Jökull - 01.12.1977, Page 53

TABLE 2 - TAFLA 2.
The concentrations (in millimoles/kg) of volatile constituents in the steam fraction
of selected samples from wells at Námafjall.
Styrkur (i millimólum/kg) rokgjarnra efna i gufu frá holum við Námafjall.
Well no. / Year and month of sampling
4 74.03 5 73.05 6 73.05 7 73.05 8 73.05 9 74.03
Samling pressure bars abs 12.8 11.2 11.2 12.0 10.9 6.9
co2 13.3 13.1 21.4 31.8 27.5 18.1
HoS 20.0 12.2 25.0 40.8 18.9 19.1
H, 14.7 4.2 23.1 41.4 16.6 13.2
o2 0.00 0.00 0.00 0.00 0.00 0.00
N, 3.3 3.6 3.6 1.0 4.7 2.3
CH4 0.18 0.55 0.73 0.30 0.82 0.19
Ar Discharge enthalpy 0.10 0.10 0.12 0.05 0.12 0.08
kcal/kg* 267 262 276 298 256 265
* Corresponds witli the initial quartz equilibrium temperature of the well discharge.
in wells 5 and 7 corresponds to a lowering of
the quartz equilibrium temperature by about
45° and 30° respectively over the period 1970—
1975 (Fig. 3). This is attributed to the precipita-
tion of quartz in the aquifers outside the wells,
where the water cools by flashing. It appears
that the flashing zone has expanded progres-
sively with time as a result of the pressure drop
in the reservoir caused by the discharging wells.
Local permeability will influence the pressure
drop in the vicinity of individual wells and
therefore influence the extent of the (juartz
precipitation. The lack of correlation between
the lowering of the silica and sodium concentra-
tions of individual well discliarges, discussed
below indicates tliat mixing of the geothermal
water with surrounding cold ground water was
not responsible for the decline in the silica
content of some of the well discharges.
For the total discharges rate of 40 kg/s for
each of wells 5 and 7, roughly 720 and 680 tons
of quartz have been precipitatecl respectively in
the aquifers around tlie two wells during tlie 6
years observation period. For a density of 2.6
g/cm:i this mass of quartz corresponds to 270 m3
for each well, which is sufficient to seal com-
pletely a rock with 10% porosity of 2700 m3.
This volume of rock corresponds to a 10 m
thick layer extending 9.3 m in all directions
frorn the well. It is thus evident that precipita-
tion of silica in the aquifers feeding discharging
wells may reduce their exploitable life time
significantly, particularly, if extensive flashing
occurs in the aquifer. For the reservoir as a
whole, such precipitation is, however, insigni-
licant.
Sodium and potassium
As the silica concentrations of the well dis-
charges decrease, tlieir ratio of Na/K increase.
The water temperatures calculated from the
NaK- and NaKCa-geothermometers (see Trues-
dell, 1975) parallels the quartz equilibrium
temperature as demonstrated for wells 5 and 7
in Fig. 3, although the forrner tend to be some-
what higher for well 5. This suggests that the
rate of re-equilibration between solution and
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