Jökull - 01.12.1977, Page 51

Fig. 2. Measured downhole temperatures and
aquifer locations in wells 4 to 9 at Námafjall.
The profile for well 9 shows low temperatures.
It was recorded shortly after completion of
drilling and before the well had attained equi-
librium after the cooling by the drilling fluid.
Other profiles represent stabilized temperatures
prior to the period of discharge. The figures in
the bottom left corner indicate quartz equili-
brium tempeatures (tqtz). If there has been a
trend towards lowering of tqtz. with time the
respective set of figures is joined with an arrow.
Mynd 2. Mœlt hitastig og lega vatnsœða í hol-
um 4 til 9 við Námafjall. Hitaferillinn fyrir
holu 9 sýnir lágt hitastig. Hann var tekinn
skömmu eftir að borun lauk og áður en holan
hafði náð að hitna upp. Aðrir hitaferlar sýna
ótruflað hitastig i holunum, áður en peim var
hleypt í stöðugan blástur. Tölurnar neðst til
vinstri á myndinni sýna kisilhita fyrir einstakar
holur. Ef kisilhitinn hefur farið lakkandi með
tima, er viðkomandi talnapar tengt með ör.
low values, when flashing occurs in the aquifer,
except at the initiation of the discharge period.
This is so because cooling, which accompanies
the flashing, leads to precipitation of silica.
Lower silica concentrations yield lower quartz
equilibrium temperatures. Flashing in the aqui-
fer may also cause the well discharge enthalpy
to increase during production, if some separa-
tion of the two phases occurs and the steam
moves preferrentially to the well. In the present
study the enthalpy of the total discharge of in-
dividual wells is always referred to the initial
quartz equilibrium temperature. The quartz
solubility data used in this study are tliose of
Morey et al. (1962) and Kennedy (1950).
For description of the sampling procedure
and the methods of calculation of the reservoir
water chemistry, the reader is referred to Arn-
órsson et al. (1978).
C.hemical characteristics
The reservoir water at Námafjall is very low
in total dissolved solids, the low chlorine con-
centrations being most noteworthy. The major
dissolved ions include bisulphide and sodium.
The chemistry of the various well discharges is
similar, but significant differences occur in the
silica ancl the total gas concentrations. These
differences can be related to the temperature
and depth of the aquifers feeding individual
wells.
It is believecl that the chlorine in the water
is derivecl from the rock by leaching. The data
of Sigualdason and Óskarsson (1976) indicate
that the Icelandic basalts are low in chlorine.
The low chlorine content of the Námafjall
waters is thus considered to be due to its limit-
ed supply in the rock. It is this supply of chlor-
ine that is most important in determining the
total dissolved solids content of the water. The
concentration and distribution of other major
ionic constituents are determined by this supply
and various solute/solute and mineral/solute
temperature dependent chemical equilibria
(Arnórsson et al., 1978).
Representative analyses of each of the well
discharges at Námafjall are presented in Tables
1 and 2. The calculated distribution of chemical
species in the reservoir water for the same
samples as in Table 1 are shown in Table 3.
CHANGES IN THE CHEMISTRY
OF THE WELL DISCHARGES WITH TIME
Factors affecting the chemical changes
Changes in the chemistry of well discharges
tapping hot-water reservoirs may occur as a
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