Jökull - 01.12.1977, Page 59

zone, ancl during the lateral movement of the
water from that zone, causes degassing of the
water. When the water cools, as a result of the
flashing, sulphide is lost from solution and
hydrogen decreases simultaneously by virtue of
its participation in a chemical equilibrium be-
tween sulphide, sulphate, and water. These re-
actions explain the lowest hydrogen and hydro-
gen sulphide concentrations in the shallowest
aquifers and in the wells, which are farthest
away from the assumed upflow zone.
Withdrawal of fluid by the discharging wells
has produced a pressure drop in the reservoir
with the effect of extending the zone of flash-
ing. Only hot water existed in the aquifers
penetrated by the wells while the reservoir was
undisturbed, but when the wells were put into
production, the pressure drop became sufficient
to initiate boiling in the aquifers. Some of the
steam, which has forrned by flashing in the feed-
ing aquifers, has escaped from the steam-water
mixture entering some of the wells with the
effect that the total gas content of their dis-
charge has decreased. This effect of degassing
will obviously be most pronounced farthest
away from the upflow zone and where pressure
dro]> is strongest in unconfined aquifers. The
discharges of wells 6 and 8 have shown the
largest decrease in the total gas content, and
accordingly, these wells are either farthest away
from tlie upflow zone or they are fed by the
least confined aquifers.
On the basis of aquifer temperature and total
gas content, wells 4 and 5 are farther away from
the upflow zone than well 7. One might there-
fore have expected a decline in the gas content
of tliese two first mentioned wells during the
exploitation period, but this has not been ob-
served. It may be that wells 4 and 5 are fed by
confined aquifers so steam separation does not
occur underground in the immediate vicinity
of the wells.
As was pointed out in the previous chapter,
the sodium content was lowest in well 7 at thc
initation of the exploitation period, but this
well contains the highest gas concentrations aiul
for that reason it is fed by water that lias lost
less steam than the waters feeding other wells.
The higher sodium concentrations of the other
well discharges coukl not be accounted for by
steam loss. Leaching of this constituent from
the rock by the cooler flashed water was offered
as a possible explanation. The withdrawal of
fluid from the geothermal reservoir by the dis-
charging wells has increased the rate of re-
charge from below. As a result less time has
been available for leaching of sodium from the
rock by the flashed waters explaining why the
sodium concentrations in the flashed water
have decreased with time. This explanation fits
the model drawn up earlier from interpreta-
tion of the gas content of the well discharges.
This model also fits the distribution of the sur-
face thermal manifestations in the area (Fig. 1).
On the basis of the reservoir model drawn up
here, furter wells in the Námafjall field should
be located in an easterly and southearstely
direction from well 7. To improve modelling
of the reservoir water chemistry it is essential
to obtain data on discharge enthalpies of indi-
vidual wells.
ACKNOWLEDGEMENTS
This article was prepared while the author
worked with Professor I. Th. Rosenqvist’s re-
search group at the Geological Institute of Oslo
University, but it is based on data from the
National Energy Authority, Reykjavík. Thanks
are due to Dr. I. B. Fridleifsson for reading the
manuscript and helpful suggestions. Financial
support from the Icelandic Science Fund is
acknowledged.
REFERENCES
Arnórsson, S., 1977: Precipitation of calcite from
ílaslied geothermal waters in Iceland. Con-
tributions Mineral. Petrol., 66, 21—28.
Arnórsson, S., Grönvold, K., and Sigurdsson, S.,
1978: Aquifer geochemistry of four high-
temperature geothermal systems in Iceland.
Geochimica et Cosmochimica Acta, 42, 523
-536.
Fournier, R. O. and Truesdell, A. H., 1973: An
empirical Na-K-Ca-geothermometer for na-
tural waters. Geochimica et Cosmochimica
Acta, 37, 1255-1275.
Gislason, G., 1973: Study of high-temperature
hydrothermal alteration at Krísuvík and
Námafjall. Unpublished B.Sc. thesis, Uni-
versity of Iceland (in Icelandic).
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