Jökull - 01.12.1987, Side 28

during its passage flashed into steam and then con-
densed again and reached a new equilibrium with the
surrounding rock. The Suðurhlíðar fluid is most clearly
so characterized but examples of this are also found in
the Leirbotnar field and the calculation to obtain the
composition of the lower fluids in well KJ-22 in Hvít-
hólar reveals a similar concentration. The chloride con-
centration is thus expected to be considerably higher in
flows derived from a liquid fraction remaining after such
boiling and such fluids are thought to constitute those of
Hveragil and the upper part of the Hvíthólar reservoir.
In some cases anomalously high fluoride concentra-
tions have been recorded. These are reported here as
F/Cl ratios and are apparently related to aquifers which
occur at the interface of acid rocks and basalts. These
are characteristic of the northern Leirbotnar and Suður-
hlíðar fields.
Geothermometry temperatures are generally in good
agreement with measured downhole temperatures. In
cases of mixed discharges and conductive cooling the
individual geothermometers yield different results. The
discrepancy may be used to infer cooling processes and
in obtaining the proportions of the different feed zones
in the case of mixing. Using geothermometers along
with sodium and chloride concentrations it was possible
to predict that bewtween 60 and 70% of the flow in well
KJ-22 in the Hvíthólar field came from the cooler lower
aquifers of the well. The composition deduced for these
lower fluids does not really tally with any of the major
groups and thus has to be treated as a separate one.
Similar composition has, however, on occasions been
observed in well KG-10 after it became blocked and
produced largely from the upper zone but still with a
lower zone component.
Fig. 17. Production characteristics of
wells in the Hvíthólar field. - Mynd 17.
Aflferlar hvíthólahola.
The monitoring of gas has shown a decrease with time
in gas concentrations and ratios such as C02/H2S which
reflect the inflow of magmatic gases into the Leirbotnar
system (Ármannsson etal. 1982). A notable exception is
well KJ-15 the flow of which is very rich in gas and shows
little signs of decrease. These results agree with those
from the fumaroles. Well KJ-15 is the northeasternmost
well and lies close to the area where the decline in gas
concentrations was smallest in the fumaroles.
Several studies have been carried out on deuterium
and ð180 ratios in the area (Darling &Ármannsson
1986). The general result is that fluids from the northern
part of the area west of Hveragil are slightly higher in
deuterium (ðD= —85 to —87%o) than those to the south
(—90 to —93%o) with no regular variation in ðlsO ratios
(—11 to —13%o) overthe area. Using values for rainfall in
Iceland (Árnason 1976) this suggests that the fluids from
the northern part of the field (Leirbotnar) is derived
from local sources, whereas those in the southern part
(Suðurhlíðar, Hvíthólar, KJ-6) might be derived from
water which has fallen as rain at a higher altitude. The
deuterium ratios are, however, not as low as in the fluids
from the Námafjall area (—95 to —100%o) 5-10 km to the
south of Krafla thus excluding the possibility of a com-
mon origin for the fluids of the two geothermal areas.
RESERVOIR MODEL
Extensive numerical modeling studies of the Krafla
geothermal area were carried out in 1982-1983. The
studies were limited to the two main wellfields, Leir-
botnar and Suðurhlíðar. The major objectives of the
modeling of these fields were to; 1) verify the conceptual
model of the field derived from the exploration results
26