fluid pressure is not required. Two subsequent injections with
the same flowrate have been modelled. The evolution of the pressure at
the entry of the fracture, as well as the input flux are given in
figure Sa. As observed experimentaly, the pressure build-up is faster
than the pressure shut-in.
Variations in the aperture of the fracture in the borehole wall are
given in figure 5b. It can be seen that the fracture does not close
completely at the vicinity of the borehole before the second injection
cycle begins.
Such a modelling capability should allow the test interpretation to
consider the whole pressure response curve, instead of a few characteristic
points. By letting us perform numerical experiments, it will also
help us understand the very highly coupled hydro-mechanical behavior of
fractured rock during a stress measurement.
5 - CONCLUSION
BRGM's programme of stress measurements by hydraulic fracturing is
still underway. However, we can expect that we will be able to attain
our aims when we finish this programme in March 1990.
The improvements we performed on the testing system we chose, and
the experience we have gained using it have provided us with a fully
operational tool and a rigorous testing methodology.
The main difficulty we have encountered in this program has been our
inability to find boreholes corresponding to the ideal technical specifications
for the method. This is due for a good part to the slowing down
of mining exploration in France. We were forced to experiment in boreholes
with too small a diameter, in heterogeneous media, with large
topographic effects. This explains why the interpretation of the test
results was never straightforward.

D7
Finally. our theoretical work contributes to the neveLopment of
interpretation methods. An analytical tool has already been developed
and tested against laboratory results for the extension of fast flowrate
reopening pressures interpretation to any geometry. Another numerical
tool for the modelling of fracture reopening under pressure simulates
this phenomenon realistically.
MF~C~
Amadei. B. (1983). Rock anisotropy and the theory of stress measurements
• Springer Verlag. Berlin. FRG.
Burlet. D •• F.H. Cornet. and B. Feuga (1988). Evaluation of the
H.T.P.F. method of stress determination in two kinds of rock. In Proc. of
the second international workshop on hydraulic fracturing stress measurements
• Mineapolis. Minnesota. USA.
Cornet. F.H •• and B. Valette (1984). In situ stress determination
from hydraulic injection test data. Journ. Geoph. Res., Vol. 89. B13. pp.
11527-11537.
Daneshy. A.A. (1973). A study of inclined hydraulic fractures. Soc.
Pet. Eng. Journ •• Vol. 13. pp. 61-68.
Haimson. B.C •• and C. Fairhurst (1969). In situ stress determination
at great depth by means of hydraulic fracturing. In Proc. 11th U.S. Symp.
Rock Mech •• p. 559-584.
Lekhnitskii. S.G. (1963). Theory of elasticity of an anisotropic
elastic body. Holden-Day Inc •• San Francisco.
Modaressi. H •• and D. Aubry (1989). Numerical Modelling for the flow
of compressible fluids in systems of deformable fractured rocks. Accepted
for the Jrd Int. Symp. on Numerical Methods in Geomechanics. Niagara
Falls. Canada.
Rummel. F •• J. Baumgartner. and H.J. Alheid (1983). Hydraulic
fracturing stress measurements along the eastern boundary of the S.W.
German block. In Proc. of Workshop on hydraulic fracturing stress measurements
• National Academy Press. pp. 3-17.
ACKNOWLEDGEMENT
This work was supported by the CEC (contract EN3G-0056-F (CD». the
Agence Fran~aise pour la Mattrise de l'Energie (convention 6-07-0010) and
the french Ministry for Industry.
LIST OF FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Location of the five stress measurement campaigns.
Results of the five stress measurement campaigns.
Topographical effect in the Salau measurements.
Variations of 0 •• around and near a borehole in an
anisotropic medium. subjected to an anisotropic stress
tensor.
Simulation of the opening of a fracture under injection
pressure.

238
•
GRAIS
GUERTING ·
CEZALLIER
SALAU
Figure 1
Location of the five stress measurement campains

USULTS
Teat. one.t.t1Oa of 0H _aaured
lite .. til ...... GeoIO&y ...ml_ ofo. ...ml_ of all clocwt.e fna _rtll Ov
(.) (....) (....) dqreea (.... )
C!ZALLI!Il 34S-410 Ortbo&neiss 10.2S - 10.') 7.') - S.6S 113 S.2
SALAU ,)S - 461 Hetaaorphic devon i •• Interpret.tion proble .. due to the site topolr.phy
11ae.tone
(vslley)
AIX 101S-1066 Interbedded 11aestone 16.6S - 2').10 14.2S - 20 347 - 29 26.3
.nd l1&nite
LE GRAIS lSS-2SS Frsctured .nd we.thered Interpret.tion proble .. due to the hi&h deoaity
rhyolites
of fr.ctures
Gt1!1lTING 694-714 C.rboniferous schists lS.S - 34.S 14 - 14.S lS2 19.0
--
Fiaur. 2 I
a •• ult. of the five .tr ....... urament campaina

Figure 3 Topograp hical effect in the Sa 1 au measur ements

241
Compression
Tension
R 0
R
= Borehole radius
= Distance from borehole
Figure 4
Variation. of a around and near a borehole in
ee
an anisotropic medium, subjected to an anisotropic
stress tensor

242
a)
b)
. ā
CL
~
... •
i •...
CL
-- II
OIl'
E
a •
...
~
•
'a
II •
0
Q.
§
E
..
•
c
E
• u
~
Q.
II
'a
•...
~
u
e
IL
0
all I
~
g
(1 t
/ ~
..,
,t
~ t
* I
:3
(

243
STABILITY OF DEEP GEOTHERMIC EXCHANGER UNDER
THERMAL -HYDRAULIC-MECHANICAL SOLLICIT ATiONS
G BERTHOMIEU, P.JOUANNA
CIvIl Engineering LabOratory, Montpelller II, France
Summary
The purpose of this stud',' In the flel!l of Hot Dry Rock geothermlCS Is to assess the effect of
Clrculotlon of /I flul!l coloor tMn the rOCk on the extenSIon of iroctur es octlng lIS a nellt
excMnger. The approach consIsts of reproouclng site condItIons as closely a:; pOSSIble In
laboratory test samples. After determination of the parameters whICh cause
thermo-nyoraullc-mechanlcal faIlure of the rOCk In the laboratory. It IS shown hOW
experImental results can be transposed to the sIte.
1 INTRODUCTION
1.1 General framework of the study
The fromework of the stu~ IS the frocturlng of rock substrotum caused by thermal
str~ses, 1 e durIng COOling or heatIng of the rOCk Uttle Is known about thIS Question of heat
fracturing of rock tooay although It Is encounteroo In Important fields such as'
- OIl prospectIon;
- storage of raCIlooctlVe wastes;
- deep geothermlcs.
1.2 Exemple of deep geothermlcs
Prevolllng clrcumstonces leo to Investlgotlon of ooep geothermlcs In Hot Dry Rock. In HDR
( I OO·C- 200·C ot 2000- 3000m), heet IS recoveroo by m~ Ing water run between two or more
boreholes In II frocture network whiCh has been creatOO or re-opene(l oenerally by hydraullc
frocturlng These iroctures oct lIS 0 heet excMnoer
Clrcu lotion In thiS exctlanoer of water that IS coloer than the rock can favour the appearance
of fresh crocks on rock foces or the extension of exIstIng crocks The stabilIty of fracture foces has·
Deen exomlneCI In prevIOus publicatIons [1.2.4,6). Scope IS limIted here to the efiect of thermal
stresses on the stoblllty of the tip of one Single frocture whICh IS olr~ subjected to hydraulIC on!l
mechanical stresses
1.3 Approech used
The devIsed methoo conSIStS In recreating In the loborotory condItIonS whIch ere lIS close lIS
posSIble to thOSe encountered In SItu Local slmilltuoe Is created on II scale of 1: 1 in 0 small 00m1l1n
oround the frocture tip
In II gIVen rock. 0 frocture In 0 laborlltory semple fulfIlls Slmullltlon condItions wilen the
mechanical. hydrllullc. thermol end even chemical condItions ot the tIP of the fracture ere loontlcal
to In SItu COn(lltlonS

244
Theorical study of the extremity of the fracture lies within the scope of failure mechanics.
The field of stresses at the fracture tip is characterised by factors K referred to as the stress
intensity factors. In faIlure mechanics, 3 mooos of opening of the fracture are considered. In the
present case, mooos II and III (shear perpendicular and parallel to the extremity of the fracture)
are negligible. Only mOO! I associated with the enlarging of the crack and to which corresponds
stress intenSIty factor KI is considered.
Identical thermomechanical stress fields at all times t at the fracture tip make stress
intenSity factors KI(t) equal in situ and in the laboratory fissured sample.
Study of the problem can be represented by the organization chart given in Figure 1.
LABORATORY
Thermomechanical parameter
at fail ure : Pe
SITE
Thermomechanical parameters
at fail ure : Ps
FIg. 1 - OrganIzation chart of stUdy
2 ROCK PARAMETERS AT THE SITE AND IN THE LABORATORY SAMPLE
2.1 Mode111ng of the heat exchDnger formed by hydrDul1c fracturing
Hydraulic fracturing may lead either to the creation of a new fracture or to the re-opening
of pre-eXisting fractures. Discussion below is limited to the creation of one single fracture assumed
to be in a continuous, hom~neous, isotropiC, linear-elastic and fragile medium. I n such a medium,
the fracture develops perpendicularly to the main minor stress and takes the form of a very flat
ellipsoid (penny-shaped crack).
In practice, the situation is generally much more complex than this picture used during
early development of deep geothermics. In the description of the methocl this scholastic assumption
has the advantage of making it possible to calculate easlly the stress intensity factor. However this
assumption ooes not reduce applicability of this approach, more complicated computations being
possib Ie in more comp lex cases.
2.2 Study pDrDmeters
Sub-indexes "s" and "e" indicate site and experimental parameters respectively.
D) Geometry (FIgure 2)
- At the site, the fracture of r~jjus CIs is perpendicular to the main stress 03s'
- In the laboratory, rock samples are cylinders of r~jjus Ra drllled in the center (r~jjus
r e) and pre-fractured longitudinally in two opposete directions to keep symmetry. A half-sample,
whith the drill hole deducted, represents a fragment of rock located at the extremity of the fracture
at the site. Height of sample He is assumed to be sufficient to constitute a plane strain problem.

245
Similitude domeln
Flg.2 Fractures at the site end In the laboratory
b) Thermomachanlcal coerrtclents and physical constants of the rock
A Ill: Lllm6 !Xl8ff1clents
t: thermlll conductivity; c:: specific heat cepoclty ; p: volumlc mess
X • t/pc: : thermlll dlffuslvlty
a : l1n88r expansion coefficient
c) Inlttal condlttons
- At the site, the frecture Is subjected to mllin stresses ° Is' 02s end 03s' It Is filled with
wllter lit pressure Pfs end temperature T fs loonticel to the in1tilll temperature of the roclc T rs'
- In the lllborlltory, the semple is ploced in II test cell end subjected to a biexial stress state
with hydrostetlc pressure Poe- The weter In the dr111ing Is et pressure Pfe end tempereture T fe
lOOnticel to the inltllli temperllture of the rock T re· T rs' The set-up requires thet T re = T rs'
d) TherlBal condlttons
The lew of evolution of the temperllture of the nuid In the frecture In situ ceo e priori be
Chosen freely, In the 1In1l1ysls below wllter tempereture T f during cool1ng is supposed to be given by :
T f • T r - 6T,th(tlt)
In whIch: 6T : rllnge of coolIng; th: hyperbol1c tangent; t, time
t: relllXlltlon time (t" 0 thermlll shock; t = 00 : st~ state)
The set-up requires thermlll stresses to be the serne In the leboratory end lit the site.

246
3 TEST APPARATUS ~D PROCl:DURE
The follow Ing dimenSIons are selected for the simulation of 6 fracture tIp opening In mooe I
In &n Infinite medium :
f e =S. IO-3m; Re= 12.1O- 2m ;aa=S.10- 3 m :He= 16.iO- 2 m.
The fractures are created artific1ally wIth a d1amond-cooted disc fI ted at he end of en erm
aM drlven by a bell connected to a variable speed motor.
3.1 Test apparatus
The test apperZltus ct)nsists of a cel l end perl0herals eble to sustain a pressure of 20 MPa
and a temperature of 200·C.
e) Test cell (figure 3)
I : Circu lar bese
2 . Rod
3 ' Tenon plate
4 : O-ring
S: Upper seal
6 : Piston feed
7; P1ston
8 : Cell air-release
, 9 : Piston stop
10 : Pis ton cir-releose
I I : Cell cyl1nder
12 . Sample
13 : Cell feed
~~~~--Il
14 : Thermocouple housing
'------u IS : Water pIpe end thermocouple housing
·_ -------1'
Ftg. 3 - Scheme of the test cell
The rock sample is held between a circular base and a piston end subjected to e confinement
pressure Poe by means 0 011 whICh can withstand high temperatures. Heat-carryIng liquid (water)
Circulates in the central borIng and in the fractures et pressure Pre'
O-rings tn Vlton Ilre used as seals between moving ports and elsa on the upper Md lower
ft£8S of the test semple between the 011 end water circuits. The piston gives a vertical force on the
sample causing friction. Prel1mlnary tests were required to evaluate these friction forces and to
determine thetempereture at .the lip of the fracture, KnowIng the temperature at the center of thO
drillIng.
This cell Is pIeced In a thermostatically-controlled heeted chamber in which the rOCK could
be laken \0 test lempareture T fa'
b) Perlpheral$ (Figure .oJ)
In ~ll1on to classic perlpherets for the setting UP and reoulatfng pressures end
temperatures. e perlpherel hes to be perfected to control the COOling 6T e of the weIer in the
frecture,

ihlS '..al.01" Is Pl"C()8lled ~ II h t:/I pressur! c cui fng pump
cnembet' Irf II v8rll1ble soeed rnator' conn«:ted to 1J rotatIng )Otnl It is ccc
~t 6I(Ct\Ct¥;Jlr connectecl 0 cold Wllter ~
he
OJUnter ClJr ren
M!f'l fran 001$100
by
Circul~tin3
poump
L:otor isea
3 w;sy cock
aile d motoT
Fi g. 4 - Scheme of the test appartltU:8
3.2 PrOC8dure
Two types of test were cerr leo out at er se lllQ up tmtlal temperal\Jre ra.n! pressure Pflt
a) Hydraulic fractur ing re1erenc:e test
Al OCIlstalt temperature, w~ pressure Pre Is ~ ncr~ In he~. el her ~p1dly or In
stl9!S. unll l roc ~Ilure CX;CUf'1. r~ r,als m8k.e It poss1bte to term l ~~lIc feHure
Drassure (Pte)crll nI to cbserve the ef(~1 of ~re pressure on the frectur ng of oct..
cool
b) Tllermal fraclurfllQ test
Undor II pr8SSUre eQUWII etlt to e cerunn proportIon of the hvo:'«Jhc faIlure pressure water
prOlTlISSlvely unt1l fll lure occurs.
.. tA80RATOAY [ ST$ ANO RESULTS
8S'tS wars arried out Cfl lImsst.ooe end fF j II semples ceo be c:ons1cilrecf M be 119
hOm~ lind lsotrop C. tanpe-eture ~ of SO·C to 200·C wes stUd
". 1 HWt Ite fr.:turlno rtfONnCO tests
Curve3 II n D In Hour 5 fU' I1IMS'ton1 6 tor ~ I! ~ the onr ~Icn of

248
(~Pe)crit = (Pfe)crit - Poe in function of temperature of the rock T reo It is observed that the
hydraulic failure pressure is much greater with a rapid rise in pressure (curve a) than with a rise
in st~ (curve b) smce water does not have time enough to enter the pores. As expected, pore
pressure plays a fundamental role in the failure of the rock.
The hydraulic failure pressure (Pre)crit 1s independent of temperature 1n the limestone
stUdied. In granite, (Pre)crit falls with the temperature of the rock as granite becomes more fragile
as the temperature rises.
12
11 .
II Fail ure bl,l hl,ldraulic frecturi ng (rapid i ncreese)
10 • Failure bl,l hlldraullc frecturil'lQ (increase in st.,)
9 • Failure by thermal frecturil'lQ
ONo failure
-II 8
~
J: 7
.....
(6)
- - •
6
B
GI
5
=-
oC3 4
3 l.Pe zone in 'Wich thermal frecturi I'IQ can occur
(b)
2
(c)
1
0
0
o 20 40 ro ~ 100 120 140 lro 1~
T re (·C)
Fig.5 - Tests on ltmestone
16
B Feil ure by hl,ldraulic frecturi ng (rapid i ncreese)
14 • Fail ure by hydraulic frecturi ng (i ncreese in stages)
• Failure bl,l thermal frecturil'lQ
12
No fail u re
-II
~ 0
II
.....
J:
8
GI
l.Pe zone in ... hich thermal frecturil'lQ
Q, 6
can occur
oC3
(b)
4
•
2
(c)
0
so 70 90 110 130 150 170 190
T re (·C)
Fig.6 - Tests on granite

249
4.2 Ther.al frecturetion tests
Figures 5 and 6 show also the values of excess pressure APe applied to the rock durIng the
cooling tests. Fet1ure by thermel fr~tur1ng IS not possIble for velues of APe below curve c. A
pressure SQuel to 70 to 90 percent of the I1ydraultc faIlure pressure IS requIred to fr~ture rock by
cooling. The l1Pe zone WIthIn whIch faIlure of the rock by thermal fr~turlng can occur IS narrow,
wher88S the cor responding coo 11 ng em p lttuCles cen be great.
Trensposltton of the laboretory results to the Site has then to be carrIed out by celculating
stress Intensity fectors.
5 CALCULATION OF KI(t) IN SITU AND IN THE LABORATORY SAMPLE
The leboretory experiments Show the Importance of the pore pressure In the failure of the
rock. ConSeQuently, celculatlon of KI should be wried out uSing effective stresses and combIning
the pore pressure field with tempereture. Before developing this complex computation, whIch has
not yet been epproached In fet1ure mechanics, we attempted a prellmary celculation of KI uSIng total
stresses.
On thIS IISSumptlon, the stress Intensity fector et either the sIte or leboratory can be
broken down as :
where
K 1° - fector reletlVe to Imtlel stete
tJ::1(t) -Increase In fector ceused by thermal stresses.
5.1 Cllculation of KID In the Initial state
It Is et!S>( to find expressions of Klo et the site or In the laboratory for the geometrfcel
conflguretlons used WIth the enalytlcel (or seml-emplrlce1) solutIons gIVen in the literature [7 for
exemple].
-Sl.1§:
-L ebor atory :
K;e· jrnre + ae).(P re - Poe)
5.2 Cllculatlon of Ii KI(t) under cooling
Determination of l1K I (t) r8QUlres numerlcel celculetlons; the number of these Is reduced
by use of dlmenslonel analysis.
a) Contribution of dillensionel amllysls [5.6]
- Sl.1§:
The set Ps of IrOIpendent perimeters Is: P s - (&s. As·lL s· Qs.l1T s . ts . Xs.ts)
Using Veschy-Bucklncj\Im's theorem (or D theorem) with an IISSumption of linear thermel
expenslon, these perimeters ere grouped In the form of ~Imenslonel terms connected to 8IUI other
by the following expression :

250
- Laboratory:
The set of independent parameters P e is : P e • (8.e. r e' Re. Ae·ll e· Q e . ~Te· teo Xe.te)
Identical calculation to that for the site gives the equation:
-,.
AI/' • f (..! r ...! R ..! t "f _ t
...._.)
-F.-=--- • ' , , 3
a cc. 6 T A a. a. 't. a
" " .
As the trials are carried out on samp les with the same geometry, the terms r e/ae and Re/ae
are fixed and the expression above is reduced to :
t.K
t 'tt
I. ( ..... )
fa cc. t.T A • 't.'
..J Q. • "
a•
2
~~-----.g ---
expressed as follows, as for the site:
Doe -ge(fite .fiZe)
b) Contribution to numerical calculation [3,6]
(a) LABORATORY
2.~ 10-6
2,010-6
1,~ 10-6
0' 0 1,010-6
O,~ 10-6
(1) • 12,6
Oa (2). 3,024
3 (3) ·1,~12
(4) .O~
0 2 3 a 4
0 1
~ 6
10-6
(b) SITE
OS
0
10-7
f(
10-8 S
10-9
0 10
OS
1
1~
03
(1) • 3,1~ 10-6
(2) • 6,30 10-6
(3) • 6,30 10-8
-'11
~2)
3)
20 ~
F1g. 7 - Ad1mans1onal graphs for tha laboratory and tha sna

251
A numeri~l eppr~h Is required to determIne functions 8s and 8e- We chose the fimte
element methcxl end the CASTEM progrem
After plotting tempereture at all Instants with the DELFI NE program, the fIeld of
dlsplecements end stresses Is computed USIng the INCA program whIch solves llnear
thermoelestlclty equetlons. The MAYA progrem is then used to calculate integral J and hence the
stress IntenSIty fector 6K I (t).
The ~lculetlons ere cerrled out In exlsymmetry for the site end In plene strain for the
leboretory [5,8].
The results can be shown es adlmenslonel graphS such as those In FIgure 7a for the
leboretory end 7b for the site. They represent the verletlon of Do In function of Dl for different
velues of D3 ( D3 • D21 D I In order to eltmlnete the time perameter).
6 TRANSPOSITION OF EXPERIMENTAL RESULTS TO THE SITE
6.1 UtIltzation of edtmenstonal graphs
Knowing the physi~l constents of the rock (Ie' a e
, Ae) and the par8fTleters of the
leboretory test which CIIUS8 the thermomechenlcal fetlure of the sample (Pfe' Poe' 6T e' le' ~), It is
possible to ~Iculete Dleend D3 e , for e rock sample of given geometry (lie, r e)' These were plotted
on Greph 8e (leboretory) end Doe deduced, giving 6K le , whence 6K le (t) since Kle(t) = Kleo +
6Kle(t)·
The Identity of laboretory end site KI (t) ~n be ensured term by term (strong eQUivalence)
or globelly (w8ll1c equlvelence).
Velue [KIW]slte ClIO then be used to find the conditions which connect the peremeters to
fetlure et the site. This Is generelly not en uniQUe solution as el8boretory trial provides IK:C8SS to a
set of peremeters 1~1ng to fetlure lit the site.
6.2 Example of appltcatlon
We teke the ~ of e grenite site et e tempereture of 100'C whose thermomech8nl~1
cherecterlstics ere as close as possIble to those of the laborlltory, I.e. :
Es ·6 10 10 N/m Z : vs· 0.3 ( As· 3.~ 10 10 N/mZ)
ts·Z4W/m.'C: ps·Z600k8/m3: cs.~J/k8.'C
a .10-~·C-1
s
The lebclretory results ~e Kle· 1.9 MPe-Im lit feilure .
(ls·1.810-6 m2.s-1)
Let us consider e frecture with e radius lis • SOm end e cooling function with e relllXetlon .
time of \ • 4005.
In ~ of II week equlvelence (K ls • K le ) curves In Figure 8 show the crltl~l cooling
velues 6T s leading to the extension of the frecture, In function of time t" for different velues of
excess pressure Pfs - 03s'
In ~ of II strano eQUlvelence, which consist of writing the eQUlllIty between the Inltlel
stetes end the stresses (Kls o • Kle o end 6Kls· 6Kle)' the th8rmel frecturlltlon ceo only ~e pIece
et the site If Drs - 03s> 0,11 MPII. For Instence, If Pts - 03s • 0,15 MPe, extension of the frtlCture
could occur wIth cooling of S'C IIPplted for IIPproxlmlltely 30 min.

252
~u 161----'\--Il--~~~~----------IG)O~IMi~
(1) ° MPa
.:.. 14 (2) 0,05MPa
G) IIts - 113s- (3) 0,10MPa
~ 12 (4) 0,15MPa
~ (5) 0,20MPa
= 10
c
-CI
CI
u
-II
u
-.-L.
U
8
6
4
7 CONCLUSION
2 ~ No failure In this zone
if ·strong" equivalence applied
(4)
-----...... (5)
O+---~~~~~--~~~~~---r~~~~
100 101 102 103
TIme ts (mn)
Fig. 8 - Transposition to the site
This study on thermomechanical fracturing of rock. is applied to the extension of fractures
acting as geothermal heat exchangers. The Question is approached by local similitude on a scale of
I: I on a small oomain, around the tip of the fracture, in situ and on a prefissured laboratory rock.
sample. By writing identity of KI stress intensity factors, in situ and on the sample, rupture
experiments mak.e it possible to predict in situ fracturing parameters.
Laboratory tests show the effect of pore pressure on rock. thermal fracturing. A rigorous
transposition of laboratory results to the site is thus dependent on the development of failure
mechanics in biphasic media, coupled with temperature.
REFERENCES
[1] BERTHOMIEU G. ,J:>UANNA P. - Wall stability of a deep geothermal reservoir under thermal
actions. 3rd Int. Seminar on the Results of European Communities - Geothermal Energy, Munich
(29 nov.-l~. 1983).
[2] BERTHOMIEU G. , JOUANNA P. - Stability of rock. faces subjected to temperature change -
Application to hot dry granite. Int. J. Rock. Mech. Min. Sci. &. Geomech. - Vol. 21, N"5, pp.
277-287,1984
[3] BERTHOMIEU G., CHEISSOUX J.L., OABERT J.L., JOUANNA P. - Stress intensity factor KI under
variable thermal conditions - 3rd Int. Conf. Num. Meth. in Fracture, Swansea, 1984 , pp.
481-494.
[4] BERTHOMIEU G. , JOUANNA P. - Stabilite d'un echangeur geothermiQu8 profond sous
sollicitations thermiQues. Revue roumaine des sciences techniques - MecaniQue appliquee - Tome
30,N"I,pp.67-74.1985.
[5] BERTHOMIEU G. , JOUANNA P. - Conditions de similitude en thermomecaniQue de la rupture -
7eme Congres Franr;ais de MecaniQue - Bordeaux (2-6 sept. 1985).
[6] BERTHOMIEU G. - Fracturation thermiQue des roches - Application a la geothermie profonde­
These d'Etat, USTL Montpellier II, July. 1987.
[7] SHI G.C. - Handbook. of stress intensity factors - Inst. of fracture and solid mechanics, Lehhigh
University Pennsylvania, 1973.

253
EEC conlnlCt n' EN3G-007S-F(CD)
IN-SITU STRESSES EVALUATED FROM MEASUREMENTS ON CORE
SAMPLES
P.J. PERREAU. O. HEUGAS, F.J. SANI'ARELU
lnstitut ~ du ~Ie. C.F.P. TOTAL. SNEA(P)
Summary
The minifnK: technique ha been extensively used 10 measure the vertical distribution
of the mioim\Dl'l horizontal in-situ IItteSS. However. its implementation is often
hindered by technical and economic coosiderations. Consequently. the delenninalion
of the stress field on die basis of laboratory measurements in orielllalCd core samples
ha been investigated. New developments and the results of three methods are
presenred: anelaslic strain recovery (ASR). differenLial strain curve analysis (OCSA).
core discing analysis. The results obtained using these methods on cores exlrII:led
from ICVeI1Il wells are desaibed and discussed. A comparison is made with data
avallable from minifracs, logs and regional teclODics. The main conclusions are that
diffemtt methods give a good idea for horizontal principal stress.
1. INTRODUCTION
Reliable methods of predicting the orientation of massive hydraulic fractures are of great
imponance 10 oil and au industries IS well IS 10 HDR activities. In order 10 predict the azimuth of
hydrauUc fracture, it is necessary 10 know the direction of the minimum horizontal compressive stress,
because a hydraulic fracture propagates perpendicular 10 this stress direction. AccuraIIc delenninalioo of
the fracture azimuth would allow optim\Dl'l location and spacing of production wells and could increase
recovery by 11\ additional 30 percent of the IOIal reservoir vol\Dl'le. The vertical distribution of die
minimum horizontal in-sib! stress is die nuijor parameter which controls the height confmement and
direction of hydraulic fractures. The measurements of in-situ stresses and of their orienwions can be
obtalned by diff~ methods:
• well tests interpreting the data analysed from micror'minifJacs.
• measurements on cores
The rninifrac IleChnique ha been extensively used 10 calculate the minimum borizonlal stress.

254
However, its implementation in the payzone and adjacent layers is often hindered by technical and
economic considerations.
The second approach is attractive in sibJations where coring is already planned since the logistics
are relatively simple and the measurements do not interfere with continued coring, drilling, and weD
completion.
Consequently, the detennination of the stress field on the basis of laboratory measurements in
orientated core samples has been investigated. New developments and the results of three methods are
presented: anelastic sttain recovery (ASR), differential strain curve analysis (DCSA), core discing
analysis.
The experimental apparatus developed for ASR measurements has made it possible to separate the
useful mechanical strain recovery and the thermal expansion effects. Six horizontal and one vertical
displacement ttansducers were used to detennine the principal mechanical strains. TIle strains required for
DCSA analysis were measured in cubes of rocks using nine strain gages. The directions and intensities of
the principal stresses were derived from the measurements. Core discing phenomena were studied on
acbJal cores and by laboratory mechanical simulation. Different disc shapes were observed. They are
related to the stress anisottopy in the horizontal plane. The discs thickness depends on the horizontal and
vertical stresses.
The results obtained using these methods on cores extracted from several wells are described and
discussed. A comparison is made with data available from minifracs, logs and regional tectonics.
2. Principle and Theory
2.1ASR
The principles of this method and several interpretation aspects have already been published. TIle
main idea is that, during and after coring a rock section, there is a relaxation of the strains. The total strain
relaxation consists in two parts: an instantaneous recovery of elastic strains and a continued timedependent
relaxation. The first relaxation can be observed by overcoring and the measured strain recovery
can be related to the totaI stress components. Voight (1968) noted that the partial recovery strain is
proportional to the total recovery strain. Then an approximate estimate of the in-situ stress state at depth
can be made by instrumenting orientated core immediately upon their removal from the borehole. If the
core is orientated, homogeneous and the vertical overtlllrden stress is known, it is possible to determine the
state of the stress: its orientation and value. Since the equations in terms of stress and strains are the same,
the problem will be set out in terms of strains.
Since the measurements displacement are 60' apart, the direction of the principal strains can be
calculated by the following equation:
Normal strains Ep. Eq and Er are measured in the directions OP. OQ and OR . Unknown angle 9 is
angle between OP and Of). Solutions for principal strains are

2.55
2
£1 +£2· ]

256
approximation a linear law can be assumed between the deformation due to die microcracks and die
effective stress tensor defmed by O'*=

257
The links with the ID-sltu stess teDsor
DilldnR criteria: Many dUcing criteria, relating strength parameters of the roc.k to the in-silU
stresseI when discing occun. have been established from the testing of small scale models of boreholes
being cored. Among die most widely used criteria, one must quocc the crircrion by Obert Stephanson and
Panel which lakes die foUowing fmn:
a, et a .. are expressed in bars and where i. and ~ are two constants depending on die roc.k and given by:
i.-230+2C
aNI
i2. 0.25 +0.0065R,
where bod! C and R, are given in bars. Note that for strong sandstones, it was shown that i. could be
expressed as it • Rc with Rc in bars.
3. Results
FJeld IestS were carried out on three cores retrieved from two wells (A and B). One core was taken
from well A and two cores from well B. Three tests (ASR. DSCA and Core Discing Analysis) were
perfmned on core from well A. two IeSlS (ASR and DSCA) were performed on cores from well B. There
was no discing phenomenon 01\ well B.
3.1 ASR
3.1.1 WELL A
The core depth is 4558 m and die core diameter is 2"5/8. Recovery behaviour of core showed bod!
expansion and contraction during 15 hours. Temperature fluctuation of the oil bath and variation of
saturation affected die observed ra:overy behavior of die core by superposing thermal expulsion and
contraction of die core. Answers of opposite transducers have been added to obtain a strain measurement
along every diameter. On raw dala,dIe following corrections were carried out I} Equilibriwn between oil
bath and core temperature was set at die 162 .... minute, being now considered as die starting point, 2} value
of desaturatioo rate was -52 10. 3 lJIIl/min. 3} value of thennal expansion coeffICient was 48.9 ~C. die
thermal expansion being isotropic. FJgure 6 shows the corrected strains with the above mentioned
comctions and the angle in plan core. In die N-E plan. the direction of die major principal strain is equal
to 172'.
3.1.1 WELL B
Two IeSlS were perfmned 00 two cores:
• The depth COliesponding to die first piece of core is at 1285.25 m. this core was
taken It the foot of the core barrel •
• The depth of die second piece of core is at 1336.13 m. 6 meters above die foot of
dlecorc.

258
• The diameter of both cores is 4".
The same corrections are then applied 10 the other well i.e. the corresponding starting points are at
the 173rd mn and 21o"'rnn, for respectively core 81 and 82, a value of thermal expansion coefficient of
40. J11llfC is taken for 82; no thermal corrections could be perfO!"l1led on 81. For the second core, the
delay was very short between the removal of this core and the rust core, consequently the data were not
stabilized and then, they have not been interpreted. Figure 7 shows the corrected strains with the above
mentioned COTeCtiOns and the angle of the major principal deformation for 82. In the N-E plan, this angle
is equal 10 232' .
3.2 DSCA
3.2.1 WELL A
Nine cubic samples have been cut in the depth interval 4552 m - 4557 m. The rigidity of this
material being rather high (E=300000 bar), it was very surprising 10 obtain rather low microfractures
intensity (curve Ii-P quite linear). It was however possible 10 measure the deformation due to the
microcracks closure. A typical curve is iUustrated by figure 8.
Dispersion of the results is iUustrated by the figure 9 which represents azimuths and dips of the
intermediate sb'ess.
The average values of the stress tensor components are on the table I. The figures used for the
computation of A are:
Pm 2500 kg/m 3
a 0.9
P R 530 bar
3.2.2WELLB
Seven cubes have been cut in a core inside the interval 1326.5 m - 1327.0 m. A typical curve
deformation-pressure is iUustrated by figure 10 while the main stress directions for one cube are
represented on figure 11.
The results obtained for the seven samples are consistent Dispersion for the direction of the
intermediate stress at is represented on figure 12 (azimuth and dip).
The average values are represented on table D. Figures used for the computation of A are:
Pm 2200 kg/m 3
z 1326.5 m
a 0.9
P R 130 bar
3.3DISCING
3.3.1 WELL A
The use of Obert, Stephanson and Panet's criterion in the case of disced cores in a dolomite of the
well A has led to the estimlllion of a ratio aria. of 1.5. Such a high latenIl stress field was confirmed by
high value of leak-ofJ tests measured on neighbouring wells, by hydrofracturing operations and various
wellbore stability analysis.
When observing disced cores, one can realize how regular and reproduceable the shape of the discs

259
is. Miguez et aI.(1986) proposed a canplelie method 10 study die geometty of die discs (fJgUre 13).
Laborarory Iiests on small scale models showed that die four poinlS Fl 10 F4 could be rdated 10 die
directions of die ~ncipallateral in-sib! stresses, die high poinlS Fl and F3 corresponding II> die direction
oldie smaller lateral in-situ stress. Studies 01 oriented cores fnxD die deep gas reservoir already mentioned
showed that a similar conclusioo could be achieved in die field.
Ocher observations wa"C made which cannot be quantified yet, such &'I the variation oldie discs'
thickness with depth or stress e.g in die super deep wen of Kola (USSR), or such as die lOIII1y lIOII
symmetrical aspect of the disc surface when the in-sib! stress tensor is inclined 011 die core axis.
DIlcIlSlIoDi aDd C:ODc1us1oDl
Through die study of corea with these methods, good indications are obtained 011 in-sib! stresses..
On figure 14, the aboYe mentioned dirmions are represented by directions found through die use of other
methods such as: for well A - well ovalization and for well B - indication of die angle of hydraulic minifracture
(Formation microscanner log).
WELL A The N-S direction of die major stress which is roughly indicated by die three methods is
in agreement with die direction of die small axis of well ovalization. This directioo also filS one of the
figures usually considered for die regional maximum stress.
WELL B ASR as wen &'I DSCA indicate die same orientation for die major deformation. This
orienwion is perpendicular 10 the major principal stress as deduced from die minifrac resulL Consequently
these methods, although indicating die orientations of die principal stresses, cannot be used, in this case, 10
classify die minor and the major stresses. Moreover one of die principal deformations measured by ASR is
negatiYe. These observations clearly show, that taking inlO account stresses is not always sufficient for
interpreting ASR and DSCA deformations. The effect of natural fractures shou1d also be considered. In
this case a wen-know natural fracture direction oriented NI40'E could explain die deformation recorded
perpendicular 10 this direction.
Discing aiteria povide a value for die ratio 0,10. while, in die case of oriented cores, die study of
shape of discs gives the direction 01 the smaller lateral in-sib! stress. The amount of information contained
in the disci Is not yet funy understood and funher research is needed. The major limilalioo of die method
Is that core discing Is not very frequent, puticularly because weak rocks such &'I chait or poorly cemented
sandstones do not produce discing when cored but develop instead distnbuled micocracks which
contribute II> rather poor core recoveries. It is felt that a beuer underslaJlding of discing conditions can
lead, on the one hand 10 a beuer estimation of die in-situ stress tensor and, on the other hand,lO a better
knowledge of die coring process.
Notations
a
1m
z
a
..
~
direction
C
F •.. .F.
Absolute stress
Average density of overburden layers
Depth
Bial coeffICient (~.9 for porous rock, ~.I for tigh rocks)
Reservoir pressure
Deformation due II> the microcracks closing in the vertical
Cohesion in tars
High and low poinlS on die disc surface.

260
k\ ~ Coefficients in discing crilerion
R.:-Rt Uniaxial compressive and tensile sttength in bars

261
01 (bII .. ) Oz (bII .. ) 03 (bII .. )
Int ..... U.t. _jor .t ..... llinor .t ......
• t .....
Table I
Results DSCA on WELL A
8, 'P1 6z 'Pz 63 'P]
1182 1328 m 141.Z" 39.3" Sl3.3" 50.9" 44.0" 114.1"
TableU
Results DSCA on WELL B
a, (bII .. ) Oz (bII .. ) 0 3 (bII .. )
61 'P1 6z 'Pz 63 'P3
II I nor at ..... _jor .t .....
Int..--dl.t.
.t.....
111 289 Z1e 148" 96" 96" 10" 23r" 82"
Fig. 1: ASR apparatus

2."
X"
Fig. 2: Positions of the gauges on the sample
Pressure
B
PA~-----t
Deformation
Fig. 3: Stress - Strain Curve Fig. 4: Examples of core discing in Dolomite
OA = Closure of Microcraks

263
Coreaxla
154mm /.,
.. ~
~30rnm //
r /;
1 //
I
i· Cortng uw-art ~
,
ZONE I.
E s.nple
g I .
154111541150 rnm
~
E
I
I
i ZONED
~~
~ ZONE 1
:l ~
I
~
i /:,
I //
Vertical component
CornpNuIon ( .... ,
~
~
-50
-40
-30
J
~
-20.
·10
J \
n ~
~
10
~ -
20.
Tr8ctlon MPIi
Fig.5: Vertical stres. component around front coring
from HASSlEU and DURVILLE (1983)

264
120.0
110.0
100.0
90.0
-

eoo
r
PlESSIOH
NORTH
!lOG
f
I
400 l
300
200 l
I
I
IlEST
EAST
100
o 20 40
Fig. 8: Typical curve for WELL A
100
IlEFORMA TION- 1E +!I
Fig. 9: Azimuths dispersion of the intermediate stress
for WELL A

_Ieol 0 5
600
PRESSION (bar)
500
400
75
80
55
.... tzant.l
90
95
100
300
........ ,\ .
:/ dovnloinl loodinl
105
110
200
100
165 160
180 175 170
150
155
1040
145
o
100
200 300
DEFORMATIONW1E+5
Fig. 9bis: Dips dispersion for the intermediate stress
for WELL A
Fig. 10: Typical curve for WELL B

267
NORTH
WEST
G'J
EAST
SOUTH
z
Fig. II: Direction. of main strelaes for WELL B
PIll .........
IIIIMA I
PIIl-411 .......
PIll ......... 11_ 3

_Unl 0 5
NORTH
liEST
EAST
75
80
85
90
95
100
105
harllantal
SOUTH
Fig. 12: Azimuths dispersion of the intermediate stress
for WELL B
. ~ 165 160
180175 1,0
150
155
140
145
Fig. 12bis: Dips dispersion of the intermediate
stress for WELL B

269
1. 2.4 "low" points
1.3 "high" points
_
of c:ootng
\
...-aInII
•
2. Definition of angles:

270
,..
CD
<
OJ
N"
a
t
(5" NORTH
::J
I
'\ NORTH
LOG ~-~-
Fig" 14: Orientations of main stress for
WELL A and WELL B

271
EEC Contract No. EN. 3G-009l-I
SI1MULAUON OF WELLS LATERA-J,O AND LATERA 4
A. Barelli and G. Cappetti
ENEL (Italian Electricity Board), National Geothermal Unit
'sJUMllU..Y
Eight stimulation jobs have been performed on the
wells of Latera geothermal field and four of them were
succesoful.
The stimulation operations planned for the two low
production wells L10 and L4 are presented in this paper.
In the case of well L10, which produces from a
siliceous rock formation, a series of acid attack laboratory
tests have been performed to select the most suitable acid
composition. Unfortunately stimulation was not possible in
this well due to casing collapse during work-over
operations.
Stimulation in well L4, with the injection of the
HCI solution at two different depths, was fairly successful
improving production capacity from 3.5 to 4.5 M\ie.
Latera geothermal field is located west of Lake Bolsena
in the Monti Volsini volcanic region (northern Latium).
Figure 1 shows the location of the field, wells and
reservoir isobaths. An uplifted structure, trending SSW-NNE and
dipping northwards, is made up of a fold of the carbonate
reservoir, covered by impermeable flysch facies formations and
volcanites.
A cross-section of the structure is given in Fig.2. The
reservoir is confined laterally in the east by a drop to 3000m
of the reservoir top, whereas in the west the reservoir is
confined by impermeable thermometamorphosed limestones.
A longitudinal field cross-section is given in Fig. 3,
showing a deepening of the carbonate structure in a NNE
direction. Small gas caps are present above the aquifer in
correspondence to local uplifts of the carbonate reservoir.
The results of the wells drilled to date in the Latera
area are summarized in Table 1: out of 15 wells only 5 are dry.
The fluid is produced from a water-dominated
hydrothermal system with temperatures of 190-240·C (typical
values are around 2l0·C). Approximately 3' by weight of C02 is
dissolved in the water.
Stimulation jobs have been carried out in several wells
of Latera field. Stimulation performed on the completely dry
veIls, Ll, L3V, L5 and L6. did not yield positive results. The
transmissivity, initially very low «0.1 Darcy·m). remained of
the same order of magnitude; extensive fractures (-10 4 m 2 ) were
induced, but did not connect with the network of natural

272
fractures. On the other hand, wells with a small injectivity
and positive or zero skin effects were stimulated successfully.
The results are summarized in Table 2 below.
Table 1. Drilling Results
I I I I I I I Electric
Well IProductivitylInjectivitylG maxlQ inj.max. ReservoirlFluidl Power
I [ (t/h)bar] (m3/h)/bar (t/h)lwith Pwh=O Temp. I I (MW)
I I (m3/h) (oC) I I (1)
__ I _I
Ll 0 0 0 0 I-I 0
L2 70-300 500 210 I I 9
L3V(2) ",0.5 ",1 30 50 210 water I 0
L3D(2) ",70 200 600 230 I 14
L4 3 2-7 200 210 I 3
L5 0 0 0 0 I 0
L6 0 0 0 0 I 0
LIO ",0.1 1 ",15 360 steam I 1
Lll >100 400 200 C02 I 0
L14 >60 400 60-70 water I 0
L14bis >400 600 1000 170 C02 I 0
VAL2 ",70 ",500 130 waterl 0
GRI 40 400 190 water I 5
GCl 0 0 0 0 I 0
GR2 55 600 1000 190 water I 8
_I
(1) Power is calculated for a double flash cycle without accounting for
future decline or eventual allocation of a productive well to
reinjection.
(2) Well drilled with partial EEC financing.
Table 2. Stimulation Results
I Injectivity (m3/h)/bar
Well I
I prior to after
stimulation stimulation
__ I
Ll I ",0 0
L3V I 1 1
L5 I ",0 ",0
L6 I 0.1 0.1
L3D I 14 ",200
L14 I ",9 ",70
VAL2 I 11 70
GR2 5-6 60
__ I
Improvement
factor
2. WELLS SE~ FOR S..llM.!.!.LA'il O~
On the basis of production, reinjection and interference
tests carried out in the Latera wells, .it has been possible to
evaluate the field potential and thus to initiate a development
programme which envisages the installation of a 40 MW power

273
plant with an expected electric production of 200 GWh/y. New
wells will therefore be drilled in order to meet the power
plant fluid requirement (1500 t/h).
Within the framework of this development programme, and
on the basis of results of previous experiments, stimulation
operations on wells L10 and L4 were planned with the aim of
1ncreasing their production. The completion and stratigraphic
profiles of the two wells are shown in Figs. 4 and 5.
3. ~IIMMLATION OF WELL L10
As shown in Fig.4, well L10 did not encounter the known
water-dominated carbonate reservoir and revealed the presence
of high temperatures .
A productive horizon with a temperature of 360·C and a
pressure of approx. 200 bar was found at a depth of 2650 m. The
well produced superheated steam with a flow-rate of 15-20 t/h.
From an analysis of the pressure transients recorded
during injection and production tests there was no evidence of
negative skin or linear flow (fracture flow); moreover, the
pressure transients observed during the injection tests showed
short time constants (15-30 min.), leading to the conclusion
that resistance to flow was mainly localized near the wellbore.
It was therefore decided to attempt a stimulation job in
order to reduce this resistance and thus to increase fluid
production. The operation was to consist of the injection of an
acid solution at high flow rate (-100 t/h).
In order to choose the most suitable acid mixture, a
number of acid attack laboratory tests were carried out on a
sample of cuttings with granulometry of 2.8-5.6mm, obtained by
mixing well cuttings taken from depths in the range of 2650 m.
The chemical and mineralogical compositions of these cuttings
are shown in Table 3.
Table 3. Chemical and mineralogical composition of the cuttings of well L10
from depths of about 2650 m
CHEMICAL COMPOSITION
Si02 • 43.8 %
CaO • 14.0 %
C02 • 10.4 %
Fe203tot.. 6.0 %
MgO 2.6 %
Al203 • 14.8 %
Na20 1.4 %
KZO 2.0 %
S 0.26%
MINERALOGICAL COMPOSITIONS
Calcite 24-25 %
Quartz -30 %
Phyllosilicates (chlorite, mica) 30-40 %
Pyrite 0.5 %
Other phases : plagioclases, amphiboles
The tests were carried out both at ambient temperature
and at 150·C with acid mixtures of HCl and HF in varying
concentrations and with a varying volumetric ratio between acid
mixture and rocks. The results of the tests, carried out in a
reactor at 150·C, are shown in Table 4. Further acid attack

274
tests were carried out operating at 20 De and in two successive
stages: a first attack with a solution of Hel and then a
subsequent attack with an acid mixture of Hel + HF in various
concentrations. The results of these tests are shown in
Table 5.
Table 4. Leboretory tests of the scid attack performed at lSO D C on the cuttings of well Ll0
Initial volumetric ratio: I Composition of Img of rock dissolved/I
mg of elements dissolved/
acid solution/rock I acid aolution I/ml of acid solution I /ml of acid solution
I I 1-,;-_-::-:-----==-_--=_-;;-
I I I Ca 5i Mg Fe Al
I I I ==--:---::-:-:;:--:::-:;-;;~"",,;--:;n;;;"
-----1:"".""'7,-----IHCl 12% I 196 177.4 0.12 0.38 0.12

275
Considering the excellent thermodynamic characteristiCS
of the fluid inside the fractured horizon at 2650m, another
directional well was planned on the same site to find fluids
with the same thermodynamiC characteristics in more permeable
zones.
4. S.tl.M!LLA1l.QtLQf..jlllL_.LAI.E.BA _ i
Well Latera 4 met two main fractured zones at depths of
about 1400 and 1700m inside the carbonate reservoir. Fluid
production was low (equivalent of approx. 3 HW).
Before stimulation, the well was completed with a 9 5/8-
casing for safety reasons.
Injection and production tests were carried out to
evaluate the hydraulic characteristics of the formation in both
conditions.
Stimulation was carried out with the injection of acid
through the drill pipes, firstly at a depth of 1400m and then
at 1700m. For each stage 40 m 3 of a solution of 28t Hel were
injected with a flow rate of 100 m 3 /h. Injection and production
tests were then repeated in order to verify the results of the
stimulation job.
Pressure transients carried out during production are
not strictly in agreement with those performed during
injection. Possibly the presence of two fractures with
different static pressures makes the standard transient
analysis questionable.
However, both production and injection tests show
improvement in productivity and tnjectivity (Table 6). The same
Table also shows that the improvement is due to an increase in
pay rather than to skin elimination as usually occurs in
atimul,ition.
Table 6. Transient analysis interpretation for well LATERA 4
INJECTION
PRODUCTION
hk I Skin I II hk I Skin I PI
(l0-12 m J) I I (m3/h)/bar (l0-12m J) I 1 (t/h)/bar
BEFORE 1--1 1--1
STIMULATION 1.2 I -3.5 I 9 0.6 I -1.4 1 3
AFTER 1--1 1--1
STIMULATION 1.8 I -3.5 I 11 1.4 1 0 1 5
1 __ 1 1 1
On the basis of the data collected during the production
teats performed before and after stimulation, the back-pressure
curves of well L4 were simulated with a computer programme
developed by ENEL (Ref.l.). Fig.6 shows the two curves.
The flow-rate predicted with a well-head pressure of 10
bar increased from 175 to 230 t/h.

276
Stimulation of completely dry wells performed with
hydraulic fracturing and acid injection did not yield positive
results from an industridl point of view.
On the other hdnd, the wells with low (but not zero)
injectivity and high skill effects were stimulated successfully.
Well L4 represents an intermediate field case between
the above two: in this case skin effect was not positive and
stimulation led to an incrcase of about 30% in the flow-rate
estimated at a well-head pressure of 10 bar with consequent
increase in the electric generating capacity from 3.5 to 4.5
M\¥e.
1. Barelli, A., Corsi, R., Del, Pizzo, G. and Scali, C., 1982 -
A two-phasc flow model for geothermal wells in the presence
of non-condensable gas. Geothermics, Vol. 11, No 3. pp. 175-
191, 1982.
2. Barelli, A., D'Offizi, S., Corsi, R., Lovari, F. and Rossi,
U. 1983 Results of the drilling exploration in the
geothermal area of Latera - Utilization project of a waterdominated
reservoir. 3rd E.C. International Seminar
"European Geothermal Update". Munich.
3. Bertrami, R., Cameli, G.M., Lovari, F., Rossi, U. 1984-
Discovery of Latera geothermdl field: Problems of the
exploration and research. Seminar on utilization of
Geothermal energy for electric power production and space
heating. 14-17 May 1984 Florence.
4. Barelli, A., Cappetti, G., Manetti, G. and Peano, A., 1985.
Well stimulation in Latera field. 1985 International
Symposium on Geothermal Energy, Kailua - Kona, Hawaii,
Transactions, vol. 9, part. II, pp. 213-219.

IT
~ ~~lat,ra
.~.
R_
In ........
N
r
FIG 1 CONTOUR MAP OF TltE TOP OF CARBONATE FORMATION
_ - 400 - Cont_ h ... ~ II
Cald,,. rim ~ Cross lection
.. 0", WIll

wsw ENE I w E
o
I 1-0 ... __-------- LATERA CALDERA
BOLSENA
·1
CALDERA
L5
L3 L3D
Ll0
Boisena Lake
I
I
I
v v v v V 'V V v V
1000
1000
o
I
+ +
r+l
L!.....2J
2
6
Fig.2 Geological cross-section A-A': 1.Volcanites; 2.Syenitic intrusion;
3. Flysch facies formation (cover); 4. Carbonate formation (reservoir);
5. Thermometamorph~sed limestones; 6. Faults.

ssw
NNE
o
v v v v y y v v y v
L14 L3-3D L4 L2 L11
I
G2
I
Gel
•
v .., .." V V ..,
SHG I
I
:-:-.'
1000
2000
..-:::s:a-,~- ,.. '-
-.- ;.
-~'"')""l\_=:c:t:-- -
--_ __
o
2km
..... ...
2
---
3 4 5 6
Fig.3 Geological cross-section 8-8' :l.Volcanites; 2.Clastic sediment; 3. Flysch facies
formation (cover); 4. Carbonate formation (reservoir); 5. Thermometamorphosed limestones;
6. Faults.

280
T ECH~ICAL
TEMPERATURE °C
PROFILE
STRATIGRAPHY
250 300 400 500
,1
" " "
v v
" " v
" "
v
" v
~ " V
500 Tuffs
~ " v v
and lavas
v
II
~ ~ v
v
v
" "
"
" v v
=
v v
1000-
~ .. ~ v
" v
III
II
-"
\9. -=-=
~
..........
- 1500- ..........
..........
.........
}, .......... Flysch:
%~
.......... shales, marls 255°C
sandstones
'~
- 200n. ... ..... , and
~ .......... limestones
h
..........
..........
..........
... .,._.-.,.
328
25""-
"' .........
- .........
-
...........
~ .........
..........
I 3000 450
4 + ..
I 1+ +
I .. Thermometa.
I J·~+l"
.. .. + morphosed 480
.... I!:I. + ~ limestones
.. .. ..
~
Fig.1
Completion and stratigraphic profile of well LID.

281
TECHNICAL TEMPERATURE °C
STRATIGRAPHY
PROFILE
150 200 300 40(1
I)z
z
. D
..
~ ::!
" '" " Tuff.
I, .. v ~
and lava.
~::

..
,...
.A
..., •
•..
:::II
•..
Do
"
~ •
I
j
N
00
N
111
14
12
10
8
II
4
2
100 1110 200 2110 Flow-rate Ct/h>
Fig.6 Back-pressure curve of the well L4. (A) Before
stimulation. (B) After stimulation.

283
EEC contracts nOEN3G-0072-F (CD)/EN3G-OOSG-D (B)/
EN3G-00SS-D (B)/EN3G009Z-D (B)/EN3G-OOSI-D (B)/
EN3G-OOSZ-D (B)/EN3G-OOSZ-D (B)
THE EUROPEAN GEOTHERMAL PROJECT
AT SOULTZ-SOUS-FORETS
O. KAPPEL MEYER (Geothermilt Consult GmbH, Hannover/FRG) (I)
A. GERARD (BRGM - MME, Orleans/FRANCE)
ABSTRACT
Within a European Geothermal Project research, teams from France and
Germany collaborate to test a site in the Upper Rhine Valley for its
suitabUlty for terrestrial heat mining (HDR energy production). Some British
scientists participate to specific tasks.
The site was chosen near Soultz-sous-Forits in Alsace, at the location of the
old oU field of Pechelbronn which was the first oU field exploited in Europe
since the ISth century.
It is situated on one of the summits of a very large thermic anomaly (ZOO km
long and ZO km width) where the mean geothermal gradient between the
surface and I SOO m is known to be higher than 6.soC/l00 m.
The program began in July 19S7 with a ZOOO m deep borehole.
Below a I 37S m thick sediment cover the granitic basement was penetrated
to a depth of Z 000 m. The temperature at the bottom of the hole is 140°C.
The geothermal gradient within the sediments is unusually high (10°C per
100 m) and diminishes to a normal after a series of factures inside the
BWldaandstein producing lOme water at 116°C with a total salinity of 9S gil.
At the depth of I SZO m hydraulically active natural fissure was reached. The
artesian outflow from this zone is O.IS 1/s, well head pressure is 1.6 bar. The
thermal water produced from the well has high chloride contents and clearly
has an identical origin with the fluid collected from the Buntsandstein just
above the Il'anite.
During the water injection tests, a second active natural fissure was detected
normally closed out but wblch seems to acquire a noticeable permeabUlty at
a well head preuure of about 40 bars.
(I) Project co-ordinatora.
The results presented in this document have been obtained mainly by or under
the direction ofl
- A. BEAUCE I Bureau de Recherches Geologiques et Mlnl6rea
- JP. CAUTRU I Bureau de Recherches Geologlques et Mini6res
- C. FOUILLAC I Bureau de Recherches Geologiques et Mlnl6rea
- B. HERBRlCH Compagnie Fran~alse de Geothermle
- R. JUNG Bund8l&Dltalt fOr Geowissenachaften und Rohatoffe
- Eo LEDOUX I Ecole des Mines de Paris
- F. RUMMEL I Befeld Mesa Systeme Gm bH
- R. SCHELLSCHMIDT I Niederalchaiachea Landesamt fOr Boderuorachung
- H. TENZER I Stadtwerke Urach
'I'beIe results will be published in detaU at a later date.

284
An extension to the 3 dimensional modelling of the main hydrothermalised
levels crossed by the well inside the granite was performed, using
sophisticated seismic techniques (VSP, tomography, etc.).
"In situ" stress measurements with "hydrofrac equiment" are executed in the
granitic section below 1 375 m. It was necessary to prepare special equipment
for high temperatures.
For the monitoring of further hydraulic investigations, including the
stimulation of some natural joints carefully selected, it was necessary to
develop a high performance seismic network. From this point of view, a great
advantage of the test site near Soult-sous-Forets is the availability of some
dozen boreholes within a radius of a few km, which are the relics of the old
oil field. Up to now three of these holes situated at distances of 150-350 m
have been opened. The depths and temperatures at the bottom of these holes
are: 840 m (l400C); 970 m (l16°C); 1 360 m (lZ3°C).
Full results concerning the interval joints network, the stress field and the
hydraulic (natural and stimulated) behaviour of the system will be known at
the end of 1988.
1. INTRODUCTION: THE KEY PROJECT OBJECTIVES
NW
Figure 1 I GPK 1 I
SE
t.r--------------~~--------~~~----~------~r_4~
KlO
o
-«lO
-~
-500
CENOZOIQUE
~
Mloclne II Eodn. :
~
[:;;] Couches de Pechelbronn
~
~ Couche rouge 0
~ Zane dolornltlque EJ
~ Zane de tronsJrlon ~
Oogger .t Lias
Rh'tlen
Keuper
Lett.nkohle
Muschelkalk
Buntsandstein
5OC..E
~ Granite
Faille principal.
JP.CAUTRU Sepfembre 87

285
1.1. The site
At Soultz-sous-Forits, nortb-east of Haguenau in the north of Alsace, a
marked thermal anomaly baa been identified wblch coIncides approximately with
the oU field of Merckwiller-Pecbelbrorm. AI early u 19Z6, Hoffman and Haas
showed that the temperature locally could exceed SooC at 400 m (by comparison,
only about ZooC would be found at this depth in a thermally normal region).
In 1984, l'Institut Mixte de Recherches Geothermiques (IMRG) prepared a
preliminary map of temperatures at 1000 m, showing that temperatures could
exceed 110°C at thia depth. Following this work, IRMG baa more recently
constructed a geological crou-section (figure 1) based on 60 deep wells and data
from seiamic reflection studies provided by Compagme Fran~aise des Petrol.., who
retain lOme interest in this oU-bearing zone.
1.Z. Development of the profect
From these studies, it became clear that neither the heat flow associated
with the anomaly nor the geological structure offered any great prospects of a
traditional geothermal exploitation using water contained in the deep waterbearinK
strata.
In practice these are too shallow, and temperatures high enough for economic
exploitation in thia particular region (> lSOOC) are likely to be attainable only
within the granite basement at depths of about Z 000 m.
Nevertheless, temperatures u high as this at such a shallow depth are an
indication of a subetantial quantity of stored heat, wblch could be of considerable
interest.
TlUS RAISED THE QUESTION OF EXPLOITATION BY THE "HOT DRY
ROCKS" (HDR) TECHNIQUE.
The results of work of this type in Great Britain and the United States have
been encouraging, although problems still remain. These concern notably 1011 of
fluid and the overall COlts of such installations. It is because the cost of drilling
and the problem of instrumentation increases 10 rapidly with depth that the
anomaly at Soultz-tlOUI-Forits ia important in the context of HDR research.
1.3. The current programme
To carry out the studies necessary to establishing the feasibility of a pUot
plant producing geothermal energy from the Hot Dry Rocks, the French and
German teams are collaborating, with support from the Commission of the
European Communities, on the following programme:
- Completion of a well through 1 400 m of overburden, penetrating 600 m
into the granite.
- Study of the fracture network in the granite (as well as its connection with
the sandstone in the cover rocks) by the most effective methods (coring,
and geophysical logging by electrical, sonic and thermal techniques).
- Measurement of the mechanical properties of the granite and (it is hoped)
the state of strell.
- DetaUed studies of the hydraulic properties and potential for fluid 1011 in.
the natural fracture system, by performing successive injection-recovery
tests and also any long-term corrosion phenomena.
The relOurces necessary for this programme have been supplied by the
Direction General de 1& Science, de 1& Recherche et du D41veloppement (DGRST),
the Commission of the European Communities (CEC), the Bundesministerium FOr
Fonchung und Technologie (BMFT), l'Agence Fran~aise pour 1& Maltrise de
l'Energie (AFME), Ie Bureau de Recherches Geologiques et Minieres (BRGM) and
the partner's own funds.
The work baa been allocated, on the German side, to five groups:
Bundesanstalt fOr Geowissenschaften und Rohstoffe (BGR), Niederslchsisches

286
Landesamt fUr Bodenforschung (NLFB), Befeld Mess System (MESY), Stadtwerke
Urach and Geothermik-Consult Kappelmeyer, and, on the French side, to IMRG,
which operates within the framework of AFME's programme RGeothermie Profonde
Generali.see R with Schlumberger, Armines, the Laboratories of Applied Geophysics
of the University of Paris VI and of Garchy and the Materials Studies division of
Electricite de France (EDF) as partners.
Z. DRILLING AND COMPLETION OF THE WELL GPK1
The well was sited in the spot envisaged, after the completion in 1986 of a
provisional geological cross-section based on 60 deep wells (drilled in 1950-1951
and reaching depths of 800 - 1 400 m) and on vibroseis data acquired by Total in
1984.
Calls for tenders were published during the first half of 1987 and the
administrative procedures completed (covering approvals for exploration of a
geothermal reservoir, for drilling and for disposal to surface water courses).
The drilling pad was constructed in May/June 1987 within an area of 1 ha
acquired by BRGM.
The drilling and completion occupied the period from 30 july 1987 to
1Z december 1987.
The final result, which meets the original technical project specification, is
shown in figure Z.
By comparison with the work foreseen in the original programme, it may be
noted that:
- A single production test was possible at the Buntsandstein-granite contact,
which showed.that the permeability of this interface was nil.
The following coring DRI LLHOLE GPK 1 IN SOU L TZ
programme was
completed:
GEOLOGY
• 1 core in the
Muschelkalk (9 m, 8Z %
recovery)
• Z cores in the
Buntsandstein (5,5 m, 9 PEQIl.SROIII.If'.
% recovery, and 9 m,
CCOO£ _ 335
90 % recovery)
390
• 16 cores in the granite,
DCl.0IITIC ZCIE
totalling 43 m, with
594
recovery rates of 90- \
JIWSIC
100 %.
IH1D
It should be noted that the
ICEII'ER
hoped-for continuous coring of
the granite, tentatively planned
in the original programme, did
not give the anticipated results
and that it was necessary to
settle for intermittent coring of
the granite.
1-------l1317
- The logging programme
as originally planned was
completed in its
entirety, both in the
granite and in the
sedimentary cover (using
radiometric, electric,
sonic and mechanical
WELL
COMPLETION
c.....",
~ ... 1I4
T rll8
• adcltlv.
Figure Z

287
tools). The excellent quality of the sonic and electrical logs (BHTV and
FMS), used for identification of fractures and fissures in the boreholes
walls, b particularly worth mentioning. Two BHTV logs were run, one by
Schlumberger and one by WBK. Schlumberger made two runs with their
FMS tool.
3. GEOLOGICAL INVESTIGATIONS AND MAIN RESULTS
3.1. Presentation
Geological investigations were performed by (in alphabetical order):
1 - J.P. Cautru, Keologist with BRGM: local geology, Keometry of the Soultz
musif - Coordination of investigations.
2 - A. Genter, thesard with BRGM: reKional Keology; with the help of P.
Martin and Y. Gros, geologists with BRGM: structural analysis of granite;
under the supervision of Ph. Chevremont geologist in BRGM: petrography of
sound granite; of A. Meunier, Professor and D. Beaufort: maitre de
conf6rence Universit6 Poitiers: petrography of alterations facies.
3 - H. Traineau, Keologist with BRGM: petrographical determination of
cuttings, drawing by computer of sedimentary lithology with the help of J.L.
Balmelle, student in BRGM.
4 - L.G. Mastin, Ph. D and B. Heinemann, Geophysicist with the Univ. of
Karlsruhe: Evaluation of caliper and televiewer data.
5 - H. Tenzel1 Geologist with Stadtwerk Bad Urach: Structural analysis with
BHTV -SABIS interpretation.
3.2. General geological results (*)
In GPK1, the sedimentary overburden was drilled (fig. 3) in accordance with
prognosb and logging correlations with neighbouring oU wells are excellent. As a
whole sedimentary layers are impervious and could form an isolating coveraKe of
the hot water circulations. The only possible water bearing formation are the
MUlchelkalk and the Buntsandstein. Nevertheless matrix porosity b weak and the
permeabUity b essentially due to fracturation.
With the fair continuity of facies and thickness of sedimentary layers and the
usumption that there are no of Permian deposits, the geometry of the overburden
allows UI to deduce the Keometry of the underlying basement. Geological
interpretation of sebmic profUes, along with local data from oU wells, permit us to
draw 5 geological cross sections across the thermal anomaly of Soultz (fig. 1). It
appears that the Soultz horst b bounded by N-S normal faults dipping to the west
for the Kutzenhausen and Soultz faults and to the east for the Hermerswiller fault.
Replica of these faults will be found deeper in the granite.
A cross section was drawn towards the East up to the Black Forest for
thermal model computations.
These Keological cross sections are being digitalised along with drill hole
data, in order to draw isohypses maps of key sedimentary beds (base of Jurassic,
top of MUlchelkalk) and of the top of the granite.
Electrofacies of granite
From wire logging we can observe in the granitic basement 3 main facies
(fli. 4).
The first one corresponding to the sound granite of -type 1-, heavy and with
hlih PEF, low uranium and high thorium content.
(i) J.P. CAUTRU

288
Figure 3
Soultz-sous-Forets
SYNTHETIC LOG OF THE SEDIMENTARY FORMATIONS
i
"I n
J.III. CNJTRU J.L.. ULMD.L! .... T'AAJr«N..I 1CJ't.'"
-DOlPIIIII •••,'........ I_ •
• _ ••"'... ILh_ .... I ... •
,.., ..,_Il h. 11_,
'U".III,.. I ... U,,-u..
. s-s,. _ Lo ,, __
-11 ... "_ •• 1 ..... "._" •
... 1 ...... IOC •• n .... ' •• -
....... IIL ..... , •
... " .•• _I .. L .......... I ... .
. un .• h ....... 111l' ........
....... p._Ir._c. .
_.1. oh_._ ..... tI
• ' "'''Il' II ,,"IOOIIOIIIU·,
......
_, ...." III 1_ll\.,
• eun. II., •• 11'0 .• h __ •
....... ,.IU •• _· ..... ,I(.
d ..... IIL, ...., ..... 1 ...
•,.llI. - .-s1DW.. '1_
.... 1_. -"Ill. ,I__ n.h ......
- LIIIlST_ "'''' ,1 __ llh •
• "ILh.
-Cl.'.IIL, .••••.• ,._.
r. .. III, .. __ ·UIIlSlIIIII.
-"1111[""""""""",,,,_,,,
1o'lMluLI., II., ..... - tun
... ,. II .. , •• Ie_ ....... IIL,.
-' lIIIII'IlU til UU'UGI", 01.,
~i'¥!:r!7:,'.~~~I:';'
- oaUlIIlI1 • .J., ..... 1 ...... ' ....
F~
~,
~~
~:-:-::::::~
. -:::"I.=:."t:'.-:,=: •• ILl.
.1_'._Lw,......,...,
... 111 .... 1 .......... 1.1.
_' ..... I ....... '~ .... _II ...

289
The other of • ype l a HaMer with a lower COllt~t In thorium ~ a higher
content In uranium.
FOT both types .anlc velocity 15 high and porosity v~ry reduced.
A thlrd electro-fac'ea ha.a been identified WMt'e porosity Is higber atId sonic
" loclty 10 r. 8y comp riaon with core taken in 5Uch a f&d~, we consider that It
it Il ' ry fractur ted and/or altef'8d. f&des.
In the d t il1;, for inDt~e between 1580 and 160S some zoaes appear bere
porOllity la AMah· u in fractured zone but where deoslty aDd soDlc velocity ue
Dormal U in • sound hoe" • By extrapolatlon of the petrographka study of core 16
we COIlIki r bat these 2iones are in the vicinity of a fractw-e wblcb was not
encountered LD the drUl hole itself.
...
u
6 RHO ~cc
UO
Figure 4
ElectrofacUts of gnnlte
_. " (
~ "
51 ....' 'Ie s .....(
00,,;,,1 rt
•
'DQ~O..["( p.o..Q' (\.,
mill
'''\ 01 Del.
•
ao\,.QtIt: Ir( " "~
L. 'lU HOItC
~ t"'I~ .lo\N4'
~ "".~'''Vt'8·''''''
~ .,... t ~·.ltl>
~ l\1(li"-
•
'I ot\.~stl( .I-c1'Od
E
LI ~ 1ro-t .tt- In
.""-'.1 .. 1'" _
E!E ("
mill " h.ff ( rI,
m ~t ..
• ~h.
D ~II
• ( "'11C
Q
11e
Figure 3b
Legeod of the figure 3
3.3. Petrogrpph,y
I. Sound gnnite
Macros

290
Figure 5
Soult~ - sous~Forets GPK 1
SYNTHETIC LOG OF THE GRANITIC BASEMENT
TyPe t "lls/It facies with .....
ll>orl"", CCO\tef>~

291
Z. Altered Ill'anite
We obHrved the auperposition of several successive hydrothermal events. The
secondary mineral phases occurring are mainly clay mlnerala. The different types
of alteratioa features are controlled by water-rock interaction in natural
fracturing IJStema. In the Soultz granite, the alteration distributlOD is linked to
fracture permeabUlty.
The two main obHrvable alteration types in the cored sectlODs are:
- Pervuive alteration, in which the rock microtexture is preserved. The
main features are the selective transformation of pre-existing
ferromagneslan minerals (biotite, amphibole) and plagioclase into
secondary clay products, iron oxydes and carbonates. These local chemical
equilibria occur widely in the granitic body. This hydrothermal event
OCC\lrl in the whole massif.
- Vein alteration, in which features of alteration depend on water rock
interaction in the natural Joints. In the cored zones, the higher the
fracturlni density, the more the wallrock alteration effect is developed.
Structural analysis of cores, well logging interpretation and chemical logging
of GPK 1 Ihowed that there were some traces of old and recent percolations in the
granitic batholith. This vein alteration is characterised by mineral transformations
in the vein itself and in its neighbouring wallrock. The vein alteration is composed
of two kinds of secondary products:
a) wSericite W alteration. In this case, sericite represents white mica (illite) or
regular mixed layer minerals (illite-smectite) with a low percentage of smectite.
This neogenesis OCC\lrl in the plagioclase, biotite and amphibole of the matrix and
in the vein deposit itself.
b) Kaolinite - white mica - corrensite assemblage. This paragenesis appears in
core 16 (1600 m deep) where the fracture density is relatively low. So it could
correspond to a wall rock of a nearby fracture.
3.4. Fracturing analYSis on cores samples (*)
Structural obHrvations on granite cores show that there are several brittle
tectonic facies (single fracture, breccia zone, protomylonite)J in particular, around
l8Z0 m deep (core 19), where GPK 1 crosses a major faulting zone. By comparison
with FMS imagery, 0.80 m of cored sections have been oriented on K 19.
In this depth interval, a Schmidt plot with thirteen fractures has been carried
out. It presents two directional famUles. The first one, N-S dipping West which
correspond very likely to the Oligocene faults. The second ore is E-W dipping
North, and represents an original azimuth. We know a Permian distensive tectonic
period which could have created these structures.
3.5. Fracturing analysis using F.M.S. (Formation Microscanner - Schlumberger
tool) (*)
The 43 meters of cores are not enough to give a statistical characterization
of fracture distribution in the Soultz granite. Nevertheless, FMS imageries
performed on complete sections are able to produce an interpretative fracturing
101 along the FMS data GPK 1 borehole. The FMS tectolog symbolizes the'
projection of all the interpretative (cf. figure 5).
On figure 5, the fracture density represents the number of Identified
structures by two meters interval. It shows that the sound granite appears more
fractured than the altered facies. In fact, the fracturing density variable has been
lmderestimated in the very fractured or altered zones. FMS imageries is a function
of electrical properties of the medium and in the fractured or altered intervals, the

292
granite is very conductive. So the imagery is dark, difficult to interpret.
Stereographic projection of all the planar discontinuities shows the main
directional families of fractures (fig. 6). After elimination of fractures which were
probably induced we find schematically the same spatial fracturing organization.
The principal directional family is oriented N 170 o E, dipping 70 0 W or 80 o E.
We observe the relative lack of vertical structures. This point could be in
relation with the technical configuration of the FMS tool (FMS is "blind" to 50 % of
the borehole surface).
During the Oligocene, the subsidence of the graben took place in an E-W
extensional regime. The N1700E structures correspond to this major brittle
tectonic period.
Secondary families could be observed in this stereogram on figure 6.
3.5. Determination of planar discontinuities and borehole breakouts using the
SABIS borehole Televiewer (*)
Well logging by the Acoustic High Temperature Borehole Televiewer (BHTV;
SABIS-System) intended in the part-project Urach, enables continuous recording of
the natural planar discontinuities at the borehole wall and sampling data on the
borehole geometry.
The orientation of strike and dip directions of the planar discontinuities is
obtained by the evaluation of the televiewer logs.
Results were expected especially concerning knowledge of the natural joint
system, the active fault pattern and the direction of the major horizontal stress.
The orientation of natural joint systems in the basement, which can be
hydraulically activated, and their relationship to the local stress field will be
investigated by comparing the televiewer data with the results of the hydraulic
tests and stress measurements.
The newly developed High Temperature SABIS-Borehole Televiewer of the
Westf1l.1ische Berggewerksohaftskasse (WBK) was to be tested and prepared in the
HDR-borehole Urach 3 by logging tests for the use of well logging in the
exploratory well GPK I at Soultz. The BHTV was tested down to a depth of 3364 m.
The in situ temperature at this depth is 143°C. The electronics of the televiewer
worked faultlessly up to an inside temperature of the tool of 15ZoC without using
an additional thermal shield.
The improved BHTV was then used for wireline logging in the Buntsandtein
and upper part of the granite of the GPK 1 well. The disperse distribution of tiny
Lignite particles in the drilling fluid and borehole cavities causes a diffuse
reflection of the sonic signals of the Televiewer. The run of the first BHTV
measurement did not show high enough amplitudes to detect the planar
discontinuities clearly. Solid mud additives should not be used during logging with
BHTV.
In the second logging programm the Televiewer had been supplied with an
additional thermal shield. Altogether 1886 planar discontinuities were measured on
the Televiewer logs.
The televiewer allows the determination of planar discontinuities and
different lithological units. The sound and cataclastic granite, in particular, can be
well distinguished by the different amplitude-reflections. The correlation of the
rock units distinguishable on the BHTV log with the density - and porosity -
measurements is good. There is also a good agreement of the BHTV log with the
focussed electrical logs (MSFL, FMS).
(*) H. TENZER

293
",S DATA - II~ MEASUREMENTS - GPKI -
Fa
• F7
Figure 6
A.GENTER. Nov.1988
nIS DATA - NATURAL FRACTURES - GPKI
•
•
---
-'WI -.. [I ,...
...--.-~--
I ......
,. - 40
.. -..
n- ..
IO-n
11-10
• - 11
-.ow •
--- ~.-~--
~- '":'::- :::
III - 131
II-III
I. - 15
IU - III
II - I.
-.ow II
F.3 N F4
E
OIIlW:Cfla ..... 'M(IURU '''"n.11S
........ , ........ , ... , ............. ...
5
5

294
Joint displacements and directions of dip-slip faults are recognizable in some
borehole sections.
The main direction of strike of the discontinuities in the granite is N-S with
dips to the E and W.
Further submaxima of joints with a NE-strike and SE-dipping as well as a SE -
and EW-strike with SW, NE - and N-dips are recognizable.
Most of the planar discontinuities show subvertical and steep dips.
Vertical structures striking N160o-l70oS are noticeable over large intervals
(ca. 135 m). These structures, which can be called "drilling induced hydraulic
fractures" might be caused by the linkage of subvertical structures. The frequent
occurrence of vertical features on the BHTV log can only be observed on a small
number of core samples. On the FMS logs vertical structures can only seldomly be
detected within one of the four pad traces.
Hydraulic tests have shown that borehole sections with vertical structures
are especially capable of absorbing water. The identification of these important
vertical structures was only possible by means of the Borehole-Televiewer. The
strike of most of natural discontinuities is also N170oS, with a W - or E - dip
(fig. 7). This suggests a strike-slip situation in the Soultz area.
PLANARE DISCONTINUITIES measured by SABIS-BOREHOLE - TELEVIEWER in the granIte of well GPK 1
Orientation 01 the naturat. Slnus-wave-llke Dlscontlnultoes and their apparent W,dth
~
." . ).:."\
w. • ~ E
, .. :~
. '. . .'
.- ..
ls
Width mrdlum
Figure 7
Width of Planar Discontinuities.

29
Figure 8a
HOT CRY FlOC!< PAOJGCT AT $OUI.ll
P\.AI DlSCO'Il\)fIJfTres m-.-d by SA81S -BORSO..E - f EY1E"N R
.n '''''9

Figure 8b
HOT DRY ROD< PROJECT AT SOUL TZ
PLANN1E DISCONTtNUITlES measured by SABIS-BORaiOlE -TELEVIEWER
in lhegrnnile of well GPI< 1
EYQtuotion by 510dlwcltur Bod Vlach
• 0'
Ron 'oncJ Pole
Oiogram •
"
" '",

297
Structures which are clearly less than I mm in size can be detected by the
Borehole-Televiewer. With processing, nevertheless, two points closer than 4 mm
to each other can not be depicted on the log. The apparent maximum value per m
of depth was therefore used for the determination of the joint widths. Wide joints
were identified particularly In borehole sections with sound granite (fig. 8a et 8b).
The different joint widths do not sbow favoured orientations that differ from the
total orientation of all joints. Joints of slight width dipping at low angles are much
more abundant than gently dipping joints of greater width (fig. 7).
The obtained results will lead to new realizations In the further development
of the Hot Dry Rock-technology.
In addition to this, natural planar discontinuities will be included in the
development of multiple frac systems and artificial heat exchange systems within
the crystalline basement.
3.6. Borehole geometry and its implicatlons for regional stress orientation (*)
The stresses In the Earth's crust produce a compressive stress concentration
on the wall of a vertical borehole which is greatest at the azimuth parallel to the
least compressive regional horizontal stress (Sh figure lOb), and least at the
azimuth perpendicular to ~. Zones of failure and spalling ("breakouts") at the
former azimuth may cause the borehole diameter to enlarge, whereas at the latter
azimuth fluid pressure In the borehole may cause vertical hydraulic fractures to
form. In this paper we use logs of the four-arm caliper tool and the ultrasonic
borehole televiewer to identify these features and use them to Interpret the stress
orientation In the vicinity of the Soultz borehole.
Caliper logs between
depths of 1 400 m and 2 000 m
were examined for breakout
Intervals using the following
criterial (1) the caliper tool
(which normally rotates due to
cable torque) must stop rotating,
(2) one of the two measured
borehole diameters must be
larger than bit size, and (3) one
diameter mUit be equal to bit
size. No Intervals were
Identified which satisfied all of
these criteria. Uslna televiewer
logs provided by the
Westfllische Berggewerkschaftskasse
(WBK), televiewer
data was reprocessed to produce
borehole Cl'OSl sections and
three-dimensional information
on borehole geometry.
Televiewer logs between 1 420 m
and 2 000 m depth showed
leveral places In which the
borehole shape was slightly oval
(figure 9, triangles), but In Cl'OSl
lections of these Intervals the
borehole wall wu smooth, not
(.) L. MASTIN and B. HEINEMANN
D
-1500
-1600
-1700
-1800
-1900
-2000
90
azlmJth
(dtV'NI tram m~Uc north'
120 150 180 210 240
o (OOglogl • *"01 cNr90 In
bCrl'hOl. dW"Ktlcn
an vertical pi ....
Figure 9
270

,~n--.. ____
298
Figure 10
Figure lOa
-0--....... _ ........ __
~ ...........................
"'.........-.......... SCALE
1:6000000

299
apalled, suggesting that the ovalizatlon wu caused by tool gouge rather than by
stress-Induced fallure.
In the televiewer logs between depths of 1400 m and Z 000 m, at leut 135 m
cootain vertical fractures wblch bisect the borehole and follow the borehole axil
for up to a few tena of meters (hollow squares, figure 9). These fractures are
generally not present in the core at corresponding depth intervala, suggesting that
they are induced by fluid pressure in the hole during drilling. 1bey are therefore
inferred to have formed parellel to SH' The orientationa of these features, plotted
in l-m depth intervals in figure 9, have an average of 169° eut of True North and
a standard deviation of 7°. Tbla iI slightly more DOrth-iouth than the orientation of
SH determined from other stress indicators in tbla region (figure lOa). It iI possible
that a more DOrth-south orientation of the mOlt compressive stress in the Soultz
area iI due to its location near the west side of the Rhlnegraben. An eut-west
tension uaoclated with the down-warping of the down-droped block within the
Rhlnegraben could, when IUperimposed on a NE-SW regional orientation of SH
rotate the local SH to a more north-south direction.
4. STRESS FIELD. ROCK MECHANICS AND PHYSICAL PROPERTIES
This work wu performed by Mesy GmbH, Bochum and Institute of
Geophysics, Ruhr University, Bochum, by and UDder the direction of F. Rummel.
4.1. Introduction
Both magnitude and orientation of the stress field at depth control the
efficiency of fluid circulation in HDR systema in the hot crystalline basement rock,
•• g.1
- the pressure in the circulation lyatem must exceed the normal stresses
acting acrou natural joints or induced hydrofractures;
- exce .. circulation preuurea may causes excelS fracture growth and thus
may result in significant fluid 10lles;
- high horizontal streues may cause the generation of horizontal fractures;
- high horizontal stress anisotropy may result in shear and induced seismicity
and may favour planar flow on a few favourably oriented flow paths rather
than flow through a confined volumetric network of existing joint ..
Besides streuea, HDR lyatems must also consider the physical properties of
the basement rock, e.g.1
- fracture mechanics parameters like macroscopic strength, fracture
toughneu or fracture surface energy will control fracture growth or
frictional sliding on existing joints u well u borehole stabUlty;
- the coefficients of heat conduction and thermal expansion determine the
poaaibUlty of thermal cracking and thus volumetric reservoir growth with
time,
- the value of heat production rate may be an indication for the heat source;
- seismic, electric or magnetic parameters are input data for geophysical
reservoir exploration.
So far our knowledge of the stress field is rather limited, particularly with.
respect to depth and in crystalline regions of high geothermal gradients u possible
HDR candidates. Also, the interpretation of geophysical logs in hot deep crystalline
environment is still belni developed and requires correlation with the results of
direct laboratory physical property meuurement ..

300
Figure 11
Schematic of borehole simulation autoclave for packer
and logging tool testing. Capacity: m 160 mm,
internal length 8 m, Z 000 bar, ZOooC

V I R ! L I • ! - ,! R F RAe - S TIT E "
1 - lIeU ncord1IIg .yau.
2 - II1ncII conuol un1&
l - 'naoun _ n ... conuol un1&
4 - IUgII ...... __ puoping .,ou.
s - II1ncII .,.t.
'-~un1&
7 - Cebl .....
• - IUgII ......-- ho ..
'-7-~_1.
10 - Celli. _ 01_
11 - Cellle hood with ___
~--
12 - Prn __ "I ..... 01 ...
13 - ....... - pull .. I ...
14 - Pocker .1-.
15 - Inton.1 with ~
T.I .. 1_
11 - Single ""'" unit

302
4.1. Stress Field and Stress Measurements
At present, hydraulic fracturing is the only method to directly measure both
magnitude and orientation of principal stresses at great depth. So far, the deepest
hydrofrac stress measurement was conducted at a depth of 5 km, although only few
data could be collected below a depth of 3 km. Many of the existing deep stress
profiles in the continental crust suggest that, at a depth of only a few kilometers,
the principal horizontal stresses approach the following values (Rummel, 1986):
SH -> Sv
Sh -> lIZ Sv
with Sv = p gz (where p is the rock density, g the gravity field)
One important conclusion from such a stress situation is that stress
anisotropy also suggests the generation of subvertical hydrofractures and the
opening of subvertical joint systems only, during forced fluid injection into deep
boreholes, which may have consequences for HDR-technology.
During injection tests in GPK 1 at the Soultz European HDR candidate site
existing fractures opened at only 50 bars well-head pressure, suggesting a minor
horizontal principal stress Sh of about ZOO bars at a depth of 1500 to ZOOO m. This
corresponds to about half of the overburden stress and is in agreement with the
general statement given above. Further detailed tests require urgently high
temperature, high pressure and gas resisting hydrofrac packer developments. A
first step towards such a packer development is the construction of a
ZOOO bar/ZOOoC packer tool testing autoclave of a length .of 8 m and an internal
diameter of 160 mm (fig. 11) and in present packer element testing.
For measurements of deep continuous stress profiles in future deep HDRdrillholes,
a wireline hydrofrac tool is in development. The system is designed for a
maximum depth of 5000 m (fig. 1Z).
Until present the spatial orientation of induced hydrofractures is still
measured by impression packer elements, since the resolution of acoustic borehole
televiewers is not sufficient to show the crack traces at the borehole wall after
crack closure. Therefore, an acoustic televiewer is in development which can work
in the injection interval between the sealing packers during crack pressurization.
Present numerical hydrofrac models used in oil - and gas exploitation are
complicated, time - consuming and not adequate for micro-hydrofrac operations
performed for in-situ stress measurements. Therefore, an analytic model is in
development which is based on fracture mechanics, includes fluid dynamic aspects
and fluid percolation into the rock, and also considers hydrofrac system stiffness.
The system even may be used on-line during the frac operation and well allow to
get insight into the hydrofrac system stiffness. The system even may be used online
during the frac operation and well allow to get insight into the actual fracture
growth process. A typical example of a pressure/frac-extension curve is presented
in fig. 13 for specified input parameters, and agrees will with observed results from
hydrofrac experiments in crystalline rock.
Physical Properties
For the present Soultz project a program of physical property measurements
on both the cores and cuttings of GPK 1 was developed. The program includes
measurements of density, ultrasonic velocities, elastic constants, fracture
mechanics parameters, and thermal, electrical and magnetic data. Presently, the
data are used for geophysical log interpretation and extrapolations for non-cored
sections of the Soultz borehole.

_______
_______
303
Figure 13
Example of bydrofrac simulation for an experiment in intact granite
at a depth of I 900 m with a horizontalstreu ratio of 0.5;
permeabUtty I ~DJ CI and CZ constants whicb determine pressure losses;
Pc critical pressure in interval affected
by system stiffneIIJ (a) pressure VS. crack length, (b) pressure VI. time
Hydraulic Fracturing simulation
80
I
,......., I ,.......,
«I II
0.. I I I 0..
~ 60
_______ L _______ ~
~
0 ::II
I I ......
I
I I
07 I I I
c:
III I I I
Q)
I I
::I '"'
I I I ::I
1/1 40 '"'
-----r-------+-------~------- 1/1
1/1
I I 1/1
III
I I I III
'"' c.. I I I
'"' c..
iii I I Ii
t
I I I
20
20
u
---r-------T-------j------- III
.... ....
I I I .t:
.... I:l I I
u
I I I
I I I
I I I
0 0
0 500 1000 1500 2000
a normalized fracture length b lleSy
Hydraulic Fracturing simulation
80 ,.......,
8
......
,......., I
II -III
_J-____ l. I ____ c..
II
...... !i 60
30
I .d
IIQ
I
07 I
I
t>...
III I
I I I
'"'
~
::I
III
III 40 20 Ilo
III
-~----+----~----+----
III
I I I -.d
'"' c.. I I I I ....
iii
I I + I I
t 20 10 .!!
III
--t----~---- I ----~--+--- Q)
....
.... I:l I I I I
I
I I ....
I
I I I -u
I I I II
0 0 '"'
0 250 500 750 1000 1250 1500
b pumping time t [s] WeSy
l1li
I:l
3

304
Until present, from the granitic section of GPK 1 (1400-2000 m) about 50 m
of core material and 600 cutting samples were recovered and extensively tested.
The granite yields rather high cutting densities increasing systematically with
depth (neglecting data from fracture zones), while the core density is almost
constant. The velocity values demonstrate typical granitic data however, with
little correlation with density data so far. Seismic velocity anisotropy of the core
material is negligible, however, absorption anisotropy reflects preferred microfrac
orientation. The magnetic susceptibility of the granite material is about 2000 It 10-5
SI units, but only 100 It 10-5 SI units in the vicinity of fracture zones. This indicates
a possibility for fracture zone prediction from cuttings while drilling. Preliminary
heat production rate data from the Soultz granite indicate a mean value of
6.Z ~W/m3, a value twice the world average for continental granites.
Further testing is in progress, particularly tests under simulated in-situ
environment (pressure, temperature, pore pressure).
5. SEISMOLOGY
5.1. Presentation
The seismological work was performed by IMRG/BRGM associated with
CNRS Garchy with the participation of:
- BEAUCE Alain Geophysicist; coordinator of the "Seismic" group;
- BENDERITTER Yves : Geophysicist;
- F ABRIOL Hubert Geophysicist
- LE MASNE Dominique: Geophysicist;
- CAVOIT Claude Electronics Engineer; realization of the electronics of
the seismic probes and network;
- CHEN Xinkai Geophysicist chinese student in 3rd cycle thesis.
Seismology plays an important role in HDR projects: through the analysis and
the location of the microseisms induced by hydraulic fracturing or stimulation, we
can hope to follow the spatial development of the main fractures stimulated or
created by the fluid injections in one well. In the case of Soultz, the siting of the
second deep borehole of the doublet will depend mainly on the orientation of the
fractures stimulated or created by the existing GPK 1 borehole.
5.Z. The seismic network
Three old vertical oil wells (GPK 1 is located on the Pechelbronn oil-field)
were reopened at the end of 1987 to install our seismic network. Located at
horizontal distances of ZOO to 400 m from GPK 1 (figure 14), they reach depths of
840, 960 and 1 360 m with bottom hole temperatures of 104, 114 and lZ4°C.
Unfortunately, only the last one nearly reaches the granitic horst under the thick
sedimentary cover.
5.3. Seismic probes: design and installation, and orientation
A prototype 3-axis seismic probe (see figure 15) has been designed following
these requests:
- functioning at high temperatures (at least lZ5°C) and rough corrosion
conditions during long periods (several months),
- good anchoring of the probe down-hole (either by cementation or by
"marbling") to avoid resonance, which is a problem arising in many cases
with clamping tools.
The mechanical part of this probe is based on the seismic probe developed by
IPG Paris for the experiments of Le Mayet de Montagne, whereas the electronic
one (including special high temperature components) was designed in cooperation
with the Centre de Recherches Geophysiques de Garchy (Garchy Centre for
Geophysical Research). The signals coming from the 3 (Z horizontal, 1 vertical) 20

305
Figure 14
Situation map
A"""
. :a", o..h 0' ,
tl,0n,
.... ptob.
S.I,,,,'C. •• 11 "'....0.. Gftd
... '~_"
.1P'"
Wollt owoy
Figure 15
Seismic probe

306
Figure 16
Orientation of the probe inside one of the seismic boreholes
x

SENSORS AMPLIFIERS ij
AMPlJF1ERS FILTERS ij ..,1
=ANALOG=CONV==JC=E=~=TER=IT=AL=:::::!.J~
HP s&ie )SO
AMPLIS FlLTRES
WI!I.LS
__ !l ___ ~ CRG/lMRG
CHANNELS
CRG/lMRG
MONITORING
DETECIlON
h
U
z
..
"
E
o
;;.
S E IS.. I C S
Network and data
aCQuisition system
STORAGB
+
+ +
+
+ + +
* Seumic hypocenters
/Seismic ray.
@) l componanb ICmIXS
Tape

308
Hz - seismometers (Mark Product LIS) are conveyed along a 7-conductor cable up
to the surface and then to the central registration station.
The calibration of these seismometers shows a fairly stable response from 20
up to 1500 on 2000 Hz. Anti-aliasing filters are operating from 1500 Hz.
One of the probes (borehole 4598) is cemented downhole (we are grateful to
the Institut Franc;ais du Petrole (French Oil Institute) for their help in choosing the
best cement formulation). The two other probes are buried downhole under a
column of tiny zirconium marbles directly poured from the borehole head.
A series of dynamite blasts in very shallow holes (2 m) has been carried out
from the surface to orientate the three down-hole probes - by hodogrammetry on
the first arrivals of the two horizontal componens of the signals. We give an
example of the orientation results obtained for the probe sited in well 4598 on
figure 16.
5.4. Data acquisition system
The acquisition system (see figure 17) developed around Hewlett-Packard
material consists mainly of:
- a fast and powerful (up to 10 channels at 20 000 Hz) Analog/Digital
Converter (HP 3852) with a dynamic range of 120 db;
- an HP 9000/350 computer built around a 32 bits microprocessor with
8 Megabytes RAM;
- storage units up to 260 Megabytes (2 x 130 Mby Winchester disk drives);
- magnetic tape cartridge unit in charge of the back up of the data stored on
the disks;
- an output unit (printer).
The software, adapted from IMRG experiments for the monitoring of an oilfield
stimulation on Total's prospecting zone at Villeperdue, was tested during the
orientation blasts and for the Vertical Seismic Profile (VSP), Walk-Away Seismic
Profile, and Offset Seismic Profile (O.S.P) described further on. It allows
continuous monitoring of the nine channels (3 channels x 3 probes), detection of the
seismic events recorded on each probe and storage of the associated signals.
5.5. Active seismic surveys
Geophysical logs performed in GPK 1 have shown that possible seismic
reflectors could be detected inside the granite (linked to micro-fissured areas).
Furthermore, in order to better define extension of fractured zones around
GPK1, and better locate possible seismic events inside the granite during hydraulic
stimulation tests with a precise velocity model, an active seismic program was
performed. The different operations were the following (see figure 14):
- one VSP at GPK 1 well;
- a 1 km-long walk-away profile with 26 blasts (spacing 40 m) along the line
between GPK 1 and borehole 4598, with 5 receivers levels between 1525
and 1425 m;
- three offset seismic profiles (OSP) at distances ranging from 500 to 900 m
away from GPK 1, and in the azimuths of the three seismic wells.
Seventeen registration points were performed (spacing 30 m) in the interval
1730-1250 m.
At the moment, the data have to be processed and it is too early to give
precise results.
Moreover, information that will be deduced from these experiments will be
reinserted in the surface exploration data profiles performed by Total (see
figure 14) in 1979 and 1984, to contribute to the re-treatment of the granite part
of the seismic reflexion sections.
This last processing will allow uS to identify the possible existence and lateral
extent of markers within this formation.

309
6. GEOCHEMISTRY
6.1. Presentation
The geochemical work was performed by IMRG/BRGM associated to some
other laboratories. The IMRG scientists are: A. Criaud, M. Brach, F.D. Vuataz and
C. Fouillac. The analytical work for the reactive tracers was carried out by Pr. M.
Dreux and his colleagues from the University of Orl~ans. The percolation
experiments are done in the department GMX of BRGM by J.F. Sureau and coworkers.
Pr. G. Michard (University of Paris) studies the water rock interactions by
the mean of experimental work in an autoclave.
The geochemical program bas been carried out since 1986, when a
preliminary Itudy on the mineral fluids from Soultz and the lurrounding area was
done. During the drilling phase in 1987, the monitoring of drilling fluids has enabled
us to detect significant permeable zones in the granitic basement. Unfortunately,
no sample could be obtained from the triassic aquifer in GPKI. In 1988, we have
sampled two brines (m 100 gil) discharging from GPKl, and from a very nearby well
(4616) tapping the Buntsanststein aquifer above the granite. Their chemistries look
very alike, raising the question of the origin of their mineralization. Then, the
geochemical group took part in the hydraulic tests. The freshwater injected in the
well during the first test was found to mix quickly with the formation water, and
also to provoke dissolution of some rock material.
In this paper we present the results concerning the abovementioned points
and, briefly, the studies that are still in progress: tracer tests, percolation and
autoclave experiments, modelization.
6.2. Geochemical logging
The monitoring of drilling fluid was carried out between depths of 1426 and
1998 m in the granite. The schematic plan of the monitoring equipment on the site
is shown in figure 18. The main aim of this technique is to assist drilling and to
complement the information by other logs (geological and geophysical). Because
the mud mixes to some extent with the formation fluids, variations of the
composition of the drilling fluid allow the forecasting and detecting of the water
and/or gas producing zones, and this generally a few meters before the drill bit bas
reached the permeable zone (Vuataz and co-authors, 1986). The continuous
recording of helium concentrations revealed the efficiency of this natural tracer to
indicate the fractured zones (see logs on figure 19). Almost all the dissolved
species (HC03-, Cl, Ca and Na profiles are given on figure 18) showed
concentrations increasing gradually, with Iteep gradients when main joints were
encountered. The pH was kept alkaline in the mud and consequently was found to
decrease when a permeable zone was reached. Because the mud was frequently
renewed, these results indicate either that the salinity of the fluids (pore and
Joints) in the basement increases with depth, or that there are more producing
zones at greater depths. Moreover, the chemical anomalies correlate well with the
other relults (drilling rate, density, resistivity). A main producting joint was
identified at 1815 m.

310
Figure 18
Schematic plan of the geochemical monitoring equipment
on the drilling site
DrUlinI mud loop
Drill pipe
lnelytie.l
eQuip .. nt
6D : 611 detector
GC : Gu chrc..lltogr.p" I
4-p., .. .,ur M. : Heli ..... &1 ,penr ... t.r ..
.. uuring device
Figure 19
Geochemical log of GPK 1 well: composition of the return
drilling fluid. All the elements are expressed in ppm;
only He is on a logarithmic scale
pH HC0 3
Cl Na ca He
1400
1500
co
P
00 "'"
I
1600
§
::c
0-
n.
l!!I
1700
1800
1900
2000

311
Figure ZO
Isotopic characteristic. of GPK 1 and 4616 fluids. The regression line is
for GPK 1 only. MKS 138 wu obtained during clrilling
I.
iii
r -50
Q
..
-80
-7 -I
6t8 0(HZO)'!I.
Figure Zl
DetaUed correlation between C1 and S04 concentrations for GPK 1 and 4616 fluids
61...-----------------..... 4616
60
ū +K5228 +KD 007
+KDOO6
205 215 225 235
5041110 /1
Figure ZZ
DetaUed correlation between Li and S04 for GPK 1 and 4616 fluids
t46
4616
142
}134
.J
1:SO
128
GPK I
"
t84!1 1930
" "

312
Sample and sampling
depth
Na K Ca Mg Ll SIOI Fe NH.
KS 138b well-head 22,2 2,43 6,18 130 119 31 36,3 28
KS 228 well-head 28,2 3,32 6,73 150 123 97 232 26,7
KD 005 1810 m 27,2 3,21 6,76 145 122 94 52,9 26,5
KD 006 1845m 28 3,28 6,96 152 126 94 7,5 26,6
KD 007 1930m 27,9 3,4 6,93 152 126 93 29,5 26,6
4616-214 well-head 27,8 3,45 8,40 140 148 110 275 24,7
Sample CI S04 HC03 F Dr I D AI CHaCOOH
KS 138b 47,4 501 501 2,7 150 - 18,8 < 0,1
KS 228 58,5 215 648

313
6.3. Chemistry of the granitic brine and related fluids
Gas and water have been collected at the wellhead and at selected depths
during the artesian discharge of GPK 1, from May Znd to June 13th. The evacuation
of 500 m3 of fluid wu necessary to clean the well, because the persistent presence
of several additives (corrosion inhibitors, completion brine) largely affected the
chemistry. Table 1 summarizes the water and gas composition for the most
recently obtained fluids. The analysis for a triassic fluid discharging from a well
(4616) located near GPK 1 (see location map, fis. 14) is also reported.
The mineralizations of the fluids are very similar and reach 98 and 10Z gil for
GPK 1 and the triassic fluid, respectively. Na and Cl are the dominant anions, and
large amounts of gas (ZO % in volume) are released. Such fluids have been described
in crystalline reeka of various contexts, with variable Na/Ca ratios (see Fritz and
Frape, 1987). The final samples (KS ZZ8 and KD 005, 006, 007) for GPK 1 are
significantly different from the one obtained during sampling operations (KS 138b),
for both chemistry and isotopes. They still contain some tritium, proving that
contamination by surface water could not be completely avoided. KS 138b should
be more contaminated with drUling fluid than the others, but is enriched in
deuterium and 180 compared to the "best" samples (figure ZO). This discrepancy is
probably due to analytical problems during the distUlation of the muddy sample KS
138b. All good quality samples are shifted relative to the local meteoric water line,
reflecting isotopic exchange with sUlcates at high temperature. The samples from
GPK 1 correlate well (r • 0.89) and enable definition of the isotopic characteristics
of the recharge, which is very close to the local and Black Forest meteoric waters.
S04 was chosen rather than Cl to compare the concentrations of the major
lpeciel for the different samples: the sulfate contents are lower and determined
with a better precision, and they offer larger relative variations between samples.
The figure Zl showl that Cl/S04 and is very similar for GPK 1 and 4616 fluids,
while moat of the ratiOl Ca/S04, Mg/SO~ Li/S04 and B/S04 are higher for the
triauic fluid than for the granitic one (figure ZZ). The GPK 1 points lay on mixing
linel indicating that a more saline fluid may exist (deeper 1) in the basement.
There is no evidence for relation between GPK 1 or 4616 fluids and the local
mineral waters (Hellons).
Thus it is likely that no actual direct connection exists between the two sorts
of fluids. But a common origin cannot be excluded, the fluids having evolved later
in separate reservoirs. This is confirmed by the Itrontium isotopes and gas
concentrations, which are identical for both fluids. Some data show a sedimentary
origin for the mineralization: high salinity, presence of ac:etate and negligible
amountl of other aliphatic acids.
6.4. Geothermometry
Sample
Quarta Quarta Cbale NaKCa NaKCaM. NelU Na/lJ Nail( Nail( Nail( 11080.
OODeI. edlab. P K .. A T .-)
KS228 138 131 86 136 223 177 233 231 212 208 186
4816-214 143 131 n 238 230 198 248 238 218 212 177
The sUlc:a geothermometer leacia to estimations of temperature for GPKI
that correspond to the measured one at 1815 m (main fluid inlet). This is consistent
with the solubility of quartz (identified on filter membranes). In the 4616 well, the
measured temperature of the main aquifer was 119°C, a value intermediate
betw_n quartz and chalcedony equilibria. It wu observed that the sUlca

314
concentration has been decreasing in 4616 during the production test, so that the
SiOZ geothermometer may be doubtful.
The calculated temperatures with other formulae (cation and isotopic
geothermometer) are widespread and in the range 177° to Z36°C for both GPKI
and 4616. The triassic fluid indicate a somewhat higher temperature than GPKl,
whatever the thermometer. All data indicate that the fluids have reached
temperatures higher in the past than at present. The 4616 fluid may be linked to a
network with higher velocities than the granitic fluid, thus explaining the
differences in temperatures.
6.5. Freshwater injection test
Z50 m3 of freshwater was injected and 118 m3 was allowed to evacuate. pH
and Cl were recorded continously, and determination of HC03, Cl, Ca and SiOZ
was carried out on site. The samples were later analysed for complete chemistry.
Immediately after the well volume (36 m3), a steep peak in pH (up to 9) and
mineralization appeared. This is due to remaining completion brine and drilling
additive probably trapped at the casing shoe (or in the annulus): this is proved by
the low ratios of SiOZ, Li and Ca relative to chloride in this fluid (see figures Z3,
Z4 and Z5). Then the mineralization was found to increase gradually, reaching
61 g/1 at the end of the test, because of the mixing with the formation water, as
shown by figure Z5. From the concentrations of chloride, it was calculated that
84 % of the injected water was recovered. The silica content does not display such
a regular behavior due to mixing. It increases relative to chloride (figure Z5), and is
only 10 % less than the initial content of the deep fluid and equilibration with
quartz. So far the extent of water rock interaction and dissolution which has
occurred cannot be precisely determined because the other reactive species (Ca,
Mg) does not display such a deviation. Another point is that the vented water
quickly became yellowish to brown. This color was attributed to the dissolution of
the tubing and oxidation of iron. Corrosion does occur, as proved by the high
amount of hydrogen generated in the tubing (see the differences in HZ in the gas
phase between bottomhole and surface in table 1). The iron concentration also
displays a peak corresponding to a remainder of chemical additives. Then the
concentration corresponds remarkably well to what would be expected on the basis
of a simple mixing between the deep fluid and the make up water. Interpretation of
the observations is still in progress.
6.6. Tracer test
We intend to test the use of a reactive tracer in connection with hydraulic
tests. The principle of the method was studied by Robinson (1985) but has given
poor results in HDR projects until now, for various reasons. Hopefully better
knowledge of the formation water chemistry (salinity, pH) will help in
understanding the results. The temperature dependant reaction is the hydrolysis of
ethyl propionate, chosen because of the temperature range and for analytical
reasons.
Efficient analytical methods have been developed for the determination, on
site, of the products which will be injected or formed during these tests. Operating
two gas chromatographies at 110° and l300C allows the determination of ethyl
propionate, propionic acid and ethanol in less than 15 mn. Among the organic acids,
only acetate is present in significant amounts in the formation fluid (see table 1). A
couple of inert tracers (iodide, whose concentration is low in the deep fluid, and
colored tracers) will be used at the same time as the ester.
6.7. Experimental work
Several themes were selected. The percolation experiments through sound
and altered cores of granite are focussed (1) on water-rock interactions at 150° C,

31S
Figure Z3
Concentrations of chloride in the fluid produced after
the freshwater injection test Oune 1988)
The peak ia attributed to rests of additives
30- +
+
+ +
20
+
;::::
co
++
U + +++
10 +
r
0 0 20 40 60 ~ 100 120
Volume m 3
Figure Z4
Correlations between Li and CI and the water discharged after
the freshwater injection test. The lower point. correspond
to the peak on figure Z3
120
A
r
100-
80
..... 60 A
:::i 40
20
0
.IJI>
A
fl>tI>
AA
A
20 CI oil
40 60
Figure ZS
Evolution of the concentrations of silica during
the ventlna phase. The lower concentrations correspond
to the peak on figure Z3
.co
80-
160
N
0
9
cii40 9
\
."
999
9
20 ~
999 9 9 9
0
20 40 60
Clo/l

316
using a freshwater as injected fluid, and (Z) on possible losses of inert tracers
(fluoresceine, I and rhodamine) because of temperature and/or adsorption on
chippings. In the second set of experimental work (done in an autoclave), the
approach of equilibrium between fluid and granite at 180 C is favored by using a
solution whose chemistry is close to that of the deep water, and selected pieces of
cores.
All these results and the use of models should be combined later to help and
understand the reactions that occur during real tests.
6.8. Conclusions
The chemical composition of the formation fluid in GPK 1 could be
determined only after most of the drilling additives had been evacuated from
GPKI. It was significantly different from the one obtained during drilling. The
origin of the brine that has been found in GPK 1 is not yet identified. Because of
its similarity with the Buntsandstein fluid, a common but remote origin is assumed.
Some results are consistent with the deep circulation from the east (or South east)
of the Rhine graben of a fluid getting its high mineralizatiqn from sedimentary
layers at higher temperatures (> ZOO°C) than their present ones. The granitic fluid
from GPK 1 may have evolved differently from the brine from 4616 because of a
slower natural flow through the fracture network.
The big differences in chemistry between formation and make up fluids has
made possible the interpretation of the first hydraulic test in terms of natural
tracers. Not only mixing of the two fluids was observed but also some evidence for
dissolution of rocks at depth. A number of data are still being interpreted. The
preparations for the reactive and inert tracer tests at selected depths have been
completed. The experimental and modelization work is in progress.
7. RESULTS OF HYDRAULIC EXPERIMENTS (*)
A longterm production test and several injection experiments were performed
in borehole GPK 1 between May and July 1988. The objectives of these tests (from
the hydraulic point of view), were to investigate the hydraulic conditions in the
basement, the inner and outer hydraulic boundary conditions, and the influence of
fluid pressure.
During the production test about 500 m3 of brine was recovered from the
borehole by artesian outflow over a time period of about 7 weeks. Several times
during the test the borehole was shut-in for some hours to observe the pressure
build up. During the injection experiments a total of about Z50 m3 of water were
injected with constant flowrates between 0.5 1/s and 3.5 1/s. After each injection
the well was shut-in for several hours to observe the decline of the fluid pressure
(figure 26). ..
Flowrate and wellhead pressure were continously recorded during the tests. In
addition to this the wellhead pressure in the seismic observation hole 4616 was
recorded to see whether the production or injection produced any pressure change
in the Buntsandstein overlaying the basement. Several temperature logs were run
during the tests to detect the water carrying fractures.
The records of the tests are not yet completely analysed but several
interesting results can already be derived:
(*) Author: R. JUNG: Bundesanstalt fUr Geowissenschaften und Rohstoffe

317
Figure 26
Recorda of wellhead pressure and flowrate during the injection
experiments in borehole GPK 1
~
0
I;)
0
•
In
c..
r
10
.c ~
g
0
In
...
I
~
0
~
....
0 100000 ;"00000 300000
sec
400000 500000
0
r.:I
on
•
en
........
-0
:t:I
-
It'I
...
~ rI
0
0 100000 200000
300000
4i)U~00
'----
500ilOil
sec
7.1. Hydraulic conditions in the basement
Among the 1500 natural joints and fractures detected by geophysical logging,
only about twenty are hydraulically significant. Most of them are almost N-S
Itriklni vertical or lubvertical fractures. The dominant feature it as a depth of
1812 m, were according to the acoustic televeiwer logs a long vertical almost N-S
striking fracture intersects an altered zone. These observations indicate that;
concerning fluid transport, the basement is highly inhomogeneous and anisotropic.
It makes therefore no sense to derive any average hydraulic property of the
basement from the results of the tests.
7.2. Formation pressure
The artesian pressure at the wellhead was about 2 bars when the borehole was
fUled with brine of density 1.07 kg/m3 and about 13 bars when the brine had been
replaced by fresh water. The artesian pressure corresponds to the fluid pressure in
the dominant fracture at 1812 m.

318
7.3. Inner boundary conditions
The inner boundary conditions can be derived from the initial part of the
pressure records during injection or shut-in. It was found that the pressure records
exhibit an almost perfect square-root of time behaviour during the first half hour
after start of injection or shut-in. The square-root of time behaviour is typical for
one dimensional flow from a plane into a space or half space (in contrary to radial
flow). Therefore the inner boundary is not the borehole wall but the surfaces of one
or several fractures. From the slope of the str'aight line (figure Z7) the surface area
of the fractures can be estimated when asumptions are made about the
permeability and the storage coefficient of the granite. Because no skin effect is
visible (pressure loss at the intersection between the fractures and the borehole)
the transmissivity of these fractures must be very high (at least some 10 Dm
(Darcy-Meter». The total storage capacitance of the fractures dV/dp (a property
which is analogous to the so called wellbore storage coefficient) can be derived
from the slope of the pressure record (time in linear scale) immediately after shutin.
It is about ten times larger then the wellbore storage coefficient calculated
from the borehole volume and the compressibility of the fluid. With this parameter
a second estimate of the fracture area is possible, when the storage coefficient S
of the fractures (storage per mZ) is assumed. The values derived from the records
are listed in table 1. The values indicate that considerably large and conductive
natural fractures are connected to the borehole.
7.4. Outer boundary conditions
The outer boundary conditions can be derived from the long time behaviour of
the pressure or the flowrate. During the production experiment it was observed
that the production flowrate became constant after about three hours after
opening the well and remained almost constant throughout the following weeks,
only slightly increasing due to the decreasing weight of the water column in the
borehole. This indicates that the main producing fracture is hydraulically
connected to a large confined or unconfined reservoir at some distance from the
borehole, which could for instance be a large fault zone known in that area. The
steady state production rate of about O.IS1/s which corresponds to an artesian
pressure of about Z.S bars can be used to estimate the longtime lossrate produced
by this connection provided this fracture is included in a HDR system. For an
overpressure of 40 bars in the fracture the lossrate would be Z.41/s (for the
viscosity of the brine). It seems very likely that similar outer boundary conditions
would be found for sufficiently large artificial fractures produced in that area.
7.5. Effect of fluid pressure
In figure Z8 the difference between borehole pressure and artesian pressure
measured at steady state conditions during the production and injection tests in
plotted as a function flowrate. It is obvious that there is an almost linear
relationship up to an overpressure of about 30 bars. For the highest flowrate the
overpressure is much less than predicted from the linear part. This indicates that
fractures start to open already at a wellhead pressure of about 40 bars. According
to the results of the temperature logs run during the injection experiments these
fractures are located at around 17Z8 m, in a borehole section where a great number
of almost N-S striking steeply dipping fissures were encountered in the televiewer
logs.

319
Figure Z7
Build-up of wellhead pressure after shut-in plotted as
function of Iquare root of time (production experiment in GPK 1)
o
If)
"-
10
.c
80 120
0.5
sec
180 200
Figure Z8
Difference between fluid pressure in the borehole and artesian
40 pressure as function of flowrate
L.. 30
o
.0
..
Q.

Site
Rock Type
Pa.s
m3/s m3/Pa8.5 m3/Pa m3 m3/(Pa.s) MPa
m
m2
Viscosity
E][periment F10wrate F.k.S' dV/dp
T C Opening
Depth F offluid
][10.:1 ][10.10 dO-7 dO-12 dO-IO Pressure
dO.:l
Soultz granite 1400-2000 production 0.15 6 2 .5.104 10 6 0.4
" " " injection 0.53 12 - 1.105 10 7 0.4
" " " shut-in 11 3 1.105 10 0.4
" " " injection 3.34 8 2 0.8.105 10 10 4 0.4
" " " shut-in 33 2 3.105 10 9 0.4
Urach Paragneis 3300-3500 injection 1.5 1.5 1 2.104 10 very low > 27 0.2
Falken granite 200-450 production 2.0 3700 9600 5.107 104 700 -3 ·1
berg
Table 1· Results of the hydraulic tests at Soultz and comparison with test results from other sites
F
S'
T
fracture area, k: permeability of the rock (k was assumed as 10-12 m2 for all sites),
storage coefficient of the rock (S' was assumed as 5.10-11 Pa- I for all sites),
transmissivity (l m3 = 1 Darcy Meter), C: specific steady state loss rate.

321
7.6. Comparison with other sites
During the last years other sites have been studied in a similar way. The
results obtained there are also listed in table 1. The experiences made in these
tests show that it Is a quite common situation to find some considerably large and
conductive natural fractures in a borehole in jointed rock. In one case (Falkenberg)
a very large and extremely conductive fracture zone was encountered. Such
fractures should be a primary target in the design of HDR systems and efforts
should be made to detect them and to define their hydraulic and geometric
properties.
8. HYDROGEOTHERMIC STUDIES
8.1. Presentation
This work was performed by R. Schellschmidt und R. Schulz from the
Geological Survey of Lower Saxony.
8.Z. The original temperature field in boreholes of the Soultz area
Figure Z9 shows the undisturbed temperature as a function of depth in
borehole GPK 1 and in three recovered oU wells (4598, 4609, 4616) in the
surroundings (max. 350 m). The maximal temperature is 104°C in well 4598 (at
838 m depth), 116°C in well 4609 (at 973 m depth), 116°C in well 4616 (at 1383 m
depth) and 140°C in borehole GPK 1 (at ZOOO m depth).
When the drUling operations were completed, in borehole GPKl, the original
temperature field around the borehole was disturbed. A stand-by time of 6 weeks
wu neceuary for a complete recovery of the original rock temperature. The only
exception Is a small interval around lalZ m depth. This zone was much more cooled
down than other borehole sections, because here a great amount of drilling mud
wu lost and entered a fault zone. The temperature at this depth was still
increasing (see fig. 30) in January 1988.
The recorded temperatures and temperature gradients of borehole GPK 1 are
given in figure 30 as a function of depth. At about 1100 m depth a decreasing of
the average temperature gradient from 100 mK/m to 30 mK/m Is clearly seen. The
decreasing temperature gradient can be explained by assuming a convective heat
transfer in the Buntsandstein/Muschelkalk aquifer (947-1377 m).
The thermal conductivity of cores of the well GPK 1 (Muschelkalk,
Buntsandstein, granite) was measured in our laboratory under original thermal
conditions. The heat flow density, the product of thermal conductivity and
temperature gradient, Is more than ZOO mW m- Z in the Muschelkalk and about
80 mW m- Z in the granitic section. This determination validates the assumption of
a horizontal convective heat transfer within the aquifers.
8.3. Influx and water loss in the open-hole section of the bore-hole GPK 1
determined by temperature measurements
The results of a production test show, that there Is only one inflow zone in
borehole GPK 1 at 181Z m depth.
The aim of the injection tests was to determine the positions of water
carrying joints and to investigate the influence of fluid pressure on the injectivity
of the borehole (ratio of injection flowrate to injection pressure). The injection
experiment. were followed by a production test for studying the chemical reaction
(BRGM) of the injected fresh water in the basement.
The thermal measurements have been evaluated by the wFlow method w and
the wShut-in method w • For these experiments it Is necessary to measure the
undisturbed rock temperature in the borehole before the hydraulic stimulation. In
addition, two or three measurements during water injection are needed when using
the wFlow method W and two or three measurements must be made during the shut-in

322
1~~
2~~
3~~
4~~
5~~
6~~
7~~
8~~
9~~
1~~~
11~~
12~~
13~~
14~~
15~~
16~~
17~~
18~~
19~~
2~~~
Figure 29
Undisturbed temperature as function of depth in borehole GPKI
and in three recovered oil wells (4598, 4609, 4616) in the surroundings
d. tho m p
o
temperature. C
2~ 4~ 6 8~ 1 ~~ 1 ~
~ ~
\' r\~
" \~ ~
1\ \ ~
\ \
" ~
\ '\ I""~
\ \ ~ ~
\ \ ~ 4598
\ ~060Kriil
'\ ~~04°C
0.033 Km- 1 \ \ ~~ 4609
.... 116 "C
\ \ \
\
\
t\ \
\ '\ ~
\ \ :~~\:- ~
\ \ '\
1\ 1\ \
\ \ \
\ \ )
\ \ '\
\ \\
14
140"C
GPKl

323
Figure 30
Undisturbed rock temperature aDd temperature gradient as function
of depth (temperature measurement on January 19th, 1988)
~ ~ - lJ 0 II ~ 1 ~ ~~ m
rr n ILL N
•
~~ 0
i
t
In
u
.
l
.
Ii
,
• \~ ~ ~ l1u I
1'1 N-
W ~ ~ I
" ~
,
:!
Il'
-
-
10 /
III
..
/
V
'" /
/
l/
V
l/
V l/
~ II
lrw J ~ w.
V
V
~ ~
--- V
-,
1M. ~L
1"\
!e
•
c
.r ~
~ f.- ~~~
•
1
~ ! ~ I , I I ~ I I ~ ~ ~ ~ ! ~ ~ ~ ~ , I

324
interval. The evaluation method has been described in detail by Michel and Haenel
(1984); it is based on a method by Murphy (1977). It was shown in 1984 that thermal
flowmeter measurements are comparable to spinner flowmeter measurements
regarding the resolving capacity (Schellschmidt and Haenel, 1987). The thermal
flowmeter is advantageous if the water flow is small and the temferature is high.
Three injection tests, using injection flow rates of 0.5 1 s- , 1.5 1 s-l and
3.5 1 s-l, were conducted in borehole GPKI. A well head pressure of 35 bars was
built up to inject water with a flow rate of 1.5 1 s-l. A water loss of 20 % was
detected at 1735 m depth; but the major amount of water (about 73 %) was lost at
1812 m depth (fig. 31). A second major outflow rate of 3.5 1 s-l and a well head
pressure of 43 bars. The water losses were about 47 % at 1128 m depth and about
53 % at 1812 m depth (fig. 32).
Further more the results of the temperature measurements immediately
after shut-in prove that the joint at 1812 m depth is connected to a very large fault
zone in contrary to the joint at 1128 m depth. This result is represented in
figure 33 (depth interval 1700 m to 1825 m). Between 1704.4 m and 1733.8 m depth
joints are observed, which are not connected to a fault zone. These joints opened at
a wellhead pressure of about 35 bars and after shut-in they closed. Because the
joints have no connection to a fault zone, the injected (cold) water was flowing out
of these joints after shut-in, running down the borehole and entered the joint at
1812 m depth.
During water injection cold water had entered at different depth into joints
of the granite around the borehole. The jointed borehole zones need more time for
the temperature recovery than the other borehole sections. By means of the "Shutin
method" it is possible to determine the distribution of water accepting joints;
even very small water losses can be detected. The temperature during shut-in as a
function of depth is given in figure 34 for the injection test with a flow rate of
3.5 1 s-l. A total number of 20 outflows have been detected, but no water
accepting joint was observed below the depth of 1812 m.
A comparison of the borehole logs proved, that only the high resolution
temperature logs detected definitely which joints are water accepting and which
are not.
9. MODELLING TECHNIQUE
9.1. Presentation
This work was performed by D. Brue1, M.C. Cacas, P. Iris and E. Ledoux from
Ecole des Mines de Paris.
In Hot Dry Rock experiments, the flow occurs through a network of extensive
fractures. Thus, the medium can not be regarded as a continuum, nor as a single
fracture. It implies the use of stochastic discrete models for interpreting and
predicting the physical phenomena involved in the system. Such a model is now
available at the Ecole des Mines de Paris. It is used for modelling and interpreting
the Soultz Hot Dry Rock field test.
9.2. The modelling technique
Fracture network generation
Obviously, the exact geometry of the fracture network involved in the Soultz
experiment can not be fully observed in a single borehole. Our modelling technique
is based on the generation of three-dimensional networks of numerical fractures
the geometry of which is statistically equivalent to the real one, inferred from insitu
observations of the fracture pattern.
The fractures are represented by disks randomly located in space (figure 35),
according to a given density. Directional sets of disks can be generated

325
Figure 31
The quotient !J.T/g as a function of depth
(Injection test on June 24th, 1988)
o
14.50
dT
K
--,--
g KIm
100 200 300 400 .500
1500
.------100 "
1.5.50
El1S.50
1600
16.50~-97"~~.~
1700 ~ 1697
~:::::::::::~93::"::::~~iiii~~~~~~~~~~
1735
17.50
.------73"
1800
1812
18.50
1900
19.50~-~---L--~----~--~--~--~--~----~~
depth, m

326
Figure 3Z
The quotient AT/g as a function of depth
(injection test on June Z8th, 1988)
1450 0
.!iT
g
K
,--
KIm
1500
240 480 720 960 1200
~ I=.. .
1550
1600
100'1(.
~
~
.
1650
1700
1750 r
1800
53 'I(.
~ -
-
~&.
=J ~
~
~ ~
----------------- 1812
-
- - --- 1728
1850
1900
1950
I
depth, m

327
Figure 33
Temperature measurement. before and during shut-in for the depth
interval 1700 m to 18Z5 m (injection test on June Z4th, 1988)
1799 129 • 9
170~.~~-)"-~\
1709.~ Yl
telllPeroture.
•
C
122. 5 125.9 127.5
mo..\-
1725+-_______ \~~~~··~~1~n~3~.~~~--------_4----------~
\~1733 ..
1\ )
1759+-__________ +1 __ ~~ _______+----------~----__ ----~
139.9
t •• per.tur •••• ur ... nt before
.hut-in while w.t.r i. inj.ct.d
with a flow rat. of 1.5 1/.
1775+-__________ ~~----~_+----------~--------~
t .... ,.t.,. ""
.... ur ... nt
during .hut-in
IBB9+-__________ ~-4------_+~~~------~--------~
1812.5 --+-_..c....... (
~=---+---- -
1~~ ________ ~ __________ ~ ________ ~ __________ ~
depth ••

328
Figure 34
GPK l: Temperatures after shut-in
+ Helium logging and drilling rates during operations
compared to geophysical logs
1115" I o·
.
II..,
'·50
",..
,...,
i IU~
+
1 1)4) ' ,.0
,'" In ,
I!~ O
1\1)
' Ml~
+
*
'f.U
r"~ +
J"" : -
f "~
•
u,
I
~OO L
IllS
\'00 ' " 00
,'1001---="'-
''''
~oo o
lGOO
• ·Pwot.«l>«- j aw. -.", T~ : I I H OS .....

105
1400
~
tc-. ....
110
1500
1600
1100
1800
1900
2000 depth ••
~.·c
115 120 125 no
--:::
lHn-E?
IlIll ',..~_ ~ 17." oInce -.t ofohut.
1 727.8 - ..c::::
173~.3-
1~7.0-
1787.~·-
~
1.12.0_L. "7
It 4h oInce -.t of ohut i
135 1&.0
\
\
Rock teaperature during ahut-in (injection teat on June 28th, 1988)

330
individually. Each set is characterized by a statistical distribution of orientations
and radii.
Flow modelling
The flow is supposed to occur through monodimensional bonds joining the
centers of the connected disks (figure 36). These bonds are supposed to be
equivalent to the set of channels inside the fracture which ensure the hydraulic
linkage with the connected fractures. An equivalent hydraulic resistance is drawn
at random in a given statistical distribution and assigned to each bond. The
piezometric head is computed at the fracture centers by solving a mass-balance
equation in each disk.
Thermal effects modelling
The heat transfer between the fluid and the matrix is simulated in order to
forecast the temperature draw-down at the production well. Each fracture is
represented by a planar heat exchanger with a given area. The conduction of heat
in the matrix is assumed to be monodimensional and perpendicular to the fracture
plane. At each time-step, an implicit finite differences method is used to calculate
the temperature profile in the direction normal to the disk. The heat conduction in
the matrix and the advection along the fracture are coupled by applying a heatbalance
equation.
Mechanical behaviour modelling
The result of the hydraulic calculation is a distribution of piezometric heads
at the fracture centers. These values may generally appear unconsistent with the
in-situ stress field. So we are developing a fluid-rock interacting numerical
modelling based on a boundary element method, in order to estimate the
mechanical behavior of the fractured rock-mass submitted to injection of fluid at
various pressure.
9.3. The model inputs
The network geometry is regarded as a model input: it is inferred from in-situ
observations of the fracture traces along the borehole wall. The bond hydraulic
conductivities and the fracture heat-exchange areas are calibrated from in-situ
local tests which are being carried out in Soultz. The hydraulic response of the
fractures to aperture variations and the in-situ stress-field are being investigated
to allow the modelling of the mechanical phenomena in the rock-mass.
9.4. Primary simulations
A few primary simulations with arbitrary fracture hydraulic conductivities
and geometry have been performed to test the model. For instance, a Hot Dry
Rock experiment was simulated in the following arbitrary medium:
- fracture orientations are uniformly distributed; all the hydraulic bonds are
assigned the same conductivity;
- the two wells are 1000 m long; the distance between them is 500 m; they
are intersected by 15 fractures on average;
- the injection flow-rate is 0.08 m3/s; its temperature is 80 o C; 10 % of the
water is lost between the two wells;
- the cumulated heat-exchange areas is 4.10 (6) mZ; the initial temperature
of the medium is 190 D C.
Seven simulations have been performed under these conditions, in different
realizations of the fracture network. The temperature drawdowns at the production
well are presented in figure 37.

331
Figure 35
THE NETWORit MODEL
Figure 36
FLUID FLOW SIIIUTlI*
Figure 37
....
I 190 0
~ 180
t 170
] 160
Ilo 150
!i
•
140
~ 130
t 120
~ ",..
110
i 100
.. 90
SIMULATED TEMPERATURE DRAW-DOWN
~
~ -.
-..;:::
r-:
--
o 4 8
-I--

332
The model described in this paper seems to be a useful tool for estimating the
range of possible behavior of a fracture system but its use is not limited to
forecasting: we also intend to use it for interpreting some of the measurements and
testing performed all through the experiment such as microseismicity induced by
the fluid injection or large-scale "channelling" of the flow.
10. ECONOMIC MODELING OF HDR(*)
Heat extraction from impermeable hot rock sections in some thousands
metres depth (HDR reservoirs) implies a complicated and expensive technological
procedure, where various site-specific geological and geophysical parameters as
well as criteria of technical installations determine the costs of the produced
energy (electricity or space heat).
For economic appraisals of industrial HDR-power plants a comprehensive
cost-benefit model was developed. The major components of the plant - a doublet
of deep boreholes, the stimulated HDR reservoir and the surface installations
including pumps for fluid circulation and the power station - are defined and
compiled in a structure diagram.
The costs for the construction and operation of the plant are determined and
considered in respect of the revenues received from the sale of the produced
electricity (or space heat). There is a mutual interaction between: the capital
investment for the equipment, the demands regarding temperature and heat
production from the geothermal resource during the production period, and sitespecific
natural conditions, like: geothermal gradient, in-situ stress field,
permeability and other properties of rocks.
A detailed description of the model is given by Kappelmeyer and Smolka (this
volume).
For the determination of the energy production cost the computer program
"HDREC" (HDR-Economic Cost evaluation program) was developed. "HDREC" is a
very versatile program and can be used as an excellent tool for sensitivity analyses.
In these analyses all computations of the model regarding the reservoir
performance the construction and operation costs of the plant and the decisive
financial parameters for the economic appraisal are executed.
(*) O. Kappelmeyer and K. Smolka: GTH Consult Gmbh

HDR - MODEL FOR COST EV ALUA nON
f'DIUIBlL IIV
lID( TOftMIIR INl IH-SltU ST1E51S
IJ&ETIOI RAIE
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(III _lIE C'fQ.E I
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T
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KIIff'ElJ£j(JI GoIIH 1liiY 1987 ....
....

334
REFERENCES
Site/Project
Hass 1.0., Hoffman C.R., 19Z6 - Temperature gradient in the Peche1bron oilbearing
region, lower Alsace : Its determination and relations to oil
reserves. Bull. Amer. Assoc. Petro. Geo1., xm, nOlO, pp. 1Z57-1Z73.
Gerard A., Menjoz A., Schwoerer P., 1984 - L'anomalie thermique de Soultz-sous­
Forets. Geothermie Actualites - N°3, pp. 35-4Z.
Gerard A., Kappe1meyer 0., 1987 - The Soultz-sous-Forets Project. Geothermics -
Vol. 16 n04 - pp. 393-399.
Gerard A., Kappe1meyer 0., 1988 - Le projet geothermique europeen de Soultzsous-Forets.
Situation au 1er janvier 1988. Geothermie Actualites,
Volume 5, n01, Mars 1988, pp. 19-Z7.
Kappe1meyer 0., Gerard A., 1987 - Production of heat from impervious hot
cristallin rock sections. Hot Dry Rock concept. Geologisches Jahrburck
n038.
Drilling
Herbrich B. - Forage geothermique de Soultz-sous-Forets. Rapports de fin de
sondage : rapport 88 CFG/03 - janvier 1988
Geology
Maget Ph., Walgenwitz F., Tietze R., 1979 - Synthese geothermique du fosse
Rhenan superieur - Rapport CCE/DGRST
Geochemistry
Vuataz F.D., Brach M., Criaud A. and Fouillac C. - Geochemical monitoring of
drilling fluids: a powerful tool to forecast and detect formation waters
submitted to Journal of Petroleum Technology.
Fritz P., Frape S.K., 1987 - Saline water and gases in crystallyne rocks. Geological
Association of Canada, Special paper 33, Z59 p.
Robinson B.A., 1985 - Non reactive and chemically reactive tracers: theory and
applications. Ph. D. dissertation, Massachusetts Institute of Technology.
Hydrogeothermometry
Michel W. and Haenel R., 1984 - Quantitative Bestimmung von Wasserinjektionen
und Extraktionen in Bohrungen mit Hilfe von Temperaturmessungen im
Hot Dry Rock-Projekt Urach, Teilprojekt Geothermik. Ber. Arch. NLfB,
95 8Z4, Hannover (Unpub1.)
Murphy H.D., 1977 - Fluid Injection Profiles - A modem analysis of wellbore
temperature surveys. Soc. Petrol. Eng. AIME, SPE 6783, 1-8
Schellschmidt R. and Haenel R., 1987 - Influx and water loss in open-hole sections
of boreholes determined by temperature measurements. Geologisches
Jahrburch, E. 39, 101-198, Hannover.
Seismology
Wohlenberg J., Keppler H., 1987 - Monitoring and interpretation of seismic
observations in hot dry rock geothermal energy systems - Geothermics
vol. 16, n04, pp. 441-445.
Green A.S.P., Baria R., 1987 - Active seismics to determine reservoir
characteristics of a hot dry rock geothermal system. Proceedings IZth
workshop geothermal reservoir' engineering - STANFORD University -
January 1987

33.5
TECHNICAL ANNEXE: HYDRAUUC FRACTURING STRESS MEASUREMENTS
THEORY AND PRACTICE
F. Rummel
Inatitut fOr Geophysik/Ruhr Universitlt Bochum
Mesy GmbH Bochum
1. HISTORY
In rock mechanics the term hydraulic fracturing is used for fluid injection
operations in sealed-off borehole intervals to induce and propagate tensile
fractures. It was first applied in 1947 in the Klepper n 0 1 borehole in the Hugoton
gas field/W-Kanau for gas production enhancement (Clark, 1949). Since then, the
technique is a standard procedure in oil and gas stimulation. In 1970 scientists of
the Los Alamos Scientific Laboratory suggested to use the method also to induce
large heat exchange surfaces in hot-dry-rock geothermal energy extraction systems
(Smith, 1970).
On the basis of the Hubbert and Willis (1957) statement, that a fracture in
the borehole wall will be initiated if the acting fluid pressure in the borehole
tensile rock strength, Scheidegger (1960, 196Z), Kehle (1964) and Fairhurst
(1964)suggested to use hydraulic fracturing as a stress measuring technique. After
detailed laboratory studies (Haimson, 1968) first in-situ hydrofrac stress
measurements were carried out by Schoenfeldt (1970) in northern Minnesota. In
Germany the technique was first used in a 30 m deep borehole near the seismoactive
Hohenzollern-Graben in 1973 (Rummel and Jung, 1975). These first
measurements led to the development of a wireline hydrofrac-stress-measuring
system at the Ruhr University Bochum (Rummel et al., 1981) which today is used
by numerous researchers in the U.S., Japan, France, Sweden etc ... The state of the
art of hydraulic fracturing for stress measuring was summarized in the 1981
Monterey international workshop (Zoback and Haimson, 1981).
New contributions towards the experimental procedure and the interpretation
of hydrofrac pressure data came from Comet (1981) suggesting to derive stresses
from stimulating preexisting fractures or joints, and from fracture mechanics (e.g.
Abou-Sayed - and Brechtel, 1978) considering the hydrofrac process as fracture
propagation rather than fracture generation within an ideal material.
Z. THE THEORY OF HYDROFRACTURING
Z.I. The Classical Approach
The classical treatment of hydraulic fracturing is based on Kirsch's (1898)
solution for the stress distribution aroWld a circular hole in a homogeneous,
isotropic, elastic material subjected to external far-field compressive stresses. It is
used in the Hubbert and Willis formula for the critical pressure at the moment of
fracture generation,
P =3~ -SH+P -P
c ~ ~ °
assuming the borehole is vertical, the vertical stress is a principal stress, and is
equal to the overburden stress, SH and Sh are the horizontal principal far-field
.tresses, the rock is homogeneous, isotropic and initially impermeable to the
fracturlni fluid and has a tensile .trength P coo and that the induced fracture is

336
oriented perpendicular to Sh' This last assumption yields the equilibrium equation
where Psi is the pressure to merely keep the fracture open after the pressurizing
system is shut-in (shut-in pressure). Po is the pore pressure in the rock mass and is
usually assumed to be equal to the hydrostatic pressure at depth z where the
fracture is induced. The azimuth of the fracture then is the orientation of SH' The
assumption that the stress concentration factors are k 1 = 3 and kZ = - 1 implies
that the rock behaves quasi-elastic. Then, the principal stresses can easily be
expressed as
which only requires to determine the rock mass density, the shut-in pressure Psi
and the fracture reopening pressure Pr = Pc - P co '
Z.Z. Opening of Existing Fractures
Rock formations at depth are characterized by the presence of pre-existing
fractures, generally joints with different orientations with respect to the
orientation of the acting principal stresses. By fluid injection into a sealed-off
borehole interval containing such a fracture, it will open as soon as the fluid
pressure exceeds the normal stress Sn acting across the fracture plane. Like in the
classical approach the equilibrium pressure to keep the fracture open can be
determined by system shut-in (Psi = Sn)'
The normal stress Sn acting across a fracture plane of given orientation is
related to the far field stresses by
S ='l.la .. (i=l,2,3)
n L lJ lJ
i,j
or
S = , l~a.
n L.. 1 1
where Ii' lj are the direction cosinus and 0i are principal stresses. Assuming that
the stress field varies linearly with depth and that the vertical stress Sv is a
principal stress leads to an equation of the form:
212
Sn = S u COS Q + 2 sin Q {[SHo + Sho + (SH + Sh)z] - [(SHo - Sho) + (SH - Sb)z] cos 2 (8 - 8')}
where a and a are the strike and dip angles of the particular fracture, S Ho and Sho
are the principal horizontal stresses at z = 0, SH and Sh are the horizontal stress
gradients with respect to z, and a' is the direction of SJr. The equation includes 5
unknowns and the solution therefore requires a minimum of 5 measurements of Sn
at various depth on fractures with different dip and strike. A more general solution
also allows that the vertical stress is not a principal stress and that the stress field

337
orientation varies with depth (Comet and Valette, 1984; Baumgll.rtner, 1987) •.
Although the method is attractive since shut-in pressure values measured are
generally rather reliable, the method can be improved. if one also uses the pressure
values at which the fractures start to open (P r ):
This could also allow to determine the pore pressure Po at depth z
.imultaneously.
Z.3. Fracture Mechanics Approach
Rocks like other materials contain pores and microcracks of various
dimensions. Therefore, when pressurizing a borehole during a hydraulic fracturing
operation the problem is to define the critical conditions for the growth of existing
cracks In the wall rock rather than predicting crack initiation. In fracture
mechanic. the stress situation for a crack is specified. by the stress intensity for a
crack tip. Crack Instability occurs when the stress intensity reaches a critical
value, the fracture toughness, which is a material property.
During the last two decades numerous fracture mechanics models have been
proposed to describe the process of hydraulic fracturing. However, a closed three
dimensional solution is not yet available. Here, I sketch a simple two-dimensional
analytical model which has shown to be useful in the interpretation of hydrofrac
work in clj'8talline rock with low permeability. The model is given in detail by
Rummel (1987) and is presently further developed (see KTB report Mesy, 1987).
In the model it is assumed. that the borehole axis is oriented. vertical, the
vertical stress is a principal stress and Sv = pgz. In the wall rock microcracks of
random lengths are distributed. at random orientations. With respect to the
horizontal far field stresses SH and Sh the most critical is a symmetrical double
crack extending radially from the borehole into the rock and oriented. perpendicular
to the direction of Sh (fig. 1).
Sh
111
-EJP - -w- -SH
-
t t t
---
t t t
Figure 1
A borehole with a symmetric double crack subjected. to the far-field stresses
SH &Del Sh &Del to fluid pressure P. Superposition concept for the derivation
of stresa intensity during hydrofracturing

338
When fluid pressure is applied to the borehole and fluid also penetrates into the
crack, the mode I stress intensity (tensile fracturing mode) in the vicinity of the tip
of this crack is given by superposition of stress intensity factor from the 4 load
sources Sh, SH, the fluid pressure P in the borehole and the fluid pressure
distribution P a within the crack of length a:
Using the general formulation of the stress intensity factor for a tension
crack of half-length a (Paris and Shi, 1965), the stress intensity factors for each
load source may be derived and superposition then leads to the following relation
for the critical borehole pressure at the moment of unstable crack extension:
where KIC is the mode I fracture toughness and ho' he' f and g are well-known
normalized stress intensity functions (e.g. Rummel, 1987).
Comparing this fracture mechanics hydrofrac relation with the classical fracrelation,
the term tensile strength P co and the stress concentration factors kl and
k? in the classical relation can be defined in the sense of fracture mechanics:
k =---
2 (h+h)
o e
The values of kl and kZ reduce to the values kl = 3 and kZ = -1 for zero crack
length as assumed in the classical approach.
For the specific case of a lithostatic stress field the frac equation is
P = P + kS
c co
f
defining the hydrofrac gradient with respect to
dP
S, dS c
Using Sr = Prgz (Pr rock density), the critical hydrofrac pressure required to
initiate unstable crack growth is given by
P = k'z + P
c· co
. dP
where k' = g (kp - p ) is the fmc fmdient with respect to depth, _c
r 0 dz

JJ9
1De relation allows to estimate pressures required for hydrofracturing at depth,
using only fracture mechanics data measured in laboratory experiments (Ktc, k,
Pce>. Taking typical values for crystalline rocks
the frac gradient is
g
k = 1.04, P r
= 2.65 -3
an
bar
k' = 0.172-
m
and the in-situ tensile strength to be expected in a 6 inch diameter borehole
(R • Scm) is P co .. 60 bar assuming
MN
K 1C = 1.7 -;:i
m
and til intrinsic crack length of some. millimeters (h .. 1). From this we might
estimate hydrofrac breakdown pressures of about 9Z0 bus at 5 km and a~ut
1780 bars at 10 km depth. These values are upper estimates. The existence of
larger cracks and the anisotropy of the stress field will reduce the pressure
subetantlally.
So far, the fracture mechanics approach considers only the instability of a
crack. It does not describe the dynamics of the crack growth or the crack extension
with time during the hydrofrac operation. This requires further to consider the
energy balance between the energy required for crack growth (surface energy,
energy losses in the form of heat and seismic radiation) and the energy available in
the pressurizing system as well as the energy input by the pumping system. It also
requires to speculate on the pressure loss at the crack inlet on the bore-hole wall,
on the pressure distribution and the fluid flow within the fracture, fluid losses into
the rock and on the fracture width as a function of crack lengthor operation time.
Various complex solutions are available and are being used in the oil and gas
stimulation industry dealing with massive hydraulic fracturing. These models are,
however, inappropriate for controlled micro-hydraulic fracturing as required for
stresse measurement (numerous tests per borehole, borehole stability, small
pumping rates, extremely small fracture width, generally extremely low rock
permeability, water as frac fluld, etc.).
Presently, we are attacking the problem and include fluid dynamics into the
fracture mechanics model described above. The model includes the following input
parametersz
- compressibility of the pressurizing fluld,
- stiffness of the pressurizing system,
- pressure lou at the fracture inlet,
- linear pressure distribution within the fracture, but variable with
increasing crack length,
- constant height and width of the fracture,
- fluid losses into the rock surrounding the borehole and the fracture plane.
A typical example for the fracture growth in granite as a result of
hydrofracturing by a wireline system is given in fig. Z. The input parameters ¥'8 as.
follows:
depths
borehole radius:
rockz
rock fracture toughness:
rock densltyt
rock permeabilit~
frac fluid:
1000 m
Scm
granite
1.7 MN/m3/ Z
Z.7 g/cm3
o IJDarcy
water

340
fluid viscosity:
1 cPoise
system stiffness:
1O-9Pa-l
pumping rate:
10 l/min
pressure loss at inlet: Z5 %
pressure distribution factor for fluid within crack, kZ: 0.01
fracture height:
1 m
fracture width:
0.1 mm
vertical stress Sy:
bar
horizontal stress SH = Sy:
bar
horizontal stress Sh = 0.5Sy:
bar
The result compares rather well with hydrofrac field results in granite
observed at various borehole locations. A more detailed description of the model is
given in the KTB Mesy report 1987.
.....,
0
0..
~
~
(.)
c..
E
::J
CIJ
CD
CI)
L-
c..
0
(.)
:OJ
·c
CJ
50
Cl)
z-1000m c1-0.7:5 e2-0.01 )'-0.:5 (PC"'11Sot-+ pc(b) L.
::J
- p1(b) (I)
G-e p1mean(b)
40 0 L.
c..
~
(.)
30 JO
C
L.
(.)
c:
c
Cl)
20 20 E
"0
c:
C
10 10 c
0 0
0 200 400 600 BOO 1000
normalized fracture length b
(I)
Cl)
~
Cl)
.....
.£:
.....,
0
CL
~
&.....I
.c
c
Cl)
E
--c..
"C
c:
c
--c..
Figure Z
Fracture mechanics determination of hydrofrac growth considering system
stiffness. Calculation for fracturing granite at 1000 m depth assuming
horizontal stresses SH = Sy,Sh = 0.5 Sy

341
3. EXPERIMENTAL HYDRO-FRACTURING STRESS MEASUREMENTS
Massive hydraulic fracturing in oU and gas stimulation projects is conducted
using injection rates of several m3 per minute and high viscosity frac fluids.
Hydrofracturing for stress determination is generally carried out using injection
rates of several liters per minute, uses water as frac fluid and the total injection
fluid volume is of the order of tens of liters. Also, the length of the sealed-off
borehole interval is small (of the order of 1 m). Generally, a double straddle packer
unit is used with inflatable rubber packers, and the unit is inserted to depth via
high pressure drlll-pipes which requires a drill rig onsite. The drill pipes also serve
as a hydraulic pressure line to both set the packers and to inject the frac fluid into
the fracturing interval. Stlll, most hydrofracturing stress measurements are
conducted be such a system.
Recently, wireline systems for hydrofracturing stress measurements are
being used (Rummel et al., 1983; Haimson and Lee, 1984; Baumg!rtner, 1987). The
wire line concept allows to take stress measurements simllar like conventional
geophysical data logging, i.e. fast and almost continuously without the presence of
a drlll-rig, and to obtain stress-log profiles. Originally a typical university
development, the present commercially designed system is capable to carry out
measurements to a depth of 1500 m at pumping rates of 10 liters per minute and
pressures up to 500 ban. A new system for 5000 m depth is presently under design.
A schematic view of such a system is shown in figure 3. At present, the strike and
dip of the induced fractures are observed via an impression packer tool including a
magnetic (or gyroscopic) orientation compass.
A typical pressure recording from a hydrofrac stress measuring operation in
crystalline rock is shown in figure 4. It demonstrates a pressure-pulse test into a
so-called -intact rock section- to measure permeabUlty, the formation break-down
and various phases of fracture propagation (refrac-phases). Typically for crystalline
rock shut-in pressures are not clearly identified by sharp breaks in the record. This
is due do the small pumping rate {,$ 10 l/min) and the high -formation permeability·
at high fluid pressures. However, the equilibrium pressure to compensate the
normal stress is clearly determined from the pressure record of a slow pumping
test (SP).
So far deep hydrofrac stress measurements have been conducted to a depth of
5 km, although only few measurements exist below a depth of 3 km. Existing deep
hydrofrac-stress data (~ 500 m) are summarized in figure 5 and figure 6 (Rummel
et al., 1986). The data suggest that the magnitude of the major horizontal stress SH
approaches the magnitude of the vertical stress (Sv), and the value of the minor
horizontal stress approaches a value of ~/Sv = 0.5.
Although the present data base is very limited particularly with respect to
depth, we may use it to speculate on mechanisms responsible for crustal tectonics.
One important conclusion could be that crustal block sliding or crustal seismicity
requires pore pressures higher than hydrostatic. Linear extrapolation of the
measured stresses to a depth of 10 km suggests shear stresses of the order of
1.5 kbar. This value is considerably smaller than expected from rock mechanics
friction experiments. Shear stresses should rapidly decrease at greater depth where
rock creep is the dominating deformation mechanism. Stress measurements iri
ultra-deep continental drlll holes may provide an opportunity to observe such a
crustal stress profile. However, this requires great efforts in the development of
stress measuring methods suitable for high pressures and high temperatures. Due to
its simplicity, hydrofracturing may be one of the techniques to be used in very deep
boreholes.

342
Figure 3
Wirellne hydrofrac concept for 6000 m deep boreholes to be developed
by Mesy GmbH Bochum
1- deto .cquilition
2- vinci! contral
3- pre •• un end flow rete control
4- pn •• un generator
5- vinci!
6- depth -.tar unit
7- cobb_
6- high pnuun hydraulic li.-.
ge 7 conductor bonholo coblo
10- hydraulic lino-cobl. cl __
11- cable head with pre •• un
trlnlducer
12- pnllMII'II reI.... vel VI
13- puoh pull valvo: pocker/injection
14- picker inflatable el.ent.1
15- injection interval with integr.ted
8HTV lor free verification
16- oriontot1on unit

343
I ~ p
~
II
~ I'
r
"So
I PI IFl IRF1I
~
I RF21
IRFJI
ISPI
1)0
....
.8. 50
(l.
IjI
o
5 10
15
t, min
20 25
Figure "
Typical hydrofrac record obtained in a 100 mm diameter borehole
at a depth of Z10 m in granite
I."'"
2
z.
I
I
• I
I
I
I
I
•
Figures 5/6
Horizontal principal.tresses versus depth measured by hydraulic
fracturing. Streues normalbed with respect to the overburden stress 5".
Data are taken from Rummel (1986)

344
4. LITERATURE
Abou-Sayed, A.S. and Brechtel, C.E.: In situ stress determination by hydraulic
fracturing: a fracture mechanics approach. JGR, 83, 2851-2862, 1978.
Baumgartner, J.: Anwendung des Hydraulic-Fracturing-Verfahrens fQr Spannungsmessungen
im geklUfeten Gebirge. Ber. Inst. Geophys. Ruhr-Universitllt
Bochum, Reihe A, Nr. 21, 1987.
Clark, J .B.: A hydraulic process for increasing the productivity of oil wells. Trans.
AIME, 186, 1, 1949.
Comet, F.H.: Analysis of injection tests for in-situ stress determination. Proc.
Workshop Hydr. Fract. Stress Measurements, Monterey 1981, U.S. Nat.
Comm. Rock Mech., Nat. Acad. Press, Washington, 1983.
Comet, F.H. and Valette, B: In situ stress determination from hydraulic injection
test data. JGR, 89, B 13, 11,527-537, 1984.
Fairhurst, C.: Measurements of in-situ· stresses with particular reference to
hydraulic fracturing. Rock Mech. Engin. Geol. 2, 3/4, 129 - 147, 19M.
Haimson, B.C.: Hydraulic fracturing in porous and nonporous rock and its potential
for determining in-situ stresses at great depth. PhD-thesis, Universitat
Minnesota, 1968.
Haimson, B.C. and Lee, Mao Y.: Development of a wireline hydrofracturing
technique and its use at a site of induced seismicity. 25th U.S. Rock
Mech. Syp., Proc. 194-203, AIME, 1984.
Hubbert, M.K. and Willis, D.G.: Mechanic of hydraulic fracturing. Trans. AIME,
210,153-158, 1957.
Kehle, R.O.: The determination of tectonic stresses through analysis of hydraulic
well fracturing. JGR, 69, 2, 259-273, 19M.
Kirsch, G.: Die Theorie der Elastizitat und die BedQrfnisse der Festigkeitslehe Z.
VDI, 42, 29, 797-807, 1898.
Mesy GmbH: Entwicklung eines Wireline Hydrofrac-Systems fQr
Spannungsmessungen in Bohhrungen bis 6000 m Tiefe. KTB-Bericht,
Bochum, 1987.
Paris, P .C. and Sib, G.C.: Stress analysis of cracks. In: ASTM Spec. Techn. Publ.
STP 381,30-83, 1965.
Rummel, F.: Stresses and tectonics of the upper continental crust, a review. Proc.
Int. Symp. Rock Stress, Stockholm, 177-186, 1986.
Rummel, F.: Fracture mechanics approach to hydraulic fracturing stress
measurements. I: Fracture Mech. of Rocks, 217-239, Acad. Press
London, 1987. .
- and Jung, R.: Hydraulic fracturing stress measurements near the Hohenzollern­
Graben-structure, SW Germany. Pageoph., 113,321-330, 1975.
- and Baumgartner, J. and H.J. Alheid: Hydraulic fracturing stress measurements
along. the eastern boundary of the SW-German block. Proc. Workshop
Hydr. Fract. :Stress. Measurements, Monterey 1981, 3-17, U.S. Nat.
Comm. Rock Mech., Nat. Acad. Press, Washington, 1983.
- and Mohring-Erdmann, G. and J. Baumgartner: Stress constraints and hydrofracturing
stress data for the continental crust. Pageoph. 124,4/5, 875-
895, 1986.
Scheidegger, A.E.: On the connection between tectonic stresses and well fracturing
data. Geofisica Pura et Applicata, 46,66-76, 1960.
Scheidegger, A.E.: Stresses in the earth's crust as determined from hydraulic
fracturing data. Geol. u. Bauwesen, 27,45-53, 1962.
Schoenfeldt, H.: An experimental study of open-hole hydraulic fracturing as a
stress measuring method with particular emphasis on field tests. PhD­
Thesis, Univ. Minnesota, 1970.
Smith, M.C.: Geothermal power. In: AlP Conf. Proc, N°19, 1974.
Zobeck, M.D. and B.C. Haimson, editors of: Hydraulic fracturing stress
measurements. Proc. Monterey 1981 Workshop. U.S. Nat. Comm. Rock
Mechanics, Nat. Acad. Press, Washington, 1983.

345
EEC contract nO EN3G-0052-D
ECONOMIC MODELING OF HDR
O. KAPPELME~R and K. SMOLKA
Geothermik Conault GmbH,
HinUberatr. 13 a, D-3ooo Hannover 1
Summary
Heat extraction from impermeable hot rock aections in some thousand
metres depth implies a complicated and expensive technological
system. (The acronym for the resource and the extraction procedure is
HDR, Hot Dry Rock).
The costs for the different components of a HDR-system are
determined. There is a close interaction between the cost for
necessary equipment, the demands regarding temperature and heat
production from the geothermal resource during the production period
(twenty years or more), and the site specific natural conditions,
such as geothermal gradient, natural stress field and other
properties. The computer program, HDREC, was developed for the
determination of energy production cost (heat/electricity) under
different conditions. Various algorithms describing the performance
of the stimulated HDR reservoir were integrated. The program serves
for an evaluation of the cost limits, sensitivity analyses (i.e. cost
at different depth) and a definition of research goals for an
improvement of the economic performance of HDR systems as well aa for
the exploration of suitable natursl conditions in the underground.
1. INTRODUCTION
In a cost benefit model the costs for the construction and
of an HDR-plant are considered with respect to the value for the
electricity (or space heat). The major components of the plant - a
of deep bore holes, the stimulated HDR reservoir and the
installations including pumps for water circulation and the power
are defined and compiled into a structure diagram which reveals the
intersctions between the various cost detsrmining factors (Fig. ,*).
operation
produced
doublet
surface
station
mutual
2. PARAMETERS AND CRITERIA
The site-specific natural input parameters: geothermal gradient,
rock permeability, joint/fissure transmissivities, in-situ stress field,
have to be obtained by measurements in a borehole. The technical criteria:
borehole depth, diameter and completion of the borehole as well as the
production rate and lifetime of the system are closely related to the
dsmands for acceptable flow impedance, tsmperature of the produced thermal
power of the system and finally the electric capacity of the electric power
station.
*) see page 333 (fig. 38)

346
3. RESERVOIR
The HDR reservoir is the central part of the model. It is cre.sted
artificially by massive hydraulic fracturing and consists of pre-existing
joints and newly generated fractures. This network of hydraulic paths
serves as a heat exchanger if a circulation is imposed. Its thermal
efficiency is determined by the rock temperature, the size and geometry of
the fracture network and the flowrate of the circulation. Fluid losses
during circulation can be high and have to be taken into account.
Algorithms which describe the above mentioned phenomena were integrated
into the model. These algorithms serve for a determination of the cost for
reservoir generation, the cost for the operation and, most important, the
performance of the reservoir regarding energy production and draw down of
energy production in the course of time. The flow and pressure potential
between inlet and outlet in the reservoir and the superposition of cooling
effects between adjacent fractures were considered (RODEMANN, 1979 ; HEUER,
1988).
4. POWER PLANT
The temperature of the fluid from a HDR reservoir is far from ideal
for heat/power conversion. Best efficiencies at relatively low temperatures
are obtained with ORC (Organic Rankine Cycle). Plants based upon ORC design
were considered for the power production (MILORA & TESTER, 1976).
5. FINANCIAL TREATMENT
The capital investment for :
- the boreholes is mostly determined by depth, which depends upon the
geothermal gradient and the envisaged reservoir temperature for the heat
production (GARNISH, 1987)
- reservoir stimulation is dependent upon the time and pump-power during
stimulation, which is influenced by the in-situ stress, the rock permeability
and transmissivities of existing fissures and, of course, by the
size of the envisaged reservoir ; it is estimated on the assumption of a
frac mechanic model for a penny-shaped fracture under constant pressure
and homogenous fluid losses due to permeation. .
- the power station is fixed by the electric capacity of an ORC-plant; a
HDR specific cost component is the investment and the operation cost for
circulation pumps, which depend upon the hydraulic impedance of injection
borehole, reservoir and production borehole. The experiences of earlier
investigations on the hydraulic behaviour of fractures in crystalline
rocks were adapted (JUNG, 19861. Due to the difference of densities
between the water in the injection- and production borehole, a buoyancy
pressure drive occurs which favours the fluid circulation and diminishes
operation cost for pumps.
The cost evaluation is based on the PRESENT and NET PRESENT VALUE
method, which are well established for economic appraisals of energy plants
(JAEGER, 1982). Their decisive financial parameters for an economic
appraisal are net present value of the total investment and levelised life
cycle energy costs. Additionally, cash flow diagrams of the specific energy
costs and the outstanding capital debt in each production year are
compiled. The time variation of the running costs and the revenues during
the production period due to a constinuous decrease of the energy flux from
the reservoir (draw down) are considered. Taxation of the income can also
be included.

347
6. COMPUTER PROGRAM "HDREC"
To execute the otherwise time-consumina numerical evaluation of the
economic model the proar8ID "HDREC" (HDR-Economic Cost evaluation progr8ID)
haa been develoDed. It is written in "FORTRAN" and can be executed on
"PC/AT"_icro computers under the operating system MS/DOS. Ita own fUe and
necessary supplementary files have a volume of about 2 MByte. Ita execution
requires about 400 kByte computing store main memory.
"HDREC" operates in conversational mode. It is composed of various
modules and is fully menu-driven. A robuat but very flexible code structure
renders a user-friendly operation. Each run of the program is uaer
specified. That is why the computations can be executed very efficiently in
a "batch-like" mode.
Input parameters in the program are :
variables in the algorithms of the model :
- site-specific geological and geophysical par8lDeters
- technicsl criteria for boreholes, reservoir stimulation and surfsce
installstions of the plsnt
- financial parameters and time dsta for the financial schedule
- cost data
various control par8lDeter specifYing the physical models
The result of the computations comprise :
the hydrsulic and thermal characteristics of the reservoir
- the electric capacity and produced electricity of the ORC-plant
- the investments of the individual components of the plant and sll
running costa during the production period
- the decisive financial parameters for the economic sppraisal.
"HDREC" is s very versatUe program. Nul tiple computation runs in one
program execution are possible. Furthermore it is an excellent tool for
sensitivity analyses. In these analyses one user specified input par8lDeter
is varied and all results mentioned above computed as a function of this
parameter.
7. EXAMPLES
The heat which is extracted from a HDR-reservoir is calculated within
the computer program for various aeometric confiaurations. Fia. 2 shows the
influence of spacina between fractures. Another critical par8lDeter of the
HDR-reservoir is the hydraulic impedance (Fia. 3). For an optimization of
an HDR-reservoir for specific natural conditions a sentivity analysis of
the cost for power production via depth is necessBry (Fia. 4). Cost of
power production durina the lifetime of a HDR-plant and the correspond ina
cash flow analyses srs show in Fia. 5 & 6.
REFERENCES
1. GARNISH, J. D. (1987). Introduction : Backaround to the Workshop. In
J. D. Granish (Ed.), Special Issu_Proceedinp of the First EEC/US
Workshop on Geotheraal Hot Dry Rock TechnoloiY. GeotheMlics, 16,
323-330.
2. HEUER, N. (1988). Wlrmeaustausch in eine. HDR-Modell .ehrersr Risse.
Internal Report at Geothermic Consult, Hannover, pp. 1-9.
3. JI{GER, P'., AMANNSBERGER, K., BERGMANN, G., GRIMM, B., KLAIB, H. ,
MELIS, M., R!IP'!NHJIUSER, I., and ZI!SING, H. J. (1982). Methoden zur
betriebswirtschaft1ichen Bewertuna rsaenerativer Eneraiequellen.
BSE-P'achtaauna, Bewertuna der Wirtschaftlichkeit reaenerativer Eneraien,
MUnchen, pp. 5-26.

348
4. JUNG, R. (1987). Propagation and Hydraulic Teating of a Large Unproped
Hydraulic. Fracture in Granite. In O. Kappelmeyer and F. Rummel (Ed.),
Terrestrial Heat from Impervious Rocks-Investigations in the Falkenberg
Ganite Massif. Geologisches Jahrbuch E 39, Hannover, pp. 37-65.
5. KAPPELMEYER, 0., and SMOLKA, K. (1988). Kosten-Nutzen-Modell einer
Anlage zur GroBtechnischen Gewinnung Terrestischer Warme aus dem HeiBen
Impermeablen Untergrund mit einer Anwendung auf die Bedingungen im
Oberrheingraben. In KFA-PBE Statusreport 1988 Geotechnik und
Lagerstatten, JUlich, pp. 343-354.
6. MILORA, S. L. and TESTER, J. W. (1976). Geothermal Heat as a Source of
Electric Power. The Massachusetts Institute of Technology Press,
Cambridge, Massachusetts.
7. RODEMANN, H. (1979). Modellrechnung zum Warmeaustausch in einem Frac.
Bericht Nieders. Landesamt f. Bodenforschung, Archiv-Nr. 81990,
Hannover.
150
,
.....................
I
I
I
~ 125
I (3) 15 ~
.s
n-'·
Fracture Spacing
I
.s
GI I
...
GI
.3 100 \ 3 ..... : 50 m
I 4 -: 10 10 ~
III
... I ,
5 -_.: 1 m p.,
GI
p..
6 -:0.1 m ii
S
GI \
E-o 75 5 GI
~
e
t:
50 0
0 5 10 15 20 25
Production Period in years
Figure 2: Thermal behaviour of a HDR reservoir
Flow rate 50 lIs; rock temperature 140°C; injection temperature 50 0 e
(1) single frac, area 5 kml, uniform flow, linear pressure potential
(2) - (6) ten parallel fracs each 0.5 kml
Distances.between fracs
(2) infinite
(3) 50 m
(4) 10 m
(5) 1 m
(6) 0.1 m

~tylDDm
0 100 1000 ~
~
! Prodw:tJoD Rate : '76 1/.
t 4000
o.e~ .r::::
!I •
0.1i~
13000
-:hact.ure
!I
~ (wttboI&1 0.4 B ~)
12000 -: RDB- S,..t.em 0.3 ~
;
.f1000
(w\ua~)
0 0
0 1 2 3 4
1raeture width In mm
o~ }
0.1
0.22
Reservoir Temperature in ·C
150 200 250
ProducUon Rate
'i"
100 Vs
~0.21 75 l/I
50 Vs
~
~ 0.20
.9
...
•
8 0.19 ...
>-
~
~ 0.18
.....
....
" " ..........
"
!iii
..............................
0,17
2 3 5 8 7
Deptn " m lm1
Fig. 3: Power demand for circulation of ·75 l/a
through injection borehole 3 ka depth, ~ 175 ..
production borehole 2.7 ka depth, • 196 mm
and HDR-reaervoir 5 ka2
(the cricical Reynold Number waa obtained for 8.5-
di .. etre in the cryatalline .ection below 1.4 ka in
the injection hole and 10 5/8- in the production
hole)
Wk- critical width at which frac extenlion pre.sure
i. reached with 90 1/. injection rate (20% lOla
rat.)
Buoyancy effect for injection temp. 50°C and
production temp. 155·C
Fig. 4: Senlitivity analyai. of power production
cost via depth for: production ratea 100; 75; 50 l/a
reservoir: 2 parallel fraca each 2 ka2, 2 am width
geothermal gradient: 8· C/I00 • down to 1.4 ka
3· C/loo • below 1.4 ka in
crystalline
interest rate: 5 %j production period 25 yeara
duration of plant design: exploration 2 y; drilling
2 y; .timulation 1 y;
powerplant 3 y

350
0.3
Cost of :
IIBII8 Power Station Maintenance
m HOR- Maintenance
PZJ Power Station Investment
om HOR- Investment
.E
1
o
>. 0.1
e'
G
oj .li 1J#.I..W.!L1JWJ.W,u.u.w.u..u.a.Ll1UllJ.4.l.W.U.J.lLlU.LUlw.}-LIJJU&+LLWW.w.ua.u.UJ,.I.IU.U..LU.fJI
o
5 10 15 20 25
TIme Blnce Begin of Energy Production in years
Fig. 5 Cost of power production during lifetime of plant ; increase of
cost due to drawdown of reservoir performance and an inflation
rate for running expenses of 0.5 % p. a.
50
::e 25
a
0
51 0
.E
~
.Q
-25
CD
0
C -50
~
Q.
0
0
-75
-100J I
0
Fig. 6
I I I 1-----,
5 10 15 20 25
lime since Begin of Energy Production in years
Cash flow analyses
Fig. 5 and 6 are based upon example in Fig. 4
75 lIs
5 \an depth and

3S1
EEC contract n° EN3G-00B1-D (B)
HYDROGP.OTHERMIC STUDIES eN HOT DRY ROCK TECHNOLOGY
R. SCHP.LLSCHMIDT and R. SCHULZ
Geological Survey of Lower Saxony
Summary
In a joint German-French HDR-project at Soultz sous
Forets the undisturbed temperature field was determined
in the borehole GPK 1 and in three oil wells in the
surroundings. The virgin rOCK temperature is about 140 ·C
at 2000 m depth. The measurements show that the temperature
gradient decreases from 100 mK/m to 30 mK/m at about
1100 m depth. The decreasing temperature gradient can be
explained by assuming a convective heat trans£er in the
Buntsandstein/MuschelKalK aquifer.
'
The results of a production test show that there is
only one inflow zone in borehole GPK 1 at 1812 m depth.
Three injection tests, using different injection flow
rates, were conducted in borehole GPK 1. A total number
of about 20 water accepti'ng joints could be detected by
temperature measurements. A second major outflow,at
1728 m depth was encountered besides the one at'lR12 m
depth. About 47 , of the injected water was lost at
1728 m depth and about 53 , at 1812 m depth.
The high resolution temperature measurements showed
definitely which joints are water accepting (partly dependent
on pressure) and which are not.
1. INTRODUCTION
Geothermal energy from hot steam and hot water reservoirs
has been used industrially in many countries worldwide for
several decades now. In the case of heat stored in deepseated
hot basement rOCK the development is different. It is the exploitation
of this energy potential that the Hot 'Dry ROCK
(HDR) concept has as its objective. This concept was developed
in the early 70s by scientists of the Los Alamos Scientific
Laboratory in the USA. Large fracture surfaces are induced artificially
in tight basement rOCK masses situated many Kilometres
below surface. Cold water is injected under pressure
into this joint system, the water is heated by the hot rOCK
masses, and returns to surface, either by a second production
well or through the injection well. The produced heat is mainly
intended for power generation. Large scale industrial use
of the HDR concept is SUbject to the question of economic viability,
in addition to the overcoming of technical problems.

352
Joint German-French project studies are planned to
investigate the Upper Rhine Rift Valley as source of economic
geothermal energy using HDR technology.
On 7 December 1987 geothermal well GPK 1 in the Upper
Rhine Rift Valley near Soultz sous Forets (France), reached a
total depth of 2000 m, with basement rock encountered at
1377 m. The well location is in the centre of the largest positive
heat flow density anomaly in Central Europe. In this
area the temperature gradients in sediment are at least
60 mK/m, at the well location (Soultzer Horst) values are in
excess of 100 mK/m.
2. TfiE ORIGINAL TEMPERATURE FIF.:LD IN BOREHOJ~ES OF THE SOULTZ
AREA
~g. 1 shows the undisturbed temperature as a function of
depth in borehole GPK 1 and in three recovered oil wells
(4598, 4609, 4616) in the surroundings (max. 350 m). The maximal
temperature is 104°C in well 4598 (at 838 m depth),
116°C in well 4609 (at 973 m depth), 116°C in well 4616 (at
1383 m depth) and 140°C in borehole GPK 1 (at 2000 m depth).
When the drilling operations were completed, in borehole
GPK 1, the original temperature field around the borehole was
disturbed. A stand-by time of 6 weeks was necessary for a
complete recovery of the original rock temperature. The only
exception is a small interval around 1812 m depth. This zone
was much more cooled down than other borehole sections, because
here a great amount of drilling mud was lost and entered
a fault zone. The temperature at this depth was still increasing
(see Fig. 1) in January 1988.
The recorded temperatures and temperature gradients of
borehole GPK 1 are given in Fig. 2 as a function of depth. At
about 1100 m depth a decreasing of the average temperature
gradient from 100 mK/m to 30 mK/m is clearly seen. The decreasing
temperature gradient can be explained by assuming a convective
heat transfer in the Buntsandstein/Muschelkalk aquifer
(947 - 1377 m).
The thermal conductivity of cores of the well GPK 1
(Musche1kalk, Buntsandstein granite) was measured in our laboratory
under original thermal conditions. The heat flow density,
the product of thermal conductivity and temperature gradient,
is more than 200 m\'1 m-2 in the t1uschelkalk and about
80 m\'1 m-2 in the granitic section. This determination validates
the assumption of a horizontal convective heat transfer
within the aquifers.
3. INFLUX MID WATER LOSS IN THE OPEN-HOLF.: SECTION OF THE BORE­
HOLE GPK 1 DETERMINED BY TF.:MPF:RATURE MF:ASURErmN't'S
The results of a production test show, that there is only
one inflow zone in borehole GPK 1 at 1812 m depth.
The aim of the injection tests was to determine the positions
of water carrying joints and to investigate the influence
of fluid pressure on the injectivity of the borehole (ratio
of injection flowrate to injection pressure). The injection experiments
were followed by a production test for studying the
chemical reaction (BRGM) of the injected fresh water in the
basement.

353
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
d. p tho '"
o
temperature. C
2~ 4~ 6~ 8~ 100 12111
~ ~
\\ ~~
\ \ ~ ~
1\ \
rig. 1 I
\ \
""
'"
~
\ 1\ I,~
\ \ ~ ~
\ \ ~ 4598
\ ~060 Krii 1 ,
:~04°C
0.033 Km 1 1\ \ ~ '\.. 4609
f-116 "C
\ \ '\
'\
\ r'\.
"
\ \ ~
\ '\ 123 4616 "C-~
\ \ \
1\ 1\ \
\ '\ \
\ \ )
\ 1\ \
Undisturbed te.perature as function of depth in
borehole GPK 1 and in three recovered oil vells
(4598, 4609, 4616) in the surroundings
\
\\
14
140"C
GPK1

100
2IIIl
3IIfII
4BIJ
5IlII
68Il
7BB
BBB
9BII
I BOO
IIBIJ
12BIJ
131111
1400
I SIll!
161l1l
171l1l
I BIll!
19IIIl
2
.........
2IlIlIl
depth ••
t.."...atur ••• C
4 6 B
'" "' '\.
"\
'" '"
""
'"
r--..
I 5 I 5
\
\
\
\
1\
\
\
t
'l
-:
.... ~.
~
~ F-
~
~
~
----.,
~
~
~
145 B
~
c:::::
C_
~
~
,j ~
.I
-
-E
7f:
~
~
\ +-
" • 211
5 B
.t .. at I graphy
PS
PI
CR
ZD
~E~
Ju
=Rh=
KP
===LTK===
MLK
GB
S
Fig. 2
Undisturbed rock teaperature and teaperature gradient as function of depth
(teaperature aeasureaent on January 19th, 1988)

355
The thermal measurements have been evaluated by the
"Flow method" and the "Shut-in method". For these experiments
it is necessary to measure the undisturbed rock temperature in
the borehole before the hydraulic stimulation. In addition,
two or three measurements during water injection are needed
when using the "Flow method" and two or three measurements
must be made during the shut-in interval. The evaluation method
has been described in detail by Michel and Haenel (1984):
it is based on a method by ~1urphy (1977). It was shown
in 1984 that thermal flowmeter measurements are comparable to
spinner flowmeter measurements regarding the resolving capacity
(Schellschmidt and Haenel, 1987). The thermal flowmeter is
advantageous if the water flow is small and the temperature is
high.
Three injection tests, using injection flow rates of
0.5 1 s-l, 1.5 1 s-l and 3.5 1 s-l, were conducted in borehole
GPK 1. A well head pressure of 35 bars was built up to inject
water with a flow rate of 1.5 1 s-l. A water loss of 20 % was
detected at 1735 m depth: but the major amount of water (about
73 %) was lost at 1812 m depth (Fig. 3). A second major outflow
was opened at 1728 m depth during the injection test with
a flow rate of 3.5 1 s-l and a well head pressure of 43 bars.
The water losses were about 47 % at 1728 m depth and about 53
% at 1812 m depth (Fig. 4).
Further more the results of the temperature measurements
immediately after shut-in prove that the joint at 1812 m depth
is connecte~ to a very large fault zone, unlike the
joint at 1728 m depth. This result is represented in Fig. 5
(depth interval 1700 m to 1825 m). Between 1704.4 m and
1733.8 m depth joints are observed which are not connected to
a fault zone. These joints opened at a wellhead pressure of
about 35 bars and after shut-in they closed. Because the
joints have no connection to a fault zone, the injected (cold)
water was flowing out of these joints after shut-in, running
down the borehole and entered the joint at 1812 m depth.
During water injection cold water had entered at different
depth into joints of the granite. These joints were much
more cooled down than the granite around the borehole. The
jointed borehole zones need more time for the temperature recovery
than the other borehole sections. By means of the
"Shut-in method" it is possible to determine the distribution
of water accepting joints: even very small water losses can be
detected. The temperature during shut-in as a function of
depth is given in Fig. 6 for the injection test with a flow
rate of 3.5 1 s-l. A total number of 20 outflows have been
detected, but no water accepting joint was observed below the
depth of 1812 m.
A comparison of the borehole logs proved that only the
high resolution temperature logs detected definitely Which
joints are water accepting and Which are not.

356
1450
o
liT
K
--,--
9 KIm
100 200 300 400 500
1500
'-------100 %--------=~ !!!!;'==--
1550
1tch5O
97%
1700
1750
93%
-----
1697
1735
1800
73%
1812
1850
1900
1950~-~--~----~--~--~----L---~--~----L---~
depth. m
rig. 3
The quotient JT/g as a function of depth
(injection test on June 24th, 1988)

357
.dT K
,--
9 KIm
1450 0 240 480 720 960 1200
1500
-
1550
1600 ~
1650
100 '" ~
--
1700
1750
1800
{
...
53 '"
~ !"""
~
~
~
~
~
~
~ - - -- - - - --
=-- -- - - - - - - - - - - - --- 1812
1850
1900 ~
1950
I I I
depth. m
rig- 4 z The quotient JT/g a. a function of depth
(injection te.t on June 28th, 1988)

358
17111111 12111 • 111
1704.4
1725
\'
) \
1709.4 :Sl
telllperatur. • o C
122.5
1720.6 ~ 1 ) ___ 1723.4
\ 1~1733'
\)
125.111 127.5
13111.111
175111
temperatur measurement before
shut-in while water is injec~ed
with a flow rate of 1.5 l/s
1775
18111111
1825
\
temperature
lIeasurement
during shut-in
depth ...
\
1812.5
-(
~
-
rig. 5
Temperature lIeasurementa before and during ahut-in for
the depth interval 1700 II to 1825 II (injection teat on
June 24th, 1988)

10S
1400
""

3W
4. REFERENCES
Michel, w. and R. Haenel, (1984). Quantitative Bestimmung von
Wasserinjektionen und Extraktionen in Bohrungen mit Hilfe
von Temperaturmessungen im Hot Dry Rock-Projekt Urach,
Teilprojekt Geothermik. aero Arch. NLfB, 95 824, Hannover
(Unpubl.).
Murphy, H.D. (1977). Fluid Injection Profiles - A modern analysis
of wellbore temperature surveys. Soc. Petrol. Eng.
AIME, SPE 6783, 1-8.
Schellschmidt, R. and R. Haenel, (1987). Influx and water loss
in open-hole sections of boreholes determined by temperature
measurements. Geologisches Jahrbuch, E 39, 101-108,
Hannover.

Granite_ater interactions in relation to hot dry rock geothermal
development
Behaviour of trace elements in water-rock interactions
Study of reactions between feldspathic rocks and heat exchange
fluids
Diesolution of feldspare: solid state chemlatry of hydrothermally
treated sanidine
Helium laotope systematics in crustal fluids from W.
snd adjacent areas
Germany
Experiments on reinjection of geothermal brines in the deep
Triassic landatones
Study of the variations in permeability and of fine particle
migrations in unconsolidated sanda tones submitted to saline
circulations
Variation in the permeability and cation exchange kinetica in
a clayey aandstone submitted to percolation of different
saline lolutions
Space and time evolution of the geochemical
from geothermal injection in an aquifer
processes arising
Teating geophya1cal exploration techniques on the laland of
M1101 (Greece)
Seilaic reconnaissance of the upper crust in the volcanic zone
of Olot (ME of Spain)
Microseisaic and seisaotectonic study of the island of Leabos
Atlas of geothermal resource I in the European Community.
Austria and Switzerland
Geothermal relources and relervel: updating of teaperature
data base

Exploration and evaluation of geothermal resources in the
Central Graben area. The Netherlands
Assessment of the low enthalpy geothermal resources of the Po
Valley Plain. Italy
Thermometry and hydrogeochemistry of the southern border of
the south Pyrenean foreland Basin
Hydrothermal activity related to recent explosive volcanism on
the island of Kos. Greece - An assessment of the geothermal
potential of the Volcania area
Study St Cugat geothermal resource in fracture granites to
heat greenhouses
Preliminary results from temperature. heat flow and heat
production studies in Ireland
An investigation of low enthalpy geothermal resources in
Ireland
Development of a two-phase flow turbine for geothermal
application
Design of two-phase flow lines for geothermal applications:
the slug flow regime

363
EEC contract no EN3G-OOS7-UK
GRANITE-WATER INTERACTIONS IN RELATION TO HOT DRY ROCK
GEOTHERMAL DEVELOPMENT
W.H. EDMUNDS', J.N. ANDREWS2, D.P.F. DARBYSHIRE3,
N. HUSSAIN2, D. SAVAGE' and T.J. SHEPHERD5
British Geological Survey, Wallingford, Oxon, OXl0 8BB
2. University of Bath, Bath, Avon, BA2 7AY
3 NERC Isotope Geology Centre, Grays Inn Road, London, WC1X 8NG
4 British Geological Survey, Keyworth, Nottinghamshire, NG12 SGG
5. British Geological Survey, Grays Inn Road, London, WC1X 8NG
Summary
Geochemical studies of fluids and rocks in the Carnmenellis granite are
providing the basis for understanding the nature of the Rosemanowes Hot
Dry Rock System. Fluid inclusion studies are helping to determine the
role played by saline fluids during previous hydrothermal (80-l70·C)
circulation. Strontium isotope ratios have been determined for a range
of rocks, single minerals, and fluids which help to constrain the
origins of solutes and the nature of mineral reactions. Comprehensive
radiochemical studies on the granite (uranium, radium, radon, and inert
gases) provide the framework for developing the use of radon and helium
to model properties (surface area, reservoir fluid volume and fracture
width) of the HDR reservoir. Experimental studies have been carried
out to investigate the reaction kinetics of principal rock forming
minerals as a basis for determining chemically reactive surface areas.
Possible heat exchange fluids for a 200·C HDR system have also been
evaluated using water-rock experiments. Fresh water flush experiments
in the HDR reservoir have provided a means of testing the overall
reactivity of the reservoir and together with the experimental studies
provide the basis for geochemical modelling using the EQ3NR/EQ6 Code.
1 . INTRODUCTION
The development and operation of a successful hot dry rock (HDR)
geothermal system will dep~nd upon an understanding of the water-rock
interactions taking place 1n the gran1t1c or equivalent hard rock
reservoir. Geochemistry is important at the exploration stage in providing
information on the nature of the granite and its contained fluids as well
as during drilling when important information may be gained from fluid
monitoring. During the evaluation stage it may be possible to use the
geochemistry to obtain information in the physical characteristics of the
reservoir (total and reactive surface area, rates of reaction, redox
processes etc). The long term exploitation and stability of the reservoir
then requires the application of the geochemical data to enable a working
model of the HDR system to be made. This paper summarises some of the
research in progress to determine the key geochemical characteristics of

3M
the Carnmenellis granite, Cornwall, and the fluids circulating naturally
within it. In addition, experimental studies are described which aim to
simulate reactions of the main rock forming minerals in the range 80-200·C
and to investigate their reaction kinetics using appropriate fluids with a
view to determining the chemically reactive surface areas. Most
importantly, the research addresses the problems of understanding the
Rosemanowes HDR reservoir during a sustained period of testing and
evaluation.
Hydrogeochemical research was designed partly to test the hydrogeochemical
model of the granite (Edmunds et al., 1984, 1985) and an
extended report on the results acquired under the first half of this
contract have been given in Edmunds et al. (1988). Detailed summaries of
the overall aims and achievements of the UK geothermal project are given in
Batchelor (1984)
repeated here.
and by Parker (1989) in this volume, and so are not
2. THE ORIGINS OF WATER AND SOLUTES IN THE GRANITE
Natural fluids occurring within granites provide a means for characterising
water-rock reactions that have taken place over time scales
ranging from months to many millions of years and over a wide temperature
range. Such reactions are natural analogues to short-term reactions taking
place over similar temperature ranges, resulting from circulation of a HDR
system. These, taken in conjunction with mineralogical and experimental
studies, constitute a basis for modelling the geochemical evolution of HDR
reservoirs.
It has been proposed in earlier phases of these investigations that
thermal brines (up to 31660 mgl- 1 TDS) which occur in the Carnmenellis
granite are of meteoric or1g1n and owe their salinity to water-rock
interactions with granitic rocks; they have evolved over timescales up to
1 Ma and possibly longer but have mixed also with shallow dilute meteoric
waters (Edmunds et al., 1984, 1985, 1987, 1988).
The thermal brines are linked to ENE-WSW trending mineralised veins and
NNW-SSE trending crosscourse faults and veins. The latter structures are
particularly well represented and the larger faults can be traced as
topographic features at the surface for 1-4 km, cutting both granite and
metasediments. Discharges of thermal water are greatest and most
continuous from crosscourses and hence it is important to understand the
origin of these fluids and their high salinities in relation to their
hydrogeological setting.
To address this problem a fluid inclusion investigation was carried out
of different mineralised crosscourse structures to determine the
temperature and composition of the palaeofluids which have utilised these
channelways. Thermometric data for inclusions in quartz and fluorite prove
conclusively that the fluids which circulateq through these fissures, on a
geological timescale, are typically low temperature (100-170·C), very
saline brines, containing 19-27 wt% dissolved salts. Moreover, secondary
inclusions with the same bulk salinity and temperature have been recognised
in all of the major granites in SW England (Rankin and Alderton, 1985).
Likewise it can be demonstrated that such fluids were intimately involved
in the higher temperature, magmatic-hydrothermal stage of polymetallic
mineralisation (Shepherd and Scrivener, 1987). Thus it would appear that
low temperature, high salinity fluids are an intrinsic feature of the
region and have been involved in many processes linked to the thermal
evolution of the granites.
The fluid inclusion thermometric data also provide a measure of the
NaCI/CaCI2 ratio. Figure 1 shows the calculated ratios for different

365
palaeohydrothermal fluids in the Carnmenellis area together with measured
values for the South Crofty Hine thermal waters. It can be seen that
present day and fossil fluids circulating within the crosscourse and
mineralised structures could be related via a simple dilution process
•••
ENE-WSW \
THERMAL WATERS • • •
CROSSCOURSE THERMAL WATERS
(c 40·e. 141 to c.4wt11o aa/ta)
x
CROSSCOURSE ORE FLUIDS
(11o-170·C. 19-27wt11o salta)
\ ENE-WSW VEIN ORE FLUIDS
A. __-'" (25O-300eC. 4-27wt11o salta)
NaCI
1:1
Figure 1. Composition of thermal waters and palaeofluids circulating along
ENE-WSW and NNW-SSE "crosscourse" structure, in granites, south-west
England
involving the mlxlng of an ancient saline groundwater with recent meteoric
waters. However, the CaC12 enrichment at the highest temperatures is
contrary to the predicted trend for fluids in equilibrium with Ca-Na
plagioclases (Dujon and Lagache, 1984) and may reflect the involvement of a
third and, as yet, unidentified fluid or significant modification by waterrock
reactions. Assuming a 10-fold dilution of palaeo-crosscourse fluids
(25 wt% NaCl-CaC12, 6180 +2 to +4 per mil, 6D -25 to +5 per mil (Harmon and
Alderton, in press; Shepherd and Hiller, unpubl. data» by meteoric waters
(8 180 -5.5 ± 0.5 per mil, 6D - 34 ± 5 per mil, Edmunds et al., 1987), the
resultant fluid composition is very close to that of the highest salinity
water from South Crofty (3.2 wt% dissolved salts, 6180 -4.6 per mil. 6D -29
per mil).
There is every possibility therefore that a deeper HDR system may
interact with older brines within the granite (in situ or drawn in) whose
chemistry and salinity are similar to those of palaeocrosscourse fluids.
Further studies of Rb-Sr and new studies of Nd-Sm systematics are in
progress to determine the origines) of the solutes as well as the clay
mineral alteration assemblages. The saline, thermal waters exhibit
anomalously radiogenic strontium isotope ratios (87Sr/86 Sr 0.728-0.733).
These have been attributed (Edmunds et al., 1986; Kay and Darbyshire, 1986)
to the hydrolysis of plagioclase because of the high susceptibility to
alteration and comparable 87Sr/86 Sr ratios between mineral and water. If
biotite were also being hydrolysed, its extremely radiogenic strontium
should have been incorporated into secondary minerals. The strontium
isotope results for thermal, saline groundwaters are compared in Figure 2
against the most up to date summary information for whole rock, single
mineral and secondary phases in the Carnmenellis granite and its envelope.
New data has been obtained for fluorites from three mines which fall in

366
the range 0.714-0.718 with the most radiogenic found in the Wheal Jane
mine. Clay mineral assemblages (Ca-montmorillonite/kaolinite, and
kaolinite/illite/quartz) from crosscourse structures in two mines have also
been analysed. The measured 87Sr/86 Sr values for the clays indicate
0·84
0·82
Elvan
D
country rocks
0·80
0·78
0·76
0·74
Do
Mylar
Elvan
Whole ro!::"
I
I
I
I
~=;r.K~-Feldspar
0·72 CoImry rocks
D
Whole rocks
D
Otz-Sericlte-Kaolinlte
Qtz-KF-Serlclte
Granite
Mylar
MINEWATER
SlderIt~~
MINEWATER ~Clays
•
Fluorite' ,
Qtz-Fluorite-Chlorite
Clays
-----
MINEWATER
Fluorite
0·70.1....---------------------------
CARNMENELlIS CARNMARTH PENDARVES S. CROFTY WHEAL JANE
Figure 2. Plot of 87Sr /86Sr for whole rocks, single minerals and mineral
assemblages from the Carnmenellis granite and mines compared with
compositions for minewaters.
that the clays are not in isotopic equilibrium with the most saline thermal
brines. For example, at South Crofty the saline waters are slightly more
radiogenic than the clays with values of 0.72764-0.73259 compared to
0.71962-0.72565. In addition, there is no consistent relationship between
the 87Sr/86 Sr values for the clays and those for plagioclase or bulk host
rocks. The evidence suggests that the clay minerals have been precipitated
from a fluid that has derived its strontium from more than one source or
that the clays are of different ages. The clays appear to be much older
than the presumed age of the thermal water.
The thermal water solutes do not bear a clear relationship therefore to
anyone mineral of the primary or secondary mineral assemblage of the
granite. It is implied, that the saline brines have been involved in
complex water-rock interactions to quite recent times and do not represent
a static hydrothermal fluid. The results of Nd-Sm studies on rocks,

367
minerals, waters and fluid inclusions should provide some additional
constraints on the ages of the alteration. In contrast to the strontium
isotope ratios 143Nd/144 Nd ratios will hardly have changed since
emplacement of the granite, the half life of 147Sm being substantially
greater than that of 87Rb.
3. RADIOELEHENTS AND NOBLE GASES IN THE HDR RESERVOIR
A comprehensive study of radioelements and noble gases is being carried
out for a range of objectives related to the understanding of the HDR
reservoir (Edmunds et al., 1988). These studies include measurement of the
three dimensional radioelement contents of the granite, uranium and radium
geochemistry, radon geochemistry and modelling, investigations of
radiogenic helium and argon, in situ neutron flux measurement and 36Cl
production.
The solution of both helium and radon by circulating fracture fluids is
of special significance in the reservoir studies, since their release is a
surface area dependent process which may be used to estimate quantitatively
the reservoir surface area (Andrews et al., 1986). The 222RD content of a
rock matrix attains a steady state value within a short time (a few half
lives of 222Rn) and hence the 222Rn flux from a fracture surface is
invariant over a long period of time. The 'He flux at a fracture surface,
in contrast, is determined by the initial amount of stored radiogenic 4He
and the duration of fluid circulation, since the latter transports He from
the rock matrix (Andrews and Hussain, 1989).
Estimates of the heat-transfer surface are important for geothermal
resource evaluation and are generally based on thermal-drawdown records. A
particular problem occurs where such records are insufficient as in the
case of newly-developed hot dry rock (HDR) geothermal doublets.
Hydrological models based upon flow and piezometric data alone cannot
unambiguously define the extent of fracture surface because the
hydrological parameters can be satisfied by various combinations of
fracture widths and lengths. The solution of 222Rn by the circulating
fluids can be used to further constrain such models.
Experimental measurements of 222Rn release into surrounding air and
water phases have been made for Carnmenellis granite specimens with well
defined surface areas. This 222Rn flux has been used in conjunction with a
simple hydrological model of the reservoir to calculate the 222RD content
of the return fluids of a geothermal doublet circulation system. For a
given production rate and piezometric difference between the injection and
production wells, the a22Rn content of the return fluid is dependent upon
the distribution of flow path lengths and fracture apertures in the
reservoir. Hatching of the calculated and experimental 222Rn contents of
the return fluids has been used to select appropriate parameters for the
reservoir model and hence to estimate the extent of the heat-transfer
surface. The model estimates the fracture width of the flow paths, total
lwept surface area and fracture volume within the reservoir.
The a22Rn model has been used to determine the changes in these
reservoir parameters for the various flow regimes that have been operated
lince September 1985. There was a gradual increase in both surface area
and fluid volume during the period of increased production from February
1986 to September 1987, together with a coincident fall in the fracture
width from about 550 pm to about 400 pm over the same period (Figure 3). A
maxu.w. reservoir sile was reached by September 1987, after which both
lurface area, volume and possibly fracture width declined following
inatallation of the downhole pump. The drawdoWD (-400 m) reduced the backpreasure
on the exit ends of the fracture paths and could explain the
observed changes. It is concluded that the best possible estimates of

368
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::::J
en
.. ..
..
0
550 ..
500
~-
-E
:2 ~ 450
..
:=- .. ..
400 II" ..
350
1985 1986
..
•
..
1987 1988
Figure 3. Reservoir parameters calculated from the radon tracer model for
the period September 1985-May 1986.

369
reservoir parameters confirm that the slow reservoir growth observed over a
two year period was probably reversed by the pumping activity. This work
continues and will be reported fully together with results on helium and
other parameters in mid-1989.
4. CHARACTERISATION OF THE CHEMICALLY REACTIVE SURFACE AREA
In a similar manner to radon the rate of dissolution
minerals/rocks is directly dependent upon the surface area
inversely proportional to the volume of contacting solution.
pH and temperature, and for a system far from equilibrium
reaction may be expressed as:-
Rate =A. k
V
of silicate
exposed and
At constant
the rate of
Where A = surface area of rock mineral, V = volume of fluid, and
k = mineral/rock dissolution rate constant. Consequently, this attribute
of water-rock reactions may be used to calculate the chemically reactive
surface area of a HDR system by coupling laboratory-derived rock
dissolution rates with the chemical compositions of injected and produced
fluids from the HDR reservoir, together with a simple hydraulic model.
_....
Figure 4. Summary of behaviour of chemical components analysed in out~ut
fluids from flow-through experiments at variable flow rate at 80 C,
reacting Carnmenellis Granite with the synthetic injection fluid. The
dashed lines indicate the concentration level of each component in input
fluids.

370
Laboratory experiments have been conducted using a flow-through
autoclave reacting drilling cuttings of Carnmenellis Granite of known
surface area with pH buffer solutions (pH = 7 and 8.3), and synthetic
versions of typical injection and production fluids from the Rosemanowes
test site, in the temperature range 60·-100·C at a pressure of 30 MFa and
linear fluid flow rates in the range 10- 5 -10- 6 msec-1. Reaction rate was
monitored by the chemical analysis of output fluids for major and trace
elements. The non-ideal fluid flow characteristics of the autoclave were
determined by the acquisition of fluid residence time distribution
information via inert tracer tests.
These experiments have revealed that the release of most chemical
components depends particularly upon temperature and flow rate, but also to
a certain extent upon time and fluid composition. The summarised behaviour
of a number of chemical components with time and flow rate in experiments
reacting Carnmenellis Granite with a synthetic streamwater (similar to the
Rosemanowes injection fluids) at 80·C and at variable flow rate is
illustrated in Figure 4. Magnesium is the only component to be lost from
input fluids at all times and flow rates, and probably reflects the rapid
growth of an aluminous smectite seen in batch experiments. Chemical
components which may be the most suitable for modelling the reactive
surface area of the present Rosemanowes reservoir are Si0 2 , Na, Li and F.
Preliminary attempts to calculate the chemically reactive surface area
of the present Rosemanowes reservoir using these techniques have estimated
a surface area of the same order of magnitude, or an order of magnitude
greater than those estimated by the radon model, depending upon whether the
laboratory determined fluxes to a surface area determined by gas adsorption
or estimated by geometric means.
5. EVALUATION OF POSSIBLE HEAT-EXCHANGE FLUIDS FOR A 200·C HDR GEOTHERMAL
SYSTEM IN GRANITE
An important consideration in the development and operation of a Hot
Dry Rock geothermal system is the selection of a heat transfer fluid and
the chemical composition of this fluid during circulation. The chemical
reaction of the circulation fluid with the reservoir rock may lead to the
undesirable corrosion or scaling of the reservoir itself, or associated
engineering structures. Two potential circulation fluids for use in a
possible high temperature HDR system in granite in SW England are a dilute
(TDS

371
appropriate to quartz solubility (300 mg/l) and high concentrations of
heavy metals such as Fe (15 mg/l), and Hn (2.5 mg/l). Hg was not totally
removed from solution as has been observed in basalt-seawater experiments
at the same temperature (Bischoff and Dickson, 1975). Solid precipitates
observed were anhydrite ( 0.21 g), a magnesium hydroxide ·sulphate hydrate
( 0.11 g), and a mixed layer smectite-chlorite clay ( 0.01 g), The
precipitation of HHSH and clay was instrumental in governing the low pH of
the fluid, which would pose problems in any HDR system through the
corrosion of borehole casing, pumps and heat exchangers. Although not
observed in the autoclave experiments, bacterially catalysed reduction of
SO. in the reservoir could produce severe sulphide corrosion of metal
structures.
The results of the autoclave experiments have shown that the use of a
local streamwater as a circulation fluid for a high temperature HDR system
would produce a relatively benign chemical environment, with the
precipitation of clay closely linked to the amount of Hg introduced in the
streamwater. The use of seawater as a circulation fluid could not be
contemplated without the removal of both Hg and S04 which are both reactive
components, although the high ionic strength of a modified seawater may
serve to inhibit mineral dissolution due to saturation effects·and may·help
to maintain pH conditions near neutral which would minimise the dissolution
of silicates and aluminosilicates.
6. TESTING AND HODELLING OF THE ROSEMANOWES RESERVOIR
A prolonged fresh water flush test of the HDR reservoir was performed
in December 1987 with subsequent tests in early 1988. The objectives were
to determine response curves for various chemical species as the reservoir
fluid was purged by dilute make-up (stream) water and to try to arrive at
a steady state composition for the reservoir fluid composition. The
results of this test are described elsewhere, but they provide an important
field scale experiment to contrast with experimental studies as well as the
evaluation of isotope and radiochemical studies (Parker, 1989; Richards et
al., in prep).
In order to be able to predict and interpret water-rock reactions in
geothermal reservoirs it is necessary to utilise computer models which
incorporate the kinetic and equilibrium features of such reactions. In
this instance, EQ3NR/EQ6 (Wolery, 1983, in press) has been applied to the
problem of understanding experimental water-rock interactions carried out
in relation to the fluid circulation tests (Falck and Savage, 1988).
The experiment to be simulated was the reaction of drill cuttings of
Carnmenellis Granite with a dilute stream water (similar to the circulation
fluid used at Rosemanowes) under closed system, 'batch' conditions at 80·C,
SO HPa at a 10:1 water-rock ratio. Details and results of the expe~iment
have been published elsewhere (Savage et al., 1987).
Results of the computer simulation revealed that it was possible to
model the experimental reaction path as defined by the changes in solute
concentration with time with reasonable accuracy. The main feature of both
experimental and computer simulation studies was the rapid reaction between
granite and streamwater such that steady-state concentrations of most major
and trace elements in the fluid phase were achieved within tens or hundreds
of days reaction time (e.g. Figure 5).

372
-
12
CI
E
14 ..
-AI calc. ..
• Mg meas.
...... 1:1 AI meas.
...... 10
-Mgcalc.
c ..
-
Kmeas .
8
0 .......-Kcalc.
as ..
6
... ..
-c
CD 4
u
c • 0
0 2 0
(J
•
[] []
0
.01 .1 1 10 100 1000
Time [d]
Figure 5. Concentrations of Mg, K, and Al in the fluid phase due to
reaction of Carnmenellis Granite with streamwater at 80·C as measured in an
autoclave experiment (unconnected points) and as predicted by EQ6
(connected points).
7. CONCLUSIONS
The geochemical investigations of the Carnmenellis granite, its fluids
and propert~es provide the context for an understanding of the Rosemanowes
HDR reservo1r. The applied significance of these results was summarised
earlier (Edmunds et al., 1988) in relation to wall rock and rock fabric
alteration, stability, production of secondary minerals, predictions of
porosity and permeability changes, fluid chemistry trends, estimation of
heat exchange area, reservoir volume and fracture width, geothermometry and
the development of models. An update on these aspects will appear in the
final report of the above contract.
8. ACKNOWLEDGMENTS
This work was also supported by the UK Department of Energy and is
published with the approval of the Energy Technology Support Unit on behalf
of D.En. This paper is published with the approval of the Director,
British Geological Survey (Natural Environment Research Council).
9. REFERENCES
Andrews, J.N., N. Hussain, A.S. Batchelor, and K. Kwakwa (1986). 222Rn
solution by the circulating fluid in a hot dry rock geothermal
reservoir. Appl. Geochem. 1, 647-658.
Andrews, J.N., and N. Hussain.
HDR geothermal reservoir.
1987. (in press).
Batchelor, A.S. (1984). Hot dry rock
United Kingdom. Mod. Geol., 9, 1-41.
(1989). Radium and helium solution in a
Pree. CEC Contractors Meeting, Toulouse,
geothermal
exploitation in the

373
Bischoff, J.L. and F.E. Dickson. (1975). Seawater-basalt interaction at
200·C and 500 bars: implications for origin of sea-floor heavy metal
deposits. Earth and Planet. Sci. Lett., 25: 385-397.
Dujon, S-C, and H. Lagache. (1984). Echanges entre plagioclase et
solutions aqueuses de chlorures sodi-calciques a differentes pressions
et temperatures (400-800·C, 1-3 kilobars). Bull. Hineral., 107,
553-569.
Edmunds, W.H., J.N. Andrews, W.G. Burgess, R.L.F. Kay and D.J. Lee.
(1984). The evolution of saline and thermal groundwaters in the
Carnmenellis Granite. Hineral. Hag. 48, 407-424.
Edmunds, W.H., R.L:F. Kay and R.A. HcCartney. (1985). Origin of saline
groundwaters ln the Carnmenellis Granite: natural processes and
reaction during hot dry rock reservoir circulation. Chem. Geol., 49,
287-301.
Edmunds, W.H., R.L.F. Kay, D.L. Hiles and J.H. Cook. (1987). The origin
of saline groundwaters in the Carnmenellis Granite, Cornwall (UK):
further evidence from minor and trace elements. In: P. Fritz and Frape
S.K. (Eds), Saline Waters and Gases in Crystalline Rocks. Geol. Assoc.
Canada Special Paper No 33, pp 127-143.
Edmunds, W.H., J.N. Andrews, A.V. Bromley, R.L.F. Kay, A. Hilodowski,
D. Savage, and L.J. Thomas. (1988). Granite-water interactions in
relation to hot dry rock geothermal development. Investigation of the
Geothermal Potential of the UK, British Geological Survey, Keyworth,
116 pp.
Falck, W.E., D. Savage. (1988).
interactions using EQ3NR/EQ6.
WE/88/44, 37 pp.
Computer simulation of granite-water
British Geological Survey Report
Harmon, R.S. and D.H.H. Alderton. (1989). Hydrogen and oxygen isotope
character of hydrothermal fluids associated with granites and
mineralization in southwest England. Appl. Geochem., (in press).
Kay, R.L.F., and D.P.F. Darbyshire. (1986). A strontium isotope study of
groundwater-rock interaction in the Carnmenellis Granite. Proc. 5th
=I~n~t~e~rn~a~t~.~~S~ym~p~. __-=W=a~t~e~r-~R~o~c~k~~I~n~t~e~r~a~c~t~i~on. Reykjavik, Iceland,
pp 329-332.
Parker, R.H. (1989). Characterisation of the Rosemanowes HDR geothermal
reservoir using an extended circulation programme. (This volume).
Rankin, A.H., and D.H.H. Alderton. (1985). Fluids in granites from
southwest England. In: ~H~ig~h~~H~e~a~t~~P~r~o~d~u~c~t~i~o~n~~(HH~P~)L-~G~r~a~n~i~t~e~s~,
=HLyd~r~o~t~h~e~rm~a~I~C~i~r~c~u~l:a~t~io~n~~a~n~d~O~r~e~~Ge~n~e=s~i~s. Inst. Hining Hetall. Symp.
Proc., 287-299.
Savage, D. H.R. Cave, A.E. Hilowdowski, and I. George. (1987).
Hydrothermal alteration of granite by meteoric fluid: an example from
the Carnmenellis Granite, United Kingdom. Contrib. Hineral. and
Petrol., 96, 391-405.
Shepherd, T.J. and R.C. Scrivener. (1987). Role of basinal brines in the
genesis of polymetallic vein deposits, Kit Hill-Gunislake area, SW
England. Proc. Ussher Soc., 6, 491-497.

374
Wolery, T.J. (1983). EQ3NR, a computer program for geochemical aqueous
speciation-solubility calculations: user's guide and documentation.
Lawrence Livermore National Laboratory Report UCRL-53414.
Wolery, T.J. (in press). EQ6, a computer code for reaction-path modelling
of aqueous geochemical systems: user's guide and documentation.
Lawrence Livermore National Laboratory Report.

375
EEC contract n e
EN3G-OOIO-F(CD)
BEHAVIOUR OF TRACE ELEMENTS IN WATER-ROCK INTERACTIONS
G. "ICRARD, R. PAUWELS et P. ZUDDAS, Laboratoire de Geocbimie des Eaux,
Universit' Paris 7
S. DUJON et M. LAGACRE, Laboratoire de Geologie, ENS
Summary
Theoretical and experimental studies of the dissolution of potassium
feldspar crystals doped with rubidium and calcium feldspars doped
with strontium show that the process follows the following steps:
1. formation of a mineral buffer that controls the major
elemants;
2. baginning of stoichiometric dissolution for the
major element (K or Ca) - trace element (Rb or Sr) combination;
3. fractionation witb enrichment of the solution, particularly in
the case of Rb, linked witb a precipitation of new solid phases;
4. obtaining of a steady state.
The study enabled estimation of the Rb/K partition coefficient
between adulaire and liquid phase (D - 0,28) and Sr/Ca between
prehnite and solution (D - 1).
INTRODUCTION
Geothermal waters from granitic and basaltic reservoirs have a
chemical composition that is related to the equilibrium between the
aqueous aolution and the minerals reSUlting from the transformation of
original granitic minerals or unaltared minerals such as quartz or
possibly potassium feldspars. For basalts, the same or similar minerals
are reformed (Browne, 1978; Kristmannsdottir, 1975).
The water composition ia therefore baSically fixed by the
temperature and the concentration of "mobile" ele.ents in tbe solution
(Hichard and Fouillac, 1980; Amorsson a.o., 1983; "icbard, 1987).
Trace ele.ents sucb as Li, Rb, Sr, Co, Hn are not controlled by an
equilibrium with a mineral phase but by tbe distribution process betvaen
.ineral and solution. The concentrations in solution depend therefore on
the concentrations in tbe original and secondary minerals and it is
surprising that they can play a role in cbemical geotbermometry (as for
example Na/Li, Fouillac and Micbard, 1979, 1981).
Consequently, we proposed a theoretical and experimental study of the
behaviour of trace ele.ents in the reactions between mineral and
hydrothermal fluid. The step fro. mineral to whole rock bas been reserved
for a later study.

376
1. THEORETICAL STUDY
While the co-precipitation of a trace element in a mineral with an
associated major ion from an aqueous solution has long been studied
(Berthelot. 1872; Doerner and Hoskins. 1925). the dissolution of a solid
solution has only been studied by geochemists since 1977. The concept of
stoichiometric saturation (Thorstenson and Plummer. 1977). discussed by
numerous scientists (Garrels and Wollast. 1978; Dandurand and Schott.
1980; Murphy and Smith. 1988). has been applied with success in the
formulation of a model for the dissolution of solid solutions (Denis.
1982; Denis and Michard. 1983; Michard. 1986). The results can be
presented in a diagram of (trace element) as a function of (major element)
(Fig. 1). where the parentheses represent the activities of the ions in
aqueous solution (Denis and Michard. 1983; Garrels and Wollast. 1978). or
in the Lippmann diagram (1982) presented by Gleynn et al (1989).
The initial dissolution is stoichio~etric. which mea~s that there is
no fractionation between trace and major elements (straight line OD.
Fig. 1). Fractionation is first observed after point p. the intersection
of OD with the curve of total saturation (Denis. 1982) that Lippmann
proposed to name "solutus". It was deduced from this that the
fractionation occurred by precipitation of a solid solution enriched in
the least soluble element. The aqueous solution is enriched in the more
soluble element and the point that represents this in the Denis diagram
moves into the triangle PDE (fig. 1). The review of the experiments of
Denis (1982) by Michard (1986) indicated that this precipitation did not in
general occur under equilibrium conditions. that point E was not
reached and that the arrest observed in the evolution of the composition
of the solution corresponded with a stationary state (point S) where the
composition of the newly formed solid solution was the same as that of the
original solid.
This known deviation of the equilibrium for co-precipitation
reactions (Riehl et al •• 1960; Lorens. 1981) has been confirmed by Galinier
(1988) who proposed among other things a method of determining the
partition coefficient for the equilibrium. Instead of dissolving the solid
solution in pure water. it was dissolved in a solution containing the most
soluble ion (for example Sr for the study of (Ba.Sr)SO ) in such a way
that the reaction stops without the occurrence of fractionation.
The discussion method. introduced by Denis and used by Galinier is
only valid under restricted conditions: no other ion than the constituents
of the solid solution should be present in the aqueous solution and no ion
present should possess acid-base or redox properties or form complexes.
It was necessary to extend the discussion in order to try to use
these results in the study of alumino-silicate minerals present in
hydrothermal mineral complexes ~a granitic and basaltic terrains.
Pauwels (1988) has shown that the Denis diagram could easily be
transformed if the mineral solid solution were part of a complete mineral
association (Michard. 1987). This condition. which holds in natural
geothermal systems. must also be respected under laboratory experiments.
In a plot of

377
the .olutus will be represented by a straight line with the following
equation (fig. 2):
(T~·'
(H+)'l
whereas the stoichiometric saturation curve is a single term function:
= K.lf (2)
where ~ • Kr represent the reaction constants of the mineralogical
buffer ofYthe major element and those of the trace element.
It often happens. however. that the newly formed minerals are not
identical to the original minerals but differ either in chemical formula
or crystal structure. As an example of the former, the couple
calcium-strontium, initially present in the plagioclase, is incorporated
in minerals such as laumontite «CaSr)AI Si& 2
+ ° 12
+ H 2
0) or prenite
«Ca.Sr)2AI2Si3010(OH)2). The potassium feldspar (K,R6)AlSi 3
0 S
occurs as
microcline or orffiose in granites but in the form of adulaire in
hydrothermal deposits. The difference in stability between the initial
mineral and the newly formed mineral enlarges the zone where
stoichiometric dissolution reactions of the initial mineral and the
co-precipitation of the secondary mineral are possible (fig. 3). The state
of real equilibrium is the same as before but, in the case of
co-precipitation outside the equilibrium zone with a kinetic separation
coefficient. the influence of the relative dissolution and precipitation
rate. on the composition of the solution in the steady state is much more
evident.
2. EXPERIMENTAL STUDY (Pauwels, 1988; Zuddas, 1988)
2.1. Preparation of .tarting material
The solids are synthetic feldspars. They were synthesised from a gel
with the ~omposition and crystallised under hydrothermal conditions
in a gold tube placed for 10 days in an autoclave at 600°C. 1000 bar.
With this method were prepared:
- a .erie. of sanidines containing 1-7 atom percent rubidium
- an anorthite with 0.5% strontium
- a labradorite (AbSOAn SO
) with 0.5% strontium.
The transformation reaction of Ca feldspar to laumontite uses quartz •
• 0 quartz wa. added to the anorthite and labradorite.
The solutions: the composition wa. calculated with the help of the
programme EQUASS. detail. of which have been described in Michard (1987),
to be in equilibrium with the minerale:

378
- quartz. kaolinite and adularite for experiments on potassium
feldspar dissolution;
- quartz. kaolinite. laumontite for anorthite dissolutions;
- quartz. kaolinite. albite. laumontite for the dissolution of
labradorite.
The first generation of starting solutions did not contain trace
elements. For the first experimental series_~ith potassi~ feldspars.
various quantities of rubidium (from 1.35x10 to 7.26x10 moles/I) were
added to the original solutions.
Temperature - water/mineral ratio
All experiments were performed at the same temperature (180 0 e) and at
the same water-mineral ratio (- 400). The temperature of 180 0 e allows the
observation of significant transformation in a timespan of less than a
year. which is not true for temperatures below 150 o e. At higher
temperatures. parasitic reactions such as the formation of muscovite
occur. selected dissolution is observed (Pauwels. 198B) and the
temperature regime is no longer representative of the geothermal systems
that we want to interpret. A high water-mineral ratio was chosen for this
first stage in order to increase the chances of observing solid
transformations. which are much more difficult to observe than changes in
the solution.
2.2. Results
Tables I. 2. 3 and 4 present some examples of results obtained during
interactions varying from 3 days up to I year.
TABLE I: Results in moles/kg for the dissolution of a sanidine with
1.5% rubidium
Time
Days
K+
10-3
Rb+
10-6
1
2
3
5
7
10
14
20
24
30
36
41
45
50
54
61
85
178
280
343
0.57
1.20
0.94
0.95
1.00
0.99
1.15
1.14
1.21
1.26
1.15
1.29
1.49
1.21
1.21
1.18
1.03
1.26
1.19
1.32
0.0
7.9
5.3
7.3
7.2
12.9
17.2
17.0
27.4
22.4
17.3
18.9
27.7
22.5
20.5
22.3
26.0
32.3
32.4
38.4
0.22 2.90
4.20 3.42
3.50 2.97
1.70 3.13
2.60 3.22
2.00 3.15
1.70 3.34
1.30 3.11
1.40 2.45
1.10 3.54 .
1.50 2.92
0.72 3.03
0.29 3.29
0.50 2.86
0.27 3.20
0.42 2.96
0.23 3.13
0.18 2.78
0.08 3.23
0.07 3.81

379
TABLE 2: Results in molee/kg for the dissolution of labradorite with
0,5% of strontiua
Time
Days
Ca2+
10-·
Na+
10-3
Sr2 +
10-6
Al(DHT. H.SiD.
10-5 10-3
o
7
15
21
28
37
44
65
81
130
0.26
2.10
5.20
5.80
8.74
2.29
1.82
1.58
3.12
6.22
4.50
5.08
6.00
6.64
6.00
5.40
6.64
7.40
6.88
9.28
0.-
2.05
3.27
3.50
7.95
3.44
0.92
1.65
2.22
3.65
4.82
1.05
0.70
0.14
0.30
2.10
2.75
2.10
1.81
1.40
2.95
3.05
4.60
4.70
4.88
3.30
3.44
3.35
4.06
4.05
TABLE 3: Hinerels detected with X-ray diffraction during the experiment of
the dissolution of anorthite
Time
Days
MineraLs
53
81
122
Prehnite + Kaolinite + Anorthite + Quartz
Prehnite + Kaolinite + Anorthite + Quartz
Prehnite + Kaolinite + Montmorillonite + Anorthite + Quartz
TABLE 4: Hinerals detected with X-ray diffraction during the experiment of
labradorite dissolution
Time
Days
81
130
MineraLs
._---------------------
Prehnite + Kaolinite + Albite + Labrador + Quartz
Prehnite + Kaolinite + Albite + Montmorillonite
+Tacharanite + Wairakite + Labrador + Quartz

380
2.3. Interpretation
2.3.1. Evidence of a stoichiometric dissolution step
The first stages of the dissolution of various feldspars correspond
always with a congruent dissolution; this means that the ratio of trace
elements and major elements passing into solution is equal to that in the
initial solid.
2.3.2. Formation of a mineralogical buffer
The presence of quartz, kaolinite in the alteration products of the
initial components, adularite (for potassium feldspars), prehnite (for
calcium feldspars) and albite (for plagioclase), as well as the
maintenance (after 1 month of reaction) of constant values for silica and
aluminium dissolved in the aqueous solution, show that after a relatively
short time water-mineral reactions take place in a mineralogical buffer
for potassium, calcium and the couple calcium-sodium (respectively for the
experiments with sanidine, anorthite and labradorite).
2.3.3. Observation of the fractionation step
This observation is particularly clear for (Rb,K)AlSi 3
0 R
• A clear
relative increase of rubidium in solution is observed in all tne
experiments after an exposure time of 10-15 days. A slight shift is
observed in the w.idth of the peaks in the IR-spectrum of the feldspar
which indicates the formation of adularite. This mineral incorporates only
a small fraction of the rubidium, which thus increases in solution.
No important fractionation between Ca and Sr has been observed in
calcium feldspars, though significant precipitation of prehnite can be
seen in the solution as well as in the solid after about one month of
interaction.
2.3.4. Attaining a steady state
In all experiments the concentration of the elements in solution
changed very little or not at all after 3 months of interaction. The small
increases that are observed in alkaline and alkaline earth elements arise
from an increase in chloride due to an evaporation of the solution (teflon
has a certain permeability to water) and to the liberation of fluoride
ions from the teflon.
Observations on the solids show an increase in the proportion of
new phases between 3 and 7 months. The stability of the composition of the
aqueous solution can be explained by a steady state where the
contributions from dissolution are counterbalanced by precipitation,
rather than by termination of the reactions.
2.3.5. Estimation of the distribution coefficient for Rb/K between
solution and adularite
Following the method developed by Galinier (1988), the
equilibration point (GE in fig. 1) can be obtained directly by adding to
the starting solution a certain amount of the most soluble element. The
amount of this element to be added (Rb in this case) is not known in
advance but must be determined in a series of tests. If fractionation in
the solution in favour of Rb is observed, then too little Rb was added; if
enrichment in K occurs, too .much Rb was added.

381
The optimal Rb concentration can thus be bracketed and the partition
coefficient
can be determined.
D -
The ratio (Rb/K) of the newly formed mineral is the same as in the
initial mineral since the solution is in a steady state. A value of
D - 0.28 was found in these experiments at 180°C. As the partition
coefficient for (Sr/Ca) is close to one, significant fractionation was not
observed in these experiments.
2.3.6. Limits in the comparison theory-experiment
A complete comparison of the experimental results and the
theoretical approach has not been possible fo~ the following reasons:
- It is difficult to measure the pH in badly buffered solutions; the
data can be affected by cooling, CO influence or contamination with
2
chloride and fluoride liberated from the teflon container.
- The thermodynamic data for pure rubidium adularite and strontium
prehnite are not known. In natural systems these extreme compositions
do not exist; there are only potassium adularites with traces of Rb
and calcium prehnites with traces of Sr. We are more interested in
the partition coefficient than in the thermodynamic properties of the
pure extremes.
CONCLUSIONS
Notwithstanding the limitations already mentioned, the following
results were obtained:
I) The dissolution of a potassium or calcium feldspar (in the
presence of quartz in the latter case) leads to the formation of a mineral
buffer that fixes the concentration of the major elements in the solution
at a set temperature and chlorine concentration.
2) The initial dissolution of a feldspar containing trace elements
follows a stoichiometric mode. Trace-major element fractionation is then
observed before a steady state is reached.
3) The secondary mineral differs from the original mineral, either in
its chemical formula (Ca) or in its structure (K).
4) An estimation of the partition coefficient between secondary
mineral and solution can be obtained.
5) One can therefore estimate the level of trace elements in solution
on the basis of the knowledge of the concentration of trace elements in
the original solution.
These first experiments should be followed-up, first at other
temperatures and for other water-mineral ratios and then with whole rock
rather than with single minerals. It may perhaps then be possible to
reach a situation where the numerous analyses of trace elements in
8eothermal solutions can be interpreted.

382
REFERENCES
ARNORSSON S., GUNLAUGSSON ~; and SVAVARSSON A. (1983) Geochim. Cosmochim.
Acta, 47, 547.
BERTHELOT M. (1872) Ann. Chim. Phys., 26, 4 08.
BROWN)!: P.R.L. (1978) Ann. Rev. ~lIrth Planel Sci., 6, 229.
DANDURANDJ.L. etSCHOTT.J. (1980) Bull. Mineral, 103,307.
DENISJ. (1982) These 3eme cycle. Universite Paris VII.
DENISJ. etMICllARD G; (1983) Bull. Mineral, 106,309.
DOERNER H.A.llnd HOSKINS W.M. (1925) J. Am. Chem. Soc., 47, 662.
FOU ILLAC C. et MICHARD G. (1979) C.R. Acad. Sci Paris.
FOUILLAC C. etMICHARDG. (1981) Geothermics, 10,55.
GALINIER C. (1988) These. Universit.e de Toulouse.
GARRELS R.M. and WOLLAST R. (1978) Am. J. Sci, 278,1469.
GLYNN P., R~ARDON ~., PLUMMER L.N. and BUSENBERG E; (1989) Geochim. Cosmochim.
Acta (soumis).
KRISTMANNSDOTTIR II. (1975) Proc 2nd U.N. Symp. Development lind Use oC Geothermal
Resources, San (o'rllncisco, vol. 1.
LAFON G.M. (1978) Am. J. Sci, 278,1455.
LIPPMANN (0'. (1982) Bull. Minerill., 105,273.
LORENS R.B. (1981) Gooch. Cosmochim. Acta, 45,553.
MICHARD G. (1986) Bull. Minerill., 109,239.
MICHARD G. (1987) in ChemiCilI TransCert in Metasmatic Processes, (H.C. Helgem ed.l, ASI
8erie nOC 218, 323.
MICHARD G. et l

Figure 1: Dissolution of a solid solution AB1 ~ in pure
water represented in the diagram (C+)-f(B+) -x
Points D: stoichiometric saturation
E: equilibrium
5: steady state
P: see text
Curves S.T.: total saturation
5.5.: stoechiometric saturation
S.D.: stoechiometric dissolution
Figure 2: Dissolution of a solid solution AM1 T in
a mineralogical buffer in a diagram -x x
(TZ+)/(H+)Z_f(~+)/(H+)Z
The precipitating mineral is the original mineral

Kr------------------------------------------,
H'
1()+'
"r:--
~-===============~~S~.S~.------------~
, ----- ---------------
II -~------------------ ... --_4
E
S.T.
\
4
__ k,/Iu:30
____ k,/1u:3
_______ k,/k.:O,34
+
+
000
1000
100 200 lOOjoun
Figure 3: Dissolution of a solid solution AMl-xTx in
a ~ineralogical buffer in a diagram
(T )/H+)Z=f(MZ+)/(H+)Z
The precipitating mineral is more stable than the
original mineral
Figure 4: Evolution of the ratio Rb/K in the solution
as a function of time for different initial Rb concentrations.
The ratios are normalised on the basis of
the ratio in the original mineral RbxKx-1AISi 3 0 S
*X = 0,07 (Rb)o = 0
A X = 0,015 (Rb)o = 0
X X = 0,01 (Rb)o =
*x = 0,01 (Rb)o =
1,35 X 10-5
7,26 X 10-5
• X = 0,051 (Rb)o = 8,4 X 10-5

385
~ contract nO EN3G-0076-D(B)
S'ruDY OF REACTIOOS BE'lWEEN FELDSPATHIC ROCKS AND HEAT EX~E FLUIDS
SlIJIII2I ry
E. Althaus and E. Tirtadinata
Mineralogisches Institut, Universitiit Karlsruhe,
Federal Republic of Germany
The aim of our investigations is to prove the idea that during the
initial phase of the reacticn between feldspirs aoo aqueous fluids an
exchange between allcali aoo alkaline-earth (Btions with hydronium ions
takes place buildill3 a hydronium feldspir at the very outer layers of
the mineral. The oompositions of the reaction fluids were measured by
atomic absorption spectrometry (AAS) and the investigations on the
solid samples were carried out by infra-red (IR)-spectrometry, x-ray
diffractometry, x-ray Guinier camera and secondary ion mass spectrometry
(SIMS) as well. That substitution of cations by 0 3
0+ ions had
occurred indeed was suggested by some particular features of x-ray
diffractograms. Clear and definite evidence was obtained from depth
profilill3 of the treated feldspar samples by SIMS.
INTROOUCTIOO
When fresh rocks/minerals oome into oontact with an aqueous solution
the followill3 processes will occur: 1) exchange reactions between allcali
aoo alkaline-earth ions with hydrogen or hydronium ions formill3 a hydrated
surface layer~ 2) dissolution processes, which means building up of a
hydrated layer and release of framework elements to the solution~ and 3)
precipitaticn of secondary minerals.
Investigations ooncernill3 the second and the third point have already
been done intensely taking into consideration also reactions occuring
during the exploitation of geothermic energy according to the hot-dry-rock
method. The first step of reaction -"the initial reaction--, however, has
not yet been treated sufficiently and the available studies of this subject
still demonstrate different aoo even divergent opinions as to the nature of
these reactions. This holds even for pipers that have appeared in the last
decade (Petrovic et al.,1976~ Chou and Wollast,1985~ Petit et al.,1987~
Hochella et al.1988).
The evidences of the exchange reactions were mainly obtained iooirectly,
e.g. from balance calculations involving the amounts of dissolved
elements in the produced fluids and the chemical oontents of the "depleted
layer- at the very outer surface of the treated solids. These indirect
methods are subject to a high degree of uncertainty, however, because all
errors in analysis or calculaticn will sum up in the net effect aoo conceal
the true mass differences that indicate that a supposed reaction has occured.
It is not easy to obtain a direct proof for these reactions, although
there have been several attempts. The most successful of them has
been published by Petit et ale (1987) who succeeded to present direct
evidence for the incorporation of Ht> in the silicate lattice of diopside
via H-depth profilill3 with the resonant nuclear reacticn method: they used,
however, polished samples. Due to the polishing the surface structure is

386
disturt:ed and deformed to depths of up to 5000~: the penetra tion depth of
the measurements, however, was only about lOOO$.. Therefore, these data are
not representative for an undisturt:ed feldspar lattice. Our samples, oowever,
have been generated by cleaving.
The first step of reaction t:etween feldspars and fluids is very important
for the overall reaction, because the hydrated or protonated surface
layer built in this first phase probably has a key function for the subsequent
reaction steps. This was the reason for the present study: its aim is
to get direct evidence for the nature of the initial exchange reaction. We
supposed that reactions of the following type take place
KAlSi 3
0 S
+ H 3
0+ ~ (H 3
0)AlSi 3
0 S
+ K+
and performed experiments under hydrothermal conditions in order to proof
it. Since, in an atmosphere that unavoidabLy contains water, it is extremely
difficult to exclude contamiriaticn by adsorpticn of H 2
0 molecules, we
decided to use deuterium instead of ordinary hydrogen in rur experiments.
The basic idea was to observe reactions between solid (alkali) feldspar and
solutions of Del in D?O (with the H analogoues being present in trace
amounts only). A:3 startIng material we chose the feldspar mineral sanidine
from Volkesfeld (Eifel, Germany) t:ecause of its nearly ideal crystal structure
and its simple chemistry. A:3 reactant fluids pure solutions of deuterium
chloride in heavy water were used (100% D). Deuterium has a tracer
functicn so that one can clearly distinguish the added deuterium from the
hydrogen t:eing present in the starting material or as contamination.
EXPERIMENTAL PROCEDURES
Apparatus, Experimental condi tions
The experiments were carried rut in rotating autoclaves. The reaction
cell consists of teflcn with a volume capacity of 9ml. The reactions took
place under the following experimental conditions: T= 200 o e: P= 16 bars:
volume of the solution: 7ml: starting fluids: O.ln DC! or O.oln DCl in O 2°.
The reaction time ranged from 1 hour to 49 days. In some of the experiments
20mg Si0 2 -speCi'ure-amorph was added to reduce the dissoluticn rate.
Starting material
Sanidine from Volkesfeld-Eifel was used for the experiments. Its
chemical analysis can t:e taken from Althaus & Tirtadinata (1987). Because
of the difficulties in measuring the surface of the relatively big (cm)
sanidine cleavage sections at the first experiment series, in this experiments
we used: .
a) very thin cleaved fragment (thickness: 0.6mm - l.5mm) for the subsequent
measurement with the infra-red method.
b) sanidine powder with grain size

387
A. Experimenta with thin cleaved fragments
The following parameters were varied:
- reaction ciJration: 1, 4, 14, 35, 49 days
- starting solutions: O.ln DCl and O.oln DCl
- the Si0 2
-amorph was added ally in a few experiments (O.1n DCl: 14-, 35-,
49 days). The disadvantage of using this finely ground powder is its
adhering ability, which makes it difficult to remove it from the feldspar
surface after the experiment. This is important because its presence
00 the surface can disturb the infrared diagram obtained from
the sanidine (Althaus & Tirtadinata, 1988).
Reaction fluids
Although the intention of the experiment is the investigation 00 the
solid samples, the reaction fluids were also measured in addition. The
results are listed in Table 1.
To prove whether sanidine dissolved stoichiometrically or not, in
contrast to the procedure reported in earlier publications this time we
refer to the Al/K-ratios instead of silK in the solutions (see last c0-
lumn). While the Al/K-ratio of the startiB;) sanidine is 1.24, we can interpret
Al/K

388
TABLE 2 Infra-red measurements on hydrothermal treated thin cleaved sanidine
fragments
Sample
Observation
thin san + O.Oln DCl
1 day no change observeable
4 days very weak abeorptioo band at -25~0 cm- l
14 days AlOOD: absorption bands:-3l00cm- &-2500cm- l
thin san + O.ln DCl
14 days no change observeable
thin san + Si0 2
+O.ln DCl
14 days } absorption bands atN3750cm- l ,N3665cm- l and
35 days - 2700cm- : OD-vibratioo of the silanol group
49 days of Si0 2
-amorph (van der Marel & Beutelspacher,
1976)
4000 3800 3600 3400 3200 :lXXl 2800 2600 2400
Wavenumber [cm-1 J
Fig. 1: IR-spectra of hydrothermally treated thin cleaved sanidine fragments
1. thin san + O.Oln DCl/lday
2. thin san + O.Oln DCl/14 days
3. thin san + Si0 2 + O.ln DCl/14 days

389
B. Experiments with powdered sanidine
The experiments were alrried oot with 2001119 of sanidine powder (grain
size

390
ionic radius
2 Theta
d-(iOl)
albite
NaAlSi °
Na+: 6.9~$.
22.06 0
4.o27$.
sanidine
KAlSi °
K+: r.3~K
21.10 0
4.2l7$.
(D 3 0)AlSij>8
(D 3
0)+: 1.s4~
21.10 0
4.2l7~
with this backgrOl.lOO the XRD-patterns particularly between 28= 20 0 -
22 0 were investigated intensively. A small amount of KBt03 was mixed with
the sample powder as an internal standard (28 (101) of KBr0 3
= 20.21 0 ,
d(lOl)= 4.39l{). Some typical diffractograms are shown in Fig. 2. Some of
them clearly exhibit the appearance of a second (iOl)-peak that can be
attributed to the formation of hydroxonium feldspar. Unfortunately, however,
the splitting of the (20l)-peak was not observed throughout all of
the experimental series and also there is no systematic correlation with
the reactioo duratioo which should be expected to occur. A possible explanation
of this unregularity is the limited sensitivity and resolution of
the diffractometer method. There is a clear indication, however, that
something has happened wi th the (201) reflection.
Instead of the x-ray diffractometer film cameras of the Guinier type
were used which have a much higher resolution for small variations in 29.
Several samples were selected to obtain a continuous series according to
increasing reaction duration: 3h, 5h, 6h, 1 day, 7-, 14-, 21-, 28-days. The
result of these investigations was dissapointing: With this method also
neither a splitting of the (201) peaks nor a shift in position could be
observed. This might be caused by:
- the contents of "(D 3
0)AlSi 3
0 8
" in the sample taken for the preparation
was not high enough to produce a detectible reflex ion
- the resolution of this method still is not high enough to select/ separate
a weak new reflection from the strong (201) peak of the starting
sanidine.
SIMS investigations
Since x-ray diffraction methods yielded inconclusive results only a
method was looked for: that could present direct evidence for the presence
of deutedum within the feldspar crystal structure. The secondary ion mass
spectrometry (SIMS) was believed to be suited for this purpose.
Some crystallites with plane surfaces were selected from the sanidine
powder. These crystallites with natural cleaved surfaces were mounted by
gentle pressing into pieces of indium foil. With this preparation method
the charging problem can be overcome (Hochella et al. 1988: Gooaens et al.
1988). Depth profiling was done for D (2amu), Al (27amu), Si (30amu) and K
(39amu). The samples were bombarded with an O-beam with l20nA primary
current and raster of 150 m. The diameter of the analysed area was 60 m.
Some representative results are presented in Pig. 3.
The results are qui te obvious: In samples treated for a short reaction
time only (5 hours) no deuterium (mass 2) was detected. After one day,
however, a small amount of D is present at the surface. The signal is much
stronger after 2 days and even more so after 21 days. In the later two
cases depth profiling shows that - in contrast to the run that lasted for 1
day only - the deuterium had penetrated into the feldspar lattice to a
considerable depth. Si (mass 30) contents are remarkably constant, while
both K (mass 39) and AI (mass 27) are depleted near the surface.

391
This dsta are taken as a strong argument in favour of the occurenoe of
the abow mentiooed reactioo, because they are in complete agreement with
the supposed mechanism. Evidently deuterium entered the feldsp!r structure
while K (and Al) was removed. Depletioo in Al is due to a psrtial dissolution
of the feldspar substance which leaves back a Si0 2
-Restschicht-, a
reactioo mechanism that has been described in detail elsewhere (Althaus et
ale 1983).
® ®
- ..... 1---+1 - ... 1----+1-
2'" 20 0 21 0 2ft 2'f' 2ff
2'"
2rf
2 THETA
Fig. 2: (20l)-reflections of some sanidine samples.
1. untreated sanidine
2. san powder + Si0 2 + O.ln DCL/3 hours
3. san powder + Si0 2 + O.ln DCl/S hours
4. san powder + Si0 2 + O.ln DCl/14 days
5. san powder + Si0 2 + O.ln DCl/2l days
CONCLUSIONS
In this work we haw tried to get a closer lnierstanding of water-rock
interactioo at the conditions of hot-dry-rock energy exploitatioo by studying
its ini tid reaction.
Hydrothermal experiment. were OIlrried out using sanidine and deuterium
chloride solutions. It is supposed that before the dissolution step, an
exchange reaction pc-ooeeda to CDlvert potassium feldspar into the hydronium
analogue at the crystal va. aolutioo interface. The results of the investigations
using different methods are as follows:
- investigations 00 thin cleaved fragments with infra-red method did not
show any change of the absorption bands compared to the starting
IIIlterial.

392
XRD investigations on powdered samples gave
tence of (D 3
0)AlSi 3
0 8
by the splitting of
However these results were not lD1ambiguous.
ciated with the x-ray Guinier method.
some evidence for the existhe
(20l)-reflection.
They could not be subs tan-
- SIMS-investigations gave a clear direct indication for the incorporation
of deuterium in feldspar along with simultaneous depletion of both K
and Al. This resul t indica tes an exchange reaction of D30 for K and a
disintegration reaction of the (Al,Si)04-network to occur simultaneously.
ACKNOWLEDGEMENl'
We thank D. Lang from the Institut fUr Petrographie und Geochemie,
Uni versi ta t Karlsruhe, for the succesful SIMS measurements and we also
thank the Institut fUr Kristallographie, Universitat Karlsruhe, for the
opportuni ty to perform the infra-red measurements.
REFERENCES
Althaus E., Faller A., Herold G., Kronimus B., Tirtadinata E., Topfer U.
(1983): Hydrothermal reactions between rock-forming minerals, rock,
and heat exchange fluids in hot-dry-rock systems. -European Geothermal
update, Proceedings of the third international seminar on
the results of EC geothermal energy research, held in Munich, 29
November - 1 December 1988: 301-309.
Althaus E. and Tirtadinata E. (1987): Study of reaction between rocks and
heat exchange fluids. -Special report CEC contractors meeting "Ge0-
chemistry", Toulouse.
Althaus E. and Tirtadinata E. (1988): Study of reaction between rocks and
heat exchange fluids.- EI:: periodic report I/1988 contract EN 3G-E5-
oo2-D(B) •
Chou L. and Wollast R. (1985): Steady-state kinetics and dissolution mechanisms
of albite.-Am. J. Sci.,285, 963-993.
Deer, Howie & Zussman (1963): Rockforming minerals,IV, 16-17.
Farmer V.C. (1974): The infrared spectra of minerals. - Mineralogical Society
Monograph 4, 148.
Goosens D., Philippaerts J., Pijpers A., Van't dack L. and Gijbels R.
(1988): Geochemistry of trace elements during interaction of water
with minerals of the Carnmenellis granite and its relevance to HDR
systems. - EI:: Periodic report contract EN 3G-0079-B(GDF).
Hoche1la M.F.Jr., Ponader H.B., Turner A.M. and Harris D.W. (1988): The
complexity of mineral dissolution as viewed by high resolution
scanning Auger microscopy: Labradorite lD1der hydrothermal concUtions.
~chim. Cosmochim. Acta, 52, 385-394.
Petit J-C., Della Mea G., Dran J-C., Schott J. and Berner R.A. (1987):
Mechanism of diopside dissolution from hydrogen depth profiling. -
Nature, 325, 705-707.
Petrovic R., Berner R.A. and Goldhaber M.B. (1976): Rate control in dissolution
of alkali feldspars -I. study of residual feldspar grains by
X-ray photoelectron spectroscopy. -Geochim. Cosmochim. Acta, 40,
537-548. -
van der Marel, aw. & Beutelsp3.cher, a (1976): Atlas of infrared spectroscopy
of clay minerals and their admixtures: Elsevier Sci. Publ.
Canp.

393
IPG-KA DEPTH PROFILE 7.11 ••
Sa"",I •• Sanldln r Sh / 1.ln DCI I .,Ililn
II·
".. • ...... 17K
a _AI
...
,... -
: II •
•
..
~
c
• .Si
~ 114
II I
@
III
II I
II I /\
10
I II n 31 41 51
tt.uurtna aycl.
..
IPG-KA DEPTH PROFILE 7.11 ••
Sa I •• Sanldln I~ / I. In DCI .,13,45
II·
"•
... a
,...
: c
•
II •
..
c 114
.Si
...
III
@
17K
-AI
III
II I
II I 10
I II
• ... 51
tt.uurtn a 1.
Fig.3: SIMs-depth profiles of sanidine crystallites with natural cleavage
surfaces.
a. san powder + Si0 2 + O.ln Delis hours
b. san powder + Si0 2 + O.ln DClll day

394
IPG-I

39S
EEC Contract No. EN3G-0079-B (GDF I
DlSOOll1l'I

396
mechanism of minerals are based on feldspar leaching since this is the
most important group of rock-forming silicate minerals. Whenever
feldspars interact with an aqueous solution an enhancement of some
elemental concentrations in solution is observed together with a
weathered mineral surface. Studies of the solution after weathering
resulted in the leached or diffusion layer hypothesis (Chou and Wollast,
1984) where one assumes a non-stoichiometric dissolution step based on
the frequently observed parabolic dissolution kinetics. This model
predicts an altered layer of a few tens of angstroms through which
reactants and products of weathering must diffuse. In a second
mechanism, based on measurements of the solid surface, a surface
reaction is assumed, which implies a stoichiometric dissolution step.
Recent studies (Hochella and co-workers, 1987; Petit and others, 1987)
emphasize the important role of high energy surface sites such as
outcrops, layer edges and microcracks. The diffusion layer model
proposed by Chou and Wollast ( 1985) accmmts for both the observed
solution and solid surface chemistry and takes the contribution of
surface defects into consideration.
The latter model predicts the formation of an hydrogen-feldspar as
a result of an instantaneous exchange reaction between alkali ions and
hydrogen (Ha()+), followed by a rapid build up of a K,Na-depleted layer
enriched in Si (for acidic conditions) and a slow diffusion of ions from
the fresh feldspar through the residual layer. This hydrated ·leached
layer has, however, never really been observed.
The aim of this study is therefore i. to look for the presence of
the residual layer and ii. to characterize it if present. For this
purpous sanidine (Volkesfeld, Eifel) grains were treated in 0.1 N
HCllHaO or 0.1 N DCI/IhO solutions using teflon-lined autoclaves. The
reaction temperatures and times were varied from room temperature to 200
·C and from 1 hour to 16 days respectively. The modified mineral's
surface was examined by secondary ion mass spectrometry (SIMS). X-ray
photoelectron spectroscopy (XPS) , scanning electron microscopy (SEM) ,
electron probe X-ray micro analysis (EFMA) and Fourier transform
infrared spectroscopy (FT-IR). XPS was applied for its surface
sensitivity (atomic layers) but it cannot provide information about the
H-content in the outer regions of the altered feldspar. The in-depth
distributions of hydrogen and of other elements were studied by dynamic
SIMS, although this technique is less suitable for analyzing the outer
surface. FT-IR was applied to examine the nature of the hydrogen
measured with SIMS: H (or D) can indeed be adsorbed as fuO (or IhO) or
bounded in the modified feldspar structure (as Si-OH or Si-oDl as
predicted by Chou and Wollast.
2. EXPERIMENTAL
Clear sanidine minerals originating from a trachyte tuff from
Volkesfeld (Eifel), were selected for their almost ideal crystal
structure and hence their stability against acid attack (Althaus, 19871.
Sanidine has the lowest dissolution rate of all feldspar minerals which
makes it ideal to study the alkali ions - hydrogen exchange reaction.
The composition of the starting material is listed in table 1.
The large crystals (up to a few cm 3 ) were crushed by a hamner.
Nylon sieves were used to select grains with sizes ranging from 0.4 DID
to ca. 2 DID which were ultrasonically cleaned in ethanol to remove small
particles adhering at the surface. 15 grains were picked at random and

397
put in a 125 cml teflon container together with 50 ml of a 0.1 N HCIIHzO
or a 0.1 N DCIlD20 solution. Most of the dissolution experiments were
carried out after addirliE ca. 140 mg of SiCa powder (Specpurel to lower
the concentration gradient of Si at the mineral/solution interface. The
autoclave was left at roaD temperature (RT, 16 'CI,or heated at 50 ·c,
100 ·c, 150'C and 200 ·C during 2 days in order to test the influence
of the reaction temperature on the dissolution behaviour of sanidine.
The relationship between the thiclmess of the observed leached layer and
the interaction time was studied by changing the time of" heating;
experiments were carried out during I, 2, 4, 8, 24, 48, 96, 192 and 384
hours at a fixed temperature of 150 ·C. The solution was filtered
throuah a 0.45 ~ Sartorius filter. The grains treated in HCl were
rinsed with bidistilled water after filtration to remove amorphous SiCa
and NaGl particles. This step was anitted in the experiments using DCl
to avoid H - D exchange at the mineral's surface. A few grains were
pressed in a pure indiun substrate to reduce surface charging during ion
bombardment when analyzirliE wi th SIMS. This method was chosen above the
more conventional way of depositing a metallic layer on the mineral's
surface which introduces various contaminants and obscures the
measurements at low sputter times. Other specimens were carbon coated
for the electron microprobe and the scanning electron microscope (SEMI
observations. The Fr-IR and XPS analyses of the treated sanidine grains
did not require any additional sample preparation. Special care was
taken to avoid surface contamination: analytical tools were
ultrasonically cleaned and gloves were used when necessary. All DCl
treated specimens were stored in a V8CUl.III desiccator to diminish H - D
exchange when exposed to air.
Table 1. Experimental conditions and surface composition of reacted
and fresh sanidine grains as measured with XPS. EPMA data
of untreated sanidine are tabulated for comparison.
lUI lUI lUI Untreated
1 2 3 aanidine
..
(a) (b)
Solution (50 1IIl) O.lNt£l 0.1 N 1£1 0.1 N 1£1
Sio. powder ( ) 143
Reaction time (hours) 21.5 21.5 2.
Concentration (atedl
C (1.) 13. 11. 17.5 7.1
0 (1.) 55. 54. 52. 51. 61.5
F (1.) 0.2
Na (l.,KLo.~. I 0.3 1.1 1.22
Al (2&,2p) 4.3 1.5 S.5 7.84
Si (2&,2p) 32. 32. 29. 25. 22.S
K (2&,2pI/ ,2pI/ I 3.4 7.1 6.31
Fe (2pI/ ) 0.1 0.13
BIt. (3dl / ' ,3d'" ) 0.1 0.11
(al XPS data
(bl : BR1A. data (Althala and others, 19871

398
3. RESULTS
3.1. XPS analyses
Petrovic am co-workers (1976) did neither observe a precipitate
nor a leached layer in their dissolution experiments with Volkesfeld
sarlidine. The XPS analyses they carried out on grains which were leached
for 300 hours at 82 ·C in aqueous electrolyte solutions with a pH
ranging from 4 to 8, show the alkali-depleted subsurface zone, if
present, to have a thickness of not more than 17 A. The authors
,however, did not include electrons with low kinetic energies ( C - 150
eV) in their spectra. These are the electrons which originate from the
first atomic layers and hence are surface sensitive: ca. 95% of the
signal. intensi ty of low kinetir. energy electrons is generated by the
first 3 atomic layers cf the specimen I..Ulder study (Scholten and
colleagues, 1985).
Three dissolution experiments were carried out followed by
quantitative XPS analyses of the treated sanidine minerals. Table 1
lists both experimental conditions and XPS data; the surface composition
of 'fresh' sanidine (see also Figure 1a) was tabulated for comparison.
a
b
.....
520.0 7.0.0
lIt.rH!RIY .v
,.,
......
11(.,-.
--
.. .....
520.0 710,0
IC.~.V
UIOO.O
Fig. 1. XPS spectrum ofa) untreated sanidine gains and b) aanidine gains treated at 200°C during 21.5 hours
in a 0.1 N HCI solution containing SiO, powder.

399
The latter corresponds well with the electron microprobe measurements
and can therefore be used as a 'blank'. Surface analysis of sanidine
~ins leached in a Hel solution containing SiOz powder (Run 1), reveal
the presence of a pure SiCa surface. Neither the surface sensitive peaks
wi th low kinetic energy (Nail and KA ... r) nor the high kinetic energy
peaks (Ku, IU p III, IU pJ I., NaA .... , Ala and Al. p) can be observed
(Firure 1b). This means that at least the outer surface (10 atcmic
layers or more) of the treated sanidine grains are depleted in Na, K and
Al to concentration levels below the detection limit of the technique
(0.1 at"). A similar experiment was carried out without adding SiCa
powder to the liquid (Run 2) in order to check wether the observed layer
of pure SiCa could be an artifact due to the precipitation of amoqnous
silica orilinatiDi from the added SiCa powder. Quantitative
calculations of the XPS spectrun of the latter experiment show Na, K and
Al still to be depleted compared to the 'blank' albeit to a lesser
extent. The presence of SiOa powder in solution plays an important role
in the life time of the residual depleted layer. This Jitenomenon is also
observed in the dynamic SIMS measm-ements and will be discussed further
on. Na and K could not be observed in the XPS spectrun of Run 3 Crable
1 ) , where sanidine fragments were exposed to the acid solution for only
two hours. Al could still be detected but quanti tati ve analysis showed
it to be depleted by a factor 5.7 compared to the 'fresh' sanidine.
3.2. SIMS analyses
XPS analyses on the reacted sanidine minerals of Run 1 suggested
that the thiclmess of the depleted layer must be larler than postulated
(Chou and Wollast, 1984). Additional SIMS measurements on grains of the
same dissolution experiment confirmed the latter: the in-depth
distributions of H, Na, Al, Si, K and Sa normalized to the Si signal.
(FilUre 2) show indeed a depletion of Na, Al and K and this to
approximately the same depth for all three elements (ca. 75 nm).
,g ... r------------,
!!
!!! ..
j
'" .
.. -s • Ill·
A I lis,·
..
.......
-<
.1·"·
...... .. • • .. • •
$pull.,. time (min.)
Fia. 2. SIMS depth prallie ~ ... diM sraUa tnaSed ia .0.1 N sa 1oIa\ioa coa&aiDiq Si~ powder .& •
Iemper,&uN ~ 2O()AIC duriDc 21.6 boan. TIle _pie _
ill • IqUIN ~ 2MI l'1li by oide. TIle 1IIu,.-i .... _
11m/mill. uai", th_ 1IIa1J'licai coaditb..
bombarded with • 100 IlA 0- beam roAered
150 l'1li ia cliameIer. '!'be &pulter rUe • S

400
One also observes a hydrated layer whose thickness extends more deeply
than the depletion of Na, Al, K and Ba. The H-profile shows a maximum
beneath the mineral surface which is believed to be caused by pumping
away hydrogen (water) of the outer layers after placing the sample in
the high vacuum chamber.
Another series of autoclave experiments was set up to test the
influence of the reaction time upon the dissolution of sanidine. If a
diffusion process is responsible for the build-up of the depleted layer,
one expects a linear relationship between its thickness and the square
root of the reaction time. The dissolution runs were carried out in 50
ml of a 0.1 N DCI solution with addition of SiOz powder. The reaction
time was set to 1, 2, 4, 8, 24, 48, 96, 192 and 384 hours respectively
at a fixed reaction temperature of 150 ·C. The treated mineral fragments
were measured with SIMS, analyzing the positive secondary ions. The
primary ion beam current was varied from 10 nA to 500 nA depending on
the thickness of the depleted layer. Figures 3 a-d show the in-depth
distributions of H, Na, Al and K normalized to the Si signal. Each
figure consists of 9 curves according to the interaction time of the
sanidine grains with the acid solution. The sputter time and hence the
depth scale was normalized to a primary ion beam current of 100 nA. The
dashed line in the figures represents an ion microprobe measurement of
an untreated sanidine mineral for comparison. The thickness of the
depleted layer increases with increasing reaction time. The surface
enrichment of Na, K and even Al observed in fresh samples and at small
interaction times ( 1 and 2 hours) are believed to be caused by
charge-induced migration during the ion bombardment. The presence of a
hydrated layer can be observed in profile 3a for the sanidine samples
which were leached for 8 hours or more. The sub-surface maxima in the
hydrogen depth profiles are observed again. The H/Si ratio at the
maximum of the profile appears to decrease with increasing reaction
times. The deuterium to silicon depth profiles are consistent with the
hydrogen profiles al though the degree of enrichment in the leached
samples is not that straightforward. It is still not clear why the
altered layer contains much more hydrogen than deuterium in spite of the
fact that the mineral grains were leached in a pure OCl/Oz 0 mixture and
contact with air was minimized. SIMS analyses analyzing the negative
secondary ions show a chlorine enrichment at the region depleted in Na,
Al and K.
Figures 4a-d show depth profiles for H, Na, Al and K normalized to
Si in reacted sanidine grains. The minerals were leached in a O. 1 N HCI
solution containing SiOz powder, this during 2 days at temperatures of
16 ·C (RT), 50 ·c, 100 ·c, 150 ·C and 200 ·C respectively. A region
depleted in Na, Al and K is observed with a thickness increasing with
increasing reaction temperature. Na, Al and K are again depleted to the
same depth. Charge-induced migration of Na, K and to a lesser extent Al
is observed again at low reaction temperatures where the residual layer
appears to be very thin. The existence of a hydrated layer is not
obvious from the hydrogen profiles shown in figure 4a, with the
exception of the 150 ·C specimen. The latter experiments were repeated
without adding SiOz powder to the solution. The thickness of the
depleted layer is increasing for increasing reaction temperatures up to
100 ·C but is very thin (or could not be measured) for temperatures
exceeding 100 ·C.
'Three dissolution experiments were carried out at 150 ·C with a
reaction time of 2 days: the first experiment was carried out in a 0.1 N

401
a ~ ~I~--------------------~
e
~
b ~ ~Ir-----------------------~
e
v; '.
-o
Z
~
..-. -, "-
..-:.':;_I................. ~ ..............L..U........
~~u.wIlL;-J....U.WIII ...
Sputter time (min.)
1.~O':;_I~LJ..Wj~ ....... .uL..~WIIII,.L......L~IIII,.-:-, .L..U~'.'
Sputter lime (min)
C ·B ~I~----------------------~
e
d ~ ~Ir-----------------------,
e
~-,
11-·
~~':;_I ................. ~ ............. ~W*~~~~.L..U~ ...
Sputter lime (min)
Sputter lime (min.)
Fi,. 3.
SnfI depth profiles normalized to the IOSi+ siana! for a) H+,
b) Na+ c) Al + ana d) • 1 K+. Each profile consists of 9 curves
aocordi~ to the reaction time: 0 1 h., • 2h., 0 4h.,
• Bh., ll. 1d., .& 2d., 0 4d., • Bd., 1Sd. 'nle
dashed line represents the in depth distribution of an
untreated sanidine arain. 'nle sputter rate is 3 rD/min. 'nle
fl"&ins were treated at 150 ·C in a 0.1 N DCllDaO solution
con~ SiOt powder.

402
a .2 I."~-----------------------'
~
I.
b] I."r------------------------.
~
U; 10
.......
o
.z
1.-<
1.-3 ~...I.LI.IllIl..-:-'-'.J.l1I1IL-UJ.1WIl--'-.LI.U'"';-.J...l.WIII
~ ~ ~ ~
Sputter time (min.)
Sputter time (min.)
I. I
C .2
~
iii
......
:;;:
I.
d] I."r-----------------------~
~
~ 10
'"
1.-<
I ....
Sputter time (min.)
Sputter time (min.)
Fig. 4.
Snf; depth profiles noI'llllLlized. to the "Sit s~ for a) tf+,
b) Nat 0) Al+ ana d) 41Kt. Each profile consists of 5 curvet!
according to the reaction temperature • 16 ·c, 0 50 "c
• 100 ·c, 0 150 ·C and • 200·C. The sputter rate
is 3 nm/min. The Sanidine grains were treated for 2 days in a
0.1 N HCllHa 0 solution containina SiC. polder.

403
OCI/DaO solution with addition of Sio. PQ'der, the second contained a
O. 1 N OCI/Da 0 solution wi th addition of Ala 0, poKier while the third one
was carried out in a 0.1 N HCI/HzO solution without any added PQ'der. No
depleted redon was observed for the experiment using the alunina PQ'der
while the sanidine grain leached in the pure HCI solution (wi thout
addi tiona! PQ'der) has only a very thin residual layer as appears from
the AI in depth distribution. The dissolution experiment containing Sio.
poKier leads to a surface layer which is clearly depleted in all
elements by a factor of ten (or morel compared to the other experiments.
3.3. FI'-IR analyses
Fourier transfonn infrared measurements were carried out on a
sanidine grain which was leached in a 0.1 N DCI/DaO solution for 16 days
at 150 ·C in the presence of Sio. poKier. Dynamic SIffi experiments show
the sanidine grain to have a hydrated layer with a thickness of ca. 700
run. The limited surface sensitivity of the infrared technique requires
highly leached grains to obtain information about the nature of the
observed hydrogen: earlier FI'-IR experiments on sanidine minerals
treated for 21.5 hours (Run 1 ,see abovel which contain a hydrated layer
as observed by SIffi (Figure 2), reveal, indeed, no difference between
treated and untreated grains. Figure 5 shows three reflection infrared
spectra of A. a freshly cleaved sanidine mineral B. the treated
sanidine grain and C. the treated sanidine grain which was heated at 150
·C in an oven during 2 hours after reaction.
17110 CIt-I AUlD ..
•
"'-----y
"000 •• 00 aeoo ."00 aaoo 8000 •• 00 ..ao ."00 aaoo aooo ,eoo
WAy ......... leN-I.)
rIC· II. Re8edioa FT-IR lpecln of A) • &eIh MDidiDe &raiD, 0) • AIlidiDe &raiD t.rea&ed al 15d'C durinc 10
d~ ill • DeI/O,O IOiliboo ccmLaiDiD, Sio, powd. ud C) liM _ &raiD .. 0) but healed ill u ewell
al l~oC dllrill, 2 boun aI\er liM di.olulioa experiment.

404
The treated l{I'ain reveals a lar~e amolDlt of adsorbed IhO (water) while
adsorbed DzO could not be observed. The presence of Al-oH, Al-DD and
Si-oH peaks can be seen but no Si-DD peak could be detected. The botmded
hydrogen (deuteritun) peaks can still be observed after heating the
sample although their intensities are diminished. The Fr-IR show the
treated grain to be deuterated only to a small extent, which confirms
our SIMS experiments.
4. DISCUSSION
A layer depleted in Na, K and Al is observed at hydrothennally
treated sanidine grains (0.1 N Hel or OCl); its thickness increases with
increasing reaction time and/or increasing reaction temperature. This
residual SiOz layer, however, was observed for minerals treated in a
solution containing SiOz poooer. Secondary electron images (SEI) of the
treated sanidine surfaces show no precipitated layer of amorphous
silica; small SiOz particles are observed at the surface but the area
covered is negligible compared to the lDlcovered area, especially in the
case of the dissolution experiments using HCl where the treated minerals
were thouroughly rinsed after reaction. Therefore the observed outer
silica layer is caused by the removal of elements (Na, K, Al) rather
than a precipitation of amorphous silica. Althaus and co-workers (1987)
decribe the dissolution of sanidine by the following reaction:
2 KAlSbOa + (SiOz) .. + 2 OCl + 4 DzO ------
2 (D30)AlSbOa + (D4Si04 ).q + 2 K+ + 2 Cl- (1)
The amolDlt of dissolved silica, mostly originating from the added
poooer, shifts the reaction to the left side and inhibits further
dissolution. Dran and others (1988) observed a hydrated alkali-depleted
region in their dissolution experiments with alkali-silicate glasses.
The authors concluded from their experiments that the silica content in
solution has a large influence on the destruction rate of the hydrated
layer at its interface with the solution and/or on the chemical
transfonnations taking place within the layer. Our XPS measurements of
Run 1 (see above, table 1) show also a total depletion for Na, Al, K, Fe
and Ba compared to the partial depletion observed in Run 2. SIMS data on
hydrothennally treated sanidine minerals wi th varying reaction
temperatures show markedly different in depth distributions for
temperatures exceeding 100 'C: the complete removal of the hydrated,
depleted region due to the high Si concentration gradient when using an
acid solution without SiOz powder, could possibly explain the absence of
the surficial hydrated zone at 150 ·C and 200 ·C. No residual layer was
observed when the amolDlt of dissolved alumim.un was increased. This is in
agreement with the assumption that the lifetime of the remaining
hydrated silica lattice only depends on the Si gradient at the
solution/solid interface. AluminlDn, however, seems to be important for
the build-up of the depleted zone: the Fr-IR analyses show clear Al-oH
and Al-DD peaks compared to the smaller Si-oH peak (the Si-DD peak could
not even be detected). It seems that hydrogen is preferentially botmded
to AlO, forming a species such as HAlSbOa. The latter species is
suggested by Chou and Wollast (1985) in their dissolution model based on
the transi tion state theory and surface coordination chemistry. It is
possible that the small amount of Al in the depleted layer, which
dropped to ca. 10- 3 times the bulk level, plays a role in the

dissolution of the residual layer at the solid/solution interface. (Xl
the other hand the observed Al content of the residual layer, although
small, could be an instrunental background: in other words the Al
content in the residual layer mIght be negligibly small.
In the model of Chou and Wollast (1985) it is (also) assumed that
the residual layer on the feldspar is slowly dissolved at the
solid-solution interface, accanpanied by diffusion of ions fran the
fresh feldspar boundary, leading to a steady-state dissolution stage.
We have fi tted a curve through the SIMS depth profiles of the K/Si ratio
in order to obtain a value for the diffusion coefficient of K. The
applied model for the calculation was taken fran Crank (1975,p 38) with
the assumption that the diffusion coefficient is the same in the
hydrated "Si~" layer and in the fresh feldspar. The boundary between
the residual hydrated layer and the fresh sanidine was taken where the
K/Si ratio reached half its maximuo value. This points also marks the
thickness of the leached layer. (Xle observes an exponential relationship
when plotting the thickness of the depleted layer versus the square root
of the reaction time. This could mean that diffusion is not or not the
only process responsible for the dissolution of the feldspar. This
exponential relationship is, however, mainly detennincd by the last two
points in the curve (for the highest leaching times) while the first
five points appear to obey an x VB. -It relationship. The same
calculation procedure was carried out on the K/Si curves obtained at
different reaction temperatures. The diffusion coefficients obtained
after the least squares fit were 1.6xlO- u , (l.HO.3)x1O- 17 and
8.Oxlo-17 ~S-I for 100 ·c, 150 ·C and 200 ·C respectively. There is a
linear relationship between lIT and 10g(D), which is an indication for a
similar diffusion mechanism. The diffusion coefficient of K in sanidine
surface layers, obtained by leaching at pH=l as a function of the
reaction temperature can be described as follows:
D = 1.366 X lo-ID exp (-57000/RT)
where R = 8.314 Jmol- 1K-l, T is the reaction temperature in K and 57000
is the activation energy expressed in Jmol-I. These values are in good
~nt with those calculated by Paces (1973) from the analysis of the
leachinar solution. When extrapolating our results to 25 ·c, we obtain a
diffusion coefficient of 1. 4x1o-. 0 ani 8"'1. This value also agrees with
Paces (1973), but is considered to be low by Chou and Wollast (1984).
5. OONCLUSIOO
A layer depleted in Na, K and Al is observed at the surface of
sanidine lCrains leached in O. 1 N HCl (or OCl). I ts thickness increases
wi th increasinar reaction times and/or temperatures when the Si gradient
at the interface is lowered by addinar pure SiOa powder to the solution.
The observation of a pure hydrated SiOa surface when feldspar is leached
in an acid environment, is in qualitative agreement with the following
mechanism proposed by Chou and Wollast (1985):
HaAl.SilOu + 5 1ft 2 Alit + HSi,Ot,- + 3 HaO (2)
There is some evidence for the existence of a hyd,roaen feldspar
(Ha Al. Si. Ot.) fran the Fr-IR measurements, al though the evidence was
limi ted to hiihly leached lCrains. The calculated diffusion coefficients

406
from the K/Si depth profiles have the same order of magnitude as ~~ues
reported earlier (Paces, 1973; Chou and Wollast, 1984). On the other
hand, an increasing amount of etchpi ts is observed with the scanning
electron microscope which indicates that dissolution actually takes
place at these surface defects. Petit and co-workers (1987) already
suggested that migration of molecular water into the surface is a key
step in the dissolution mechanism of silicate minerals. The latter is
confinned by the large amount of adsorbed HzO observed in the FT-IR
spectra.
REFERENCES
ALTHAUS E. and TIRTADINATA E. (1987) Study of reaction between rocks and
heat exchange fluids. Proc. Conmission of the European ConImmi ties:
contractors meeting "geochemistry" ,Toulouse, 1-17.
CHOU L. and WOlLAST R. (1984) Study of the weathering of albite at room
temperature and pressure with a fluidized bed reactor. Geochim.
Cosmochim. Acta 48, 2205-2217.
CHOU L. and WOlLAST R. (1985) Steady-state kinetics and dissolution
mechanisms of albite. Amer. Jou. of Science 285, 963-993.
CRANK J. (1975). The mathematics of diffusion, Clarendon press, Oxford.
38-39.
DRAN J-C., DELLA MEA G., PACCAGNELLA A., PETIT J-C. and TROl'IGNON L.
(1988). Aqueous dissolution of alkali-silicate glasses: reappraisal of
mechanisms by H and Na depth-PrQfiling with high energy ion beams.
Phyg. Chem. Glasses, in press.
HOCHELLA M. F. Jr., roNADER H. B., TURNER A. M. and HARRIS D. W. (1987)
The complexity of mineral dissolution as viewed by high resolution
scanning Auger microscopy: Labradorite under hydrothermal conditions.
Geochim. Cosmochim. Acta 52, 385-394.
PACES T. (1973) Steady-state kinetics and equilibrium between ground
water and granitic rock. Geochim. et Cosmochim. Acta 37, 2641-2663
PFI'IT J-C., DELLA MEA G., DRAN J-C., SCHOIT J. and BERNER R. (1987)
Mechanism of diopside dissolution from hydrogen depth profiling.
Nature 6, 705-707.
PRl'ROVIC R., BERNER R. A. and GOLDHABER M. B. (1976) Rate control in
dissolution of alkali feldspars--I.Study of residual feldspar grains
by X-ray photoelectron spectroscopy. Geochim. Cosmochim. Acta 40,
537-548.
SCHOLTEN J. J. F., PIJPERS A. P. and HUSTINGS A. M. L. (1985) Surface
characterization of supported and nonsupported hydrogenation catalysts.
Catal. Rev. Sci. Eng.27(1), 151-206.

EEC contract no. EN3G-0030-UK
HELIUM ISOTOPE SYSTEMATICS IN CRUSTAL FLUIDS
FROM W.GERMANY AND ADJACENT AREAS
E. GRIESSHABER, R.K. O'NIONS and E.R. OXBURGH
Department of Earth Sciences,
University of Cambridge, Downing Street,
Cambridge CB2 3EQ, U.K.
SUMMARY
Helium isotope results are presented for groundwaters
sampled from Southern Germany and its adjacent areas. They
are discussed within the overall tectonic framework established
for tge ~ea, and in particular, the relationship
between the ~e/ He distribution and areas of active extension
is highlighted.
In detail, there are important systematic variations of
helium isotope composition and water chemistry which may
lead to the characterisation of the mantle helium bearing
fluid end-member. It is likely that more detailed studies
which combine rare gas isotope geochemistry with water and
gas phase chemistry will provide insights into the transport
mechanisms for mantle volatiles in the continental crust.
1. INTRODUCTION
A regional pattern of mantle 3He distribution in W.Europe has been
established through the analysis of helium isotopes 1n groundwaters,
natural gases and geothermal fluids.
The most striking feature of this pattern is the clear relationship
~etween tectonio setting and the presence or absence of mantle-derived
He (Hooker et al., 1985a; Oxburgh et ale 1986; Deak et ale 1988; Hooker
et ale 1985b; Matthews et ale 1987). For example in all areas actively
undergoing extension, such as the Rheingraben (FRG), the Pannonian Basin
(Hungary) and ~ome parts of the Massif Central (France), near-surface
fluids oarry a ~e component which is clearly of mantle derivation. In
oontast to this, areas which formerly underwent extension, such as the
Mesozoio extension of the No~th Sea, a mantle derived helium component is
not resolvable. The helium ( He) that is now present in the North Sea and
some other tectonically s2~~1~38areas ~~~ been generated within the
radioaotive decay series of ' U and Th in the continental crust.
The relationship between tectonic style and helium isotope composition
is displayed partioularly well by the Neogene sedimentary basins in
the vioinity of the Alps. Whereas Neogene sedimentary basins for which
extension appears to have been at least in part responsible for their
origin (Pannonian Basin, Rheingraben) contain mantle-derived ~e, no such
Signature is evident in the Neogene basins produced by loading such as
the Molasse and the Po Basins.

~8
These and other features of distribution of mantle-derived 3He in nearsurface
fluids have been discussed at some length by Oxburgh and O'Nions
(1987) and O'Nions and Oxburgh (1988).
Groundwater has proved to be a particularly useful medium for sampling
helium present in various parts of the continental crust. With the
exception of very young (post 1950) recharge waters, which are 'contaminated'
with bomb-tritium, they have the capacity to provide unique spatial
information on the distribution of helium in the continental crust.
So far comparatively limited effort has been made to integrate the large
body of chemical and stable isotope data available for groundwaters with
the rare gas data, in order to gain insight into the provenance, mixing,
temperature of equilibration and depth of water penetration. However, on
the large scale, helium accumulation rates in aquifers of regional extent
such as the Great Artesian Basin, Australia and the Auob Sandstone,
Namibia (Torgersen et al. 1985) and helium discharge rates in major
aquifers from the Pannonian Basin, Hungary have been used to demonstrate
that radiogenic helium is lost from the continental crust at about the
same rate as it is being produced.
In this paper the value of integrated studies that includes water
chemistry and rare gas isotopic measurements is demonstrated for groundwaters
of southern Germany and adjacent areas.
The contrasting tectonic character of different parts of western Germany
together with the extensive chemical data available for the groundwaters
sampled, provide an excellent opportunity to gain deeper insight into
processes controlling the passage of helium and other volatile species
into and through the continental crust.
2.THE REGIONAL DISTRIBUTION OF 3HE/4HE RATIOS IN WEST GERMANY
The helium isotope results currently available for groundwater
samples for southern Germany and its adjacent areas are shown in Figure
1. The samples are subdivided into five groups. One group covers the
range of 4 < (R/Ra)c < 6, which should be compared to mantle-derived He
sampled at mid-ocean ridges with (R/R ) ... 8. Other divisions are made
for samples with 2 < (R/R) < 4, 0.5 a

The location of this region to the west of the Egergraben has been noted
previously. The KTB deep drilling will provide an excellent opportunity
to examine this situation in more detail.
3. GROUNDWATERS FROM SELECTED AREAS
In this section a more detailed appraisal is made of the data available
from the Rheinische Schiefergebirge, Schwarzwald and the southern
Rheingraben. The helium isotope systematics established for groundwaters
in these regions are compared to other aspects of groundwater chemistry
including halogen contents and temperatures derived from the silicaeaturation
thermometer.
3.1 RHEINISCHES SCHIEFERGEBIRGE
Samples from this area may be subdivided into three major groups:
the East and West Eifel, the Vogelsberg, and the HunsrUck-SUdrand St~rung
(Figure 1).
(R/R) values range from close to the assumed mantle end-member
((R/R) ~c8) down to values characteristic of the radiogenic helium
produgtIon ratio in the continental crust ((R/R ) ~ 0.04).
(R/R) values and helium concentrations ro~ groundwaters are compared
ina rigure 2. Overall there is little correlation except for the
tendency for samples with the lowest (R/R) values to be associated with
the highest helium contents. However, Yfcsamples from the three areas
denoted above are considered separately, some clear correlations emerge.
This is particularly true for samples from the East and West Eifel, where
the highest (R/R) (or the equivalent percentage mantle He) are associated
with the ioSest total helium contents. Similar relationships are
evident for samples from the Vogelsberg and the HunsrUck-SUdrand St~rung
but over more limited ranges of total helium contents. In each area
mantle He appears to be mixed with a much more abundant radiogenic component
of crustal He, and therefore is most easily resolved in groundwater
with lowest total helium contents.
There are some interesting relationships between the total helium
contents and other featues of goundwater chemisty. An example is shown in
Figure 3, where He and Cl- contents are compared. [He] - [Cl-] variations
are distinct for each of these three different regions shown in Figure 2.
In _each case, an increasing He content is accompanied by an increase in
[Cl]. This suggests that to a first approximation the groundwaters in
eaoh area may be describeg by a simple two component mixing model. One
end-member with high [Cl ] and high total [He], and by reference to
Figure 2., predominantly radiogenic helium, and the other end-member with
~ery low [Cl-] and low total [He] and a high percentage of mantle-derived
He. From these relationships it now appears that in both regiOns
(East/West Eifel, Vogelsberg) the mantle He component is associated with
low [Cl-] groundwater which is mixing locally with high saline groundwater
dominated by radiogenic helium of crustal origin.
A secound feature of the groundwater chemistry of the Rheinische
Schiefergebirge is shown in Figure 4, where [Cl-] is compared with silica
content, expressed here as a silica saturation temperature given by
Truesdell (1976). These temperatures will be less than the highest temperatures
experienced by groundwaters, if any silica precipitation has
taken place at low temperatures.

410
However, noting these limitations, there is no obvious relationship
between the silica saturation temperature and [CI-] contents. In contrast
there is some suggestion from the data in Figure 5 that the low salinity
groundwater end-member associated with the strongest mantle He signature
has equilibrated with silica at higher temperatures (Figure 5).
3.2 SCHWARZWALD AND SOUTHERN RHEINGRABEN
Although there are some similarities between groundwaters in the
Schwarzwald and in ~he Rheinische Schiefergebirge, particularly in the
occurrence of HCO and CI -rich varieties in both areas, the mantle
helium component ar (R/R) values, are generally much lower in the
Schwarzwald (Figure 1). W~e~e mantle helium components are elevated such
as in the southern"Rheingraben samples, they again appear to be associated
with the low t9tal helium contents (Figure 6). The ralationship
between [He] and [CI ] for Schwarzwald, southern Rheingraben and the
Urach samples are shown in Figure 6. As with the Rheinische Schiefergebirge,
some distinctions between the different geographic areas can be
made. The highest [CI-] contents occur in the Urach samples, and there is
just" a crude relationship between [CI-] and [He]. However, with the
exception of the southern Rheingraben samples which have the lowest
(R/R) values, [He] and [CI-] contents, there is no simple relationship
betw~eg [He], [CI-] and (R/R) in the Urach and in the Schwarzwald
samples.
a c
CONCLUDING REMARKS
1. The contrasting tectonic situations in W.Germany and adjecent
areas'provide some key insights into the release of mantle-derived fluids
into the continental crust.
The results both affirm the first order observa ion between active
continental extension and presence of 3
m~tle-derived He in the near surface.
They clearly show that the mantle He signature is distributed over
wider geographic areas than surface volcanics, suggesting that melts are
distributed over much broader regions at depth than at the surface
(Oxburgh and O'Nions 1987; O'Nions and Oxburgh 1988). Of particular
interest are the results from the Oberpfalz, which imply the presence of
melts and mantle-derived fluids at depth beneath the area, but without
any other observable physical expression such as heat flow or seismicity.
2. A preliminary examination of He abundances and isotope composition
in relation to groundwater chemistry, shows some important features.
These include the association of the highst mantle-derived He signal with
the lowest salinity, presumably shallow groundwaters in the East and West
Eifel and Vogelsberg. In each of these regions the results approximate to
a two co~ponent system in which one end-member has a high salinity and
high [CI] gggteUt, associated with high total [He] which is dominantly
radiogenic He of crustal origin, and the other component ~s low
salinity, low total [He] and a high percentage of mantle-derived He. In
each case, the end-members appear to be characteristic of relatively
small geographic areas.

411
3. The mechnisms responsible for the transport of He and other
volatiles through the crust are still poorly understood. Careful studies
of rare gas isotopes in chemically well characterised groundwaters such
as those described above offer a real possibility for advance. Given the
comparatively straightforward relationship outlined, it should prove
possible to characterise the chemistry of the fluids carrying the mantle
helium signature into the groundwater. Coupled with information on
groundwater ages and flow regimes, the rates of migration and accumulation
of these components may be addressed.
REFERENCES
1. Hooker, P.J., O'Nions, R.K. and Oxburgh, E.R. (1985a). Helium isotopes
in North Sea gas fields and the Rhine rift. Nature, 318, 273-275.
2. Oxburgh, E.R., O'Nions R.K. and Hill, R.J. (1986). Helium isotopes in
sedimentary basins. Nature, 324, 632-635.
3. Deak, J., Horvath, F., Martel, D.J., O'Nions, R.K., Oxburgh E.R. and
Stegna, L. (1988). Helium isotopes in geothermal waters from
North West Hungary. In: Case Study on Pannonian Basins, eds.
F.Horvath and L.Royden. Am.Ass.Pet.Geol.Spec.Pub. pp. 293-297.
4. Hooker, P.J., Bertrami, R., Lombardi, S., O'Nions, R.K. and Oxburgh,
E.R. (1985b). Helium-3 anomalies and crust-mantle interaction in
Italy. Ceochim. Cosmochim. Acta, 49, 2505-2513.
5. Matthews, A., Fouillac, C., Hill, R., O'Nions, R.K. and Oxburgh, E.R.
(1987). Mantle-derived volatiles in continental crust: the Massif
Central of France. Earth and Planetary Science Letters, 85, 117-
128.
6. Oxburgh, E.R. and O'Nions, R.K.(1987). Helium loss, tectonics and the
terrestrial heat budget. SCience, 237, 1583-1588.
7. O'Niona, R.K.
oontinental
press).
and Oxburgh, E.R.
crust. Earth and
(1988). Helium, volatiles and the
Planetary Soience Letters (in
8. Torgersen, T. and Ivey, G.N. (1985). Helium aooumulation in 4
groundwater
II: A model for the acoumulation of the crustal He degaSSing
flux. Geochim. Cosmochim. Aota, 49, 2445-2452.
9. Truesdell, A.H. (1976). Summary Seotion III, Geoohemioal Techniques in
Exploration. Proc. 2nd. U.N. Symp. on the Development and use of
geothermal Resouroes, San Franoisco, 3-29.

411
ISOTOPIC COMPOSITION OF
HEUUM IN GROUNDWATER -
SOUTHERN GERMANY AND
ADJACENT AREAS
o 10 100
....
' c.
- 0
UltAeH
g
MUNCHFH
IR/R.I. Yel" ..
• ~ - 6
l - ~
C) o· S - l
~ 0-' - 0, .
o < 0-01
m Q ... t,,,,,,y/T.,,,.,, volo.ol ,
~"'Io,""ullo
o Ho,."ollo "'"111'01
FJ&vc J .. Gentral map shOW1n.g heUum isotape resuhs ablntnfld (or gt'OundwotC'l'
s.n.mp1e;, rro", M)uthcm Cemumy And adJactn1 Areas tn rel&tlOn to pnnctpk
r."lures orU •• regloM) geolo~ _
Hellum .datA are pr_ntcd ~. (R/RoJc valu~. whcre I( b Ihe 3110/ 4110 ",Uo or 0 ..
w mpl. nOITn.~d 10 Ille ~ellum ISotope ratio 01 01, (1 .4 x IO-Il). Th. tubIC",pt c
denutei cornalot1 (or air conlamlnaUOn.
Tho dau. ..... lP'Oupcd Inlo rour major ~p. WlttI Ii > (R/Ra! ~ • > tR/1Wo > 2, 2 >
IRfRAIc > 0.5 Ilnd 0.5 > (rt/~ > 0.1.
Nol. Ihal ItQnUc helium ot

80
RHEINISCHES SCHIEFERGEBIRGE
eo
E
~
i 40
%:
.!!
-C
III
==
~ 20
0
0
~c
0
0
0
0
••
0
.~
o6\ •
"-
0
• • •
\ . ~
0
0 E •• W. Elfel
• Vogelaberg
• H. S. Storung
~o
200 400 800
He content (cc(STP) He/cc) x 10- 7
800
Fliure 2. Comparison of helium isotope composit1on (expressed in terms of
percentage of mantle helium present) and total helium content for groundwatel'S
from the Rhe1n1sche Schiefergebirge (FRG). Samples are d1stlngu18hed according to
the geographic areas shown on the figure. In each area there Is a clear decrease of
percentage mantle helium in groundwater with increasing total helium content.

po.
I
«:)
800 RHEINISCHES SCHIEFERGEBIRGE
cO·39
-------
_
0·09 c
.... 640
/ 1·02 (R/R.le
480
/
/ c1·15
c E. & W. Eifel
e Vogel.berg
• H-S. Storung
_ 320 c3.14
c
E
.,
I
0·7
e
-
0·13
-e eO.83
o 4·05
U
e1·56
160 c3·41
l:
/
2·32
5.0 .. 3 • 4 / _.0·09 ..
e3·17 1·42.,.0·81
3·4- 1·80 /2.64 1.78 ,/
5.00~~:e ____ ~ __ ~ __________ ~ ______ ~~~4-__________ -L__________ ~
1600 3200 4800 6400
8000
Figure 3. Comparison of helium and chloride contents for groundwater from the
Rheinische Schiefergebirge. The same geographic groupings are used as In Figure 2.
(R/Ralc values are shown against each sample point. In each of the three areas there
Is a general Increase of chloride content with Increasing helium content. These
relationships suggest that a simple two component mixing model may be applicable
to each of these areas.

6
(II
~
•
£
-at
E
U
12000
1000
RHEINISCHES SCHIEFERGEBIRGE
\ 0.1 (R/R.,c
.1.5
• H.S. Storung
\
• Vogel.berg
o
.0.1
.0.1
0.8 \
1000 •
.0.1
3000
\z0.7
1.4·
.0.1
.1.0
E •• W. Eifel,
Rhelngraben
.0.&
1.&
• 2.5
0
02.&
• 2.&
"-
3.4.
00 150 200
"
Filare 4. Comparison o( chloride contents and SiO:z contents expresSf'd as quartz
baturaUon temperature calculated using the expression gtven by Truesdell (1976)
(or samples from the Rheinische Schiefergebirge. There is no clear distinction
between the apparent temperatures calculated (or the highly saline end-member
With dominantly radiogenic helium. and the low saline end-member in which the
mantle helium component is most evident.

80
70
RHEINISCHES SCHIEFERGEBIRGE
[] E. & W. Eifel
• Rhelngraben
/l. Ardennes
60
E 50
~
'i)
:I:
.! 40
-C
CO
:!: 30
fI!.
20
6.
[]
[]
[] []
[]
• /l.
[]
/l.
[]
[]
[]
[]
• •
•
[]
[] []
•
[]
•
•
10
6. 6.
•
0
0 30 60 90
/l.
T Quartz
Figure 5. Comparison of the bellum isotope composition expressed in terms of
pelcentage mantle bellum component (see caption to Figure 1.) and apparent silica
saturation temperature.The samples are from the Rheinische Schiefergebirge and
the Ardennes (Belgium).
A crude correlation is apparent between the silica saturation temperature and
percentage of mantle bellum in the groundwater.
120

...
I
-
3000
o 2500
u
~ 2000
II
:z::
0: ~
t- f,..
rn 1500 «0
- 00·02
~ ~.~
~ 1000 0'55
g /
II 00'1
:z:: 500
0·93
~8 ••
1'58 0·17
0·27 001·73
SOUTH RHEINGRABEN AND BLACK FOREST
0·93
•
1000 2000 3000
CI, mg/iltre (H 2 0)
0·93 (R/R.le
o BI.ck For •• t
(Schw.rzw.ldl
• S. Rhelngr.ben
Fliure 6. Compartson of helium and chlortde contents In groundwaters from the
southern Rhelngraben (FRG) and the Schwarzwald (FRG). IR/Ralc values are shown
against each data point.
There are distinct groupings of data according to region as Is evident for the
Rheinische Schiefergebirge. The southern Rhelngraben samples have the lowest
salinity. lowest helium content and highest mantle helium Similar to the
observations In the Rheinische Schiefergebirge (FIgures 2. 3). However. the overall
variation between these parameters In the present data-set are less apparent than
In the Rheinische SchIefergebirge.
4000

____
________
4000
SOUTH RHEINGRABEN, SCHWARZWALD, URACH
3000
6
N
~
4D
...
=
--CI
E
0
2000
0.2
0.13
• •
• 0.18
1000 • 0.62
0.93 .0.17
• .0.01
0.8.
1.52 .0.3
o L-__________
•
~~~~LL ~~~
0·1
o 50 100 150
_L ____________ ~
200
T Quartz
Figure 7. Apparent silica saturation temperatures and chloride contents for
groundwaters from the southern Rhetngraben and the Schwarzwald. There is no
clear relationship between these two parameters in the present data set.

419
EEC Contract no. EN3G-0006-F(CD)
EXPP1UHERTS ON REINJECTION OF GP.OT1IERMAL BRINES
IN THE DEEP TRIASSIC SAlmSTORES
A. Boisdet - J.P. Cautru - I. Czernichowski-Lauriol - J.C. Foucher
C. Fouillac - J.L. Honegger and J.C. Hartin
Institut Hixte de Recherches G~othermiques (BRGH/AFHE),
BP 6009, F 45060 Orl~ans Cedex 02
ABSTRACT
After the favourable results obtained in exploitation of the Dogger
geothermal aquifer in the Paris Basin, the Triassic hot-water-bearing
sandstones were tentatively tapped. The extent and higher
temperature of this second objective make it a hopeful target for
operators. However, difficulties encountered in the Ach~res. Cergy
and Helleray-St Denis en Val operations (respectively in the
Yvelines, Val d'Oise and Loiret Depts), especially in reinjection
tests, led the CEC, AFM! (Agence Francaise pour la Ha1trise de
l'Energie) and BRGH (Bureau de Recherches G~ologiques et Hini~res) to
finance a programme for the investigation of:
a) factors limiting injectivity in sandy clayed aquifers,
b) conditions for perennial exploitation by doublets.
Percolation tests with the geothermal fluids of Helleray and
Villefranche/Cher were performed on site on microcores of industrial
materials and on natural cores taken in variably clayey sandstones.
The experiments used an experimental loop specially designed by IHRG
for field tests on operation sites.
The main experiments are described and their results with comments
are given here with respect to:
a) petrology and porology of materials before and after testing,
b) hydraulics - variation of permeability during the experiment,
c) chemical characterisation of the liquid, gaseous phases and
suspended particles; fluid treatment.
Finally, a procedure is described for processing fluid before its
reinjection into the aquifer.
I. IIn'ROIlUCTION
1.1. Difficulties encountered duriOl exploitation of the Triassic
After the success of the first geothermal operations on the Dogger
exploitation of the Triassic reservoir was envisaged. The Triassic,
though less well-known than the Dogger, presented a particularly
attractive target because of its higher temperature and much wider
extent in Europe. However, the operations at Cergy-Pontoise (1980),
Helleray (1980) and Ach~res (1982) showed the difficulty of applying,
without adaptation, the technical solutions used successfully on the
Dogger. In particular, the Triassic reservoir, characterised by matrix
permeability as opposed to the fracture permeability of the Dogger, has
aaymmetrical behaviour - while a good producer, it may react badly to
reinjection without treatment of the reservoir and/or the fluid. Figure
1 shows the location of these three operations.
a)~. The low production flowrate (70 m'/h) here has induced
operators to exploit the Dogger, which has a hydraulic potential greater
than 200 m'/h.

420
b) Melleray. The Melleray operation pTesented f ctOTS that
distinguished it. both technically and organisationally, from the othe
operations. in particular the use of heat fOT greenhouses yidoned the
field of utilisation which had hitherto been confined to district
heating. Many injection problems were encountered.
To investigate the problems, the CEC (DCXII) in 1982 T4?qu(l5tcd that
3.n injection test be carried out. The test revealed a puz zling
phenomenon in that the injectivity j,ndex: of the ,"escrvoir rapidly fell
very considerably. The operational di(ficulties led to the decision to
shut down the doublet.
c) Ach~res. The Triassic here . tapped from 1900 m down , revealed flui d
at high temperature (78"C l. but low discharge (25 m'/h). It was
therefore decided to exploit the Dogger instead (56'C, 20n m~/h).
P'i&ure 1. Location map (modified aft.er the "Atlas ot Ceothel1ll81
Resources in the European Community", R. Haenel and E. Star08te.
cds., CBC, 1988)
,- Tempelolulc ·CJ
,-
...".
"r= PouOg

421
__ 101
iii .. M.IIe",IU Souh.IZI ~11I
I'.,. Aan,4'
I'r.nce I'r ... 11. .. 1, Iloa_k UK
Plllh. CUYI W4II11 W.II211 W.II" W.III
'M! Ctor.",uunJ 100 1211 16 10 7:11
pli 1I,on "''''' 100 00 1.10
GIN' 028 02 2- 0 ..
TIJS"" 301 .00 14 2l1li .034
Io"twaup ........ C •• I ..'
IIlI IU 01 ttl 11
CII, "
.. l 411 • 12 11 40
lIl. 12 sa 971 4Il 110
/\, • II, • II. I I 182·· Ol 2 -I
......, I...,,, ..... '"",nil
'.
II 000 211000 14100 1'000 SlUG
c.
K ]:1\1 ]:Ia IN no IZlI 11120 2114 13!1OO l810
~. ... III 1111 2!16O 161
h n ...• 210 42
MIO, n III. .. Sll
1'1 20000 neoo ,IW 181 000 13115
!WI, 11 .. 2M 21 1400
IICUJ 311 Il1O 113 ..
..
Ga. liquid rolla
Th. high measured value of H2 Is probably owing to corrosion reactions
••• Th. high Iran content will also be 0 result of corrosion
s.ut.tt._,..15J
UK
W-..!oot>IwoIll
TlO
1.0
.240
1,
11
174
18
,-
,llOG
1110
102
, I
311
1&900
12:10
11
(1) Vuataz at .1. (1988). (2) IMRG data. (3) AGIP (1988)
(

422
validate the treatment to be employed. It was tested on a second fluid,
that of Villefranche sur Cher, where Gaz de France (the French Gas
Board) has an installation tapping the Triassic.
Although the greater part of the in situ experiments were carried
out at Helleray, it can be seen in Table 1 that the fluid there has
physico-chemical properties entirely comparable with those described for
the Triassic elsewhere in Europe. Consequently, the results and
conclusions of the research undertaken in the framework of this contract
should be applicable to other European geothermal fields.
2. TECHNOLOGICAL AIm INSTRUMENTAL ASPECTS
2.1. Description of the percolation test loop (Fig. 2)
The main characteristics of the loop are as follows:
a) Percolation of cores (diam. 4 cm, length 9 cm) of permeability close
to 1 mD.
b) High rates of circulation in the cores (of the order of cm/s, similar
to those encountered in the reservoir in the vicinity of screens.
c) Pressure in the circuit up to 220 bars, with the possibility of
establishing a counter-pressure down-flow of the cells enclosing the
cores.
d) The removal of particles from one sample and their deposition in a
second can be studied.
e) Connections are built in for injection of a tracer, attachment of a
particle counter and collection of gas.
f) Fluid can be thermostated above the cores so that both production and
injection conditions can be simulated.
g) A differential pressure sensor on the first cell increases the
precision of the absolute pressure sensors.
h) Continuous recording of key parameters (flowrate, pressure,
temperature), surveillance of clogging of samples and filters, and
safety devices enabling the pump to be stopped.
i) Installation in an independent transportable unit able to operate on
any operation site by direct connection to the geothermal circuit.
The operation of the entire percolation system and the functioning
of the geothermal exploitation are monitored by a multichannel data
acquisition unit, a computer and a teletransmitter.
2.2. Description of the water treatment loop
The object of this pilot unit is to select and optimise systems of
physico-chemical treatment of the fluid. It includes all possible types
of treatment of a fluid: circulation (volumetric pump), regulation of
pressure (offtake), degassing, settling, injection of chemicals etc.
(injection pump, mixer) and filtration.
Two measurement systems are allowed for, on entry to and exit from
the system, for analysing and quantifying the changes in the fluid.
They include sensors for pH, Eh, pressure and temperature, and allow
connection of any other measurement system (particle counting, gas
content, etc.).
3. RESULTS OF PERCOLATION TESTS
3.1. Recapitulative table
Sixteen cores were tested in all during
tests made on the Helleray and Villefranche
conditions are given in Table 2.
the various
sites. The
percolation
experimental

423
figure 2. Diagrammatic repreaentation of percolation loop
• •
'U .... f ...... 10" ca..., ..... I) ..... II -_
c;. ........
..... 'r .. .,. .................. IJ "-t,u .. '.'-(IoP'
s....~t
ek ............ (P_UIIt, -.--.)
NI .......'IU.II ..............,...........
1.'." wei ... (lO,
_
".. .....
I., .., " .......... (22)1t, 17
!o I Cellllfl'. _II eM""" .edt .......
figure J. Water treat.ent diagram
II
.....
__ (lXIIt)
: 1 ...·,.. jT. .....
c Itt)KU .. 01 ....
.. t.~._
141 o.r. ..... PO'PI···· Pt.uw .......
0.-._ (tP)
.. (...-)
Of'
01' ......... ,. ....... __
'0' " •... '-... .. "'.--.
N1·H Z
c;.. ..... I ......
I. _lG-UI.) CI,C Z
~_heat .... ,.. ......
__ --
7,' Pt .... ,..........................
10 It
,
II
II
_-
~ .... _.t.... ,..t_
• n...-tot ".. .....
,-
II Clot ............... 10
,--
ChoIoo _
U,I7 .......... k:,...... n -_
.....

424
Site
Experiment
n'
Mawrtau
""' ... ty
I Gradell VUIII" 12
Il
Core length
(nwn)
90
flowra~~otal
A __
Lft~::o"' Pru.:.....
_(;~,g;lUty tempel'otute
duration ('e) (1') (1')
00/ 1&0'200 1&
3&h
M
K
I.
I-
~:
K
A
V
2 Grades V01Ilcli 12
Il
3 Gr~dc.V""lVII 12
Il
4111 AcruliLh 20 49
& GrhGI:II VUIIKe"a 13
II
6 Cre. dn VOMIL .. 12
Il
•
Gm.6 meul~ 23
9 Grl!s"mculct 23
90
16
90
90
1&
90
90
UOO/ 1&0'200 SO
l80h
12·2&·315:11 1&0'200 SO I 1,1
74h
2·48 12·24·37·SOr' 30
ISOh
Z-4·8-12·251 &2-M SO U 10
39&h
2·4·8 12·251 110.110 SO U 10
U2h
41l1lh U'U U I 1,2
10114 h 8.$08.0 32 I 1.2
IU Grc.amt:uln 23
30
Ol38h to" 6.& 28 U 1,2
V
I
I.
I.
~:
~'
K
A
N
C
H
K
20 Grl'." mcule. 23
21 Grh.6mcule. 23
2~121 GrclI H mcuicii 23
23 &re... 22
IIoVIndsLoneli
24 (21 Bercll 22
I>Mndl>wneli
25(31 HcrCII 22
lNInru.LDnn
26131 Il.cr-ea 22
.... nd"t.,unL ..
40
30
311
30
90
90
90
2122h I U U 0,1
2I2Oh 0,8 U I 0,1
316h 2 23,& I 0,1
BI 80 U I 0,1
25h eL 1,2
\II 200 24 I 0,1
43h eL 1.2
101 90 118 I 0,1
1,5h ell.2
101 80 U I 0,1
3h
I'll.2
III Town water ~upplr
121 Artificially Noel $oturoted water
131 Woter with oxidant (NoOCI)
Table 2. Results of percolation experiments at Helleray and Villefranche
3.2. Geological monitoring of the percolation experiments
Both the industrial material (Aerolith) and natural samples (clean
and clayey sandstones) to be tested were selected according to their
petrological characteristics. They were investigated by chemical
analysis, X-ray diffractometry and in particular chips were examined by
the scanning electron microscope and porosity was measured using the
mercury porosimeter before and after the percolation experiments, in an
attempt to be able to visualise the degradation of the pore spaces
inferred to be reflected by the observed reductions in permeability.
The relative variations in mercury porosity appear to owe as much
to natural variations in the material from one point to another as to
the percolation tests themselves.
No variation in the shape of grains and pores was clearly
established, either in the Hill Sandstones or in the Berea Sandstones,
which appear unaffected by the injection tests. Large changes were
however observed in the Vosges Sandstones. Before percolation (Photo 1)
the pore spaces are clearly visible and sharply defined in all their
fineness and complexity. Overgrowths of authigenic quartz with crystal
faces, hexagonal crystals of kaolinite arranged in fine columns and
honeycombs and coatings of illite are readily identifiable in their
finest detail. After percolation, however (Photo 2), the same sandstone
is unrecognisable. A thick coating of clay minerals clogs the spaces
and all detail of shape is obscured. The pore space appears to have
been reduced by breakdown of the illites and the deposition of colloids,
possibly those in suspension in the percolation fluid.

4
.3. Variation in peneability of cores durin& tests 8, 9 aod 10
The variation in permeability of the cores is shown on PiguTe 4.
yhere it can be seen to decrease progressively Yith time, though the
detailed variation is affected by the percolation flowrate and the size
ot the pores in the upstream filter.
CRES DeS VOSGES - Carri~re de Bust ( Bas-Bhill)
PhOtO 1. HE!
Before percolation
Appearance of pore system
(1) Authigenic quartz
(2) Kaolinite
(3) Illite (honeycomb and
coating)
Note the fineness and
clarity of pore spaces
Photo 2. HE8
After percolation
Clogged appearance of pore
system
Grains covered with a thick
coating of clay minerals
KXPERIKBftT 8. Perm bility (K) decTeased by 20% in 3.5 days. from 9.6
to 7.7 m1l11d rcies. An injection of nitrogen made at the end of
th expe 1ment covered the surface of the core Yith bubbles and
c used n abrupt lowering of permeability fro 7.7 to 7.0
mi1l1darcics.
!RIMBNT ,. The !louy te was increased x2.S. A very rapid decrease
o n tisl perme&b11 ty Ko at fiTst. was followed by a much slower
deere se. but the e~periment ~ halted after hours because the
upat cam pressure ncreased to 215 bars.

426
EXPERIMENT 10. The flowrate was reduced to 4.5 kg/h, but using a 25~
filter. An abrupt decrease of permeability from 9.8 to 6.3
millidarcies (36%) occurred in 24 hours.
The results of the hydraulic tests show:
• the importance of the effect of filtration of fine particles upon
the variation in permeability of the core;
• the sensitivity of AP to variations in temperature;
• the importance of the presence of gas in the fluid •
•• 1'1 .. :
1..-:::,. / '''PK -gem-.kKlh- Upstream fIIt.r 11'1
.~ \ . :;:;i':-~';"':\::_:':'l::/'''~/'-'-' \
.. \

427
necessary to test a number of oxidants and a variety of experimental
conditions in order to develop an oxidation procedure. Two kinds of
conclusion have resulted:
a) Chemical. NaOCl has been shown to be the fastest-actins and most
efficient in closed cell, but large amounts of oxidant would be
necessary for a continuous-flow fluid.
b) Technical. Installation of a settlins tank at the level of the
chemical injection pump in the pilot setup, by prolonsins the reaction
time and the use of a more restricted medium, would provide better
conditions for iron oxidation and the aggregation of particles.
The on-line tests also showed the necessity for installation of a
different device for filtration after oxidation. The classical system
of frontal filtration was shown to be inefficient. The integration of a
system of tansential micro-filtration in the pilot scheme would
therefore increase its capacity owins to the reduced risk of cloggins.
4.2. ModelliDl of deSaBsiDl and the chemical variation of the fluid in
the Helleray seothermal loop
The complete degassins of the fluid that is recommended before
reinjection requires that particular attention be paid to the risks of
scalins in the geothermal loop, mainly because of the increase in pH
entrained by the degassins. Scalins has been evaluated usins the
TPDEGAZ geochemical model (Czernichowski-Lauriol, 1986, 1988) on the
basis of two exploitation schemes, according to whether the fluid is
desassed before or after the heat-exchanger.
The results of modellins show that there is a risk of calcitescalins
in the surface equipment and in the reservoir consequent upon
reinjection; but this risk is reduced if the fluid is degassed after the
exchanser. Given, however. that the maximum degree of saturation
remains low, the use of crystal srowth inhibitors should suffice in
scalins prevention.
5. CONCLUSIONS
The considerable research carried out at Helleray and at
Villefranche have enabled the main lines of physical and chemical fluid
treatment. required to improve the injectivity of the Triassic
clayey-sandstones, to be defined. In view of the chemical homogeneity
of Triassic fluids (high ferrous iron, hiSh gas/liquid ratio) the
results should be transposable to other European sites. Treatment should
include the followins steps:
a) DeSassins at atmospheric pressure with separation of the saseous
phase. This avoids one of the major causes of injection difficulties
clossins of the reservoir by sas bubbles. The risks of scalins that
this is likely to entrain are minimised if this step takes place after
the heat exchanser.
b) Oxidation. This enables agsregation of fine colloidal particles so
as to facilitate settlins and filtration. Javel water (NaOCl) has been
shown to be a sood oxidiser that can sive fast, controlled oxidation.
c) Settlins. This eliminates the heavier particles, facilitatins
subsequent filtration. This can be done either before or durins the
stage of oxidation. In the latter case, oxidation and the enlargement
of particles is favoured by a lonser reaction time and a more restricted
medium.
d) Filtration. This eliminates the remainins particles. Tansential
filtration on a ceramic membrane appears to be more useful than frontal
filtration.

428
e) Injection of inhibitors.
• Corrosion inhibitors, to eliminate those particles directly
attributable to corrosion;
• crystal growth inhibitors, to prevent mineral deposition that may
occur as a result of degassing.
Pilot water-treatment and percolation setups were designed and
tested during this study. It is essential that these be used for any
project for the exploitation of Triassic fluids in order to test the
effectiveness of the treatment on a fluid in continuous flow and to
adjust the various parameters that need to be controlled.
6. ACKNOWLEDGEMENTS
It has been possible to carry out the Triassic research programmes
thanks to the invaluable assistance of M. Brach, S. Detoc, A. Menjoz,
F.D. Vuataz, D. Robelin and P. Marteau. The authors thank the AFME,
BRGM and CEC, who financed the studies, and Gaz de France, who aLLowed
access to the Villefranche site VR52. J. Kemp, BRGM, is thanked for the
English translation.
7. REFERENCES
CAUTRU J.P., ROBELIN Ch. (1984) .- Etude d'un r~servoir argilo-gr~seux.
Cas du Trias du doublet g~othermique de Melleray (Loiret) .- BRGM Report
84 SGN 124 IRG/GEO, 34 p.
CZERNICHOWSKI-LAURIOL I. (1986). Degassing of geothermal fluids: a
geochemical model - Geothermal Resources Council, TRANSACTIONS, 10,
113-118.
CZERNICHOWSKI-LAURIOL I. (1988)
Mod~lisation de l'~volution de la
chimie des fluides g~othermaux lors de leur exploitation par forages
These de Doctorat de l'Institut National Poly technique de Lorraine,
Document du BRGM n D 159, 196 p.
DETOC S. (1987) - Etude des particules et de l'oxydation du fer dans Ie
fluide g~othermal du Trias a Melleray - Rapport de stafe d'ing~niorat
ISIM-IMRG.
DETOC S. (1988) Etudes pr~liminaires de traitement des fluides
g~othermaux (cas du Trias a Melleray) Rapport de stage de DEA
"Sciences de I' eau et am~nagement", 50 p.
FOUILLAC C., VUATAZ F.D., BRACH M., CRIAUD A. (1988) Water rock
interactions in a Triassic sandstone aquifer. Detailed study of a low
temperature geothermal system, Melleray, France.- Proceedings of the
contractors meeting and workshop on geochemistry, Antwerp, 5 november
1986, report EUR 11362 EN, 51-62.
VUATAZ F.D., CZERNICHOWSKI-LAURIOL I., FOUILLAC C., DETOC s. (1988).­
Chemical study of a low temperature geothermal fluid in a Triassic
sandstone aquifer : scaling potential and fluid treatment (Melleray,
France) .- Workshop on deposition of solids in geothermal systems,
Reykjavik, Island, Aug. 16-19, 1988, 8 p.

Contract EN3G-0032-F
STUDY OF THE VARIATIONS IN PERMEABILITY
AND OF FINE PARTICLE MIGRATIONS
IN UNCONSOLIDATED SANDSTONES
SUBMITTED TO SALINE CIRCULATIONS
J. BAUDRACCO - Universit6 Paul Babatier,
Laboratoire de Min6ralo.ie, U.A. 67,
39, al16es Jules Guesde, F-31400 - TOULOUSE.
ABSTRACT
Bamples of unconsolidated clayey sandstone were submitted to
percolations with NaCl and CaC12 solutions with ionic stren.ths I =
0.01 and I = 2 at 20, 60 and 90·C.
The permeability deoreased as a function of time for all the
samples examined. When the temperature rose from 20 to 90'C,
permeability decreased for the I = 0.01 solutions, but it increased
for the I = 2 solutions. The fluid oirculations were accompanied by an
entrainment of fine particles, that was all the .reater as the
solutions beoome more diluted and fluid.
This behaviour, explained by the phenomenon of clay
flocculation-defloooulation, is .overned by the values of the
attraotion and repulsion potentials between particles. The calculation
of the forces present show that the electrokinetic phenomena .overn
flooculation and mi.ration of fine particles in sandstone.
INTRODUCTION
The exploitation of oil wells and leothermal doublets always
shows a deorease in flow rates with respect to time. It is, therefore,
important to understand the permeability variations under the effect
of saline solutions and to determine the oonditions under which the
deterioration of reservoirs oocurs durin. exploitation SO that they
may be avoided or reduced.
Followinl examination of consolidated sandstones (Baudracco and
Tardy, 1983) the study of the permeability variation mechanisms was
oontinued by submittinl unoonsolidated sandstones to v.ried saline
solution oiroulations.
1. DESCRIPTION OF THE SANDSTONE
The sandstone studied was a Triassio sandstone taken near Larne
(Northern Ireland) at a depth of 1245 m. It consisted of 95X quartz,
2X feldspars, IX halite and 2X fine particles of which 0.7X was
illite, 0.2X smeotite, O.lX kaolinite, the remainder mainly consisted
of quartz. The sandstone was reduced to powder by crushinl it in a
mortar. The powder obtained consisted of 2X partioles < 2~m, 6X
between 2 and 63~m, 68X partioles between 63 and 125~m and 24X
partioles ) 125 ~m. The CBC of the fine fraction was 50 meq/l00 I •.
After leaohinl, the dried and homolenized powder was placed in
the peroolation oell and oompacted so as to oonstitute a bed with a
porosity of 39X. As the powder was renewed for each test, the
permeability to air was measured in each case before percolation; it
varied between 805 and 840 mdy whioh indioates a hilh delree of
homo.eneity of the beds.

430
2 - DBSCRIPTION OF THB APPARATUS
The percolation apparatus (Baudracco, 1978) consists of a
pressure generator and regulator, a sample holder, instruments for the
continuous measurement and recording of rock permeability and filtrate
flow rate.
The powder in the sample holder was placed on a membrane whose
average pore size was 1.2~m (Fig. 1).
~~~~~\S~~---distributing
ferrule
~~~~~~~~ _____ water-tightness joint
~~~~~::~-protection grill
------thermocouple
T'-ofH1' ....... "'---unconsolidated material
~~~~~~~~::===filter
= frit
collector ferrule
3. ANALYTICAL PROTOCOL
Figure 1. Schematic diagram of the sample holder
for a powder bed
The sandstone was submitted to circulations of NaCI and CaCI,
solutions with an ionic strength of I = 0.01 and I = 2 at 20, SO and
90·C.
The sample, previously impregnated with the solution used, was
submitted to a percolation pressure that was selected so as to obtain
an initial filtrate flow rate of approximately 10 cm l per hour. The
tests lasted for 25 hours.
4. RBSULTS
4.1 - Permeability variations with respect to time.
Generally speaking, the permeability to water decreases with
respect to time and stabilizes after approximately 20 hours. So, we
have retained the permeability coefficient measured after 25 hours of
experimentation (k, ; Table I).
4.2 - Permeability variations with respect to temperature
The permeabilities measured after 25 hours (k,) decrease for
solutions at I = 0.01 and increase for those at I = 2 when the
temperature is increased. For I = 0.01, the reduction is linear ( k =
-1.S4T + 149) for NaCI, it is hyperbolic (k = 4300/T -45) for CaCl,.
For I = 2, the increase is therefore linear for NaCI (k = 0.35T -
1.37) and hyperbolic for CaCl, (k = -584.1 lIT + 39.15).

431
Table I. Values in millidaroies, of permeability coefficients
measured after 25 houra testin, (k,) and ratios (H) of
k, I •••• I/k, I" coefficients.
Solution
IIf (JDdy) IIf (JDdy) R
I • 0,01 I • 2
20'C 170 10 17
N
...
u 60'C 26 29 0.9
~ 90'C 3.2 33 0.097
20'C 117 5.5 21.3
...
u 60'C 50 20 2.5
z 90'C 2.5 30 0.083
"
4.3 - Permeability variations with respect to concentration.
For a ,iven cation, when the ooncentration increased from I =
0.01 to I = 2, the permeability decreased (H = k'I ••.• I/k,I •• > 1) for
solutions at 20'C, but inoreased (H ( 1) "for solutions at 90·C. At
60'C, the solutions aoted in opposite directions (H > 1 for NaCl and H
( 1 for CaCl.).
It oan be noted that, for all the tests performed except for
that with CaCl. at I = 0.01 and 60·C. the permeabilities to CaCl. were
,reater than those to NaCl : k,Ca > k,Na.
4.4 - Entrainment of ,els of fine particles in the filtrates.
The filtrates oolleoted durin, the tests were sometimes reddish
in oolour due to the presenoe of siliooferrio ,els. The followin, were
systematioally obtained (Table II) :
- ,els, in the first two fraotions oollected at 20'C and in the
first fraction oolleoted at 60'C for the solutions at I =
0.01. But, no ,els were present in solutions at 90'C nor in
those at I = 2.
- fine partioles «1.2~m) in the first fraction collected at
90'C for I = 0.01, at 60'C for I = 2, in the first two or
three fraotions for the other solutions.
So ,els and mineral partioles were not present in filtrates when
the temperature and the solution ooncentration inoreased.
5. INTERPRBTATION
1 - Permeability
Permeability appears to be a factor that depends on the porous
environment. variable with time, the nature and the conoentration of
salt and the temperature of the solution. These variations have
already been analyzed (Baudraoco and Tardy, 1987).

432
Table II. Weight (in mg) of particles collected in the first
fractions of the filtrates j fractions containing a
reddish gel.
*
Solution
Particle weights in the successives
fractions (mg)
2 J. 1:
90·C 36 0 0 36
CaC1 2
60·C 41* 2 44
....
20·C
~
46* 2* 49
0
90·C 10 3 0 13
H
NaCl 60·C 27* 7 3 37
20·C 98* 20* 11 129
90·C 0 0 0 0
CaC1 2 60·C 4 0 0 4
N 20·C 7 0 8
u
H 90·C 0 0 0 0
NaCl 60·C 3 0 0 3
20·C 11 0 12
5.1.1 - The decrease in permeability in time has been observed
by virtually all investigators in the field. The explanations put
forward (adsorption of the liquid on the walls of the porous
environment, accumulation of fine particles, swelling of olays,
interaction of colloids) are varied. In our opinion, it is the result
of the displacement of free fine particles which block the finest
pores and the physisorption of water on the clays.
5.1.2 - For the majority of reports, permeability decreases when
the temperature increases. The phenomena mentioned (roughnesses
created by the thermal stresses, expansions, quartz-water
interaotions) cannot explain these variations.
5.1.3 - The bibliographic information concerning the influence
of the concentration is considerable and sometimes oontradictory. This
parameter seems to be closely linked with the methodology and with the
order in which the solutions intervene.
In our opinion, the permeability variations are due to different
arrangements of the clay particles within the porous environment. This
parameter appears to be governed by the flocculation-de flocculation
mechanism of clays as it related to the variations in the thickness of
the double ionic layer existing on clay particle surfaces.
5.2 - Flocculation-deflocculation : Reminders
Flocoulation or deflooculation of clays occurs as a
the particles' attraotion and repulsion potentials.
funotion of

433
5.2.1 - Attraction potential. In the caBe of two flat particleB
with thiokneBB 6 and separated by a diBtance 2x, the attraction
potential iB equal to (Verwey and Overbeek, 1948)
1.[1 1 2]
VA • - 48w x2 + (,.+6)2 - (x+6/2)2
A = Hamaker conBtant
It can be Been (Fi•. 2, curve V.) that the attraction potential
is only important for short diBtanceB; beyond 5 - 10 A, it iB
virtually nil.
~
.... s
.....
~
...
30
~ 10
~ -10
~
15
~ -30
(A)
-50 VA
Fi.ure 2.
VariationB in the attraction (V.),
repulBion (V.) and reBultant (V. +
V.) potential ener.ieB with reBpect
to the diBtance between particleB
for NaCl (I = 0.01, 20·C).
5.2.2. RepulBion potential. ThiB dependB on the nature of the
eleotrolyte and on the interaction of particleB.
5.2.2.1. For a mono-monovalent electrolyte (NaCl) and in the
oase of two partioles in interaotion it is .iven by :
u
z-u
Kx • 2 • - 2
[
,(.-u. . 2) - ,(.-u. arc ain e - -2-
)]
where :
u •
itT
I Z
.~.
itT
elliptio inte.ral of the firBt de.ree caloulated from Jahnke-Bade
table ••

434
On the outside of the particle, the potential is
eu/ 2
• _e;:..z_I_2_+;.....:._+~[.!.:( e:. ẓ _/_ 2 _-~1~) ..::e~-Kx_~]
where: 'x, the potential at distance x ; '0 , the surface potential
; e, electron charae ; v, ion valency; k, Boltzman constant; T,
absolute temperature; E, dielectric constant of the solution. 11K
is the thickness of the double ionic layer.
5.2.2.2. For a mono-divalent electrolyte (CaCl.) and in the case
of two particles in interaction, the potential is aiven by
2a [
Kx .. - --- C F (k':!!2 ) - F (k. arc sin }.o)]
4Jaa3+1
where a = e U ; b = e" ;}.0.~q2-a+b/q; q2. (4a3_1+~aa3+1)4a2 ; kasin a;
F : elliptic intearals accordina to Jahnke-Bmde notations,
calculated from numerical tables
On the outside of the particle, the potential is aiven by
{3 [(e b / 2 +1/3) + (eb/2 - {3) e- Kx ]
e a / 2 • ----~----------~--------------~
(e b / 2 + V3) - [(eb/ 2 - V3) e-Kx ]
where, in this case :ea/2z~2eU+l; u· ~1jIx; eb/2.~2eZ+l; z .. ~o: K~e::i'2)1/2
x : distance from the wall.
In the hypothesis that all the superficial charges of the clays
are saturated with cations, the potential variation curves are
represented for NaCl ( to = 140mV) and CaCl. ( '0 = aOmV) in Fiaure
3.
a
, ..
> 0
~ 150 "
to ''':
:;1
H
~ 100
~
'0
50
50 100
DISTANCE (A)
Fiaure 3. Variations in potential with respect to distance
a) flat double layer in interaction: 1) NaCl ; 2) CaCl.
b) flat double layer: 1) NaCl ; 2) CaCl.

435
The reaultant potential (Fia. 2, Curve VA + VI, tor NaCl) ahowa
that the toroea ot attraotion are the stronaer tor ahort diatances,
and that beyond theae distances the torces ot repulsion dominate.
On the outside ot the partioles, the potential is not
partioularly sensitive to the ettects ot heat, but the intluence of
the oonoentration is very olear (Fia. 4) j this is also the case
concernina the thickness ot the double ionic layers (Table III).
Thus, the ionio toroe, more than heat, encouraaes the
tlocoulation ot olays. Furthermore, and in aareement ~ith the
experimental results, this appears to be areater with calcium than
with sodium (Fia. 4, Table III).
150
..... ~
I 100
80
• NaCl I • 0,01,
• "
t "
A NaCl I • 2,
o CaCl2 I • 0,01,
A CaCl2 I • 2,
20·C
60·C
90·C
20·C
20·C
20·C
50
25
50 75
100
DISTANCE (A)
Fiaure 4. Variations in potential with respect to distance tor
the tlat double layer tor NaCl, I = 0.01 at 20'C (e),
60'C (.), 90'C (t) j NaCl, I = 2 at 20'C (A) and CaCl.
tor I = 0.01 (0) and I = 2 (A).
Table III. Values (,10- 11 m) ot 11K "thickness" ot the double
layer tor the studied solution.
NaCl
CaCl2
I • 0,01 I • 2 I • 0,01 I • 2
20 30,6 1,83 21,5 1,44
60 29,5 1,87 21,0 1,43
90 28,7 1,89 20,4 1,41

436
5.3 - Migration of fine particles
In porous envtrDnments the potentials are submitted to three
different forces (Khilar, 1981; Houi, 1986) : the hydrodynamic force
Fa ; the force of attraction, FA ; and the force of repulsion F. '
(Fig. 5). Their respective intensities have been evaluated by'taking
into account, in order to simplify the calculations, a spherical
particle in interaction with a plane wall (pore wall or large
particle) •
Figure 5. Schematic diagram showing a clay
particle attracted to the pore wall.
(After Khilar, 1981).
5.3.1. The hydrodynamic force depends on the form and the
dimensions of the particles and of the collector (wall). Various
experiments have been proposed (Table IV). The values calculated
taking into account our experiment~l
conditions (flow Q = 10 om"/h,
mean velocity V = Q/S •• ; S = cross section; • = porosity) are
relatively low, with three authors, F./r. is 1.8 10-' N/m. With high
velocities, of the order of magnitude of those encountered in
exploitation (F./r.)., _ 4.4 - 8.8.10- 4 N/m.
Table IV. Formulae and values of hydrodynamic force F. (~, V
viscosity and velocity of liquid; r. particle
radius ; fIR) : function of particle and collector
radius) •
Authors
Formulae
Calculed values
STOKES
TARDOS et a1 (1978)
GOREN (1970)
..
GOLDMAN et a1 (1967)
O'NEILL (1968)
¥HILAR (1981)
FH - 6 11 rp \1 V
FH - 60,87 rp \1 1,5V
FH - 6 11 rp \1 feR)
FH - 1,7.6 11 rp \1 V
1,07 .10- 7
5,1 .10- 7
0,71 .10- 7
1,82 .10- 7
1,81 .10- 7

437
6.3.2. The forces of attraction and repulsion are derived fro.
the oorrespondin, potentials (F. = dV./dx, F. = d V./dx). For the
model retained (spherical particle captured by a flat collector) the
formulae proposed by Honi, and Hul (1971) and by Khilar (1981) were
used
(A : Hamaker constant)
FR - 128 wrpkT [y2(1+ ~ y2) (e-Kx/1+e-Kx) - ~ (O,75Kx/eos h4 ~x +
1,5 ain h ~ leos h 3 ~ + tan h ~ - 1 ]
= tan h (Z/4) ; Z = Ve •• /kT
where 'Y
thickness of the double layer.
•• : zeta potential
11K
F. depends on the nature, the temperature and the concentration
of the solution as well as on the distanoe between particles. The
values of F. are low (Fi,. 6) so that, for distances < 20 A, the layer
represent in, the variations of the total force (FT = F. + F.) is very
olose to that of F ••
.....
B
-Z
...."
!.
I
0,1
0,05
..
""
..
-III:
0,01
-0 ,~1-r-=::::;:::::9~~~--
20
DISTANCE (A)
Fi,ure 6. Variation in the attraction (F.), repulsion (F.) and
resultant (F. + F.) force with respeot to the
distanoe. For comparison (F./r.) •••• 10-'N/. ;
(F./r.) •.•.• 10- t N/. ; (F. : hydrodynamio force; exp
experimental; H.V. : hi,h velocity) ; F.a calculated
with A = 2.10-1IJ (Verwey and Overbeek) and
A = 1.78.10-··J (Khilar).

438
Therefore it appears that the hydrodynam}c force is much lower
than the electrical forces: (FI)••• only becomes equal to FA for a
distance equal to 0.4~m and (FI)I', at 90 A.
Thus, in our testa, the eleotrokinetic forces are preponderant
and ,overn the behaviour of the porous environment. For the
concentrated solutions, the double ionic layers of olay particles are
reduced, thus decreasin, the repulsion forces and increasin, the
attraction forces : at impre,nation of the samples the flocculant
particles and the environment behave as if they contained only small
quantities of clays. It can thus be understood that, for NaCl and
CaCl. at I = 2, the permeability increases with the temperature as a
consequence of viscosity. In this oase there is only a small quantity
of particles in the filtrates.
For diluted solutions, the repulsion forces are the more
powerful at low temperatures, the clays remain dispersed and the
finest ones can be transported by the liquid flow. When the
temperature increases, flocculation occurs. It is then normal that,
for these solutions, lower permeabilities and quantities of entrained
particles are found at high temperatures than at low temperatures.
6. CONCLUSION
The behaviour of unconsolidated sandstones submitted to the
percolation of varied saline solutions was studied.
In all the tests, permeability decreased with time and depended
on the type of salt, its concentration and its temperature. Durin,
leaching, the sandstone freed fine partioles with ,reater ease at low
temperatures and at low ionic strengths than at high temperatures and
at ·high ionio strengths.
It has been possible to explain these permeability variations
and particle migrations by means of electrokinetic phenomena and of
forces present in the environment.
BIBLIOGRAPHY
Baudracco, J. (1978) Contribution a l'~tude de l'alt~rabilit~ des
roches sous l'action des eaux naturelles. These Sc., Toulouse,
241 p.
Baudracco, J. and Tardy, Y. (1983). Influence de la temp~rature, de la
nature et da la concentration de solutions aqueuses sur la
perm~abilit~ de gres. Rapport des Communaut~s Europ~ennes,
Ener,ie, EUR 8766.
Baudracco, J. and TARDY, Y. (1987) Variations de la perm~abilit~ de
reservoirs 'r~seux soumis a percolation de solutions saline9.
Essais conduit sur ,res meubles. Sci. Geol. Bull., 40, 4, p. 313-
330.
Goldman, A., Cox, R. and Brenner, H. (1967) Slow viscous motion of a
sphere parallel to a plane wall. Chem. En,. Sci., 22, 637-645.
Goren, S. (1970) ~he normal force exerted by creepin, flow on a small
sphere touohin, a plane. J. Fluid. Mech., 41, 3, 619.
Honi" E.P., Mul, P.M. (1971) Tables and equations of the diffuse
double layer repulsion at constant potential and at constant
char,e. J. Colloid Interf. Sci., 36, 258-272
Houi, D. (1986) Modelisations exp~rimentales et th~oriques de
l'accumulation de particules a la surface d'un filtre. These
Soience, INP Toulouse. . .
Khilar, K.C. (1981) The water sensitivity of Berea sandstone, Ph. D.,
Univ. Microfilms Intern ••
O'Neill, M (1968) Adhesion of small spheres to plane surface. Chem.
En,. Sci., 23, 1293-1300
Tardos, G., .Abvaf, N. and Gutfin,er, C. (1978) Dust deposition in
,ranular bed filters, theories and experiments. J. Air Pollution
Cntol Assoc., 28, 1, 354-360
Verwey, E. and Overbeek, J. (1948) Theory of the stability of
lyophobio oolloids. Elsevier, Amsterdam.

439
Contract EN3G-0032-F
VARIATION IN THE PERMEABILITY AND CATION EXCHANGE KINETICS IN
A CLAYEY SANDSTONE SUBMITTED TO PERCOLATION OF DIFFBRBNT
SALINE SOLUTIONS
M. AOUBOUAZZA and J. BAUDRACCO
Universit~ Paul Sabatier, Laboratoire de Min~ralo~ie,
U.A. 67, 39, all~es Jules Guesde, F-31400 - TOULOUSE.
ABSTRACT
A sample of Berea clay was submitted to successive
percolations of CaC1I, MgC1I, KCl and NaCl solutions with an
ionic strength of I = 0.01 at 20·C.
Generally speakin~, when the solution was changed there was
a sudden variation in permeability. The exchanges of cations
with the same valence (Cal+/M~I+, K+/Na+) seemed to be
governed by Lan~muir type "isotherms" ; as far as the
exohan~es with different valences (Mgl+/K+, Na+/Cal +) were
concerned, they were more complex. In the first case,
adsorption and desorption velocities were the same and were at
their maximum at the start of the tests; in the second case,
the exchange was slower and the monovalent cation had the
hi~hest velocity in all cases.
INTRODUCTION
Durin~ exploitation of geothermal or oil-bearing wells, the
variation in fluid composition is often accompanied by a
variation in the rate of production. In order to understand
this phenomenon, percolations were performed with different
saline solutions on a clayey sandstone. The results obtained
are ~iven below.
1. MATERIALS AND ANALALYTICAL PROTOCOL
The experiments were performed on a Berea sandstone
oonsistin~ of 85~ quartz, 5~ feldspaths, 1~ carbonates, 2~
opaque minerals and 7~ clays includin~ approximately 5~
kaolinite, 1~ illite and 1~ montmorillonite and mica needles.
The sample studied (40 mm diameter and 40 mm hi~h)
permeability to air of 600 mdy and porosity of 21~.
had a
It was submitted to successive percolations of CaC1I,
M~Cll' KCl and NaCl solutions with an ionic stren~th of I =
0.01 and then of water at 20'C usin~ an experimental device
that has already been presented (Baudracco, 1989). The
quantity of cations present in each of the fractions of
filtrate collected was determined. The experiments, performed
at a constant percolation pressure that was set so as to
obtain an initial filtrate flow rate of 10 cm'/hour, lasted
for 25 hours.

440
2. RESULTS
2.1 - Permeability
Permeability to water decreased with time in the case of
exchanges of cations with the same valence (Caz+/Mgz+,
K+/Na+), but increased.in the case of exchanges of cations
with different valences (Mgz+/K+ and Na+/Caz +). It was also
noted that when the solution was changed there was a sudden
variation in permeability (Fig. 1).
2.2 - Cation adsorption and desorption
Figure 1 gives the variation curves for the concentrations
of cations held.back or freed during the exchanges. These
curves are related to ·isdtherms and we will call them
"isotherms" as our dynamic system is not in a state of
equilibrium.
The Caz+/Mgz+ and K+/Na+ exchange "isotherms", i.e.
exchanges of cations with the same valence, are symmetrical
and of the Langmuir type, whereas the Mgz+/K+ and Na+/Caz +
exchanges were highly assymmetrical in the monovalent cation
direction. :
The quantities of cations held back by the porous
environment (in milliequivalents per liter of filtrate) were
1.61, 1.57; 9.81 and 9.21 respectively for Caz +, Mgz+, K+ and
Na+. They were roughly the same for the monovalent cations (=
1.6) and for the bivalent cations (= 9.5). It then becomes
apparent that 1 Ca Z + was replaced by 1 Mgz+, 1 K+ by 1 Na+,
but that 1 Mgz+ was replaced by 3 K+ and 3 Na+ replaced 1
Ca Z + •
Furthermore, for the cations with the same valence, the
adsorption and desorption velocities were the same and were at
their highest at the start of the experiment (Fig. 2). This
maximum was higher and the exchange took place at a higher
veloci ty for monovalen:t cations than for bivalent cations. In
the case of cations with different valences, the exctanges
were slower and the adsorption or de30rption velocity of the
monovalent cation was higher than the desorption or adsorption
velocity of the bivalent cation.
3. INTERIRETATION
3.] - Permeability
At the present stage of our work, it is difficult to
explain the permeability variations obtained. However it
should be noted that, with the exception of the Na+/Caz +,
permeability increases with time when a cation is replaced by
a cation with a smaller hydrated radius - Mgz+ (rB = 0.345
nm)/K+ (ra = 0.124 nm) - and that in the reverse situation -
Ca Z + (ra = 0.317 nm)/MgZ+, K+/Na+ (r. = 0.183 nm, Sutrll, 1946)
- it decreases. Study of the influence of these substitutions
on the mean di~ensiori of the pores is currentl~ under way.

~I
3.2 - Cation adsorption and desorption
The symmetry of the adsorption-desorption "isotherms" of
the cations with the same valence should permit the
application, for these exchanies, of the laws relative to ion
exchanie equilibrium in the double diffused layer of colloidal
suspensions. This is currently beini verified.
4. CONCLUSION
The behavior of a sample of Berea sandstone submitted to
successive circulations of CaCla, MgClI, KCl and NaCl
solutions at I = 0.01 and 20·C was studied. The first results
show the close relationship between the permeability
variations and the cation exchanie phenomena at the level of
clays.
The exchanie "isotherms" for cations with the same valence
seem to show that the laws relative to ion exchange
equilibrium in the double layer linked with clays are
applicable. Our work is therefore beini oriented in this
direction.
BIBLIOGRAPHY
Baudracco, J (1989). Study of the variations in
permeability and of fine particle miirations in
unconsolidated sandstones submitted to saline
oirculations. Geothermal Update. 4th International
seminar on the results of EC Geothermal Energy
Research., Florence, 27-29 April.
Sutra, G. (1946). Sur Ie dimension des ions
~lectrolytiques. Journ. Chim. Phys. 190-326.

442
e) Ca++/Hg++ b) Hg++/K+
1
14
2 ~
:; ....
. ...J
a " B ....
w
0
""
a:l
~ w
~
z a:
a 6
B e..
w
H
I--
w
>
"" a:
....
I--
Z 6 I--
4
W
U
...J
z
"" w
a
4 a:
u
2
2
6 20 24 0 4 B 12 16 20 24
TIME (H) TIME (H)
1 n c) K+ /l>Ie +
14
>-
:; 21:;
...J
a " B ....
w
O~
~ u.:
~
z 6
a:
a
....
B e..
w
I--
a:
w
""
>
6 ....
I--
Z ~ I--
W
4
U
...J
""
z ."...,..
a
u
:;
2
) 2
0 '" 0
0 4 8 12 16 20 24 0 4 8 12 16 20 24
TIME (H) TIME (H)
e) Ca++/H30+
10 14
12
a &
w " >- I--
* KlK.
~ H
10 ...J
z
....
a
a:l
6
Ca++
H
B ""
w
I-- ~
a: a:
I-- "" w 0 Mg++
z
e..
w
u 4 6 w
z .... >
•
K+
a
U
I--
4
...J
"" w
2 a: 0 Nl
2
/' .. ...
~
~
~
0 0
0 4 B 12 16 20 24
TIME
(H)
4 a:
w
Fig.1.- Variations of relative permeability and cations concentrationa during
the successives percolations.

443
a) Ca++/Mg++
b) ++/K+
4 4
%
.....
cr
~3
3
~
H
152 2
-I
W
>
1 1
0
3 6 9 12 15 0 3 6 9 12 15
c) K+/Na+ TIf1E (H)
d) Na+/Ca++ TIME (H)
10 4
%.....
cr
A
~ 9
3
~
15
-I 2 2
W
>
1 1
~A
0 0
0 3 6 9 12 15 0 3 6 9 12 15
TIME (H)
Figura 2.- Variations of adaorption and desorption velocities of the
cations during the successives percolations.
TIME (H)

EEC contract nO EN3G-0068-F (CD)
SPACE AND TIME EVOLUTION OF THE GEOCHEMICAL PROCESSES
ARISING FROM GEOTHERMAL INJECTION IN AN AQUIFER
A. COUDRAIN-RIBSTEIN, P. MERY and A. VINSOT
Centre d'Informatique G~ologique
Ecole Nationale Sup~rieure des Mines de Paris, France
Summary
Injection of hot water in an aquifer may induce dissolution and
precipitation of minerals. The importance of these phenomena is
examined with the help of numerical experiments. The mathematical
model is the result of the coupling of a fluid flow and mass
transport code (Goblet, 1981) with a geochemical speciation code
(Coudrain-Ribstein, 1988). Simulations are presented for the case of
an interseasonal heat storage where water is injected at 60°C.
Laboratory' experiments are carried but in order to identify the
reactions which may occur during this storage.
1. INTRODUCTION
In several types of thermal exploitation of the underground, the heat
bearing fluid travels accross a field of variable temperature, for example
(i) in classic geothermal exploitation when the cooled fluid is reinjected,
(ii) in hot dry rock exploitation when the cooled fluid is injected, (iii)
in heat storage when the heated fluid is injected. The solubility of
minerals depends on the temperature. Thus, geochemical reactions may occur
during the transfer of the solution and it is interesting to attempt to
evaluate the importance of these phenomena in order to determine when they
might modify the hydraulic properties of the aquifer.
Our purpose is to present a general model capable of taking into
account transport of solutes and geochemical reactions arising from a
variable temperature field. The speciation (distribution of the chemical
elements among the different species) must be determined for accurate
prediction of reactions throughout the fluid motion.
The case study presented here, is a thermal storage in the aquifer of
the Fontainebleau sands in Saint Quentin (Yvelines, France). The projeot,
managed by the company NAPAC, can be summarized as follows : it consists of
two wells and surface installations. During the summer, the water is pumped
out at the initial temperature (12°C) from the cold well, heated on the
surface to 50-60°C and reinjected into the hot well.
During the winter, the water is pumped from, the hot well~ the heat is
extracted to warm the school buildings, and the cold water (12°C) is
reinjected into the cold well.
Several previous studies have been devoted to the quantitative
examination of the dissolution-precipitation phenomena. For example,
Desplan (1979) computed the quantity of calcite that might be dissolved or
precipitated around injection wells taking the difference between the

445
equilibrium compositions at the temperature of injection and at the initial
temperature. Because of the unsteady evolution of the temperature and the
variation of the fluid velocity around the well, this quantity may only be
an approximate average to a large volume of rock.
An accurate study of the geochemical phenomena was made by Kam (1986)
on the project of heat storage at Plaisir (Yvelines, France), where the
temperature increase is about l50'C. However, these phenomena were not
related to a precise location and volume of rock. The model presented here
makes it possible to calculate, with a given set of assumptions, the
evolution in space and time of the chemical composition of a solution in
motion and of the quantitites of dissolved or precipitated minerals.
2. MATHEMATICAL MODELLING
The model used to simulate the transfer of fluid and heat is METIS
(Goblet, 1981) which solves the balance equations by the finite element
method. Since the hydraulic regime is assumed to establish itself very
quickly, the mass balance equation is :
di v (V) • 0 ( 1 )
with V: Darcy velocity (m.s- I).
The unsteady tempersture distribution is described by the following
equation where convection and dispersion are taken into account :
-+- _ CJT
div (A· grad T - ¥ f V T) so'lf· - (2)
dt
with T : temperature
;A· : thermal conductivity of fluid-filled medium
(W ••-I.K -I) ; ~ f : heat fluid capacity (J .m- J .K-I) ; r· : heat capacity of
fluid-fUled medium (J.m-J.K-I).
The model STELE used to simulate the transfer of reactive chemical
elements is created by coupling METIS with CHIMERE, a geochemical
speciation code (Coudrain-Ribstein, 1988). Since the complexity of the full
integration of geochemical calculations into a transport code may produce
inefficient tools (e.g. Garven and Freeze, 1984), we elaborated a general
method which allow8 us to clearly pose the chemical constraints and the
equations of speciation and transport and to minimize computer time and
core storage. This method is based on the use of chemical components as
defined by Morel (1983): each chemical species is expressed as a linear
combination of components. The set of components is a mathematical basis of
the vectorial set of chemical species. The advantages of this method become
strongly apparent when the components are chosen for the specific geochemical
system under considerstion. The main features of STELE on the
basis of the example are as follows.
To define the initial state, the number Nes of the species tsken into
account is 19 :
IH2, H+, mf", HCl, H2COJ, HCO)-, COJ--, H4SiO", HJSiO,,-, Cr+, CaCO J ',
CaHCOJ+, K+, Al J+, C02(g), calcite, chalcedony, kaolinite, K feldspar}.
The number Nr of the independent reactions among them is 12, and the
number Nc of components is 7 (Nc • Nes - Nr). A chemical basis allowing to
describe our system can be, for instance :
B • {H~, H+, C02(g), calcite, chalcedony, kaolinite, K feldsparj (3)
On the one hand, the use of this notion of components sllows us to
reduce the number of unknowns when computing the speciation. Assuming that
the reactions in the aqueous phase are at equilibrium, which is true in
most cases, the activity of the secondary species, not chosen as
components, can be expressed as a function of the component concentrations,
activity coefficients and thermodynamic constants. The numbar of unknowns
is then reduced from Nes to Nc. Moreover, choosing species with a

446
prescribed activity as components reduces the number of unknowns ; choosing
species with dominant concentrations as components reduces the numerical
problems (see for example the of H20 in Miller and Benson, 1983). One
non-linear mass balance equation is established for those components whose
activity is unknown, stating that the total amount of each component is
constant, whatever its distribution among the aqueous species :
TOT(E i) = Ij "'ij C j (4)
with TOT(E i) : total concentration in the aqueous phase of component Ei ;
j : index on aqueous species; '" ij : stoechiometric coefficient of the
component E i in the species E j ; C j : concentration of the species E j.
In our example, the composition of the aqueous solution in its
·initial state may be reconstructed when water is at chemical equilibrium
with calcite, chalcedony, kaolinite, a potassium feldspar and a partial
pressure of carbon dioxide gas of 10- 1 • 5 atm. Assuming that H 2 0 activity is
constant and equal to 1, there are six prescribed conditions, for computing
the initial composition 1 H201 =1 ; Peo2 =10- 1 • 5 Icalcite 1=1
Ichalcedony ~1 ; 1 kaolinite 1=1 ; IK feldsparl=l.
Only one unknown remains, the activity of H+. With the basis (3), the
equation is : TOT(H+)=O. This first computation provides the total amount
of all other components (TOTO (C0 2 ), TOTO (calcite) , etc. where the
subscript 0 designates the initial state).
During a storage phase, when the temperature rises to 60 o C, 7 10-4
moles of calcite per liter of solution could precipitate. To prevent such
precipitation in the surface installations, 1.5 10- 3 mol.1-1 of HC1 will
be added. Thus, HC1 and C1- must be added to the previous set of species,
and the dominant species C1 to the previous basis
B' = l H20, It, C1 -, C02(g), calcite, chalcedony, kaolinite,
K feldsparJ (5)
Classic general formulas exist (e.g. Coudrain-Ribstein, 1988) to
compute the total concentrations of each component as a function of a new
basis. Inside the aquifer, we assume that the system is closed to C02(g)
and that dissolution and precipitation of chalcedony, kaolinite and
potassium feldspar may occur. The calcite is less than 0.4 % of the rock
mass, thus the equilibrium is probably reached in upper formations and we
allow only precipitation to occur. To validate this previous conceptual
system, laboratory experiments are being carried out and some results are
presented below. The equilibrium composition at the local temperature is
defined by 5 prescribed conditions: IH20 F1 ; Icalcitel=l Ichalcedonyl=l
Ikaolinitel=l ; IK feldspar 1=1.
The set of 3 mass balance equations to be solved is :
TOT(W) = TOT(H+) n-I ; TOT(C1-) = TOT(C1-) n-I ; TOT(C0 2 ) .. TOTO(C0 2 ) where
the subscript n-I designates the previous time step.
On the other hand, this notion of components allows us to reduce the
number of unknowns when computing the transport. Assuming that each aqueous
species has access to the same porosity and has the same dispersion
coefficient, it is only necessary to solve the transport equations (6) for
the total concentration of the components when TOT(E i ) varies. Taking into
account convection, dispersion and chemical reactions, they are expressed
as follows (e.g. Marsily, 1981) is :
f --+- - J ;')TOT(Ei)
div ~ D grad TOT(E i) - V TOT(E i) + wi = w (6)
. 2 -I ~t
with D=d+av : dispersion coefficient (m.s ); d : diffusion coefficient
(m 2.s- l ) ; '" : intrinsic_Idispersion coefficient (m) ; w: porosity ; ~
geochemical flux (mol. 1 .s-I) which corresponds to the transfer between
the solution and other phases and is computed by the relation :

447
1--1/dt * fTOT(E j) - TOT(E j)*J (7)
with dt*: kinetic coefficient (s) ; TOT(Ej) * : total concentration at
equilibrium with respect to solid phases. This firat order kinetic relation
is used because of ita aimplicity. However, it could be replaced by any
other availsble relation. When the mass balance equation (6) is solved it
is assumed that the diaperaivity of the pore velocity is small; otherwise
another model must be used to accurately describe the transport processes.
for instance a stochastic transport model (Coudrain-Ribstein, 1988).
In our example, computing the transfer with the basis (5), the
conditions are: TOT(H20). TOTO(H20) ; TOT(H+). TOT(C1-) ; TOT(C0 2 )
TOTo(C02). Because of the stoechiometric dependence between kaolinite and
feldspar, and because we used the same kinetic coefficient for both, it is
only necessary to compute the transfer of one of them. Several previoua
simulations have shown that the solution is always undersaturated with
respect to calcite. Thus, the total concentration of the calcite component
is constant. Finally, three mass balance equations (6) remain, those with
TOT(H+), TOT(chalcedony) and TOT(kaolinite).
At each time step, the successive calculations are (i) speciation
(set of equations 4) at the local temperature and computation of the
geochemical terms at each node (equation 7) ; (ii) solution of the mass
balance equations (6) where geochemical terms are explicitly introduced.
3. SIMULATION
Because of the symmetry around the injection well, one may limit the
simulated zone to one radian (Fig. I), with no flux through the vertical
boundaries. The porous layer is homogeneous and the parameters are :
Q - 5 m 3 .h- 1 ; h _ 50 m ; rOI •• 100 m ; dr • 1 m ; w • 0.1 ;
• - 50 m ; d - 10-9 m2.s- 1 Ol(f • 4.18 106 J.m3.K-I
K'- 2.22 lOS J.m 3.K -I ; A. •• 2.13 W.m-I.K-I ; dt* _ 1 month
with Q : injection rate h I aquifer height ; rOI. : radius under
consideration ; dr : space step.
Injaction .all, TI- 60·C, Q- 5. l /h
.. : : . . . . '. .'. : :' i' ..
::.: ;--::'::'.~::.=:;':;
: :.: :I e :- '-:' .......................... Ie 'III
b -
50 •
r ••• -
100 •
)'
Fi&Ure I : Configuration under consideration in the si.ulations
The geochemical processes in the aquifer are driven by two forces
the variations of temperature and variations of total concentration of Cl-.
The temperature front reaches 25 • after 6 aonths, when the non reactive
chellical element (Cl-) front reaches 50 ••

448
After 6 months, the balance is such that chalcedony is leached in the
15 m around the hot well and precipitated farther away (Fig. II). The
maximum dissolution is in the range of 0.4 10-3 mole per cubic decimeter of
fluid-filled medium in 6 months. With our assumptions, after 10 years, the
porosity near the injection well could increase by about 2 %. The maximum
precipitation related to a greater volume, is only 0.05 10-3 mol.dm-3 • The
maxima increase with the kinetic coefficient and for example, are about
three times greater when dt equal to 10 days.
~ in 10- 3 mo1/dm 3 of Cha1cedony
0.04
o.
- 0.10
- 0.20
- 0.30
1:
.oi.
_ It) dt
100 III
50
radius in III
0
Figure II Computed quantity of precipitated and dissolved chalcedony
after six months of injection versus distance from the well
Even if the kinetic coefficient for chalcedony is three times less
than the one for kaolinite an potassium feldspar, their involved quantities
are about one hundred times smaller. Moreover, with our set of assumptions,
the dissolved and precipitated quantities of chalcedony are independent of
the reactions with these minerals. That was demonstrated on the basis of
simulations carried out with a geochemical system limited to chalcedony and
water. But the field studies are mainly based on the study of the water
composition. Thus, in order to validate the model, it is necessary to
compute this composition that is strongly dependent on the geochemical
system.
4. LABORATORY EXPERIMENTS
Two percolation experiments were carried out to identify the actual
reactions caused by heating or acidification which occured between the
water and the sand.
Water from the aquifer was injected into a column of cored sand. The
changes in the water composition since its sampling were limited and known.
The water flowing out of the column was regularly sampled in order to
analyse the major elements (CA++, Na+, Mg++, K +, H 4
Si0 4
, S04-- , C1-, HC03-)
and mesure the pH. The temperature was automatically mesured.

449
The experiments lasted respectively for 500 and ~50 hours. After
about one hundred hours, we observed thst the outflowing water remsined
with a stable composition which was the same as the incoming one.
When this stable composition was obtsined, we heated or scidified the
inflowing water. We observed thst only the concentrstion of stlics and to a
amaller extent, of potsssium differentiated from the inflowing one. This is
quslitatively consistent with the calculations which are presented above
with a aimple chemical system.
The quotient of (K+) snd (H4Si04) in exceas over the incoming
concentration was not constant ; so we might say that more that one mineral
reacts. Thus we looked for a simple system of minerals which was able to
explain the observed concentrations. Therefor we computed, with the model
CHlMERE, the equilibrium between the analysed inflowing water and several
sets of minerala at the temperatures of the experiment.
The figure III presents the observed curves of the concentrations of
TOT(K) and TOT(Si) during the first experiment. It also presents calculated
points with the following conditions:
- equilibrium with kaolinite (Si2A120 s(OH) 4),
- conatant undersaturation of -0.2 with respect with microcline
(KA1Si30 8)
- little and constant oversaturation ( < 0.1) with respect with
cristobalite (Si02)
- closed system with respect to C02(g)
where the thermodynamic basis is those compiled by Fritz (1981) the
saturation coefficients are expressed by logarithm (equilibrium corresponds
to 0). Microcline and kaolinite were observed in the sand before
experiment.
The proposed system can approximatively reproduce the observed
tendances. The same system but with undersaturation of -0.15 with respect
with cristobalite gives a good approximation for the second experiment.
Calculations with STELE can be tried with this system even if it is not
completely satisfying. Actualy the definition of such a system is
complicated and moreover, not sufficiently constrained. But numerical
experiments in good agreement with laboratory may allow to delimite and
test a field of hypothesis.
5. DISCUSSION AND CONCLUSION
Along a flow path, the chemical composition of the fluid changes as
the chemical and physical conditions (composition of rocks or gases,
pressure, temperature) vary. Thus, the movement of the fluid allows
transport of chemical elements from one place (dissolution) to another
(precipitation). In the general case, in order to account for such
phenomena one must simultaneously take into consideration transport
processes (diffusion, dispersion, convection) and geochemical reactions. In
most cases, it is necessary to compute the speciation at least of the major
elements in order to describe the evolution of the precipitation and
dissolution phenomena. To minimize computing time and memory snd to clearly
state the balance equations, it is very useful to choose the chemical
components (mathematicsl basis) for the specific geochemical system under
consideration.

, r--j -------II
.450
Silica
concentration (10-4 mol/I)
~5r-----------------------------~
_tar at 30-C
T·SO
ecldlfk:8tlon
1
1
10
J
l
o~----~------~------~----~
290 340 300 440 490 Potassium
time (hours)
2.ocloncentration (10-4 mol/1)
__ at 30ec T·ea T·eo
.,ldlfloatlon! !
I I ~~~~
1.0
Ir~a _E:
0.5
0.0 ~
1/ obe.wd
o
______________---'-______-'-____----l
2;0 340 390
time (hours)
490
Figure III
One percolation experiment : silica and potassium concentrations
in the outflowing water versus time ; column : length
23 cm ; diameter: 6.5 cm ; flow rate: 100 ml/h

4.51
The model built accordina to this method is illustrated here by a
case study of injection of hot water to store heat in an aquifer. The
numbers derived from this simulation should be used with some caution in
view of the approximate nature of the calculations. They do show, however,
that silica transport and deposits may be significant.
The main difficulties in studying a real case are (i) to define the
active geochemical system (which reactions and reaction rates to take into
account), (ii) to account for space heterogeneity of the porous medium
(Matheron snd Marsily, 1980), (iii) to compute the evolution of porosity
and permeability as a function of dissolution and precipitation. At
present, the coupled mathematical models allow us to simulate the
associated phenomena in a rigorous way with respect to heat and mass
balances. They are the tools for making numerical experimenta allowing us
to test the importance of each parameter or assumption on the geochemical
processes. To support such a simulation, the geothermal exploitation must
provide sufficient data (field atudies, rock sample analyses, laboratory
experiments) to identity the geochemical transfer.
REP'ERENCES
1. COUDRAIN-RIBSTEIN, A. (1988). Transport d'AlAments et rAactions g60chimiques
dans les aquifAres. ThAse en Science, Univ. Strasbourg, 381 p.
2. DESPLAN, A. (1979). Les rAactions g60chimiques lora de l'exploitation
d'un doublet gAothermique. Th~se Doctorat, Doc. BRGM nO IS, OrlAans,
168 p.
3. FRITZ, B. (1981). Etude thermodynsmique et mod6lisation des rAactions
hydrothermales et diaaAnAtiques. Th~se en Science, Univ. Strasbourg,
197 p.
4. GARVEN, G. and R.A. FREEZE (1984). Theoritical analysis of the role of
around water flow in the genesis of stratabound ore deposita. Am. J.
Sci., 284, 1125-1174.
5. GOBLET, P. (1981). ModAlisation des transferts de masse et d'Aneraie en
aquifAre. ThAse Docteur IngAnieur, Ecole Nat. Sup. Mines et Univ. Paris
VI, 225 p.
6. KAN, M. (1986). Simulation physicochimique de l'Avolution hydrothermale
des milieux poreux ou fissurAs. Th~se Doctorat, Univ. Strasboura,
234 p.
7. de MARSILY, G. (1981). HydrogAOlOfie quantitative. Masson, Paris.
8. MATHERON, G. and G. de MARSILY 1981). Is transportation in porous
.edia always diffusive? A counter example. Water Resour. Res., 16 (5),
901-917.
9. MILLER, C.W. and L.V. BENSON (1983). Simulation of solute transport in
a chemically reactive heteroaeneous system : model development and
application. Water Resour. Res., 19 (2), 381-391.
10. MOREL, F.M.M. (1983). Principles o~ aquatic chemiStry. Wiley
Interacience, New-York.
Acknowledaementa : This work was supported by
European Communities (DG XII),
la Recherche Scientifique" and
pour la Mattriae de l'Energie".
the Commission of the
the "Centre National de
the "Aaence Fran~aise

452
Overview paper on 9 EC contracts listed in Tabl~ 1 of this paper
TESTING GEOPHYSICAL EXPLORATION TECHNIQUES
ON THE ISLAND OF MILOS (GREECE)
J. WOHLENBERG
Lehr- und Forschungsgebiet fur Angewandte Geophysik, RWTH
Lochnerstr. 4-20, D-5100 Aachen, F.R. Germany
*
Aachen,
Summary
A series of geophysical research projects was carried out on Milos
(Greece) to test and improve the various exploration techniques and
strategies on the known geothermal reservoir of the island. Eight
research teams from four countries (F, UK, GR, FRG) executed field
experiments in joint efforts between 1984 and 1988. Magnetotelluric
(MI, AMT and AAMT), electric-selfpotential and seismological methods
were applied to achieve a better understanding of the correspondence
of the internal structure and geometry of the geothermal reservoir
with the change of relevant physical parameters.
The experimental phase is over and some of the Final Reports are
already available. There is a very close relation between the
observed field parameters and the seismic activity with the contours
of the geothermal reservoir of Milos.
Following the Travale Testsite Experiments (1979-1983), the Milos
Testsite Proj ects within the Commission's third R&D-Programme is
the second venture of this type. The results are in press as Special
Issue of GEOTHERMICS, Pergamon Press.
INTRODUCTION
Within the framework of its second R&D programme from 1979 to 1983
the Commission of the European Communities funded a series of geophysical
research projects in the geothermal energy sector. The main target of
these projects was to test and improve geophysical exploration methods as
well as to contribute to a better understanding of a known geothermal
reservoir. The project "Testing Geophysical Methods in the Travale
Geothermal Field" was launched by the Commission in 1980 (Wohlenberg,
1985).
* Rheinisch-Westfilische Technische Hochschule

453
The results of these projects, which were carried out by 12
scientific teams from 4 countries (United Kingdom, France, Italy and
Federal Republic of Germany), are summarized in Barbier et a!. (1985).
These studies once more underlined the fact that there is no single
geophysical exploration technique specially suited to investigation of
geothermal systelllS. The best prospect of obtaining good results comes
from combining several methods, the choice of which must take into
account the different characteristics of the individual types of
reservoir (Wohlenberg, 1982). Even if the results of the special projects
in the Travale Testfield could not be considered as very convincing, one
achievement was that the scientific tealllS agreed on an optimum common
research strategy, on compatible instrumentation and especially on a
uniform data processing and interpretation concept. By this process of
cooperation, standardisation and comparison, a relllSrkable improvement was
achieved, particularly of the electrolllSgnetic (magnetotelluric)
exploration technique.
The experiences with the integrated Travale project emphasised the
importance of continuing with the same type of experiments during the
Commission's third Rand D programme from 1984. After discussion of the
logistic possibilities and, in particular, of the geological and
geophysical conditions, the Greek island of Milos in the Aegean Sea
became the new testfield. Beeween 1984 and 1988 eight groups of
scientists from the United Kingdom, France, Greece and the Federal
Republic of Germany worked on Milos. As in the Travale Project, efforts
were concentrated on electrolllSgnetic techniques. In addition, and with
equal emphasis, seismological investigations became an important part of
the project. Mapping of the electrical selfpotential (SP-method) also
formed part of the Project.
GEOLOGICAL AND GEOPHYSICAL SETTING
According to current geological thinking, Milos is part of the outer
arc of the Aegean volcanic belt, as are the islands of Santorini and
Nisyros and the geothermal reservoirs of Methana (Fig. 1). Seismic
reflection and refraction' investigations identified a complete
continental-type crustal structure underneath Milos, causing a volcanism
which is characteristic of subduction zones (Fig. 2). This volcanism, of
phreatic type, controls the shape and morphology of the island (Fig. 3);
volcanic rock types cover all of the non-volcanic basement formations.
Thi. volcanism i. responsible for the strong geothermal anomaly of the
island, the exploration and eventual exploitation of which lIISy play an
important role in the geothermal energy concept in Greece.

454
~C!!
" 0;.,.
C!I,
,
'",
?O!'
, "'?
,~
""~
o 60 km~,
~ ",
------"
Scale '--_ _"-
1IIIlIIII ACTIVE VOLCANIC-ARC
• RELATIVE MOTION OF THE AFRICAN AND AEGEAN PLATES
~ GEOTHERMAL AREA
,. /
,
~ : Kilos island testsite, part of the volcanic arc of the
Aegean zone
While the feeding system of the geothermal anomaly of the previous
testsite (Travale) consisted of deep reaching hydrothermal convection
cells, the cause of the Kilos geothermal reservoir is probably the
presence of one or more magma chambers. Shallow fractured regions,
forming the reservoir above the molten rock volumes, are filled with hot
water and vapour. These fractured and saturated formations were the
actual target of the geophysical exploration because they were supposed
to be characterized by their very specific physical properties.

"55
'101

. Sf>
The mapped geothermal gradient of M.llos (Fig. 4) strongly suggestll
t hat the thermal anomaly doe8 not affect the whole island. The most
iuteresting area to explore is the central eastern part of Milos. Several
boreholes (Zepheria Ml, M'2, M3. MZl in Fig. 4) had confirmed t he
exiaten~e of an extended temperature anomaly. Tbe pattern of the apparent
electric resistivities of the eastern pa~t of Milos i8land (Fig. 5;
Duprat. 1973) I s i n good sgreelllent with the surface contours of the
geothermal anomaly. The regions with extremely low specifi c resi8tivities
coincide with zones of the steepest geothermal gradients. The Bouguer
gravi~ map8 contr ioute no essential direct information on the geothermal
reservoir; neither do the earlier seismological investi gati oos. When tho
Milo8 project started. little was known of the seismicity of the island
(Drakopoulos and Delibs8is. 1973). and a significant 1mprovement of the
knowledge of the geometry and internal structure of the Milos geotbe1"llllll
reservoir was expected from th~ two seismological studies.
I PII1.o&-ISI.AKO
~1'~~101
TII .. ",al gradllnt conI: ...
6 Borlhcl,
~ Geotbermal gradient map of K os island (Tsokas. 1985)

4S7
Results of DC-geoelectric mapping of Hilos island.
Electrode spacing AB/2 - 100 m (Duprat. 1973)
THE HI LOS PROJECTS
The exploration strategy on Hilos had to be different from that for
the Travale Testsite. The geological situation is different. Tectonic and
especially volcanic processes control the geological history of the
bland and therefore more emphasis had to be put on the seiSlllological
experiments. Other geophysical investigations such as. for example. the
determination of the geothermal gradient. the Bouguer gravity. and DC
geoelectrical soundings had already been carried out sufficiently as
mentioned above. Magnetotelluric experiments again had a high priority
since the specific geographical conditions of an island and the most
interesting geotectonic island-arc position of Hilos promised new
experiences regarding the application of the HT-techniques.
Fractured rock. especially if lubricated by fluids as expect!!d in
the Hilos geothermal reservoir. produces small earthquakes along active
microfaults and cracks. Localizing these earthquakes allows
identification of the pathways along which hot fluids rise (Wohlenberg.
1982). Fractured rocks and magma reduce the velocities of seiSlllic signals
travelling through such zones. Therefore the observation of traveltime
residuala of local and distant earthquake signals by a dense seiSDdc
atation array can identify geothermal targets (seismic tomography).

458
Hydrothermal fluids generally are extremely saline and therefore
good electrical conductors. This means that geoelectrical and
electromagnetic exploration methods. like DC-soundings. SP
(selfpotential)- and IP (induced polarisation)-mapping and KT
(magnetotelluric)-experiments. contribute essentially to investigation of
the size and structure of a geothermal reservoir.
The investigations which were integrated in the Milos Testsite
project are listed in Table 1. At the time of writing this paper the
situation of the Milos project is that field experiments are complete.
that some of the Final Reports are available and that the interpretation
of results is still being discussed. The results of the individual
research projects are being published as a Special Issue of GEOTHERMICS.
Pergamon Press in 1989.
Research groups partfcfpatfng fn the Mflos project
Organisatfon
Project leader
Tftle of the Project
Contract nO
BRCM, OrlAsns (F)
A. Beauce
Test of an fntegrated methodology
for hfgh temperature geothermal
exploratfon
EN3C-0021-F
RWTH Aachen (D)
Instftut de Physfque
du Clobe (F)
J. Wohlenberg
A. Hfrn
Sefsmologfcal Exploratfon of
the Mflos Geothermal Reservofr
EN3C-0015-D
EN3C-0078-F
BCS, Edfnburgh (UK)
S. Crampfn
Shear-wave splfttfng observatfons
on Mflos, Creece
EN3C-0098-UK
TU Braunschwefg (D)
C. Musmann
Actfve Audfomagnetotellurfcs on
Mflos for determfnatfon of
electrfcal conductfvfty dfstrfbutfon
and thefr correlatfon to
geothermal anomalfes
EN3C-0024-D
Frefe Unfveraftit
Berl fn (D)
V. Haak
A dense mappfng of electrfcal
conductfvfty
EN3C-0023-D
Unfversfty of
Edfnburgh (UK)
V.R. Hutton
Magnetotellurfc measurements I
(a) a feasfbflfty study and (b)
geothermal exploratfon fn an
actfve volcano-tectonfc envfronment
EN3C-0008-UK
EN3C-0026-UK
ICME. Athens (CR)
C. Thanassoulas Applfcatfon of the self
potentfal (SP) technfque
EN3C-0025-CR
Table 1

4S9
The seismological working groups (BRGK. Prance; IPG. Prance; RWTH.
FRG) .. tabUshed a large number of digital recording mobile se1B111ic
stations. The station networks operated simultaneously or during
overlapping periods in 1986 and 1987. They operated mostly in an event
recording mode. The experiments during the first field campaign in 1986
were concentrated on the central and eastern part of the island since the
seismic activities were expected to be strongest around the geothermal
anomaly. This assumption was confirmed: local sei8lDic events occurred
almost only in the eastern part of Milos. Consequently. during the second
field campaign in 1987 the station arrays again were established around
the geothermal anomaly in the eastern part. The station networks of the
working groups RWTH (FRG) and IPG (Prance) are given in Pig. 6. While the
local earthquake activity during the recording period in 1986 was rather
MILOS-ISLAND
_'"" l
rti'
]i''''
24'20'
24'])'
STATIONS
e 1986
.,987
• 1986,1987
Recording sites for the sei8IDological experiments during
the field campaigns 1986 and 1987 (Ochmsnn et al •• 1988)

4 ()
lov, the experiments in 1987 fell into 8 period of extt~m~ly estrong
seismic activi~y. The station array of RWTH (PRG), for exampl~, recorded
more toao 1200 seismic events during A period of 8 weeks. Only the best
recorded of these were treated furtber . Pig. 7 gives an example of tbe
epicentre distribution derived from the RWTH station array. The
seismological inves t igations inc lud~d an analysis of the velocity
distribution within the study area, an analysis of the focal mecbanism,
(HId a tomograpbiesl inversion of traveltime res1dualB. A very special
investigation dealt with the phenomenon of shear-wave polarisation and
tbe related she.ar-oiav& 8plittLn.g effect. These studies ver earr1 d out
by the working g~upa BGS (UK), IPG (F) and R~B (YRG).
MILOS- ISLAND
(G reece)
•
.
• • •
" ..
•
..
•
..
o 2. 3·rt. .
..
.' ,
. . ....
Distribution of the recorded seismic events during field
experiments 1986/87 from the RWfH station array (Ochmann
et a1., 1988)
, .
The decision as to where the magnetotelluric record1ng instruments
should ~ placed 00 the island .... as simLlar to the. for seismology, and
thus the HT- experlments also were concentrat.d iu tbe eastern and
southern part of Milos. Before tbe usio MT~campa1gQ 8tar~ed. a
fei3sibility st\Jdy was carried out by a team from the University of
Edinburgh (UR) . The a1m of tbis pre- site invutigatl.oll WAS to f ind out
whether the island character would give MT-technjq\Jes A cbacce to b~
esuccessful at all (Dew 8, 198 7) .

461
In autumn 1986 working groups from Universities of Braunschweig
(FaG), Edinburgh (UK) and Frankfurt/Berlin (FaG) and from the Geological
Survey of France, BRGH (Orleans) carried out HT-measurements in the
frequency range from 100 Hz to 10000 sec. The experiments were
accomplished mostly simultaneously so that direct comparison of data from
different teama was achieved. Besides the passive HT-recordings the group
from Braunschweig University conducted active HT-experiments. This
working group paid additional attention to studying the influence of the
coastal effect on HT-measurements (Drews et al., 1988; Beauce et a!.,
1986; Dawes et al., 1987). As an example of the results of the very
successful HT-experiments, Fig. 8 presents a contour map of the lowest
observed resistivities (in Ohm.m) from the active HT (AAHT) studies.
~ I Countour map of the lowest resistivities (in Oro-.m) from
AAHT-experiments on Milos island (Drews et al., 1988)

462
Such low resistivities are not the result of penetrated sea waters
because this would lead to unrealistic salinities. As Dupral (1973)
pointed out. the only way to get such low resistivities is higher
temperature. A comparison of Fig. 8 with the temperature gradient map
(Fig. 4) and the results of the DC-geoelectric measurements (Fig. 5)
after Duprat (1973) are in very good agreement: low resistivities from
the MT-experiments correlate with areas of high temperature gradients and
contour almost the known geothermal reservoir. .
A very necessary contribution was the SP {self potentia1)-mapping
carried out by IGME in 1986/87. A total of 191 km of SP-profiles was
observed on Milos mostly on the eastern part. The SP-survey was
complemented by magnetic measurements along the same profiles. The
compiled SP-map was very successful in correlating with features of the
known geothermal field on the eastern part of the island. Comparison of
the SP-map to other geological-geophysical information proved a close
relationship between SP-anoma1ies and areas of high geothermal activity
(Thanassou1as. 1987).
FINAL REMARK
The two geophysical testsite experiments in Trava1e 1979/1983 and
Milos 1984/1988 were extremely useful as a platform for improving
geophysical exploration techniques aimed at investigating the geometry
and structure of geothermal reservoirs. testing and calibrating
instruments and comparing uniform data processing and interpretation
concepts. Finally it was a European effort and a multi-disciplinary
dialogue and this. too. was an important target of the Commission of the
European Communities.
REFERENCES
Barbier. E •• J. Woh1enberg. and P. Ungemach. (Editors) (1985). Testing
Geophysical Methods in the Trava1e Geothermal Field. Geothermics.
Special Issue. Vol. 14. No. 5/6.
Beauce. A •• H. Fabrio1. D. Le Masne. and J. P. Deriaud (1988). Test of an
iutegrated methodology for high enthalpy exploration on the island
of Milos (Greece). EEC Final Report. Contract No. EN3G-0021-F.
Booth. D. C •• S. Crampin. S. Sch1eper. N. Ochmann. and J. Woh1enberg
(1988). Shear-wave splitting observations on Milos. Greece.
Geothermics. in press.
Dawes. G. J. K •• D. Ga1anopou10s. and V.R.S. Hutton (1987). Magnetotelluric
Studies. Milos. Greece. 1986. EEC Final Report. Contract
No. EN3G-0026-UK{H).
Drews. C •• N. Ffirch. H. M. Maurer. G. Musmann. and P. Weide1t (1988).
Active Audiomagnetote11urics on Milos (Greece) for determination of
electrical conductivity distribution and their correlation to
geothermal anomalies. EEC Final Report. Contract No. EN3G-0024-D.
Drakopou10s. J. C •• and N. D. De1ibasis (1973). Volcanic-type Microearthquake
Activity in Milos. Greece. Anna1i de geofisica. m!.,
131-153.
Duprat, A. (1973). Geoelectric survey on Milos island and Susaki area.
Public Power Corporation of Greece.
Fytikas, M. (1977). Geological and geothermal study of Milos island.
Geological and Geophysical Research (IGME), Tom. XVII Vol. No.1.

tikas, H., and G. Marinelli (1976). Volcanology and petrology of
volcanic products from the island of Milos and neighbouring islets.
Journal of Volcanolo and Geothermal Research, 28, 297-317.
~, A., and H. Sachpazi 1988. Seismology on Hilos-rsland (Greece).
EEC report, Contract No. EN-EG-A2-141-F.
nmann, N., D. Hollnack, and J. Wohlenberg (1988). Seismological
exploration of the Hilos geothermal reservoir, Greece. Geothermics,
in press.
lpazachos, B. C. (1973). Distribution of seismic foci in the
Hediterranean and surrounding area and its tectonic implication.
Geophys. J. R.A.S., 33, 421-430.
thanassoulas, C. (1988). APplication of the SP technique over Milos
geothermal test site. EEC Final Report, IGME, Greece.
Tsokas, G. N. (1985). A geophysical study of Milos and Kimolos islands.
PhD Thesis, University of Thessaloniki, Greece.
Wohlenberg, J. (1982). Zur geophysikalischen Exploration von Wirmelagerstltten.
Statusreport Geotechnik und Lagerstitten, Proj ektleitung
Energieforschung, KFA JQlich.
Wohlenberg, J. (1985). The Geophysical Exploration of Geothermal
Reservoirs: The Travale Test Site (Italy). Geothermics, Vol. 15,
No. 5/6, 615-621.
463

EEC contract n 2 EN3G-0074-E
SEISMIC RECONNAISSANCE OF THE UPPER CRUST IN THE VOLCANIC ZONE OF OLOT
(NE OF SPAIN)
J. GALLART 1!* and A. HIRN 2
1: Dep. Geologia Dinamica, Geofisica i Paleontologia,
Universitat de Barcelona, 08028 BARCELONA
*: present address: Institut J. Almera - C.S.I.C., Barcelona.
2: Institut de Physique du Globe, 4 pl. Jussieu, 75005 PARIS
Summary
A geophysical reconnaissance based on seismic refraction profiling has
been carried out in the region of Olot (NE of Spain), the youngest quaternary
volcanic area of the Iberian Peninsula. The methodology designed to
provide the structure of the upper crust under the Volcanic Zone (V.Z.)
and its lateral variations has been tested in this study. A global model
coherent for the profiles covering the axis of volcanism shows low seismic
velocities for P waves, less than 4 km/s in the first 400m under the V.Z.
In the surrounding Eocene sediments higher values are measured, differing
from East (5.3 km/s) to West (4.6 km/s). Comparable variations of electrical
resistivity have been obtained in a complementary survey. The basement
is located at about 2 km depth, deepening from E to Wand from N to S, and
higher velocities, 6.6-6.7 km/s, are correlated below an horizon at 5 km
depth. The S-waves analysis from individual 3-component stations provides
independant structural constraints. High Vp/Vs values are obtained at small
depths. ~he polarisations derived from particle motions show several converted
P-S phases, originated mainly on top of the basement. The area beyond
eastern boundary of the V.Z. displays anomalous high energetic arrivals
and complex particle motions corresponding to local effects that
should be further investigated from denser seismic measurements.
1. Introduction
A geophysical reconnaissance of the area of Olot (NE of Spain), site
of the youngest volcanism of the Iberian Peninsula, has been undertaken in
the geothermal research framework of the CEC.
A study based on refraction seismics has been planned in cooperation
between the University of Barcelona and the Institut de Physique du Globe
de Paris. The aims were not only the knowledge of the structure of the
upper crust, but also a test on the ability that such a moderate-cost seismic
technique can achieve in detecting the structural variations in a complex
zone up to now poorly sampled at depth.
2. Geological framework
The neo-quaternary volcanism of Catalunya extends in a SE-NW direction
from the Mediterranean to the area of Olot. This intraplate volcanism is
related (Arana and others, 1983) to the Neogene extensional tectonics that
affected the NE of Spain as well as larger areas of western Europe. This

tectonics is controlled by NE-SW normal faults that delimit Miocene sedimentary
basins, together with secondary NW-SE fault systems responsible
for the volcanism.
The Olot volcanic zone (V.Z.) is located southwards of the Pyrenean
structures (see Fig.1). A succession of E-W folds and NW-SE faults delinea
te there a central volcanic block surrounded mainly by Eocene materials,
with some quaternary sediments (Mallarach, 1983). The volcanism, probably
of deep (mantle) origine, has been dated from petrographical, geochemical
and other analysis ( Arana and others, 1983; Donville, 1973), and extends
from 0.11 My up to 10.000 years.
3. Geophysical data
Few geophysical data are available for the Olot volcanic zone, oil
prospections having always avoided this area. Seismic catalogues (Fontsere
i Iglesies, 1971) show the existence of an important seismicity, with
several destructive ~arthquakes in the XVth century, but also with long
calm periods. Nowadays, a clear lack of local seismicity is reported, well
documented since 1985 from instrumental recordings.
Analysis of residuals from regional and teleseismic events recorded
in a local array (Gallart, Olivera and Correig, 1984) provided some first
evidences for structural differences within the upper crust between the
V.Z. and surrounding terrains.
An aeromagnetic map of Catalunya recently compiled (Zeyen and Banda,
1988) shows a clear positive anomaly centered in the V.Z. indicating a
downwards extension of the high susceptibility magnetic materials. No significant
gravity anomalies are found in that area ( Torne, Banda and Casas
1988).
High regional heat flow values have been reported (Albert, 1979), although
exact values are uncertain due to the lack of conductivity measurements.
On the other hand, no hot-temperature fountains nor other heat sources
have been found close to the V.Z., due perhaps to a very continuous
surface folding lid (Riba, 1975).
4. The seismic experience : aims and methods
The global structure of the upper crust under the volcanic zone and
its lateral variations was the main goal to achieve in the present seismic
experience. As 3-D tomographic techniques were not feasible here due to
the lack of local sources and vertical seismic profiling economically
unapproachable, a set of seismic refraction profiles was designed as shown
in Figure 1.
The V.Z. extending WNW-ESE for about 10 km and less than 5 km wide,
the main profile was settled along the axis of volcanism. A method inspired
on the marine Expanding Spread Profiles (ESP) has been tested, with both
the shotpoints and the recording device placed successively at increasing
distances in order to sample always the same zone but for incresing depths.
Five shots of about 100 kg each have been fired along the main line
(Fig.1) : A, C close to the ends of the V.Z.; B in the middle; D, G westwards,
with a maximum offset of 36 km. Lateral variations have been checked
from shotpoints H, F and E recorded, respectively, on a perpendicular
direction, outside the V.Z., and in a N-S fan profiling. Thirty autonomous
3-component stations were deployed in a very reasonable density (one each
SOOm) considering the logistics constraints. All the planned experience
could be carried out successfully, in a short period of time.

~va.CIHIC
~*~
Figure 1
Locati on of sei smic profiles and ahote within a
geologi cal sketch.
5. Global interpretation of the seismic profiles (P waves); velocity model
for the upper crust
Some examples of the recorded shots for ti~ main profile WNW-ESE are
shown in Figures 2 and 3. The interpretation has been carried out with the
usual techniques of synthetic seismograms and ray-tracing for highly heterogeneous
structures. Different programmes have been tested and an example
from Rayamp is presented in Fig. 3,b,c corresponding to the profile from
shot 0 (Fig.3a).
For shots A (Fig.2a), D (Fig.3a) and G • out of the V.Z. and recorded
towards the East, the surface velocities lie between 4.4 and 4.7 km/s.
and the basement under the V.Z. is defi ned at about 2 km depth, wit h an
apparent velocity of" 5. 25 kIn/s. The reverse profile from shot C (Fi g .2b)
shows higher surface velocities, about 5 . 2 km/s and 5.3 km/s at 400m depth
extending under t he V.Z. The apparent refracted velocity for the basement
is here slightly less than 6 km/s .
In t~e uppermost part of toe crust under the V.Z., sampled from shot
B r ecorded in both senses, lower velocities are f ound: 3.0 km/s 1n the
f irst 100m in the centr al part, and 3 .75 krn/s up to 400rn c3epth where the
velocity increases to 5.3 km/s.
For the more distant anots D. G, a clear secondary phase i s correlated
from distances of 18-20 I

467
I
-1~~--~~~--~-r~~~-T--~~~--~~~--~~--r-~~
o 10 10
:I
91
Q.
••
j
0
~O
--lll
-1
-10 -10 .0
Figure 2. Examples of data for the main profile : record sections from
shots A (2a, above) and C (2b, below). Vertical components are
displayed with a reduction velocity of 6.0 km/s. Distances
expressed in kilometres and times in seconds. The correlated
phases are discussed in the text.

468
2
-1
10
20
SO
0
DISTANCE (Km)
0
E
~ 2
J:
l- S
~
w
0 4
CS
8
2
a
..::
.~
.
•
~,
-.;
• •
a
-
115
17
21
DISTANCE (Km)
Figure 3. Record section from shot D West of the V.Z. (3a up) together
with the fitting of the interpreted model with the Rayamp program
in arrival times (3b centre) and synthetic seismograms
(3c down). The squares are the observed arrival times.

469
The model presented in Figure 4 provides a good fitting of the correlations
observed for the 5 shots of the main profile, except for the local
anomalies discussed above.
The lateral profiles from shots E, F, H confirm the main features of
this model and provide some evidences for lateral variations such as a
thickening of the basement also from North to South ( delays of up to 0.2s
in the corresponding arrivals on the fan profile).
,
a D A
4
•
IZ
4..15 4.1115
,
t--z. V.--t
•
DISTANCE 000
III 20 Z4
'~
13.15 I
3.75 5 ...
5.30
'!i.25
,
c
5.30
..
N
1.0
•
n
1.7
•
Figure 4. Distribution of seismic velocities with depth in a model coherent
for the shots of the main profile, which location is indicated,
as well as the extension of the volcanic zone. Numbers
correspond to P-velocities in km/s.
6. Structural constraints derived from S waves of individual seismo rams:
Vp Vs ratio, converted waves, anisotropy
A second approach of structural interpretation has been attempted
through the S-waves data available on the 3-component recordings. In the
record sections of the horizontal components it is not possible to correlate
and define S velocities from the few S waves visible, specially in
the V.Z.
However, for the individual seismograms with clear S arrivals, the
relationship k ~ Vp/Vs can be estimated through the ratio Ts/Tp • The results
indicate that k changes mainly with the distance to the shotpoint
and not with the profile considered. In,the first 5 km distance, very high
values around 2.0 are found; between 5 and 20 km values of 1.85-1.90 are
usual, and decrease to 1.77 after 20 km.

470
Z 1---1
~
LI---.J
•
0.70 8 0.80 0.80 8 0.90 0.80 8.1.00 1.00 8 1.10 1.10 8 1.20 1.20 8 1.30
Figure 5. Particle motions for stations B05 (up) and Fl2 (down), with the
corresponding seismograms. Arrows indicate polarized arrivals
of P and PS type. Other details of the motions are explained in
the text

471
The analysis of the particle motion from the digitized seismograms
constitutes an useful method in such a complex area to discriminate in
late arrivals between those corresponding to pure P reflections in structursl
layers, and those associated to converted phases P - S when crossing
shallower horizons.
In the V.Z. of Olot several P-S conversions have been found. The Figure
5 illustrates 2 examples with a P-S phase directly visible in the horizontal
seismograms and clearly confirmed in the particle motion. In the
Fiaure, together with the seismograms, the 3 classical planes are shown :
plane of incidence ZL (up); perpendicular ZT (middle) and horizontal LT
(down). Each box displays the motion during 0.1 s. In the examples, stations
85 and F12, inside and outside the V.Z. respectively, first arrivals
are vertically polarized (see plane ZL) and are followed 0.25 or 0.3 s
later on by an horizontally polarized phase from the same azimuth (see
plane LT) : it is a phase of "sv" type, or a converted P-S wave. Taking
into account the delay between P and PS phases, the conversion should take
place when crossing the basement near the station, and the relative variations
in such a delay correspond probably to variations in the basement
topoaraphy.
In 2 other cases, the converted PS wave displays two consecutive perpendicular
azimuths that seem to correspond to splitted phases of type
"SH and SV", and this has been reported as evidence for anisotropy (Crampin
and others, 1984). Additionnal data are needed in the V.Z. of Olot
to further developments of such anisotropic analysis.
7. Complementary measurements: vertical electrical soundings (V.E.S.)
A complementary survey of 13 V.E.S. with Schlumberger device has been
carried out to provide independent constraints on the properties of materials
near the surface by measuring their electrical resistivity.
The data, interpreted with an a priori information algorithm (Pous,
Marcuello and Queralt, 1987), indicate that basaltic rocks have resistivities
about 10000 l1.m , and pyroclastic materials show ~ slightly higher
than 1000n.m • The thickness of volcanic materials, not uniform, is less
than 100 m , usually with non volcanic small layers interbedded. In the
Eocene terrains East and West of the V.Z., differences in electrical resistivities
are detected, with values of ~~ 500 l1.m obtained in the E, whereas
in the W, ~, 200 £L.m • Such variations seem to be coherent with the
seismic velocities obtained near the surface.
8. Discussion and conclusions
The study was aimed as a reconnaissance on the structure of the upper
crust in the volcanic zone of Olot, and as a test on the ability of the
methodology adopted of high density seismic refraction profiling.
Althouah the E.S.P. geometry designed has been less sensitive to specific
depths than in marine surveys due to the more discontinuous sampling,
a alobal model for the upper crust in the volcanic zone and surrounding
terrains has been obtained. This model (Fig.4) is coherent with the correlations
observed in the 5 profiles covering the main axis of volcanism.
Strona lateral variations are associated to the V.Z. where velocities
less than 4.0 km/s have been observed in the first 400 m • The thickness
of the basaltic rocks outcropping there is less than 100 m, with small
clay layers interbedded, and recent geological work (Mallarach, 1988 personal
communication) indicates that 100 - 200 m of lacustrine sediments
exist below the basalts.

472
The seismic basement is well defined under the V.Z. at 2 km depth,
deepening from E to ~ and also from N to S as suggested by lateral profiles.
In the upper crust, a layer at 5 km depth marks an increase in
velocities to more than 6.~ km/s.
An individual analysis on the S phases visible in some of the 3-component
recordings provides independent structural constraints. Very high
values of the ratio k = Vp/Vs are obtained near the surface, decreasing
to more usual values in the basement.
Pure P phases and converted P-S waves can be discriminated from particle
motion analysis. A significant number of P-S phases has been detected
in this experience, most of them converted through the basement/sediments
discontinuity. The study of P-S waves provides an additionnal methodology
to derive crustal features under the stations and to detect lateral
variations of short wavelength provided a dense array is available.
The splitting in PS phases observed in 2 records could be an evidence
for anisotropic ~eatures in the upper crust, but more data are needed
to obtain reliable results.
An anomalous zone has been observed in the eastern transition of the
V.Z., with few high energetic reflected phases and complex particle motions.
Some local structures may focus the energy, and transitional
effects should also be considered. This region could be the goal for a
further seismic study with high~r density measurements.
References
Albert, J.F. (1979). El mapa espanol de flujos calorificos. Intento de
correlacion entre anomalias geotermicas y estructura cortical. Bol.
Geologico y Minero,90, 36-48.
Arana V., A. Aparicio, C. Martin Escorza, L. Garcia Cacho, R. Ortiz, R.
Vaquer, F. Barberi, G. Ferrara, J.F. Albert, y X. Gassiot (1983).
El volcanismo neogeno-cuaternario de Cataluna. Caracteres estructurales,
petrologicos y geodinamicos. Acta Geol. Hispanica, 18,1, 1-17.
Cramp in S., E.M. Chesnokov and R. Hipkin (1984). Seismic anisotropy the
stats of the art: II. Geophys.J.R.astr.Soc., 76, 1-16.
Donville D. (1973). Geologie neogene et age des eruptions volcaniques de
la Catalogne orientale. These. Univ. Toulouse
Fontsere E. i J. Iglesies (1971). Recopilacio de dades sismiques de les
terres catalanes entre 1100 i 1906. Fundacio Salvador Vives i Casajuana.
Barcelona, 547pp.
Gallart J., C. Olivera y A.M. Correig (1984). Aproximacion geofisica a
la zona volcanica de Olot (Girona). Estudio local de sismicidad.
Rev. de Geofisica, 40, 205-226.
Mallarach J.M. (1983).
E. 1:20.000 •
Carta geologica de la regio volcanica d'Olot.
Pous J., A. Marcuello and P. Queralt (1987). Resistivity inversion with
a priori iriformation. Geophysical Prospecting, 35, 590-603 •
Riba O. (1975). Geotermismo de la zona volcanica de Olot. Nota preliminar
sobre posibilidades geotermicas. Bol. Geologico y Minero,LXXXVI-I,
45-62.

473
Torne M., A. Casas and E. Banda (1988). Cartografia geofisica en Catalu­
~a II) El mapa gravimetrico. Rev. Sociedad Geologica de Espana, 1-2,
81-88.
Zeyen H.J. and E. Banda (1988). Cartografia geofisica en Cataluna I) El
mapa aeromagnetico. Rev Sociedad Geologica de Espana, 1-2, 73-80 •

474
EEC contract no EN3G-0087-GR (Part I)
MICROSEISMIC AND SEISMOTECTONIC STUDY OF THE ISLAND OF LESBOS
N.D. DELIBASIS and N.S. VOULGARIS
Geophysics and Geothermy Division-Geology Departement
University of Athens
Summary
The results of a research project aImIng at the evaluation of the
geothermal field on the island of Lesbos are presented in this study.
Data analysis showed that the whole island of Lesbos hosts earthquakes
of small magnitude with the exception of the Eressos area. Comparison
of the epicenter distribution with the tectonic network of the island
revealed that all macro and micro tectonic features host at least one
epicenter and that fault system trending NW-SE is more active than the
other main fault system of the island of NE-SW direction. An
additional characteristic of the epicenter distribution on the island,
is the presence of a main arc-formed central seismic zone, which
divides the island into two parts. Earthquakes with focal depths
greater than 5 km are predominant in the concave part of the zone (NE
part of the island), as opposed to those at the convex side. The
existence of a large number of microearthquakes with shallow focal
depths (0-2 km) in the central part of the island, could be possibly
attributed to the circulation of hot material through the tectonic
network of Lesbos.
1. INTRODUCTION
Within the framework of a research project for the uncovery and
exploitation of the geothermal field of the island of Lesbos, undertaken by
PPC-Greece with the support of the Commission European Communities, a
microseismic activity study and seismotectonic analysis of the island, were
carried out by the Division of Geophysics and Geothermy of the Geology
Department of the Athens University.
Field work operations were initiated on the 25 of November 1987, with
the installation of nine portable one component seismic stations (MEQ 800)
on the island. As it can be seen from figure 1, the above mentioned 9
stations covered an area of about 900 km Z in the central part of the
island. Distance between the stations varied from 8 to 12 km. A tenth
station of Sprengnether type, with three components (Z, N-S, E-W) was
installed in the center of the area in question, in the village of st.
Paraskevi. All stations operated at 72 - 84 dB, as dictated by the ground
noise at every site, until January 30, 1988.
2. DATA ANALYSIS
From the large number of earthquakes recorded during the two month
operational period of the network, those with a P and S arrival time
difference less then 20 sec, were analyzed. In that way it was possible to
obtain information from an area larger than the actual area of interest.

47S
25.78 26.28 26.78
39.110
i
~~------------~~---------------+38.~
38.90
25.78 26.28 26.78
Figure 1. Location of seismic stations on Lesbos island.(8cale 1:1000000)
During the calculation of the focal parlllleters of the earthquakes
recorded, two computer prograas were used: HYPO 71 on a PDP11 and BYPOCENT
on an IBM-PC computer. In order to select the appropriate velocity .odel
two .odels were taken into account as well as the results of a refraction
profile that was carried out on the island at the end of the field work
operations, using 6 seis.ic stations. The first velocity .odel was
proposed, for the Aegean area, by Makris (1977) after refraction shooting
in the Aegean and the second was proposed by Panagiotopoulos (1985).
~ co.bining these .odels with the results obtained fro. the seis.ic
profile for the topmost layers, the following .odel (Table I) was used in
the calculations.
TABLE I
DEPTH (0) VELOCITY (o/sec)
0.0 l.8
0.5 5.1
3.0 6.0
lO.O 6.6
33.0 7.7
40.0 8.1
Magnitude calculations were based on duration .easureaents according
to the for.ula:
Mt-l.31logD+O.OO1lA+O.73 (Kiriatzi and Papazachos 1984)
80.e aagnitudes (Ms) were also calculated for selected events using
aaxi.ua deflection .easureaents and calibration data according to the
for.ula:
M-1.04loga + 1.4llogA + O.lO (Papazachos-Vasilicou 1966)
were A is the epicentral distance and a is the real aaxi.ua ground aotion.

476
3. EVALUATION OF THE RESULTS
From the data obtained during the time period that the seismic station
network was in operation, it has been possible to locate the epicenters of
1007 events in the broader Lesbos area. The magnitudes of those events
varied between 0.0 and 4.0 R and their depths from 0 to 58 km.
A total of 698 epicenters are located on the island (figure 2) and a
number of 254 epicenters with focal depths less than 5 km are presented in
figure 3. As it can be seen from both figures there is a seismic zone,
dividing the island into two parts. This main seismic zone runs through the
areas of Thermi, Kalloni, st. Paraskevi and towards the NW to Petra where
it turns north in the area of Hithimna to be connected with an underwater
seismic zone. By comparing the two figures it is possible to distinguish
that within the main seismic zone (hatchured area), no earthquakes
shallower than 5km are observed. A similar phenomenon is also observed to
the NE of the main seismic belt
In the central Lesbos area, which is also the area of interest from
the geothermal point of view, 337 events were located with magnitudes
varying from 0 to 3.4 R. Due to network geometry the accuracy of the focal
parameters calculations was verY satisfactory for this part of the data
set.
In order to investigate the areas where no events shallower than 5km
were observed and the direction and dip of possible seismically active
faults a cross section was prepared (figure 4).
Since field operations and data analysis provided a large data set of
good quality, it was decided that the calculation of the Poisson ratio
would be attempted using the Wadati method. It is the first time that this
was done for an area of geothermal interest in Greece.
The values of the Poisson ratio (0) were calculated for a number of
events located in various areas over the island. Earthquakes located
outside the central area of the island, indicated values of 0 from 0.22 to
0.32 with a correlation coefficient of 0.998, while earthquakes located in
the center part of the island provided values varying from 0.28 to 0.35
with a correlation coefficient of 0.997. Correlation of 0 values of the
earthquakes with the corresponding focal depth revealed that, depths
smaller than 5km are related with 0 values between 0.30 and 0.32, while
depths greater than 5km correspond to higher 0 values from 0.30 to 0.35.
Another phenomenon observed during the " analysis of the" seismological
data, was that a number of seismic stations in the network, did not record
S-waves or that in some cases the amplitude of recorded S-waves was smaller
than that of the P-waves. In order to provide an explanation for these
observations, the seismic ray paths were traced from the epicenter
locations to those stations were absorption of the S-waves has been
observed. In that way it was possible to establish the intersections of
those ray paths (figure 5). Since 223 events displayed such an anomalous
behavior, it was not possible to include all of them in the diagram and for
that reason only a few of them were selected in order to cover the whole
area from every direction. In figure 5 it can be seen that all the
intersections of the ray paths lie in the center and northwest part of the
island in the areas were the Poisson ratio appears to have the highest
values.
4. SEISHOTECTONIC ANALYSIS
As it was mentioned previously, the whole area of the island of Lesbos
is covered by epicenters of small magnitude earthquakes. There is only a
small area in the west part of the island that shows no seismic activity.
This area, the area of Eressos, is surrounded by events of focal depths

25.78
39.50
25.98 26.1B 26.38 26.58 26.78
+---------+---,-----+---------~~------+---------+39.5O
39.30 39.30
(!)
[!)
...
Depth Dcal
< 3
3

25.78 25.98 26.18 26.38 26.58 26.78
39.50 39.50
m
•
!!fl •• 6
•
m
39.30 39.30
39.10
(!)
(!)
(!)
[!)
(!)
0
!
(!) m 39.10
Depth (kill
< 2
2

479
ws
EN
~---@~ __ ~~~~-@~~~-a~--------~O
(!) If) CD
_-lID ~
I!J ~. • I!J I!J
I!J - C ~I!J!III
~~' ~
I!\ ~ I!J I!J 3
rI I!J I!J II
5
Figure 4. Seismic cross section. Section location is shown in figure 3.
Exaggeration ~:1 in both directions.
greater than 50. Events with such focal depths are observed allover the
island while events with focal depths less than 50 are limited aainly to
the concave side of the aain seismic zone. During the two month period that
the network was in operation 444 events with focal depths greater than 5km
were recorded on the island in contrast to the 254 events of saaller focal
depths. This could indicate that surface seismicity is controlled by the
deeper earthquakes.
Comparing the epicenter distribution with the tectonic structure of
the island of Lesbos, we note that every fracture is the host to at least
one epicenter. and by examining the seismic cross section prepared along
the island (figure 4), we observe that it is possible to distinguish not
only the direction but also the dip of the seismic lineaments foraed by the
distribution of the epicenters. In figure 6 the active seismic lineaments
have been plotted along with the proposed dip and dipping direction. The
aajority of these seismic lineaments trends NW-SE. This observation
underlines the fact that the fault system directed NW-SE is more active
than the second system trending HE-SW.
The seismicity center observed in the east part of the island, in the
area of Laabou Hili lomi, could be possibly attributed to an intersection
of two seismic lineaments with different directions, since seismic activity
is more pronounced usually at such intersection points.
5. CONCLUSIONS
It is an obvious conclusion, both fro_ the analysis of the earthquakes
recorded during the period December 1987 - January 1988 and the previous
seismic history, that the island of Lesbos displays a high seismic
activity. This high seismicity emphasizes the notion that the neotectonic
evolution of the island is continuing.
Examination of the epicenter distribution in the central area, as well
as over the entire area of the island, reveals the existence of a curved
principal seismic belt, that divides the island into two parts. Starting
from the area east of Therai, this belt trends west and in the area.of st.
Paraskevi, ia divided into two parallel branches, directed towards the NW.
In the area included between the two above mentioned branches, no events
with focal depths shallower than 50 have been observed. In addition to
that characteristic aain zone, other seismic belts of saaller size also
exist. These zones characterized as second order zones, are aainly observed
to the north of the aain aeismic belt. In the areas included within these
second order zones, no events with focal depths shallower than 50 have
been observed.

480
25.78
26.28
39.ijO +---------------------~----------------~39.ijO
38.90
25.78 26.28
~--------------------~-----------------+38.00
Figure 5. Ray paths of events with S-wave absorption phenomena. Filled
squares correspond to station positions and circles to epicenter locations.
(Scale 1:600000)
25 78 26.28
-···~------------~--~r---------------t39.ijO
39.ijO +
~ __________________ ~ ______________-+38.90
38.90
25.78 26.28
Figure 6. Active seismic lineaments on the Lesbos island. (Scale 1:800000)

481
Furthermore, in the broader area to the north of the lIII.in seismic belt,
attenuation or absorption of the S-waves has been observed on the seismic
signals transmitted through that area and the values of the Poisson ratio
are highest in that area.
The large number of microearthquakes, with shallow focal depths (0 - 2k1a),
observed along the fault lineaments, the absence of those shallow events in
some limited areas and the attenuation phenomena, could be possibly
explained by the circulation of hot IIII.terial through the tectonic structure
of the island.
From the overall processing and analysis of the seismological data of the
island of Lesbos, we arrive to the conclusion that the area presenting the
highest probability for the uncovering of a geothermal field, is the
central and northern part of the island.
References
Delibasis, N. D. (1982). Seismic wave attenuation in the upper mantle
beneath the Aegean region. Pageoph,120, 820-839.
Drakopoulos, J. K., and N. D. Delibasis (1973). Volcanic .icroearthquake
activity in Hilos - Greece. Ann. di Geophisica,XXVI, 1, 131-153.
Fytikas, H. D., O. Juliani, F. Innocenti , P. Hanneti, R. Harruoli, A.
Peccerillo and L. Villar (1985). Neogene volcanism of the Northern and
Central Aegean region. Ann. Geol. des Pays Helleniques, 46, 104-124.
Jacobshagen, V., and J. Hakris (1974). Zur Geodynamic Griechenlands und del'
Agais. Nacht. Deuch. Ges, 2, 78-85.
Jongs.a, D. (1974). Heat flow in the Aegean sea. Geophys. J. R. astI". Soc.,
37, 337-346.
Katsikatsos, G., D. Hataragas, G. Higiros, and E. Triandafillou (1982).
Geological study of Lesbos island. Special Report, Institute of
Geolological and Hinning research.
Kiratzi, A. A., and B. C. Papazachos (1984). Hagnitude scales for
earthquakes in Greece. Bull. Seism. Soc. America, 74, 969-985.
Hakris, J. (1978). The crust and upper IIII.Jltle of the Agean region obtained
from deep seismic soundings. Tectonophysics, 46, 269-284.
Hacropoulos, K. C., and P. W. Burton (1981). A catalogue of seismicity in
Greece and adjacent areas. Geophys. J. R. astr. Soc., 65, 741-762.
Panagiotopoulos, G. D., and B. C. Papazachos (1985). Travel times of
Pn-waves in the Aegean and surrounding area. Geophys. J. R. astI". Soc., 80,
16.5-176.
Papazachos, B. C., and A. Vasilicou (1966). studies on the IIII.gnitudes of
earthquakes. Progress report in seismology and physics of the earth· s
interior 1964-1966. 17-18.
Praget, H. (196.5). Presentation
Volcaniques de l'ile de Lesbos.
512-527.
d'une
Ann.
esquisse geologique des terrains
Geol. des pays Be 11 eniques, 16,

482
EEC contract nO EN3G -
0013D
ATLAS OF GEOTHERMAL RESOURCES IN THE EUROPEAN COMMUNITY,
AUSTRIA AND SWITZERLAND
R. HAENEL
Department of Geophysics, Geological Survey of Lower Saxony
Summary
This Atlas presents for the first time on a common basis the available
information on Geothermal Resources in the 12 EC Member States,
Austria and Switzerland. The document consists of more than 400 maps
and a detailed text. It is based on results obtained primarily from
EC research contracts. The Atlas presents the main characteristics of
all the geothermal reservoirs investigated, such as the depth,
thickness, temperature at the top of the aquifer, geological
cross-section and - as far as possible - porosity, permeability,
transmissibility and salinity. Supplementary information is usually
given on the distribution of boreholes, fault zones, facies changes,
nearby oil and gas reservoirs, piezometric levels, etc. Syntheses of
these detailed results lead to the presentation either of available
Geothermal Resources in the region (expressed in joule per square
metre) or of "Geothermal Potential Areas" which appear qualitatively
suitable as sources of geothermal energy. These comprehensive
detailed maps on a national or regional scale are preceded by maps on
a European scale demonstrating the relationship between geological
structures and geothermal reservoirs, updated heat-flow densities and
updated temperatures at 1000 m and 2000 m depth and reviewing the
areas of geothermal resources in Europe.
1. INTRODUCTION
The first requirement for the exploration of geothermal energy is
the understanding of temperature distribution below the surface. In
addition, to determine the resources, it is necessary to measure also
hydrogeological and rock characteristics of the subsurface. The
acquisition of data for the assessment was occasionally complicated or
even prevented because of the absence of information, unreliable data,
the confidentiality of data, etc. The ambitious aim to catalogue both
resources and reserves had therfore to be restricted - for the time
being - to the assessment of the resources only. Even this would have
been impossible without the excellent teamwork and enthusiasm of all
the research teams involved.
The Atlas of the Commission of the European Communities is one of
the major achievements of the Community's Geothermal Energy Research
and Development Programme, and represents the results of work in many
countries over several years and is the first attempt in the world to
present comparable data on geothermal resources both within and across
national borders. The Atlas, consisting of 110 Plates representing more

483
than 400 .aps, is a funda.ental docu.ent for tbe exploration of geother.al
energy (Baenel .. Staroste, 1988).
2. PEfINITION AND ASSESS"E"T Of RESOURCES
The ter.s resources and reserves are defined as follows (HuffIer ..
Cataldi, 1978; Baenel 1983):
Resource. are tbat part of the geother.al energy which .ight be
extracted econo.ically and legally at so.e specified ti.e in tbe near
future. There are various categories of resources, reflecting the degree
of certainty in each case. The resources include also reserves.
Reserves are the known resources de.onstrated by drilling or by
geoche.ical, geophysical, and geological evidence and whicb can be extracted
econollically and legally at tbe present ti.e.
Reserves are subdivided further in ter.s of their geological assurance
into proven (evidence fro. borehol es), probabl e (evidence fro.
geological, geophysical, and/or geoche.ical investigations), and possible
(only geological evidence) reserves.
The 10Mer tellperature li.it of userulness for geother.al energy is
also open to definition. In theory, an~· groundwater with a te.perature
above 0 ·C can be used for heat pu.ps. BOMever, heat pu.ps cool the water
passi ng through the., and in practice, the lower li.i t for te.perature
supply is set by the consideration that the water rejected fro. the heat
pUlipS lIust not be so cool as to have an environ.ental i.pact on lakes,
rivers or near-surface groundwater.
Undar norlllal conditions, the fluid extraction is governed by the
hydraulic conductivity or trans.issivity. It is not usually possible to
deterlline hydraulic conductivity during exploration because a borehole and
costly pUMping tests would be necessary. It is easier to deter.ine errective
porosity. Therefore, the assessllent of resources is based on the
arrective porosity.
fro. experience the depth for econo.ical heat extraction rarely
axceeds 3 k. under present day condi tion. To obtai n an esti .ate of the
total heat content of an area at that depth, the accessible resource base
down to 3 k. (ARB expressed in joule) in low enthalpy areas can be
3
deterMined by using the following si.ple equation:
• volu.e fro. tbe Eartb's surface to 3 ~. deptb •• 3
.ean density of the rock col u.n. kg/.
• .ean specific heat capacity. J/(kg ~)
• te.perature at 3 k. deptb. ·C
• surface te.perature, ·C.
The heat in place (B in joule) contained witbin a given aquifer can be
o
deter.ined using a volu.e .odel of heat extraction (Huffler .. Cataldi.
1978) :
where: P
T t
• 1
A
and sub.cript.
1 )
( 2 )
• effective porosity. unitless
• te.perature at tbe top of tbe aquifer. ·C
• net tbickness or thickness of the aquifer ••
• surface area under consideration, .2
• and w refer to tbe rock .atrix and water respectively.

484
It is obvious that only part of H can be recovered. This is exand
pressed by a ter~ called the recovery fac~or Ro' The product of R
R
are the resources. 0
()
In most cases, the Rater has to be reinjected after use, either
because it is highly saline or because there is a need to maintain pressure
in the aquifer. The resource at wellhead for a doublet (i. e. a related
pair of extraction and injection boreholes) is given empirically as
folloRS (Lavigne, 1978):
Ri th:
Rhere:
R H
o 0
R o
T temperature of the reinjected water.
r
3
4
In order that the data from each country should be reported on a
comparable basis in this Atlas, the CEC expert group recommended that a
value of T = 25 ·C should be used for the purposes of calculation.
If J'nly a production Rell Rithout reinjection is considered (a
singlet), the recovery factor is (Gringarten, 1979):
( 5 )
Only a part of the geothermal energy resources can be exploited economicall
y at present. Thi s can be expressed by i ntroduci ng a recovery factor
R 1
. The reserves can then be calculated as folloKS:
( 6 )
The recovery factor R1 is difficult to determine. It depends on sitespecific
geological conditions as Rell as on the cost of the installation
of a singlet or doublet. Hore details are presented by Brook et a!.
(1979), Cataldi et a!. (1978), Haenel (1983) and Ineefeldt et a!. (1983).
The main data necessary for resource assessment are the thickness
z, the temperature T, and the effective porosity P. The depth of the
aquifer is also an important parameter because of the drilling cost. Data
Khich can be obtained easily are the densities ~ and ~ , the specific
heat capacities c and c , the area A, and the meanmannual \urface temperature
T. Other m param:ters, such as permeability, transmissibility,
salinityOetc., are of value in improving the assessment of the resource
and their knoRledge is essential to evaluate the reserves.
3. SHORT HAP DESCRIPTION
The first 5 Plates of the Atlas give a general vieK of the results
acquired on an European scale and represent the geodynamics and geothermal
perspectives, the heat-flow densi ty, the temperature at 1000 m depth, the
temperature at 2000 m depth and a revieR of geothermal resources based on
available information including all geothermal installations at present
operational or under construction. They are folloRed by more than 100
Plates of national maps. Host of the countries start Kith a Geothermal
Thematic Hap at a national scale indicating geological structures of
interest, springs and boreholes producing geothermal Kater (see Fig. 1) as
Rell as the geothermal installations. The next Plate sholls at one glance
the areas of geothermal resources and/or the geothermal potential areas
and finally, a Plate Rith temperatures at 500 m depth.

485
The three lIaps at the national scale are folloRed by regional lIaps
representing lIainly the essential characteristics for each acquifer, such
as depth, thickness, tellperature and - as far as possible - porosity,
per.eability, transllissibility and salinity. Syntheses of the results lead
to tbe presentation of geotberllal resources, expressed in joule per square
.etre, or of geotherllal potential areas. To set these data in context,
also lIaps dellonstrating the relationsbip betReen geological structures and
geotber.al reservoire are presented.
An exa.ple is given for the Valendis Sandstein of tbe Northern Basin
of the Federal Republic of Ger.any; Fig. 2 and Fig. 3. Tbe structure of
this for.ation, Rhich 1S also an i.portant oil reservoir, is characterized
by a R1de, east-aest directed syncline. Its geotherllal resource~ are close
to the clayey line in the south, and .axi.uII values of 2.5 GJ/. have been
assessed. The Rater consists of a bighly saline brine aitb te.peratures of
about 50 ·C at an average reservoir deptb of 1000 11. The probable reserves
2
have also been calculated and allount to 0.2' GJ/II .
4. SUHHARIZED RESULTS AND A SHORT OUTlOOl
Geother.al resources have been investigated in pro.ising areas. They
are presented on national and European scales and su •• arized in the
folloNing table Rbich de.onstrates the present state of knoNledge:
Country
Belgiull
Den.ark
Fed. Rep.
of Gerllany
Greece
Spain
France
Italy
The Netherlands
Portugal
Uni ted U ngdo.
Austria
SRi t:l8rland
Surface
kll 2
21 "0
150
9760
100800
87.0
5200
290
25000
4520
.000
ARB3
18
10 Joule
7170
H
HOO
32800
18U65
'950
70
6000
1980
1350
HIP H o
10 18 Joul e
O. 6
770
HI2
0.9
229
.100
1.1589
78
'.3
400
H5
100
Resources H1
10 18 Joule
O. 1
206
1H
O. 2
55
830
Outside these areas, no assessllent of geotherllal resources can be lIade
aith current data. This of course does not exclude the existence of
further resources. The fact that certain aquifers have been presented
Nithin a given area should not be taken to preclude tbe existence of
others !lithin the sa.e zone. In Italy the indicated value corresponds to
the highest val ue of heat in pI ace, H, assessed ai thi n one area of the
Hai n Reservoi r. 0
Finally, the resource .aps are thought to be a basis for scientists
and eng1neers, as Rell as for govern.ental and industrial decisions
lIakers. Furtberllore, it is hoped that the present docullent is encouraging
other groups to continue this Rork, to i.prove the procedure of assessllent
and to establish also a lIethod for an evalutation of reserves, Rhich could
be acceptable by IIOSt of the groups, aorking in tbis field.
21

486
5. REFERENCES
Brook. C. A .• L H. Harine~ D. R. Habey. L R.
L. J. P. Huffler (1979). Hydrothermal
reservoir temperatures ) 90·C. In:
Resources of the United States -
790. p. 18 - 85.
Silanson. H.
convection
Assessment
Guffanti. and
systems wi th
of Geothermal
1978. ~U~.~S~.~G~e~o~l~o~g~i~c~a~I~~S~u~r~v~e~y~C~l~·~r~c~u~l~a~r
Cataldi R .• A. Lazzarotto. L. J. P. HuffIer. P. Squarci. and G. Stefani
(1978). Assessment of geothermal potential of central and southern
Tuscany. - Geothermi cs. I. p. 91 - 1 31 .
Gringarten. A. C. (1979). Reservoir lifetime and heat recovery factor in
geothermal aquifers used for urban heating. - Pageoph .• 1!1.. p. 297 -
308.
Haenel. R. and E. Staroste (Ed.) (1988). Atlas of Geothermal Resources in
the European Community. Austria and Silitzerland. Hannover (Th.
SChafer) .
Haenel. R. (1983). EC Project on the evaluation of the community potential
of geothermal energy. - In: European Geothermal Update (Eds. Strub. A.
S. and P. Ungemach) - D. Reidel Publishing Comp.. Dordrecht. Boston.
Lancaster. ~ 21 - 38.
Kleefeldt. H .• I. Koppe. and R. Haenel (1983). Evaluation of geothermal
energy resources and reserves in selected areas of the Federal
Republic of Germany. - In: European Geothermal Update (Eds. Strub. A.
S. and Ungellach. P.) - D. Reidel Publishing Comp .• Dordrecht. Boston.
Lancaster. p. 82 - 89.
HuffIer. L. J. P. and R. Cataldi (1978). Hethods for regional assessment
of geothermal resources. - Geothermics. I. p. 53 - 90.

Figure 1: A cut or the Geothermal Thematic Hap or the Federal Republic or Germany.
The'map .how. the locationa or apringa/boreholea, volcanic areaa and geological
regional atructurea. The apringa/boreholea indicated with a running number are
explained in a .eparate table in the Atla •.

488
FEDERAL REPUBLIC OF GERMANY, Northern Sasln
. --
-~o o
• 1000
100
S 10 16 20 '2) 3J
10'
10'
lOJ
'\' . ;
[.\ •'.'
' ;~ ~ .
~L
'~ ,
' .....
Pigure 2 : De pth, temperature, salin ty, cross sect on t - I'
and a cross plot POl"oSlty/p@rme a b1 l 1ty o r the
aQuir@C' • Valendis Sandsteln' .

4 9
• L' __ -.L_-:'-l'" , I
o •
.. =-----

4W
EEC contract n° EN3G-00II-D (B)
GEOTHERMAL RESOURCES AND RESERVES:
UPDATING OF TEMPERATURE DATA BASE
R. SCHULZ and K.H. WERNER
Geological Survey of Lower Saxony
Summary
Temperature data of about 7200 wells in the FRG were registered
and stored with additional informations in a
computer data bank. Values, which have not been measured
in temperature equilibrium, are automatically corrected~
thereby the type and the quality of the measurements are
taken into consideration. Depth profiles of the temperature
and its gradient, maps of the location of the wells
and maps of temperature isolines at optional depth and
scale can be produced by a computer procedure. Data of
lower quality are smoothed by weight functions depending
on their own quality and their distances to data of higher
quality. The data base will be continuously updated.
The average temperature at depth of 1 km and more is
higher by 5 to 10 K than the temperature in older maps.
Therefore the geothermal potential is higher than expected
until now.
1. INTRODUCTION
The true subsurface temperatures are important input data
for the assessment of geothermal resources and reserves. The
purpose of the project was to update the temperature data base
for the Federal Republic of Germany (FRG). The following steps
had to be taken:
- Storage of all available temperature values in a computer
data bank~
- determination of the virgin rock temperature~
- computerized construction of temperature depth profiles and
temperature-isoline maps.
The project started on 01.01.1986~ its duration was 30 months.
The results are summarized in a final report (Werner and
Schulz, 1988).
2. TEMPERATURE DATA
The temperatures of the subsurface are measured by different
methods, especially by
- logging in wells with equilibrium temperature or in wells
with a disturbed temperature field caused by drilling,
cementation, hydraulic circulation etc~
- testing (drill stem test, sampling, etc.)~

491
- single point measurements in mines or tunnels:
- bottom-ho1e-temperature (BHT) measurements immediately after
drilling.
Temperature data of about 7200 wells (status Oct. 1988)
in the FRG are available:
Logs of equilibrium temperature
Logs of disturbed temperature field:
Temperatures of tests
Temperatures of mines
Wells with BHT
including BHT measurements
515
116
282
758
5525
with 3 or more values 152
with 2 values 1252
with 1 value and with shut-down time 2749
with 1 value and without time information: 5366
Since RHTs can be measured in one well at different depths during
the drilling operation, the sum of the measurements are
larger than the number of the wells.
The temperature data together with further necessary informations,
like location, depths, altitude, etc., and a
classification number, dependent on the type and quality of
the measurement, are stored in a computer data bank. For this
purpose a new program system was devo1ped (Bo11erhey and Werner,
1988), based on a geological data bank system (KUhne,
1983).
3. CORRECTION METHODS FOR BHT VALUES
As seen above the most temperature values are BHT measurements.
These temperatures are disturbed by drilling with
mud circulation and have to be corrected, since the virgin
rock temperature (VRT) is requested. Dependent on the quality
and quantity of the measurements different correction methods
are implemented.
If 3 or more BHT values in one depth at different times
after cessation are available, the thermal stabilization
method for a cylindrical borehole can be used (Leblanc and
others, 1982). In this approach the temperature of the mud at
the circulation stop (t ~ 0) is assumed as a constant, which
varies by t. T from the VRT. The BHT values describe the temperature
stabilization during the shut-down time, and the VRT
is given by
VRT - BHT (t) - t. T (exp (-al /4 K t)-l}
where VRT - virgin rock temperature (·C)
(1)
BHT measured bottom hole temperature (·C)
t.T - initial temperature disturbance (K)
a - borehole radius (m)
K - thermal diffusivity (ml/s)
t - time after cessation of drilling (s).
The VRT is calculated by a fitting method varying t. T and K : K
is the effectiv thermal diffusivity of the mud and the
surrounding rocks.
If 2 BHT values are available only, a line source approach
is used. If the negative heat transfer during the
circulation time should be considered, the VRT is given by
VRT - BHT (t) - q/(4lT ).) 1n «t + s)/t) (2)
where q - heat flow rate per length unit (\i/m)

492
A = thermal conductivity (W/mK)
s = circulation time (s).
Eq. (2) is the base for the Horner-Plot (Horner, 1951).
If the circulation time can be neglected, a line source
approach with an explosion heat sink is used (Lachenbruch and
Brewer, 1959)
VRT = BHT (t) - 0/ (4n At) ( 3 )
where 0 = heat per length unit (J/m) •
In most cases 1 BHT value is available only. Then Eq. (1)
can be used again, but the unknown temperature disturbance and
the effective thermal diffusivity have to be estimated. The
numerical base for a statistical determination, using BH~
measurements with 4 and more va1ues~ is too weak at present,
therefore an estimate ofK = 1.2 10- 1 m2/S is used: t,T is
estimated dependent on the BHT values and the surface temperature.
In the records of the wells, drilled before 1970, the
time after cessation is often not registered. Therefore this
time also has to be estimated by comparision with wells in the
surrounding area. These temperature data have low quality and
have to be handled carefully to avoid geological misinterpretation.
4. MAPS OF TEMPERATURE DISTRIRTJTIO~
Temperature data can be presented very impressively by
maps of the temperature distribution at distinct depths. One
purpose of the project was to produce such maps automatically
by a computer procedure.
The density of the temperature data varies considerably.
Because of the oil and gas exploration there is a high density
in the North German Basin, in the Upper Rhine Graben and in
the Molasse Basin in South-Germany. On the other hand there
are no deep wells in the Rhenish Massif, in the B1ac~ Forest
and in the Bavarian forest. With regard to the heterogeneous
density and the various quality of the data the representations
of the temperature distribution should be confined to
regional areas. \ie have chosen areas of about 90 km x 90 km,
corresponding to geological maps in a scale of 1:200.000
(CC-maps). For each area special investigations were carried
out to produce the most probable iso1ine map.
Dependent on its quality each temperature date can be
supplied with an optional weight factor: e.g. an equilibrium
temperature with 1.0, a test temperature with 0.8, and BHTs
with factors from 0.7 down to 0.1 dependent on the used correction
method. Additionally a weight function and a radius
of action can be chosen, so within this circle temperature values
of high quality are not changed and values of low quality
are smoothed. With these weight functions the temperatures
are calculated at equidistant grid points and the iso1ine
program starts.
The construction of iso1ine maps is illustrated by an
example: the map for the area ~C 3926 Braunschweig (North Germany).
There are more than 400 wells, which are deeper than
1500 m (Fig. 1), that isa very high density. But most of the
temperatures are BHTs, generally with 1 measured value and
without a known shut-down time. There are just 8 wells with
temperature measurements of high quality, and 34 BHTs with 2

493
.. f>-
+ +
5850000
+ + + + * +
+ +
+ + +
+
5840000
+ ~+J ~
+ + .... + +
..
++
1+ + ;,(:~.
+
5830000 it +At
+
.,.
5820000 +..-+ +
+
+ ~+ +
++ +
+
+ + +
+
+
+
•
+
+
+ 11 + + +
5810000 + *
5800000 +
+ .*
5790000 +
+
+
5780000
+
5770000 +
!At •
~
-

494
5850006 >- + y
2Hf.
5840000
+ +
2H
3Z
5830000 ... +
5820000
JOO JOO
...
+
230
ZH
+
zftl;H
5810000 ZH ZH
+
110 +
5800000
+
5790000 2H
+ 110
110
+
5780000
5770000
AJ --
Y
3580000 3600000 3620000 3640000 3660000
Fig. 2: Area CC 3926 Braunschweig (distances in m)
Location of wells with temperature data of acceptable
quality at 1500 m depth: status: Nov. 1988.
Classification:
110: equilibrium temperature
230: disturbed temperature field
300: testing temperature
3Z: BHT with 3 values, correction according to Eq.(l)
2H: BHT with 2 values, correction according to Eq.(2)
2L: BRT with 2 values, correction according to Eq.(3)

495
5790000
5780000 ~
1c::,
~60-
~65.
7 _
3600000 3620000 3640000 3660000
Fig. 31 Area CC 3926 Braunschweig (distances in m)
Map of the temperature distribution at 1500 m depth:
statusl Nov. 1988.
Weight factors:
1.001 equilibrium temperature
0.001 disturbed temperature field
1.001 testing temperature
0.801 BHT with 2 or more values
0.401 BHT with 1 value and shut-down time
0.161 BHT with 1 value and without time
Smoothing functionl exp (-Ric) where c - 3000 m

496
CD
o
- '"
UI
.....
-co-
...... 0
=-=8
3'
*
N
N
UI
o
-ON
.....Cl
en
o '"
o
...
51
8
Fig. 4: Area CC 3926 Braunschweig
Depth profile of average temperature: status: Nov.1988
Weight factors and smoothing function: see Fig. 3
Depth (m) Number of wells Range ( • C)
500 819 18.3 - 38.4
1000 682 27.0 - 65.2
2000 119 53.0 - 103.2
3000 8 88.1 - 128.4
4000 5 112.7 - 165.2

497
or more values (Fig. 2). If we use these values only, large
areas have to be blanked. If we use all data without the
weight functions, a diffuse and unrealizable temperature
distribution is yielded. Therefore weighted and smoothing
functions have to be used. Fig. 3 shows the isoline map for
the chosen weight factors. Isolines will be plotted only if a
temperature date is available within a radius of 10 km: otherwise
there are breaks and blankings.
With the same weight functions a depth profile of the
average temperature is constructed (Fig. 4). The remarkable
change of the gradient at a depth of 2500 m is caused by the
reduction of the number of wells (from 119 down to 8). This
effect must not be connected with a possible geological
structure.
~his example shows, that maps or profiles of temperature
cannot be interpreted without knowledge of the quality and
quantity of the data. An interpretation with regard to geological
formations, deep convection systems etc. is allowable
only if an extensive analysis of the data situation is made.
The isoline program uses rectangular coordinates. For
large areas the conformality is violated. Maps of the temperature
distribution of the FRG (Fig. 5) are produced by combining
the computerized isoline maps by hand and fitting, if
necessary, the isolines at the boundaries of the single maps.
The solid lines are within a circle of a radius of 10 km from
a measured well, the dashed lines are interpolated. Areas
without any temperature data are blanked.
Compared with older maps (Wohlenberg, 1979: Haenel, 1980)
more details are included in the new maps caused by the greater
data base. But these maps give an overview only: anomalies
in larger areas apparently isotherm cannot be excluded. An
example is the anomaly to the north of Munich, which was
unknown until exact temperature logs were carried out in three
new wells.
The average temperature values are higher by 5 K at a
depth of 1000 m (Fig. 5), and by 10 K at 2000 m in the new
maps than in the older ones. In the older maps the correction
of the BHTs was not carried out.
So the maps of the temperature distribution present only
the information at the present state. The data base will be
continuously updated and new maps can be produced very fast,
if new or corrected temperature data are availa9le.
Fig. 5
Federal Republic of Germany
Map of temperature distribution at 1000 m depth below
ground level: status: July 1988.

498
.. f'·-
--~--
-- ~ .
"I
,/
t
\
I
\
l ,
,'"
.... -~_\ "­
\
I I
'-.
I,.
.,.---
- ...

5. REFERENCES
Bo11erhey, M. and K.H. Werner (1988). TEDAHM - Programmsystem
zur Erfassung und Oarste11ung von Temperaturdaten
aus Bohr10chmessungen. - NLfR-Archiv Mr. 103 144
(unpublished report).
Haenel, R. (Ed.) (1980). Atlas of Subsurface Temperatures in
the Eurorean CommunIty. Th. Schafer, Hannover.
Horner, D.R. (1951 • Pressure build-up in wells. Proc~ Third
World Petroleum Congress, 34, 316.
KUhne, K. (1983): DASP - Ein System zur Verwa1tung und Auswertung
geowissenschaft1icher Oaten. Geo10gisches
Jahrbuch, A 70, 41-59.
Lachenbruch, A.H., and M.C. Brewer (1959). Dissipation of the
temperature effect of drilling a well in Arctic
Alaska. Geological Survey Bulletin 1083 - C,
73-109.
Leblanc, Y., H.-L. Lam, L.J. Pascoe, and F.W. Jones (1982).
A comparison of two methods of estimating static
formation temperature from well logs. Geophysical
, 30, 348-357.
Woh1enberg,
prospectinr J. (1979 • The subsurface temperature field of the
Federal Republic of Germany. Geo10gisches Jahrbuch,
E 15, 3-29.
Werner, K.~and R. Schulz (1988). Geothermische Ressourcen
und Reserven: WeiterfUhrung der Temperaturdatensammlung
(Final Report). NLfB-Archiv Nr. 103 143
(unpublished report).

soo
CEC contract no. EN3G-077-NL
and no. EN3G--0097-NL
EXPLORATION AND EVALUATION OF GEOTHERMAL RESOURCES IN
THE CENTRAL GRABEN AREA, THE NETHERLANDS
J.P. HEED ERIK
TNO Institute of Applied Geoscience
and R.M. VIERHOUT
ComprimoBV
Summll1Y
In The Netherlands, research on the possibilities for exploitation of geothermal energy
was started about 10 years ago. The fIrst exploratory geothermal well, however, was
completed in 1987. The well was drilled by TNO-Institute of Applied Geoscience within the
framework of the National Research Programme on Geothermal Energy and Energy Storage
in Aquifers (NOAA) on behalf of the Management Office for Energy Research (NOVEM,
formerly PEO). The research was funded in part by the Directorate General for Research and
Development (DG Xll) of the Commission of the European Communities.
Previous inventory studies carried out between 1979 and 1984 gave a global picture of
possible locations for geothermal exploitation. The primary aim of the geothermal exploration
well was to acquire reliable geological data and reservoir characteristics for the entire profile
of the hole. The geological monitoring programme together with core analysis studies
provided accurate lithostratigraphical and lithological descriptions of the various reservoir
formations relevant to geothermal energy.
Geochemical information on the chloride content of the formation water at various depths
was obtained by testing the pore water in sediment samples. During the testing of the well
water samples were collected for complete analysis.
The geothermal reserves of two potential reservoirs are estimated from the results of the
test well. The results and conclusions of the geothermal study of the Central Graben were
recently set out in the final report. the principal results of which are summarized and
explained in this article.
1. INTRODUCTION
In the Netherlands, research on the exploration and exploitation of geothermal energy was
started about 10 years ago. In the framework of the National Research Programme on
Geothermal Energy and Heat Storage, between 1979 and 1984 a large number of studies
were carried out on energy storage and geothermal energy:
research on temperatures at great and shallow depth;
- an inventory survey of the geothermal potential of different geological formations;
a study on the reservoir characteristics of potential aquifers;
- two feasibility studies for geothermal energy projects in Spijkenisse and Delfland.
On the basis of the results and conclusions of these studies, in 1985 the National Research
Programme on Geothermal Energy and Aquifer Energy Storage (1985-1989) was

SOl
fonnulated. Major components of this research programme are; a study of the interactions
between operational and geohydrological requirements, a more detailed smvey of aquifers
suitable for energy storage and geothermal energy exploitation, and investigation of the
geothenna1 reserves of identified potential fonnations.
,. ,
....,., II ,·OOCII CWlU'
~
• L~_.-nco.
...-• .-.roo
-------
IIIDOeI TIGILIN
.,..."""
------- -------
"lOCI"
a.

502
The first exploratory geothermal well in The Netherlands was completed in 1987. It was
commissioned by the Management Office for Energy Research (NOVEM, fonnerly PEO)
and drilled by the TNO Institute of Applied Geoscience as part of the National Research
Programme on Geothermal Energy and Energy Storage in Aquifers (NOAA). At all stages
this work was done in collaboration with the Geological Survey of The Netherlands (RGD),
the Utrecht State University (RUU) and Comprimo B.V. (engineers & contractors). The
research was funded in part by the Commission of the European Communities.
2. GEOLOGY
The primary aim of the test well was to acquire reliable geological data and reservoir
characteristics for the entire profile of the hole. The final depth was originally planned to be
1550 m below mean sea level. But detailed study of the Asten (1) well-logging data revealed
adequate reason for continuing the hole to a final depth of roughly 1650 m
Figure 2:
Tectonic situation of the region and position of the "Asten 2 Block" on the edge
of the Central Graben (Source: National Geological Service).

503
At forehand the following potential formations were identified:
approx.depth
- Breda Formation 940 - 1000 m
- Voon Sand (Veldhoven Formation) 1200 - 1400 m
- Basal Dongen Sand (Dongen Formation) 1500 - 1530 m
- Houthem Formation 1650 - 1670 m
Subsequently, the test well results, particularly the core tests, indicated that it waS worth
investigating the Berg Sand. The test well provided accurate lithostratigraphical and
lithological descriptions of the various reservoir formations relevant for geothermal
exploitation. The lithostratigraphic position of these beds is given in figure 1.
Tectonic activity during the history of geological development, specially during the
Tertiary and Quaternary, has brought about an extensive fault system in the Asten region.
The faults are partly in a NW-SE and partly in a WNW-ESE direction, so various blocks
have developed. Downthrow of the Central Graben area along the NW-SE fault system has
resulted in differences of more than 1000 m in Tertiary base depths. The Tertiary base is at a
depth of some 850 m west of the Central Graben and at about 600 m to the east. The
maximum Tertiary base depth in the Graben is almost 2000 m.
In terms of geological structure, the fault system of the Central Graben consists of several
faults which can be tracked over great lengths (the peripheral Peel fault is the most
important), interspersed with a large number of smaller faults at varying slips. In many cases
the crust part between the faults is broken down into smaller blocks. The test well is in such
a block. The bordering NW-SE faults are 2.5 kilometres apart and the distance between the
N-S faults is roughly 2 kilometres. The faults and the Tertiary base contours are shown in
figure 2.
ctiorides (mg/I)-
o
25000 50000
o+-------~-------J
- freshwater saltwater mt (150 mg I)
500
[
i I
1000
•
1500
2~ •
2000
Figure 3:
Otloride contents of the formation water in Asten.
(Source: Vening Meinesz Laboratory, Utrecht National University)

504
3. GEOCHEMISTRY
It is usually difficult to obtain good well data on the composition of formation water at
various depths. Water is produced only occasionally and even then not until the final depth is
reached. The Utrecht National University (RUU) developed a method to fill the gap in
geochemical information - i.e. testing the pore water in sediment samples. The suitable
sample material for this testfug was core material, "roller bit" lumps, stabilizer and lumps
carried up and "little lumps" sampled on the vibrating screen. Sample quality deteriorated in
that sequence. The samples were checked for drilling mud contamination.
The calculated chloride content of the pore water at various depths is given in figure 3. As
chloride is the major component of formation water below the "fresh-salt" water limit, the
chloride gradient is also representative of the total content of dissolved matter. The "freshsalt"
contact (chloride content approx. 150 mg/l) in Asten is at roughly 320 metres. The
minimum values in the curve give an impression of the salt content during an earlier
sweetening phase while the maximum values are an indication of the present increasing
salinity of the arenaceous layers.
Chemical analysis of water samples collected during the testing of respectively the
Houthem Formation and Berg Sand confirmed this picture. Although the salt content of the
formation water from the Berg Sand is higher than from the Houthem Formation, the
chemical composition of both waters is highly comparable.
Figure 4 shows a hydrogeological profile orientated according to the most probable
direction of flow of the deep groundwater. It is assumed that the deep groundwater is
supplied from the southeast. The deep groundwater in the Central Graben is a "mixed water"
of fresh infiltration water from the Ardennes and the Eifel and saline to very saline thermal
formation water percolating upward through crevices and zones with karst development and
discharging into the Tertiary cap of the Central Graben. The salinity increases in a northwesterly
direction. In the Central 'Graben, the fresh-salt contact (150 mg/l chloride) and the
1000 mg/l isochloride contact are hundreds of metres apart.
o
NW
Andel
Maas
As1en
Roermond
Maa!
Grans
20
800
1600
2400
m
Central Graben
ISSS3 consolidated rock
b' ="~ isochloride plane
I
o
I
50 km
Roerdal High
• 60000 mg/l
streamline with chloride content
Figure 4:
Hydrogeological profile across Asten orientated according to the most probable
direction of flow of the deep groundwater.
(Source: Vening Meinesz Laboratory, Utrecht National University)

50S
4. FORMATION EVALUATION
Porosity, pore content, permeability and temperature are the principal characteristics of
geological formations which together determine the geothermal potential and, to an important
extent, the reservoir behaviour .. Pettophysical interpretation methods play an important role
in the process of formation assessment which uses, among others, data from· geophysical
well-logging and core tests to evaluate these characteristics.
Porosity and permeability of the cored sections are measured in the core samples. By
correlating core porosity and permeability across the cored sections with the calculated and/or
- from various geophysical well-logging data - derived porosity and permeability.
porosity and permeability values can be determined for the uncored sections.
The average or representative porosity and permeability were calculated for the various
reservoirs considered for geothermal application as well as the total transmissibility of the
reservoir consisting of various beds. The so-called productivity index can be calculated with
the steady-state flow equation. The productivity index is a value indicating the production of
the relevant reservoir at a pressure drop of one bar. The results of the formation assessment
can be summarized as follows:
Table L Reservoir characteristics of the formations studied.
Formation Depth Thickness Porosity TransmissiVity Productivity Temperature
(m) (m) % mDm (m 3 /h1bar) °C
Breda 939 - 998 59 40 47.130 13.00 35.5
Voort 1196 - 1415 219 32 1.490 0.50 49.2
Berg 1494 - 1513 19 35 6.120 2.50 59.5
Dongen 1518 -1530 17 24 70 0.03 60.0
Heers 1628 - 1636 8 38 1.870 0.75 62.3
Houthem 1647 - 1665 18 23 nla 0.60 62.9
Two of the formations investigated can be considered for geothermal exploitation. i.e.:
- Breda Formation (Kakert Bed) with a productivity index of Pi = 13 (m 3 /h)!b8r and an
average temperature of 35.5 0c, and the
- Berg Sand with a productivity index of Pi = 2.5 (m 3 /h)/bar and an average temperature of
59.5°C.
5. GEOTIIERMAL RESERVES
The geothermal reserve of an area depends on the temperature gradient which allows
calculation of the temperature at any required depth and on the reservoir characteristics which
dictate the recoverability of the hot formation water. In The Netherlands the temperature
gradient varies from place to place between 3 °Clloo m and 4 °Clloo m. The average
temperature gradient in the Asten block is approximately 3.2 °Clloo m
The thermal reserve of a specific reservoir is determined by the reservoir volume. the
amount of formation water available and the average temperature, the water density and the
matrix. as well as by the heat capacity of the water and the matrix. Figure 5 shows the
temperature distribution across the Asten block for the reservoirs considered for geothermal
energy production. Two cases are distinguished in determining the usable heat ~T;
geothermal energy production, with installation of a heat pump and without installation of a
heat pump. The minimum injection temperature without a heat pump will be approximately
30°C, while the injection temperature with a heat pump can drop to 17 0c. Not all the heat

506
available can be produced. In France it is customary to use a production factor of 0.33. As
there are no supporting studies on the situation in The Netherlands, a conservative
production factor of 0.25 is assumed for this research.
Breda Formation
ITIIIll > 25"C
E3 >3O"C
~ >35"C
rum >4O"C
~ >45"C
~ >SO"C
~ >SS"C
~>6O"C
mIDI >65"C
~ >7O"C
Figure 5:
Temperature distribution in the Central Graben for two potential geothermal
reservoirs.
Using the above criteria to calculate the geothermal reserves in the Asten fault block leads
to the following result:
Formation
Breda Formation
Berg Sand
Total of Asten fault block
Expressed in tonnes oil equivalent
Expressed in natural gas equivalent
Geothermal reserves
without heat pump with heat pump
27.5 x 10 15 J 43.4 x 10 15 J
7.6 x 10 15 J 12.0 x 10 15 J
35.1 x 10 15 J 55.4 x 10 15 J
838 x 1()3 TOE 1323 x 1()3 TOE
998 x 10 6 m 3 (S1P) 1575 x 106 m 3 (STP)
A rough estimate of the geothermal reserves in the Central Graben can be made by
extrapolating the test well results for the Asten fault block. It is assumed for such
extrapolation that the reservoir characteristics of both formations are continuous in the
Central Graben. To allow for the uncertainty in this assumption, the reserves were multiplied
by a risk factor of 0.25.

507
The reserves in the Central Graben are:
Formation
Breda Formation
Berg Sand
Total of Central Graben
Expressed in tormes oil equivalent
Expressed in natural gas equivalent
Geothermal reserves
without heat pump with heat pump
310 x 10IsJ 490 x 10lsJ
90 x 10IsJ 140 x 10lsJ
400 x 10lsJ 630 x 10lsJ
9.5 x 1()6 TOE 15 x 1()6 TOE
11.4 x 10 9 m 3 (STP) 17.9 x 10 9 m3 (STP)
The disadvantage of this method of quantifying geothermal reserves is that a limited
allowance is made for economic recoverability of these reserves. To counter this objection,
the geothennal reserve can be expressed in the so-called heat capacity of the formation at a
specific location. The heat capacity is expressed in the energy supplied per unit temperature
and pressure drop and is a function of the reservoir characteristics of the formation, the
temperature-dependent liquid properties and the well radius and radius of influence of the
well system.
The radius of influence of the well system is estimated at approximately 1000 m. If it is
also assumed that the well is finished with a 6" liner, the heat capacity can be calculated at
any required location in the Central Graben with the adapted equation for steady-state flows.
It is also possible to compile a heat capacity map of the area for any relevant formation. The
thennal capacity of a production well in a geothermal-doublet can be determined at any
required location with the heat capacity map presented in figure 6, in conjunction with the
information on the productivity of the formation (table 1) and the temperature map given in
figure 5.
Figure 6:
Heat capacity map of two potential geothermal reservoirs in the Central Graben
(kW/bar "C).

508
The heat capacities of the two potential fonnations in Asten are:
Breda Formation
Berg Sand
Heat capacity
21.3 kWlbar·C
4.0 kWlbar·C
If the required pressure drop L\p and the difference between production and injection
temperature L\T are known, the capacity of an eventual geothermal-doublet can be calculated
quite easily:
L\p
L\T
Thermal capacity
Breda Formation
with heat pump
lObar
18.5 ·C
3.9MW
Berg Sand
without heat pump
20 bar
29.5 ·C
2.4MW
6. POSSmILITIES FOR GEOlHERMAL ENERGY PRODUCTION
Two possible geothermal reservoirs have been identified; the Berg Sand and the Breda
Formation. However, for geothermal energy production from the Breda Formation at a
temperature level of roughly 35 ·C the boundary conditions of the energy consumers could
make it necessary to use a heat pump. The power requirements of the various pumps - head
about 250 metres and a pump capacity of respectively 50 and 230 m 3 /h - are so high that
co-generation could become attractive in economic tenns.
Various energy consumers categories have been considered for the evaluation of the
economic and technical feasibility of geothermal energy production, either from the Berg
Sand or from the Breda Formation, i.e. greenhouse market gardens, apartment buildings,
hospitals, etc. Calcuiatiohs 'based on the following assumptions; lifetime 25 years, internal
interest rate 4% per year and a fixed payback time of 10 years, show that at this moment the
equivalent gas price of geothermal energy amounts to approximately two times the present
natural gas price. Although geothermal energy production at present is economically not
feasible, in the near future other criteria, such as environmental considerations, may playa
decisive role in the decision making. The evaluation of the feasibility shows that - in
general - geothermal energy production in the Central Graben area from the Breda
Formation with using a heat pump and co-generation unit is more feasible than production of
geothermal energy from the Berg Sand.
REFERENCES
Doorn, Th.H.M. van, et al., 1985. Geothermal Energy Exploitation and Large Scale Heat
Storage in Tertiary and Lower-Quaternary Formations (in Dutch). Report 85 KAR 02
EX, 108 pp. Geological Survey of The Netherlands, Haarlem, The Netherlands.
Dufour, F.C., et at, 1983. Feasibility Study and Preparation of an Evaluation Well for the
Geothermal Demonstration Project Delfland (in Dutch). Report OS 83-29, three parts.
TNO Institute of Applied Geoscience, Delft. The Netherlands.

S09
Haak. A.M., Heederik, J.P. and Vierhout, R.M., 1985. Preliminary Investigations for Pilot
Borings in Asten (in Dutch). Report OS 85-36, 37 pp. TNO-Institute of Applied
Geoscience, Delft, The Netherlands.
Heederik, J.P. and Huurdeman, A.J.M., 1988. Exploration and Evaluation of Geothermal
Resources in the Central Graben Area, The Netherlands. Geothermal Prospects of the
Asten Project Proceedings and Information 40; Comminee for Hydrological Research
TNO, The Hague, The Netherlands.
Mot, E., et aI., 1984. Report on the National Research Programme on Geothermal Energy
and Energy Storage, 1975-1984 (NOA-I) (in Dutch). 118 pp. Energy Research Project
Management Office, Utrecht.

510
EEC Contract nO EN3G-0031-I (S)
ASSESSMENT OF THE LOW ENTHALPY GEOTHERMAL RESOURCES
OF THE PO VALLEY PLAIN ITALY
G. GHEZZI,. R. GHEZZI and H.P. MARCHETTI
GE.T.AS.s.r.l. '. Pisa
Summary
The geothermal potential of the Po Plain has been assessed down to
1000 m depth. Geological and hydrogeothermal data have been collected
from hydrocarbon and water wells and stored in a Data Bank.
Temperature and thermal gradients have been framed within the complex
structural picture of the area,. where the northern front of the
apenninic folds faces a southward plunging pedealpine homocline.
Temperature Haps are set out for depths of 300,. 500 and 1000 m,
b.g.l. The lithostratigraphic units have been subdivided into
continuous and discontinuous aquifers or aquicludes. Permeability and
net pay of continuous aquifers are defined according to the measured
·values. The Transmissibility is so obtained and contoured in order to
supply a first general evaluation of the hydrogeological potential.
The synthesis is given by Thematic Haps constructed for depths of
300, 500 and 1000 m. They show the main characteristics of
temperature,. water salinity and Transmissibility, and the location of
the most promising areas where relatively high temperature coincides
with favourable reservoir characteristics. The low enthalpy resources
and possible thermal yield of a 2-wells plant have been tentatively
quantified for one of these areas, located in northern Lombardia.
1. INTRODUCTION
The Po Valley plain is located in a foredeep basin area characterized
by low geothermal gradients and rather uniform temperatures.( Italian
Working Group,. 1984). Nevertheless it represents one of the national
priority targets for the exploitation of the low-enthalpy geothermal
potential. Due to the remarkable concentration of possible users for
various types of projects,. a mUltiple utilization of water thermal energy
could be feasible,. also in combination with conventional sources. In this
frame,. the evaluation of the geothermal potential executed in the study
covers the subsurface section not deeper than 1000 m b.g.l. '. aiming to
define the thermal conditions of favourable shallow reservoirs whose
exploration and exploitation would require low investment costs.
The study has been carried out on the basis of the interpretation of
about 1300 hydrocarbon wells (AGIP) and water wells '. selected among more
than 2000. All the useful information supplied by the wells has been
stored in a Data Base Bank type DB-3-Plus. The general scale of the maps
elaborated under the contract is 1:200.000.

51\
2. GEOLOGICAL PICTURE
The subsurface geology of the Po Valley Plain is rather well known
after the great number of seismic and well data collected by AGIP for
hydrocarbon exploration. Only a part of this wide information was
accessible for the present study: mainly published data,. well logs and
papers on the subsurface stratigraphy and structure. Anyway these studies,.
based on the lithostratigraphic,. geophysic and micropaleontologic analysis
of all the significant hydrocarbon wells,. allowed to submit a detailed and
updated breakdown of the subsurface formations and a definition of the
relationships between sedimentation,. structure and paleogeography.
Pieri and Groppi (1981) showed that the structural pattern of the
basin was a result of the relationships between the external buried front
of the Northern Apennines (Fig. I), consisting of three main folded arcs
(from west to east: the Monferrato,. the Emilia arc and the Ferrara-Romagna
arc), with the foreland area,. consisting of the pedealpine and Veneto
homoclines. The Apennine folded belt '. of northern polarity, overthrusted
on the south dipping homocline (upper part of Fig. 2). The front of the
Apenninic structures is located in the subsurface of the Po plain,. several
km north of the Apennine foothill. The age of the movements ranges from
Messinian to Pleistocene,. with main phases during Upper-Middle Pliocene.
The homocl1nes are fairly uniform and continue with two main breaks,
located in the south of the Prealpi Lombarde and in the north-eastern
sector of the Veneto Friuli plain.
The lithostratigraphic picture is quite complex due to the lateral
and vertical variability caused by different deposition environments,.
tectonic evolution and rate of subsidence. Above the Mesozoic to Eocene
carbonatic substratum deposition took place during two main complex
sedimentary cycles (Dondi and D' Andrea,. 1986):
- pre-evaporitic and evaporitic Oligo-Miocene cycle. Lower offshore to
deep basin marls (Gallare Group) and marly-arenaceous turbidites
(Harnoso-Arenacea Fm) are prevalent in the Oligocene-Tortonian sequence.
Only along the pedealpine margin coarser submarine fans deposits are
present (Gonfolite Group). During the Messinian,. deposition of
evaporites (Gessoso - Solfifera Fm) and of coarser clastic sediments
( Sergnano Fm) takes place,. due to the general salinity crisis of the
Mediterranean Sea and the increase of emerged lands.
- Post-evaporitic Upper Messinian-Pleistocene cycle. Hypohaline
sedimentation takes place after the Late-Messinian transgre~sion.
Clastics prevail,. with turbiditic sediments (Fusignano and Sartirana
Fms) in the deepest part of the basin and with sand (Cortemaggiore,
Caviaga Fms) in the coastal and marginal areas. During the Pliocene a·
progressive subsidence occurs with a consequent increase of basin areas
where thick turbidites are deposited (Porto Corsini and Porto Garibaldi
Fma). The lower offshore is characterized by pelitic sediments (Santerno
Fm) and the coastal shelf by sand (Cortemaggiore Fm). At the end of the
Pliocene,. the basin shallows up to its recent emersion: the filling
consists of shelf sand (Asti Fm) and later of continental (mainly
alluvial) deposits.

512
For the purposes of this study 26 lithostratigraphic units have been
singled out above the carbonatic substratum. The formation sequence has
been assessed for 500 hydrocarbon wells and 300 water wells,_ on the basis
of either AGIP information or electrical and lithological logs correlations.
An example of formation correlation is given in the lower part of
Fig. 2. A composite interpretation of a well log is given in Fig. 3.
3. HYDROGEOLOGY
According to the lithostratigraphic breakdown, two main
hydrogeological systems can be identified in the subsurface of the area:
1) Discontinuous aquifers system. It includes the formations of the
carbonates sequence,_ which in this area is older than the Upper Eocene.
The carbonates generally exibit a pronounced fracture permeability so
that their geothermal potential can be significant. Anyway due to the
discontinuous and erratic distribution of the permeability and to the
lack of measured values,_ no quantitative estimates of the productivity,
or any other hydrogeological parameter, appear to be reliable. The
occurrence of carbonates within the investigation depth (1000 m) is
confined to the Alps foot-hill of Lombardia and Veneto,_ to the area
surrounding the Euganei-Berici hills and to the easternmost Friuli
plain.
2) Continuous aquifers system. It includes the permeable formations of the
post-Eocene sedimentary cycles. These can be mainly considered as porous
media even if in some case a fracture permeability may be present.
According to the lithological and electrostratigraphical characteristics
and to the occurrence of water or hydrocarbons,_ the 26 units
forming the sedimentary sequence have been subdivided into 6 aquiclude
and 20 aquifer units. An example of hydrogeological interpretation of
electrical and stratigraphical logs is shown in Fig. 3. The resulting
aquifer geometry, reported in 15 cross-sections, reflects the above
mentioned structural and sedimentary complexity. (Lower part of Fig. 2).
The shallower complex is the multilayer aquifer of the Continental
Quaternary, its thickness ranging from a few metres up to more than 500 m.
The wide extension and the occurrence of several aquifers,_ often very
permeable, make this complex of greater interest. The Continental
Quaternary generally rests on the ,Plio-Pleistocene Asti sand aquifer, and
the 1000 m isobath line falls within it over two-thirds of the area. The
most interesting aquifers, of Lower Pliocene to Messinian age (Caviaga and
Cortemaggiore Fms, Sergnano Fm and its lateral equivalents) are separated
from the Asti Fm by a thick aquiclude (Santerno clay Fm). Their occurrence
within the range depth of 1000 m is confined to the anticline tops and to
the northern belt of the Pedealpine homocline. The deeper aquifers belong
to the Oligocene-Tortonian units (Gonfolite,_ Ottobiano,_ Serravalle and
Marnoso-Arenacea Fms): they seldom occur above 1000 m depth and often show
low permeability and lateral.passages into the Gallare marl pelitic unit.
The last shows up as the basal aquiclude,_ separating the Continuous System
from the underlying carbonate sequence.
For the units of the Continuous Aquifers System, a tentative

513
quantification of the hydrogeological potential has been carried out by
extrapolating punctual measured parameters. The parameters used for the
evaluation are the permeability and the net pay thickness. For 10 out of
the 20 aquifer units,. permeability values were supplied by core analyses
or production tests. Extrapolations and assumption of values for the
aquifers devoid of known parameters have been set out according to all the
information about lithology, facies,~eposition environment,. vertical and
horizontal variability. All the retained values of permeability have been
taken,. on the basis of conservative criteria,. near the minimum of the
measured ranges. The net pay values have been evaluated from the available
Resistivity and SP Logs for each formation and,. within a formation,. for
each main chronological interval. Statistical average net thickness as
percentage of the gross formational thickness has been assumed for each
formation in the areas where the values appeared to be fairly uniform. The
results have been assembled in 5 hydrogeological maps, relevant to the
sequences of: Pleistocene '. Upper-Middle Pliocene '. Lower Pliocene, Upper
Miocene,. Oligocene to Middle Miocene. Each map shows:
- distribution of aquifers and aquicludes; areas of missing sedimentation
(or erosion) in the given chronological interval; differentiation of the
aquifers unit according to the formations.
- average net pay (h,l of the gross thickness) and permeability (K) of the
aquifers,. differentiated within each unit on the basis of the known
punctual values and main horizontal variation trends.
Table I shows, for each aquifer, the range of K measured values and of K
and h(l) retained values. The net thickness has been expressed in
percentage in order to supply a probable value of h in all the cases where
only the gross thickness of a formation was known.
The values of K and h have been utilized in order to calculate the
parameter Kxh (Transmissibility). Three maps of Transmissibility have been
constructed,. for the depth intervals 0-300 m, 300-500 m, 500-1000 m. The
following methods were adopted for the Kh definition and contouring:
- for each well,. in the given depth interval,. the existing formations have
been singled with their relevant values of K and h. The Kh values
relevant to each formation are added and give the punctual Kh.
- the punctual Kh values have been contoured by keeping in account all the
geological apsects which may lead to variations of the parameter.
- in the Kh computation of the a-300m interval the freshwater aquifers
have been disregarded, due to the preferential purpose of these layers
in supplying water for civil and agricultural use. The values shown in
this map refer only to the sections saturated by salty water.
The maps represent a theoretical approach to the quantitative
evaluation of the potential productivity of aquifers suitable for a
low-enthalpy geothermal utilization. From a practical point of view, these
just show the distribution of the most promising areas under a potential
hydrogeological aspect but they do not allow a direct quantification of the
recoverable production. The last depends,. as known,. on many factors other
than the Kh, such as porosity, compressibility storage coefficients,.
salinity, viscosity and temperature of the water. These factors are of

514
very uncertain evaluation,_ expecially in the depth range of this research,
and any tentative estimate would only be an hazardous attempt.
Table I - Permeability and net pay values of the aquifer units
AQUIFER UNIT
PERMEABILITY VALUES RANGE NET PAY RANGE
Measured (mD) Retained (mD) % of gross th
CONTINENTAL QUATERNARY
ASTI Sand
PORTO GARIBALDI Sand
PANDINO Sand
DESANA Sand
PORTO CORSINI Sand
MAGNAGO Sand
CAVIAGA Sand
ERACLEA Sand
CANOPO Sand
CORTEMAGGIORE Sd,_gr
SERGNANO Gravel
BORECA Cglm
FUSIGNANO Sandstone
SARTIRANA Sandstone
SERRAVALLE Sandstone
OTTOBIANO Sandstone
CAVANELLA Sandstone
GONFOLITE Cglmtc Ss
MARNOSO ARENACEA Mly Ss
500-12000
10- 6000
20- 530
70-380
250-700
200-2000
200-1000
80
500-2000
25-125
15-45
20
20-40
7.5-15
50
100-200
75
40
100-250
100-400
200
25
25
25
30
70
30-60
7.5
50
40-75
50-70
40
50-70
30-45
55
50-75
50
70
60-75
50-60
60
30-60
50
45
60
70
35-70
20-45
50-150
80-220
To complete the assessment of the hydrogeological characteristics,_
the depth of the freshwater-saltwater interface,_ defined by AGIP in most
of the hydrocarbon wells (Fig. 3), has been contoured over the whole
surface of the plain. The attitude of the interface is also reported on
the cross-sections. (Lower part of Fig. 2).
4. TEMPERATURE
The underground temperature has been assessed for depths of 300, 500
and 1000 m. The distribution at 1000 m depth (loW.G. ,}984) has been
updated for the Po Plain by processing data from recent AGIP wells and
revising old data. The information derives from temperatures measured in
about 450 hydrocarbon wells which provided three types of data:
1.DST vaLues recorded in pooLs. They do not need corrections and give a
reliable measurement of the reservoir temperatures.
2.BHT values estimated from extrapolation of temperature recovery curves.
The Fertl-Wichmann (1977) method is used by AGIP for extrapolation.
3.Unstabilized measurements taken ·during drilling stops. The emprical
statistical correction of. Squarci and Taffi (loW.G. ,_ 1984) has been
applied to the values. Most of the old wells supplied this type of data.
The few values from water wells have been valuable for defining
temperatures at shallow depths (300 and 500 m) as data from hydrocarbon

SIS
wells are very scarce from depth of less than 800 m.
Different methods of interpolation have been adopted to assess the
temperature at the three given depths:
- when shallow data are not available •. the temperatures at 300 •. 500 and
1000 m have been estimated by assuming a steady-state vertical conductive
heat transfer from the measured values. An example of this procedure
is shown in Fig.4. The thermal conductivities are evaluated on the
basis of lithology. In many cases also the effects of convective heat
transfer are evident: in the example of Fig.5 the convection can be
clearly observed. with relatively high temperature and low gradients
over remarkable depth intervals. Apart from the carbonate aquifer. these
effects appear also to occur in the Miocene formations. and are
particularly developed in conditions of structural culmination.
- when reliable shallow data from water wells are available •. the estimates
of temperature at 300 and 500 m have been corrected to account for
convective effects due to cold water flow at shallow depths. The
resulting temperatures are generally lower than expected from conductive
interpolation between deep data and the surface. This also allowed for
the evaluation of the average error at shallow depths in the
reconstruction based only on values deeper than 800 m; various
comparisons showed that these estimates are about 3°C higher than those
corrected with shallow data •. and in some cases as high as 5 0 -6°C.
Temperature Haps have been completed for depths of 300 •. 500 and 1000
m b.g.l.; an example is given in Fig.6. The maps reveal the existence of
wide zones with relatively low. uniform temperatures separated by smaller
well defined zones with positive anomalies. These are located:
- along the pedealpine belt betweeen Novara and Padova. Particularly
conspicuous is the anomaly on the eastern margin of the Euganei Hills
with temperatures of 80°-90°C measured at 300 and 500 m. Another anomaly
can be observed between Como and Bergamo (Fig.6) with values of about
30 •. 40 and 60°C at 300 •. 500 and 1000 m depth. respectively. Smaller
anomalies can also be found east of Milano with values of 50°C at 100Om.
- in the eastern Veneto plain •. between the Tagliamento delta and Grado.
Two maxima are observed at 300 and 500 m with values of 40°C and 55°C.
- in the central part of Emilia-Romagna •. where there are several positive
anomalies extending NW-SE. The strongest one is located near
Ferrara •. reaching 50°. 70° and 90° at 300 •. 500 and 1000 m depth.
- along the southern margin of the plain (values as high as 35°.45°.7.0°C).
The position and trend of most of the positive anomalies appear to be
tied to the structural culminations of the pre-Pliocenic substratum (Della
Vedova. Pellis.. 1980) and associated to the uprising of the fresh
water-salty water interface. This points at convective effects with a
relatively fast upflow of thermal and mineralized water. favoured by
faults and fracturing. Convective heat transfer is stopped at the base of
the widespread Pliocenic Santerno clay Fm which is intrinsically
impervious and probably very ¥eakly fractured also where the post-Miocenic
tectonics is remarkable. The most significant anomalies are associated to
shallow depths of the fractured carbonatic substratum (Ferrara•. Vicenza •.

516
Euganei-Berici Hills,_ Tagliamento and Grado). Minor anomalies,_ as those in
the southern margin of the Po Valley and in the central part between Pavia
and Cremona, appear to correlate with structural highs of Miocenic
formations in the Apennine folds. These are probably linked to the
distensive faulting which is found in areas of anticline hinge. On the
other hand, a widespread cooling is observed in areas where the
Plio-Quaternary sequence reaches its maximum thickness.
5. THEMATIC MAPS
The synthesis of all the mentioned data is given at the regional
scale by means of Thematic maps relevant to three depth intervals: 0-300
m; 300-500 m and 500-1000 m b.g.l. The maps show the distribution of the
three basic parameters: average temperature of the interval,_ water salinity,
hydrogeological potential quantified as Transmissibility (Fig. 7).
Owing to the uneven distribution of check points,_ the reliability of
the reconstruction differs from place to place. For the temperature
distribution and the fresh water-salty water interface the error range
should be low (about 3°C for the temperature and maximum of 100 m for the
interface in areas devoid of wells), but it may be high for the
Transmissibility, whose quantification is affected by a number of
assumptions and not supported by sufficient check points. The numerical
values of Transmissibility must be regarded as probable or, as is more
frequent, possible values, but the qualitative evaluation of the
hydrogeological potential and the relationships between the different
sectors of the plain are valid and consistent with the most updated
picture of the subsurface geology. Therefore,_ the Maps provide,_ also under
this last aspect,_an outline of the most promising sectors for the future
development of the low enthalpy geothermal resources.
As it can be observed in the sketch-example of Fig. 7,_ relevant to
the depth interval from 500 to 1000 m, some areas are characterized by an
entirely impervious sequence (sector between Pavia and Piacenza; south
west of Bergamo), corresponding to the occurrence of the Santerno clay Fm
and Gallare marl. The central sector shows average temperatures of 30° to
35° C with small local positive anomalies,_ and possible Transmissibilities
around 10-15 Om. This condition is determined by the occurrence in the
whole interval of the Asti sand Fm; Le. 100 to 200 m of porous layers
with permeability of 50 to 150 mD. A few Km south of the Milano parallel,
all the aquifers of the interval are salty water bearing,_ while north of
it fresh water can be found down to a depth of 750 m b.g.l.
A very promising area is located mid way between Milano and Como; it
is defined by the occurrence of the Sergnano gravel Fm with temperatures
from 35° to more than 50° C. Due to its favourable conditions,_ and to the
relatively high number of measured parameters in the relevant aquifer
unit, this sector has been selected as a pilot area for future
developments and a tentative evaluation of the probable geothermal
resources has been executed.
The following hydrogeothermal parameters have been used in the
relevant calculations:

SI7
Depth interval
Temperature zone
Helin temperature
Permeability
Net thickness
Transmissibility
Porosity
The heat in
500 to 1000 m
35· to 55· C
45·C
0.400 D
80 m
Surface
Air temperauture
Formation density
Fm sp. heat. cap.
32 Om Water density
0.20 Water sp. heat cap.
place Ho,. associated to t\~ aquifer section
Ho - 1.08 x 10 J
160 Km
12.5·C 3
2.55g/cm
0.84 KJ/Kg K
3
1.00 glcm
4.19 KJ/Kg K
results:
Considering a reinjection temperature of 25· C the Recovery factor Ro is
equal to 0.20 and the 1~ntified resources Hl - Ho x Ro,. riPult~:
H - 2.16 x 10 J or, for unit surface: 1.36 x 10 JIm
1 .
Provided a maximum drawdown of 100 m and 12" well exploiting the whole
interval,. a likely yield is about of 17 lIs. So,. the extractable l~nergy
from a 2 wells plant with a lifetime of 30 years is: E - 1.35 x 10 J
and the t~ermal yield supplied by the plant: W - 1.4 HWTh
Calculations executed by considering a lower temperature ( 35·C) but
higher Transmissibility ( 44 Om) for the interval from 300 to 500 m in the
same area and formation, lead to a thermal output of 1.1 HW. The
exploitation with a 2-well system of the whole interval from 300 to 1000
m, by using the conservative values of 38·C and 35 lIs,. could yield up to
2.0 HW • Keeping in account the comparatively low cost of wells not
deeper ihhan 1000 m, this result is of significant practical interest.
ACKNOWLEDGEMENTS
The temperature assessment here described has been executed with the
scientific coordination of C. Calore,. R. Celati,. P. Squarci and L. Taffi.
REFERENCES
AGIP (1977). Temperature sotterranee. Inventario dati raccolti dall'AGIP
durante la ricerca e la produzione di idrocarburi in Italia.
AGIP Hineraria (1959). Campi gassiferi padani. Atti del convegno
giacimenti gassiferi Europa Occidentale. Acc.Naz.Lincei,. 2. pp. 45-497.
Cremonini,. G.,. and F. Ricci Lucchi (1982). Guida alIa geologia del margine
appenninico-padano. Guida geol. reg. S.G.I. Pitagora,. Bologna.
Della Vedova,. B. '. and G. Pellis (1980). Deep thermal trends for the Po
Valley from Agip temperature measurements in gas and 011 wells. Boll.
GeoL Teor. Appl. '. 22,. 129-137.
Dondi, L.,. and H.G. D'Andrea (1986). La pi anura pad ana e veneta
dal1 'Oligocene superiore al Pleistocene. Giorn. Geol. '. 48/1-2,. 197-225.
ENI (1972). Acque dolci sotterranee. Inventario dati raccolti dall'AGIP
durante la ricerca e la produzione di idrocarburi in Italia.
Fertl,. W.H.,. and P.A. Wichmann (1977). How to determine static BHT from
well log data. World 011, 184,.105-106.
Italian Working Group (1984). Assessment of EC geothermal resources and
reserves. Italy. Report to EC Commission,. Contract EGA.AY.115 I (S).
Pieri,. H.,. and G. Groppi (1981). Subsurface geological structure of the Po
Plain,. Italy. C.N.R. - P.F.G. '. RF 414.

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522
EEC contract n 8
EN3G-0089-E
THERMOMETRY AND HYDROGEOCHEMISTRY OF THE SOUTHERN BORDER OF
THE SOUTH PYRENEAN FORELAND BASIN.
M. FERNANDEzti:, A. FREIXAS(l), X. BOSCH(l), X. BERASTEGUI(l)
and E. BANDA •
(1) Servei Geologic de Catalunya, D.P.T. i O.P., Generalitat
de Catalunya, cl Diputaci6, 92, 08015-BARCELONA.
(2) Institut Jaume Almera, CSIC, cl Marti i Franques, sin,
08028-BARCELONA.
SUMMARY
The preliminary results of an ongoing project to
determine the geothermal and hydrological characteristics
of the Osona depression (Catalonia, Spain) are presented.
A detailed geological cartography was followed by 36
geothermal gradient determinations, 63 comp'\ete wat~r
chemical analyses and 30 stable isotope (0 1 and H )
analyses. The mean geothermal gradient is 38 mK/m with
considerable variations. The scattering indicates a
complex groundwater regime. Recharge and discharge zones
have been identified in the north-northeastern part and
southern half of the depression respectively. A few
sampled boreholes show anomalously high gradients.
Hydrochemical and isotopic analyses do not give
indications for geothermal processes. These results are
interpreted in terms of local deep water circulation ~nd
mixing of thermal fluids with shallow waters. Brines
present in the area are believed to mask the resulting
hydrochemistry. Any attempt to estimate the reservoir
temperature has given unreliable results. Further
analyses are being carried out to confirm or modify these
results.
1. INTRODUCTION.
A geothermal reconnaissance of Catalonia using a
selection of available water wells showed the existence of
three anomalously high thermal gradients near the village of
Mont-rodo in the Osona depression. Further evidence of
geothermal manifestations had already been found and studied
(IGME, 1984; Fernandez and Banda, 1988) some 20 km to the
south. To study thermal . characteristics of the Osona
depression and the possible relationship with the thermal
manifestations mentioned above it became obvious that a more
complete investigation of the area was needed.

523
In this sense a project including geothermal and
hydrological objectives was designed and it is being carried
out with the financial support of the CEC and the Servei
Geolbgic de Catalunya. The preliminary results from thermal,
geochemical and isotopic analyses allow a first approach to
the main characteristics of the area.
2. GEOLOGICAL SETTING.
The study area is located near the eastern border of the
Eocene and Oligocene Ebre Basin, in the Osona county, forming
a morphologic depression (Fig. 1). Stratigraphically it is
mainly made up of Eocene sediments lying unconformably on
Palaeozoic metamorphic and igneous (manly granitic) rocks.
Locally, Triassic limestones and clays crop out. The Pre­
Eocene basement is dipping 5° to 9° to the NNW (IGME, 1983).
It is structured by a NW-SE trending normal fault system,
which is believed to have controled the Cenozoic
sedimentation. Northeastwards from the study area, some
faults bounding the Neogene basin of La Selva are related to
and responsible for the Neogene Quaternary volcanism. In some
localities the opened fault planes are filled with basalts.
Relationships between faulting, Eocene sedimentation and
Neogene Quaternary sedimentation and volcanism indicates a
deep role of such faults.
The Eocene sediments onlap the basement to the SE with
the sedimentary units from the bottom to the top being:
Pontils Gp, made of argilaceous sandstones and conglomerates
(red beds), considered to be impervious: Tavertet Fm., lying
on the Pontils Gp, made of sandstones and sandy limestones
and considered to behave as an aquiferous formations: Malla­
Banyoles Marl Fm., behaving as an impervious body. The main
water reservoir is the Eocene Folgueroles Sandstone Fm.,
showing in its outcrops a due to porosity karstification. The
seal rock is the impervious Igualada-Vic Marls Fm., lying on
the Folgueroles Sandstone affected by a penetrative N-S
trending vertical joint. The overlying Eocene materials are
the Milany Sandstones and Marls: the Centelles Sandstone and
the Artes red beds. These overlying units are considered not
to play an important hydrological role for the purpose of our
study.
3. GEOTHERMAL MESUREMENTS.
A total of 36 geothermal gradient determinations have
been made in the study area. water wells with a depth range
of 60 to 430 m and with a water column of more than 50 m·have
been used. The geographical distribution of wells has been
conditioned by their availability, depth, and absence of
pumping equipment to secure thermal equilibrium with the
medium. The thermometric device is composed of a platinum
resistence thermal probe in combination with a four - wire
cable which has a sensitivity and precision of 0.01 K and
0.05 K, respectively. Temperature readings have been made
every 10 m.

524
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S25
4660000
4655000
4640000
46)5000
46)0000 r------.------.,--,
4625000
U5000 2"15' HOOOO H5000
Fig. 2.- Distribution of thermal, chemical and isotopical
water-point samples. Symbols correspond to different
analyses: (+) chemical, (x) chemical and isotopical, (0)
thermal, (c) thermal and chemical, (¢) thermal, chemical
and isotopical. Geothermal gradient: empty symbols «30
m.K/m), half full symbols (30-35 m.K/m), full symbols (> 35
m.K/m) •

526
The mean geothermal gradient is 38 mK/m, with a standard
deviation of 20.9 and minimum and maximum values of 6 and 115
mK/m. On the other hand, regional thermal gradient is 35 mK/m
(Fernandez and Banda, 1989) and the estimated thermal
conductivity for materials filling the depression is 1. 7 to
2.4 W/m K that yi~lds a conductive regional heat-flow density
of about 70 mW/m • Figure 2 shows the geothermal gradient
distribution.
The most remarkable facts are the following: Low gradient
values « 30 mK/m) are located in the northern and eastern
borders, on the Folgueroles Sandstone Fm. Their temperaturedepth
logs show a slight trend to gradient increase with
depth (Fig. 3a). The mean thermal conductivity inferred,
assuming a conductive heat-flow density of 70 mw/m2, is 2.7
to 3.3 W/m K, which is too high. These facts could be
explained by a downwater circulation in such areas.
Normal gradients (30 to 35 mK/m) are distributed over the
central part of the basin, in the vic Marl
thermometric logs are linear with a deduced
conductivity of 2.0 to 2.3 w/m K in accordance
conductive regime.
Fm'
The
thermal
with a
The distribution of high gradients (> 35 mK/m) is very
irregular through the central and southern part of the basin,
difficulting the identification of the discharge area. The
thermometric logs show a linear shape even at great depths
(> 400 m) as it is shown in Figure 3b. However, the inferred
thermal conductivity is too low (1.1 to 1. 6 W/m K) for a
conductive pattern as the only heat transport mechanism.
Moreover, the majority of high gradients are associated to
the NW-SE extensional fault system. These faults could be a
good conduct to allow the ascension of deep water that
circulates through the deepest sedimentary layers and perhaps
the Palaeozoic basement.
The water wells associated to the thermal anomaly of
Mont-rodo have gradient values between 65 and 115 mK/m. Two
of the four wells show a linear temperature-depth evolution,
while in the other two a gradient decrease with depth is
observed, showing a clear upwater circulation (Fig. 3c,d). In
spite of this, a deep origin of water is necessary to explain
the temperatures measured in these wells.
4. HYDROGEOCHEMISTRY.
The 63 sampled points were chosen out of a set of more
than 300. with the purpose of obtaining a wide spectrum of
the possible different underground waters, criteria such as
geographical distribution, different geological formations,
possible hydrogeological units and means of sampling, were
considered. The aim of this sampling campaign was to relate a
certain hydrogeochemistry and isotopy to the wells, showing
thermal anomalies. According to this, springs, producing
wells and abandoned wells were sampled (Fig. 2). Temperature,
conductivity, pH and alkalinity (potentiometrically), were
mesured in the field~ Lab~~atory_ dete~inat~ons lfclud~~:
CondUc$ivity, pH, HC0 3 ' S04 ' Cl , N0 3 ' N02 , Ca , MG ,
Na+, K , O.M., F-, B, Li+, and Si0 2
•

527
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.. ..
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110
110
100 110
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iliAD I OT "'''It)
Fig. 3.- Thermometric logs corresponding
zones. A) Recharge zone, B) Discharge zone, C)
rod6 anomaly.
to different
and D) Mont-

528
As shown in Figure 4, most of the points are of the
calcium-bicarbonate type, with low to moderate conductivity
(about 700 f'S/cm) (see also Fig. Sa,b). These correspond to
springs and producing wells that drain the Folgueroles
Sandstone· Fm. (with carbonate cemment); a few other points
can be classified as calcium-sulfate, with conductivity
values ranging from moderate to high (about 1200 f'S/cm) (Fig.
Sf, Sg). These are associated to wells drilled in the
overLying beds were gypsum is present; a third group
consists of very high conductivity waters (up to S7S00
f'S/cm) (Fig. Sh). They are related to marly bodies - were
halite casts have been described and gypsum is known to be
present-, of very poor hydraulic properties. They all are
wells that never produced water and that have high contents
of SH 2 •
.'•....- ."'.. ". 0.1.:' •
.
..
.
rCa" 100 " r Co " rU-_ 100 r U
r ICOI H+COfl
Fig. 4.- Distribution on a Pipper diagram of the 63
samples analized. Symbols as in Fig. 2.
In this context, the Mont-rodo wells are related to the
first group but show a slight Sodium-bicarbonate (Fig.
Sc,d,e) trend. This seems to be the only hydrogeochemical
quality that might be related to the thermal anomaly, for
neither Li+, B, F- and Si0 2
nor the geothermometers based in
the Si0 2
or in the relation rNa/rK show any particular value.

529
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IIS/C. 25'C
Calch. 5" '16
blc.,.bonIIt. '2.' ".5 Cil C0 , HC0 h_p. ·C
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a b L.~nd
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calel .. 603 634 1390
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d
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luUat. 14.6 155
II
,•••• q
it
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clorl ... 143
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Fig. 5. - stiff diagram showing the four different types of
groundwater.
5. ISOTOPY.
A selection of 30 points out of the 63 was made in order
to analyse stable isotopes 0~8 and H2. Geographic
distr ibution, di fferent hidrogeochemistry classi f ication,
geothermal anomalies and expected local meteoric
representation (springs) were the criteria used for this
selection. It was expected that waters infiltrating in
different recharge areas would show different isotopic
content. Furthermore, occasional shifts of the value could
be related to geothermal processes.
As shown in Fig. 6, the points fall close to a regression
line with a regression coeficient of r=0.92 -the three brines
not taken into account- considered to be the local meteoric
water line. This line dips less than the Meteoric Water Line,
found in neighbouring areas (IGME, 1984). .
The isotopic analyses do not show shifts or trends
attributable to geographical distribution. However, the three
brine~ show 0 18 positive shifts due to high residence time
and H positive shifts related to production of SH 2 • The high
geothermal gradient of Mont-rod6 wells are thinly scattered
among the other wells. In a thoroughly studied neighbouring
area (Valles-Penedes graben), two thermal springs with tlit
t8mperatures 0/ 60'C and 70·C have isotopic values of 80 :-
8 /00 and 8 H :-60% 0 (IGME,1984). lfis is in accordance to
the 8 values -considering the 0 positive shift- that

- 9 - 8
I
- 7 - 5
- "
r Springs 1101; )( Wells 191
• High geothermal gradient
• Normal geothermal gradient
o LOll geothermal gradient
- 40
:.
0
J:
III
- 50 ~
N
:z:
"Q
- 60
-18 -2
Fig. 6.- Relation of 0 and H for 30 groundwater samples.

531
meteoric waters would have if a forced convection model is
accepted four that anomaly (Fernandez and Banda, 1988).
6. CONCWSIONS.
The interpret ion of the results mentioned above lead to
the following preliminary conclusions:
- High scattering in geothermal gradient data indicates a
complexe groundwater regime probably related to the basement
structure of the depression, the presence of faults and the
multilayered configuration of the aquifer.
The distribution of low gradient values and their
thermometric shapes (dG/dz > 0) allow to delimit the recharge
zone at the Northern and Northeastern margins of the
depression.
- Oscillations of high frequency in thermometric logs of
wells located at the margins of the depression evidence
important variations of permeability according to a
multilayer configuration and fractured or karstified rock
fabrics.
- Discharge zones are identified by the distribution of
high gradients in the Southern half of the depression, always
associated to the NW-SE fault system.
- Hydrochemical and isotopic analyses from high gradient
water wells do not indicate geothermal processes.
Brines present in the area might mask a
hydrochemistry more clearly related to processes connected to
geothermal anomalies.
Attempts to estimate the reservoir temperature by
application of different geothermometric techniques have
failed yielding unsuitable results.
- Hydrochemical and isotopic results could be explained
by a mixing process of thermal fluids with considerable
amounts of shallow waters. However, this is contradictory
with thermometric measurements. In fact, linear shape of
temperature-depth logs indicates a deep origin of water and
little mixing process with cold shallower waters.
In order to substantiate the results obtained so far
further analyses are currently being carried out. It is
expected that the contradictory geothermal and geochemical
and isotopic results will be clarified and a more complete
hydraulic model will be obtained.
REFERENCES.
Fernandez, M., and E. Banda (1988). Aproximaci6n a la
anomalia geotermica de La Garriga-Samalus (Valles-Penedes).
Acta Geo1. Hisp., 23, 1,1-20.
Fernandez, M., and E. Banda (1989). An approach to the
thermal field in Northeastern Spain (in press).
IGME (1983). Mapa geo16gico de Espana 1/50.000. Hoia n" 332.
Madrid, 45 p.
IGME (1984). Proyecto de inyestigaci6n geotermica en el
valles mediante sondeos de reconocimiento y sintesis
hidrogeotermica. 130 p.

532
Contract nO EN3G-DD67-F
HYDROTHERMAL ACTIVITY RELATED TO RECENT EXPLOSIVE VOLCANISM ON THE ISLAND
OF 1(0S, GREECE
AN ASSESSEHENT OF THE GEOTHERMAL POTKNTIAL OF THE VOLCANIA AREA
J.M. BARDINTZEFF *, P. DALABAKIS **, H. TRAINEAD *** and R. BROUSSE *
* Laboratoire de Petrographie-Volcanologie, bat. 504, Universite
Paris-Sud, 91405 .Orsay, France
** I.G.M.E., Athens, Greece
*** B.R.G.M., I.M.R.G., B.P. 6009, 45060 Orleans, France
Summary
Altered xenoliths, sampled by the 0.25 - 0.12 Ma. old non-welded
ignimbrite deposits on Kos Island (Greece) are witnesses of a fluid
convective system in the basement. The conjunction of a potential
heat source, high temperature hydrothermal parageneses, pervious bed
formations, permanent tectonic phenomena and successive fracturations
and possible feeding in marine water, augurs favourably geothermal
potentialities. The hydrothermalized area of Volcania may be related
to a geothermal field, as evidenced by mineralized and sometimes
degassing waters.
1. INTRODUCTION
Kos Island (Dodecanese, Greece), located some ten kilometers far from
the Turkish coast, is the most eastern island of the volcanic Aegean arc
(fig. la). Only three islands of this arc evidence present volcanological
activity: Milos, the famous Santorini and Nysiros.
Kos Island is covered by several pyroclastic deposits, which witness
high volcanic production rates up to recent periods.
2. TERTIARY AND QUATERNARY ERUPTIVE EVENTS ON 1(0S ISLAND
Two eruptive episodes have occurred since the Tertiary era. A Miocene
10.4-7.5 Ma old subduction-related early episode has produced a welded
ignimbrite formation exposed throughout the whole island (Keller, 1969). A
recent < 2.7 Ma old episode is related to the present subduction of the
African plate under the Aegean island arc and associated emitted products
can be observed only in the central and western parts of Kos Island.
At the beginning of the youthful volcanic activity, 2.7-1.6 Ma old
rhyolite and dacite extrusive domes precede two major explosive events
(Dalabakis, 1987). First, a 0.55 Ma old (Boven and others, 1987) pumice
tuff-ring yields a crescent shape and is associated to Kefalos caldera
(fig. Ib). This crisis ended with growing up of Mt. Zini obsidian rhyolite
dome and its partial collapse as Merapi~type avalanche. A second

533
Milos
o
"Sanforinl
A.,.an S.a
~
9 xx!- ,
N
t
~YOli
NY';'OSQ
P11. 1. a. Kos Island located in the Aegean arc. b. Volcania area
located in Kos Island. Possible eruptive centres are indicated: 1 •
eruption of Kefalos (0.55 Ha ) and associated caldera (in barbed line), 2
- eruption of KOl (0.25 - 0.12 Ha ).

534
P6 PS ®
• • P4 •
P8Dp3
sn •
P
P2
N
1
Fig. 1. (continued). c. Water sample localization around Volcania area
(encircled). Star submarine degassing at Paradise bubble beach. d. The
'area .of Volcania. Open circles (Zl to Z14) - hydrothermalized zones,
triangles - wells (PI and P2) and spring (511), f - fault.

S3S
generation is made up of 0.2S-0.12 Ha old non-welded ignimbrites of Kos
which represent the result of one of the most powerful explosive eruptions
recorded in the eastern Mediterranean area (Dalabakis, 1986; Stadlbauer,
Bohla and Keller, 1986). A 20 km diameter caldera, located between Kos and
Yali (a neighbour island ten kilometers in the South), is supposed to have
resulted from this explosion (fig. Ib). Deposits of 3S to SS km3 in volume
have covered about SOOO km2 of the Aegean Sea. Inland, ignimbrite deposits
display the following sequence: (a) up to 3S cm thick basal
phreato-Plinian ash-fall unit, (b) up to 3 m thick ground-surge unit, and
(c) up to 20 m thick non-welded ignimbrite flow unit.
3. XKHOLI'mS, WITNESSES OF THE BASEMENT
Numerous plutonic, metamorphic and sedimentary xenoliths, 6 to 10 %
in volume, can be found inside recent ignimbrite deposits and can be used
as tracers of the basement. Granite as well as biotite microgranite
xenoliths are interpreted as co-magmatic with rhyolite pumice in Kos
non-welded ignimbrite deposits (Dalabakis, 1988).
Alteration processes between xenolith rocks and saline fluids (may be
of marine origin) are evidenced by successive hydrothermal parageneses:
(1) diopside-sphene-biotite paragenesis in plutonic xenoliths indicates a
deuteric stage yielding thermodynamical conditions of 0.9-1.2 kb water
pressure and 6S0-680·C,
(2) in metamorphic and sedimentary xenoliths, hydrothermal parageneses
evidence temperature and pressure variations with distance from the pluton
(a to c):
(a) muscovite + adularia + chlorite + talc (from 0.3 to 0.6 kb of water
pressure, from 300 to SOO·C),
(b) actinolite + epidote ± biotite, the more frequent paragenesis,
(c) pyrite + bornite + haematite (more than 300·C).
A hypothetic caldera is supposed between Kos and Yali, above a
magmatic reservoir. At a subvolcanic level, a hydrothermal convective
system model is proposed (fig. 2).
Important heat retention at shallow levels in the island bedrock is
provoked by long time-duration granite crystallization processes until
recently. Microgranite sills have been emplaced all around into fractures
of the caldera. The geothermal process begins with the segregation of a
dense and saline fluid phase from the magmatic liquid. Intense fracturing
by active seismo-tectonics can favour the percolation of marine waters at
depths, increasing the geothermal potential. Hydrothermal fluids actively
circulate in the whole system.
The conjunction of (a) a potential heat source, (b) high temperature
hydrothermal parageneses witnesses of fluids circulation, (c) pervious bed
formations, (d) permanent tectonic phenomena and successive fracturations
and (e) possible feeding in marine water, augurs favourably geothermal
potentialities in this area.
Since the Antiquity, numerous hot springs have been recorded in the
island of Kos and used for medical purposes, especially by famous
physician Hippocrates.
4. VOLCANIA AllEA
Volcania area a suggestive name I - located 1 km N E far from the
rim of Kefalos caldera, is of special interest (Bardintzeff and others,
1988, 1989). Inhabitants have related that thermal waters sprung out 30
years ago and now remains a strong sulphur smell. This area is a 1
km large basin with 14 small circular hydrothermalized zones forming two
crossed alignments, trending NI0S· for the major line (fig. Id). Circular

CALDERA
I
----r IIme.tone-.--..----
meteoritic or
marine water
I I
GRT+DIO+£PT
DIO+£PT+ACT
- contact metamorphl.m
metamorphic rock.
MAGMATIC RESERVOIR
I
\
PYR+BOR+H£M
ACT+£PT.zBIO
MiJS+ADiJ+CHL+TALC
DIO+SPH+BIO
hydrothermal activity
Fig. 2. Hydrothermal convective system model at Kos Island. Arrows -
fluid percolations. Metamorphic and hydrothermal parageneses: ACT -
actinolite. ADU - adularia, BIO - biotite, BOR - bornite. CHL- chlorite,
DIO - diopside, EPT - epidote. GRT - garnet, HEM - haematite, MUS -
muscovite, PYR - pyrite, SPH - sphene.

537
zones, yielding diameters of 5-20 m, constitute minute 1 m high reliefs
and are covered by whitish strongly altered deposits along with native
sulphur. The 70 m large main central zone, located at the crossing of the
two alignments, contains subaqueous fumarolitic 1 • thick organic
matter-enriched layered deposits overlying fumarolitized pumice tuffs of
Kefalos caldera.
In Philippines, some areas have been described (Bogie, Lawless and
Pornuevo, 1987), displaying some similarities with altered zones of
Volcania. A few tens to a few hundred meters across, 11 areas coexist
within 6 kml in Southern Negros with cold-gas emission and associated
intense argillic alteration. Shallow pools of rainwater may accumulate in
hollows. If these overlie active gas vents, the water may become acid but
without representing the outflow of water of the convecting system. Their
distribution is related to geologic structure. These features, named
"kaipohan" are interpreted as nonthermal manifestation of hydrothermal
systems.
S. VATER GEOCHEMISTRY
Ten samplings of water have been performed in the neighbourhood of
Volcania area (fig. 1c; table). Temperatures for 7 wells outside the
Volcania basin range between 19.3 and 20.5·C, similar to surface water
temperatures, and pH between 5.7 and 7.6. Another well (PI), located in
one alignment 200 m far from the central zone, contains acidic water (pH -
2.8, 20.3·C) which regularly releases H2S. At 60 m far from the well, a
spring (Sll) produces 20.0·C hot and 7.7 pH water. Near the coastal line,
an old Artesian drill (AD) produces 23.4·C hot and 6.5 pH water with
intermittent degassing. At 1 km in the south-east, gas bubbles escape
safely from sea bottom as alignments near the so-called "Paradise bubble
beach". All measured temperatures, somewhat low, may result from cooling
down during water ascent.
Water chemical analyses (table) show mild mineralizations. Waters are
bicarbonate calcic, except PI which is sulfate calcic and AD which is
chloride calcic according to Piper's classification (fig. 3). Si02
contents (65-110 mg/l) are comparatively high for waters yielding not so
hot temperatures. High Ca contents as well as high Hg contents can be
attributed to rock-water interactions, especially during the crossing of
the important carbonate sequence. Na/CI ratios (0.65). a little higher
than in the sea (0.55), confirm rock-water interactions. Significant
amounts of nitrates, nitrites, ammonium and phosphates in water analyses
substantiate pollution by fertilizers.
Cationic geothermometers, when applied to waters which have
chemically reacted with host rocks, yield unrealistically high
temperatures. Temperatures obtained by the Na/K geothermometer (Arnorsson,
Gunnlaugsson and Svavarsson, 1983) range from 145 to 225·C and those
obtained by the Na/K/Ca geothermometer range from 200 to 2S0·C .. Lower
temperatures obtained by silica geothermometers (85-11S·C in equilibrium
with chalcedony, Arnorsson, Gunnlaugsson and Svavarsson, 1983, and
11S-140·C with quartz, Fournier, 1977) see. more likely.
6. COIICLUS ION
Indications for near-surface percolation of hydrothermal fluids, such
as nstive sulphur deposits, silics-rich water, have been found. A sulfate
calcic vater with degassing has been also observed. These surface
geotheraal aanifestations could be regarded as a zone of lateral leakage
fro. the hydrother.al convective system, which developed above cooling
ugaa chamber.

538
T'C pH gaz condo
PI 20.3 2.8 +++ 1830
P2 19.4 7.6 + 940
P3 20.4 6.0 ++ 990
P4 19.4 6.0 1120
P5 20.5 7.1 1900
P6 19.6 5.7 1390
P8 20.0 6.9 1430
P9 19.3 6.4 1085
511 20.0 7.7 805
AD 23.4 6.5 ++
Ca2+ Mg2+ Na+ K+ NH4+
PI 6.56 1.33 5.92 0.40 0.12
P2 5.75 2.76 3.34 0.26 0.006
P3 6.82 2.76 2.63 0.22
P4 7.04 4.03 3.52 0.21 0.017
P5 14.51 5.05 6.80 0.32
P6 9.46 2.46 5.89 0.29
P8 7.03 5.67 5.83 0.26
P9 6.50 3.16 4.32 0.19 0.017
511 2.92 2.38 4.40 0.15
AD 19.11 5.35 11.96 0.43 0.011
Cl- 5042- TAC N03- P043- N02- F- Si02
PI 6.91 9.06 0.048 0.003 0.001 0.005 110
P2 3.39 1.54 6.15 0.50 0.054 0.008 90
P3 3.24 1.06 8.20 0.23 0.005 85
P4 2.54 1.46 10.40 0.097 0.005 90
P5 5.64 10.93 8.65 0.048 0.006 75
P6 6.06 5.41 5.75 0.032 105
P8 6.77 2.91 8.20 0.016 70
P9 4.94 1.31 7.40 0.048 0.008 80
511 4.23 1.06 4.30 0.081 0.010 0.011 65
AD 20.03 10.51 8.28 0.045
Table. Physico-chemical water analyses of the Volcania"area from wells
(PI to P9), spring (511) and artesian drill (AD): temperature in ·C and pH
measured in the field, possible degassing, conductivity in }JS/ em, cation
contents in meq/ L TAC - total alcalinity in meq/ L silica content in
mgt L • Li contents are lower than 50 }Jm/ L for all water samples.
Analyses: Laboratoire d'Hydrologie et de Geochimie Isotopique, Universite
Paris Sud, Orsay, with the exception of AD, Club Mediterranee, Kefalos.

Pi,. 3. Water che.Ical analyses plotted in the Piper's diagram. Same
Iymboll al in table.
S39

540
ACKNOVLEDGHEMTS
We are grateful to B. Bonin, L. Chery, A. Criaud, J.C. Fontes, A.
Gasparini and F. Vuataz for helpful discussions.
REFERENCES
Arnorsson S., E. Gunnlaugsson and H. Svavarsson (1983). The chemistry of
geothermal waters in Iceland. III. Chemical geothermometry in geothermal
investigations, Geoch. Cosmo Acta, 47 , 567-577.
Bardintzeff J.M., P. Dalabakis, H.Traineau and R.Brousse (1988). Volcania
(lIe de Kos, Gr~ce): une zone hydrothermalisee A inter@t
geothermique?, 12° Reunion des Sciences de la Terre, Lille, p. 10.
Bardintzeff J.M., P. Dalabakis, H. Traineau and R. Brousse (1989). Kos
Island (Greece): recent explosive volcanism, hydrothermal parageneses
and geothermal area of Volcania, Continental magmatism general
assembly, I.A.V.C.E.I., Santa-Fe.
Bogie, I., J.V. Lawless and J.B. Pornuevo (1987). Kaipohan: an apparently
nonthermal manifeatation of hydrothermal systems in the Philippines. J.
Volc. Geoth. Res., 31, , 281-292.
Boven A., R. Brousse, P. Dalabakis P. and P. Pasteels (1987). Geological
and geochronological .evidence on the evolution of Kos-Yali-Nysiros
eruptive centres, Aegean arc, Greece, Terra Cognita, 7 , 328-329.
Dalabakis P. (1986). Une des plus puissantes eruptions phreatomagmatiques
dans la Mediterrannee orientale: l'ignimbrite de Kos (Gr~ce), C. R.
Acad. Sci. Paris, 303 ; II, 505-508.
Dalabakis P. (1987). Le volcanisme recent de l'tle de Kos, Th~se de
doctorat de l'Universite Paris Sud Orsay , 1987, 266 p.
Dalabakis P. (1988). Enclaves lithiques et granitiques de l'ignimbrite
recente de Kos (Gr~ce): mise en evidence d'un circuit hydrothermal de
haute temperature, 12° Reunion des Sciences de la Terre , Lille, p. 40.
Fournier R.O. (1977). Chemical geothermometers and mixing models for
geothermal systems, Geothermics, 5 , 41-50.
Fournier R.O. and A.H. Truesdell (1973). An empirical Na-K-Ca
geothermometer for natural waters, Geoch. Cosmo Acta, 37 , 1255-1275.
Keller J. (1969). Origin of rhyolites by anatectic melting of granitic
crustal rocks. The example from the Island of Kos (Aegean Sea), Bull.
Volc., 33 , 948-959. -­
Stadlbauer E., M. Bohla and J. Keller (1986). The Kos-Plateau-Tuff
(Greece): a major ignimbrite eruption that crossed the open sea, Int.
Symposium of Volcanology , New Zealand.

541
IBC contract n R IH3G-0034-B
STUDT ST CUGAT GKOTHBRMAL RBSOORCB IR FltACTORB GRANI'l'BS
TO HEAT GRBKNBOUSBS
J. truAIz
BNlIBR - Spain
Su.ary
A project bas been developed to confira the existance of a geother­
.. 1 resource situated at a depth of between 170 and 1000 • in the
highly fractured granitic fault zone of the eastern border of the
Vall~s Graben at Sant CUgat, Catalonia, and to construct a preli­
.inary aodel of the behaviour of the reservoir.
1. INTRODUCTION
The HI Spanish border is structurally affected by the extensive
Buropean rift systea of the Mio-Pliocene age.
The ValUs basin, though aore than 200 Ita long, is li.ited
a&ylletrically by tyO faults 3000 and 1000 • deep respectively yith
granite edges and arcose filling. In 1979 ENHER began to undertake a
series of investigations into geotheraal prospecting, yith the
objective of using the potential geotheraal resources in the future.
The folloYing studies have been carried out in Vall~s in the period
1979-19831 general and detailed geology; deteraination of the geometry
of the graben by aeans of graviaetry and seisa1c geophysics;
hydrogeological survey of the basin; deteraination of the geothermal
anoulies by geochea1cal and geophysical aethods such as electrical
vertical soundings, SP, MT, audio-MT and seis.ic noise.
The first test yell vas drilled in 1983 on one of anoaalies
detected, Sant CUgat. It ended at 400 aetres and roughly 60 ll C yith a
high degree of peraeability. The geotheraal reservoir consists of a
highly fractured gran; te yi th excellent secondary peraeabili ty due to
the present seis.ic activity in the graben and the hydrothermal
alteration.
This first drilling bas indicated that a loy enthalpy geothermal
resource exists in the fractured granite.
2. PROJICT DBSCRIPTION
Th. developaent of th. project vas designed in several phasesl
Phase 1 would consist of puaping tests at different floy-rates.
T.st durations would a,. about 72 to 120 hours with puaping tiaes of 6
and 12 h and r.cov.ry ti_s of 12 and 18 h. The chosen floy-rates yere
roughly 1 100, 200, 300 and 400 "/h.
Phase 2 vould a,. a long duration puaping test of about 2 aontha at
a constant floy-rate to a,. deterained frOil the abort ti_ puaping
tests.

542
During each pumping test, and under shut-in conditions after recovery
for a time equal to pumping, the following tests would be carried
out: temperature and pressure measurements, chemical analysis of fluid
samples and surface flow-rate measurements.
Phase 3 would be the pre-modelling of the reservoir based on a
hydrogeological synthesis of the local area where the reservoir is
placed.
Phase 4 would be the preliminary study of the geothermal reservoir
using ISAPB and other specific model SPB programmes that would show
an interpretation-definition of the reservoir geometry pointing out
possible boundaries; assessment of exploitation parameters using a
doublet and preliminary modelling of the reservoir behaviour during
production of roughly one year.
Finally, operations control, coordinating with the ENHBR project
manager.
3. liORK CARRIED OUT
The following aspects of the above programme have been carried out:
HydrogeoLogicaL study of the area of the thermaL anomaLy of St
Cugat del Valles, covering the following: geological mapping, climatology,
inventory of water sources, surface hydrology, underground
hydrology: extractions and balance.
- Short duration production test with four variations of flow rate;
the parameters obtained in this test were: load losses in the production
well PB-3, transmissivity, storage coefficient •
. - Long duration production test with a constant flow of 80 l/sec
over 50 days, giving the following data: transmissivity, storage coefficient,
simulation of the behaviour·of the water during an exploitation
of one year with flow-rates of 30, 50 and 70 litres per second
according both to a specific model SPB for this case and to the ISAPB
model as well.
- Determination of the bubble point pressure; measurement of the
gas/liquid volumetric ratio at atmospheric pressure; measurement of
filtration with characterisation of particles and bacteriological
sampling.
- Geochemical follow-up of the two prodUction tests, including
variations in the chemical composition of the water produced during
both tests, short and long, and their duration in time.
4. HYDROGEOLOGICAL STUDY
Characteristics of the waters
The possible water sources in the zone are: - Quaternary, mainly
alluvial terraces formed by silts and sands, in the Riera of Rubi with
a depth of 10 m, a permeabili ty of between 100 and 200 metres/day and
with an effective porosity of 10 to 15 %. - Miocene, principally pods
formed by conglomerates and sandstones mixed with clays, with notable
lateral changes in the facies. It llay reach a maxi!BWI depth of 900 a,
the transmisivity is very low (1-5 .. /day), Witt positive porosities of
between 1 and 5 %. - Granite constitutes the ge thermal aquifers of the
area. It has only been examined during the dril ings made by ENHBR. It
appears to be very extensive and is related to the important batholith
of the Catalanides. Petrographically the aquifer has been defined as a
milonitised granodiorite with silica, quartz, calcite and fluorite.

543
Hydro.eteorological Balance
Accordina to the data available, the follovina par&lleters .. y be
taken as repre.enting an average yearl
644 _
Rainfall •••••••••••••••••••••••••••••••••••
Potential evapotranspiration ••••••••••••••• 807 _
511 _
Actual evapotranspiration ••••••••••••••••••
Defic! t for plants ••••••••••••••••••••••••• 296 _
Useful rainfall •••••••••••••••••••••••••••• 132 _
Average filtration
4 to 7 %
Piezometry
The only fact mown about the grani tic aquifer is that the level
has decreased fro. 5 • in 1973 to 42 • in 1986, and this is due to the
extraction. that are being carried out by mne Berta, a mning explotation
clo.e to the yell.
Hydraulic action
In the Quaternary aquifer, recharge is due to the direct infiltration
of rain-vater falling on the aterials of the aquifer and i. also
due to infiltration of .urface vater and natural discharge fro. the
M;ocene. The pump;ng of the weLLs may be cons;dered as the ma;n
outlet.. In the Miocene aquifer, recharge is due to infiltration of
rain-vater as yell as transfer fro. another aquifer. Hydrogeological
unit. ay be con.idered as outlet.. The gran;t;c aquifer .eems to be
linked to the fault plane and it. mlonitised zone, but it is believed
that a group of faults divides the possible system into blocks. The
hydraulic potential of the granitic aquifer could increase vith depth,
and it i. though that the puaped vater fro. the yell. has it. origin in
a deep flov which ri.e. through the rift planes. According to so.e
e.tiate., the Berta .ine ay extract more than 7000 ../day fro. the
granitic aquifer.
5. DBSCRIPTION 0' VBLL TEST
Short-duration te.t
It co.pri.ed 4 multiple-.tep dravdown te.t. at constant yield,
.eparated by recoveries of the .... duration (12 hour. puaping/12 hours
te.t). The folloving point. vere notedl The dravdown on test yell PB-3
vas loy in co.parison vith the yield obtained. All the yells drev down
to the .... extent when yell PB-3 vas producing, and this vas in spi te
of the different di.tance. fro. PB-3. At the conclusion of this shorttera
te.t a long-duration te.t (SO days) at a yield of 80 1/. (288
"/h) vas progrUMd.
Long-duration test
The mo.t i.portant fact. deduced during the multiple-.tep dravdown
te.t verel De.pite different di.tances involved (16, 68 and 205 .) fro.
PI-3 to the ob.ervation vella PI, P2 and PP4, all ahOY the .... curve.
of variation of dravdown, amplitude and synchronization. The yell head
temperature of vell PI-3 .tayed .table and high (58 8 C) during the test.

544
6. CHOICE OF INTERPRETATION METHOD
Discussion model,
et a1. 1974)
single vertical fracture method (A C Gringarten
The aquifer at Sant Cugat occurs in granitic rocks, probably highly
fractured in a manner consistent vith the regional structure. Observation
veIls PI, P2 and PP4, situated in the same direction vith respect
to the veIl, shov exactly the same dravdown amplitude vhen they are
influenced by pumping from veIl PB-3 in spite of the difference in
distance. The hydrological study indicates that large faults cross the
area. These results indicate that the Theiss model cannot be used to
describe the behaviour of the veIls (the influence of pumping decreases
vith distance in this model). A better model vould be to describe the
hydrodynamic behaviour of a unit cut by a single vertical fracture
intersecting veIls PB-3, PI, P2 and PP4. (Figure 1).
The calculations vere carried out using Gringarten's model vith the
folloving assumptions: The aquifer is homogeneous, isotropic, of infinite
lateral extent and constant thickness over the vhole area influenced
by pumping. The matrix produces vater immediately, folloving a drop
in pressure of the fracture. The pumping veIl is situated in an environment
vith a vertical fracture that is narrov in relation to its
length and to the distance to the observation veIls.
The softvare interpretation vas known as ISAPB (semi-automatic
interpretation of pumping test) vhich takes into account the interpretation
methods mentioned above, vith variations in yield and the
corrections for weLL effects.
The main use is as follovs: Using the simpliest hydrodynamic system
best adapted to the real case under consideration, the operator adjusts
the hydrodynamic parameters so as to model a theoretical curve giving
the best possible fit, on a graphic screen, betveen the calculated
dravdown curve and the measured curve.
The long term pumping test calibration vas undertaken in the
folloving sequence:
The observation veIl PI: determination of the transmissivity Tx
and transmissivity contrast C during recovery, then determination of
the storage coefficient and fracture half-length from the dravdown.
Then the veIl PB3: calibration of the head loss coefficient
(invariant drop in level) and possible adjustment of other parameters.
The parameters used vere those determined in test veIl PB-3
observation veIl PI, i.e.:
and
X f
S
T x
C
B
T Y
• 4900 m Fracture half-length (m)
1.9 x 10 -4 Storage coefficient
• 6.7 x 10 -3 mals Transmissivity along the fracture
axis (ma Is)
10 Transmissivity contrast (C • T x IT Y )
1475 s 2 1m 5 for veIl PB-3 Quadratic head loss
• Transmissivity perpendicular to the fracture (ma/s)
Calibration on test veIl PB-3
The behaviour of the test veIl is given in Figure 2. Simulations
have been made of pumping at constant yield over 212 days (7 months)
folloved by a recovery period of 153 days (5 months). The yields used

S4S
vere 30, 50 and 70 lIs. The behaviour of the production yell is given
in Figure 3.
The aaxi.u. dravdoVDS reached after 212 days are given in the table
belovl
VKLL PI-3
30 lIs
8.2 •
50 lIs
15.1 •
70 lIs
23.3 •
7. DISCUSSION ON SPBCIFIC HODBL FOR SANT COGAT GBOTBERHAL P'IBLD
SIMULATION PUMPING BARRIERS (SPB)
The hydraulic behaviour of the granitic aquifer located in the Sant
Cugat area is illustrated in the block diagr .. of Figure 4. The granitic
aquifers a.erge vith a pyra.1dal shape .-Gng deposits of Hiocene
sedi.ents and slates that, for practical purposes, can be considered
imper.eable.
Fro. that hypothesis and taking into account the li.ited geometrical
feature of the thermal anoaaly, it .. y be deduced that the water
flow wst be ascendant and vi th a deep origin, it Seell8 possible to
dray up a schematic .odel of the subsoil flov (Figure 5). It can be
estiaated that the equipotential surfaces are planar and horizontal
belov a certain depth. This indicates that the piezometry that vould
correspond to these plane surfaces vould be constant and independent of
the distances to the pumping yell.
Despite the existing imper.eable boundaries, vhich are assumed to
be asyaetrical, at shallover depths the equipotential lines are curved,
defining a concave surface. Piezometers of equal depth, at the same
distances fro. the Yell, might thus have different piezometries and, on
the other hand, piezometers of different depths, at different distances
froa the pumping vell, .ight have practically equal piezo.etric levels.
Bach point of the equipotential lines in the proposed aodel is
defined by tvo par .. etersl depth and distance to the pumping yell.
Close to the Yell, the depth of shallov piezo.eters could be a more
important par..eter than the distance fro. the pumping yell. Bence, it
could happen that the piezo.etry effect could not be defined by the
8uperficial distance fro. vell to piezoaeter, because the corresponding
equipotential .. y give rise to appreciable distortions in the dravdoVD
vith reference to the real distance betveen both the pumping and observation
points. In such cases, as in this schue, less error aay arise
by .easuring the distance fro. the yell to the different piezometers
along the equipotential lines rather than fro. the surface, i.e. fro.
the bottom of the yell to the bottom of the piezo.eter.
Parameters and li.its of the aquifer
Being deeper and further away fro. the pumping vell PB-3, the dravdoVDs
measured in PP-4 should be less affected by the effect of partial
penetration. Therefore the par..eters of the aquifer, calculated on the
basis of that piezometer, should give the .ost reliable aean values for
the area researched. Vhen such values of transa1ss1vi ty and storage
coefficient are applied in the Jacob's hypothesis in order to calculate
the distances over which the dravdoVDS aeasured in PI and P2 are produced,
the result is that the distances fro. the pumping vell PB-3 to
the piezometers PI and P2 would have to be taken as 150 and 185 .etres
respectively. Such values are apprec:iablely greater than the real distances
on the ground. On the basis of the lIOdel proposed, it turns out

546
that these distances can be equated to a good approximation with those
between the bottoms of the wells and the shallow piezometers, i.e. PI
200 m, P2 216 m and PP4 230 m.
Examination of both the drawdoYD and the recovery data shows that
the effects of impermeable boundaries and/or pumping by others on the
same aquifer can be detected.
The parameters derived for the granitic aquifer are those that
apply before the boundary effects appear. Their values, applying the
hypothesis indicated previously, are summarized as follows:
PUMPING
RECOVERY
T iii' /day
5 x 10 -4
PE-3
2023
PI P2 PP4
2024 2050 2045
2 2,2 3,5
PE-3
2133
PI P2 PP4
2027 2126 2095
2,3 1,9 2,1
50, the aquifer parameters chosen are: T _ 2000 lIi'/day; 5 _ 2 x 10 -4
Effects of the impermeable limits
The geometry of aquifer, as well as the flow model that can be
derived, leads to drawdoYD curves in which boundary effects with a
complex behaviour appear. After the initial section of such curves,
from which the parameters quoted above have been determined, successive
inflections appear which correspond apparently to changes in the transmissivity
and storage coefficient until the values reach roughly T -
200 m~/day and 5 - 3.10 -~ • Beyond this point, the curve cease to
change. It seems then that over the ten days of pumping a deep aquifer
was characterised without further boundaries, that could be described
(at least over the duration of test) by the T and 5 values indicated.
Apparent evolution of T and 5 during the lOng term test
During the process of calibration by means of the interactive calculation
programme, it proved useful to include the following images in
the pumping test; the time of appearance of each during the pumping
test is indicated.
IMAGE5 OF THE VELL DURING THE PUMPING
AND TIME5 VHBN THEY BECOME APPARENT
Image nil
Distance to well (m)
Time (min)
1
2
3
4
5
6
7
8
9
10
740
2020
2800
3300
3750
4000
6200
12000
14800
15300
35
260
500
700
900
1000
2500
9000
14000
15000
The first two images must be produced by real barriers in the
aquifer close to the surface itself, over distances of 300-400 m and

547
1000 a froa the extraction vell. The other i.ages could be either real
barriers or undefined reflections.
Calibration of the pumping tests
The progr.... developed for calculation of the dravdoYD takes account
of the par ... ters derived previously I Transmss1vity, T. 2000
.. /day, storage coefficient, S • 2 x 10 -4 •
Calibration of lOng te~ test
Using the par ... ters listed previously, the .adel fit for the longte~
tes t .. y be seen in Figure 6.
A satisfactory fit for the pumping vell vas obtained both for
pumping and recovery, particulary after 10 days of pumping.
The calibration of piezo.ater PI shovs a msutch of roughly 2,5 %
in one part of the pumping as vell as in the mddle section of the
recovery. The piezo.aters P2 and PP4 (Figure 6) are .adelled vith a
.ore than satisfactory precision during the pumping tilDe, but P2 shovs
a aaxiaum aisutch greater than 15 % over the second half of the
recovery period.
Overall, these results suggest that the chosen aodel, although
notably complex, can be adjusted vith a very satisfactory precision.
Comparison
Vith the aia of coaparing both .odels, ISAPB and SPB (Si.ulation
Pumping Barriers), an identical test vas carried-out on SPB (Figure 7)
and on ISAPB (Figure 3). This involved pumping for 212 days and
recovery for 153 days vith production flovs of 30, 50 and 70 lIs. In
addition, one veek's vorth of pumping for 12 hours/day at a flov of 70
lIs vas .adelled. The coaparison of results vas as follovsl
Pumping to cons tan t flov
30 lis 50 lis 70 lis
Drav- lSAPB SPB (a) % lSAPB SPB (a) % ISAPB SPB (a) %
doYD
Vell
PB-3 8,2 9,1 -0,9 10,9 15,1 16,2 -1,1 7,3 23,3 24 -0,7 3
8. GBOCBBKICAL SURVBILLAHCI OF PUMPING TEST
The che.tcal analyses of the theru.l. vater of St. Cugat at our
disposal prior to the present production test shov a stability in tiae.
Also the tritium analyses vhich are available, indicate apparently this
stability.
Geocheaical control carried out
On the basis of a planning previously dete~ned s .. ples vere taken
in the high output pumping test and in the long t~ pumping test and
proceed vith conventional cheaical analysis, chea1cally, isotopically
and for dissolved ......

548
9. EVALUATION AND INTERPRETATION OF ANALYTICAL DATA
The chemical composition of the pumped water shows notable stability
both in time and in relation to the volume of water extracted. The
only notable variation is an abnormal increase in the salinity when
beginning the prolonged pumping, the causes for which are not yet clear
and may be due to the influence of pumping in the Berta Hine.
Tritium contents in the water samples are always at extremely low
levels, under 0.3 + 0.2 T.U., which indicates water infiltrated before
1952. No Sign was detected of mixing with more recent waters.
The analytical results of measurements for deuterium and oxygen-18
gave assurances of the meteoric origin of the water without any deviations
attributable to thermal processes. It also appears from the concentration
of oxygen-18 that the recharge zone is within the Litoral
Range.
The water-rock relationship study indicates that the geothermal
water is oversaturated with quartz, albite, microcline, moscovite,
ferric biotite and ferric chlorite; undersaturated with anorthite and
in equilibrium with magnesium biotite and magnesium chlorite. It can
also be said that the water is almost in equilibrium with regard to
calcite and fluorite. This study shows that the water is stored in a
granitic reservoir. The geothermal calculations for various chemical
equilibria coincide, indicating a temperature at depth of 100-lOSR C.
However, some sodium-chloride facies are also noted in the fluid
composition, which causes a suspicion of a possible relation, during
circulation at depth, with evaporitic materials, possibly Trias. This
same phenomenon is observed in other thermal waters in the graben.
The study of the gases dissolved in the water has shown that it is
atmospheric air which has lost an important part of its oxygen. The
concentration of CO 2 and H 2 S is very low or non-existent, indicating
the absence of damaging phenomena for future exploitation pumping.
The low amount of gas indicates a bubble point close to atmospheric
pressure. Since the water is almost in equilibrium regarding calcite,
there should be no problems of aggressivity-incrustation in the exploitation
of the geothermal fluid nor should there be any phenomena of
precipitation of sulphate.
The stability noted in all the geochemical and isotopical data
allow us to conclude that we are in the presence of a relatively extensive
geothermal field, recharged by meteoric waters, which could be
exploited continuously in accordance with the limits noted in the hydrodynamic
study, without any appreciable problems related to its geochemical
composition.
10. SPECIAL TEST
Additional surveys were carried out to complement the geochemical
work, with the aim of determining bubble point pressure, filtrability
characteristics of samples and bacteriological· sampling. The main conclusion
may be summarized as follows: A figure of 1.4 bar pressure at
bubble point was obtained, related to a temperature of 58R C. From the
filtrability measurements, it is noted the content in solid particles
is very low, and may be due in this case to the lack of total development
in the PE-3 production·well, with the particles observed being
primarily clays. No bacteria were noted in the pumped water, a very low
o 2 and CO 2 content, and the non-existence of H 2 S, with pH showing
a tendency to alkalinity as has been already reported.

Th,or. Ci\ Ott urtbl huH
--'
F'lgur. ,
~lIIogen,olN isotopic .qv , te~ ",iltl Infini'e
lat .1 x cn$ion .nd (olUt.nt thicltn $S
W.U, .nd plUOllllttf$ in 'he hull
01'9ra block or ttl.
souuOUnOU1g th.rm~1
.00000,lous.
__ 1'kllnl
-_ • • otol:
... & "tic
F t
UltitR-SAN CUG~T
Figure 2
DEL VAL LE S
XPERIHENT AL AND C ALCULA lED BEHAVIOUR OF
Pi 1 (jEOTl1ERMAl WELL
L q,nd> Fl IOI".t, 80 lIs
o £lIptrimen ,\ datu
- (iltul~hd M'ft (singh! holt 1Y16deU
e:r
.~~----------------------------~
PARA"ETtRS OF THE SIHULA
IIAII ~ __ ten ,1M11d ...
h.-tt
• F.ult, h.llltn9lh 4.900m
• Tr ~ ·ssivity (onlrut: 1
-AA\ulf.r tr-,n","uivify :] 1 -, *i
-s r~ cot fident : _S lO·i
•

550
Pumping ... u PE-3
Time: 79.3 days
SIMULA lION-SPB
I1lxillun dra .. do .. n
23.1911 Isillulatedl
24.05 .. Irull
pZ Pill ••• t ..
T.: 79.3 days
l1uilun dra"do"n
13.1011 Isillulat.d!
13.1'" It.iIIl
11" Pi ... meter
Tim.: 79.3 days
I1nillun drawd ... n
13.84m Isimulat.dl
14.11m IruU
.P~ Pi ...... ter
Ti.e: 79:3 dlYs
l1axillun dra .. d ... n
13.77. Isimulat.dl
13.10. It.aU
Calibrati.n .1 pUllping t.
c.nstant II ... (80 lis
Figures 6
Calibrati.n ., pumping t.

.5.51
EEC Contract No. EN3G-0065-IRL (GDF)
PRELIMINARY RESULTS FROM TEMPERATURE,
HEAT FUN AND HEAT PRODUCTION STUDIES IN IRELAND.
(.J. BARTON, A. BROCI and A.D. SIDES
Applied Geophysics Unit, University College Gal¥BY, Ireland
Summary
An assessment of existing surface data ¥BS done to locate sites for
scientific drilling in the main Irish granites. Four sites were
selected, two in the Gal¥BY Granite, and one each in the Leinster and.
Barnesmore (Donegal) Granites. Physical and chemical property
measurements vere made on surface and core samples and heat flow
measurements were made in the boreholes. Preliminary results give
the following heat flow and heat production values for the four
boreholes. Gal¥BY Granite: 77mWm-2. and 7pWm- s : 65mWm- s and 4)1Wm-a •
Leinster Granite: BOmWm-a. and 2)lWm- s • Barnesmore Granite: 85mWm- s
and 5pWm- a • These results are discussed in the light of comparisons
with the UI.
1 Introduction
Previous work under Contract No. EG-A--022-EIR has provided limited
temperature and heat flow data for the Republic of Ireland which were
confined mainly to that part of the country underlain by the Carboniferous
Limestone (Brock and Barton, 1984). This work extends the measurements to
the main Irish granites for which heat flow data vas entirely absent. To
this end funds vere made available for special scientific drilling in the
granites. When studying granites heat production data is also required,
and the project involves geochemical lleasurements on both core and surface
samples. In addition, temperature and heat flow measurements are also
being made in non-granitic areas in order to enhance the earlier dataset.
A map showing the borehole coverage is given in Fig 1.
2 Granite Studies
There are three large granite batholiths in the Republic of Ireland,
called respectively, the Gal¥BY Granite, the Leinster Granite, and the
Donegal Granite (see Fig 1). All are of late Caledonian age (c 400 My).
The Gal¥BY Granite with an outcrop area of about 600km s is intruded into
Dalradian and Ordovician .etamorphic rocks and unconformably overlain to
the east by Carboniferous Limestone. It is composed of diorites,
granodiorites, ada.ellites and leucogranites. The Leinster granite covers
an area of about lSOOkm t and vas emplaced into Lover Palaeozoic
greenschist facies rocks. It consists of five plutons arranged en-echelon
along a N-NE axis. The Donegal Granite consista of several bodies of
which one, the Barnes.ore Granite, bas the highest surface heat production
at about 4)1W.-'. It is a 811811 predoainantly ada.ellite body intruded
into Dalradian lleta.orphics.
Since the funds available for drilling vere It.ited it ¥BS t.portant
to select the drill sites with great care. In the Gal¥BY Granite a range
of geophyaical and geochem.cal surface data vas already available. In

552
particular. detailed field gamma-ray spectrometer measurements (Feely and
Madden. 1987; Madden. 1987) had identified relationships between
lithology and heat production which suggested that drilling should take
place in two distinct granite types. A high surface heat production
leucogranite (the Costelloe Murvey Granite) was selected for one borehole.
and an adamellite (the Errisbeg Town1and Granite) with widespread outcrop
for the other. Final site selection was made after surface geological and
geophysical surveying for the absence of large scale joints and fractures.
together with considerations of long term access and water supply. A
similar approach yielded one borehole each in the Leinster and Donegal
Granites. Limited surface information suggested that the Northern pluton
in the Leinster Granite had the highest heat production in the batholith
and this was selected for drilling. In the Donegal area the choice fell
upon the Barnesmore Granite which had the best indications for high
surface heat production.
Drilling was by continuous coring using the wire1ine method and
gave good recovery of BQ (35mm) size diameter core. The two boreholes in
the Galway Granite were drilled in March/April 1987. and the boreholes in
the Leinster and Donegal Granites were drilled in November/December 1987.
Three of the boreholes were cased near the top and capped. The fourth (at
Barnesmore) in severely fractured rock was cased to the bottom. Some time
after drilling the boreholes were logged for continuous temperature.
natural gamma. resistivity. and SP (as appropriate). In addition. a
stepped log using a thermistor probe was taken for heat flow measurements.
The core from each borehole was sampled at 5m intervals for physical
and chemical property measurements. All samples were measured for
density. thermal conductivity. and potassium. uranium and thorium content.
Selected samples were taken for INAA. AA. XRF. CR39. and micro-probe
studies. The thermal conductivity measurements were made at Imperial
College. London. and the radio-e1ement measurements using a laboratory
gamma-ray spectrometer were done at the Risoe Nations1 Laboratory.
Denmark. Surface samples in the vicinity of the borehole were also taken
for physical and chemical property measurements in order to compare
surface and downhole results.
In addition to the boreholes drilled specifically for this project ~
number of commercial boreholes have been obtained in or near the main
granites and in Carboniferous limestone areas. Some data is available
from these boreholes. but it is at present not complete. and in this paper
the emphasis will be upon the four special granite boreholes. A
description of the data from these boreholes is given in Section 4.
3 Temperature and Heat Flow Studies
Work during phase 2 of the European Community geothermal energy
programme resulted in reports from British and Irish workers (Whei1don et
a1. 1985; Rollin. 1987; Brock and Barton. 1984) which together list 18
heat flow values for Ireland. They range from 52mWm-~ to 87mWm-& with a
mean of 67mWm-& and are shown in Fig 1. There is a tendency for the
values to show a northwards increase across Ireland. but there are also
local variations. A substantial sub-group of boreholes are close to the
Iapetus suture zone (see Fig 1) and have a mean heat flow of
71mWm- 2 which is higher than might be expected for Phanerozoic terrain.
The Iapetus suture zone is also the site of a recently discovered
electrical conductivity anomaly (Whelan and others. in press). and it is
possible that there is an association between the anomaly and the enhanced
heat flow.
During the current phase of the geothermal programme a number of new

553
boreholes have been obtained for temperature and heat flow measurements in
addition to the special granite boreholes, and a programme of temperature
measurements in these boreholes is now under way. In addition, heat flow
calculations will be attempted using the limited data available from deep
hydrocarbon wells. The aim is to increase the number of heat flow values
from 18 to between 25 and 30 and to improve the geographic coverage (see
Fig 1). All the preserved boreholes will be visited twice in order to
check that equilibrium has been achieved. Since the work is not yet
complete the heat flow values presently available are necessarily
preliminary, but the initial results from the new granite boreholes are of
considerable interest, and will be described in the next section.
4 The Granite Borehole Data Set
--rn what follows the geological, geochemical, and heat flow data from
the four granite holes will be described borehole by borehole.
4.1 Ros!. Hhll No.1 (RM1, Fig 2a)
This borehole with a final depth of l37.8m was drilled in the high
surface heat production Hurvey facies of the Galway Granite. Host of the
core consists of coarse grained leucogranite similar to the surface
outcrop. The radio-element profiles show considerable variability in U
and Th, but with a more constant I. The zones of variable U and Th appear
to correlate with zones of hydrothermal alteration and pegmatite veining.
The heat production values also display scatter, but the mean is high at
about 7pWm"' • There are no discernable depth trends.
The lower part of the borehole shows an uncorrected thermal gradient
of 21.3°C km- I and a mean thermal conductivity of 3.7Wm- I I-' which yields
an uncorrected heat flow of 79mWm-~ • An approximate correction for the
recent climatic optimum and the Little Ice age reduces the heat flow to 77
mWm- 1 •
4.2 Camus No.1 (CS1, Fig 2b)
The borehole was drilled in the Errisbeg Townland Granite which is
the most widespread of the granite types in the Galway Granite batholith.
It reached a depth of 125m and although the core is clearly of the
Errisbeg Townland Granite type it displays considerable variability with
biotite and felsic layering. Below 105m pegmatites and zones of
hydrothermal alteration are common. In contrast the radio-element
profiles are fairly uniform, resulting in a heat production of about
4pWm"· for most of the length of the borehole.
The average thermal conductivity and temperature gradient in the
lover part of the borehole are 3.4Wm- 1 1-' and 19.6°C km-' • The climate
corrected heat flow value is 65mWm- a which is surprisingly low for a large
granite batholith.
4.3 Sally Gap No.1 (SG1, Fig 3)
The borehole was drilled in the adamellites of the Northern Pluton of
the Leinster Granite and reached a final depth of 218.6m. There is
substantial lithological variation in the core with aplite and pegmatite
veining, and much hydrothermal alteration particularly from 90m downwards.
The radio-element profiles indicate that U and I levels increase with
depth, whilst high Th values occur in the middle section of the borehole.
As a result the heat production values which are below 2pWm- 1o near the
surface vary between 2.5pWm-' and 3.OpWa-· at greater depth.
The Dean thermal conductivitr and temperature gradient in the deepest
so. of the borehole are 3.3 Wa- 1-' and 25.3°C km-' respectively. The

554
climate corrected heat flow is BOmWm- 1 which is in interesting contrast to
the lower values from the Galway Granite which has a much higher surface
heat production.
4.4 Barnesmore No.1 (BM1, Fig 4)
The borehole is in the Barnesmore Granite which has the highest
overall surface heat production in the Donegal Granite complex. It was
drilled at an inclination of 69° and had an inclined length of 202.8m.
The core generally consists of highly fractured, haematite stained, medium
grained adamellite, commonly transected by aplite dykes and associated
pegmatite zones. The U and Th concentrations show slow variations with
depth which do not correlate significantly with lithology, whilst the I is
very uniform. The heat production lies between 4pWm-~ and 6pWm-~ for
most of the core and is somewhat higher than the (rather limited) surface
measurements.
The true vertical depth temperature gradient in the lower part of the
borehole is 23.4°C km- I and the mean thermal conductivity is 3.8Wm- 1
I-I.
The climate corrected heat flow is 85mWm- 1 • This value, however, is very
preliminary, since unlike the other boreholes a large topographic
correction will be needed. The correction has not yet been done but it
can be expected to lower the final value.
4.5 Discussion
It must be stressed that all the above heat flow results are
provisional. None of the boreholes have yet had a second visit, and the
calculations and corrections will be subject to refinement. Nevertheless
the general level of the values will be unlikely to change substantially,
and a number of trends are already apparent.
The granite heat flow values are all higher than the values in
adjacent non-granite areas. The heat production potential of the granites
has a clear influence on the heat flow. But the heat production - heat
flow relationships are not consistent. The Leinster granite shows low
surface heat production with high heat flow, whilst the substantially
higher heat production in the Galway Granite does not seem to be
accompanied by an equivalently higher heat flow. The depth distribution
of heat production on a scale length of kilometres must be very different
in the two granites. Similar discrepancies have shown themselves in the
United Kingdom granites (Webb et aI, 1987) and it will be important to
study the underlying geochemical aspects in detail. The large geochemical
data set acquired during the course of this project will be crucial.
5 Conclusions
A large data set involving physical, geochemical, and radio-active
measurements on surface and core samples has been accumulated with a
special emphasis on the main Irish granites. All the granites show heat
flow which is enhanced with respect to adjacent non-granite areas, but
which show unusual heat production-heat flow relationships. The Leinster
Granite is a low heat production granite (co ~Wm-3 ) with a relatively
high heat flow (8OmWm- 1 ), whilst the high heat production Galway Granite
(4-7pWm-$ ) shows relatively lower heat flow (65-77mWm- 1 ). The cause must
be sought in the heat production-depth relationships and it is expected
that the geochemical data will allow comparisons with a similar situation
in the UK granites.

555
Acknowledgements
Dr H. Feely, Hr E. McCabe and Hs H. Cahill, University College Galway
for geological and geochemical support; Hr R. Aldwell and Dr P. O'Connor,
Geological Survey of Ireland and the Department of Energy, Dublin for
drilling support; Hr D. Inamdar and Hr T. McIntyre, Geological Survey of
Ireland for assistance with physical property determinations; Hr J.
Wheildon, Hr J. Gebski, and Hr J. Dare, Imperial College for thermal
conductivity measurements and Dr H. Iunzendorf, Hr P. Sorenson and Hr P.
Ingemann Jensen, Risoe National Laboratory for radio-element
determinations.
References
Brock, A. and Barton, I. (1984). Equilibrium temperature and heat flow
density measurements in Ireland. Commission of European Communities,
Brussels, EUR 9517.
Feely, H. and Madden, J.S. (1987). The spatial distribution of I, U, Th
and surface heat production in the Galway Granite, Connemara, western
Ireland. Irish Journal of Earth Sciences. !!.. 155-164.
Madden, J.S. (1987). Gamma-ray spectrometric studies of the Main Galway
Granite, Connemara, West of Ireland. PhD Thesis, National University
of Ireland.
Rollin, I.E. (1987). Catalogue of geothermal data for the land area of
the United Iingdom. Third Revision, 1987. British Geological Survey
Report, Ieyworth.
Webb, P.C., Lee, H.I. and Brown, G.C. (1987). Heat flow - heat production
relationships in the VI and the vertical distribution of heat
production in granite batholiths. Geophysical Research Letters, ~,
279-282.
Wheildon, J., Gebski, J.S. and Thomas-Betts, A. (1985). Further investigations
of the VI heat flow field (1981-1984). British Geological
Survey Report, Ieyworth.
Whelan, J.P., Brown, C., Hutton, V.R.S. and Dawes, G.J.I. (in press). A
geoelectric section across Ireland from magnetotelluric soundings.
Physics of the Earth and Planetary Interiors.

;56
Heat Flow Boreholes
3 0
• UK data
• UCG and existing data
o Future sites
72
• Heat flow In mWm- 2
1
~
69
• 0
69 ~
irish Grid 100kmE
o
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OJ
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/
/'"
/'"
60 •
,,/
/'"
/
/
/
o
Caledonian
granites
1_ '1 Iapetus Suture
-' zone
Fig
Borehole Localion Map

TC __ lit-I
4
•
I
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· e 10 -
i
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Q •
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1020 ao 40 10
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557
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;
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.:
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iii
','
LlcalND
T C ... ",.,••• Conduct"",
T .. T •• p., .......
H.' ... H ••• ',oclttetton
...... IIt ... geologlc., ....
c., 1 ....... ,.., Ora .. "_
I ... H,Clro,,,., •••• M.'.' ....
(ta) 1 .... "1.&., Town'-net Qran_.
I ... H., ..'ot".,.... a"a,atto"
Fill 2 Dill tor (8) Ros I Mhll No 1 (RM 1 J borehole
Ind (bJ Camus No 1 (CS 1) borehole

558
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H P K U
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559
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0;2~3~~.~~~ ________;2__.~~1~~'~1~0__'~2~1••
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10 - -'-
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lU
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----....! ~----
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LEGEND
T.C. - Th." .. , Conducl"''',
T - T."'''.r ......
H.P. - H •• I Producllon
.... p_ geologic.' ke,
1 - O.e,b .. den
II - Ad._ ....
3 - Hydrolher .... ' ."er.llon
Fig 4
oa.a 10' Ba,nesmore No 1 (BM 1) borehole

560
ER»-OO66-I1lL
AN INVESTIGATION OF LOW ENTHALPY GEOTHERMAL RESOURCES IN IRELAND
P. M. BRUCK AND F. X. MURPHY
Department of Geology, University College, Cork, Ireland
Summary
A reappraisal of known warm springs in Ireland and the discovery
of a number of additional ones enabled the selection of several
sites with geothermal potential for further investigation. Field
mapping and detailed geophysical surveys in the areas adjacent to
these springs has permitted the formulation of hydrogeological
models accounting for their origin. The warm springs are confined
to carbonate terrains and are invariably associated with fault
zones. Exploratory boreholes have confirmed the importance of
faulting. The primary permeabi1ities of these Carboniferous
limestones are generally very low. However, they often have high
secondary permeabi1ities due to faulting. Limestone solution
within the fault zones and along adjacent joints has enabled. the
formation of an interconnected network of cavities and f iss u res.
Weathering and dolomitization of the limestone along the faults has
also contributed to the permeability. These zones provide conduits
which allow warm water to migrate to the surface from depth. The
location of geothermal production boreholes is thus constrained by
the presence of such structures.
1. INTRODUCTION
The main aim of this project is to assess the potential for the
exploitation of low enthalpy geothermal resources in the Republic of
Ireland with the aid of several 500m deep boreholes. As an initial part
of the study, a reappraisal of all known warm springs (Fig. 1) was
undertaken. These had been located in earlier EEC funded projects
(BrUck and co-workers, 1986; Burdon, 1983). The geological setting of
each spring was examined and hydrogeological models were constructed to
account for their origin. The most promising springs from the point of
view of temperature and geographical location were selected for further
examination. Detailed field mapping of the district around each spring
was undertaken together with geophysical surveys with the help of the
Applied Geophysics Unit of University College, Galway.
In addition a number of previously
dis·covered, particularly in Co. Limerick,
and Char1ev1l1e-K11mallock regions (Fig.
geophysical surveys were also undertaken.
unrecorded warm springs were
in the Newcast1ewest (Fig. 2)
3), at which geological and
Several tepid springs were

561
also noted. These have temperatures of 2-3 0 above that of normal
shallov circulating groundvater. Detailed investigations. as outlined
belovo vere undertaken at a number of sites selected by this initial
study.
2. THE BALLYNAGOUL SPRINGS
In the Charleville-Kilmallock region of Co. Limerick a number of
vana springs (lS.S0C) vere known to be localized along a fault at the
foot of Inocksouna Hill (BrUck and co-vorkers. 1986. Cooper and
co-workers. 1983). During the present study additional vana springs
vere discovered a short distance south of lnocksouna at Ballynagoul.
Three springs with a constant temperature of 16.SoC occur in a cluster
and a fourth with a temperature of IS. SoC lies about SOo. to the east
(Fig. 4) • The total discharge frOli the four springs is about 22.000
litres/hour.
It vas decided that further examination of the site vas justified.
A detailed geophysical survey vas undertaken. This revealed the
presence of a thick layer of glacial drift across eastern Ballynagoul.
The bedrock surface vas shown to be highly irregular. Vertical
electrical soundings indicated that the overburden often exceeds 20m in
thickness. A lov resistivity zone vas encountered in the vicinity of
the main cluster of springs.
Three exploratory Scm diameter cored boreholes vere drilled in
December 1987. BG 1. vas located within the main cluster of springs
and vas drilled to a depth of 110m. It penetrated a sequence of
Dinantian dark grey biomicrites with interbedded argillaceous horizons
and shale wisps. In general the limestone is highly impermeable with
porosities of less than O.S%. Hovever. between depths of 60-8Om. a
strongly weathered and dolomitized zone vas encountered. Although this
vas a narrow diameter borehole it yielded a large voll.lllle (20.000 -
23.000 l/hr) of vana (l6.S0C) vater under strong artesian pressure.
Borehole BG 2 vas located ISO. NW of BG 1 and vas drilled primarily to
provide structural and lithological control. It penetrated a lover
stratigraphic level within the Dinantian than that of BG 1. comprising
pale grey thickly bedded relatively non-argillaceous calcarenite. A
trachytic intrusion vas encountered at the base of the borehole (75m).
The limestone vas unveathered and contained fev open fissures or
cavities. nevertheless a small artesian flov of vater with a temperature
of 12·C vas obtained. The third borehole. BG 3. vas located close to
the eastern spring. It vas drilled to a depth of 42m and penetrated a
sequence of Dinantian dark grey biomicrites with abundant. closely
spaced thin chert bands or nodules. A very small artesian flov of tepid
vater (11.8°C) vas also obtained.
A detailed aicropalaeontological study vas subcontracted. The
results (Jones. 1988) prove that BG 2. encountered the lowest
stratigraphic level ranging frOli aid-late Arundian to lower Holkerian.
BG 1 and BG 3. encountered limestones of Holkerian-lower Asbian age.

562
The age dating indicates that a fault with a minimum displacement of
150m separates BG 1 and BG 2. It is considered that this fault provides
the conduit which allows deeply circulating warm waters to reach the
surface from depth.
At the time of writing a 500m deep borehole is being drilled within
this area, located between BG 1 and BG 2 at a site which geophysical
surveys have shown to be a zone of low resistivity. This borehole
should provide valuable information concerning the nature of the
aquifer, the mechanism by which the warm water reaches the surface and
temperatures at depth, as well as giving an indication of the volume of
warm water present.
3. THE HYDROCHEMISTRY OF THE BALLYNAGOUL WATERS
Water samples from the Ballynagoul springs and boreholes were
regularly analysed. The chemistry of all the springs and boreholes is
virtually identical and is similar to that of the Knocksouna springs lkm
to the NW (BrUck and co-workers, 1986). The results of the analyses
have been plotted on a trilinear Piper diagram, enabling the
hydrochemical facies to be determined (Fig. 5). The analyses all lie
within the calcium-type and bicarbonate-type fields indicating that the
water has been derived from a carbonate aquifer which the high Mg
content suggests is extensively dolomitized.
4. THE NATURE OF THE AQUIFER
Many of the springs in both Munster and Leinster (see 7 below)
discharge from within, or close to, the top of the Dinantian Waulsortian
limestone and are often associated with faults. The primary
permeability of the limestone is very low. However, during the Variscan
orogeny Ireland experienced weak to moderate compression and broad open
folds with thrusts and numerous cross faults were produced. The
Waulsortian limestone lacks discrete bedding surfaces and consequently
deformed in a more brittle manner than the underlying and overlying
well-bedded strata. Faults were developed and these now represent
localized zones of high secondary permeability. Fault zones within the
Waulsortian tend to be strongly brecciated and their wall rocks are
usually intensely jointed. Secondary dolomitization has often occurred
along these zones due to the migration of Mg-rich fluids. This
indicates the importance of faults in increasing the permeability and
localizing fluid flow. In addition, the dolomitization of the limestone
has also contributed to the permeability.
A detailed study of joint and fracture patterns within the limestone
of the Newcastlewest and Charleville-Kilmallock regions was conducted.
N-S joints predominate (Fig. 2). Where the Waulsortian outcrops the
joints are often opened out by limestone solution; sometimes aperatures
several centimetres wide have developed. It is considered that these

563
fora a labyrinth of interconnected fissures which enables fluid
circulation to take place. The increased joint frequency associated
with faults also provides a significant contribution to the secondary
permeabilities.
5. A HYDROGEOLOGICAL MODEL FOR THE BALLYNAGOUL SPRINGS
The Ballynagoul springs occur on the northern limb of a major
syncline. The recharge area is believed to lie further south in the
Ballyhoura Mountains, an anticlinal inlier of largely Devonian "Old Red
Sandstone" with a core of Silurian sedilllentary rocks (Fig. 6).
Rainwater readily infiltrates through the weathered bedrock surface and
percolates into the less weathered sandstone along joints, bedding
surfaces and through intergranular pore space. Migrating down-dip the
water eventually encounters a major fault on the northern limb of the
Ballyhoura Anticline which juxtaposes Old Red Sandstone against
Waulsortian limestone. The water migrates across the fault zone into
the limestone and continues its downward flow towards the hinge region
of the syncline where it becomes heated as a consequence of the effects
of the natural geothermal gradient. This warm water then migrates
up-dip along the northern limb of the syncline. The presence of a
significant fault provides a conduit which allows the wara water to
rapidly escape to the surface. The conduit may be produced by "piping"
(Mayo, Muller and Ralston, 1985) due to localized hydraulic cleaning
and/or solution channelling within the fault zone.
6. MALLOW
During the previous phase of EEC funded geothermal research, warm
springs which occur at Mallow, in north Co. Cork, were studied in detail
(BrUck and co-workera, 1986). Consequently it was considered that
further work in this region was unnecessary. However, as a result of
the EEC study, Cork County Council drilled a 75m deep demonstration and
production well in 1986, in the grounds of Mallow Mart which yielded a
very large supply of wara water (19.5°C). A pump rate of almost 4,000
l/sec. produced a drawdown of less than 2.5m over a 43 hour period.
In 1988 sufficient funds were obtained to deepen this borehole to
500.. The aim was to determine whether zones of warmer water occur at
depth. The borehole was fully cored and is located entirely within the
Waulsortian limestone. The core was logged by the authors as part of
the present project and downhole geophysical measurements were made by
the Applied Geophysics Unit, University College, Galway. ~e core of
Scm diameter was almost invariably fresh and unweathered below a depth
of 90.. Cavities and fissures were absent and there was little evidence
of limestone solution and dolOlllitization. Temperature logging of the
borehole revealed the presence of anomalously high temperatures in the
upper 90m with a cooler aone between depths of 62-78m (Fig. 7). Below
this the temperature initially dropped, then stabilized at 120-15Om

564
before gradually increasing again due to the geothermal gradient. At
the base of the hole a temperature of approximately 18°C was obtained.
Thus no significant increase in temperature over that present at the top
of the borehole occurred.
These results indicate that the warm water must travel laterally for
some distance at shallow levels utilizing the increased permeability due
to weathering and solution channelling which are often intense close to
the bedrock surface in limestone terrains. It is considered that the
water is derived from the same source that supplies the Lady's Well warm
spring located just 200m to the east (BrUck and co-workers, 1986). It
thus seems likely that significant water circulation is confined to a
fault zone at depth. A "pipe-like" conduit which has developed along
this allows the water to migrate towards the surface where it discharges
at Lady's Well and the Mallow Spa as well as circulating through the
weathered bedrock close to the surface. The failure of the deepened
borehole to encounter any further inflow of warm water indicates the
difficulty of obtaining water at depth in a discontinuous aquifer whose
permeability is secondary as a consequence of fracturing, do10mization
and limestone solution.
7. WARM SPRINGS IN LEINSTER
Warm springs in Leinster, also all located in Dinantian limestones,
were previously studied by the firm Minerex Ltd., on behalf of the
Geological Survey of Ireland (Burdon', 1983). As part of the present
study three springs, Ki1brook, St. Gorman's Well and Louisa Bridge (Fig.
1) were selected for further examination with the aim of siting a sOOm
deep borehole at one locality. Ki1brook is the warmest spring in
Ireland (23°C) and has a consistently large discharge. A detailed
geophysical survey conducted in the area around the spring indicated the
presence of a very thick cover of glacial drift which sometimes exceeds
sOm. Two low resistivity zones were identified which are considered to
represent saturated faults within the bedrock. The spring occurs at
the intersection of these faults. The fault intersection must represent
a zone of greatly increased permeability in which a "pipe-like" conduit
has developed. This allows the warm water to reach the surface from
depth.
At Louisa Bridge it was unfortunately not possible to conduct a
geophysical survey due to cultural interference. There is little
bedrock exposed in the region. Thus geological controls upon the origin
of the spring can only be surmised. However, it is considered that a
cross fault, transecting the Ce1bridge Syncline, provides the conduit
which allows the warm water to reach the surface from the syncline core.
Field mapping was conducted in the region around St. Gorman's Well.
It is readily apparent that the spring occurs on or close to the faulted
contact between the Wau1sortian limestone and the overlying Visean

S6S
"Calp" limestone. Further work has yet to be undetaken at this site and
on the basis of the results obtained from all these three localities a
decision will be made upon the location of a 500m borehole to be drilled
in 1989.
8. CONCLUSIONS
This study has shown that in Ireland warm springs are confined to
limestone terrains. They are associated with faults, particularly
within the Waulsortian limestone, although some springs discharge from
the upper contact of the Waulsortian or close to faults within the
overlying well-bedded Visean limestone.
The primary permeabll1ties of these limestones are generally very
low. They often have a porosity of less than 0.5%. However, during
Variscan deformation, the limestones particularly the Waulsortian,
behaved in a highly brittle fashion, producing faults with broad zones
of breccia and intense jointing within the wall rocks. These acted as
zones of high secondary permeability. The circulation of Hg-rich fluid
along some faults resulted in the dolomitization of the wall rocks which
increased the permeability. Solution channelling and opening of
fissures along joints also made a significant contribution to the
permeability. Thus with time, an interconnected network of fissures and
cavities developed in close association with fault zones, allowing water
to circulate to depths at which it became heated due to the geothermal
grsdient. The upward migration of the warm water was again localized by
fault related zones of high secondary peremeability. Rapid migration to
the surface takes place along "pipe-like" conduits produced by hydraulic
cleaning and limestone solution along fault zones.
Large volumes of warm water are considered to occur in the hinge
regions of limestone cored synclinal structures such as those of the
Charleville-Kilmallock area and the Celbridge Syncline. However, the
siting of production wells in such areas is highly speculative since a
large volume of warm water will be extracted only if a zone of high
secondary permeability is intersected.
REFERENCES
BRUCK, P. M., C. E. COOPER, M. A. COOPER, K. DUGGAN, L. GOOLD, and D.
J. WRIGHT, (1986). The Geology and Geochemistry of the Warm Springs
of Munster. Ir. J. Earth Sci., 7, 169-194.
COOPER, C. E., P. M. BRUCK, K. DUGGAN, L. GOOLD, and D. J. WRIGHT,
(1983). The Warm S ri s of Munster Ireland: Final Re ort. Dept.
of Geology, University College, Cork. RS 83 9.
BURDON, D. J. (1983). Irish Geothermal Project, Phase 1. Unpublished
report to the Geological Survey of Ireland. Minerex Ltd., Dublin,
150/75/15.
JONES, G. L. (1988). Micropalaeontology report on boreholes Be I, Be 2,
BG 3, Ballynagoul, Co. Limerick. Unpublished report, Conodate,
Dublin.
MAYO, A. L., A. B. MULLER, and D. R. RALSTON, (1985). Hydrochemistry
of the Meade Thrust Allochton, southeastern Idaho, U.S.A. and its
relevance to stratigraphic and structural groundwater flow control.
J. Hydrol., 76, 127-61.

N
t
100 km
St. OOffn . ,, "' • • l"t/'OOh
•
~ •• I .. 8.ld, •
• Kno kloune
• aaUynagoul
FIG UR E
Loe. tiOrl rn ap showing ma in wo.rm >.aprlrtge atudlGd
N
t
NEW.CASTlE
5 km
N o mVfion
Calp li m~ ' I ~no
W OUI.Of fi'o n
limOl fone
• Sp. ing. 0\)0'. ,,0 C
• WOI.f W.IIJ. a bove 11° C
FIGURE 2
Geological map of t he NawQaella WODt r gl o n Dhowlng t he
looallon o . warm springs and boreh o lell with aR oma loualy hl Oh temper aturG '

567
N
t
i
C04P
'
\ "
\ '"' I
"'" " Wouho,Uo
lo".' h i. Sf\ol_
: . , o. s
• WOo $ " '"9.
CHARlEVlue
IQuA
showing
III rm s pri n g IOOlllltl ••
IQURE • O.ologle&1 map of a allynagoul , howlng wum .prlng.
and bor.I\Dr. ,It.,

568
o IIALLVNACOUL
IGUAE li
Hydfochemlcal analYS86 tor
a llI llynagotJl and Knockaol.Jnll
• KNOCKSOUNA 15 · C 20
plottod on trilinear dlAltrllmo
o
wT D «
u.pth
Im.t •••,
~oo
c
.
:;;
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.;
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E
;
~
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.
CJ
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;
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z
.. ~
~ .....
II)
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II) '"
;;; J
...

569
EEC Contract nO EN3G-0048-F
DEVELOPMENT OF A TW

570
Austin and others (1973, 1980), Pempie and others (1982) and
Liangguang and others (1987) assessed the feasibility of electricity
production from geothermal liquid dominated resources. Bussac and
colleagues (1984) presented the technico-economic study of two-phase
flow thermodynamic cycles and the method for calculating nozzles
expanding saturated liquids or liquid-steam mixtures. Dehaine and
colleagues (1988) designed, implemented and experimented a test bench
which confirmed the interest of the Total Flow Concept through the
validation of a six equations model computer code (design of single
and two-phase nozzles), the identification of the power losses and the
ways to improve the system efficiency (at present 27 • on total flow
isentropic energy).
2. BENCH DESIGN
--------
A technico-economic study (Bussac and others, 1984) assessed the
influence of different parameters on the thermodynamic two-phase cycle
and on the design of the components of the system, for real geothermal
resources in the world. A computer code was used to model the twophase
expansion in the nozzle, pointing out the influence of the steam
quality and the rate of non condensable gases at the inlet, the diameter
of droplets (Weber number), the length of the nozzle. It showed
the possibility of achieving thrust efficiency in the range of 75 to
92 • as previously measured by Alger (1975).
A Pelton-type impulse turbine was chosen for its ability to cope
with higher jet velocities (300 to 600 m/s) and rotational speed
(8 000 rpm after overspeed qualification) than in its usual hydraulic
utilisation. The stress corrosion and erosion phenomenas could be
solved with this technique.
For the design inlet conditions selected (150 0 C , 4.76 bar , steam
quality 0 to 20 .), the liquid phase occupy less than 0.1 • of the
volume flow rate at the nozzle exit (steam quality 20-30 '). The recovery
of the kinetic energy of dispersed dropLets has a strong infLuence
on the totaL efficiency as the Liquid phase represents 70 to 80% of the
totaL kinetic energy.
B -
Turbine housing and other devices
A twofold experimental program was settled for testing the injection
nozzles by measuring the thrust and jet velocity at the exit,
related to the inlet conditions (temperature, pressure, flowrate) and
so validate the computer code. A method for assessing the performance
of,the nozzles on the turbine bench is presented in 3. The test bench
aimed at simulating a geothermal resource by generating either pressurized
water (up to 200 0 C) or a steam-water mixture (20 • of steam at
the nozzle inlet). In both cases the design conditions, - chosen as
representative of many resources - were l50oC, 4.76 bar at the inlet
and 40oC, 0.074 bar at the outlet. Safety and easy operating
conditions guided the conception.

S71
The turbine housinq contains the turbine and the nozzles and is
directly connected to the condenser. Its design must take into account
the influence of aerodynamic and hydrodynamic losses on the net power
to be produced. If droplets hit the bucket with no relative speed,
evacuation of water from the wheel can disturb the jets coming from the
nozzles. Comfort (1977a, 1977b) has estimated the influence of
droplets diameter and their adhesion to the blades on the theoretical
efficiency for an axial flow impulse turbine. The wheel rotation at
very hiqh speed (8 000 rpm , 200 mls) causes aerodynamic frictions in
the mist (droplets in the steam) which influences very much the enqine
efficiency.
3. BENCH TESTING
A -
Nozzles test bench
For the assessment of theoretical hypotheses reqardinq the interest
of the total flow concept, it is necessary to characterize the
nozzle thrust coefficient alone and with the turbine.
A special bench was implemented for measurinq the thrust on a
balance with constraint qauqes. This method was unsuccessful as the
measures were not reproductive from one test to another but it qave
qualitative informations about the coherence of the jet and allowed to
verify the adaptation of the nozzle for the design conditions (sinqle
or two-phase inlet). For the latter confiquration, steam and hot
pressurized water were qenerated in two different boilers for a better
requlation of the characteristics of the two fluids (pressure, temperature,
flowrate). The mixture of water and steam - representative of
actual qeothermal resources on the one hand, producinq more isentropic
enerqy than pressurized water only on the other hand - is achieved by
the pulverization in sprayinq nozzles of the water in the ambient
steam at the inlet of the injection nozzle. Because of the head loss
in the sprayer, the water must be sub-cooled viz more pressurized at
the same temperature (150 0 C). For sinqle or two-phase inlet injection
nozzles the critical flowrate was found the same as foreseen which
validated the model.
For that phase of research a method was settled for measurinq the
liquid phase velocity which was to measure the time of transfer of the
droplets between to points of the stream separated from a known distance,
by means of optical probes measurinq absorption of liqht in the
two-phase stream. The photodiodes were connected to an acquisition
card. Two softwares were developed for acquisition of the measures,
filtration and treatment of the signal. By measurinq both total nozzle
thrust and droplets velocities and knowinq the total flowrate and the
steam quality at the nozzle exit, it would have been possible to know
the speed of both phases (liquid and vapour) and consequently the slip
between the two phases. These interpretations were not possible as the
probes were covered with the mist and water droplets so the signal was
not qood enough to be treated. This type of measurement is quite
difficult because of the hiqh velocity of the flow (400 to 500 ';s).

572
Different configurations of the test bench have been built
according to theoretical design and the results of experimentations.
A first turbine housing contained the nozzles, so there were a
large distance around the wheel, causing significant wheel friction
losses in the mist. The interpretation of physical phenomena in the
buckets were not possible because the slip between phases - steam and
water - could not be assessed, but it is nearly sure that all droplets
hit the bucket with annulation of their relative speed, causing a
reduction of 50 % of the recovery on the liquid phase which represents
70 to 80 % of the total kinetic energy. The best experimental result
was 20.2 % efficiency for the following conditions
- mass flowrate 0.63 kg/s of pressurized water in 4 nozzles
- inlet temperature 154°e, pressure 5.3 bar
- outlet temperature 52°e, pressure 0.080 bar.
A second turbine housing was built, also with an horizontal axis
Pelton turbine but the radial free space around the wheel was reduced
to 21 mm which caused a reduction of 80 % of aerodynamic power losses
(there is a cubic relationship with the rotational speed). At the same
time, a new type of losses was encountered, involving the liquid phase
which disturbed the jet from the nozzles (deviation and slow-down).
More, as in steady flow water is evacuated at the wheel periphery, a
liquid ring is created between the housing and the wheel. These
hydraulic losses are proportional to the flowrate which means that no
advantage was found from the increase of the number of nozzles. Some
devices were made to try and lessen this drawback but were not
successful. There are other considerations, for example the weather
conditions : hot and wet air influenced the cooling tower behaviour.
Possible scaling could lower the heat exchange coefficient of the
condenser, increasing consequently the outlet pressure of the nozzle
which causes irregular functioning especially in two-phase mixture.
Experiments also showed that too closely placed nozzles could interact
unfavourably. For all these reasons, the performances of the system did
not increase with the number of nozzles and a new housing configuration
was decided.
This third test bench made it possible to use either single-phase
or two-phase nozzles, the same Pelton turbine having a vertical axis.
New tests were carried out after the modification of the swivelling.
The operation mode was quite easy for starting as well as for normal or
emergency stop, all physical conditions being steady (temperatures,
pressures, flowrates of water and steam). A simplified model of the
flow energy conversion from the nozzle inlet to the wheel allowed
interpretations about the nozzle and the buckets efficiencies. The
exploitation of the curves giving the dependence of the torque upon the
rotational speed for several numbers of nozzles leads to a nozzle
thrust efficiency of 71 % which would be the result of a thrust
efficiency of 95 % on the steam (30 % of the flowrate) and 60 % on the
liquid phase, assuming that there is no loss of relative speed in the
bucket for the steam and there is no recovery for the water.

573
A slip between the two phases is quite plausible considering that
the model takes into account three correlations, one of which concerns
the Weber number (diameter of droplets). The effect of droplet size on
wheel efficiency was shown by Comfort (1977a; b) who calculated the
trajectories of droplets from 0.5 to 5 microns entering the blade at
427 m/s. It pointed out that research efforts should be made to reduce
droplet diameters to less than 1 micron which is not easy to verify by
the usual granulometry technique because of the quality of signal in
the mist with water projections.
At this state of development, the identification of power losses -
some depend on the nozzle exit pressure, the remaining on the total
brine flowrate - led to the estimation of the net power to be produced
for the same engine comprising two, four, six or eight nozzles which
would have a nominsl efficiency of 27 % without improvement of the
nozzles or the process and for the following conditions :
- mass flowrate : 1,2 kg/s of 20 % steam - 80 % water in 8 nozzles
- inlet temperature 150·C, pressure 4.76 bar
- outlet temperature 40°C, pressure 0.074 bar.
Measurement of the droplets size (granulometry) and velocity (laser
Doppler anemometry) would confirm the above interpretation. A new
design and testing of nozzles could lead to 92 % thrust efficiency
provided that the liquid phase gets out the nozzle at 90 % of the
isentropic expansion speed. The net efficiency of the engine would
reach 35 % of the total isentropic energy of the brine.
Considering the loss of 70 % of the total kinetic energy - corresponding
to the liquid phase - the maximum efficiency of the engine
would be 6S %, provided the nozzle efficiency is 100 % for both phases.
Improvement of the circulation of the water in the buckets would
valorize even more the total flow concept, but it is not likely to
recover on dispersed droplets. It is essential that the evacuation of
water is such that it does not interfere with the two phase flow jet.
4. MATERIAL SURVEY
Special attention must be paid to prevent brine chemistry hazards
(scaling, corrosion) usually encountered in geothermal processes whose
effects can be different depending if rotating or heat exchange devices
are concerned.
In the Total Flow Concept some other problems could arise like
stress corrosion, erosion, because of high velocities (around 500 m/s)
in the blades. Scale control and material evaluation for corrosion and
eroaion resiatance have been studied in Lawrence Livermore Laboratory
by Tardiff (1977). Encrustation can result from the expansion of the
fluid (carbonates, silica, sulphurs) by carry over in the vapour phase
(acale in aeparators, turbine. steam ducts. condenser) or'the liquid
phase (injection well, ducts).
Experience in geotheray or other hostile environments make it
possible to choose the right material according to the salinity. the
type of non condensable gasea (H S, CO etc ...) found in the brine.
2
Inhibition methods have been successlully tested for scaling and
corrosion. Film protection of aurfaces. control of partial pressure of
CO have also been studied. Apart from the Pelton wheel which is under
2
more important mechanical stresses than in its usual applications (low
speed on bucket and lower rotational speed, water only), all other
materials should be considered as for the conventional flash cycle or
ORCS.

574
5. CONCLUSION
As noticed by Austin and Ryley (1980), total flow systems remain a
promising route for exploiting many geothermal resources not fit for
other processes because of high salinity, adverse thermochemistry or
too low power to be produced, but could be the only solution for
generating electrical power (1 to 5 MWe) in remote areas like islands
where population are not connected to the grid. More research,
experimentations and demonstrations need to be done but yet there
exist a potential market for this concept even with performances lower
than expected.
1. DIRECT 8G'ANSI(JoI WITH USE OF STEN1 CK.Y
T
LIQUID
5
ATM. PRESS~
2. RANKINE CYa..E WITH AUXILIARY RlJID
T~8G'ANSI(JoI
VAPOUR
GEt£RATOR
5
lbf;
T
2~
TlRIIIE
AlM. [

REF ERE N C E S
Alqer, T, 1975 -The performance of two-phase nozzles for total flow
qeothermal impulse turbines- presented at the 2nd U.N. Symposium on
Development and use of qeothermal resources, San Francisco, calif.
May 20-29
Austin, A.L. Hiqqins, G.H. and Howard, J.H. (1973) -The total flow
concept for recovery of enerqy from qeothermal hot brine deposits-,
Lawrence Livermore Laboratory, Report UCRL-5l36
Austin, A. L. and Ryley, D.J. (ed.) (1980) -Status of the development
of the total flow system for electric power from qeothermal enerqy-,
in Kestin, J. et al. SOurcebook on the Production of Electricity from
Geothermal Enerqy, (ads.), u.s. Dept. of Enerqy, Washinqton D.C., Pub.
DOE/RA/405l, pp. 504-540
Bussac, F. Gillant, P. Sagnes, P. and Schleqel, R. (1984) -oeveloppement
d 'une turbine diphasique pour la production d 'electricite .i
partir de la detente de saumures qeothermales-, Final report NT 84 Ea
01, E.E.C., Research Contract No. EEG-2-0l5 F(G)
Comfort, W, 1977a -Interium report on performance tests of a total
flow impulse turbine for qeothermal applications- Lawrence Livermore
Laboratory, UCID 17411
Comfort, W.J. 1977b -The design and evaluation of a two-phase turbine
for low quality steam-water mixtures-, Lawrence Livermore Laboratory,
UCRL 52281
Dehaine F., Schleqel R., Unqemach P. (1988) -Turbine diphasique pour
applications qeothermiques-, Final Report NT 88 Ea 01, E.E.C.,
Research Contract No, BN3G-0048F (CD)
Lianqquanq H., Chujun G., Wembo F., Zhian w., Fenqshan Z. (1987) -A
comparison of enerqy use efficiency of total flow and flashad steam
methods for qeothermal power qeneration- Geothermal Resources Council,
Transactions, vol 11, pp 437-443.
Pempie P., Potiron A., Mauconduit M. and Grossin R. (1981) -Faisabilite
de la production d'electricite a partir de sources q80thermiques
a moyenne enthalpie (10o-200 0 C), final report NT 81 Cg 60, E.E.C.
research contract No BGC-2-002-F (CD)
Tardiff G., 1977 -Chemistry and materials technology for utilization
of hiqh salinity brines- LLL, inter society enerqy conversion conf.,
washinqton, D.C.

576
EEC contract n. EN3G-0047-1
DESIGN OF TWO-PHASE FLOW LINES FOR GEOTHERMAL APPLICATIONS
THE SLUG FLOW REGIME
P.ANDREUSSI, A.MlNERVINI, G.NARDINI
A.PAGLIANTI
University of Pisa, Italy
SUlIDDary
In geothermal fields, the steam-brine mixture can be transported
from wellheads to a central plant by a two-phase flow pipeline. As
indicated by field experiments, in this pipeline slug flow conditions
are LikeLy to occur for a wide range of vapour and brine fLowrates. In
the present work, the slug flow regime has been carefully analyzed in
a small diameter (2"), low pressure facility, which allowed
measurements of the mean liquid hold-up and of pressure losses. A
mechanistic model of slug flow has also been developed.
1 . INTRODUCTION
In the exploitation of geothermal energy, it can be economically
advantageous to transport steam and brine from the wellhead to a central
separation facility.
When this procedure has been adopted at Latera Field, the first
water-dominated reservoir in Italy, it has been observed that the
two-phase transmission line steadily operated under the so-called slug
flow regime (Sabatelli, Andreussi and Minervini, 1988).
This experimental observation is interestin~, as the available flow
maps indicate that other flow regimes should occur in the pipeline. A
possible explanation is that in long pipelines, characterized by
significant slope variations, the slug flow regime occupies a much wider
region of the flow map than observed in small scale experiments.
In the present paper, the slug flow regime is studied in detail in
a low pressure loop (2" ID, 17 m long). These experiments will be
completed with high pressure data, obtained in a new loop (2" ID, 70 m
long, 20 Bars) now under construction.
The experimental measurements are used to test a model of slug flow
based on a mechanistic analysis of this flow regime. This model has been
implemented in a computer code developed for the design of gas-liquid
pipelines in geothermal applications

577
2. PLOW HODEL
The well established model of slug flow due to Dukler and Hubbard,
(1975) and Nicholson and coworkers (1978) is based on the assumption that
slug flow can be represented by a sequence of identical slug units.
These units are characterized by the following parameters, (see also
Pig.l):
H - mean liquid hold-up in the slug
S
Hp(x) - local liquid hold-up in the film
Vs
- mean liquid velocity in the slug
Vp(X) - local liquid velocity in the film
Vc(X) -
local gas velocity above the film
L - S
L - P
L -
U
slug length
film length
LS + Lp , length of the slug unit
V T
- translational velocity of the slug unit
In the analysis of intermittent flow it is usually assumed that the
gas velocity in the slugs equals the liquid velocity and that the liquid
film following the slug does not contain dispersed bubbles.
Kinematic and geometric parameters are related to the liqUid and gas
superficial velocities, V SC
' V SL
' by the continuity equations
!7
J Vp(x) H,(x) dx + Kg Vs Lg } (1)
o
L
P
J VC(x) :1 - Hp(x): dx + (1 - HS) Vs LS} (2)
o
The continuity eQuations reLative to an observer traveLLing at the
translational velocity of the slug unit are
(v T - Vp(x» Hp(x) - (V T
- V S ) HS (3)
(V T
- Vc(x» (1 - Hp(x» - (V T
- V S
) (1 - HS) (4)
Using Eqs. (3) and (4) and defining the mean film hold-up as

578
L,
~..~ fo H,(x) dx
(5)
Eqs. (1) and (2) become
(6)
(7)
Summing (6) and (7), it is obtained
V H - V SL + V SG - V S (8)
where VH is the mixture velocity.
The mean hold-up in the slug unit defined as
H L + H L
H L '" S S "
LU
can be obtained from Eqs. (6) and (8) as
H - .L
V SL
+ HS (V T
- V H
)
(9)
(10)
This relation indicates that HL can be expressed as a simple
function of HS .and V T , as weLL as the superficiaL veLocities of the
liquid and the gas.
The pressure gradient, neglecting acceleration losses due to gas
expansion, can be expressed as
dP .. ( dP) + ( dP ) (11)
dx dx ,. ~G
In (11), gravitational losses are given by (for the case that
liquid density, p »p , gas density)
L G
( dP )G - HL P L
g sin ~ (12)
dx
where ~ is the pipe inclination,
and frictionaL Losses, (dP/dx)F' have been expressed by NichoLson and
coworkers (1978) as
V L
( dP ), - ( dP )B + 2 fs P s S S +
dx dx' . D~
1- P L
H,(L,).IV T
- V,(L,)IIV S
- V, (L,)I
Lu . (13)

579
In Eq. (13) the term (dP/dx)B represents pressure losses along the
gas bubble and can ususlly be neglected. The first term is related to
di.tributed lo.ses along the slug, fs being the friction factor and Ps
the mean density in the slug. D is the pipe diameter. The second term is
rel.tive to the acceleration of the liquid film at the slug front to the
mean velocity within the slug.
The function. Vp (x) and Hp(x) can be determined from the momentum
balanc. along the liquid film
2
'f P 'f P
.J! (V, ~) + g D ~ (~ ~) + ~ i i +
dx dx P L
A P L
A
+ H, P g sin
• - ( dP )B i - 0 (14)
P L
L dx
wher. TJ and 'fi . are the shear stresses at the wall and at the gas-liquid
interface, Pp is the wetted perimeter, Pi the length of the interfacial
chord, ~ the ratio of the distance from the film surface to the center of
.t.tic pre.sur. to the pipe diameter. In Eq. (14) the terms 'fiand (dP/dx~
can be expre.sed by the equstions proposed for stratified flow by
Andr.ussi and Persen (1987). It is useful to mention that these terms
have. limited influence on the solution of Eq. (14).
The integration of Eq. (14) can proceed up to . x -~, where
Eq. (1) is satisfied. Dukler and Hubbard (1975)and Nicholson and
coworkers (1978) integrated Eq.(14) (in a simplified form) in order to
det.rmin. \'p(x) and &rex), and assumed in Eq. (1)
L,
f (15)
o
Th. us. of Eq. (15) simplifies the computations, but also introduces
.ignificant change. to the calculated pressure losses. Por this reason
Eq.(15) i. not .dopted in the present model.
In order to solve the model equations so far considered, it is
n.c •••• ry to introduce independent correl.tion. for three of the four
p.ramet.r. VT' HS' Ls,vs (.lug frequency). Considering recent work on
.lug flow, VT can be expres.ed .s (Bandiksen, 1984)
wh.r. V 0 beccx.. negligible and C ,. 1.2 for large values of the mixture
velocity or, •• suggest.d by Bandiksen (1984), for
lr - VH > 3.5
-18'"11
(16)

580
The liquid hold-up in the slugs can be obtained by the correlation
proposed by Andreussi and Bendiksen (1988)
V - V
H M LD
(18)
S V M + VLG
where V LD
and V LG
are functions of geometry and physical properties.
As suggested by Nicholson and coworkers (1978), the third input to
the model can be a constant value of the ratio LS/D. This choice is
supported by the present measurements, which indicate that this ratio
changes very little in the range of gas and liquid f10wrates
investigated.
The complete model considered in this paper is compared with that
proposed by Nicholson and coworkers (1978) in Fig. 2, where the pressure
gradient is represented as a function of the gas velocity for three
liquid flows. As can be seen, the differences are appreciable. For V SL
=
0.5 mis, VSG = 10 mls Nicholson et a1. obtain a pressure gradient which
is almost twice the value relative to the present model.
These differences are essentially due to the use of Eq.(15), while
the different correlation used for HS in the two models produces
significant effects in the calculation of slug and mean hold-ups, but has
only minor effects on the pressure gradient.
3. EXPERIMENTAL SET-UP
The apparatus includes an inclinable bench 17 m long the slope of
which can be varied continuously in the range +7% by means of a motorized
support. The test section consists of a transparent p1exig1ass tube with
inner diameter of 50 mm and maximum length equal to the length of the
bench. The tube is made of carefully flanged 2 m long interchangeable
sections mounted on the bench by precision supports.
The liquid, water in present experiments, is circulated by two
centrifugal pumps of different size. Air is supplied from a high pressure
line. Air and water are metered by two sets of rotameters. The air outlet
pressure was in all cases close to atmospheric conditions.
Liquid and gas are fed to the pipe through a Tee section. At liquid
entrance, stratified flow conditions are created by a thin diaphragm
which separates liquid inlet (from below) from gas inlet (along flow
direction in the pipe).
The void fraction in the liquid slugs has been measured by a
conductance probe made of two ring electrodes mounted flush to the pipe
wall. A detailed description of the behaviour of this probe is reported
by Andreussi and coworkers (1988).
4. ANALYSIS OF RESULTS
4.1. Mean hold-up and void in the slugs
In Fig. 3 the mean liquid hold-up relative to all inclinations and

581
two liquid flow rates is compared with theoretical predictions, Eq. (10).
As can be seen, the agreement between theory and experiments is very good
when the correlation by Andreussi and Bendiksen (1988) is used for Hg.
The correlation by Nicholson and coworkers (1978) gives worse
predictions, in particular at large VSG (errors up to 40%).
From Fig. 3, it is clear that the mean liquid hold-up in slug flow
is about independent of pipe inclination. It is interesting to notice
that in the same range of gas velocities, the liquid hold-up in
stratified flow is a very strong function of pipe inclination.
The mean void fraction in the slugs has been compared with the
correlations proposed by Andreussi and Bendiksen (1988) and Nicholson and
coworkers (1978) in Fig. 4. According to Andreussi and Bendiksen (1988)
the effects of small changes of pipe inclination on Hg are proportionally
small. This is quite clear from Fig. 8. It is also evident that the
empirical fit proposed by Nicholson and coworkers (1978) for a different
fluid pair (air-light oil) cannot be extended to air-water flow.
4.2. Pressure gradient
The experimental measurements of the pressure gradient are compared
with the present theory in Fig.5 for the case of horizontal flow. As can
be seen, systematic deviations between theory and measurements occur, with
errors increasing from 10% at low VSG up to 40% for large values of the
gas superficial velocity. Other published data, like those reported by
Nicholson and coworkers (1978), show similar deviations when compared
with the present theory.
These data are fitted reasonably well by the model developed by
Nicholson and coworkers (1978). This model also gives a better fit to
present data than the complete model considered in this paper.
The main difference between the two models lies on the use of Eq.
(15), which has been adopted by Dukler and Hubbard (1975) and Nicholson
and coworkers (1978) in order to simplify the numerical solution of model
equations. It has been shown in Fig. 2 that the use of Eq. (15)
introduces appreciable changes to the computed pressure gradient, which
becomes larger than the value relative to the complete model. This
explains the better fit obtained with the approximate model, but this
improvement appears to be only related to a mathematical approximation
rather than to a better physical description of the phenomenon.,
The differences between experimental and calculated values of the
pressure gradient can be related to a number of possible reasons. For
instance, it can be noticed that the large scatter of slug translational
velocity may cause an appreciable variation to the computed pressure
gradient when this is calculated as the mean of the pressure gradient
relative to each slug unit rather than as the pressure gradient relative
to a slug unit traveling at the mean translational velocity.
It is also possible that in present experiments the flow is still
under developing conditions, even if a few tests have shown that the
pressure gradient does not vary appreciably over the last 8 m of the

582
pipe.
5. CONCLUSIONS
The main flow parameters characterizing intermittent flow have been
determined by a conductance probe method and pressure drop measurements.
A theoretical model has also been developed and tested with the
experimental results.
The agreement between theory and experiments is good for the mean
hold-up and slug hold-up measurements, while calculated pressure losses
are systematically lower than the measured values. These deviations are
also present in the data published by Nicholson and coworkers (1978), but
are hidden by an approximation made to the model.
Further work is under way in order to improve the flow description.
In this work each slug is considered as a transient phenomenon as it is
possible that fully developed intermittent flow only occurs in very long
pipes, or, eventually, never occurs.
6. REFERENCES
Andreussi,P., and L.N.Persen (1978). Stratified gas-liquid flow in
downwardly inclined pipes. Int.J.Hu1tiphase Flow, 13, 565-575.
Andreussi,P., A.Di Donfrancesco and H.Hessia (1988). An impedance method
for the measurement of liquid hold-up in two-phase flow.
Int.J.Hu1tiphase Flow, In press.
Andreussi,P., and K.Bendiksen (1988). An Investigation of void fraction
in liquid slugs for horizontal and inclined gas-liquid pipe flow.
Submitted for Publication.
Bendiksen,K.H. (1984). An experimental investigation of the motion of
long bubbles in inclined tubes. Int.J.Hu1tiphase Flow, 10, 467-483. •
Duk1er,A.E., and H.G.Hubbard (1975). A model for gas-liquid slug flow in
horizontal tubes. Ind.Eng.Chem. Fundam., 14, 337-347.
Nicho1son,H.K., K.Aziz, and G.A.Gregory (1978). Intermittent two-phase
flow in horizontal pipes: Predictive models. Can.J. Chem.Eng., 56,
653-663.
Sabate11i,F., P.Andreussi, and A.Hinervini (1978). Esperimenti in campo
su1 f1usso di f1uidi geotermici (acqua-vapore) in una tubazione. In
F1uidodinamica Hu1tifase ne11'Impiantistica Industria1e UNICOPLI,
Hi1ano. pp. 187-203.

583
I.
Ls
LF
Vr
X 1
~
HF (x) • VF (x) ---
Fia· 1 - An illustration of the slug unit.
1041~---------~------~------~------___ ~ ______ -r ______ -,
- - - NICHOLSON et a1.(1978)
--- Complete theory
'P = 0° D=0.05m
)(
"C
......
a..
"C
VSL=1.5m/s
-----
1~~2-----~----0~.5----~~1~.0---------'~~------------~5~---------1~0-.----~20.
VSL • rr./s
Pia· 2 - Comparison between the .adel by Nicholson and
coworkers (1979) and the present theory.

584
0.8 r-------,-----.,-----..------,
-- Present theory (Ip= 0)
--- NICHOLSON etal (1978)
0.7
0.6
0.5
0.4
0.3
0.2
VSL=1.56 m/s
/
I ~
VSL =0.71 m/s ,
,
o - 3.06°
0- 0.76°
t::. 0°
~ 0.3°
\.
\.
Ip
0.1
"­ ...... ...... ......
...... ---
°0~---~5----~1~0---~1~5---~20
VM • m/s
Fig. 3 - Mean liquid hold-up. Comparison between theory
and experiments.

en
:x:
I
0.75
" 0.50
0.25
tp
o -3.06 0
o
6.
0.3 0
/
/
/
/
/
I
/
/
/
/
GREGORY et 81, (1978)
ANDREUSSI and BENDIKSEN,(1988)
/
/
" " "
./
./ ,/
,/
O~~-----L--------~------~------~
o 5 10 15 20
VM 1m/51
Fig. 4 - Gas void in the slugs. Comparison with existing
correlations.
1~r-------~----------~--------r---~
...
E
~
3
10
VSl,m/s
o 0.71
o 1.13
6. 1.56
'V 3.0
2
100·~.4------1~.0~--------~4~.0~----~10~.0~~
VSG, m/s
Fig. 5 - Comparison between experimental and cal
culated pressure losses,

PART II -
DEMONSTRATION PROJECTS

589
ENERGY DEMONSTRATION PROGRAMME OF THE EUROPEAN COMMUNITIES
GEOTHERMAL ENERGY
G. GERINI, V. KARKOULIAS, I. GALANIS
Commission of the European Communities
Directorate-General for Energy
New and Renewable Energies Unit, Geothermal Sector
The fundamental objective of the Community Energy Policy Is to ensure
for the CormIunlty adequate energy supplies at acceptable cost, levels
of security and, of Increasing Importance, levels of environmental
Impact. This policy, which Is based on components like reduct Ion In
dependence on oil, diversification of Imported energy sources and
Increased use of Indigenous energy sources, renewable energy sources,
efficiency In energy use, has taken this stand following the -Oil
shocks- In 1973 and 1979. Further Improvements have been set out for
the Community In the 1995 obJectives, adopted by the Energy Council of
8 November 1988. In brief, one of the main conclusions of this Council
concerning the Renewable Energies (mainly: geothermal, hydro, solar,
biomass and wind) denotes that the promotion of technological
Innovation through R&D and Demonstration should be continued.
The fact that renewable energies are Indigenous with low environmental
Impact and usually have low operational costs have made them a favoured
energy form under Community Energy POliCY. This poliCY Is set out In
the Council Recommendation of 9 June 1988 on renewable energy sources
(RES). The main recommendations made to Member States are:
to Introduce, legislative and administrative procedures to help
overcome obstacles to the renewables;
to continue efforts on R&D and Demonstration programmes;
to promote:
a) cooperation
explol tat Ion
b) the transfer
among
of RES and
of technology
Industries
producing equipment for the
to encourage contractual terms for the sale of electricity
generated by pr Ivate producers from renewable energy lIources to
distribution companies.
Despite the unfavourable market conditions over the last years, most
countries maintained their commitment to the development of RES.
Industry has also Invested a considerable amount of money In RES. The
European Community have committed substantial funds to research,
development and Demonstration of technologies uti I Ising RES.

590
The Community's Energy Demonstration programme, which Is defined as the
successful completion of R&D phase, and which Is In operation since
1978, has become the largest of Its kind In the world.
For the Community however, the primary goal Is not just to achieve a
successful demonstration. Its much broader objective Is to stimulate
the market penetrat Ion of technology and the reappllcat Ion. Between
1978 and 1988, 2440 demonstrat Ion projects In RES were subml tted of
which 851 were selected with a total Community support of ± 266 million
Ecus.
PRESENT STATUS OF THE GEOTHERMAL ENERGY DEMONSTRATION PROGRAMME
The major obstacles to the development of geothermal energy are the
Initial costs, which combined with the geological uncertainties
generate financial risks of considerable Importance. In some projects,
once the geothermal resources have been located by drilling, plugging
and corrosion problems may occur. The economic conditions and the
competitiveness of geothermal energy development have declined over the
last years, resulting from the fairly abundant supply of conventional
energy resources at relatively low prices.
The objective of Community support Is therefore to help overcome this
Initial risk, which Is particularly significant for the private sector,
and to encourage the development of techniques and technologies to
reduce costs and resolve some of the technical problems which can be
encountered.
In the geothermal energy sector (GE), 274 demonstration projects were
submitted In eight call for tenders (1978-1988), of which 130 were
selected with a total community support of ± 62 million Ecus. Of these
130 projects, 53 have completed the phase supported by the Commission
and 20 of these projects are fully completed and are now In operation.
Some have been unable to begin Implementation of their project, because
of financial or administrative problems, and some projects have been
abandoned. Drilling difficulties, unfavourable hydrogeological
conditions or water quality and re-Injectlon problems have also led to
projects being abandoned, but even If they are fal lures, these projects
have nevertheless provided very useful geological and geothermal
Information. The projects of high enthalpy for electricity generation
are generally very successful.
Table I shows the distribution of the demonstration geothermal projects
according to the type of uti Iisation. The distribution Indicates that
heat del Ivery to district or private heating (65%) Is the most
Important application In the low enthalpy. The heating of greenhouses
(16%) Is a more recent development. In the last three cal Is for
proposals, many projects deal with technology development and
Improvement.

591
Table I.
UTILISATION OF GEOTHERWAL ENERGY IN "
Space heating 65,3
Electricity generation 15,1
Greenhouses heating 15,9
Industry-technology 2,7
Aquaculture-pisciculture 1,0
The low enthalpy potential Is fairly widely spread throughout Europe.
In France district heating Is the main application. In Italy health
resorts and agricultural uses are prevalent. In the near term the main
low enthalpy projects within the EC will be eventually focused on
district heating, agricultural and Industrial application. Efforts will
be pursued on electricity production with ORC-Installatlon. At present
geothermal energy of low enthalpy provides about 295.000 TOE In the
Community.
The high enthalpy projects within EC countries are concentrated In
Italy, France (French overseas territories), Greece, Portugal (Azores)
and Spain (Canary Islands). Italy has a large capacity (over 800 1.tW)
already Installed or under construct Ion. Developments are expected In
Greece, France, Portugal and Spain.
The first results of the geothermal programme given by the most
advanced demonstration projects are very encouraging and for several
projects, commercial exploitation has started. The programmes have
shown that the supply of geothermal energy Is of considerable local
significance In certain regions of the Community.
The greatest part of Community support has been given to the drilling
phase for the exploitation of new geothermal reservoirs, as well as to
projects Incorporating a solution to unsolved problems associated with
submerged pumps and heat exchanges, and to Industrial processes.
FUTURE OF THE GEOTHERMAL DEMONSTRATION PROGRAMME
The future of the geothermal demonstration programme (after 1989) will
depend on decisions to be taken In the course of 1989 by the Commission
and the Counc II.
The priorities for a technology programme In the field of geothermal
energy could be as follows:

592
exploitation of new geothermal resources.
development of new technologies and techniques for underground
operations and surface equipment In order to reduce costs.
development of new methods for solving corrosion. scaling. re­
InJection and plugging problems.
As far as applications are concerned. electricity generation and
thermal utilisation should be further developed. Special emphasis
should be given to space-heat Ing. heat Ing of greenhouses and
agricultural premises. pisciculture. aquaculture and Industrial heat
uses. Production of electricity through ORC Installation should also be
supported for further development as well as combined uti I Isatlons. In
order to approach the Ideal exploitation of the geothermal resource.
The development of geothermal exploitation In the Community wi I I
strengthen the geothermal energy Industry and bring benefits for
employment and balance of payment. particularly If the possibilities of
development In third countries are considered.
It Is our main objective In this conference to present:
1. the geothermal demonstration activities In the Member States.
2. the state of the technology.
3. the legislative and administrative environment. and
4. actions and ways to overcome the obstacles. having as aim the
promotion and development of geothermal energy In the European
Community.

Sea.tOD 4: OYerYi_ of seo~be~ re~ce de_lo.-n~
mid de.JDa~r.~tOD
Overview of the geothermal demonstration activities in Spain -
Projects in the Madrid and Basque region
Overview of the geothermal demonstration activities in
Catalunya
Geothermal demonstration project in Denmark
Geothermal demonstration activities in Belgium
Overview of geothermal demonstration activities in Germany
Overview of high enthalpy projects in Italy
Overview of low enthalpy projects in Italy
Progress on geothermal research & system implementation in the
United Kingdom
Milos demonstration project
Geothermal energy in Greece -
Potential and exploitation
Bilan de la filiare g60thermique en France - Actions de
dbonstration

595
OVERVIEW 0,. THB GI!O'I'HXRMAL DEMONS'!'RATJON ACTIVITIBS IN CATALUNYA
A. MITJA, J. ESTEVE and J.J. ESCOBAR
Direcci6 General d'Energia. Generalitat de Catalunya.
J.F. ALBERT-BELTRAN
Tecnologia y Recursos de la Tierra (TRT S.A.) Madrid.
Summary
A summary of the demonstration activities developed in Catalunya in the
geothermal energy field is presented. There exist now six geothermal energy
demonstration projects which are an outcome of the exploration done mainly
by the Instituto Geo16gico y Minero de Espana (IGME), the Empresa Nacional
Hidroelectrica del Ribagorzana (ENHER) as well as the result from the
Geothermal Program of Catalunya undertaken by the Directorate General of
Energy of the Generalitat de Catalunya (autonomous catalan government).
These implementations, which are in different development phases, present
different geothermal resource applications: industrial zone supply, greenhouse
utilization, building heating and proteinic algae culture. All the projects
have been subsidized by the Commission of the European Communities (D.G. XII,
D.G. XVII) wlthin their corresponding programs.
1. GEOLOGICAL SETTING.
The structure of the catalonian 1 itoral zone is determined by a big
system of horsts and grabens NE-SW, product of the neogene distension which
continues, from west to east, from the Ebro depression till the Balearic
islands. From south to north, the distensive system comprises the provinces
of Tarragona, Barcelona and Girona, with a very similar tectonic disposition,
although in southern part of Tarragona interfere iberic structures, and in the
northern part of Glrona interfere with the Pyrenean range.
An important characteristic to emphasize is the crustal thinning which
can be observed in this sector. In fact, whereas in the Ebro depresssion the
Moho discontinuity is situated at 30 km depth, in the Catalonian coastal zone
this discontinuity rises to 15-20 km. To this fact is added a volcanic
activity which has been active periodically in time from the Neogene untill
the last Ouaternary phenomena in the Olot area, wich have been dated at 9000
years only. This volcanism is basaltic of the Inter-plate type, similar to the
one of other central-euro~ean depressions, without hardly any magmatic
differentiations and with a clear mantle origin.
The presence of geothermal anomalies is closely related with these recent
crustal dynamics. which also shows itself in the form of a pronounced seismic
activity along the entire Catalonian coastal zone. While the mean value of the
heat flow in Europe Is 64,4 mWm- 2 (Cermak. 1979), in the Iberian Peninsula
ranges to 81.6 mwm- 2 , and the highest values (110-120 mwm- 2 ) are found along
these coastal grabens of the NE Mediterranean area (figure 1) (Albert-Beltran,
1979 and 1988).
Taking into account the granitic
grabens and their arcosic filling,
preferential ways for the thermal water
active secondary permeability produced
nature of the major part of these
the borderfaults constitute the
to reach the surface due to having an
by the mechanic effect of the fault

596
,my.uuite) and by the pronounced hydrotermal alteration of its minerals.
Most of the projects (Samalus, Sant Cugat del Valles, Jafre de Ter and
Montbrio del Camp) are related to these structural features with anomalous
rates of heat flow (figure 2). Only the Lleida geothermal well is placed on
the Ebro basin with normal gradients.
2. GEOTHERMAL EXPLORATION (YEARS 1973 - 1987).
The geothermal exploration in Spain began in 1975 with the IGME
(Instituto Geologico y Minero de Espana) "Inventario Nacional de
Manifestaciones Geotermicas" (National Inventory of Geothermal Indications)
which was a summary of the hot springs and wells with thermal water within the
spanish territory, both insular and peninsular.
As far as Catalunya is concerned, the first study of the existing thermal
springs is due to J. Bataller (1933) having only a descriptive c~aracter. The
first prospective academic works that consider the geothermal phenomena as a
possible energy source were done by J. Albert-Beltran in 1976.
As a consequence of the time- and space-convergence of these two actions
(the National Inventory and the more concrete and complete knowledge of the
catalan situation with respect to the rest of the Estate), the IGME, in 1976,
began its first set of regional studies, choosing the Valles graben as a pilot
zone to develop the initial prospect ions.
During the 1976-1984 period, the geothermal studies of all th~ catalan
neogene grabens (Valles, Penedes, La Selva, Olot and Emporda) was completed.
A survey of the Pyrenean range hot springs was also included in this activity.
In all the structural units mentioned, a geochemical survey of the
thermal springs was carried out (chemical analysis, gases, isotopes,
calculation of the thermodynamic equil ibria, etc.) conducting afterwards
studies in both structural geophysics (mainly seismics and gravimetry) and
other of more detailed nature (MT, audio-MT, SP, electrical resistivity,
gradient slimholes, etc.).
In 1984 and 1985, the Empresa Nacional Adaro de Investigaciones Mineras
(ENADIMSA), having a budget allocated from the PEN (National Energetic Plan)
to help developing geothermics in Spain, continued the exploration surveys in
the area of La Selva undertaking, among other works, three reconnaissance
slimholes up to 400 m depth.
The Spanish Administration prospective activities in Catalunya culminated
in the period 1981-1986 with the drilling of ten 500-1000 m depth slimholes
(continuous coring) in the Valles area. All of those gave positive results
which allowed the assessment of geothermal reservoirs at 80° - 120°C in Caldes
de Montbui and La Garriga - Samalus. The overall investment made in geothermal
prospection by the IGME in Catalunya during the 1976-1986 period amounted to
210 million pesetas (about 1,5 million ECU).
Another important geothermal activity carried out in Catalunya is that
undertaken by the Empresa Nacional Hidroelectrica del Ribagorzana (ENHER). In
1979, ENHER, in agreement with the IGME, carried out a prospection of the part
of the Valles basin not studied to that moment using the same research
methodology mentioned above (geochemistry, geophysics and gradient slimholes)
so that by the end of the surveys, a common interpretation of the data related
to the depression could be made. This job was eventually done by ENHER.
The results confirmed the. two anomalies already discovered by the IGME
in Caldes de Montbui and La Garriga - Samalus, and detected a new one in Sant
Cugat deJ Valles. ENHER participated in the drilling of the Valles soundings,
joining IGME.
Independently of these actions, ENHER has carried out other geothermal
activities obtaining ex~luration permits in several parts of the Pyrenees,

~~ ... g into effect preli.inary prospect ions in La Maladeta aassif zone (Les,
Arties, Tred6s, 801, etc.) during the years 1983-8S, as well as in the Reus­
Montbrl6 area (1984-86) where an 80°C, less-than-100 • deep anoaaly was found.
On the other hand, the Generalitat de Catalunya (autonOlM)us catalan
government) ended up in 1986 the writing of the Catalan Geotheraal
Exploitation Program which is a ca.pilation .. 1 the available data about
geotheraal resources existing in the autona.ous terrJtory. In the progra., the
aain actions to be taken by the Catalan Administration in order to develop the
geotheraal energy exploitations were defined.
3. GEOTHERMAL DEMONSTRATION PROJECTS IN CATA~U~YA.
During the ten years 0f exploration in Catalunya, important hot water
reservoirs at 80-120 o C have been discover~d in the best studied zone: the
Vali's graben. Information about the location of other potential deposits yet
to be assessed in the Pyrenees, La Selva, Emporda, Penedes and the Reus-Valls
grabvns has been a very :mportant outcome of the activity as well.
In that sense, takiny advantage of Spain's Integration in the European
Community, ENHER Obtained a subsidy from E.C.·s DG XII to study Sant Cugat del
Val ,~s reservoir, being this the first spanIsh geothermal implementation to
rece've financial help from the Europear. A

598
tree ~02 which is stored in a regulation vessel. Part of the geothermal heat
is to be used to grow up proteinic algae, the habitat of which is to be
mclintained between 25 and 30°C; the rest of the heat will be used in the algae
drying procedure. The endoyenous CO 2
will be utilized as a nutrient and pH
regulator in the Spirulina metabolic process.
The system will be a two-phase implementation: " 0.5 Ha experimental
pilot plant which will opera' e during the first two years and, once the
commercial viability of the project is shown, a final 2 Ha plant will be
implemented. The investment needed amounts to 80 million pesetas (0,6 million
ECU) for the first phase and 200 million pesetas (1,4 million ECU) for the
second one.
The algae produced in the factory will be used up as direct nutrients
in piscifactories, cosmetic components and in the manufacture of dietetic
food.
The project also include~ the monitoring of the hydraul l~ evolution of
both the thermal and geochemical well characteristics, as well as the control
of the algae production technology. This monitoring will allow the assessment
of the hot water reservoir and the analysis of the techno-economic viability
of t~lS geothermal energy applicat'on.
4.2. ~leida project.
A 1.1B2 m, 20 lis, 54°1:: we': ',as been drilled near the Clly of I.leida.
The hot water comes from tho ~us"helkalk limestones which are t·.e Ebro
depression substratum in tha' area.
Right now, the well is C(JI,nectt'.:l to the city's Maternal Hospital heating
system, a recently built fac]l It~ 'he heat distribution of which is based on
a radiant floor tyPt! of implem""'at "n.
There are two main phaseb 'r, 'he geothermal water exploitation project.
In t~e first one -to be carried ..,,' within the next two years- 600 million
pesetas (about 4,3 million EC~) WI!' be invested to supply heating and hot
water to 1.100 brand new dwell Jr'gs I"cated nearby the well, in the districts
of SecA de Sant Pere and PardlhYt's.
T~e second phase has a t;vt' year implementation horizon and will need
a 1.400 million pesetas (10 mill ion ECU) investment. The goal is to supply hot
water to a set of 4.300 dwellings, most of them existing; therefore, their
heating systems will need modlflcation.
The economic viability studies show a very difficult financial situation
in pure business terms since payback periods of around ten years are found.
Nevertheless, the enterpr ise has very high social prof itabil i ty. In that
sense, the sponsors of the project are nowadays searching financial public
institution involvement; should t.~is not happen, the foreseen timing could be
much delayed, if not the own existence of the project endangered.
4.3. ~,amalus project (Valles .9.I)ental, Barcelona).
In this granitic zone, lhltially studied by the IGME, a 905 m, 100 lis,
81·~ well was drilled in March 1988; the thermal water static le~el is settled
at 128 m.
The work in the geothermal water distribution network is already under
way. It will be used to heat up greenhouses devoted to grow ornamental plants,
to be utilized in a health resort to be built in Samalus, and the rest will
be directed, in a district heating fashion, to La Garriga industrial zone,
located 3 km away and in a height above sea level 130 m below that of the
geothermal production well.
The foreseen investment tv implement the geothermal water distribution

599
facilities lays around 300 million pesetas (2,1 million ECU) whereas that
ne led for the greenhouses, hotel and industrial applications is estimated to
aluount 3.000 mi 11 ion pesetas (21 million ECU).
4.4. Other projects.
At the moment of the wrl' ng of the present communication, the Montbri6
del Camp (Tarragones) well 6ite was being prepared with the goal of beginning
drilling in January 1989. Th~ Arties project (Pyrenees) will be undertaken by
ENHER after that of Montbr16 .s f!nished.
s. REFERENCES.
Albert-Beltran, J.F. (1976). E~tudio geotermico preliminar de Cataluna. Tesis
doctoral. Facultad de Geologia ~e Barcelona. 460 pp.
Albeit-Beltran, J.F. (1979). lIeat flow and temperature gradient data from
SpaIn. In Terrestrial heal flow in Europe. V. Cermak-L. Rybach editors. pp.
261-266. Springer-Verlag.
Albert-Beltran, J.F., E. Banda, E. Clavell, C. Garcia, I. Pinnaga, and J.
Sanchez-Guzman (1988). Heat flow in Spain. In Atlas of Geothermal Resources
in the European Community, Austeia and Switzerland. R. Haenel-E. Staroste
editors. Public. EUR 11026 (E.C.). 75 pp and 110 plates.
Bataller, J.R. (1933). CondiclOne~ geo16gicas de las aguas minerales de
Cataluna. Publ. num. 8. Laboratorio de Geologia. Seminario de Barcelona. 90
pp. Barcelona.
Cermak. V. (1979). Heat fiow map of Europe. In Terrestrial heat flvw in
Europ~! V. Cermak-L. Rybach editors. pp. 3-40. Springer-Verlag.
ENHEP (19/9-88). Non published rapports about geothermal research in Valles,
PyreneeR, Montbri6 and Lleida.
IGME (197~-86). Non published rapports about geothermal research in Valles,
Penedes, La Selva, Olot, Emporda and Pyrenees.

600
r.:"1 Balamenl hen:inia
~
100 200 KII ,
Figure 1. Heat flow .a f thE' Ibt!rian Pen insula (mWm-2)

60 1
10 20 Itt
Rl8lS DE FmD!
D~~ D~ A=. D~ lZl r-
OSlO !O
"
Vll.ANO~ IIA C£1.T1tV
• Volcda~
C
O~
o lIIaoIoo:
:::::l~
- p.aa
- Glulilles~
ON
• /laI
tolq\lr 2. C ol09ic:al tr " 1:)1 ~ .!II." .. lU A~ ion of the In o t her al projects

Sa (1'10 11.1
o
Sont CU9at
del VQLI~s 0

Table 1.- Geother •• d.~O".tr.t1on
project. 4n Cat.lunya .upported by the CEe.
-- . -- - - ------------------------------------------------
PROJfCT PROPOSfR VfAR OF PROPOSAL fLIGI BLf COST (fCU) CEC CONTRIBUTION
-.-.------ -. -------.---------------------------------------
fN'1fR 1986 193.528 44"; (OG XII)
Ja'",. ae r.r INPROGfSA 19B6 486.679 40"; (OG Xliii)
S.",.,u& INPROGESA 19B8 968.960 40"; (OG )11111 )
Montbr16 de' C.mp EI04'1ER 19811 403.12: 40"; (OG Xliii)
EN"ER '986 1.172.720 40"; (OG Xliii)
Arti •• ENHER 1987 607.143 40" (OG Xliii)
Table II. Main cn.~.ct.rt.t1c.
0' geotn.rmal project. in Catalunya.
GfOY.·ERMA~
RESERI/OIR
TEMPERATURE
( • C)
WELL DEPTH
(m)
FLOW RATE
(1/a)
TV PE 0 F
WATER
T .0.5.
(gIl)
GAS
UTILIZATION
Sint Cuglt
Granite
58·
400
100
NI-Cl
2.3
Gr •• ",nou •••
Ja'''. 01 Ter
Eocene l1m •• ton ••
51·
970
20
CI-S0 4
4.6
'ree CO 2
Prote",.dc a'g ••
(Sol rul In,)
Same''':'.
Grlnlte
81·
905
100
NI-C0 3
H
0.5
Induatry.
G,. •• nnou •••
Lleldl
Trll •• 1 c 1·1m •• ton •• 54·
1182
20
NI-CI
3.5
Ho.pltll n •• ting
lIontl>rl6
oel Clmp
Granite - 80·
T,.1 ••• 1c 1i", •• ton ••
not orllleO
NI-CI
3.7
Gr •• nhOu •••
Arti ••
Grantte >40·
not drill eO
NI-C0 3
H
0.2
Balnaoth."'ap)'
-----------------------------------------------------------------------------------------------------------------------
8)
IN

604
GEOTHERMAL DEMONSTRATION PROJECT IN DENMARK
A. Kahler, Dansk 01ie og Naturgas A/S
Summary
The demonstration plant in Thisted with an absorption heat pump
started operating Kay 1988. The plant is expected to produce approx.
80.000 GJ/year heat to the district heating network in Thisted at the
present heat demand. The heat pumps extract the heat from 130 m 3 /h
15' saline 44 C brine from a Gassum sandstone layer in 1250 meters
depth. The brine is pumped to the surface by a 160 KW submersible
pump and reinjected in the sandstone layer.
The use of an absorption heat pump results in low operation costs for
the heat pump as the absorption heat pump is driven by heat from
boilers and the heat ~rom the boilers is fully recovered. Prob1eJIIS
concerning corrosion and stabile injectivity in the sandstone
reservoir seem to have been avoided.
The demonstration plant is an expansion of a pilot plant with an
electric heat pump erected by Dansk 01ie og Naturgas A/S in the years
1982 to 1984. Dansk 01ie og Naturgas A/S has a sole concession for
the exploitation of geothermal energy in DenlllBrk. The expansion of
the pilot plant to the demonstration plant was performed in the years
1987 to 1988 as a joint venture with the district heating company in
Thisted: Thisted Varmeforsyning A.m.b.a.
1. Introduction
In Denmark, which has no vu1canic activities, the temperature increases
by approx. 30 C per km. The temperature is thus approx. 100 C in
3 km depth. It was reservoirs at this temperature level Dansk 01ie og
Naturgas A/S was looking for, when the company in 1977 was granted a sole
concession for the exploitation of geothermal energy in DenlllBrk.
After the drilling of 3 wells to approx. 3 km at Ars, Farse and
Thisted where the hot water was found, but in reservoirs with poor
transmissivities, it was decided to establish a pilot plant utilizing a
high permeable Gassum sandstone reservoir at 1250 m's depth in the last
well drilled close to the district heating network in Thisted.
The development in the geothermal exploration in DenlllBrk has thus
moved from deep reservoirs with poor transmissivities to low enthalpy high
permeable reservoirs with good transmissivities,but demanding heat pumps.

2. Exploration .. ll in Tbisted
The ori,inal tar,et re.ervoir for the exploration _11 in Thist.d,
which is nov a production _11, v.. tha Su,.rall: .andatone for.ation at
approx. 3 b'. depth. A drawina of the _11 is .nclo •• d. Tha _11 va.
drillad to 3287 • in 1981, but 3 cora. fro. the tar,.t zone and 10, •
• howad, that tha per.eability v .. to low to allow a production.
At tha ._ tt.e tha 10,. lrMlicat.d, that a zone in the upper part of
tha Su,.rall: zone (1849-2114 .) .iaht be .uitabl. for a production. A pump
ta.t va. parfor.ed and tha ta.t ..... d pro.i •• ina, but the trans.i •• ivity
droppad fro. 80 to 20 Da durina the 20.000 .3 vu-p t •• t - probably b.caus.
of fine •• i,ration.
Tharaaftar tha intare.t conc.ntrat.d on the Ca •• u. .andaton. r ••• r­
voir at l230-l270 below tha ,round, which va. vu-p t •• t.d in 1982 and
found .uitabla for tha production. Th. r ••• rvoir ba. the followin, data:
Thickne .. :
Trana.isdvi ty:
SaUni ty, brine:
Dena ity , brine:
Bubbla point, brine:
37 •
100 Da
15 ,
1100 kl/. 3
4-5 bar
3. Pilot plant
Tha pilot plant in Thi.ted va. ar.cted in the y.ar. 1982-1984 vith
.. parata production and inj.ction v.ll-dt •• locat.d 1.5 b apart. Both
vall. _re ,ravel packed (a dravina of the cotapl.ted vella is .nclo.ed)
and tha plant .tartad op.ratin, in .ept•.b.r 1984. The pilot plant produced
2l.000 GJ in the flrat year of production. The lay-out is .hovn at
the enclo.ed dravin,.
The .ub.. r.ible pump in the pilot plant at 40 IV pump.d 46 C, 15,
.aline ,eother.al brine fro. the re.ervoir at 1250 .'. depth to the .urface.
1.3 HW heat va. tranaferred to the di.trict h.atin, network at 65 C
by a .in,le electric heat pump coolina 30 .3/h ,eo the rae 1 brine fro. 42 C
to 14 C. The cooled ,eother.al brine va. then reinjected throuah the
injection _11. The brine va. filtered to 1 .icron at the production vell
.ite and the injection _11 .ite.
4. DellOnatration plant
B .. ed on the experience. fro. the operation of the pilot plant it
va. decided to expand the plant to the pre.ent dellOnstration plant vith a
,eother.al flow of 120-140 .3/h. The .xpansion of the plant ha. been p.rfor.ed
.. a joint venture between the di.trict heatin, co~any in Thi.ted
and Dansk Olie 0, Natur,a. A/S and i •• upported by EEC. The dl.trict heatin,
cotapany ha. erected the nev h.at pump plant and Dansk Olie 0, Natur,a.
A/S ba. expanded the ,eoth.r.al loop.
At fir.t it va. planned to expand the h.at pump plant vith .everal
pi.ton heat pump. in .erie., which would live a hiah .fficiency. Due to
the co.t of electricity for electric h.at pump., it va. however decided
to expand the plant vith an ab.orbtion h.at pump to reduce the .lectricity
co.t.
The enclo.ed dravina of the concept for the deIIOnatration plant
.hould be .elf-explanatory except _ybe for the ab.orbtion h.at vu-p to
the left of the electric heat pump. The plant is dedaned vith a _11

606
distance of 1. 5 kID, which should secure a constant temperature for the
produced geothermal brine for 25 years. Then a new production well may
have to be drilled.
The absorbtion heat pump is of the LiBr type. It consists of an evaporator,
an absorber, a generator, and a condensor. The evaporator contains
clean water boiling at a low pressure and temperature, receiving
heat from the geothermal brine. The absorber contains a LiBr/water solution
and the LiBr salt sucks and absorbs vapours from the evaporator
releasing absorbtion heat to the district heating water.
The generator contains a LiBr/water solution, which is pumped to the
generator from the absorber. The generator removes the water in the LiBr
solution by boiling of water vapours to the condensor and returns the
concentrated solution to the absorber. The condensor supplies condensation
heat to the district heating water and feeds the evaporator with
clean water.
The demonstration plant is expected to have 6000-7000 full load hour
equivalents per year as an incinerator plant covers the heat demand most
of the summer. The effects for the plant can be read from the enclosed
drawing. The plant has not yet been operating at full load as the hot
water boiler needs further modifications, which should be finished at the
end of November 1988.
The coefficient of performance for the absorbtion heat pump is 1.6
to 1.8 (driven by fully recovered energy, that should have been produced
to the district heating network anyway at the same costs) and the coefficient
of performance for the electric screw heat pump is 3.3 to 3.5. The
electric power demand for the absorbtion heat pump is approx. 10 KV. The
advantages of absorbtion heat pumps are:
- a very low electric or mechanical power demand (less than l' of the
heat output - circulation pumps and valves are the only moving parts)
- low maintenance costs due to the few moving parts
- the driving energy can be delivered from new or existing high temperature
boilers or steam generators and the driving heat is recovered 100,
The disadvantages are the high -driving- heat, which is completely recovered,
but increases the total heat flow and may thus limit the number
of full load hours in periods with reduced heat demands. Further the driving
heat should be produced at boilers for 160 C water or steam instead
of ordinary boilers for 100-120 C water or steam.
The plant in Thisted has been designed with special consideration to
the risk of corrosion and damaging of the sandstone reservoir (the salinity
of the geothermal brine is 15'). The following actions have been
taken:
- plastic tubes are avoided and an overpressure in the system is secured
by a1arma and N2-batteries in order to avoid oxygen penetration
- uncoated carbon steel tubes are used to distribute corrosion evenly
and stainless steel has been used for moving parts in the brine flow
as balls in ball valves
- the geothermal brine is" prior to the injection" pumped to the sewer system
until it cleans up

607
- the geothermal brine is prefiltered to one micron in bag filters at
the production well-site and again filtered to 1 .icron in cartridge
filters at the injection well-site
At start up part of the produced brine is returned to the production well
to reduce the pressure chock in the reservoir and reduce the brine outlet
to the sewer system.
s. Operation experiences
The delaOnstration plant was tested in February 1988 and officially
started .. y 1988. The pilot plant started operation in September
1984. The operation experiences can be su..arized as follows:
- the stainless Grundfos submersible pump part in the pilot plant perfor.ed
well without scaling, erosion or corrosion, but the IaOtor for
the submersible pump, froll another manufacturer, broke down several
times. This problem should be solved now.
- the new IaOnel coated submersible pump at 120 m 3 /h at 30 bar from FAGRO /
REnA operates satisfactorily. The change of manufacturer was not due to
dissatisfaction with the Grundfos pump, but because
Grundfos had not developed a sufficient large pump at the time.
- the electric screw heat pump from Sabroe has been operating well with
the expected efficiency
- the absorbtion heat pump from Ludvigsen & Hermann / Sanyo operates satisfactorily
- oxygen penetration into the plant has been avoided (corrosion rates
are below 0,1 l1li per year in carbon steel)
- the filter bags and filter cartridges last IaOre than one IaOnth once
the system ha. been cleaned up (the clean up demands 1 to 5 sets, 1 to 3
sets for a restart and approx. 5 set. after a change of the pump)
- the geothermal brine looses approx. 3-4 C from the reservoir to the
wellhead at 30 .3/h and 1-2 C at 130 .3/h
- the pressure in the production well stabilizes at 3-4 bars below the
undisturbed pressure in less than a week at 130 m 3 /h
- the pressure in the injection well stabilizes at 8-10 bar above the
undisturbed pressure in less than a week at 130 m 3 /h
- the sand production during start up is reduced by the gravel pack and
the partial return of the produced brine to production well during start
up
6. Economy
The accounts for the expansion of the pilot plant to the delaOnstration
plant have not been closed, but the cost. for the erection of the
plant ..aunt to approx. (.o08y of the year):

608
Production well to 3287 m
Injection well to 1242 m
Pilot surface plant
Expansion to de.onstration plant
Invesc.ent costs
28,400,000 DICK
10,300,000 DICK
13,300,000 DICK
18,000,000 DICK
70,000,000 DICK
The invest.ent costs for the expansion of the pilot plant to the
demonstration plant has a satisfactorily pay back time, but the total invest.ents
are too high for a satisfactorily pay back time at the present
heat demand in Thisted.
The invest.ent costs for a new geothe~l plant at a sufficient heat
de.and at the district heating network, with 2 gas boilers, two 1200-1500
m wells and two absorbtion heat pu.ps extracting approx. 6 MW heat from
the geothe~l brine can be estt.ated as follows:
Production well, test
Injection well
Production plant
Injection plant, pipeline
Miscellaneous
Invesc.ent costs
15 DICK million
12 DICK million
30 DICK million
8 DICK million
5 DICK million
70 DICK million
At 5' interest, no inflation, 25 years operation time and an annuity
loan, this invest.ent cost can be substituted by a yearly payment of
DICK 5,000,000 per. year. By a sufficient heat demand, the yearly operation
time can be set to 8000 h and the yearly heat production can be set
to approx. 50 GWh or 180,000 GJ.
The operation costs contain only maintensnee and power for the submersible
pu.p and circulation pumps, as the driving heat for the heat
pu.p is recovered completely and the production of the driving heat may
be based on fuels already used at the district heating central(s). The
following yearly expenses and heat production costs can be computed:
Capital costs:
Power, 350 KV:
Haintensnce:
Yearly total costs
5,000,000 DICK/year
1,200,000 DICK/year
2,000,000 DICK/year
8,200,000 DICK/year
Heat production costs including capital costs:
46 DICK/GJ or 0.16 DICKjKVH
At a 15 year's currency for the loan, the heat production costs including
capital costs become: 55 DICK/GJ in the first 15 years and then 18
DICK/GJ.

37 • .J
18 5 /8·-226. •
PRODUCTION WELL
L
~
l
MISTED
7·,23 lb /ft, It-55, B~C
301 • ~ opofplDlp
319 • Bottca of pIDIp
190.
313.
INJECTION WELL
20·-72 •
13 3/8·-255 •
'-.... G&a 11ft -.1".a
-> and .andrela
13 3/ 8·-1167 •
~
jj
~
~
lOU ~9 5/8· Liner top
7"' 29/26 lb/ft
M80 B~C injection
at ring ---..1+
CaaiDq window
1236 •
16· h ole
20/40 _ah aend
1273.
I
(
I
I
'\
~
I
I
6.0"· a creen,
.012•
alota
16"' hole
20/40 _.h
a and
(
/
/
'\
'\
~
9 5/8"'-1207 III
I __ ~~ 6.063"' acreen,
.012" alota
, 5/8·-2583 •
I
'lD at 1242 III L - -u; __ If - .J
I
8 1/2· opeDhole
'lD at 3287 III
FIGURE 1

610
GEOTHERMAL PILOT PLANT IN THISTED
1.3 Mif
0.4
30 M'/H 1,. o BAR
y
- ---
FIGURE 2
DEMONSTRATION PLANT
FIGURE 3

GEOTHERMAL DEMONSTRATION PLANT IN THISTED
3.8 _
1.3 _
300 Ma/H /
'l'O DIID. DM'I.:; 8Y8'f. --+---:;;;:~t~~~ ..... ;-t+;;;;;;;r-;;;~"",;,- /
72·C !50·C /
0.3 .. /
/
/
/
/
/
/
/
-.
mIn'm~­
~ A.M. B.A.
IWf81t OLD ()Q
IIUORGU Al8
-.
,...... ..... -~i\~---t'"""'""1H t--4-----~~---..... "* 1.!5DC
o BAIl
y
y
i'JI-2
FIGURE 4
'1'B-3

612
GEOTHERMAL DEMONSTRATION ACTIVITIES IN BELGIUM
LIE SUN FAN and N. VANDENBERGHE ::
Belgian Geological Survey, Brussels
Summary
In the Hainaut area, the geothermal use of the Saint-Ghislain and the
Douvrain wells is briefly described.
The drilling of a deep production well and a shallower reinjection well
in North Belgium are described together with the extensive reservoir
testing results of both wells. Also the results of a test loop between
the two wells are described.
The conditions under which the semi-doublet can be operated successfully
are described.
A few years ago the Dinantian limestone reservoir in the Hainaut area
was put into production. Between 1972 and 1981 three geothermal wells are
drilled in the Mons area, at Saint Ghislain, Douvrain and Ghlin. The three
wells have high artesian flow rates, around 10Om 3 /hour, and reservoir temperatures
of about 70°C. The reservoir depth varies between 1335m and
263Om. The geothermal significance of the wells was extensively discussed
by Delmer and co-workers (1982) while data for this area were summarized
in the EC Atlas of Geothermal Resources (1988, p.20).
Both the Saint-Ghislain well and the Douvrain well are in production while
the use of the Ghlin well is still under study. At Saint-Ghislain, since
1986, several public buildings have been heated with the low enthalpy geothermal
source through a 6km long loop of pipes, enveloped in polyurethane
(25Omm inner diameter, 400mm outer diameter). For external temperatures
above +5°C, the geothermal heat alone can satisfy the energy demand of the
buildings. Yearly savings in gas are estimated to be about 1 million m 3 •
After leaving the heat exchanger the water still has a temperature of 40°C,
and is further conducted to a nearby greenhouse in which it heats the air
at the outlet of the greenhouse it has been cooled to 35°C. Finally the
water is further conducted over 1500m to a waste water treatment installation
; there it is used to preheat the extracted muds, helping the production
of methane from these muds.
In Douvrain about 10000m 3 is produced per month and the heat is used for
the air conditioning in a nearby hospital.
The new data on the development of a doublet system at Merksplas-Beerse
in the Campine area in North Belgium are further presented in this paper.
1. THE MERKSPLAS-BEERSE I WELL
a. The well
The Merksplas-Beerse I well was drilled as the first well of a planned
doublet for low-enthalpy geothermal energy. The site is located in the
Antwerp Campine area in North Belgium (fig.1).
:: presently at University Leuven, Belgium

613
The geothermal reservoir is located in a fractured and karstified limestone
at the top of the Dinantian (Vandenberghe, Poggiagliolmi, Watts,
1986). A detailed structural and isohypse map of the top of the limestone,
based on several seismic surveys (Dreesen and co-vorkers, 1987), allowed a
selection of geologically acceptable geothermal development sites. The
location of a potential user and the local surface conditions finally determined
the location of the first well.
After drilling through soft sands, clays and chalks of Tertiary and
Mesozoic age, the well reached the top of the Carboniferous at 1006m depth.
The top of the Dinantian reservoir was reached at 1630m depth.
The drilling was continued till a total depth of 1761m (fig.2). Within
the Dinantian two reservoir sections could be established, one at the top
(163Q-1656m) and one at the base (1740-1745m), each time associated with
other lithologies in addition to limestone (fig.3).
b. The reservoir testing
After acid treatment of the open hole Dinantian section gas shows were
observed. Therefore the well was properly tested (DST) and down hole samples
were taken after nitrogen lifting. The gas is solution gas with
bubble point between 200 and 400 psi at the reservoir temperature of 72°C,
and a GLR of about 1 at standard conditions ; the composition is largely
dominated by carbon dioxide (up to 90 vol %, the remainder being methane
and nitrogen.
A geothermal pumping test was carried out with a submersible pump at
200m depth. Pumping took about 50 hours (in several steps) and recovery
afterwards was recorded over 97 hours (fig.4). Water level was measured
and amerada's recorded the pressure at 1608m near the base of the casing.
The maximum flow rate that could be produced with a draw down till 190m
depth was 72m'/h. The productivity index was 5.4m'/h/bar (at 70°C, 0.53
cps). The permeability determined from Horner plots was 2-3 Darcies.
Both a direct calculation and a model calculation show that more than
eighty percent of the reservoir pressure drop during pumping is related
to some sort of skin, maybe formation damage but more probably related
and inherent to the fracture nature of the reservoir.
The enormous amount of saline water (about 130 gr NaCl type/lit) from
the pumping test was stored in a plastic lined earth-wall basin. After
proper chemical treatment the 'o1II.ter 'o1II.S reinjected in the well. The injectivity
index was 3.65m'/h/bar (at 15°C, 1.2 cps). During the pumping test
the interference with a gas storage site in the same limestone reservoir
was measured in two wells at 9-10 kIll distance from the geothermal well.
Interference was observed and therefore the ~mal interference during
injection and subsequent production at the gas storage site was calculated.
The interference amounts to several bars and obviously influences the
operational design (a.o. pump depth, possibility of single well system).
c. The reservoir stimulation
In view of the very high skin pressure drop an open hole foam acid
stimulation was applied to the upper reservoir level. The deeper part of
the well was sand filled. Individual testing of the lower reservoir level
during the DST (0.34m'/h/bar loP., 30mD) had shown that permeability and
productivity were much inferior compared to the testing of the whole section
with the two reservoirs (geothermal testing). An injectivity test
was carried out before and after the stimulation and pressure variations
were continuously recorded during the testing together with accurate flow
rate measurements. Although the injectivity increased by 10 to 20%, the
skin pressure did not decrease. Skin related pressure in this testing

614
amounts to about 35 % of the total pressure increase during injection.
Apparently currently available stimulation techniques can not substantially
improve the reservoir characteristics and it is suspected that the complex
nature of a fractured reservoir, partly cemented by calcite and locally
karstified, might be a natural cause for the relatively low productivity
in an otherwise highly permeable and laterally continuous reservoir.
2. THE MERKSPLAS-BEERSE II WELL
a. Description of the modified doublet system (semi-doublet system)
As a result of the downward evolution of oil prices, coupled with the
relatively low productivity of the Merksplas I well, the financial support
for drilling the second deviated well to complete the doublet system
dwindled.
In order to complete this geothermal demonstration project in northern
Belgium, a new concept then was developed by using a semi-doublet system,
with a single producing well and a reinjection well in a shallow chalk
reservoir.
The prediction of the chalk reservoir evolution under injection (o.a.
fracturing conditions) was calculated. The reinjection problems were studied
(in collaboration with Labofina) by means of a core flooding test and
a compatibility test between the formation water-rocks, and the injected
brine-rocks. Cretaceous chalk core, and water, was available from the
neighbouring Meer, Turnhout and Loenhout wells.
A programme was designed consisting of three parts
- The drilling of the reinjection well down to the Cretaceous reservoir at
800m.
- Testing, including in-situ well tests and laboratory tests.
- A pilot loop test between the two wells.
b. The well, sampling and testing
The well was situated about 132m away from the first well. This distance
was chosen in order to avoid damage of the Merksplas-Beerse I casing
(9"5/8, 36 lb/ft) by the injection pressure.
The top reservoir is at 695.5m and the 7" casing was set at 708.6Om. The
reason to drill deeper into the reservoir was to protect it from overlaying
potentialy damaging unconsolidated fine sands.
A leak-off test showed that the reservoir is very porous with the possible
occurrence of natural fractures.
Drilling into the reservoir chalk continued till TD 800m in 6" diameter.
The reservoir was drilled with a flush medium only composed of water with
a small amount of calcium chloride. No bentonite has been used. Cores
were taken in certain intervals, but core recovery was poor due to the
presence of flintstone layers of varying thickness and the friability of
the chalk formation. Side wall coring was attempted in order to improve
the recovery. The well was logged and cleaned by airlifting. The well
production was tested by airlift and electric submersible pump. Results
show that the water temperature is 35°C, initial water level stabilizes at
+1.20m (GL) and the Productivity Index (PI) is 4m 3 /h/bar. Water samples
were collected periodically for laboratory test purposes.
Downhole fluid sampling was carried out for chemical, physical, compressibility,
and compatibility analysis.
The laboratory water analysis were performed identically both for the
Cretaceous formation water and the reinjected Dinantian brine from the
first well (samples were obtained during the pilot loop test).

615
Density fln. water 20°C
Salinity
Conductivity
pH
Dried residue at 105°C
Viscosity
Total Bacteria
particles content I ml
Particles size
Distribution 1 A 2 ~
GLR
C02 vol %
CH~ vol %
Compatibility
Merksplas I
(Dinantian)
1.09 g/ml
120 gil
120 mS
5.53-6.06
140 gil
0.93 cm (32°C)
negligible
2.6 10 6
max. 4.5 ~
74-90%
0.7-1.6
80-93
8-15% max.
Merksplas II
(Cretaceous )
1.01 g/ml
13.50 gil
21 mS
7.45-8.1
13.5 gil
0.81 cp (35°C)
10' col/ml
3.4 10 6
max. 4.5 ~
87-100%
Both waters are compatible as long as the
doublet is in closed system (oxygen free)
and the C02 kept in the solution.
c. The Pilot Loop Test
This full scale simulation loop test between production and injection
wells immediately after the drilling of the reinjection well was designed
in order to get a representative brine for various test purposes. This
was due to the fact that, during the drilling of the Merksplas 1 well and
the several testing programmes, at least 10.00Om' of treated surface stored
production water, have been reinjected back into the reservoir after treatment
with biocide and oxygen scavenger, furthermore several chemical additives
still remained in the reservoir as a result of the foam acid simulation
test. For evacuation of at least 20.00Om' of water with a capacity
of 75m'/h, a three weeks continuous loop test was necessary.
The test was realised with a test skid, equipped with an automatic
backwasking cartridged type filter, chemical treatment injection pumps,
heat exchanger and a Roto-Jet injection pump (fig.5). A continuous inline
water quality monitoring was also installed at the upstream and downstream
sides. An electrical submersible pump at 250m depth was used in
the production well.
Conventional core analysis and core flooding test were carried out, including
electrical measurements (cementation factor) mercury injection
(pore size data), permeability variations with pore water composition and
SEM analysis. SEM was used to compare flooded and unflooded core plugs
and the potential for mobile fines to migrate and the nature of the fines
present.
The reservoir is layered, with a net pay thickness of 164 ft and an
average porosity of 31%.
The air permeability is about 122 md. The liquid permeability using chalk
reservoir water in only 30 to 60% of the Klinkenberg corrected air k,
whereas liquid permeabilities using Dinantian brine vary between 71 and
118% of the liquid permeability using chalk reservoir water. Variations
are related to grain size distribution.
Permeabilities obtained from well test transient analysis, over the
whole reservoir interval, are higher.
Well test transient analyses indicate permeabilities around 156-25Omd during
production test, 200-250 md during injection with clean water and
255-265 md at the final stage of brine injection (pilot loop test) (fig.6).

616
The pore size of the Cretaceous rocks is very large attaining occasionally
30 pm and frequently interconnected by the smaller pore size of approx.
0.1 pm. Threshold entry pore radius varies between 0.15 and 1.8 pm.
However, solid filtration is still absolutely necessary and it is suggested
that a 2-3 pm filtration should be used in the start up period accompanied
with a proper design so that the pressure drop across the filter
should not cause the release of C02.
The irreducible water content measured was 5-10%. Core flooding tests
suggest that chemical scaling does not seem a probable damage factor, that
mechanical plugging could occur and that brine injection is preferential
over water injection.
The main results
- Main circuit : Q
Pwh producer
Pwh injector
T produced
T injected
Oxygen level
= 75m 3 /h (water level: about -200m GL)
= ± 10 bars
= 24 bars when starting but gradually deminishing
to 19 bars in 3 weeks time.
is assumed that this pressure drop is
caused by a gradual dispersion of the skin.
71°C
32°C
= 0.02 ppm at the upstream (70°C)
10-25 ppb at the downstream (32°C)
Bacteria = negligible
- Second circuit (heat exchanger) : Q = ± 6Om 3 /h
T-in = 10°C
T-out (for future consumer) ~ 68°C
- Total power used Electrical Submersible Pump = 122.67 Kw
Rotojet
135.00 Kw
Total Kw used
= 257.67 Kw
- Produced Heat capacity: Q = 75m 3 /h
Delta T = (71°C - 32°C) = 39°C
For a heat exchanger efficiency of 9S%, the
heat production represents 3.2 MWth.
- Pressure loss - At surface injection line (132m) about 8 bars.
- Through the injection tubing about 0.5 bars.
- Through filter installation and heat exchanger about
1 to 2 bars.
CONCLUSIONS
It has been demonstrated that the semi-doublet system, with a deep
production well and a shallower injection well, can be successfully operated
in the particular case of the Antwerp Campine area. Compared to the originally
plannet doublet with two deep wells, the maximum injectivity of
the shallow reservoir imposes a maximum production rate and hence a reduced
energy production. This disadvantage is offset by the lower drilling costs.
Eventually drilling costs could even be lowered more if using a single well
with an isolated production tubing in the centre and an injection through
the annulus of a larger diameter upper part of the same well. Obviously
such semi-doublet will cause a pressure drop in the production reservoir,
that might lead to an increase in pumping costs in the future.
If further use of geothermal energy will be made in the area, obviously
the interference of all these factors will have to be considered, including
the value of the chalk reservoir itself as a potential geothermal resource.
It

617
REFERENCES
Delmer, A., Leclercq, V., Marliere, R. et Robaszynski, F. (1982). La geothermie
en Hainaut et Ie sondage de Ghlin (Mons-Belgique). Ann. Soc. Geol.
Nord, CI, 189-206.
Dreesen, R., Bouckaert, J., Dusar, M., Soille, J. and Vandenberghe, N.
(1987). Subsurface structural analysis of the Late-Dinantian carbonate
shelf at the northern flank of the Brabant Massif (Campine Basin, North
Belgium). Toelich~. Verhand. Geologische en Mijnkaarten van Belgie, 21,
1-37.
Vandenberghe, N., Poggiagliolmi, E., and Watts, G. (1986). Offset-dependent
seismic amplitudes from karst limestone in northern Belgium. First
Break, 4(5), 9-27.
Vandenberghe, N., Boonen, P., Voets, R., Dusar, M., Bouckaert, J., and
Lie Sun Fan. The Merksplas-Beerse I and II veIls. Internal Report, Belgian
Geological Survey.
~ LOCATION MAP
of the
MERKSPLAS-BEERSE
Doublet
II
I5lu"
THE NETHERLANDS
BELGIUM
ANTWERPEN
•
.
eA
a M[RIl5PlAS
TUANHOUT
IlERY
. •
oaSTNAU(
POID£RL[I
..
..
. - ..... '"
! ~
·
.·
:'
.

618
-'."UOl
RT +33,89m TAW Bitsize Casing
0
L
PERMANENT DATUM :
.21.15.- ..
26" 20" 94 lbl ft
100
.; m
200
.:
: 17"'1l 13"~8 545 lb/ft
..
300 :
356m BOC - :
:
400
~
:
; ....
~
500 510 III DV 4: ·:~512 >-
- a:
:. ~
:; m < 0
12"}Iz 9"\ 36lb/ft - u
I-
600
a: ..
i
.. ...., "Q
: e:
l-
0
700
•
800
900
: 1/1
.. ::::I
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852m SOC - - Q/
0 u
991 m DV - .I
1000 ~1006
III
1100
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8"".l 7" 23lb/ft -::::I l:
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1300
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I
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1200 27lb/ft
1400
z
~
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•
'- Z
.t:.
.,
~ •
Q.
Q. II:
1500 ~ ::::I
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:
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z
1600 ... 1627,5
m
11/1 Z
6"~ OH ·c~ ~
1700 '-0,- z
".0 Q/ c(
:. '-- z
'--- 0" -
1761 m
-IU
C
Fig. 2.
MERKSPLAS - BEERSE I WELL.

619
DEPTH
PENETRAnON FRACTURE
CHRONO. 1m) LlTHOLOG. RATE LOG
OR q !PCilV'ntilt (Fl ' SCH UM)
~
~~
~~
. ~
~~ r65
ml ' erg" I dolomrf.
' $Olldy ) ~hol"
~ dolo," l,m~stOD#
~ Ilmt'ston.
~ colc .. dol shol~
(lip" l.mHt~
£>:. :1 sonds ton~
Fig 3. The Dinan Ian reservoir lithology end
the froc ure reservoir location .

P(bar)
163
162 --- . ..
.. • •• • • • •
• •
•
161
160 k = 20
S = 69,9
159 ~Pa = 12.2 bar
158
157
156
•
155
154
153
152
151
•
•
10
100
tp. ~t
1000
~t
Fig.4. Horner plot for ... 0 pumping step at the rate of 72 m 3 /h .

~, -'SECONDARY LOOP
15 WC '.,-C I-
1-....... ---_-­ ~
I
Wat ...
-.pIing
FIlTER
9,5 bar EXCHANOER ~ bar
..... __ --Il2-C
W.Q.C. • Wat., Quality Control
G.M. • Gas Monitor
:::J
o
~ ~~
~ ~C
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42bar :;; , .. 111"' ....
~-JET W
P\JIIP I-
~----------6m----------~
ESP
250111
1------------------132 m distanc. ----------------~
CRETACEOUS
7O!I1II
100111
REINJECTION WEll
(t.411)
117601111
DINANTIAN LINES TONE
PRODUCTION WELL (t.41)
Fig. 5.
THE PILOT LOOP TEST MERKSPLAS II

622
Fig. 6. Injection flow profile test "I
1+-+------67!i
HIr--------+ 700
1-I-.!-____ = ___ --72!i 124m. 8"
.......,..::....-------+ 750
k"-E~-----77!i
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6D
OVERVIEW OF GEOTHERMAL
DEMONSTRATION ACTIVITIES IN GERMANY
R. SCHULZ
Geological Survey of Lower Saxony
Summary
A short overview of geothermal demonstration projects in
the FRG during the period of the last 4 years is given.
All the project sites are situated in Southern Germany.
In Bruchsa1 (Upper Rhine Graben) a doublet in working
order exists now. There were problems caused by the high
salinity of the formation water. The connections for the
consumers are at a planning stage.
Host demonstration activities take place in the South
German Molasse Basin, where the Ma1m is the main aquifer.
The Sau1gau project was the first one using energy in the
low-enthalpy range. Projects in the Ha1m aquifer will be
successful, if the karstic limestone can be found by
drilling.
A new project starts at Weiden (Upper Palatinate).
1. INTRODUCTION
The geothermal resources and reserves in the Federal Republic
of Germany (FRG) have been estimated for selected aquifers
in the North German Basin, the Upper Rhine Graben and the
Molasse Basin. The results are published in the Atlas of Geothermal
Resources (Haenel and Staroste, 1988). The geothermal
energy is utilized in a few localities in the FRG only. A complete
list of this utilization at the status of 1985 is given
by Haenel (1985).
In the present paper an overview of the projects, at that
time either planned or under construction, and in addition of
new projects is given. Table I shows the results of the dri1-
lings belonging to the current projects in the FRG. All localities
are situated in Southern Germany. It is known, that
there is a very high water salinity in the deep seated aquifers
of the North German Basin. Planning the projects in this
area is delayed until the results of the Bruchsa1 project will
be received, where a similar water type exists (see first line
of Table I).
Table II shows the present and the planned future status
of the geothermal projects. Three projects only produce heat
for utilization at the present stage. In the table the real
annual average heat production is given: the technical feasibi1ities
can be higher.
Three of the projects, each in another area and in another
geological formation, will be briefly described in the
following.

EC Re9:ion Max. Max. Max. Solid Flow '" ~
Year Depth Temp. Aquifer Depth Temp. Content Rate
r~oca1ity-we11 (m) (OC) (m) (OC) (mg/kg) (1/s)
0-
UEEer Rhine Graben
x Bruchsa1 GB1 84 1932 116 L.'I'riassic 1720-1870 115 127300 10
x GB2 85 2542 132 L.'I'riassic 2286-2463 129 22
Molasse Basin (West)
(x) Au1endorf-Laimbach 1 81 2076 97 U.Jurassic 1162-1215 55 555 20
Bad Buchau 1 82 795 48 U.Jurassic 606-782 48 433 60
x Bad Wa1dsee 1a 86 2322 95 M.Miocene 424-604 30 616 7
Ravensburg 83 2100 80 M.Miocene 376-536 34 524 15
Sau1gau TB1 77 648 43 U.Jurassic 619-625 42 479 32
x GB2 83 915 54 U.Jurassic 725 43 455 3
x GR3 81 928 52 U.Jurassic 583-618 41 553 50
Molasse Rasin (East)
Erding 1 83 2359 69 U.Jurassic 1815-2295 66 650 55
Munchen-Schwabing 1 (89) (2500) (90) U.Jurassic (1800-2300) (75)
UEEer Palatinate
(x) Weiden 1 (89) (2000) (60) L.Permian (800-1100) (32) (5)
Table I: Geothermal energy projects in the FRG (1985-88): values for planned projects in
paranthesis.
EC: supported by the EC

Region
Locality
Status
Present (OCt. 88)
thermal output
(Gwh/a)
H
Future
Status
Upper Rhine Graben
Bruchsal
Molasse Rasin (West)
Doublet in working order
E
long-distance heat supply
(castle)
Aulendorf
shut-down
thermal bath
Bad Buchau thermal bath with heating 2.75
utility water for a hospital
Bad Waldsee long-distance heat supply 3.6
(sanatorium)
Ravensburg shut-down
Saulgau thermal bath with heating 3.9
Molasse Basin (East)
E
P
hospital heating
deepening the well
unknown
hospital heating
Erding
MUnchen
Upper Palatinate
planning for utilization
planning for drilling
thermal bath w. heating
long-distance heat supply
(airport building)
thermal bath w. heating
Weiden planning for drilling «4.7)
thermal bath
Table II: Status of new geothermal energy projects in FRG (see Table I).
Thermal output means the real average annual utilization; the feasable
technical utilization can be higher.
H: withdrawal of heat: E: heat exchanger: P: heat pump

626
2. UPPER RHINE GRABEN
The Upper Rh1ne Graben is very suitable for geothermal
projects because of the good hydraulic conductivity and the
high temperatures in the deeper seated aquifers. The site of
the Bruchsal project is north of Karlsruhe and near the main
fault of the Graben.
Two wells have been drilled at a distance of about
1200 m. Both wells have been successful (see Table I): hot
water of a maximal temperature of 129"C can be extracted in
the Buntsandstein aquifer (Lower Triassic): the flow rate is
about 20 lIs. The high salinity of the formation water necessitates
a doublet. The well GB 1 will be the extraction, the
well GB 2 the injection well. It is demonstrated by pumping
tests, that there is a hydraulic connection between both
wells. The installation, especially the tubing between both
well heads, exists (Fritz, 1988). So the doublet is now in
working order (Table II). It is planned to utilize the geothermal
energy for a long-distance heat supply. The consumers
will be a sports center and the Castle of Bruchsal. It was
also intended to produce electric power by the use of the socalled
Organic Rankine Cycle (ORC). But this plan will not be
pursued further, as the costs are too high.
The life time of the doublet can be predicted by model
investigations (Schulz, 1988). The thermal breakthrough time
is 33 years for a flow rate of 20 lIs and further parameters
given for the Bruchsal project (e.g. aquifer thickness 25 m).
But the real thermal utilization time is much longer. This
time is defined by the extraction temperature and its cooling
dawn. If a relative decline in temperature of 10 % can be
tolerated, that is 7 K in the Bruchsal project, the utilization
time is more than 200 years. This value is valid for an
unbounded aquifer. Since the wells have been drilled into the
main fault of the Rhine Graben, it has to be assumed, that the
aquifer is bounded in its horizontal extension. But even if
the extension width is 1200 m only, the utilization time is
80 years (Fig. 1). So the geothermal doublet in the Bruchsal
site will probably work during a long period of time.
3. THE MOLASSE BASIN
The previous investigations indicate that the highest
geothermal reserves are situated in the Molasse Basin of
Southern Germany. In a current research project the reserves
and resources will be assessed more accurately for the Malm
aquifer (Upper Jurassic). The first results of the hydraulic
and thermal situations are published (Werner, 1987: Jobmann
and others, 1988).
Host demonstration activities in Germany take place in
the Halm aquifer of the Holasse Basin (Table I). An interesting
project is the geothermal demonstration project in Saulgau.
This project was the fist one in the area of the FRG
using geothermal energy in the low-enthalpy temperature range.
The purpose was to establish a pilot project preparing the
later use of the low-grade saline water including the supply
of heat and water for domestic use. To realise this project
the geothermal wells GB 2 and 3 were drilled and the necessary
infrastructure was installed. Important results are obtained

627
Working time of • doublet (years)
Fig. 1,
Normalized extraction temperature as a function of
the working time of a doublet (after Schulz, 1987b).
Te extraction-, Ti injection-, To aquifer-temperature
Model: aquifer with limited horizontal extension:
b width of the aquifer,
a distance of the two wells (1200 m).
Used Parameters (Bruchsal projekt):
Flow rate: 20 lis:
thickness of the aquifer: 25 m:
thermal conductivity of the rocks: 3.4 W/m K.

628
by exploring the Malm section, by means of stratigraphy,
facies and fracture analysis, by the geohydraulic investigations
and by the regional exploration of the flow system in
the t1alm. For establishing a hydrogeothermal balance of the
Malm aquifer, the temperature field, the thermal conductivity
and diffusivity of the Malm rock were determined. The results
are summarized in Fritz (1987).
The planned doublet cannot be realized: The well GB 2 is
a failure, it produces water at a flow rate of 3 lis only
(Table I). This low flow rate is caused by the nearly complete
absence of the karstic section within the Halm limestone
(Werner, 1987). The distance between the other wells TB 1 and
GB 3 is 400 m only: therefore the life time of the doublet
will be relatively short, even if a natural ground water flow
is assumed (Schulz, 1987).
The experience of the Saulgau project and the results of
further projects and investigations show, that geothermal projects
in the Molasse Basin would be exceptionally successful,
if the karstic section of the limestone within the Halm aquifer
could be found by drilling.
4. THE UPPER PALATINATE
A new geothermal project is planned by the Stadtwerke
Weiden i.d. Oberpfalz (Upper Palatinate).
The aim of the project is to produce warm water for a
thermal bath and to save energy for a leisure-time center. The
drilling of the well is planned. There is hope to find an
aquifer in the conglomerate zone of the Rotliegendes (Lower
Permian), probably in 800 m down to 1100 m depth. Pump tests
in the well will show, whether the flow rate will be high
enough.
5. ACKNm-lLEDGEMENTS
For their helpful cooperation, especially for their support
with detailed information, I wish to thank Dipl.-Ing.
J. Fritz, Urach, Dr. J. Werner, Freiburg, Dipl.-Geol. P. Stier,
~mnchen, and Mr. Meyer, Weiden.
6. REFERENCES
Fritz, J. (Ed.) (1987). Erdwarmebohrung in Saulgau, Gemeinschafts-Demonstrationsprojekt.
Kommission der EG,
Report EUR 11290 DE.
Fritz, J. (1988). Geothermikbohrung GB Bruchsal 2 zur Reinjektion
hochsalinaren HeiBwassers. In Projektleitung
Biologie, Okologie, Energie (Ed.), Status report
1988 Geothermik"und Lagerstatten. KFA JUlich.
pp. 259-270.
Haenel, R. (1985). Present status (1985) on utilizing geothermal
energy in the Federal Republic of Germany. In
C. Stone (Ed.), 1985 International Symposium on Geothermal
Energy, International Volume. Central Press,
Sacremento. pp. 69-76.
Haenel, R. and E. Staroste, (Eds.) (1988). Atlas of Geothermal
Resources in the European Community, Austria and
Switzerland. Th. Schaefer, Hannover.

629
Jobmann, H., R. Prestel, R. Schulz, G. Strayle, J. Wendebourg,
J. Werner (1988). Hydrogeothermische Energiebilanz
und Grundwasserhaushalt des Halmkarst im sUddeutschen
Holassebecken, Projekt Isotopenhydrologie und
Hydrogeothermie. In Projektleitung Biologie, Okologie,
Energie (Ed.), Status report 1988 Geothermik und
LagerstKtten. KFA JUlich. pp. 247-257.
Schulz, R. (1987). Analytical model calculations for heat
exchange in a confined aquifer. Journal of Geophysics,
61, 12-20.
Schulz, R. (1988). Analytische r10dellrechnungen zum Warmeaustausch
beim Dublettenbetrieb. In Projektleitung Biologie,
Okologie, Energie (Ed.), Status report 1988
Geothermik und LagerstKtten. KFA JUlich. pp. 259-
270.
Werner, J. (1987). Das Forschungsvorhaben "Hydrogeothermische
Energienutzung und Grundwasserhaushalt des tlalmkarsts
im sUddeutschen Holassebecken" - Ziele und
Zwischenergebnisse. Zeitschrift deutsche geologische
Gesellschaft, 138, 399-409.

630
OVERVIEW OF HIGH ENTHALPY PROJECTS IN ITALY
G. ALLEGRINI
ENEL (Italian Electricity Board), National Geothermal unit
INTRODUCTION
Italian geothermal fields producing fluids which
can be used for the production of electricity are located
in the area between the Apennines Mountains and the
Tyrrhenian Sea. The most exploited areas up to now are in
Tuscany. It was, in fact, in the Larderello area, which is
situated in the district of Pisa, that the electrical
exploitation of geothermal fluids was tested for the first
time in the world, after geothermal fluids had been used
for many decades both for the production of boric products
and as a source of heat or mechanical power. Only at the
end of the Fifties' was geothermal research extended
outside the traditional boraciferous region leading to the
electrical utilization of geothermal fluids in Monte
Amiata. Finally, in the Seventies', as a consequence of the
oil crisis, new boosts for research enabled the discovery
of new geothermal fields of industrial interest in Latium
and Campania. The new prospects opened up by geothermal
activities in Italy have meant, in the Eighties', a
substancial thrust to the industry (proportional to its
expected results) in both the technical and organizatonal
fields.
Three major technological aspects were investigated
in detail:
1) improving the methodologies of surface investigation and
of data analysis in order to reduce the mining risk that
greatly affects production costs.
2) having drilling rigs and technologies available which
are capable of reaching greater depths economically.
3) setting up a unified project of a geothermal power plant
whi.ch is extremely flexible and adaptable to every
geothermal fluid and which enables us to anticipate as
much as possible the return of investment by a quick
installation in the field.
As regards organization, the aim was to use human
and financial resources in a logical succession of
Research, Exploration and Development Projects in order to
permit easy programming during the period to harmonise
development of all activity.
The results of this effort are evident from the
following analysis of the situation as of December 1987 and
those programs already planned regarding our aims and the
activities for the Nineties'.

631
SITUATION AS OF DECEMBER 1987
On the 31st/12/87 the installed geothermal power
was 517 MW and the production of electricity had reached 3
TWh (table 1). Five new power plants of 20 MW each were
already on line, three of them having started operating
during 1987. At the end of 1988 two more new power plants
of 20 MW each went on line: therefore, a total of 7 new
units was reached.
Larderello
Also in reference to the 31st/12/87 we are now
analyzing the situat,ion for each area, beginning from the
Larderello area, which includes the largest geothermal
fields with the oldest ~nd highest exploitation. It extends
approximatelY 250 km and includes the fields of
Larderello, Gabbro, Castelnuovo, Serrazzano, Sasso,
Monterotondo and Lago Boracifero.
All in all there are 400 MW installed (and 11 more
have been installed in 1988), equal to 77% of the total
geothermal power installed, using in total approximately
2800 t/h of steam. 25\ of this power is in plants of recent
installation, fed with admission pressures compatible with
reduced specific consumptions. No less than 60\ of the
power is, however, engaged in plants Which, during the next
years, will be the subject of renewal projects with the
complete substitution of the principal machinery and an
increase of the entire power installed of about 150 MW. By
the beginning of the Nineties' all the plants will be
automated and teleconducted from a single teleconduction
Center in order to remove the need for the continuous
presence of personnel in the power plants.
To reduce the decrease of flow rate and of pressure
in the field of Larderello, which was subjected to the
highest drainage during these years, it was planned to
compensate for the lack of water with the reinjection,
using both the water discharged by the power plants and the
brine produced by. the wells of Travale-Radicondoli field,
30 km away from Larderello.
Moreover, deep drilling executed in the schistose,
quartzitic basement underlaying the first productive
reservoir revealed the presence of a diffused permeability
which allows the production of steam with higher pressure
and temperature.
For all these reasons (renewal of the plants,
recharge of reservoirs and deeper wells), we can assert
that the traditional production area will not only maintain
present production levels, but will also contribute
80 - 100 MW of the increase of production foreseen for the
end of this century, as will be discussed in the following
paragraph. This occurs in addition to the contribution of
new development programs in the boundary areas of this wide
zone, which are partly in progress (Monteverdi) and are

632
partly in an advanced exploration phase (Selva, Carboli,
etc.).
Travale-Radicondoli
In this area of about 30 km 2 , 70 MW were installed
on 31/12/87 (and another 20 were installed in 1988), after
having" dismantled 18 MW of noncondensing plants, which had
been installed in the seventies. They are recently
constructed plants with reduced specific consumptions and
fed with high pressure steam.
Next to the geothermal field which is operating at
present, a cold water front exists, deriving from the
inflow of meteoric water in the reservoir through outcrops
of the rocks. The rate of the inflow is proportional to the
decrease in pressure of the reservoir caused by steam
production. For this reason the collection and transfer of
the water available in the Travale field was planned to
increase reinjection into the Larderello field. Further
exploration wells are planned for a better definition of
the surface extension of the geothermal field under
exploitation.
Monte Amiata
At the end of 1987 only a newly installed unified
20 MW plant was added to the already existing 22 MW of
noncondensing groups; but no less than 80 MW were available
in the steam found within the development project of
Piancastagnaio, not used until now because of authorization
difficulties. The above mentioned project, however,
foresees the installation of 6 more plants of 20 MW each by
beginning of the 1990' s (the realization of two of them
began at the end of 1988). In this area, in fact, there is
a reservoir at a depth of between 2500 and 3500 m included
in the metamorphic basement and containing a fluid at 330-
350 ·C and approximately 200 bar of pressure. The
completion of this project, the formulation of a new
development proj ect in the Bagnore area (where the
exploration drilling discovered a second productive
reservoir at approximately 3000 m) and, finally, the deep
exploration planned for the near future in the Abbadia San
Salvatqre area, will constitute one of the main elements of
the geothermal program of "the Nineties', discussed below.
Latera
The last power plant to be examined among those of
table 1 is the noncondensing unit of 4.5 MW installed at
Latera as a demonstration for characterizing in quantity
and quality the geothermal field. The latter, where Enel is
the operator on behalf of the Enel-Agip Joint Venture, has
an extension of" 20 km 2 and produces a two-phase fluid at
about 210-220 ·C, with TOS of 10-12 g/l, principally

633
alkaline chlorides and alkaline-earth bicarbonates and with
a noncondensable gas content varying between 3 and 7\ by
weight. Research and experiments carried out in this new
field have been partly financed by the EEC by means of
various contracts, the results of which were demonstrated
at the "Geothermal workshop" of 13-14 October 1986.
The area of Latera is of particular importance for
the Italian industry: it is, in fact, the first area
outside Tuscany where electricity is produced exploiting a
water-dominated field. In this area the installation of a
40 MW power plant is planned, made of two unified groups of
20 MW each, fed respectively with flash steam of high and
low pressure, and with the adoption of a direct-contact
reboiler, with a thermodynamic cycle particularly suitable
for brines with a high and variable content of
noncondensables gas.
AIM FOR THE YEAR 2000
By the end of this century the· aim is to reach the
highest possible utilization of geothermal resources for
electricity production, using the technology available and
as economically as possible. The aim of the program for the
above date is to reach the value of 9 TWh/year for
geothermal electricity production.
According to the organization previously described,
all the activities to be carried out are linked within
(Deep) Exploration Projects, which are the result of
Surface Exploration Projects, and Development Projects,
which are obtained from the former. Moreover, there are
Renewal Projects, aimed at older fields and plants.
Taking into consideration the situation as of
December 1987 described above, the carrying-out of this aim
means the drilling of new wells for about 950 Jan and the
installation of new power plants in order to increase the
total installed power by about 1000 MW, also taking account
of the dismantlings foreseen in this period, equal to 250
MW. Figures 1 and 2 show the development of installed power
and energy production according to the program mentioned
above. Figure 3 shows the location of geothermal activities
in progress and planned for the future.
However, it is necessary to bear in mind that only
60\ of the 1000 MW, by which the power installed on
31/12/87 should be increased by the year 2000 and which
should increase the' annual energy production by about 6
TWh, is assigned to Renewal and Development Projects of
which a great deal is known and whose estimate can be
considered reliable. The further installation of 400 MW,
which has been hypothesised on the basis of new
Development projects, should come from the favourable
results of intense Research and Exploration foreseen in the
very first years of the program in areas where thermal
anomalies have been found.

634
Therefore, a more reliable evaluation could be
carried out only after the above mentioned research and
exploration.
From our present knowledge it can be deducted that
about 70% of the future use of high enthalpy will develop
in the Tuscan areas and only 30% will come from Latium and
Campania. In the latter regions, in fact, there are in some
cases technical difficulties still to be resolved, before
being able to start commercial exploitation.
CONCLUSIONS
It can be seen that the work in Geothermal Research
and Development over the next decade is decisive and the
expected results are very important. Also the financial
costs are considerable: they can be valuated at
approximately 3000 billion lira at the monetary value in
1987. Regarding technological development, apart from
continuing commitment to the above mentioned fields, it is
also necessary to devote resources to the development of
the following aspects:
1) cycles and methodology for the economic exploitation of
particularly scaling and corrosive fluids or of
particularly complex hydrothermal systems (binary
cycles, use of scaling and/or corrosion inhibitors,
integrated cycles of chemical, electrical and thermal
production, etc.).
2) technological solutions suitable for the further
reduction of the negative impact on the environment
both during the construction of plants (wells and power
plants) and during the operating phase.
It is very important, however, to define reliable
standard times for authorizations in order to exploi t to
the maximum the efficiency gained with the organization of
the proj ects; therefore, the opportuni ties offered by law
No.896/86 should be exploited to the full. This law
specifically regulates geothermal activity and provides
total invol vement by the Local and Central Administration
at all levels during planning approval.

635
lAkE 1
1l1ll.IM ['[01l£1IW. PlM:I PUIII5 III LII£ AS If O£IlJIIU 1987
lID. of Uoll I.hl 1o".II.d &r." klliUI. 1.1., 51 ...
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I 14.5
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1981 55
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LA L.!CCIA I 1 8 I 65 I. I 190 1983 -
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SUl-TOIIli.
LAIOt:I£LLO 2'1 400.1 310.2
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I(

It
TWh
~
• INSTALLED POWER
1100
-
• AALABLE POWER
. • •
,~
-. -
• ~
•
I~
• '"
•
•
A
A
~
• •
•
•
•
•
1tt
... "
n H
"
" lOOt
,..
•
"
n
H
"
- " ,..
Figure 1: Planned development of the geothermal installed
power and of the available production power in Italy
Figure 2: Planned development of the production
of electricity from geothermal resources in Italy

637
Figure 3 -
Location of high enthalpy areas in Italy

638
OVERVIEW OF LOW ENTHALPY PROJECTS IN ITALY
C. SOMMARUGA and G. VERDIANI
"Valorizzazione Risorse Geo~ermicheu
Vl.a Sl.smondi, 62 - I 20133 I'Ii.lano
SUllll!lilry
Italy holds, with 631 MWt, a prominent place in the world on the low e~
thalpy geothermal use. The re.cent energy crises encouraged the develoE
ment for direct appll.catl.ons. Thence, AGIP and ENEL carried out some sp!,
ce heating projects u,sing deep wells: "Metahopoli" (operating) "Ferrara~
and "Vicenza". The very high drilling cos~s and the low crude oil price
howewer slaken.the research in'deep aqul.fers. Toe operators are addres
, ,
sing to not very deep aquifer exploitation using ,when necessary, tne heat
pumps. 'l'he main distrl.c~ hea~ing projec~s, opera~ing or l.n progress, are
k ADano", "Bagno dl. Romagna-, nAqqui", "urosse~ou and ~Ll.nate Al.rport" (u~
der study), and the wl.de greenhouses systems of "Amiata" and UPantani _a!!
ded ~o older but developing projects of "ADano" (spa, resort and greenno~
se heating) and "Larderello/Caste.Lnuovo u (district, greennouse and indu
strial heating).
In 19~6 the law on the geothermics came into force. It encourage the
direct uses by contributes to mining operations. Up to 2000, inCTease of 5\
p.a. (target of about 1100 MWt) is possible if the State incentives both for
mining and planting works will be prompt and adequate.
1. GENERAL VIEW
In European Community, Italy and France hold the firs,t places on the di
rect use of low eqthalpy geothermal energy, before Spain and Greece. It
is necessary anyway to note that the italian capacity for low enthalpy geo
thermal uses is 631 MWt, but 376 concern balneology, based on the traditio;
dated back to the Roman Age.
On the world classification Italy holds the fifth place behind. JaP4n,
Hungary, Iceland,and U.S.S.R (including spa).
GEOTHERMAL NON-ELECTRIC USES
ITALY
E.E.C.
rest of
EUROPE
rest of
WORLD
total
WORLD
631
221
MWt
x 1000 TOE/y
1325
465
4516
1583
6409
2252
12250
4300

639
2.0.OVERVIEW OF THE MAIN PROOECTS
The basic data of the main italian projects regarding space and gree~
houses heating are summarized in the annexed tables. Here they are integr!
ted with some comments and notes concerning aquaculture and industrial pr~
jects too.
The pointed out power and energy values referre to the only geotheE
mal energy rate utilized in the project, free of the integrative and servi
ce energies. Besides they are calculated considering the real utilized temp!
rature step. As regard the thermal spring uses, the wastes temperature is
considertd to be 15°C.
2.1. DIRECT OR INDIRECT (NON-ELECTRICAL) USES OF HIGH TEMPERATURE FLUIDS (90
-l60·C) available from steam wells or hot water (not suitable to electr!.
city generation) and from geothermal electric power plant wastes.
At present, 140 MWt high temperature waste fluids, available from ENEL ge~
thermal fields are utilized only in very low part. Regions of interest I Tu
scany, Latium, campania and Sicily.
Larderello/Castelnuovo and Travale/Radicondoli areas. Some realiza
tions, started on the fifties, at present have a more than 50 MWt total ~
wer. They utilize geothermal fluids to district heating (12MWt) ,greenhouses
(12 MWt) and industrial heat process (27MWt, for the boron minerals treat
ment by CO2 and NH3 contained in geothermal fluids). An ENEL center for ge~
thermal energy direct uses is in Bulera/Pomarance.
Pian Castagnaio/Amiata. The heat waste of the back-pressure geothe~
electr1c plant (15 MWe)is utilized, by heat-exchangers, to heat in winter!
bout 25 ha of greenhouses (in increase). other complementaries uses are un
der consideration. Available powerl 90 MWt (SO MWt are utilized in wiB~er
to the greenhouses).
Not realized projects. Up to this time the following projec~s haven't
been realizedl
~I local apace heating and industrial process to extract potassium
salta from geothermal brines,
VIllcano islandl old projects (years '50-'.60) to produce alum, to extract suI
phur (Frash metod) and recent projects to desalt sea_ater bY geothermal heat.
2.2. EXPLOITATION OF -HOT SPRING- SYSTEMS (15-l00·C and more)
The italian hot spring whole is made up by 750 -thermal sites-,a third
of which haa a good tourist organisation with 420 spa and 2000 hotels. The
total of the people living in these centers is about 900,000 units. The an
nual clienta are 2,700,000 for 20,000,000 days. Approximately 130,000 peE
aons work direcly or indirectly in this job. The drilled wells are about 600
whose half ia active.
~ projects, etc. (Euganean Hills). The hot spring system is utili
zed, by 200 operating wells,to heat nearly 130 spa and hotel •• The high mass
flow (2600-3400 m3 (h) causes, although under con~rol, the water stratum low!
ring and see_ to cauae aome local subsidence phenomena. To avoid these pr~
blema it ahould be neces.arythe waste reinjection. Besides the increasing

640
ITALIAN GEOTHERMAL PRO.JECTS
: ......... .
..... -....... .
UII PROJECTS
I-TORUO
2-ACOUI TERKE
l-NEIUOPOll, uun
4-REGGI0 E., GUASlAllA
S-YILLAYEILA, YICEIlA
6-ABUO. PAOOVA
7-GRAOO, TAGlIUE"IO
I-ROOIGO
, i-FERRARA
10-BAG"0 01 ROKAGU
II-LAROERElLO, CUTElIUOYO,
RAOICO"OOLI •• tc.
12~AKIATA, P.CASTAGUIO
Il-GROSSETO. ROSElLE
14-ALFIIA. LATERA
15-CUUO,ORHEIElLO
U-I'UTUI. CIYlUYECCHIA
I7·CESAlO
1I-110F£TE. ISCHIA
II-LUlU'
20-TARA1l0
ZI-YULCUO
2 '"___-.,;1~90:__ ___:2:JqO k.
DEYELOPMOT (1988)
... 0 SPACE HE AT I"G
~. UlE :1:. P:LOA;~EEs; : uS[ S
* *
El. POVER pun
-
A 6- GREUHOUSES
• 0 UOUSTRY
C AOUACULTURE
. SPA. RESORTS
0 Iu.plnd,. or
Iblndon,.
(C.s .... ,.g •• G.Yo,dle.1)
GEOTHERMAL EM~IROMME'TS
and .qui,.,. t ..,. "ithh l k. dll,
I
ACTI¥[ MAGNATIC PROYUCES (15-'o50'C)
RECUT COIlUEMUL RIFT (15-150'C)
:i::" SEDIRUTARY IUI~ (l5-100'C)
OROGEUC IEL T. PLATFORM.
CRISTAll UE USSIF (O-IOO'C)

641
PROGRESS OFF THE MAIN ITALIAN GEOTHERMAL PROJECTS
--- PROGRESS OPERATUG :U PROGRESS: STARTUG 'UIIDER STUDY' SUSPENDED o~
PROJECT
, , ABUOOIIED
~
, , It-'DICONDOU.! '
/TRAVALE
AMIAlA I~U , , , ,
ELECTRICITY lARDERELlO,etc, , , lATERA -
: AlFIU
: CESUO
: IIOFETE -
--------- --------~---------;---------1---------~---------
D 1$ T RIC I ACOUI TERME - : HRRARA - GROSSETO/ : lIU TE A IRPORl: CESUO -
HEAliNG BAGNOAO"AGU-:nCENU- /ROSELLE- :POMARUCE :CESU

642
energy demand could be satisfied utilizing partly the waste fluids (37 b C).
There are some greenhouses in Galzignano and Valcalona and others (under ex
periment) in Abano.
Grosseto .and Roselle projects. These involve two integrated .projects
(with heat pumps fed with energy derived from natural gas) to heat, by hot
spring waters, about 6000'''equivalent-dwellings" (buildings, schools, .facto
ries, barracks) and lha greenhouses. 1000m 3 /h fluids are available !T-400C)
Pantani (Civitavecchia). This project is carried out by some private
persons to heat 52 ha greenhouses, by means 4 geothermal wells (1 for rein
jection) less than 600 m deep (T- 56"C).
other projects. In Bagno di Romagna, a district heating project is ~
perating sinoe 1987 using down-hole exchangers too.In AcquiTerme a district
heating project operating from 1988, utilize hot spring (T-700C). In San
Casciano de' Bagni is starting a project to space heating. The Grado project
forecasts the district heating by 32°C geothermal fluids and sporadic util~
zations (space heating and aquaculture) near the ~uths of' Tagliamento river
(Venetia, Friuli) are operating or. ,in progrt.ss (T'"'30-45 0 C, from 250-500 m) •
2.3 DEEP AQUIFERS EXPLOITATION (Po Valley, 100o-2500mdepth, 40-l00"C)
They concern clastic salty upper aquifers and carbonate fresh or sal
ty, lower aquifers. AGIP and ENEL plann~d_aome' pilot projects differe~
tiated owing the aquifer and fluid salinity I some of their utilize abandoned
hydrocarbon wells.
"Metanopoli" (Milan) project~It is operating from 1986 to the heating
of dwellings and offices by a "doublet" in clastic salty aquifers with me
thane. The not very great production (50 m 3 /h) of hot water is partly compe~
sated by means of 100m 3 /h of natural gas.
Linate Airport project (Milan). It is forecast to heat the . terminal,
the air-parking (defrost), and the runway (for fog dispersal) by means of 1-2
doublets and natural gas integration.
Ferrara/Casaglia project. It represents the biggest italian district
heating project,by a doublet with max. 400m 3 /h of salty hot water (lOaOC).
The reservoir is in carbonate rocks 1300 m deep about. It is under study the
utilization of the waste fluids (from 60 to 45°C) and the drilling of a se
conddoublet to increase the volume of dwellings to be heaten from 2,.6 to 3.4
millions m 3 • other utilizations are foreSeen too.
Vicenza and'Villaverla projects. TheY,are two projects exploiting a
carbonate aquifer, bearing hot fresh water, by "singlets". Vicenza project wj II
be operating in 1989 to district heating. Villaverla' project (starting) re
gards industrial use (laundry, etc.). The utilized fluids are16m3/h (59°C)
the availability is 100-160 m 3 /h.
Rodigo ,project is starting to heat a first module of greenh:luses, dri
ers and aquaculture ponds,using an abandoned hydrocarbon deep well in a qery
deep carbonate reservoir (4000m). The available mass-flow of freshwater is
:

643
other projects are under study (i.e. -geo-Torino-,etc.), in compet!
tion with natural gas heating.
2.4. EXPLOITATION OF NOT-DEEP,"COLD- AQUIFERS (by heat-pump, o-lOOOm depth,
7-35-C). Unlike other countries (France, SWeden, U.S.) the similar ge~
thermal uses, in Italy, are rare. Only in Po Valley a few factories utilize
own water wells. Some projects are in progress in Reggio Emilia and in
near Guastalla, exploiting resources from fresh water wells (60m deep, 30
m 3 /h, l6-C) or heat from fresh water aqueduct network.
3. LEGAL ASPECTS
After nine elaboration years, on 1986, Dec. 25 st , the law N.896 about
-The Discipline of the Research and exploitation of the geothermal resour
ces- came into force. It fills a gap because it finally arrive to regulate
an activity till now controlled by old and inadequate mining laws.
The new law distinguishes the geothermal resources in nationals (over
20MWt, administrated by the State (or by the -Special Statute Regions-' and
~ (below 201'lWt, administrated by the State or by the Spec.5tat.Regions
for exploration and by the Regions for exploitation). Finally there are Ii!
tIe utilizations (below 3 MWt, with wells not more than 400 m administrated
by Regions. The law forecasts moreover, some financial incentives I 20-30'
of the well cost, for productive wells, and 75-80' for dry wells. To this
aim 35 milliards lire have been set apart for the 1985-89 period. By the
-Financial Law, 1987-, they have been increased with other 30 milliards.The
italian State, besides, prepare (by means of ENEL, ENI, CNR, ENEA)an assessment
of the geothermal resources in Italy. We remember also the -law 3OS/82-which
foresees financial facilities for plants.
4. GEO'niERMAL ACTIVIY DEVELOPMENT FROM 1975 TILL NOW.
Up to 1975 the direct uses of the geothermal energy in Italy consisted
of spa and resorts in Abano, Ischia and some other hot spring area. Besides
in Larderello the geothermal heat was exploited for the space and greenhouse
heating and for industrial process.
The recent energy crises encouraged the developnent of the geothermal r!
searchs mostly by AGIP and ENEL. These companies carried out three pilot pr~
jectsl -Metanopoli-, Ferrara- and -Vicenza-. The high operative costs Lel!p!1
cially drilling costs), the difficulty to fire! users and the increas:tng
use of the natural gas nevertheless produce a slowing down of the geothe!
mal act! vi ty.
Now the attention of the pperators is addressing to the not very deep
aquifer exploitation (by the heat-pumps) in thermal spring systems, not ve
ry hot but with minor investments and mining risks (i.e. Grosseto, Bagno di
Romaqna, Acqui Tarme, Grado, S. Casciano dr/ Bagni, etc.). To this purpose the
EEC/GETAS is preparing an assessment of the not deep (o-lOOOm) aquifers
in Po Valley and Venetia-Friuli Plain.

Italy (1988-1990)
MAIN SPACE HEATING AND DOMESTIC WATERS
GEOTHERMAL PROJECTS
PROJECTS
STEAN WAST£.I(9D-150 0 C)
- LARDERELLO
- CASTELNUOYO Y.C.
- PIAl CASlAGUIO
DEEP WATERS (~0-90'C)
- NET ANOPDLI
-FERRARA
- YICENlA
HOT SPRIIGS (25-100'C)
- ABANO, EUGANEI
- ACOUI T.
- BAGNOlll ROMAGU
- GROSSE TO, ROSELLE
- othe .. (35 IpIS)
Ictive top r ... .. plaited h.lted net geoth net
mh! DEPTH
~ TENPERATURE YOLUNE ~ ENERGY
SPRINGS
N. • .3/h inlet-outlet. .3 NWt TOE/,r
~ Co I 1000 (p .. k) SAYING !...!..!...Ll
.everal 300-1000 160 95 359 6 7500 oper.tlng fro. 1950
Ily,r.1 300-1000 97 75 250" 5.~ ISH • 210 . (19SS), 25 lupplled b, EIEL 11950-19S5)
liver •• 90 ~O 100 2.1 (SOO) hut-eachange .. InltaUed
doub let 190,0 50· 62 30 3S5 5.6 490 on linl fro. 19S5. Salt .Ito.. froo clastici.
·In Idditlon: 100 .3/h of g ...
doublet 1300 400 100 60 2&00/3400' 14-(1S) 10000 on lin. In 19S9/90. Silt .Iter fro. clrbontlcs.
linglet ISOO 100 65 40 IHO 6.5 3000 on line In 19S9. Fresh •• ter fro. cirbonltlci.
200 250-1000 ~340 75 37 2000 102 50000 huting of 130 hotel-IPI (Itart: 1'900-1930)
2 0-200 30 70 25 175 1.5 490 on line fro. 19S6. In pr09re ..
4 0-150 gO 25/42 IS 164 1.5 170
3 0-140 42 10/20 1376 IS 2370 In progre .. (scheduled:1990/92)
lev,rl1 0-1000 ISO S 1000
"'OLD"
VATERS (l0-25'C)
- REGGIO E., GUAS.lALLA
tot .1
lev,r.l 17- 70 15 10 150 (2) (500)
--- -- ---
250 9079 7B500 (C,So'.arugl, G.Yerdianl dec.19SJ!)
NAU PLANNED PROJECTS: MILANO LUATE AIRPORT (55°C, 1500. dopth, 1-2 doubloh, 5~10 MWt ,lOth.,lpICO hOltln" dHroltlng, fog-disp ..
III); GRADO (JO'C, 200. depth, '111 flo. 290 .3/h, IpIC. huting (560,000. 3 ), 2.1 MV gloth.,1250 TOEfy); S.CASCIANO DrBA:
~. (3S 0C, hot Ipring, ... 1 flo. 75 .3/h, IP"CI blltlag (170.000 .1), 1.5 MV glOth., ~60 TOEfy), etc.
NAil SUSPENDED OR ASANDONED PROJECTS: CESARO. PADOYA,

Ital.y (1988-1990)
MAIN GEOTHERMAL'GREENHOUSES
..
~
-- .. .,.
"OJECI, :=
""r•• I, 'hU !.ill.: .ill.!. .ill!ill! UU" : ... .. " ... a
.. :;
(I'to) 1'" ~ .. °C b IWt
... -
~o a
~ .... - ~ HOW UT[ U .. a a La
. ... , I I E
.... .. .. . - .. ~ ":
... ....
• 1000
.... ...
a ..
.
: .. -
0
:! - -: Iflf./,r
-... •
,
I. t" : " ..: ..:
•• !
- -
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(11 (l l_t .. I·I.II (4)
... t., ,,,throal (,roJoet) Clword ,,,th... 1 SAnD a
:; I [ I
• • I 5
'I-.COII I.
• I 0 - • 7D 0 0 0 0 - SI l88l;' ....... 0.05 b. 0.08 In
( •• l .... ' e.s .... r •••• .,., • ., ..
l. -100150 , I 3117 40 '(10) • so 31 0 I 0 .1 0.3 :M 15 O. dr, .11-•• 11 d.,t.4 -(·).... lhU.
"-.. liD I. fIf. - 300 I 35 O.Ol - 10.5 SR
UL 1I 511110
• l 300 7. 12 3D l.' 1.3 JO 1I
•
TALCALOII 00 I O-JOO , 7D 0 0.1-1 1.5 0.3 10 -
,. - AlI.U(,.ClIlI, •• h) 00 - 700-IODO lDOD 5 81 41 'U 3'" 5l 'lO 30 31 5/100 D" i'I.h •• td,,·lIr,.t 4l b( nil !)
CASI[UUOYO T.C. 0 700-1000 31 S 115 IS
0.1' Z.5 Z.l • 4.' 0.1 (15) 1 IlIrlid I. "st-Il
'5 75
LA'" 0 - 700-1000 l.5 S li5 100 O.DI) (0.01 ".011 - ,
PO ..... C[ (......) 0 I (lDOO) 35 S 104 40 0.5
•
~., 0.3 M. IlIlI. SOlCOl
IADICOIDOlI 00 lISO lSO S .5 IS 7 7
, 7 35 (70) - ,r".~••,,", ht h41
II - ca ... O DE I 0 35
• •
3. U 0.17 0.17 0.5 0.1 15
, ., ... 0
4" 4SO .00 , 41-55 ZI 5l 5l l4 lO Jl 5DO - • I r'IoJ •• tI ..... 1I
So - 5110.1. 0 I 0
• st-II ! 0.5 ! .(1.5 O.l (10) S
total IJ \05 14 .n 10
(1)- 'I. 'Iod... t; l .. L •• hrd,; ".'.utl••; ,..10....,; L .. Elth.; S.-Slrdl.I.; [I.£oil, I.; CI.CII ... II.
IlDO (.... d.d ~al ...)
(l)-I.~htorl.. h I-llIrtl." o.d ... Io.Io,; o. ..... tI.g; [...... lud.l·
(l)- .... t .. ia.; ,.,••• 10,; I. ' .. th ...... lIt ..; 5-lt ... r1l14.al or CI.dlOlli (hl,h 10th"., 'hldl)
(4)_ ........ It ..., o.drlo", M.I, ... hltl.,; 1-1.11
"OPOS[O "OJ[CI5 1('11 .COUI 1.(1 .. II ,ro .... d.70'C. I.UII; (£1) .. '.001101.'11 (lS·-37°C. D.HI); (h) "OSS[IO (40·C. I hi. Ioll,rlte4 I), (h)IOIIUOIOloo
(l'04·C. Ill ... 0.7 hI; (I.) SU .. I .. (U'C. H.1t (C.).ISCI" (40·-loo·C.);(U) FE ..... ( ... tll ('ro. I). SO'C lolot. JO'C •• tlot): .tc.

646
5. PREVISIONS·AND SUGGESTIONS.
An annual increase of 5 , upto 2000 should lead the italian geothermal
capacity (for non-electric uses) from the present 225 MWt to 700 MWt. The 5 , V!
lui! is superior to italian historical average (2-3.5 ') and intermediate to
the forecasts of 3-7' (Cataldi, Sommarug&, 1985). Considering the plentiful
availability of the resource, this value does not represen.t an impossible aim.
It is necessary however that the policy must be finalized to a promotional
and technical information action, by means liI.f prospective areas inventory and
pecu.liarities of expected fluids. This in order to study thei r best utiliza
tion according to local social -and economical conditions.
There is, besides, the financial aspect representing always a ·big re
straint for similar projects. In fact, even if they give after long time B~
me benefits, they require immediately some fairly high investments. In 1989
the EEe geothermal programme of financial contributes willbeover.Thence it
is essential that the State contributions~ foreseen both in geothermal law
and in other laws are prompts and a·dequates. In order to support the oper!
tions starts, it is very important the contribute must be given just after
the project approval and not after the end of the. work.
Italy (1975-2000.>
DIRECT USES OF GEOTHERMAL ENERGY
PEAl GEOTHERMAL CAPACITY (MWt) 0
YEAR
TOTAL
UERGY
with hlln .. l. SPACE AGRICULT. lRDUSTRY BALIEOLOGY SAYII&
In.l. lid. HEATING AOUACULT •. SERVICES TOURISM 10DD TOE/yr
10TE
1175 510 124 10 7 27 3)6 117 ~htorl.al
IUO 517 141 107 7 27 376 110 •
IU5 571 U5 III 7 27 378 210 •
lUI 830 254 131 57 30 378 231 •
---- --------- ------------------------------ --....a. ______ -------
un 715 325 170 120 35 390 263 problblt
• 765 350 185 135 45 395 281 po .. lblt
2000 1130 700 350 185 100 430 415 po .. lhlt
0) referred to t .. ~.rlture 0' 15'C for bal •• ology I.d outlet t .. perlturl
i. III other u ....
HIlt
~500
,..
·tooo
HIlt
ULHOLOGY
°1~1~75------------1~98-0-----------1-9~85-------19~8-8--------1-99~2-------------------2-00~OO
HISTORICAl I (PROBABll) I
POSSIBLE
POSSIBLE

647
6. SELECTED REFERENCES
A.I.R.U. (1987).11 riscaldamento urbano in Italia.
~IP
(1988). Riscaldamento Urbano Geotermico a Metanopoli (Milano).
Amm. Provo Pisa (1986). Atti della Giornata di Studio sulla Geotermia (Pisa,
16 mar.).
cataldi R. and C. Sommaruga (1986). Background, present state and future pr~
spects ot geothermal development. Geothermics, Vol. 15,359-383.
C.E.C. (1988). Atlas of Geothermal Resources in the European Conaaunity.
C.E.C. (1988). Conaaunity Demonstration Progranne in the sector of Geoth. En.
Com. Acqui T. (1984). Proqetto di teleriscaldamento da ricupero geotermico ad
Acqui Terme.
Com. Ferrara (1984). Geotermia, proqrannazione energetica del territorio. Atti
del Convegno Internazionale (Ferrara, 11-13 apr.).
C.N.R.-P.F.E. (1979-1987). Atti dei seminari informativi (Unita op.Geoterm~.
C.N.R.-P.F.E. (1982). Manifestazioni idrotermali italiane.
!".A.O. (1985). Application of Geothermal Energy and Industrial thermal efflu
ents in Agricolture. Report of 1985, CNRE Bull., No 6.
Federelettrica (1986). Conv. Naz. -Ruolo degli Enti Locali nella sfruttamento
delle fonti energetiche rinnovabili-.(Padova, 10 apr.).
Ferrara G.C., Luccioli F., Palmerini G.C., Scappini U. (1985). Update report on
geothermal developDent in Italy. GRC, Intern. Symposium, Int. Vol., 95-105.
Leoni P. (1987) • 11 teleriscaldamento geotermico della citta di Vicenza.
ziario Ordine Ingegneri di Verona.
LeSIllO R. and C.Sommaruga (1988). Energia geotermica a bassa entalpia.Geoloq:a
Tecnica, 1/88.
Piano Energetico Nadonale (P.E.N.), (1981,1985,1988/9)
SOmmaruga C. (1981). L'utilizzo dei tluidi a bassa entalpia quali fonti ener
getiche integrative. Energia e Materie Prime, No 3-4.
SOmma ruga C. (1984). Le ricerche geotermiche nell' Isola di Ischia. Energia e
Materie Prime, No 33-34 •
SOmmaruga C.(1985). 11 riscaldamento con fluidi geotermici. Industria Infor
mazione, easale M.
SOnaaaruga C. (1985).Sistemi geotermici antinebbia e antigeloper gli aero
portio Le Scienze, No 208.
SOmmaruga C. (1985). 11 proqetto geotermico di ~rosseto. I.e Scienze No 201.
SOrdelli C., Corbellani P., Facchini U., SOnaaaruga C •• Bazzanella G. L'impiego t
gricolo dell'energia geotermica. Genio Rurale, 1./6, 25-32.

648
PROGRESS ON GEOTHERMAL RESEARCH & SYSTEM
IMPLEMENTATION IN THE UK
w.s. Atkins & Partners
Woodcote Grove
Ashley Road
Epson
SUrrey KT18 SBW
United Kingdom Geothermal Energy Re~earch commenced in the mid-
1970's and has included extensive work to identify, test and
exploit aquifer resources and to determine and test Hot Dry Rock
systems.
1- GEOTHERMAL AOUIFERS
Progress on aquifer resource studies was reported in ETSU for
the Department of Energy in 1986 (Ref.1).
2 - RESOURCE STUDIES
Basin studies. The British Geological Survey identified the
major sedimentary basins as the area~ of geothermal
potential, and carried out a series of studies on them,
making a preliminary appraisal in 1980 and a more detailed
as~essment between 1980 and 1986. Their work has included
desk studies collating existing data, geophysical surveys in
promising areas, heat flow measurements and the acquisition
of data of geothermal value from commercial boreholes drilled
for other purposes.
Deep drilling. The basil:l studies provided a broad
assessment of the potential of specific areas. However, the
depth and thickness of formations of intere~t·could only be
estimated and the key factor - the permeability of the rock -
was largely unknown. Deep exploratory drilling to establish
these factors has therefore been necessary in the regions
showing promise. Three exploratory boreholes were drilled
during the programme - in the We~sex Ba~in at Harchwood, in
the Northern Ireland Basin at Larne and in the East Yorkshire
and Lincolnshire Basin at Cleethorpes.
The Harchwood borehole appeared to be successful on pump
testing, with a good flow at 72°C, giving a potential of
about 4 HW of heat. Subsequent further testing of the
reservoir after the Southampton borehole was drilled,
however, indicated that the reservoir was probably bounded by
faults which would place severe limits on the extent to which
it could be exploited.

649
The Larne borehole was disappointing. Two formations were
tested but the one that was expected to provide the major
resource was shallower than expected (and therefore cooler .
about 40°C) and the deeper and warmer formation (at about
65°C) proved virtually impermeable and therefore
unexploi table. This experience illustre.ted the risks in
drilling even e.t moderate distance (35 km) from a previous
borehole which he.d e.ppee.red to be very productive.
There were also two target formations at the Cleethorpes
borehole. The prime.ry te.rget we.s thinner the.n expected e.nd
therefore unable to provide a resource in the
Grimsby/Cleethorpes urbe.n e.ree.. The secondary. she.llower.
target proved to be e. very good e.quif er. but wi th e.
temperature of about 50°C.
In addition to the deep boreholes drilled e.s pe.rt of the
programme. boreholes drilled for other purposes (for example
hydroce.rbon explore.tion) were. with the consent of the
companies involved. tested for their geothermal potential. In
7 such boreholes the permeabilities of the Permo-Triassic
sandstones were good and the temperatures of the we.ter
bearing formations were in the range 40-85°C. They were
however in rural areas.
Size of resource. The detailed basin studies. supported by
the information gained from the exploratory deep boreholes,
have greatly increased the knowledge of the deep geology and
hydrogeology of the UK. and the data have enabled the
geotherme.l resource to be quantified. The approach used has
followed standard practice established by the European
Commission. The starting point was the total amount of heat
stored in the rocks and the fluids in them down to a
practical drilling depth. Constraints were then imposed to
take account of future technological and economic conditions.
and the estimate was further reduced by a factor to take
account of the most likely assumptions rege.rding the method
of exploitation. The identified resources calculated in this
manner, for three temperature ranges, were :
OVer 60°C
40 600C
20 . 400C
183 Htce
1771 Htce
2285 Htce
Al though these are large. there is one last factor to be
taken into e.ccount: the coincidence of high heat load density
and resource. This is very poor, with the higher temperature
resource restricted to three areas only - around Bournemouth.
Crewe and Antrim - and the 40 - 60°C resource. in adj e.cent
areas and also on the north-east coastal belt between
Scarborough and Skegness. around Cirencester and in Northern
Ireland. The exploItable resource is therefore much smaller
than the above figures.
Because the towns are relatively small. e. total of not more
than e.bout 100 geothermal schemes is likely in these areas.
One typical geothermal scheme is capable of saving about
3.500 tonnes of coal equivalent p.a. and on this basis the
100 schemes would save some 0.35 Htce p.a. This is a similar
figure to that estime.ted by ETSU in 1985 of a national
contribution of 0.25 Htce p.e.. by the year 2025. but is
considerably less than the 4 Htce p.a. envisaged in 1976 when
less was known about the extent of the resource e.nd its

650
coincidence with centres of high heat load. It would only be
increased significantly if new loads were to be sited in the
areas on the basis of the distribution of the resource.
3- HOT DRY ROCK
Progress on the Hot Dry Rock Programme is set out in ETSU
Project Profile 058 (Ref.2J
The Department of Energy's Geothermal HDR Programme has been
under way since 1976. The centrepiece of the programme is the
project being carried out by the Cambourne School of Mines at
the Rosemanowes Quarry in Cornwall. The site was chosen for
two main reasons. First, it is on the exposed surface of the
Carnmenellis granite batholith, where heat flow in the rocks
is known to be twice the UK average. Secondly the rock is
known to contain a natural jointing system that permits the
flow of fluids and is amenable to enhancement techniques that
increase flow levels to those required for economic heat
extraction.
The project has been planned in three phases :
Phase 1 shallow depth experiments to assess the
feasibility of enhancing the permeability of rock
Phase 2 - intermediate studies to determine the feasibility
if creating a viable HDR reservoir
Phase 3 the establishment of a deep (6000 metre)
commercial prototype system.
The Cambourne project has involved a wide range of scientific
and engineering activities, including a number of novel ideas
and techniques. Activities have included drilling, logging
and microseismic techniques ; reservoir mechanics
geochemistry crosshole seismics and vertical seismic
profiling ; instrumentation, computing and modelling ;
resource evaluation and heat flow studies for the Cornubian
Granites.
The programme over the next 3 to 4 years will concentrate on
further experimental work using the existing reservoir at
Rosemanowes and will aim to reduce the uncertainties
associated with commercial depth systems. At a depth of 6 to
7 kilometres the water temperature echievable is around
250°C. The most attractive option for exploiting this
technology is the generation of electricity. Current
assessments put the estimated generating cost from a
commercial deep system in the range 3 - 6 p/kWh.
The cost of the project is £27 million - 85% funding from the
Department of Energy and 15% funding from the Commission of
the European Communities. Completion of Phase 2 is expected
September 1988. .
The Department of Energy are currently seeking private sector
partners for Phase 3 of the programme.

651
4- IMPLEMENTATION OF THE FIRST GEOTHERMAL PROJECT IN
SOUTHAMPTON
The Southampton project was the result of an initiative
between the Southampton City Council and the Department of
Energy to demonstrate the use of Geothermal Energy following
the identification of the resource by the Harchwood tests. A
company was subsequently formed (The Southampton Geothermal
Heating Company) by two French companies IDEX and STREC with
particular experience in Geothermal Aquifers and District
Heating and a joint co-operation agreement made between the
Company and Southampton City Council.
The drilling work was financed by the Department of Energy
with support from the EEC and was completed in November 1981.
5- EXPLOITATION OF THE CLEETBORPES GEOTHERMAL RESOURCE
The Cleethorpes Well which was drilled as part of the
Department of Energy's Resource Testing Programme is situated
in an area designated for horticultural development
investigation is currently being made into the possible
exploitation of the Sherwood Sandstone Aquifer for greenhouse
heating. The Sherwood Sandstone is at a depth of 1300-1500 m
and was cased over during the development of the well to the
lower Basel Permian Sands. Windows will need to cut into the
casing at about 1450m and the area cleaned of cement by water
jetting. The Sherwood ie expected to produce brine at 50 0 C
having a salinity of 80 gIl. The commercial production rate
would be in the order 30 - 50 lIs.
If the resource can be successfully demonstrated the
Cleethorpes Borough Council will be interested in further
opportunities for exploitation of the considerable energy
potential of Geothermal Energy from the Lincolnshire Basin
Ref.l
Ref.2
Geothermal Aquifer Department of Energy R
Programme 1976-1986. Published by ETSU for
Department of Energy in 1986.
D
&:
the
Project profile 058 Geothermal EnergylHot Dry Rocks
Cambourne Proj ect. Published by the Department of
Enbergy in 1988.
Ackno"ledgement
Acknowledgement if given to ETSU and the Department of Energy
for permission to reproduce extracts for the two documents
Raf.1 &: 2.

652
MILOS DEMONSTRATION PROJECT
E.E. DELLIOU
Mechanical Engineer
Public Power Corporation
Direction of Alternative Energy Forms / Geothermal Division
10, Navarinou Street, 106.80 Athens, Greece
~ummary
On.Milos island a high enthalpy, water dominated geotherrMil
field of high salinity exists. At 1985, a 2MW geothermoelectric,condensing
type, pilot power plant was installed
on the island. This plant has been provided by Mitsubishi
Heavy Industries under a contract with Public Power Corporation
(PPC). During long term'operation trials of the
unit (turbogenerator, steam gathering and brine transmission
systems) unforseen problems arised in both steam and
brine cycles due to remarkable scaling phenomena. The
sequence of events in identifying the scaling problems, the
technical approach applied to remedy them and data,conc~
ing the plant I s operation are reported:ln this presentation.
1. INTRODUCTION
Geothermal exploration began in Greece in 1970. The surface
geothermal exploration was conducted by PPC in collaboration
with the Greek Institute for Geology and Mineral Exploration
(IGME) and with the assistance of OECD experts. Milos island
has been a site of major drilling activity due to its particular
geothermal interest. Milos is a volcanic island of the Cyclades
Archipelago lying at the southwestern edge 'of the Aegean
Volcanic Arc.
Following previous surface exploration five deep wells, all productive,
partially penetrating the existing high enthalpy geothermal
reservoir, have been drilled on the island. The drilling
of the first exploratory well, MZ-1, started in Milos, in 1975,
in Zephyria area. Another drilling took place, in 1976, near
Adamas, for a second exploratory well MA-1 (FIGURE 1).
Since these two exploratory wells, were proved productive, PPC
decided, in 1979, the drilling of three production wells. So in

653
1980-1982 the production wells M-1, M-2 and M-3 were drilled in
Zephyria area (FIGURE 1). The DG-XVII of the Commission contributed
to the drilling expenses with an amount of 87.600.000Dra
, I , , • ' ..
FIGURE t. MILOS ISLAND - GEOTHERMAL WELLS LOCATION.
Following the completion of these wells, short term deliverability
tests and measurements,proved the capability of the wells to produce
a two-phase high enthalpy and salinity fluid (TDS up to 120.000 ppm).
Later, on 1985 the first geothermoelectric pilot power plant
was installed, using M-2 as production well and M-1 as reinjection
well.
2. PRODUCTION WELL M-2 CHARACTERISTICS
Short term deliverability tests gave few preliminary data concerning
the production capacity and the fluid charactericts
of the well. In order to use M-2 well, as production well
for the geothermoelectric pilot plant, more reliable information
was necessary. So systematic well testing and brine studies
were performed by the Contractor Mi tsubishi. During the discharge
testing, a number of brine samples were collected and analysed.
Most representative results of chemical analyses on samples
taken at 8 bars separating pressure, are given in TABLE I.
TABLE I: ANALYSIS OF HOT WATER DISCHARGED FROM WELL M-2.
Conltltuent Na K Ca 101\1 S04 CI 8 F As LI Sb NH~ 51°2
Conc.ntration
(ppm)
28750 7780 3630 6 60 71610 65 2.6 5 63 87 53 1150

654
The Milos deep geothermal reservoir includes a resource eonsisting
of concentrated and modified sea water with high silica
content. Major constituents are sodium, potasium and calcium
chlorides, and a significant concentration of silica.
3. MILOS 2MW PILOT GEOTHERMOELECTRIC POWER PLANT.
PPC and Mitsubishi Corporation signed in 1984 a contract for
the provision and installation of a 2MW geothermal power plant
of condensing type, which would meet the electricity demand of
Milos island. HORS QUOTA prograrrrne finances this project with 50}6 of the
total cost. The initial design of the plant was based on preliminary
data about fluid's chemistry, available at-that time.(As
proved later, after the above mentLoned systematic tests the
actual silica concentration was found 40% higher than the concentration
initialy measured during the preliminary tests).
r
1 u .."... I
I IILIU'CI \
Ir ----------~
I
---==-- ----u.:...::'i',.. vagi r€:}, I
'dl CDNI'IIIOL
--, po.o r-~~ I-J
JJ"o2~ .~~
Y .....
'-------

6SS
In order to solve the scaling problems, the Contractor, redesigned
some parts of the installation and made the following
modifications (FIGURE 3); he:
2 2
- Increased the separation pressure from 8kg/cm abs to 25kg/cm
abs. So a new cyclone separator, with nominal separation
pressure 25kg/cm2 abs was installed before the initial cyclone
separator of 8kg/cm2 abs.With the above modification the
steam-brine separation takes place in non-saturation conditions.
- Reduced the main steam pressure and temperature after the
25kg/cm 2 abs separator (H.P.Separator) from 25kg/cm2 abs and
224°C to 8 kg/cm 2 abs and 170°C (1.e. to the steam inlet conditions
for the installed turbine). The reduction of the
temperature is carried out by injecting water inside the
steam pipe-line between the two cyclone separators and is
performed for combined effects of cooling and scrubbing (irT:qJroving
of steam puri ty) •
--.... " .... u.
~,..o· ...... n.ow
----........... LI ••
--IfU. 10..111
FIGURE 3. MILOS. 2 IoIW GEOTHERMAL POWER PLANT (modi fi~d).
After the completion of the above modifications at the site
the power plant went on-line in December 1986.
4. STEAM CYCLE - SCALING PHENOMENA
From the very begining of the plant operation (semi-commercial
status) severe scaling in turbine was observed (overhaul period
for the turbine less than one month). At first the turbine sca-

656
ling was attributed more to accidental "carry-over" than to
the steam purity.~, t~Contractor applied some improvements,
such as:
- Wider -pitch nozzles at the turbine
- Counter - measures for prevention of brine over-flow, due
to (probable) fluid level instability in the separator.
After the completion of the above mentioned improvements and
the turbine cleaning by sand-blasting, the plant went on-line
again,in October 1987.
The main concern this time was to evaluate the progress of the
turbine scaling by observing the increase of the steam chest
pressure for some loads after a certain continuous operation
period. The data collected during a continuous operation of
the turbine are illustrated in FIGURE 4 and 5.
j'
i'
f·
II Afl.r 21 da,.
• AI t.r 10 days
. . .
lUIII_MI" OPl!lU.tlOII lINt I DAYSI
FIGURE 4. STEAM CHEST PRE~E
INCREASE WITH TIME.
Q£ D.I U ..
our~ur LOAD (NW.'
FIGURE S. STEAM PRESSURE INCREASE
IN THE CHEST CF THE TURBINE.
2D
FIGURE 4 , shows the steam chest pressure increase versus time.
FIGURE 5, illustrates the required steam pressure, in order to
obtain the desired output (up to 2MW) after a certain time of
turbine continuous operation. Moreover, in the same period,
high level vibrations were observed at turbine bearings.
Taking into consideration that the design pressure limit for
the turbine operation is 10,5kg/cm2 abs and since the overhaul
period for the turbine was proved to be not longer than 2 months,
the Contractor, applied some new modifications (FIRGURE 6):
- Installed a wire-mesh inside the low pressure separator, in
order to "catch" mainly the iron ions.
- Installed two spray-water nozzles inside the moisture sepa-

657
rator , in order to apply "water-washing" to the turbine.
t;J
r---- ----,
I
I
I
: .--0--------- J
I I
\-::.~~r
t.*"~
[~--===!::>r
----------~
1 COiftO\ I
.....,..- -----,;i:~... '''ge r-er, I
.......... r--~~ ... J
~
......
L~
,.... .NlJlmo•
L ____ g;: _..,
I I
I
,,-------1
\ .:7"_. If .7:!,,~.
• ..·1
__ ...... 'ea .. \I ..
==-=-=:. "0 .,.. ... ~ ..
----•• ra ,"'.' 1.1.'
--pc.- "' ...
FIGURE 6. MILOS. 2 MW GEOTHERMAL POWER PLANT ( •• Isting).
FIGURE 6 illustrates the actual installation of Milos 2MW geothermal
power plant. The steam quality obtained after the last
modifications is shown in TABLE II.
TABLE II: CHEMICAL ANALYSIS OF STEAM BEFORE TURBINE
Constltu.nt 510 2
Cl- F."" H2S COl" H2 CH,
Conc.ntratlon 0.1 0.9 0.03 500 10.000 TracH Trac.s
(ppm)
In Milos power plant, steam condensate was sprayed inside the
moisture separator, last February (1988),through the installed
nozzles. During this procedure the steam chest pressure dropped
gradually in a short period (about 6-7 days) to the nominal
level (FIGURE 7).
--
I "..."
I . ...
l.N8 ,....
! 1 ••_' c.o~ ....,........ I -
........... _ L&ll ....-c.t.......
\M'I
~
I :----
!. I
I
~ • 1-'1 .. . i
I.,..., ..... • • • 'I'
•• , II~''''''''' "".
1-
FIGURE 7. ·"TER-WASHING. EFFECT ON TURBINE'S STEAM
CHEST PRESSURE.
I

658
It is worth to mention that no vibrations or other problems
were observed during the "water-washing" period.
The "water-washing" method for turbine cleaning on-line, is
quite new, since it has been applied, at first, some years ago
in geothermal power stations at the GEYSERS area (USA). So,the
experience allover the world on this technique is very lmdted
up-to-date.
Under these circumstances, the adoption of "water-washing" method
at the Milos power plant is still under consideration.
However, its re-evaluation and optimization has to be further
investigated, for its final adoption (in case the conventional
steam scrubbing methods will be proved not functional).
5. REINJECTION SYSTEM.
In Milos geothermal power plant, the high temperature direct
reinjection system is used.
From the very begining, the high silica content in the brine,
created major scaling problems and consequent difficulties in
disposing such a fluid. A pond test carried out on August 1985,
with fluid separation at8bars, proved that heavy scale was deposited
in the piping and surface equipment. After that, a high
pressure (25bars)separation pond test was conducted. Its
target was the assessment of the suitability of the fluid separation
and brine transmission under high pressure, as appropriate
operation process. As a guide to evaluate the suitability
for reinjection of the produced brine, a number of criteria
were established (TABLE III).
.
TABLE III· BRINE QUALITY CRITERIA PRIOR TO REINJECTION
IT E loot CRITERIA ACTUAL TEST RESULTS
Suspendl!d Solids 5ppm 1 ppm (with dlrl!ct filtr~tion )
SIlica supl!r - satur~tlon a 900 -1000 ppm (~t 205 -22SoC whl!rl!
cornpondlng thl!oretlc~1
Suling rUe 1 nvn/yr 0.&-1.2 mm Iyr
Dissolved Oxygl!n 10 ppb

659
Although the brine quality data measured before the well M-1
met the criteria set, the most critical pieces of the surface
installation (reinjection pumps, control valve, orifice meter
etc.) and the first 100 meters of the brine transmission line
were found suffering from sev~scaling. FIGURE 8 illustrates
the scaling rate in the reinjection line.
..
~O
!
..
• 0
~
.
~
!!
~
.. JO
..
! .
..
..
~
zo f
~
oX 10
~
0
100 ISQ zoo 2'0
RII~Jlctl •• Z58AA SIP'~H I •• PI.,. ll.gth (.)
Pu"", Out lIt Sco II
FIGURE 8. SCALING RATE IN REINJECTION PIPE.
On July 1988 the following modifications at the reinjection piping
were carried out:
- Flanges were added at the piping between the reinjection pump
outlet and the H.P. hot water collecting tank level control
valve, for easy cleaning.
- The first 60 meters of the reinjection brine line were replaced
by a new pipe of 8" diameter (instead of 5").
From the reinjection experience gained so far, it might be concluded
that the well performance has not been affected by deposi
tion of solids in the reinjection well M-1 or in the surrounding
reservoir formations.
6. CONCLUSIONS
Scaling control in separated high salinity brine imposes high
pressure separation with subsequent energy losses.
Milos geothermal power plant meets the
of the island, so a change in demand
change in the load of the plant.
whole electricity demand
results to the same
Although higher purity steam was produced in Milos power plant,
severe turbine nozzle scaling remains still a great problem.
"Water-washing" of the turbine on-line might be a solution,
under the above kind of operation, in case severe scaling

effects are otherwise unavoidable.
Direct reinjection of spent brine under high pressure and
temperature seems to be succesful in higher temperature formations
without indications of reservoir clogging.
REFERENCES
The above presentation has been based on internal PPC's technical
reports, and on reports submitted to PPC by the power plant
Contractor, Mitsubishi Ltd, of Japan (assisted by GEOSPAC Ltd)
and by the Consortium VIRKIR/NEA of Iceland acting as PPC's
consultant.

661
GEOTHERMAL ENERGY IN GREECE
POTENTIAL AND EXPLOITATION
by Dr. Sp. Kyritsis
Center for Renewable Energy Sources
Frati 6, Fousa, GR-19400 Koropi Atti.k:is, Greece
ABSTRACf
Greece has very favourable conditions for geothermal energy.
The south Aegean active volcanic arc is an area characterised by high temperature reservoirs.
The recent active extentional tectonics which operate mainly in Central and Eastern Greece,
have positive heat flow anomalies.
The results to date of geothermal exploration show or make probable the existence of
geothermal reservoirs of high, medium and low enthalpy in many areas of the country.
Concerning High and Mediwn Enthalpy, the known geothermal fields are those of Milos and
Nisyros in the south Aegean, and there are very promising fields in some other Aegean
islands and less promising ones in still others.
Concerning Low Enthalpy, the I.G.M.E. research programme has yielded many geothermal
fields distributed almost allover the country, especially in Northern Greece and the island of
Lesvos.
The currently verified potential yields, 36OOm 5 /h of water with temperatures between 40
and 9O"c. The probable potential of the fields is estimated to be 3 to 4 times more.
The current exploitation of the geothermal potential is very low, limited to the high enthalpy
fields in a small 2MWe pilot plan, and to some agricultural applications, especially
greenhouse heating for low enthalpy geotherma1 potential.
The expected applications in the near future relative to the past, are very PJ:Omising. For the
high enthalpy fields, the P.P.C. is preparing a 20 - 30 MWe installation, and for the low
enthalpy fields, the applications under way, mainly for space heating (houses and
greenhouses), are considerable.
To facilitate applications, a new government policy is under preparation, which will provide
new facilities and motivation for research and applications.

662
BILAN DE LA FILl ERE GEOT~~~~~Q~~M~~~~CE
ACTIONS DE D~~ONSTRA~~9~
Agence Fran~aise
Jean LEMALE
pour la Maitrise de l'Energie
L'industrie de la geothermie en France a grandi tres vite,
l'essentiel des operations a ete realise entre 1978 et 1986. Plus
de 200 000 equivalents logements sont actuellement chauffes par la
geothermie, dont 85 % dans l'agglomeration parisienne. Energie de
"type capitalistique, lao" geothermie subit actuellement de maniere
severe les retournements recents de la conjoncture economique
baisse de l'inflation et surtout la baisse des couts des energies
traditionnelles. L'aquifere le plus sollicite est la structure
calcaire du Dogger, l'eau qu'il contient est plus ou moins'
en sels et sulfures, lequels gene rent dans certaines zones des
problemes de corrosion et de dep6t prejudiciables [ la bonne
exploitation des operations. Un effort de recherche et
d'experimentation important a permis de realiser des progres
significatifs, notamment sur la comprehension des phenomenes
perturbateurs, les methodes de diagnostic de l'etat des puits, les
methodes de rehabilitation ainsi que de prevention. Un systeme de
telesuivi de la quasi totalite des operations, mis en oeuvre par
l'AFME constitue un outil fondamental pour les maitres d'ouvrage,
les organismes de recherche. Il permet d'avoir une image precise de
la filiere et de l'evolution de ses principaux parametres
caracteristiques. Outre l'aspect dejA tres positif de l'impact de
la geothermie sur l'environnement, de nouvelles conditions
favorables au developpement de cette filiere necessiteront
l'amelioration de sa competitivite, la diversification de ses
utilisations ainsi que l'integration de ses objectifs dans une
politique europeenne.
charge~'
1. BILAUOMMAIJ~!
Parmi les energies dites nouvelles, la geothermie est certainement
celle qui au cours des dix dernieres annees a connu le developpement le
plus remarquable. En effet, sur l'ensemble du territoire fran~ais on
compte A ce jour 66 operations en fonctionnement, correspondant A un
total d'un peu plus de 200 000 equivalents logements et 200 000 TEP
substituees, pour un investissement total d'environ 3,5 milliards de F.
Il faut cependant souligner que l'utilisation de la geothermie est
presque exclusivement reservee au chauffage; de plus, la repartition
des operations se trouve concentree sur seulement deux secteurs
geographiques (Fig. 1). Le'plus important le Bassin parisien avec 54
operations (doublets) en fonctionnement, et un second avec une dizaine
d'operations (puits unique) le bassin Aquitain surtout dans le region de

663
Bordeaux. Le tableau 1 r~sume l'ensemble des 86 op~rations r~alls~es en
France compte tenu a la f01S des r~gions et des taux de succ6s ou
d'echec. Les tentatives effectu~es en dehors des deux grands bassins
sediment aires se sont sold~es par des ~checs. La figure 2 montre
l'evo1ution de la g~othermie de 1974 a 1986. On re16ve une croissance
tres nette de 1978 • 1984 puis le ralentissement et l'arret en 1986.
2. SITUATION ACTUELLE
--i'arret-dll-developpement de la g~othermie trouve son
dans l'6volution du contexte dans les trois domaines
economique, financier, technique.
explication
suivants
2 .1. As.pl!ctL~cQn~miJllleLet __ fi!l~nsier!
La majorit~ des operations ont ~t~ mont~es sur de l'habitat
existant par des maitres d'ouvrages publics. Pour inciter les abonn~s a
se raccorder au reseau de chaleur, les contrats d'abonnement pr~voient
en gen~ral une clause de sauvegarde qui stipule que leur chauffage ne
leur coutera jamais plus cher avec la g~othermie qu'avec leur ancienne
~nergie. Avec la baisse des couts des ~nergies, les produits
d'exploitation ont chute considerablement, alors que les charges totales
restaient pratiquement identiques. En effet, la partie pr~pond~rante des
charges est constituee par les annuit~s d'emprunt. La majorite des
projets ont ete r~alises dans les ann~es 80-83, p~riode ou l'inflation
etait particulierement elevee en France. Les taux d'interets non indexes
sur le marche mon~taire semblaient supportables, vu les pr~visions
retenues concernant les taux d'inflation des vingt ann~es a venir.
Aujourd'hui le differentiel inflation-taux d'int~ret penalise lourdement
les ma1tres d'ouvrage. Les ren~gociations souvent annonc~es n'ont
malheureusement pas abouti pour la majorite des op~rations.
2.2. Aspects tech~~~~!
11 est necessaire de distinguer les difficult~s li~es aux
equipements de celles plus specifiques li~es au syst6me d'exploitation
et notamment des phenomenes de corrosion-d~p6ts.
2.3.1. LIUL~~i"p'!!ments de ~a b~c14L.9~othermale
Les equipements utilises derivent g~n~ralement de ceux utilises
dans l'industrie petroliere. Les puits p~troliers, pour des raisons de
gisement ont une duree de vie qui n'exc6de pas quatre' cinq ans. Les
conditions d'exploitation different ~galement, notamment par la
necessite d'utiliser pour la g~othermie des debits beau coup plus
importants. La duree de vie des pompes de production immerg~es, prevue
initialement. 'ans s'est en fait av~ree, rapidement beau coup plus
courte que prevue. Les incidents les plus fr~quemment rencontr~s ont eu
pour origine des phenomenes electriques. D'autres types de pompes
present ant la particularite de n'avoir pas de parties electriques
immergees ont ete utilises : les pompes a arbre long (une seule en
service), les turbopompes. Ces derni6res doivent co.penser leur moindre
rendement par un cout de maintenance moins eleve. La premiere turbopompe
a ete aise en service • Keaux dans le cadre d'une operation de
demonstration de la communaute europeenne: depuis 10 op~rations sont
equipees de ce nouveau syst6ae de production. On cons tate actuellement
une diminution des incidents sur l'ensemble du parc des pompes de
production que l'on peut attribuer, d'une part • un effort des

664
constructeurs pour adapter leurs materiels aux exigences de
l'exploitation des eaux du Dogger et d'autre part • une meilleure
organisation de la maintenance favorisee par la creation de societes
specialisees. Differents materiaux ont ete utilises en surface pour
vehiculer l'eau geothermale d'un puits a un autre, parmi lesquels on
peut citer: l'acier ordinaire, l'acier inoxydable (316L), la resine
epoxy, l'amiante ciment. Les canalisations en acier ordinaire sont
soumises aux phenomenes de corrosion • des vitesses plus ou moins
rapides fonction de la localisation sur la boucle (zones de turbulence)
et de la nature physico-chimique du fluide vehicule. Les canalisations
en acier inoxydables ne sont pas, dans certaines conditions. l'abri de
la corrosion (corrosion par piqures). Les canalisations en resine ont
l'avantage de ne pas etre affectees par les phenomenes de corrosion, par
contre des faiblesses mecaniques notamment au niveau des jonctions
collees ont eu pour consequence des ruptures de canalisation. En ce qui
concerne les aut res equipements, il faut signaler la bonne tenue des
echangeurs a plaque de titane.
2.2.2. ~~!!.J)_Mnom_~~_~ __ ~~.9_r_~Q_~_Q.~=-d~'p_Ql
Les phenomenes de corrosion-depot dependent a la fois des
caracteristiques physico-chimiques du fluide, de son mode
d'exploitation, de l'existence ou non de bacteries et des phenomenes
electriques induits par l'environnement des tubages. Les elements
determinants contenus dans le fluide sont la "salinite tot ale" et la
teneur en sulfures dissous. La salinite tot ale des eaux du Dogger du
Bassin parisien varie entre 8 g et 34 gIl, son influence sur la
corrosion est importante. La teneur en sulfures dissous varie dans
rapports beaucoup plus importants de 0,5 a 100 mg/l (fig. 3), la
concentration est nettement plus elevee dans la partie de 1 'aquifere
situee au Nord de Paris. Les problemes significatifs de depot sont
rencontres pour des teneurs superieures a 20 mg/l. L'essentiel du
mecanisme repose sur la formation de la mackinawite Fe (l+x)Sx
principalement, et de chlorures de fer hydrates Fe2 (OH)3 Cl. Ce
mecanisme consomme le fer du tubage, pour produire des depots de
produits de corrosion dont le volume est notablement superieur a celui
du fer des tubages ayant participe a la reaction; il y a donc
globalement diminution de la section a l'interieur des tubages. C'est
l'action des chlorures qui empeche la formation d'une couche de sulfure
de fer protectrice, la corrosion n'a donc aucune raison de s'arreter un
fois commencee. Les gaz dissous (H2S, Co2) qui peuvent etre liberes soit
au niveau de la pompe, soit dans les parties superieures lorsque
l'exploitation s'effectue au-dessous du point de bulle sont susceptibles
d'accelerer les phenomenes de corrosion (modification du pH). Des
analyses chimiques de depot, effectuees sur divers sites ont permis
mettre en evidence une activite bacterienne. Cette activite a pu etre
mise en evidence de maniere indirecte sur un site par 1 'augmentation
tres importante (de 0,2 a 10 ppm) des sulfures. Des souches
bacteriennes, peu connues, differentes des suflato-reductrices ont ete
decelees. Des circulations de courant ou des differences de potentiel
sur les faces externes ou internes peuvent etre a l'origine de corrosion
acceleree. En l'absence de protection cathodique des courants vagabonds
parfois import ants en milieu urbain peuvent attaquer de maniere
fulgurante la face ext erne des tubages (cas Maisons Alfort a proximite
de ligne de chemin de fer Paris-Lyon). Sur la face interne, la formation
des
de

665
des dep6ts n'etant pas homogene, la proximite de zones de metal nu avec
des zones ou des dep6ts de sulfures existent peut provoquer (l'ensemble
baignant dans le fluide geothermal) des effets de pile accelerateur de
corrosion.
Les percements de tubages par corrosion identifies sont encore peu
nombreux. Des mesures d'epaisseur residuelle realisees sur des forages
situes dans des zones critiques laissent craindre des perce.ents •
courte echeance si des mesures efficaces ne sont pas prises' temps
(rechemisage, injection d'inhibiteurs efficaces). Les phenomenes de
dep6t associes aux phenomenes de corrosion ont une incidence sur
l'exploitation des installations geothermiques. En effet, les sulfures
de fer dont la formation peut intervenir des le sabot du puits de
production, se deposent tout le long de la boucle geothermale. Ces
dep6ts generent des pertes de charge par diminution des diametres et
augmentation de la rugosite limitant les debits possibles aux differents
niveaux de la boucle au puits de production (augmentation du
rabattement), au niveau des filtres et de l'echangeur (colmatage), au
niveau du puits d'injection (augmentation de la pression de
reinjection). Pour remedier • ces phenomenes des methodes curatives et
des methodes preventives ont ete mises en oeuvre. Plusieurs methodes
curatives ont ete experimentees, ce sont en general des solutions
lourdes et couteuses. Elles ne pourront 6tre renouvelees qu'un nombre de
fois limite. Les methodes preventives actuellement utilisees consistent
en la mise en oeuvre de produits inhibiteurs dont les premiers resultats
sont encourageants, mais demandent a etre confirmes sur une duree
suUisante.
2.2.3. ~e reservoir
A 1 'occasion des operations de rehabilitation sur les puits
recouverts de depot il n'a pas ete note de degradation des
caracteristiques du reservoir. Par contre des cavages importants ont ete
constates aux niveaux injecteurs. Ce dernier phenomene qui n'a pas
actuellement de consequence sur l'exploitation, peut trouver son origine
par une redissolution du cal caire de la formation du fait du
refroidissement du fluide geothermal. Lorsqu'il y a formation de depot,
des particules solides non incrustees s'accumulent au fond du puits de
reinjection. Une accumulation import ante est susceptible de masquer les
niveaux producteurs (injecteurs) et aggraver les difficultes de
reinjection. Il n'a pas, • ce jour, ete constate d'interferences
thermiques entre puits d'un me me doublet ou entre doublets voisins. Des
interferences hydrauliques bien que limitees ont ete mises en evidence
entre certains doublets situes dans des zones • haute densite
d'ouvrages. Ces interferences peuvent 6tre favorables ou defavorables
selon les dispositions respectives des doublets. Une baisse du niveau
piezometrique au niveau du puits de production peut cdnduire • une
obligation de baisse de debit.
3. ACTIONU!. RE~.!lE!.CB_~ET 1?~ELOP.l~ENT ~~CESSITEES PAR LA
SITUATION DE LA GEOTBERKIE BASSE ENTBALPIE
Pour faire face - aux diU,rent.- prahle.es rencontrea au cours de
l'exploitation des operations de geothermie, une mobilisation des
partenaires s'est progressivement realisee. L'action qui en a decoule
s'est concretisee par un effort important d'organisation et
concertation, d'experimentation. la de recherche. Les actions

666
entreprises se situent ! differents niveaux suivi technique du
fonctionnement, diagnostic, rehabilitation, prevention, optimisation du
fonctionnement. Des groupes de travail reunissant maitres d'ouvrage et
professionnels de la geothermie ont permis sur les themes precites de
faire Ie point sur l'etat des techniques et des experimentations
realisees et ! poursuivre.
3 .1. ~uJ.~J_~e_~~Il_:l~~_eJ; __ dj,agno!l-.ti~
En contrepartie d'une autorisation diexploiter delivree par l'Etat,
Ie maitre d'ouvrage doit fournir periodiquement un certain nombre
d'informations! 1 'Administration. Le suivi des installations doit
permettre une gestion optimale des ouvrages en temps reel, mais
egalement ! long terme. Les principales fonctions faisant l'objet du
suivi sont :
la securite generale des installations (rabattement, pressions,
boucle,
epaisseur casing ••. )
les performances hydrauliques du reservoir et des puits
(pression de gisement)
• Ie suivi physico-chimique et bacteriologique du fluide
(corrosion)
Ie rendement de l'extraction thermique (~T'rKwh geo'fKwh
Hec)
Ie rendement des systemes de pompage ( hydraulique, electrique).
Parallelement et afin d'aider les partenaires des operations
(maitre d'ouvrage, administration, bureaux d'etudes, orqanismes de
recherche) dans la connaissance des phenomenes regissant Ie
fonctionnement de la boucle geothermale, l'AFME a mis en place un
dispositif de telesuivi. Ce systeme permet un suivi continu et
systematique des principaux parametres de fonctionnement de 50
operations au Dogger du Bassin parlSlen. Ce systeme original, premier
exemple de gestion.centralisee de reservoir geothermal est operationnel
depuis plus de 2- ans, il permet entre autre au maitre d'ouvrage
d'interroger son installation! distance de n'importe quel lieu equipe
d'un Minitel. L'AFME elabore des statistiques mensuelles par site et
globales, elles sont diffusees ! chaque maitre d'ouvrage qui peut ainsi
situer les performances de son installation par rapport aux autres. Le
traitement approfondi du volume considerable de donnees issues des
differents sites ont permis aux organismes de recherche d'ameliorer
leurs modeles de gestion du reservoir du Dogger et d'apprehender de
maniere rationnelle les quelques phenomenes d'interference apparus dans
des zones! haute densite d'exploitation. Du plus l'analyse sur chaque
site des correlations entre les parametres de fonctionnement de la
boucle a debouche sur la mise au point de systemes de diagnostic
automatique des dysfonctionnements. Le systeme mis en place est encore
riche de possibilites, outre les parametres de (temperatures, pressions,
puissances) fig. 4 actuellement mesurees, il est prevu un elargissement
du champ de mesure sur un certain nombre de sites, (operation aidee par
la CEE). 11 s'agit notamment de la mise en place de capteurs de
pressions annulaires, capteurs chimiques et corrosimetres destines !
saisir en contiu les fluctuations de pression de gisement et du chimisme
du fluide de formation.

667
3.2. !t6h_abiJ.iJation
L'incrustation de d6pots de sulfures sur les puits conduit a
limiter les debits du fait de l'augmentation des pertes de charges.
L'evolution des courbes (pression, debit) a l'injection et • la
production sont particuli6rement significatives. Pour redonner aux puits
leurs caracteristiques nominales, il faut eliminer ces d6pots par un
procede approprie. Diff6rentes techniques ont 6te utilisees jusqu'. ce
jour :
• curage m6canique - avec descente d'un outil de forage, ou outil
special de scraping; ces m6thodes n6cessitent la aise en oeuvre d'une
machine de forage et de son environnement
• cur age hydraulique - cette technique plus 16g6re que la pr6c6dente a
ete experiaentee pour la preaiere fois en JUln 88 sur le site de La
Courneuve. La technique consiste a detacher les d6pots sous l'action
d'un outil de jetting injectant un fluide sous pression. Elle necessite
un artesianisme residuel suffisant pour pouvoir remonter les particules
detachees des parois.
En cas de percements, plusieurs solutions peuvent etre envisag6es :
• le recheaisage semble la technique la plus efficace, elle peut etre
mise en oeuvre sur des chambres de poapage en 13" 3/8 ou des tubages de
9" S/8. La reduction de diametre et corr61ativement de debit rendent
plus difficilement envisageable l'application de cette solution sur un
tubage 7". Dans ce dernier cas seule la solution consistant a Tealiser
un nouveau puits peraettra la poursuite de l'exploitation. Par ailleurs,
le choix d'un aateriau composite resoudrait de aaniere import ante les
phenom6nes de corrosion. Deux operations de aise en oeuvre de aateriau
coaposite aidees par la CEE sont programm6es a court terme, un
recheaisage sur un puits existant a Maisons Alfort, realisation d'un
nouveau puits avec un tubing en composite a Bondy.
3.3. ~~~ch1!iq'!.ll~4L ~evl!n~ion
Pour faire face. ces phenom6nes de corrosion-depot, des solutions
consistant • traiter le fluide sont en cours d'experiaentation. Trois
principaux types d'inhibiteurs ont ete utilises inhibiteur de
croissance cristalline - inhibiteur de corrosion - bact6ricides. Les
inhibiteurs de croilsance cristalline bloquent le d6veloppement
cristallin du sulfure de fer. Ce sont en general des sulfonates ou
polyacrylates possedant des propri6t6s dispersantes ou dilatantes. Les
inhibiteurs de corrosion utilises sont des amines grasses de type
aromatique ou aliphatique. Par leurs proprietes filmantes, ils protegent
les surfaces metalliques contre la corrosion. Des formulations combinant
les effets de ces inhibiteurs sont actuelleaent testees. Le aode
d'injection peut Ie faire de aaniere continue ou discontinue (squeeze).
11 est pr6f6rable d'injecter les produits a la base du puits de
production. Troil syst6mes d'injection en fond de puits ont ete
experi.ent'l lur trois litel differents : a Bondy le tube de traiteaent
de fond de puitl (TTFP) experiment' etait constitute par un tube acier
ISS de diaaetre 1" de meae nature que le tubage. Au bout d'une saison de
chauffe, le tube a eta totalement detruit par la corrosion. A
AulnaY-loul-Boil, le TTFP elt constitute d'un tube acier de diaaetre 1"
1/4 danl lequel coulilse un tube en elastoaere de di .. etre 1/2", ce tube
devrait etre remont' et exaain' • la fin de l'actuelle saison de
chauffe. A La Courneuve le TTFP est conltitu' de tubes de r'sine
furanique visl'l bout • bout par longueur de 6 m. Une rupture de ce tube

668
a ~t~ constat~e en octobre 88, Ie diagnostic de cette rupture n'est pas
encore connu.
Chacune des ces exp~rimentations ont fait l'objet d'une campagne de
mesures approfondie. Les vitesses de corrosion mesur~es sur des coupons
de corrosion disposes en tete de puits ont diminu~ considerablement, (de
1 000 ~/an l 50 ~/an l Bondy, de 1 770 ~/an A 49 ~/an l
Aulnay-sous-Bois). Les sp~cialistes neanmoins s'interrogent sur l'action
des agents filmants (inhibiteurs de corrosion) sur des surfaces
recouvertes de d~p6ts et caract~ris~es par un coefficient de rugosit~
~leve. En mati~re des m~thodes d'injection une exp~rience d'injectiDn
par squeeze va etre tentee sur l'op~ration de Cachan (op~ration aid~e
par la CEE). La technique utilis~e consiste A forcer dans la formation
une grande quantit~ de produit l haute concentration. Le produit se
desorbe ensuite lentement et effectue son action, la periodicite peut
etre hebdomadaire ou mensuelle. A Cachan, la premi~re phase de
l'exp~rience en cours, a consist~ A tester la methode et les produits
sur des carottes repr~sentatives du r~servoir.
3.4. Q~j,J~~Jla~~~m du t~I!.C;!t

669
en France sont bien evide .. ent conditionnees par l'evolution de
l'environnement energetique. Du point de vue tecbnique, la resolution
satisfaisante des problemes de corrosion-depOt est une condition
prealable • de nouvelles exploitations au DOGger, tout du moins dans les
zones. baute teneur en sulfures. Le concept consistant • traiter le
probleme "materiau" plut6t que d'enoraes quantites de fluide par des
metbodes cbimiques apparait plus satisfaisant • l'ArK!. 11 est beaucoup
attendu des experiences de mise en oeuvre des materiaux composites. Un
cabier des cbarges et les notes de calcul correspond antes ont ete
elaborees sous l'egide de l'ArKE. Parallelement une reflexion est menee
pour recbercber des applications energetiques non traditionnelles et des
applications non energetiques. Parai ces dernieres on peut citer les
domaines suivants extraction de litbiua ou d'autres matieres
premieres, loisirs et tourisme, sante, cbimie et biocbimie de syntbese,
culture d'algues nutritionnelles (proteines). L'utilisation des
aquiferes d'eau potable du Bassin parisien (Albien, Neocomien) offre
encore des perspectives insuffisaament exploitees, soit dans le cadre
d'une exploitation combinee de l'eau et de la cbaleur, soit en stockage
intersaisonnier de cbaleur. Le projet des Assurances Generales de France
proprietaire de deux iaaeubles de bureaux d'environ 30 000 m2 cbacun,
beneficiant d'une aide de la CEE devrait demarrer en 1989. En debors de
l'equilibrage tberaique par stockage des excedents extraits de la
climatisation, le doublet permet de aettre • la disposition des
Autorites de la Protection Civile, une ressource d'eau potable de
secours en cas de catastropbe ecologique sur l'agglomeration parisienne.
CQ"g,J!~t~1!
La geotbermie basse entbalpie en France constitue certes un domaine
industriel encore jeune, aais qui a considerablement evolue au cours des
dernieres annees compte tenu de l'effort realise par 1 'ensemble des
partenaires des operations. Tous les problemes ne sont certes pas
resolus de maniere definitive mais les resultats concrets obtenus a
l'issue des dernieres experimentations permettent d'envisager l'avenir
de la filiere geotbermique avec une certaine serenite. Co .. e l'a deja
souligne la Direction des Energies Nouvelles de la CEE, il faut a la
geotbermie et en particulier dans le domaine basse entbalpie, une
politique de soutien • long terme, determinee et volontariste qui la
rendra peu sensible aux aleas conjoncturels, et qui permettrait
d'assurer • cette ressource un r6le significatif et non plus marginal
dans le bilan energetique co .. unautaire. L'ArKE avec l'enseable de ses
partenaires oeuvrent dans ce sens.

IlOADEAlIX STADIUM
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Sea.l00 5 -
Tecboology. ob.uc.l.e. aDd actlO1U1 to pro.»te
clevelo~Dt of geotber.al energy
Technical proble.s to exploit geothermal energy (high and low
enthalpy)
Exploration et exploitation en g'othermle dans les pays de la
Communaut' europ6enne - Quelques aspects de la l'gislation et
de. ragle. administrative.
Economie de la g60thermie dan. la CEE
Obstacles and recommendations to promote geothermal energy
development

6n
TECHNICAL PROBLEMS TO EXPLOIT GEOTHERMAL
ENERGY (HIGH AND LOW ENTHALPY)
R. CORSI
STEAM s.r.l. Energy and Environmental System
Summarv
The main technical prolems to be afforded during the
exploitation of a geothermal resource are basically linked to
the chemical composition of fluids.Scale formation and
corrosion are infact major drawbacks in geothermal
operation. The problems arise in each part of the gethermal
exploitation cycle(production casings, utilization and
reinjection plants) and involve both low and high enthalpy
geothermal resources. Uniform corrosion, sulphide
scaling, stress corrosion cracking on turbine blades, plugging
of wells and distribution nozzles, silica scaling are
phoenomena to be investigated during the utilization of a
geothermal resource. The paper provides a brief analysis of
these technical problems and a brief discussion about their
possible solution.
1. INTRODUCTION
Geothermal installations are simpler than thermal ones.They do
not involve the sophisticated equipment necessary to produce
heat by burning solid, liquid or gaseous fuels. Nevertheless
the particular chemiophysical properties of geothermal fluids
generally create economical and technical constraints to an
easy development of geothermal energy.Geothermal fluids
contain in fact dissolved gases and solids whose thermodynamic
behaviour during the utilization cycle may cause troubles by
attacking the metal surfaces (corrosion), by causing scaling
and pollution ( if released to the atmosphere) .
Since the chemical characteristics of geothermal fluids vary
greatly between reservoirs and, to a lesser degree, even
within the same reservoir, the technical problem.an user has
to face, are generally site specific and very often the
solution is not "standard". Furthermore "problems" may be
identified in each part of a geothermal installation ( from
the production to reinjection wells passing through the
utilization plants) and at different thermodynamic conditions.
It follows that a general solution to these problems cannot be
found and that specific studies and investigations are
necessary.
Aim of the present note is to give a general description of
the technical difficulties directly linked to the use of
geothermal energy in order to promote technical discussion on

678
the specific topics and hence to try to reduce constraints yet
limiting the geothermal development.
2. TECHNICAL PROBLEMS IN THE PROPUCTION WELLS
Scaling growth and uniform and stress corrosion cracking are
the main problems to be solved both in low and high enthalpy
geothermal wells.
2.1 Scaling
It is generally agreed that the four major classes of scaling
are: silica and silicates, carbonates, sulphates and
sulphides.Although scaling problems are site specific, some
general remarks may be attempted:
a) calcium carbonate scaling
is generally a problem for medium temperature resources
(reservoir temperature < 240 C ) but also low temperature
reservoir ( Bruchsal,Szeged etc) may present this problem.
- It produces very fast plugging in the production wells
(sometime within few hours the well may result completely
clogged as is the case of Torre Alfina) .
- The CaC03 deposits are generally smooth and do not generate
additional pressure drop because of the increase of the pipe
surface roghness. As a consequence the flow rate of the CaC03
encrusted wells diminishes very slowly at the beginning and
then dramatically when the inner diameter becomes small
because of the scaling growth.
- The source mechanism advocated to explain the plugging
process is the following:
Reservoir solution cointains dissolved carbon dioxide. The
amount of C02 present in a solution at equilibrium is ,
proportional to partial pressure of the C02 in contact w.ith
the solution according to the Henry law. When production of
well starts th~ bottom hole pressure decreases and the
following equilibrium shifts to the right
2HC03
= H20 + C02 + C03
The CaC03 concentration increases and may cause CaC03
precipitation according to the value of calcite or aragonite
solubility products. The CaC03 precipitation thus begins with
flashing. If flashing takes place in the productive wells we
will have in hole scaling, if it begins in the formation, we
will have formation plugging ( as it is the case of Kizildare
geothermal field).
- A discussion of the different types of approach to eliminate
this problems is reported by Corsi 1988.Among these it is
generally agreed that the most economic and reliable solution
is the utilization of phosphonate type scaling inhibitor
injected down hole by means of the injection plant depicted in
fig 1 and described in detail by Corsi 1985 and Pieri et al

679
1988. The tests carried out by ENEL in the Kizildare
geothermal field confirmed the previous results so that this
approach can be considered reliable and economic. The
inhibitor concentration needed to prevent scaling may range
between 5 to 20 ppm leading to an economic impact (in medium
temperature resource 200 230 C) on the producible
electricity less than 10 mills per kwh.
b) Sulphide scaling
- Sulphide scaling occur both in low and high enthalpy
reservoirs
- It produces generally slow plugging of the production wells.
The sulphide scaling is generally hard and rough and is
basically formed by heavy metal sulphides (iron in low
temperature and lead, zinc, antimony in high temperature
reservoirs) .
These deposits generate high pressure losses by a
combination of casing inner diameter reduction and increase
of roughness coefficient of the casing. The pressure drop in
sulphide encrusted casings are similar to those of a sand
coated hole: the friction factor increases with the increase
Fig. 1. Simplified scheme of the downhole inhibitor
injection equipment. (1) Inhibitor tank, (2) low pressure
filters (60 um), (3) metering pumps, (4) high pressure
filters (20um), (5) tube drum and whinch, (6) incoloy tube,
(7) stuffin box

680
of Reynolds number so that the production reduction may be
very high also for small thickness of scaling. As an example
fig. 2 shows a comparison between the characteristic curves of
an high enthalpy clean well ( bottom hole temperature 260 C,
1300 m depth, 9 5/8 production casing) and the same well
having the same diameter but the encrusted casing. The
roughness coefficient was calculated according to Gudmusson et
al 1977, assuming a roghness height of 5mm. This value of
roughness height is common and was measured after the first
production tests in the Asal wells in Gibuti.
-In high enthalpy wells the precipitation of heavy metal
sulphides occurs in the geothermal waters having an high heavy
metal content due to the stability of metal chloride complexes
at such temperature. It is now well established that the
sulphide scaling mainly occurs after the fluid flashing and
that the quantity of deposit is limited by the stock of
sulphide in the solution. During flash in fact:
1) C02 and H2S evolution and possible hydrolisis of C03-­
causes a slight pH increase leading to the conversion of
some HS- ion to S-- ion. The latter favours the precipitation
of heavy metal sulphides.
300
• c\eanwell
• encrusted well
200
100
O~--~~---L----L----L--~
1 2 14 16 18 20 22
Well Head Pressure (bar g)
Fig.2 -
Comparison between the output curve of a
clean and an encrusted well having the
same diameter.

681
2) temperature reduction may cause preci~ation of most heavy
metal sulphides even at constant pH and S ion concentration.
In low enthalpy wells of the Paris basin the precipitation_of
iron sulphide is supposed to occur via the corrosion of the
iron of the casing. The mechanism would be the following:
1) corrosion of the casing by means of electrolitic process
Fe a Fe++ + 2 e-
2) reaction of iron ion with the H 2 S dissolved in the solution
to form different type Of sulphide~ +
(1+x)Fe++ + HS = Fe(1+x)S + H
The high concentration of hydrogen sulphide present in the
Dogger basin waters seems to be derived by the bacterial
reduction of sulphate to sulphides.
- Theoretically two main methods exist to eliminate sulphide
scaling in high enthalpy ~:
1) recycling the incondensable gases in the wells ( as shown
in fig. 3 ) to prevent H2S to escape from the solution
2) chemical inhibitor injection by utilizing the plant shown
in fig.1
No experimentation is reported for the first one: the
associated technological problems may descourage the testing
of this approach. Some work has been carried out by ENEL on
the second approach , but until now no results are reported :
the work is still under progress.
- In low enthalpy wells the solution to the sulphide scaling
problem seems to be strictly linked to the solution of the
corrosion problems. The utilization of fiber glass liner
Compr ••• or
Iftqc\·oo.
'--__ __o-

682
(Ungemach et al. 1988)
materials appears to be
economics of these type of
c) Silica scaling
or of other corrosion resistent
the solution to the problem. The
approach has to be verified.
In hole silica scaling occurs only in very high temperature
and very saline geothermal wells ( Cerro Prieto, Milos, Salton
Sea etc) and is often associated with heavy metal sulphide
scaling.
- Its characteristics are similar to those of sulphide in hole
scaling: it is rough and hard and produces slow plugging of
the production wells.
- The precipitation of silica is due to the temperature drop
and the concentration of the fluid during the exploitation. A
brief description of the mechanism of silica precipitation is
reported by Corsi 1988.
- No reliable methods to prevent in hole silica scaling are
reported and the periodical mechanical cleaning appears the
only reliable solution. Further work is necessary to
investigate the possibility of using in hole scaling inhibitor
injection.
2.2 COrrosion
Well casing problems have historically resulted from thermal
stress, Sulphide Stress Cracking (SSC) ,cementing inadequacies
and internal or external corrosion of the casing. The main
problems with drillstem have been wear and erosion, localized
corrosion and corrosion fatigue.
Geothermal fluids cointain in fact several chemical species
that have significant effect on the corrosion of metallic
construction materials. These key species, which were
identified from corrosion literature and data on chemical
composition of fluids, are as follows : oxygen, chloride ion,
hydrogen sulphide, carbon dioxide species, ammonia species,
sulphate ion.
A quantitative prediction of their corrosive effects is
complicated by the following factors:
the interaction of two or more chemical species may give
different ,results from those obtained with the individual
species.
the importance of a species depends on the form of attack
(uniform corrosion, pitting, crevice corrosion, cracking or
corrosion fatigue). A complete review of the corrosive effects
of some species is reported in Ellis et al. 1981 and 1983.
- Dissolved carbon dioxide , hidrogen sulphide and chlorides
represent the most common trouble especially in low
temperature reservoirs.
- Dissolved C02 lowers in fact the pH so increasing the
susceptibility to corrosion of carbon steel and high strenght
low alloy steels.
Furthermore dissolved C02 can provide alternative proton
reduction pathway further exacerbating carbon and low alloys
steel corrosion.
The presence of ferrous ion and C03 ion lead to the
formation of ion hydrogenocarbonate Fe (HC03) 2 which is highly

683
• 6070ppmCl
5 • 133500 ppm CI
->c.
-
E 4
-CD
co 3
c::
c
0
.;; 2
g
0 1
(J
0
0 100 200 300
Temperature (C)
Fig. 4 Corrosion rate of carbon steels in deaerated sodium
chloride brines at total pressure of 70 bars.
Carbon dioxide partial pressure sufficient to produce pH 5
(Ellis et al. 1983)
water soluble. The increasing temperature may however form
stable iron carbonate films which slow the corrosion rate so
that the carbon steel corrosion rate diminishes with the
temperature if in presence of C02 as shown in fig 4.
In the figure is also shown the effect of chloride ion.
- From the above considerations it may be concluded that the
presence of carbon dioxide leads to very high corrosion rate
in low temperature geothermal wells while may reduce greatly
the corrosion rate in high temperature wells.
Dissolved H2S produce very high corrosion rate on certain
copper and nickel alloys so that these materials have to be
avoided in geothermal applications.
The effect of H2S on iron based materials is less
predictable: accellerated attack occurs in some cases and
inhibition occur in other. The HS- may infact' act as an
inhibitor stimulating the recombination of hydrogen atoms
produced as result of the proton reducing process occurring at
metal surface. These atom may enter the metal and yield two
types of corrosion either blistering or
embrittlement, depending on steel mechanical strenght and
stress intensities. This risk however is limited when dealing
with soft carbon steel grades adapted to H2 S

684
service. This type of steels are however strongly exposed to
general uniform corrosion enhanced by the presence of chloride
ions.
Furthermore the need for low strenght steels to resist
Sulphide Stress Cracking conflicts directly with the need for
high strenght to resist thermal stresses. There is therefore a
need for high hot toughness steels,with high yield strenght,
resistance to SSC and increased resistance to localized
corrosion. Fortunately, the same metallurgical features which
lead to improved resistance to SSC in casing also lead to
improved toughness and resistance to corrosion fatigue.
- The solution to the problem ( at least in high temperature
wells) is therefore an accurate selection of materials for
casing and drillstem according to the relative presence of the
key species mentioned above. A wide and complete review of
these materials is reported by Ellis et al. 1983.
- As pointed out in the previous, uniform corrosion seems to
be an heavier problem in low temperature geothermal wells.
The utilization of alternate materials ( fiberglass or some
other non metallic materials) for liner or casings appears in
principle a good solution as pointed out by Ungemach et al.
1988.
- To reduce the metal loss rate of the drill pipe , amine
based corrosion inhibitors have been developped for the
geothermal environment. This corrosion inhibitor are however
expensive and their use has to be examined case by case.
3 TECHNICAL PROBLEMS IN THE UTILIZATION PLANTS
Efficiency reduction in heat exchanger devices and in power
plant machineries ,in separators, possible failures in turbine
blades and glandes, are the main problem to face in
utilization plants. They are mainly caused by scaling and
corrosion.
3.1 Scaling and deposits
The mechanism of CaC03 and sulphides scaling formation and
its characteristics were discussed previously ..
This type of scaling is found in pipelines, pressure
separators and heat exchangers and may result in changes of
their performance.
The encrusted separators lose efficiency promoting water
droplet entrainment in vapor phase and cause additional
pressure drop in vapor phase so reducing the overall
efficiency of the power plants.
The formation of deposits on pipes and heat exchangers may
result in changes in their heat transfer and pressure drop
characteristics. The extent to which deposition will affect
heat transfer and pressure drop will depend on type of
roughness beeing formed as described by Gudmusson et al. 1977.

68.5
The ultimate result is however a loss of efficiency of
conversion plants so that scaling preventive measures have to
be studied to reduce the problem.
- The methods to prevent CaC03 and H2S scaling in surface
equipment are the same as discussed above.
- Silica scaling is certainly the most dangerous in surface
and reinjection equipment. In fact nowaday no proven and
economic method to prevent silica encrustations is known.
Maintaining fluid temperature higher than amorphous silica
saturation temperature and hence carrying out reinjection at
high temperature is often the only praticable method. This
practice may prevent an efficient energy recovery so that a
lot of methods of silica scale treatment were studied ( Corsi
1988, ELC CNR contract 1988) . Instead of attempting to prevent
the formation of scale, these methods allow deposition of
scale within specially designed equipment by carefully
selecting the incremental steps in reducing the pressures and
temperatures between the wellheads of the producing wells and
those of reinjection wells. For example, if the scaling
problem is located in the flash tank, a special flash
crystallizer may be erected (Awerbuch et al. 1983). If the
scaling problem is located in the reinjection lines special
thickener , filters or retaining tank were studied (ELC - CNR
contract 1988).
A sketch of the possible silica scaling treatment methods is
shown in fig 5. The feasibility of the different types of
treatment has to be carefully examined by means of a technical
and economic comparison of the different alternatives taking
into account the peculiar characteristic of the resource.
- Scaling is also found on the blades of the turbines and may
damage the turbines glandes.This type of scaling is basically
constituted by silica,chlorides and borates. It is caused by
water droplets carried along and then evaporated on a
substratum. In high temperature superheated steams some
volatile elements may also be present as gas phase (chlorides
and silica ). In this cases the steam becomes also very
aggressive. This type of scaling is very dangerous since may
cause blades failure and outages of the power plants.
- In water dominated geothermal reservoirs the problem may be
solved utilizing very efficient separators so eliminating the
entrained droplets.
The case of the superheated steam is a little more
complicated. The scaling elements may be present in gas phase.
It is so necessary eliminate the elements by spraying a liquid
solution in the superheated steam in order to solubilize and
eliminate in liquid phase the scaling elements. The following
separation of the liquid phase completes the process.
- Scaling may also be found in the cooling water circuit, in
the nozzle of the cooling water distribution system, in the
cooling tower filling, in the nozzles of spray condenser.
This is colloidal sulfur, formed by the oxidation of H2S which
comes into contact with atmospheric air.
- The problem is generally solved utilizing "splash" type
cooling tower fillings and adequate nozzles and distribution
systems.

686
r 1
PRODUCTION
WEll
SUJRRY
ULTRAFILTRATION
FLOTATION
STEAM
CENTRIFUGAL SEPARATION
FILTRATION
CASEB
RETAINING TANK
-==-__ SILICA RECOVERY
Fig. 5.
POSSIBLE ALTERNATIVES TO ELIMINATE SILICA SCALING.
3.3 Corrosion
Pipelines, separators ,heat exchangers present the same type
of corrosion problems as described in the previous paragraph.
On the contrary machinery of the geothermal power plant may
present particular type of corrosion cracking depending on the
type of contruction material.
Geothermal steam may infact cointain chloride ions. These may
be present as gas phase ( in some superheated geothermal
reservoirs) or as liquid droplets present in the steam phase
because of low efficiency of the separators.
The chlorides may induce serious chloride stress corrosion
cracking and pitting on the turbine blades.
This problem may be solved as previously outlined i. e.
"washing" the steam: an alkaline solution is introduced in the
steam stream so that the acid soluble component of steam are
retained in the liquid phase and then separated by means of an
efficient separator.
- The inefficiency of separators are also responsible of the
erosion corrosion phoenomena which are very often found on the
last stage of geothermal turbines. In this case utilization of
appropriate turbine drainage systems may also be useful to
eliminate the problem.
- Other problem directly linked to the presence of H2S is the
corrosion of cooling towers basins. This phoenomenon is due to
the combined presence of H 2
S and thiobacillus thiooxidans in
the cooling water. These ""bacteria are infact capable of

687
oxidizing the H2S to H2S04 so reducing the pH until the cement
is corroded.
The addition of sodium hydroxide or of particular type of
byocide may solve the problem.
4. WASTE TRATMENT PLANTS
The presence of dissolved gases and elements potentially
troublesome to the environment (arsenic, boron, sulfur etc)
forces the geothermal user to adopt antipollution measures in
geothermal installation. These are basically reinjection
plants and H2S abatment systems.
- Silica scaling and plugging in the reinjection lines and a
solid product to dispose (colloidal sulfur from H2S abatment
plants, amorphous silica from water tratment plants) are the
main problems for the operation of such a type of plants.
- Seismic risk is another potential problem. Infact, where
. reinjection causes the pore fluid pressure to exceed the
hydrostatic pressure, seismic activity may be induced if there
are preexisting faults or major fracture areas near the
reinjection zone.
An accurate monitoring of the seismic activity is hence
necessary to foresee and prevent seismic problems.
- The description of the water and gas treatment plants is
outside the scope of the present note.It is here sufficient to
recall that environmental impact problems may sometimes
represent a serious constraint to the geothermal industry
development.
5 CONCLUSION
The main technical problems limiting possible geothermal
development are summarized in the tables 1,2,3.
Among the different problems here examined, silica and
sulphide scaling in high temperature reservoirs and iron
sulphide scaling in low temperature reservoirs seem to be the
major technical problems and should be further investigated to
find suitable technical solutions.
Environmental aspects may also represent a serious constraint
to the geothermal industry development and major attention
should be paid to reduce geothermal energy environmental
impact.

688
REFERENCES
Allegrini,G.and Benvenuti G.C. (1970) Charac'teristics and
geothermal power plant protection collateral processes of
abrasion, erosion and scaling) U.N. Symp. on Develop.and util.
of Geoth. Resour.,Pisa Geothermics Sp. Iss.2, 865-881
Awerbuch 1.,Van der Mast V.C., Roger A.N. (1983). Geothermal
Scale Control by Cristallization. Proc.of the 7th Annual EPRI
Geothermal Conference and Workshop
Corsi,R. Engineering aspects of CaC03 and Si02 scaling(1988)
Proceedings of the NATO Advanced Study Institute on Geothermal
Reservoir Engineering Antalya ,121-141 Kluwer Academic
Publisher
Corsi,R., Culivicchi,G.Sabatelli,F. (1985) Laboratory and
field testing of calcium carbonate scale inhibitors. G.R.C.
Symp. Geoth. Energy Haway, Trans., Vol 9 pp. 239-244
ELC-CNR (1988) Applicazione di trattamenti fisici per 10
smaltimento di fluidi geotermici residui; Progetto Finalizzato
Energetica
Ellis,P.F.,Conover,M.F. (1981) Materials selection guidelines
for geothermal utilization systems.
Contr.No DE-AC02-79ET27026 prepared for US DOE
Ellis,P.F.,Smith,C.C.,Keneey,R.C.,Kirk,D.K.,Conover,M.F.
(1983) Corrosion reference for geothermal downhole materials
selection. Contr.No AC03-81SF11503 prepared for US DOE
Gudmusson,J.S.,Newson I.H.,Bott,T.R. (1977) Rippled deposit:
formation and pressure drop effects. AERE Harwell and Nat.
Lab. AERE-R 8703 Unclassified
Pieri, S.,Sabatelli, F.,Tarquini, B. (1988) Field testing
resul ts of downhole Scale Inhibitor Injection. Proc. of
Workshop on Deposition of solid in Geoth. Syst. Reykjavick
Ungemach, P., Roque, C. (1988) Corrosion and Scaling of
Geothermal Wells in the Paris Basin. Proc.of Workshop on
deposition of solid in Geoth. Syst. Reykjavick

TAB 1 - GEOTHERMAL WELLS
Problem
Reason of the problem Errect or tbe problem Geotb. Reservoir Possible Remedy Typical range or
tbermod. cbaracL
Iron sulphide ICaling
Corrosion of the IlCeI
casing in presence of
H2S. Formation of FeS.
Presence of high
conccntralion of H2S in
the native fluid.
Reduction of production
due 10 boIh reduction in
diameter and surface
roughness modifacalion
Paris basin
OIange casing III8ICriaIs
Scaling inbibiton
Temp.: 60: 80 C
Heavy mccaI Sulpude
Scaling
Dissolved gas escape
during production in presence
of heavy melal ions
A. above
Milos, MofeIC,
Gibuti, SallOn sea ,
AmialaeIC.
Mainlain bigh pressure
in the wen
Scaling inbibilOr.
Mechanical cleaning.
Temp. >250
Calcium carbonate scaling
Dissolved gas escape. Ph
increase
Fast plugging of the
wells
LaIel8, Torre AlflJl8,
Kizildare,
Axr.ocre, ele.
lnbole acaling inbibilOr
Chemical and Mechanical
cleaning
Temperarure < 250 C
Low and mediwn
enthalpy. Surface
water influx
Uniform corrosion
Casing sleCl conswnplion.
Risk of water COIItami-
nation and well collapse.
Down bole pump failures
Aggressive geolh. walen:
low pH; High Temp;
presence of CO2, H2S, Cl,
oxigen ele.
Paris basin, Salton
See, Milos. Gibuti
Allow ICaling growth
Change maIeriaIs
PralicalIy risk with
any lemp. Ph below
S al wen head.
lnlen11ediale lemp.
Stress corrosion
aacking
Use of bigh SlrOIghl rnalCriaI
in presence of Cl,
H2S
Failure of casings 01\ drill
pipes during drilling
I..ardcre1Jo
Ulili7JItion of new
maIeriaIs
High lemper., super.
beald Sleanl resav.

TAD 2 -UTILIZATION PLANT
0-
8
Problem Reason of the problem Effect of the problem Geoth. Reservoirs Possible Remedy Typical range or
thermo char act.
Scaling in the heat ex- See table 1 for CaC03 and High pressure drop Paris basin CaC03 scaling inhibitor
changers Sulphide scaling through the heat exchan- Periodical cleaning
ger. Reduction of heat
(mech and chemical.)
load
sand injection".
Scaling in the pipes Table 1 Table 1 Tablet Table 1
Scaling in the turbine Droplets carry-over Turbine shaft damage LardereUo, Milos, Improve separation High lemp. water and
blades and glandes.; Blades failures pipes ele. Philippines efficiency steam dominated reservoir.
Valves sticking Frequent outages Washing steam
Efficiency reduction in H2S oxidation .Formation Clogging of the fIlling Geothermal power H2S abatment
the cooling tower of coUoidai sulphur. and of the distribution plant Splash type fIlling
system Algae growth system nozzles ParL type of nozzles
Utilizzation of biocyde
Turbine blades failures Chloride Stress Corrosion Turbine damage as above Steam scrubbing to re- Superheated steam
Cracking Forced outages move chloride ions with high chlorides
Separation efficiency content
improvement
Cooling tower system Low Ph in cooling water Damage to cooling wa- as above Sodium hydroxide
corrosion Presence of sulfooxidans ter structure addition
bacteria
Utilization ofbiocyde
Uniform corrosion rate High chlorides, H2S, Damage in aU utilization Alternate materials
C02, dissolved oxigen plants A void oxigen contam.
Silica scaling in separa- Concentration Temp. drop Frequent mechanical Cerro Prieto High temp. in separa- High temp. water
tors during exploitation cleaning of separators Milos tor dominated reservoir
Outages AsaI Flash crystallizer T>250C

TAD 3 -
WASTE TREATMENT PLANTS
Problem Reason of the problem Erred of the problem Geotb. resevoir Possible remedy Comment
Surfa:e wa/er c:oruami- ChrmicaI composition of Reservoirs under Reinjection Reinjection has also
Dalioo geodL walen not compa- explowion beneflClll effect on
lillie wilh surface walen
die reservoir life.
Air COIIlaminaIioo Incondensible gases Human and animal life Reservoirs under H2S abtmenl
10 !be IIIJDOSpbere damage explolalion Use of high chimneys
Silica IC8ling Temperature drop during Reinjection lines clogging Salton see, Milos, WaJrz trealmClll plant It represents one of die
uti1izalion cycle Reinjection wells clog- AsaI Acidificalion drawback 10 the geothenn.
ging
Reinjection at high lernp. energy exploilation
Iron sulphide IICaling SceTable 1 As above Paris Basin Filtratioo
Change type of materiaJ
(filx7 glass)

692
EXPLORATION ET EXPLOITATION EN GEOTHERKIE
DANS LES PAYS DE LA COKKUNAUTE EUROPEENNE
QUELQUES ASPECTS DE LA LEGISLATION
ET DES REGLES ADKINISTRATIVES
Jean BARBIER
Service d'Information sur l'Energie
c/o BRGK BP 6009 45060 ORLEANS CEDEX FRANCE
Resume
Le developpement de la geothermie depend non seulement ,des
conditions de lancement des operations, mais aussi du bon fonctionnement
economique des installations realisees en contexte de concurrence
energetique la geothermie doit rester competitive. Differentes
disposition concernant la commercialisation des produits (electricite,
chaleur) ainsi que certains contrats d'assurance ou de garantie, sont
examines.
1.INTRODUCTION
Comme pour toute industrie, les dispositions legales ou
administratives sont susceptibles de jouer en geothermie le role de
frein ou au contraire de constituer un atout important. Une
harmonisation des regles est d'autre part souhaitable, chaque fois
que possible, afin de faciliter l'action des differents acteurs
concernes par l'energie geothermique. C'est pourquoi la Commission
des Communautes Europeennes s'est penchee A plusieurs reprises sur la
question: un document de travail recent (decembre 1986) a notamment
fait le point sur ce sujet, en degageant plusieurs propositions de
recommandations relatives aux aspects legislatifs et A la promotion
de la geothermie (1).
Les principaux points mis en relief sont repris dans les lignes
qui suivent, avec quelques developpements. En effet, l'attention
avait surtout porte sur 1 'exploration, essentiellement pour la basse
enthalpie semble-t-il (references ~xplicites aux donnees provenant de
forages petroliers). 11 a paru interessant de ne pas ecarter la
haute enthalpie, c'est-A-dire la production d'electricite, et de mettre
l'accent sur les questions de commercialisation. En effet dans un
contexte de concurrence severe entre sources d'energie, le marc he a un
role determinant: les conditions de la vente de l'energie ont donc sans
nul doute, pour la geothermie comme pour les aut res energies
alternatives, aut ant d'importance que la reglementation sur la
demonstration ou sur l'exploration.
L'on s'est inspire, pour la haute enthalpie, du meme document de
synthese que mentionne ci-dessus : celui-ci traitait aussi en effet des
petites centrales hydrauliques. Or, selon l'enquete realisee dans

693
lei diver I paYI de la Coaaunaute, la legislation sur la vente
d'electricite elt en general identique, quel que soit Ie aoyen de
production adopte. La reglementation concernant la geotheraie n'aurait
donc pas • etre distinguee de celie relative • la ainihydraulique, en
dehors des questions de protection de l'environne.ent bien entendu. Par
ailleurs pour 1 'exploitation de la basse energie, plusieurs
renseignements interessants ont pu etre tires d'un audit sur certaines
operations geotheraiques du Bassin parisien, audit dont la supervision a
ete confiee • l'1gence francaise pour la aaltrise de l'energie (2).
Certel l'etude ne porte que sur un seul pays, aais Ie noabre de sites
etudies (plus de vingt), la variete des contextes juridiques, ainsi que
Ie caract ere recent de ce travail, lui conferent un interet certain : de
larges passages lui ont ete empruntes.
2. PRODUCTION ET DISTRIBUTION D'ELECTRICITE
Les pays actuelle.ent concernes sont ceux du Bassin
mediterraneen, Italie et Grece: d'autre part Espagne, Portugal et
France Ie sont egaleaent, sinon par leur territoire .etropolitain, du
moins par les lies volcaniques d'outre-mer qui y sont rattachees. De
plus l'existence d'un programme co .. unautaire de Roches Chaudes
Seches (BDR), qui n'exclut nullement pour l'instant la production •
long terae d'electricite, peraet d'accorder quelque attention l ce
sujet, aeme dans l'Europe aoyenne ou nordique.
2.1. La distribution
Le cas general pour un petit producteur d'electricite est une
situation d'autoconso .. ation l'explication est • chercher dans
plulieurs causes.
Tout d'abord, un producteur prive de courant ne peut pas
utiliser Ie reseau public
l'electricite qu'il produit,
afin
sauf
de transporter et
rares exceptions. Ce
distribuer
producteur
doit donc ceder Ie courant • un producteur-distributeur public. Si
les tarifs d'achat lont souvent pre-etablis et publies, en
particulier dans l'Europe du Sud, les negociations se font plut6t
dans l'Europe du Nord au cas par cas avec les diverses compagnies
diltributrices. Les obligations lont pour l'essentiel' la charge du
producteur: elles decoulent notaaaent de contraintes relatives au
courant produit, en liaison avec la securite du reseau. Dans certains
pays une obligation luppleaentaire est iaposee, celie d'etre
autoconsoaaateur: seule la production excedentaire peut etre cedee
(1).
Les tarifs d'achat eux-a6aes sont parfois consideres co .. e peu
incitatifl par les investisseurs, et sont en tout cas plus faibles
que lei tarifs de vente aux usagers : la situation d'autoconsoamateur
elt cette foil dictee par l'interet econoaique.
2.2. La production
Plulieurl cas lont • considerer. Tout d'abord vient celui des
paYI o~ la geotheraie de haute enthalpie est sous regiae ainier
Portugal, Espagne, RF1, France: la Grece quant • elle a une legislation
Ipecifique. Un projet en ce lens est en cours en Italie, aail la loi n'a
toujourl pal ete proaulguee. Pour les autres pays et en dehors du
Luxeabourg, pour lesquels l'energie geotheraique n'a pas de regiae
legal, l'exploitation depend de lois plus generales relatives au
10US-IOl, • la production d'eau souterraine ou
(1).
• l'activite de forage

694
Un permis d'exploitation est necessaire dans tous les pays ou
une ressource de haute energie a ete identifiee. Dans certains pays
toutefois seule une compagnie nationale habilitee peut beneficier d'un
permis, la question de producteurs independants ne se pose alors pas.
2.3. Les contraintes relatives a l'environnement
Les dispositions principales sont celles qui ont trait a la
reinjection, compte tenu des caracteristiques chimiques tres
particulieres des fluides de haute temperature. Necessaire non
seulement au maintient de la production mais aussi a la sauvegarde de
l'environnement, la reinjection est dans la pratique obligatoire.
Heme les gisements de vapeur contiennent des elements qu'il faut,
apres condensation du fluide geothermal, eliminer. Les dispositions
administratives de reinjection sont dans la plupart des pays liees a
l'octroi du permis d'exploitation.
2.4. L'exploration
Dans tous les etats membres un permis de
indispensable; la duree du permis n'est en general pas
France, Grece et Espagne ou elle est de trois ans et en
de cinq ans (1).
recherche est
definie, sauf en
RFA ou elle est
3.PRODUCTION ET DISTRIBUTION DE CHALEUR
Les operations de geothermie presentent un certain nombre de
caracteristiques qui n'ont pas toujours ete per~ues au moment de leur
montage (2) :
- c'est tout d'abord au plan de la production une entreprise
industrielle, donc :
une entreprise a risques techniques qui exige une gestion
technique tres rigoureuse et une capacite de decision rapide
face aUK incidents;
une entreprise dont les immobilisations sont tres importantes
et dont les possibilites d'investissements doivent etre
notables pour faire face en permanence aux problemes de
renouvellement, d'adaptation et de modernisation des
installations ;
une entreprise du secteur primaire dont la valorisation de la
production depend essentiellement des cours internationaux.
- c'est ensuite souvent un service public de distribution d'energie
calorifique, ce qui suppose une gestion attentive et efficace des
contrats d'abonnements, ainsi que du fonctionnement du reseau de
chaleur et des installations des abonnes.
Les caracteristiques de cette entreprise sont donc bien
eloignees du simple changement de chaudiere, auquel on a parfois
pense reduire une operation de geothermie (2).
3.1. La vente de chaleur
La vente de chaleur se fait dans le cadre de dispositions
legales ou contractuelles. Ces dispositions concernent donc aussi la
geothermie, d'une fa~on qui n'est pas toujours neutre.
Ainsi, la legislation peut imposer un comptage de l'energie
fournie ; dans le cas de la geothermie ce n'est pas toujours aise, la
mesure du debit n'etant pas tres fiable surtout en regime diphasique.
De meme, une tarification possible pour les modes traditionnels de
chauffage comporte un terme fixe d'une part, ainsi qu'un terme variable
selon la consommation de combustible d'autre part. D'une certaine
maniere ce terme proportionnel reflete les consommations

695
mail aUlli lei aleal, cliaatiques co .. e econoaiques (variations
conjoncturelles du prix des coabustibles fossiles, etc.). Or pour un
producteur de chaleur d'origine geothermique, il s'ajoute une autre
sorte d'aleas qui est Ie risque ainier : corrosions, pannes, colmatages
ne sont pas toujours aisement previsibles. Si ce risque est aal calcule
ou aal couvert, il peut en resulter des difficultes financi~res
inextricables pour Ie promoteur de l'operation geotheraique. En effet
les dispositions contractuelles peuvent s'opposer A ce que les frais
correspondants soient equilibres par l'ajusteaent des charges payees
par les usagers.
Les operations comportent en effet au moins quatre aodes de
recuperation possible des charges (2) :
Operation realisee par un proprietaire (Maitre d'ouvrage) pour
ses besoins propres,
Repartition des charges entre les utilisateurs, par exemple
entre locataires ou differents organismes groupes en une
mAme entite (syndicat) ices organismes repartissent ensuite ces
charges entre les divers utilisateurs ou bien les prennent en
compte dans leur budget comme dans l'item precedent {hopitaux)
Operation A caract~re commercial avec contrats d'abonnements
fondes sur un tarif, lequel comporte des param~tres de revision
Operation A caract~re co .. ercial dont les contrats comportent
des clauses de sauvegardei Ie cont pour l'usager ne peut exceder
Ie prix que celui-ci aurait paye pour l'energie que celui-ci
utililait avant la geotheraie.
Une large part des difficultes financi~res actuelles dans les
operations de geothermie est due au jeu de cette clause de
sauvegardei il ne faut toutefois pas oublier que celle-ci, qui peut
paraitre exorbitante presentement, a certainement contribue l rendre
possible Ie montage des operations vis-A-vis d'une client~le dejA
equipee de aoyens de chauffage (2).
Les aleas geologiques de l'exploitation n'etant pas toujours
couvrables d'un point de vue financier par les ventes de chaleur, un
Iysteme de garantie a ete mis en place.
3.2. Un syst~me de aarantie geothermique l long terme
L'objet est de couvrir Ie proprietaire de l'installation (Maitre
d'ouvrage) pendant une longue duree, quinze ans par exemple, contre
les consequences directes des phenom~nes geothermiques doamageables
lur la boucle d'exploitation i ceci vise nota .. ent la baisse de
temperature de la ressource ou la baisse de production des puits. Cette
gar an tie couvre (2) :
les incidents reparables, avec reaboursement du cont des
reparations prises en charge,
lei sinistres partiell, avec reaboursement pendant la duree du
sinistre d'une partie de l'annuite d'amortisseaent,'
les cas de sinistre total, avec reaboursement de la part de
l'inveltisseaent non aaortie.
Dans tous les cas il y a application d'une franchise pour chaque
sinistre, de l'ordre de 80.000 ICUs environ.
D'un point de vue financier, la couverture est assuree par :
• Ie Maitre d'ouvrage jusqu'A 80.000 ICUs (franchise),
Ie fondl de garantie de 80.000 • 600.000 ECUs,
un pre.ier groupe d'assureurs de 600.000 A 4.000.000 ICUs,
un second groupe d'assureurs de 4.000.000 A 4.500.00 ICUs.

696
La garantie est accordee en contrepartie d'une cotisation egale
A 3% du montant total garanti, cotisation versee A la signature du
contrat. Le syst~me parait actuellement fragile, ayant ete mis en place
en fonction d'hypoth~ses que les difficultes techniques rencontrees
viennent remettre en cause. Differentes voix se sont elevees pour
demander une modification des r~gles de fonctionnement de ce fonds de
garantie le fonds risque de s'epuiser, au rythme actuel des
indemnisations, et les Haitres d'ouvrage se retrouveraient avec la seule
garantie des assureurs c'est-a-dire avec une franchise de 600.000 ECUs
A leur charge pour chaque sinistre (2).
3.3. La protection de l'environnement
Elle est d'autant plus necessaire que la distribution de chaleur
ne peut se faire que dans des regions a forte densite de population ; la
protection de l'approvisionnement en eau, notamment celle des nappes
d'eau souterraine, est indispensable. Or, meme pour des operations de
basse enthalpie, la nature du fluide geothermal est souvent telle qu'un
melange avec les nappes d'eau douce alimentant les agglomerations doit
etre absolument prohibe. C'est pourquoi la legislation, par le moyen des
permis d'exploitation, prevoit parfois la possibilite d'interdire et
donc d'arreter l'extraction du fluide geothermal en cas de risque de
contamination. Pour une raison identique, l'injection d'inhibiteurs de
corrosion est tr~s surveil lee : certains de ces inhibiteurs sont des
composes amines qu'il est exclus d'introduire dans les eaux destinees A
la consommation, ce qui risquerait de se produire en cas de percement
des tubages par exemple.
La reinjection du fluide geothermal permet d'eviter un rejet en
surface et une pollution chimique ou thermique du milieu naturel.
Cette reinjection, lorsqu'elle est necessaire, est soumise A
autorisation dans tous les pays ou la geothermie est utilisee: comme
pour la haute enthalpie, cette autorisation est accordee dans le cadre
du permis d'exploitation. Seule la loi fran~aise oblige la reinjection
dans le reservoir d'origine dans les autres pays cette unicite
production-reinjection n'est pas indispensable (1).
La geothermie, en se substituant aux combustibles classiques,
reduit la contamination de l'atmosph~re en poussi~res et en polluants
gazeux. Toutefois la legislation ne lui est pas de ce point de vue
particuli~rement favorable. En effet, les r~glements prevoient
habituellement une limitation instantanee des rejets, un debit
instantane etant aisement mesurable. Or les generateurs classiques
restent la plupart du teaps en place, pour l'appoint ou le secours.
L'emission instantanee de polluants lors d'arrets momentanes de la
geothermie (grands froids, pannes) est donc inchangee, alors que la
quantite annuelle est globalement reduite d'un facteur de deux ou
plus. Une r~glementation fondee sur une limitation annuelle des
rejets dans l'atmosph~re serait de toute evidence A l'avantage de la
geothermie.
3.4.L'exploration : necessite d'une aarantie
L'exploration en geothermie de basse enthalpie necessite des
investissements qui sont du meme ordre de grandeur que ceux des puits
petroliers. Les chances de succ~s, c'est-A-dire de trouver une nappe
d'eau chaude, sont beau coup plus grandes en revanche la valeur de la
production est plus faible, sans commune mesure, ainsi que la marge
beneficiaire quand elle existe. En d'autres termes si les

697
inveltille.entl lont de type petrolier, l'economie d'une operation Ie
rapprocherait plut6t du type minier : danl ces conditions un operateur
ne peut Ie permettre Ie rilque de perdre beau coup en cal d'echec, avec
l'elpoir de gagner peu (I'il gagne) en cas de SUCC~I.
C'elt pourquoi la geothermie de basse enthalpie n'a pu se
developper que lorlqu'a ete mil lur pied un syst~me de garantie pour
1 'exploration. Ce Iyst~me prevoit Ie reaboursement en cal d'echec de
la quali-totalite del depensel de forage bien evide .. ent cette
garantie n'a ete accordee que lorsque les chancel de succ~s, tant au
plan geologique qu'au plan economique, n'etaient pas trop faibles. De
faeon annexe une aide lupplementaire a ete consentie au cas par cas
lorlque Ie reservoir geologique necelsitait, pour Atre atteint, des
depensel plus importantel qu'il n'etait possible de prevoir
initialement. Cette dilposition doit Atre prise avant Ie debut des
travaux, de maniAre. pouvoir prendre des decisions tr~s rapidement
en courl de forage Ii necellaire (arrAt ou poursuite du forage par
exemple).
Rappelonl enfin qu'au plan administratif, un permis
d'exploration est indispensable dans tOUI les pays de la Communaute.
REFERENCES
(1) Anonyme.- Rapport lur lei legillations et mesures de promotion
des Etats membres pour l'energie geothermique, et
elements de base pour une recommandation communautaire.
Document de travail CCE XVII-SNRE-ACKDP-no 129/11/86
(2) Anonyme.- Rapport du Comite de coordination pour les operations
de geothermie en Ile-de-rrance (23/11/87, confidentiel)

698
ECONOMIE DE LA GEOTHERMIE DANS LA CEE
CHRISTIAN BOISSAVY - EXPERT AUPRES DE LA
DIRECTION GENERALE DE L'ENERGIE DE LA COMMISSION DES
COMMUNAUTES EUROPEENNES
L'economie de la geothermie est liee au type de gisement exploite. Elle
depend donc du site geographique, des caracteristiques du fluide
geothermal, de la mise en oeuvre et de l'epuisement de la ressource. En
ce qui concerne le gfte geothermique les parametres fondamentaux sent la
profondeur du giserrent, la temperature, la productivi te et le type de
fluide.
L'economie depend des solutions techniques retenues, et surtout de la
necessite ou pon d'injection. Pour 1 'utilisation, les parametres different
selon que l'on traite d'usage direct de la chaleur (geothermie basse
enthalpie) ou de production d'electricite (geothermie haute enthalpie).
Les parametres economiques essentiels sent lies a l'environnement financier
et en consequence a la nature du montage de 1 'operation, a la derive
de l'inflation, a l'evolution du coat des combustibles de reference ainsi
qu'a la duree des operations qui pesent lourdement sur leur rentabilite.
L'economie et la rentabilite de projets de demonstration en basse et
haute energie ayant re9u un financement communautaire sont discutees et
montrent qu'en raison de la structure particuliere des coats en
geotilermie, caracterisee par des investissements tres lourds, il est
essentiel de contenir les depenses de fonctionnement et de maintenance.
1/ MESURE DE LA RENTABILITE DES OPERATIONS DE GEOTHERMIE
La mesure de la rentabili te des operations de geothermie depend de
deux types principaux de parametres, ceux techniques lies a la
resseurce geothermale et ceux strictement economiques.
1.1 Farametres techniques
Ce sont les deux parametres fondamentaux qui conditionnent
l'economie des projets geothermiques. La profondeur du gisement
est surtout liee a la valeur du gradient geothermique et la
productivite des forages aux parametres physiques du reservoir.

699
Ces deux donnees sont influencees, pour la premi ere par, Ie
contexte geoclynamique ou la variation du gradient geothermique
oscille entre des valeurs normales de l'ordre de 2 A 4°/100 m
dans des zones de plateformes stables jusqu'a des valeurs anormales
de 20 A 30°/100 m dans des zones geodynamiques actives et
en part i- ulier celles ou s 'exprime une activi te volcanique
rec ente, pour la seconde par J Ie contexte geologi que et structural
qui condi tionne Ie type de reservo ir (permeabil i te de
matrice et/ou permeabilite de fractures).
Gradient de temperature et profondeur de l'aquifere
Il influence fortement les coats qui varient non lineairement
avec la profondeur. Cela est surtout vrai pour les gttes
geothermiques A basse temperature (la figure 1 IIDntre la relation
entre Ie coOt du Mwh geothermique et la temperature de
gisement pour des gradients de 2 A 5° C/l 00 m et les figures 2
et 3 l'influence directe de la profondeur sur Ie coat brut des
forages.
Transmissivite et facteur de skin
La transmissivite (produit de la permeabilite par l'epaisseur
productive du reservoir) determine la productivi te du forage
pour une valeur donnee d'energie de pompage.
Le facteur de skin r'eflete les condi tions de reservoir A proximite
immediate du forage, il est positif lorsque des particules
colmatent les pores et/ou fractures dans la formation et negatif
quand Ie reservoir est bien developpe favorisant ainsi la
producti vi te du forage. La figure 4 IIDntre I' effet tres important
de la transmissivi te sur Ie coat du Mwh geothermique pour
un doublet geothermique.
Porosite/fracturation et pression de gisement
Porosi te et fracturation affectent la productivite des forages
mais conditioment egalement les espacements qui doivent ~tre
preVUB entre forage de production et d'injection dans Ie cas de
doublet. La pression de gisement Quant A elle conditionne la
possibili te de production artesieme des ouvrages et l'utilisation
de pompe de surface pour les forages d'injection.
Les caracteristiques chimiques du fluide geothermal interviement
fbrtement dans Ie design et l'economie des projets. II
s'agit essentiellement de la salinite qui conditionne ou non la
necessi te de rtHnjection) de la composition chimique permettant
ou pas l'exploitation en puits unique) de la teneur en gaz
valorisante ou penalisante pour l'exploitation et de la teneur
en particules solides et produits de corrosion qui influence les
coats de maintenance et d'entretien.

700
Le coat global de mise en production des fluides hydrothermaux
de basse, moyenne et haute temperature est fortement influence
par la specificite du srte - zone geologiquement bien connue ou
les forages sont dits de developpement ou zone d'exploration,
mais egalement par la necessite ou non de recourir au systeme du
doublet qui multiplie environ par 2 Ie montant des investissements
sous-sol.
Influence de l'utilisation de surface
Les investissements de surface sont lies a la transformation et
a la distribution de l'energie. Les utilisations de surface
si tuees a I' aval d' un separateur ou d' un echangeur de chaleur
sont directement influencees par la puissance energetique du
srte geothermique exploite.
Les facteurs d' echelle jouent alors un role important en haute
comme en basse energie. Pour les basses temperatures Ie probleme
est de definir un optimum entre la capacite thermique des forages
et la de man de d'energie du consanmateur (debit et temperature).
La reduction de la temperature de retour avant injection
ou decharge et l'augmentation du temps d'utilisation reduisent
evidemment les coQts (figure 5). Les centrales geothermiques
sont toutefo is limi tees a des puissances bridees par la
necessi te de raccorder plusieurs pui ts a la centrale geothermique
dans un environnement pas toujours favorable, dans Ie cas
de la haute enthalpie et par I' obligation lorsqu' il n' existe
pas, de construire un reseau de distribution de chaleur
connectant tous les utilisateurs (1 a 60 MW) en basse enthalpie.
1.2 Parametres economigues
L 'economie de l'energie geothermique est strictement liee au
coQt des energies conventionnelles qu'elle substitue. En ce qui
concerne la haute enthalpie elle doit se mesurer principalement
au charbon et au nucleaire. Pour les utilisations non electriques,
la competition s 'effectue avec les combustibles
fossilles (gaz, huile, charbon).
Les econanies d'energie realisees par l'exploitation d'une
operation geothermique peuvent conduire ou non a un profit
di rectemen t proportionnel aux coats des energies de reference.
L'evolution des prix montre en 1988 des coQts voisins de ceux de
1978 sauf celui du charbon (figure 6 - coat energie CEE de 1978
a 1988).
Environnement financier
II est compose de deux facteurs principaux Ie premier
correspond a la nature du montage financier (montant des subventions,
aides, taux et duree des prets, etc ... ), Ie second est
lie a l'evolution de la derive des prix.

701
Ce parametre influe ~norm~ment sur Ie r~sul tat ~conomiQue des
op~rati ons de g~othermie, en effet les projets aujourd 'hui en
fonctionnernent ont ~t~ mon~s dans les ann~es 80-83 en p~riode
inflationiste (voir figure 7 - Inflation en Europe et ~volution
des taux d'emprunt
en France), les recettes
r~elles des op~rations de g~othermie en Ecus constant sont donc
en g~n~ra 1 sensiblemen t inflki eures aux hypotheses envisag~es
dans 1 es Hudes de fa i sabi 1 i te •
Elle influence plus fortement les op~rations de g~othermie HT,
en effet entre la phase de forage d'exploration permettant de
mettre en ~vidence une ressource exploitable et Ie premier Kwh
g~othermique produit, il peut s'~couler parfois une dizaine
d'an~es Qui p~sent lourdement au niveau des immobilisations de
capitaux.
1.3 Rentabili t~ et temps de retour
Investissenent
II faut noter que l'exploitation de l'~nergie g~othermique
comporte essentiellement des investissements lourds et un coat
marginal d'exploitation faible, c'est i\ dire une structure de
coat tres differente de celIe des ~ergies classiques auxquelles
elle peut se substi tuer.
La partie SOlIS sol des investissements peut se d~composer en
deux phases : les coats d 'exploration (inventaires, forage de
reconnsiss!l'lce, ~tudes, g~ophysique, etc ••• ) et des coats de
mise en production (forage d'exploitation, systemes de pompage,
~changeurs, s~parateurs, etc .•. ). Les premiers sont faibles
compar~s aux seconds mais differents selon Ie type de r~servoir
g~othermique. IIs sont marginaux dans Ie cas d'op~ration basse
~ergie en milieu al!dirrentaire mais peuvent repr~senter une
partie non n~gligeable de l'investissernent total en haute
enthalpie (6% du coat du Kw g~othermique install~ pour ENEL).
Les coats de forage de production et d'injection sont en revanche
pour un reservoir i\ une profondeur donn~e, proportionnels i\
la puissance g~othermique recherch~e. II repr~sentent 20 i\ 60%
du nontant total des investissements en basse ~ergie et 50 i\
70% en haute enthalpie.
Les investissements de surface peuvent @tre di vis~s en deux
parties: une li~e i\ la production/injection du fluide
g~otherma 1 et i\ son tran sport (pompes et ~changeur en basse
~nergie, s~parateurs, d~canteur, condui tes de vapeur en haute
~nergie) et l'autre li~e i\ la transformation et i\ la distribution.
En basse ~ergie, il s'agit du reseau primaire, de la
station g~othermique, des soua-stations et des modifications i\
apporter aux chaufferies existantes dans Ie cas de chauffage
urbain. En production d'~lectricit~, il s'agit d'une centrale
avec groupe turb~l ternateur et de son raccordement au r~seau
de distribution existant de courant ~lectrique.

702
Charges d'exploitation
Ces charges comprennent : les depenses d' entretien courant de
l'installation, les provisions pour Ie remplacement de certains
equipements, les charges d' electricite, les assurances, taxes et
frais divers.
Les coats d' explo i tati on spec ifiques a la geothermie sont en
general peu eleves, leur maitrise est donc essentielle pour la
bonne rentabilite des operations, toute derive venant penaliser
les gains obtenus, par la substitution de combustibles dont Ie
coat est variable.
2/ BILAN ECONOMIQUE ET RENTABILITE DE QUELQUES OPERATIONS DE
DEMONSTRATION
Les operations presentees sont de types var les, les forages geothermiques
de ces operations etant productifs, ce sont des succes techniques
qui mon trent des appli cat ions variees et des resul tats economiques
tres differents. II s'agit des projets de demonstration de :
Metanopoli - GE 02/79/ITALIE - doublet geothermique en reservoir
sableux, eau tres salee, chauffage urbain et production de gaz.
Lamazere - GE 19/80/FRANCE - Pui ts unique - Reservoir d' eau douce -
chauffage de serres.
Bordealx-Pessac - GE 46/81/FRANCE - Puits unique reservoir d'eau
douce - chauffage et production d'eau chaude sanitaire pour des logemen
ts collectifs.
Latera - GE 29/81 et GE 116/83 ITALIE - Doublet de forage dans un
champs geothermique a eau dominante - production d'energie electrique.
Southampton - GE 69/81 et GE 607/83 ROYAUME UNI - Pui ts unique en
reservoir sale - Chauffage et production d'eau chaude sanitaire.
2.1 METANOPOLI - GE 02/79/ITALIE
L'operation realisee ill Metanopoli utilise l'energie geothermique
pour chau ffer une zone res identielle appartenant au groupe ENI ill
San Donato Milanese au Sud de Milan.
Le reservoir geothennal est consti tue des sables et conglomerats
situes ill 2 km de profondeur. La salinite du fluide (70 gil) et la
necessite de maintenir la pression dans la nappe ont conduit ill la
foration de deux puits separes par environ 1 100 m aux niveaux des
couches productrices. La productivite des puits est de 50 m3/h
d'eau ill 62°C contenant 100 m3/h de methane. Cette production est
obtenu a l'aide d'une pompe electrique immergee a 600 m de profondeur,
en effet la transmissivite du reservoir est faible et indui t
un rabattement du niveau d'eau tres important.

703
Les installations de surface comprennent un systeme de recuperation
du gaz qui est distribue par SNAM et un echangeur de chaleur
A plaque classique permet la distribution de chaleur dans Ie
rllseau de chauffage urbain existant.
L'operation a ete mise en service en 1986 et est exploitee depuis
une annee.
Les problemes existants dans les doublets exploi tant des reservoirs
detritiques sont ici maitrises et les indices d'injectivite
des pui ts evoluent favorablement.
Les besoins du reseau (4,8 Gcal/h) peuvent @tre assures A 40% par
la geothermie sans tenir compte du gaz naturel produit. La quantite
totale d'energie econamisee est d'environ 800 TEP par an.
Les investissenents (5 330 K Ecus) se decomposent comme suit
forages et tests (3070), equipement de puits et realisation (800),
centrale geothermique et installation de surface (1 460).
Les coOts d'entretien de maintenance et d'electricite representent
220 K Ecus et les economies rlialisees par rapport A une installation
conventionnelle utilisant Ie gaz naturel sont de 500 K
Ecus.
Si l'on prend en compte Ie support de la Commission 440 K Ecus, Ie
calcul du temps de retour brut de cette operation est de l'ordre
de 17 ans.
On voit done ici que dans Ie cas d'un doublet profond avec un
gradient geothermal anonnalenent bas (2,2 0 /100 m) et malgre la
presence de methane qui contribue pour 30% A l'economie realisee,
la rentabili te de l'operation est faible. Ceci est dO A la
trensmissivite tres basse des formaticns rencontrees (1 A 2 Dm)
qui penalise fortenent l'economie du systeme et ce malgre une
reussite technique complete de l'operation.
LAMAZERE -
(GE 19/80/FRANCE)
La zone horticole geothermique de Lamazere a ete implantee au
coeur du Gers afin de creer un p61e de developpenent economique
createur d'emplois.
Le forage unique si tue A l'ouest d' Auch a He realise en 1982 et
capte l'eau douce des sables inframollassiques A 1 200 m de
profondeur. Le debit de 160 m3/h et la temperature de l'eau A 57°C
IErmettent de chBJffer grAce A un bassin de regulation, un appoint
gaz pour les jours les plus froids et une technique de distribution
de chaleur localisee A basse temperature, 7 A 8 hectares de
se rres chaudes.
L'operation fonctionne depuis fin 1982 avec seulement 3 hectares
de serres construi tes. Le debit d' eau geothermale produi t depuis
lors a ete de 1 850 Milliers de m3 soit un debit moyen horaire de
45 m3/h et les caracteristiques d'exploitation conformes awe
previsions des etudes.

704
De 1984 a 1987 la production d'energie geothermique a ete de 847
Mwh/an ce qui represente 96% des besoins en energie des serres et
681 TEP economises annuellement si l'on prend en compte l'energie
electrique necessaire au pompage.
L'investissement total a ete de 2 370 K Ecus (1982) superieur de
24% au coat previsionnel (1980), ce depassement a ete dQ a une
inflation forte et des surcoOts lies aux aleas geologiques durant
la realisation du forage. Les differentes subventions ont atteint
un montant de 815 K Ecus, Ie reste de l'investissement a ete
assure sous forme d'un pd!t a 10,5% sur 20 ans. Le montant des
investissements hors creation de serres s'eleve seulement a 1 571
K Ecus.
En 1987, les charges d'exploitation (electricite, P2 + P3 forage,
remboursement d'emprunt, taxes et frais de gestion) s'elevent a
223 K Ecus a comparer avec une economie de 653 TEP (7592 Mwh).
Le coOt de la TEP geothermique a Lamazere est donc d' environ 342
Ecus, son prix de revient actuel, sans tenir compte des investissements
est de 158 Ecus ou encore de 0,014 Ecus/Mwh.
Cette operation techniquement reussie realise difficilemEmt Ie
petit equilibre car Ie nombre d'utilisateurs previsionnels n'a
jamais ete atteint. Une superficie double de serres raccordees
permettrait meme au prix actuel du gaz naturel, une rentabilite
acceptable de 1 'operation.
SOUTHAMPTON -
GE 69/81 et 607/83 ROYAUME UNI
L'operation geothermique de Southampton etait basee au depart sur
la realisation d' un doublet de forage au Trias devant assurer Ie
chauffage de b8timents publics et la fabrication d' eau chaude
sanitaire pour Ie centre de la cite. Le premier forage realise en
1981 ayant montre une productivite faible et l'existance d'une
barriere de permeabili te dans Ie reservoir, il rot alors decide
apres de nombreux tests hydrogeologiques d' exploi tel' Ie pui ts de
production seul et de rejeter l'eau geothermale salee (125 gil)
dans l'estuaire voisin de la Test.
L'ouvrage existant produit 40 m3/h d'eau geothermale a 72°C en
tete de puits gr8ce a une turbo-pompe installee a plus de 600 m de
profondeur en raison de la faible productivite des sables du Trias
captes entre 1730 et 1800 m.
L'energie geothermique est utilisee par Ie biais d'un echangeur a
plaque de 2 Mw, assurant 60% des besoins du reseau de chauffage
urbain. L'appoint secours est realise par un generateur diesel et
une chaJdiere fuel classique. Dans un futur proche, Ie reseau de
distribution doit s'etendre et la temperature de retour etre
progressivement abaissee avec une pompe a chaleur.
L'operation fonctionne industriellement depuis debut 1988 apres
une annee de tests sans probleme technique et avec des parametres
d'exploitation comparables aux etudes previsionnelles.

705
L'investissement total y compris les ~quipements de production,
r~seaJ de chauffage urbain et station conventionnelle exclus, a
H~ de 2120 K Ecus et assure une production annuelle de 16 200
Mwh. Les coats de maintenance et d'entretien et d'~nergie ~lectrique
sont ~valu~s A 172 K Ecus.
Le temps de retour brut en tenant compte d'un financement du
projet sur 20 ans A un taux d'emprunt de 10,5% est de 7 A 8 ans.
Le coOt du Mwh g~othermique ressort A 0,026 Ecus A comparer au
coOt actuel de gaz en Grande Bretagne qui est de 0,028 Ecus/Mwh.
Ce projet qui contribue A une ~conomie d'energie de 1 400 TEP, est
donc un succes ~conomique qui devrait s'am~liorer dans Ie futur
avec Ie raccordement progressif sur Ie r~seau de nouveaux utilissteurs.
La rentabili~ de cette op~ration aurait ~t~ bien moins
bonne si la foration d'un puits d'injection avait ~t~ n~cessaire.
LATERA -
GE 29/81 et 116/83/ITALIE
Le champ geothermique de Latera se situe A l'ouest du Lac Bolsena
dans la partie Nord du Latium. Les recherches g~othermiques ont
commence en 1970 dans cette zone et un champ A eau dominante a ~t~
mis en evidence par ENEL. Les puits produisent dans cette zone un
melange d'eau et de vapeur A 190°C en surface.
Sur les nombreux puits r~alises, deux sont actuellement utilis~s :
L 3D (producteur) et L2 (injecteur). Ils captent des formations
carbonat~es s' ~tageant entre 500 - 1500 m de profondeur recouvertes
~ r les laves et tuffs des volcans de Bolsena et Latera.
Des phenomenes de d~p8ts-incrustation de CaCo3 ayant ~t~ observ~s
lors des essais, une injection d'inhihiteur en fond de puits est
realiB6e en continuo La production (450 t/h) est utilis~e apres Ie
separateur dans une turbine de 4 Mw. Le puits injecteur est situe
A 2 km et Ie fluide diphasique est d'abord mis A pression atmospherique
avant d 'Etre pomp~ vers Ie pui ts d' injec tion.
La zone est equipee depuis 1979 par 9 stations micrOB~ismiques qui
connect~es A la station de !%,oduction d '~lectrici t~ permettent
d'arr~ter l'injection si un microsHsme est enregistr~. L'op~ration
fbnctionne en continu depuis octobre 1985 sans problemes
techniques et sans avoir decele une activite s~ismique anormale.
Les investissements ont ~te d'environ 3 200 K Ecus -(1 800 pour Ie
forage de production et d' injection et 1 400 K Ecus pour les
installations de surface).
La production annuelle d'~lectricite d~livr~e sur Ie r~seau est de
12 000 Mwh contribuant A economiser environ 2 600 Tonnes d'~quivalent
petrole. Le temps de retour brut de 1 'op~ration est de 8
ans.

706
Le coat du Mw
electrique produi t par la centrale de Latera peut
@tre estime a environ 30 Ecus et donc competi tif par rapport au
coOt moyen du courant electrique sur Ie reseau italien qui est de
27 a 33 Ecus.
La construc tion d' une centrale de 15 Mw est prevue en raison des
bons resultats obtenus sur cette uni te experimentale de 4,5 Mw.
BORDEAUX-PESSAC -
GE 46/81/FRANCE
L'operation geothermique de Pessac visait au chauffage et a la
fourni ture d' eau chaude sani taire pour environ 1 500 logements
collectifs chauffes par panneaux de sol et utilisant Ie fuel comme
combustible.
La nappe exploitable a l'aplomb de la region bordelaise appartient
a un ensemble de multicouches du Cretace inferieur qui contient
une eau faiblement mineralisee ce qui permet son rejet a l'egout
apres utilisation. Le forage de 1 085 m de profondeur realise en
1982 a rencontre les formations sableuses prevues et produit 150
m3/h d' eau a 48°C avec une pompe elect rique immergee pilotee par
un variateur de frequence.
La geothermie assure 100% des besoins jusqu 'a une temperature
exterieure de + 5°C. Deux echangeurs a plaque de 2 700 Kw et deux
pompes a chaleur d' une pui ssance totale de 475 Kw permet tent
l'exploitation et Ie rejet de l'eau geothermale a 28°C.
Les installations de surface ont ete realisees en 1982 et l'opera
tion fonctionne depuis c inq saisons de chauffe sans problemes
techniques particuliers.
L'investissement total (1982) de 2 600 K Ecus a ete finance par
des prets (1 400 K Ecus) et par differentes subventions regionales,
nationales et europeennes.
Le coOt d'exploitation annuel incluant (les combustibles, la
maintenance et 1 'entretien des installations) est passe de 690 K
Ecus sans geothermie et pompe a chaleur a 220 K Ecus des la
premiere annee de fonctionnement contribuant a une economie annuelle
brute de 470 K Ecus.
La charge financiere annuelle (remboursement des emprunts) etant
de 220 K Ecus, 1e temps de retour reel de l'operation est de
l'ordre de 10 ans et de seulement 5 ans pour les utilisateurs en
tenant compte des subventions.
Ce projet techniquement et economiquement satisfaisant permet
annuellement une economie d' energie de 1 'ordre de 1 500 tonnes
equivalent petrole, il montre que me me dans une zone climatiquement
privilegiee, la solution du chauffage geothermique conduit
a une bonne rentabilite economique meme dans l'environnement
energetique concurrentiel actuel.

707
Conclusions
Ces cinq projets de demonstration en fonctionnement montrent des
rentabilites economiques tres variables (8 A 17 ans) sans tenir
compte des subventions dont ils ont pu beneficier. Les parametres
in ternes influencan t ce tte rentabil i te sont spec ifiques A
chaque operation et donc une regIe d' ensemble es t diffic i Ie A
etablir.
Les performances enr~istrees sont satisfaisantes et elles
contribuent A une economie d' energie prima ire d' environ 7 000
TEP/an au bilan energetique de la CEE. L'investissement par TEP
econcmisee est ici en moyenne de 2 200 Ecus, ce chiffre parait
eleve, mais doit i!tre apprecie en fonction des structures de
coOt particulier de la geothermie.
Un des points significatifs et encourageant pour toutes ces
operations est la fourniture sur Ie marche d'un kwh geothermique
Ie plus souvent competitif par rapport aux aut res sources
d'energie soulignant ainsi les possibilites de developpement de
ce type d'operation ou les coats d'exploitation faibles, s'ils
sont bien maitrises, contrebalancent les investissements lourds
au depart.
EVOLUTION ET PERSPECTIVES
m======~=================
L'energie geothermique peut @tre utilisee dans des applications tres
vari6es, elle contribue en 1989 pour 0,8 AIM TEP un bilan l!nergetique
communautaire et represente 0,2% de la production d' energie
primaire dans la CEE.
La geothermie ainsi que les autres sources renouvelables d' energie
traverse actuellement une pliriode delicate en raison de la baisse
importsnte du coOt des combustibles conventionnels.
Les parametres techniques de chaque operation ont une importance
considerable, ils conditionnent tres fortement la rentabilite economique
de la geothermie et si l'on a difficilement une influence sur
les caracteristiques d'un reservoir geothermique, on peut en revanche
tirer Ie meilleur parti de cette ressource des lors que les investissements
lies A la mise en production ont ete realises.
Une optimisation des ope-ations doit donc s'effectuer
- au niveau de la conception car il est indispensable de privilegier
les utilisations en cascade et les applications complementaires
qui permettent une duree annuelle d'exploitation plus longue et
une substitution d'energie plus grande sans investissement
SOU!l-BOl supplementaire.

708
- au niveau de l'exploitation, ou il faut ameliorer les performances,
(l'optimisation de l'exploitation des doublets au Dogger en lIe de
France a conduit par exemple a ameliorer en deux ans de 20% les
taux de substitution pour atteindre 90% en 1987), et egalement
mettre en place des sui vis d'exploitation rigoureux et des analyses
periodiques de fonctionnement pour maintenir les coats de maintenance
et d'entretien qui peuvent venir penaliser la rentabilite des
opa-a tions.
Au niveau de la ressource, les meilleures operations existantes sont
celles ou la reinjection du fluide n'est pas necessaire, c'est donc ce
type d' operati ons geothermiques (pui ts un ique dans de s zones a fort
gradient et eau douce) qui peuvent aboutir a la reduction des
investissements initiaux et donc a une meilleure rentabilite des
operati ons.
II faut noter egalement que la couverture du risque mlnler initial qui
reste un facteur limi tatif au developpement de l'exploitation de la
geothermie necessi te des struc tures d' investissement adaptees qui ne
sont pas encore en place dans toute la CEE.
Au ni'leau du marche existant, en basse enthalpie, Ie debouche principal
de la geothermie reste Ie chauffage urbain (plus de 80%), mais
la competition avec les autres sources d' energie y est forte et
l'efficaci te de la geothermie rendue difficile dans la me sure ou Ie
dimensionnement du reseau n'est pas toujours adapte et en raison de
l'absence d' un pilotage fin indispensable pour bien gerer l'exploitation
de systemes aussi specifiques.
Pour la haute enthalpie, il n 'existe pas de probleme particul ier,
ecooomiquement Ie coat du Kwe dans la CEE n'a pas subi la m@me
reduction que les combustribles traditionnels maintenant ainsi la
bonne competitivite de l'electricite geothermique. La seule limitation
est liee a l'absence dans certaines zones (les rles par exemple) d'un
reseau permettant d'acheminer Ie courant electrique produit.
La geothermie a neanmoins des obstacles varies a franchir pour
continuer son developpement dans un contexte concurrentiel tres difficile
depuis la fin de 1985. Au plan financier, la couverture du risque
geologique est indispensable dans les zones d'exploration et il faut
nettre en place des emprunts a long terme dont Ie taux et la duree
sont actuellement des facteurs trop contraignant pour les operations
geothe rmiques.
En geothermie les temps de retour moyen sont de l'ordre de 10 a 12
ans, mais l'on doit tenir compte de sa facilite d'integration au
niveau du milieu ainsi qU'a son efficacite prouvee dans la lutte pour
la defense de l'environnement. Dans ces conditions la geothermie
represente dans certaines reglons de la communaute une source
d' energie localement importante qui peut ~tre developpee a un coat
compet i tif •

7~
BIB L lOG RAP HIE
==================
A.F.M.E. (1983) - Le guide du maftre d'ouvrage en geothermie - Manuel et
methodes - n08 - 186 p.
Anrstead H.C.H (1981) - "La geothermie, exploration, forage, exploitation"
- Edition du Moni teur - 380 p.
Bakelolell, C.A. (1979) - Low temperature, direct use geothennal energy
costs - Geothennal resources council transactions, vol. 3 - p. 23-26.
Barbier, E. (1987) - Geothennal energy in the world energy scenario
Geothermics, vol. 15 - nO 5-6 - pp. 807 - 819.
Barbier, E. (1985) - Geothermal energy in the context of energy in
general and electric power supply - National and International aspects -
Geothermics, vol. 14 - nO 2-3 - pp. 131 - 141.
Barbier, J. (1986) - Les ressources geothermiques basBe enthalpie dans
1es pays membres de la Communaute Europeenne - Rapport CEE DG XVII -
99 p.
Boisssvy, C. (1987) - Le programme de demonstration de la CEE en energie
geothermique, Geothermie Actua1ites - vol. 4 - n04 - pp. 30 - 34.
Book, N.L, L.J. Groone, C.A. Bakewell and E.H. Herron (1981) - Economics
of low te'lperature, direct use application of geothennal energy - Energy
- Vol. 6 - pp. 317 - 322.
Cataldi, R., C. Sommaruga (1986) - Background, present state and future
of geothennal development - Geothermics - Vol. 15 - n03 - pp. 359 - 383.
Cataldi, R. (1987) - Exploitation de l'energie geothermique dans Ie monde
- Situation actuelle et perspectives d'avenir - Impact:
Science et Societe - Unesco, Edition Er~s - nOl48 - pp. 347 - 361.
CEC.DG.XVII (1988) - Energie en Europe - nOlO - 84 p.
CEC.DG.XVII (1988) - Commlnity demonstration program in the sector of
geothermal energy - 33 p. Sesame sheets.
CEC.DG.XVII (1983) - Economics and optimization of geothennal district
heating in EC Member States - 69 p.
CEC (1987) - Bulletin of energy prices - 85 p.

7\0
Gudmunson, J.S., G. Palmason (1981) - Wor1 survey of low temperature
geothermal energy utilization - Geothermal division report.
Gudmunson, J.S., (1982) - World users of low temperature geothermal
resources in 1980 - G.R.C. - Vol. 6 - p. 441 - 444.
Gudmunson, J.S., (1985) - Direct use of geothermal energy in 1984 -
Geothermal resources council - pp. 3 - 18.
Gerini, G., et C. Boissavy (1988) - Le programme de demonstration de 1a
CEE en geothermie - Jigastock transactions - vol. 1 - pp. 9 - 17
Glen, E.M, J .W, Tester and G.A. Graves (1980). Economics of geothermal
energy. American Nuclear Society-Topical meeting transactions.
Harrisson, R. (1983).
developments. UK costs.
241-246.
Cost modelling of low enthalpy geothermal
European Geothermal update. Ed. D. Ridel. pp.
Lema1e J., and M. Pivin (1987). La fi1iere geothermique, premier bi1an.
Deuxieme edition. AFME-80 p.
Lott, X. (1985) La rentabi1ite financiere de 1a geothermie. Jiga transactions.
pp. 85-91.
Towse, D. (1976) Economical Geothermal heat as an Alternate fuel.
transactions of AIME. Vol. 260-pp. 322-326.
Tribou1et, A. (1988). Resu1tats techniques et economiques de deux operations
en exploitation it Paris et it Ivry-Sur-Seine. Jigastock transactions.
Vol 1. - pp. 475-479.
Varet, J. (1984) Le coat de 1 'energie geothermique. Hydrogeologie et
geologie de l'ingenieur, nO 2. pp. 145-160.
Weissbrod, R. and W. Baron (1979). Modelling the impact of resource and
economic conditions on the competitiveness of roo dera te temperature
geothermal energy resources. Geothermal Resource Council transactions,
Vol. 3.- p. 773-775.

CO

712
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~,O
COUTS UNITAIRES EN
FONCTION DU SKIN ET DE
Ll TRANSMISSIVITE
-1,0
-0,5
~ 20dm SO
.,~ 5...-7 40
••••• •••". 30 C ha1eur en Mwh X 103
•••••
..
~ ••••••'- 100dm 20
,
CHAUUR PRODUITE
lO
XJO ISO 200 m3/h
Figure 5
COUTS UNITAIRES EN
Cout en Ecu X 1(y2 kwh
• 2.0
FONCTION DE LA CHARGE
ET DE LA TEMPERATURE
DE
RETOUR
L-Facteu. de cha.ge
0,5
L = 1
~" .... , ...... .
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"TRENDS or REAL PRICES or ENERGY
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714
Figure 7
EVOLUTION DES TAUX OOEMPRUNT
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( Eurostatis tique 19m)
Amee

715
OBSTACLES AND RECOMMENDATIONS TO PROMOTE GEOTHERMAL ENERGY DEVELOPMENT
R. CARELLA
AGIP S.p.A., Milan, Italy
Summary
Geothermal energy is a relatively abundant, local resource with
generally limited environmental impact. The more widespread water
resources with a temperature of 100 0 or less can be utilized for
heating purposes. When in certain areas (Italy and Greece in
mainland EC) temperatures exceed 150 0 , electricity can be generated.
Electricity production from geothermal energy compares cost-wise with
traditional fuels but, because of low energy prices, non electrical
projects are presently less appealing. In the long term however
geothermal resources deserve to play a role. To achieve full
commercial penetration, especially for non electrical applications,it
is however necessary to maintain availability of financial and other
forms of support both from governments and EC, with continued R&D.
After description of the main obstacles encountered, some
recommendations are hereunder presented.
1. DEVELOPMENT STATUS
Except for electrical uses in Italy, the development of geothermal
industry in EC, like the start-up phase of any new business, has been slow
and market penetration presents many difficulties.The support and
financial backing by some governments and by EC has played an essential
role in helping to demonstrate the viability of geothermal technologies.EC
support has been organic and long-lasting; up to 1987 funding amounted to
almost 60 million ECU (out of a total of 225 for alternative energy
sources) •
Current situation and short term prospects (Table I). In 1986 use of
geothermal energy worldwide was more than double the use of Bolar and wind
energy combined.
Even if electrical capacity of geothermal plants at
represents only 0.25% of the world total, it corresponds to the
about 5 million people. In some countries (El Salvador;
Phillippines) it contributes an important percentage (up to
their needs.
5020 MW
needs of
Nicaragua;
25-30%) of
Within EC only Italy has a large capacity, both already installed
(518 MW) and under construction or firmly planned in the near term (over
300 MW). It represents the fourth position in the world, after having been

716
for many decades the first. Concerning the rest of Europe, Iceland has an
established position with 39 MW which will not rise substantially in the
near future. The interesting potential of Greece and Turkey (which have
only few MW installed) will be reflected in some new small-sized plants
but will not have in the short term any significant impact on the
countries' electricity production. The situation in the rest of the world
is quite differentiated. At present USA dominates the world market (over
2200 MW), followed by Phillipines (894 MW) and Mexico (655 MW).
For what concerns non-electrical uses in EC, they are developped
almost exclusively in Italy and France. In France district heating is the
main application. In Italy health resorts and agricultural uses are
prevalent. In the near term the main activity in Italy will be focused on
district heating and in Spain on agricultural applications and
subordinately district heating; Greece will develop some agribusiness
projects. Elsewhere some demonstration plants will be built.
In the rest of Europe, Iceland and Hungary have large direct
applications of geothermal heat.
Non electrical uses in the rest of the world are evidently quite
differentiated; the predominant role of Japan must be mentioned (over 65%
of the total, almost exclusively used in health resorts).
The contribution by geothermal energy to world non-electrical heat
requirements, amounting to 4.6 million TOE/year (1) is very modest (about
one thousandth of world needs).In some countries however its role is
important (85% of greenhouses for vegetables and 45% of greenhouses for
ornamental plants are heated by geothermal water in Hungary). By
comparison roughly 20 million TOE/year are provided by other renewable
resources (excluding fire-wood) of which 14-15 from biomass, about 4 from
solar and 0.4 from wind.
2. OUTLOOK FOR THE FUTURE
Renewable resources, including geothermal energy, have a valid
potential and a role to play in the long term, taking into consideration
their local character, relative abundance and generally limited
environmental impact and considering the available energy scenarios which
assume that beyond the year 2000 the classical (fossil) fuels may not be
able to meet all the world needs and will be costlier.
It must be stressed that geothermal heat use, by substituting fossil
fuels,is an efficient way to reduce noxious emissions in the atmosphere.
It is therefore a very useful instrument in defense of the environment.
For what concerns the present and the near future, development of
geothermal energy, as of other renewable resources, is influenced in a
negative way by the low prices of energy. Such influence is felt strongly
in projects for non-electrical uses in EC, which have come virtually to a
(1) With a minimum reference temperature of 15°C;using a more conservative
lower limit of 35°C the amount is reduced to 2.5 million TOE/y.

717
stand-still in France (which was the country where these applications had
made a relevant market penetration). Geothermal electricity production
up to now has suffered only marginally, due to its being generally
competitive in Italy where most of the capacity has been developed.
Renewable resources are sometimes seen with scepticism because of an
excessive initial optimism on lead time required for getting technologies
to maturity and on cost of production; their ability to penetrate the
energy market was also overestimated.
It must be taken into account however that, contrary to what happens
for many other renewables, geothermal energy technologies are sufficiently
mature and that costs (Table II) are often much lower than those of other
renewables, being in fact of the same order of magnitude as those of
traditional fuels in the field of electricity generation. One can thus
expect a certain growth in the near term, especially for high temperature
resources. Progress in the non electrical heat will be slow, but with
possibilities of market penetration especially where frontend costs can
be reduced and incentives exist.
3. STATE OF THE ART & OBSTACLES
TECHNOLOGY
The state of advancement in the geothermal sector varies according to
the areas considered: in general the field of hydrothermal resources
(where heat is extracted from hot natural waters produced through wells)
is mature with improvements, not very difficult to attain, needed for what
concerns surface equipment and facilities and progess required to better
locate the resources.
The non hydrothermal resources (whereby heat would be extracted from
the underground hot rocks by means of water artificially injected therein)
need a strong and substantial R&D effort, especially on the production
side, to became mature.
HYDROTHERMAL RESOURCES
The main problem on the resource side is to increase the probability
of finding in the underground adequate permeability and hence good
productivity. Optimal location of wildcat wells is particularly difficult
in the case of fractured reservoirs (common in high temperature fields).
Whilst location of hot areas is generally easy when the anomalies are
superficial, there can be problems in finding deep hot spots, for example
when a substantial flow of cold water affects the shallower formations.
The lack of detailed knowledge on local extent and thickness of
pays and on flow-rates and chemical composition of fluids hampers the
development of projects for non-electrical uses.
As concerns production of geothermal fluids the main obstacles
encountered have been:
- difficulty in following up the productive pays when permeablity is due
to fractures or is very variable laterally;

718
problems in fracturing successfully in case of high temperature
reservoirs;
- insufficient flow rate and inadequate injectivity in sandstones (which
are a common aquifer for low-enthalpy fluids) when reservoir is deep.
On the equipment side, down-hole pump failures are not uncommon in a
high temperature environment and corrosion in presence of aggressive fluids
influences operations both in the field and in the surface down-stream
facilities.
For what concerns power plants, problems arise in case of a
corrosive nature of impurities in steam which may cause fatigue or stress
cracking in turbines and possible scale formation.
In relation to the binary cycle type of generators, not enough
experience is available on large-scale plants.
Non-hydrothermal resources
These are the resources that in the long term (beyond 2000) may
allow a substantial increase of available geothermal reserves, provided
the presently existing technical problems can be solved and the price of
traditional fuels increase.
Many nations are making an effort in the field of hot dry rocks
(USA, Japan, Great Britain, France and Germany) with local government
funding and lEA and EC support.
Hot dry rocks R&D is multidisciplinary and requires the cooperation
of industry which has the know-how in some key sectors of research and can
field test the results of laboratory research, possibly on existing dry
but hot geothermal wells. Industry could benefit both in the specific
geothermal field and in oil operations where targets are getting deeper
and thus hotter and where fractured rocks are often the reservoir.
The field of hot dry rock R&D is very broad and includes all the
topics previously discussed for improving productivity in hydrothermal
systems and covers reservoir physics and modelling, heat transfer problems
and chemistry of injected and produced fluids, microseismics, etc.
ENVIRONMENT
High enthalpy geothermal resources are often located in protected
areas (national forests, volcanic parks, etc.) where operations are
severely limited or forbidden.
In some countries where spas are very popular (Japan; Italy, for
example Abano) or where hot springs are a strong tourist attraction (New
Zealand) proximity between thermal springs and geothermal fields causes
worry for the depletion of the springs by production from geothermal wells
and thus precludes or limits further industrial development.
Heat use of fresh warm waters must take into account priority uses
(for example for drinking and/or irrigation purposes).
In some unstable areas microseismic events are occasionally
associated with reinjection of geothermal fluids in the underground and
are being checked by timely monitoring.

719
ECONOMICAL
The need for high front-end capitals (be it in the exploration or
in the production stage) can limit growth of geothermal business.
Sizable investments are in fact needed before construction of the
plant, mainly to pay for the one or more wells needed, which must undergo
extensive testing and field development requires the drilling of
reinjection wells when the fluids are saline. A number of dry holes may
have to be drilled and reinjection wells are often needed.
Back-up equipment for peak load or for emergencies increases costs
of plants.
MARKET
When considering the future of the geothermal business one has to
remember that the average size of a geothermal power plants is to 10-50 MW
and that the largest and unique field (Geysers) in USA does not exceed
2000 MW (the size of two typical nuclear or fossil fuels plants); the
capacity of a non-electrical geothermal plant corresponds to savings
between 100 and 5000 TOE/year.
It is thus evident that one cannot generally expect a significant
impact on the overall energy market sizewise, but rather a "niche"
penetration through dissemination of several small specialized projects in
the most favourable sectors.
Penetration problems arise especially in the non-electrical sector.
District heating is dependent on development of distribution networks and
these are mainly high-temperature and thus adapted to use of fossil fuels,
cogeneration and biomass incineration rather than to geothermal heat.
Installation of low-temperature heat networks will be also difficult where
an efficient gas distribution system is in place (Holland, Italy) or if
preference for individual heating systems will prevail. Competition from
alternative heat sources will be strong also in the agribusiness field.
As indicated in an evaluation report to EC (1) replication is
hindered by the fact that most of the activity is carried out by
municipalities which have not the scope to commercialise and that there is
no established geothermal industry, the development of which is considered
vital for a full exploitation of the potential in EC.
For I mlrket to develop for this relltively new resource, which
has yet limited applications with few years of operative experience,
information and formation is critical. The public is not generally aware
about the nature, importance and availability of the geothermal resource.
Very little is also known about the potential uses and applications as
well as the relevant benefits and problems. Too often information on
available financial support is also laking.
Training progra.s are very limited.
(1) P.Caprioalio - "Evaluation of EC energy demonstration programme"­
June 1988.

720
Finally there is little integration
producers, distributors and end-users and
business available is often too heavy.
between geothermal fluid
competion for the limited
4. RECOMMENDED ACTIONS
The suggestions put forth herewith are related to the problems and
obstacles encountered and take into consideration the present state of the
art.
Actions are needed at the government and EC level:
in the short term to ensure further development, within realistic
limits, of hydrothermal resources;
in the long term to ascertain if it is possible to
hydrothermal systems, which would allow a substantial
available exploitable reserves.
4.1 SHORT TERM
exploit nonincrease
of
TECHNICAL ACTIONS
Looking to the technical side the following actions have
priority:
A) Carry out and up-date an inventory of the resources which can be used
realistically within the framework of the present and short/mid-term
energy market in a given country.
A geothermal assessment must thus consist in a geological, thermal
and hydrogeologic study; with different degrees of detail,it has been
carried out by EC at the Community level as well as by certain countries
(Italy, France) at the national level.
Even if such an inventory is essential, it is not sufficient and it
must be completed with an assessment of existing applications of
geothermal energy and potential users, taking into consideration climatic,
market and other constraints.
Coupling resource data and information on existing and potential
users will allow to establish the location and framework within which the
resource can be practically used.
Standard technical solutions and economical prefeasibility studies
will define possible alternatives to be taken as model by interested
parties.
For what concerns new investigations, which are essential to ensure
progress towards the optimization of the use of the resource, there is
need to assess previously unexplored areas by surface surveys and
especially by drilling selected deep information wells.
Another very useful activity is the testing of abandoned wells (for
oil and mineral exploration, geognostic, etc) in order to obtain specific
data on flow-rates and other physico-chemical parameters of the fluids.
For non-electrical uses such actions are essential because
variations in characteristics of the reservoir and the waters are common
and the potential users need to know the situation on, or as near as
possible to, the possible location due to the fact that the resource must

721
be used in place.
B) Techniques for locating fractured reservoirs in high enthalpy
geothermal systems must be studied and experimented; finding hydrothermal
fluids with commercial flowrates is of immediate interest and represents
the main problem to be solved in order to reduce within reason the risk of
geothermal exploration. Several possible avenues could be studied; one
interesting approach is the use of induced or possibly natural
microseismic monitoring.
C) On the equipment side one of the main problems is the control of
corrosion and scaling.
The following factors should be given attention:
- use of the most appropriate down hole and surface material to withstand
aggressive fluids (particularly more rugged pumping systems);
- use of cost-efficient inhibitors and other techniques for mineral
precipitates prevention or disposal.
Careful and timely maintenance of the equipment will also be
necessary. In particular in power plant operations attention should be
given to increased reliability by adequate initial material selection
and by frequent in-service inspection, careful washing if needed,and
metallurgical check-ups.
For what concerns large binary plants, demonstration could provide
needed practical advances.
Especially for what relates to agricultural uses, one must develop
and use efficient, low cost, reliable and simple systems, which can be
easily operated by farmers. Research on this aspect is being carried out
in many countries in the Mediterranean area (France,Italy, Israel).
OTHER RECOMMENDATIONS
Aside from the technical actions indicated above, many more aspects
must be addressed to:
In the critical field of non-electrical uses there is need to put
the accent on projects which rely on the cheapest available geothermal
heat.
For example: tapping shallow hot aquifers (like some agricultural
projects in Italy and Greece); using heat available donwstream of
geothermal power plants (greenhouses of Mt. Amiata in Italy) or
recoverable from abandoned dry holes or depleted oil and gas fields
(agricultural applications in Rodigo and laundry drying in Villaverla,
both in Italy); drilling shallow wells in spa areas, thus increasing the
availability of hot waters for new uses (Bagno di Romagna, again in
Italy).
Another approach is to upgrade the value of geothermal fluids by
trying to promote joint sale of heat and a more rewarding application.
For example spa waters can heat hotels and other buildings and be
used for bathing and mud treatments, as well as be an ingredient of
cosmetics. Fresh warm waters can sometimes be so pure as to have the

722
potential, after extracting heat, to be sold as drinking water (as
indicated by analyses of Vicenza waters in Italy) or as waters for
irrigation (as in Southern Tunisia and in Israel).
Apart from optimization efforts, there is need for close coordination
between fluids producers and down-stream plants operators (or better for a
single management).
To ensure initial penetration in the energy market,the following
economic support actions are needed:
- provide for an insurance guarantee for dry holes drilled both for low
temperature fluids (as is the case in France, Italy and Switzerland) and
for rank wildcats targeted to high enthalpy resources (following the
Japanese example);
- cover the costs of the inventory, including the drilling of information
wells and the testing of dry holes drilled for other purposes;
- ensure that the price paid by the utility to the fluid producer is
equal to the cost of the higher priced fuel substituted and possibly
take into account hidden social costs of other electricity production.
the construction of new utilization plants must be stimulated with
financial non refundable contributions (following Italian example)
and/or granting loan facilities and/or favourable tax terms; such
actions to be maintained until adequate market penetration is
achieved;
- it must be ensured that no disparity in tax treatment hurts
geothermal development.
On the practical side, government agencies should set an example in
promoting geothermal energy uses by adopting such systems when possible in
public properties.
Development of a specific geothermal industry, bringing together
know-how from other business sectors is essential.
Information and planning will playa critical role in ensuring
adequate development of geothermal resources.
At the EC level there should be established a professional
association to work-out, express and promote a common interest position.
Nation-wise, dissemination of data and information on the amount of
the resource, its location, quality, possible uses and cost of projects,
as well as on available funding and financing is essential.
A very useful approach has been the French example, whereby a special
government organization has been entrusted the responsibility of diffusing
information and of evaluation of projects which ask for support.
Monitoring of construction of the same, ensuring follow-up of operations
and proposing and testing solutions for problem areas have been recent
developments, critical for a healthy development of geothermal business.
Training programs may help in forming technical personnel and in
management of plants.
Another critical requirement is that the legal steps necessary to
develop a geothermal project (especially non-electrical)must be simplified
as much as possible both for exploration and exploitation of the resource
and for down-stream operations. Again France is a good example.

723
While some actions will be best taken at the regional and lower
local level (information, specific planning, project implementation) other
(broad energy choices, financial support, research on major operating
problems, etc.) should be centralized. Some problems will have to be
tackled at one or the other level, according to their importance and the
policies of the country involved.
Even if in the medium term a certain market for geothermal energy may
develop in Europe (geographically more extensive for what concerns low
enthalpy resources; more concentrated but quite important for electricity
generation) ,know-how and specialized personnel may better be employed if
one will take advantage of business opportunities abroad,where geothermal
prospects are numerous and diversified (see table III).
It will be however necessary to this purpose to provide in all or in
part financing for the foreign geothermal projects (with national or EBI
development funds and possibly with EC support).Activity abroad will be
favoured by a concertation between the European firms interested in such
markets.
In the set of actions indicated above EC will have an important role
to play in ensuring support, dissemination of information and harmonizing
of policies.
LONG TERM
The long term actions concern mainly investigations on exploitation
of resources alternative to the classical hydrothermal systems, that is
mainly hot dry rocks (HDR). The R&D effort should enable us to verify if
it is possible to operate technologically and economically HDR systems or
if geothermal exploitation will be limited to natural hydrothermal
resources.
Notwithstanding the fact that two projects to HDR projects are
already under way in Europe (one British, one Franco-German) no research
is dedicated to shallow really hot rocks (above 250 0 C), the only ones that
have a chance of commercial development even if in the long term. Such a
research could be carried out with the help of industry and with
government funding and EC support in Italy where geological conditions are
favourable and there is a specific know-how in operations at high
temperatures.

724
TABLE I
GEOTHERMAL USES (1987)
=============~========
ELECTRICAL CAPACITY (1)
(l4We)
Installed
capacity
!
Capacity under
construction or
in advanced
plannina (+)
EC EUROPE (and overseas territories) 529 315
_~!~hi~~_!ta~!_______________________ t-______ (5_1~~_____ 1 __.____ ~(_30_5~) ______ ~
REST OF EUROPE (excluding URSS) I 60 I 11
_~!_~~~:~_!~~~~~~ _____________________ L ______ i~~l ______ J _____ (.;...0...,;) ____ -4
REST OF THE WORLD ! 4431 I 1178
of which USA I (2212) I (436)
__________ :~~~~ _____________________ _+-----:2~5}----___t--. ____ ~(~1~38~) ______ ~
TOTAL I 5020 I 1504
3 I I
10 TOE/year 6800 2000
I
I
I
•
(+) - Di Pippo estimates that 75% of the projects could be on line by 1992.
(2)
NON ELECTRICAL USES
(loNt)
I Total Space I Agriculture
I
I
heating land aquacul ture
I
I
I I
Health
resorts
I I I
Other
EC EUROPE I 1325 720 I 202 376 27
of which Italy I (651) (113) I (135) (376) II (27)
I
I
----------~~~~:~--+--~~~~l...-+--.!.~~l...----+-----~~l...-----t---..:(-3~)--4-~( 0"";)-1
REST OF EUROPE I 3652 I 1138 I 931 I 1031 I 552
(excluding URSS) : : : I
I I I I
of which Iceland I (1306) : (945) : (77) : (209) (75)
Hungary: (1540): (75) : (565) I (581) 1(319)
------------------+---------~------------~--------~-----~--~~~--~~--~
REST OF THE WORLD:
I
8083 :
I
1105 :
I
825 :
I
5145 I 1008
of which USA : (1198) : (333) : (148) I (157) I (560)
Japan : (4764): (49) : (50) : (4475) 1(190)
------------------.---------~------------~----------------.--------.---+~~~
I I I I I
TO~AL : 13060 : 2963 : 1958 : 6552 I 1587
10 TOE/year (_)! 4590 : 1100 : 590 : 2200 ! 700
• I I I I
(1) - Di Pippo "International development in geothermal power production"
ASME-GRC Geothermal Energy Symposium, 1988 (modified)
(2) - Sommaruga, unpublished, 1988 (modified). Temperatures above 15°C for
health resorts used and actual temperature for other uses.

Energy cost
(S/KWh)
100.00
TABLE II
CO"PARATIVE ELECTRICITY PRODUCTION COSTS
10.00
Gasoline
1.00
0.10
0.01-'-----,-------,--------..-------
100W 10KW 1MW 100MW
Rated capacity
B. van der Toom "Renewable Energy - A Global View" Shell paper, 1987 {modified}

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KEY TECHNOLOGY REQUIREMENTS
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4 . Explored Fields Possibly Ready for Production Drilling
S. Proven Fi elds Undergoing Production Drill i ng, Plant Construction or Oper ~ t ion •
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727
IBDU or ADTBORS
ALBERT-BELTRAN, J.F., 595
ALLEGRINI, G., 630
ALTHAUS, E., 385, 395
ANDREUSSI, P., 576
ANDREWS, J.N., 363
ANORITSOS, N., 74
ANTONELLI, G., 119
AOUBOUAZZA, M., 439
ATKINS, w.s., 648
BANDA, E. 522
BARBIER, J., 692
BARDINTZEFF, J.M., 532
BARELLI, A., 271
BARTON, K.J., 551
BAUDRACCO, J., 429, 439
BERASTEGUI, X., 522
BERTHOMIEU, G., 243
BILLAUX, D., 232
BOISDET, A., 419
BOISSAVY, Ch., 698
BOSCH, X., 522
BRACH, M., 109
BRERETON, N.R., 213
BROCK, A., 551
BROUSSE, R., 532
BRUECK, P.M., 560
BUONASORTE, G., 98
BURLET, D., 232
CAPPETTI, G., 271
CARELLA, R., 715
CAUTRU, J.P., 419
COLIN, F., 46
CORNET, F.H., 189, 205
CORSI, R., 677
COUDRAIN-RIBSTEIN, A., 444
COULSON, I., 154
CRIAUD, A., 84, 109
CULIVICCHI, G., 10
CZERNICHOWSKI-LAURIOL, I., 84,
419
DALABAKIS, P., 532
DARBYSHIRE, D.P.F., 363
DEGOUY, D., 20
DEHAINE, F., 569
DELIBASIS, N.D., 474
DELLIOU, E.E., 652
DIRVEN, P., 36
DOHERTY, P., 154
DRESSELAERS, J., 36
DUJON, S., 375
EDMUNDS, W.M., 363
ESCOBAR, J.J., 595
ESTEVE, J., 595
EVANS, C.J., 213
FERNANDEZ, M., 522
FIORDELISI, A., 98
FOUCHER, J.C., 419
FOUILLAC, C., 84, 109,419
FRElXAS, A., 522
GAl.ANIS, I., 589
GALLART, J., 464
GARELLI, C., 94
GARNISH, J.D., 3
GERARD, A., 283
GERINI, G., 589
GHEZZI, G., 510
GHEZZI, ~., 510
GIANNlHARAS, E.K., 63
GIJBELS, R.H., 395
GOOSSENS, D.A., 395
GRIESSHABER, E., 407
GROSSIN, R., 569
GUIDI, A., 119
HAENEL, R., 482
HARRISON R., 154
HEEDERIK, J.P. 500
HEUGAS, 0., 253
HIRN, A., 464
HONEGGER, J.L., 56, 84, 419
HUSSAIN, N., 363

728
JOUANNA, P., 243
JOURDAIN, M.J., 46
KAPPELHEYER, 0., 283, 345
KARABELAS, A.J., 74
KARKOULIAS, V., 589
KIRITSIS, Sp., 661
KOUTSOUKOS, P.G., 63
LAGACHE, M., 375
LEHALE, J., 662
LOUWRIER, K., 3
MAHLER, A., 604
MARCHETTI, M.P., 510
MARTIN, J.C., 109, 419
MARTIN, M., 20
MENJOZ, A., 84, 109
MERY, P., 444
MICHARD, G., 375
MINERVINI, A., 576
MINETT, S.T., 154, 170
MITJA, A., 595
MORTIMER, N.D., 154, 170
MOSNIER, J., 205
MOUZA, A., 74
MUNIZ, J., 541
MURPHY, F.X., 560
NARDINI, G., 576
PHILIPPAERTS, J.G., 395
PIATTI, A., 129
PIEHONTE, C., 129
PIJPERS, A.P., 395
ROJAS, J., 109
RUMMEL, F., 335
SANTARELLI, F.J., 253
SAVAGE, D., 363
SCHELLSCHHIDT, R., 351
SCHLEGEL, R., 569
SCHULZ, R., 351, 490, 623
SHEPHERD, T.J., 363
SIDES, A.D., 551
SMOLKA, K., 345
SOHHARUGA, G., 638
STAROSTE, E., 3
SUN FAN, L., 612
SZEGO, E., 129
TAS, H., 36
TIRTADINATA, E., 385
TRAINEAU, H., 56, 532
VANDENBERGHE, N., 612
VERDIANI, G., 638
VIERHOUT, R.M., 500
VINSOT, A., 444
VOULGARIS, N.S., 474
O'NIONS, R.K., 407
OXBURGH. E.R., 407
WERNER, K.H., 490
WOHLENBERG, J., 452
PAGLIANTI, A., 576
PANDELI, E., 98
PARKER, R., 141
PAUWELS, H., 375
PERREAU, P.J., 253
XYLA, A.G., 63
ZUDDAS, P., 375
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