The Hengill Geothermal System, Conceptual Model and Thermal ...

Franzson et al.
fracturing related to the SISZ. This thermal mining appears
to be most effective at a depth range of ca 1400-2000 m.
4.5 Hverahlíð Field
The three wells in Hverahlíð field in the southern part of
Hengill also show contrasts in formation and alteration
temperatures as shown in Figures 14-16. Well 21, which is
located near to the only thermal manifestation, shows
consistently higher formation temperatures than the
alteration, while well 26 shows lower formation
temperatures in the three depth levels. Well 36, on the other
hand, shows equilibrium at the depth of the caprock, a
slightly higher formation temperatures at 1000 m depth, but
then a pronounced temperature reversal below 1500 m
depth. It is tempting to suggest that the thermal mining at
the base of the well is related to the most active part of the
fissure swarm as shown in Figure 16, similar to what has
been proposed for Nesjavellir. The higher formation
temperatures in the upper part of the system compared to
the alteration temperatures, however, raises hopes that a
relatively recent heating episode has occurred in that area,
unrelated to the aforementioned volcanic fissures. That
upflow channel has not yet been identified.
Figure 13: Nesjavellir field. A comparison between
formation temperature (Tf) and alteration
temperature (Ta) at ~1500 m depth. Red colour
indicates Tf > Ta, green indicates Tf ~Ta and
blue Tf < Ta.
4.3 Bitra Field
The much lower formation temperatures in Bitra, as
compared to the alteration temperatures, are seen at all
depths except in the easternmost well where the two
temperatures are in equilibrium below 1500 m (Figures 14-
16). It is tentatively suggested here that this very effective
thermal mining is due to the combined permeability of the
fissure-swarm and the superimposed fractures of the SISZ
(c.f. Figure 2). It is also suggested that this thermal mining
continues further west into the eastern part of
Skarðsmýrarfjall.
4.4 Hellisheiði Field
Figures 14-16 show a similar relation between the
differential temperature and the geological features in the
Hellisheiði field. A recent heating occurs within the depth
range of the caprock along the volcanic fissures (Figure 14).
This heating is most pronounce at the southern slopes of
Skarðsmýrarfjall, but is only vaguely detected in wells in
central and eastern Skarðsmýrafjall. It is of interest that
alteration temperatures at shallow levels seem to be higher
than the present formation temperatures in the western part
of the field. The deeper section in Figure 16 shows that a
large part of the Skarðsmýrarfjall has suffered thermal
mining (cooling), and only in a very localized part in
between the 5000 and 2000 year old volcanic fissures do we
still see evidence of formation temperatures higher than
alteration temperatures. The thermal anomaly appears also
to be present in Stóra Reykjafell further to the southwest. In
general, thermal mining seems to be more pronounced in
the eastern part of the Hellisheiði system and it is suggested
that it may be due to higher permeability in that region
compared to the area west of the volcanic fissures due to
Figure 14: Hellisheiði, Bitra and Hverahlíð fields. A
comparison between formation temperature (Tf)
and alteration temperature (Ta) at ~cap rock
depth. Red colour indicates Tf > Ta, green
indicates Tf ~Ta and blue Tf < Ta.
Figure 15: Hellisheiði, Bitra and Hverahlíð fields. A
comparison between formation temperature (Tf)
and alteration temperature (Ta) at ~1000 m
depth. Red colour indicates Tf > Ta, green
indicates Tf ~Ta and blue Tf < Ta.
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