25th International Meeting on Organic Geochemistry IMOG 2011

O-84
Prediction of gas volume and dryness in shale gas systems:
kinetic modeling of kerogen and retained hydrocarbons thermal
cracking in Barnett And Posidonia shales
Françoise Behar 1 , Daniel Jarvie 2
1 IFP Energies Nouvelles, Rueil-Malmaison, France, 2 Worldwide Geochemistry Company, Humble, United
States of America (corresponding author:francoise.behar@ifpenergiesnouvelles.fr)
The aim of the present study is to predict the volumes
of the different sources of gas generated during
kerogen cracking in the Ro range 0.5 to 3.0 and to
add the contribution of generated gas from secondary
cracking of retained hydrocarbon compounds in
source rocks after the main phase of primary
expulsion. For that purpose, two immature kerogens
from the Barnett Shale (US) and from the Posidonia
Shale (Germany) were selected. First they were
artificially matured by closed system pyrolysis in the
temperature range 250-350°C and with durations
between 1 to 216h. Then, for late gas generation,
kerogens already matured at 375°C/24h, were heated
between 400 and 600°C.
The following kinetic schema was optimised (Behar et
al., 2008a) :
Kerogen = non HC gas + HC1 + Asp. + kerogen 2
Asphaltenes = non HC gas + HC1 + Resins + prechar
Resins = HC1 + prechar
HC1 = HC2 + prechar
The optimised kinetic parameters show that the
asphaltenes are as unstable as the kerogen from
which they originate. Their maximum yields are 64 –
65% for the two samples, with those of the resins
being respectively 19 and 28% for the Barnett and
Posidonia. The associated gas generated with oil
does not exceed 5% and is mainly wet. The maximum
yield of late gas is respectively 6% and 4% for the
Barnett and Posidonia kerogens. It is almost pure
methane since the ethane yield does not exceed
0.4% and neither propane and butane are produced.
For the two shale gas kerogens, a full compositional
kinetic model (Behar et al., 2008b) was used to
predict both the mass balance and chemical
composition of the retained compounds after the main
phase of oil generation and expulsion. Then, from this
mass balance, it was possible to predict the gas
volume and dryness during further thermal maturation
for Ro range between 1.0 and 2.7 %Ro. Results are
shown on Fig. 1 for the Barnett shale. In the Ro range
between 1 and 1.5%, the gas dryness rapidly
increases from 65 to 80% with a maximum yield of
4%. Then, an additional contribution of 3% of dry gas
leads to a final gas dryness at almost 90% These
results are in excellent agreement with observed data.
dryness dryness (%) (%)
yield yield (%) (%)
6.0
4.5
3.0
1.5
Gas generation
0.0
1.0 1.5 2.0 2.5
100
90
80
70
60
Ro (%)
Gas dryness
50
1.0 1.5 2.0 2.5
Ro (%)
wet gas
dry gas
Fig. 1: Prediction of gas volume and dryness under
geological conditions in the Ro range from 1 to 3%.
References :
Behar, F., Lorant, F., Lewan, M.D., 2008a.
Elaboration of a new compositional kinetic schema for
oil cracking. Organic Geochemistry 39, 764-782.
Behar, F., Lorant, F., Mazeas, L., 2008b. Elaboration
of a new compositional kinetic schema for oil
cracking. Organic Geochemistry 39, 764-782.
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