Jökull - 01.12.1977, Blaðsíða 50

LEGEND:
Fig. 1. Location of drill-
holes in the Námafjall
geothermal field (from
Gudmundsson et al.,
1971).
Mynd 1. Staðsetning
borhola á jarðhitasvœðinu
við Námafjall.
hyaloclastites. Other rock types have not been
encountered in the drillholes. Rock alteration
in tlie drillholes has been described by Gislason
(1973).
Exploitation of the Námafjall geothermal
field started in 1967 and reached the present
production in 1970. The wells have been pro-
ducing continously over the study period apart
from niinor stops during maintenance opera-
tions. The total discharge rate of wells 4 to 9
is of the order of 200 kg/s with an average
discharge enthalpy of sorne 270 kcal/kg. An
account of the development of the geothermal
field ancl its exploitation lias been given by
liagnars et al. (1970).
WATER CHEMISTRY
Introduction
When the deep, hot reservoir water flashes in
the drillholes and in the aquifers feeding them,
the volatiles are mostly transferred to the steam
that forms. At the same time the pH cf the re-
maining water increases as well as its content of
non-volatile constituents. The gas content of
steam samples depends on the original gas con-
tent of the reservoir water, the enthalpy of this
water, possible loss/gain of steam during flash-
ing underground, and the sampling pressure.
The content of non-volatile constituents in
samples of flashed water is determined by the
same factors. It is therefore clear, even if the
total discharge enthalpy had remained the same,
that the analytical concentrations of the collect-
ed water and steam samples cannot be used to
detect chemical changes in the well discharges
that may occur with time, because the sampling
pressures for individual wells varied front time
to time.
If the enthalpy of the total discharge was
known, the ratio of water to steam at any sampl-
ing pressure can be calculated easily (see Arn-
órsson et al., 1978), which in turn allows evalua-
tion of the total discharge composition from
analyses of water and steam samples collected
separately on the wellhead at a known pressure.
In the present work the only nteans of estimat-
ing the enthalpy of individual well discharges
was frorn the silica content of the water assunt-
ing equilibrium with quartz. As pointed out by
Arnórsson et al. (1978), this method may yield
48 JÖKULL 27. ÁR