Jökull - 01.12.1977, Page 49

Changes in the Chemistry of Water and Steam Discharged
from Wells in the Námafjall Geothermal Field, Iceland,
During the Period 1970-76
STEFÁN ARNÓRSSON,
NATIONAL ENERGY AUTHORITY, REYKJAVÍK, ICELAND
abstract
The chemical composition of borehole fluids
in the Námafjall geothermal field, northern Ice-
land, has changed significantly during the ex-
ploitation period 1970—1976. Withdrawal of
fhiid by discharging production wells has pro-
duced a pressure drop in the reservoir and ex-
tended the zone of flashing. The chemical
changes have resulted from the increased flash-
mg in the reseruoir. These changes include
lowering of the concentrations of silica by as
much as 40°/o, lowering of sodiwn and potassium
by 0—30% and 5—50% respectiviely, and the
lowering of total gas concenlrations by 0—45%.
The lowering of the silica concentrations is
considered to be due to the precipitation of
quartz from the flashing water in the aquifers
feeding the wells. The flashed water, which
flows from the major upflow zone, tends to
leach sodium from the rock. During the ex-
ploitation period sodium concentralions have
fallen in all but one luell. This is explained by
progressively less effective leaching caused by
accelerated replacement of the flashed water by
unreacted water from below. The water tem-
peratures derived from the NaKCa-geothermo-
meter have clecreased lo about the same extent
as the quartz equilibrium temperatures. This
indicates that re-equilibration upon cooling be-
tween the solution on one hand and quartz and
the respective NaKCa-silicate on the other is
about equally fast, at least for the temperatures
and water compositions in question. The lower-
lng of tlie gas content of some of the well dis-
charges can be explained by increased loss of
steam underground from the aquifers feeding
the respective luell. From the observed variation
ln the chemical composition of the well dis-
charges, it is deduced that the main upfloxu
zone is near well 7 or to the east of that well.
Flashed water flows frotn ihe upfloiu zone in a
westerly direction ancl the wells xuithdraw water
and steam from this body of fluid.
INTRODUCTION
The Námafjall geothermal field is located in
northern Iceland abont 5 km east of Lake Mý-
vatn. Surface tliermal manifestations cover some
5 km2, being most intense on the low liyaloclas-
tite ridge of Námafjall and just east of it. So far
10 wells have been drilled into the Námafjall
geothermal reservoir. They are all located west
of Námafjall and near the western margin of
the field as deduced from surface thermal mani-
festations (Fig. 1). Their depth varies from 340
to 1800 m.
The present paper describes changes in the
chemistry of water and steam discharged from
6 of the wells (nos. 4 to 9) that have been re-
vealed by repeated sampling during the period
1970—1976. The otlier wells were not included
in the study because they were not productive
or had so low discharge rates that they were not
exploitable. Tlie deepest well, no. 10, was drill-
ed at the end of 1975, and is therefore not in-
cluded in this study.
The depth of the wells studied ranges from
637 to 1312 m. The highest downhole tempera-
ture, 289° C, has been measured at 1100 m
depth in well 7. Downhole temperatures and
aquifer locations (recorded by loss of circula-
tion fluid during the clrilling) for all tlie wells
are presented in Fig. 2.
Only basaltic rocks are exposed in the im-
mediate vicinity of the geothermal area. They
include basalt lava flows, pillow lavas, and
JÖKULL27. ÁR 47