Jökull - 01.12.1987, Page 27

by cooling of the flowing fluids which results from boil-
ing.
CHEMICAL CHARACTERISTICS OF WELL
FLUIDS
The chemical properties of the well fluids have been
monitored regularly especially to obtain information on
the influence of the magmatic activity in the region, as it
affects the gas content of the geothermal fluid and the
associated tendency to deposit iron sulphides and sil-
icates which blocked some of the best producers among
the wells (Ármannsson et al. 1982).
The chemical regime is complicated and it may be said
that of the 22 wells which have been sampled no two
have fluids with the same chemical composition. There
are, however, certain characteristics common to fluids
from all wells and others which characterize groups of
wells. These have been used as an aid in the devel-
opment of a model of the area. The fluids from all the
wells are relatively dilute (TDS <1500 ppm) and most of
them can be shown to be close to a neutral pH at depth.
The fluids have been divided into 6 groups according to
their chemical chasracteristics (Table 4) and each group
is thought to give information on origin and upflow
zones.
A comparison between measured temperatures Tmeas
and carbon dioxide geothermometry temperatures TC02
(Arnórsson & Gunnlaugsson 1985 and Armannsson et
al. 1982) has been used to infer whether magmatic gas
was present in the fluids of a particular well. This is
possible since the modified magmatic gas is predom-
inantly carbon dioxide. Another parameter which has
been used to monitor the same effect is the C02/H2S
ratio which tends to be high where magmatic gas occurs.
From Table 4 it is apparent that only the Leirbotnar
wells have been severely affected, the northern ones
acutely but the southern ones to a varying degrees.
The very low chloride values found in some fluids are
thought to originate from water which has some time
TABLE 4 KRAFLA FLUIDS. DIVISION ACCORDING TO CHEMICAL CHARACTERISTICS OF
THE RESERVOIR.
Group No.
1 2 3 4 5 6 7
Leir- Leir- Leir- Hvera- Suður- Hvít- Hvít-
botnar botnar botnar gil hlíðar hólar hólar
Upper Lower Lower Upper Lower
zone zone.N zone.S part part
Drawing wells
mostly 2, 3A,5 15 13 and 14, 17 21
— 7 and 8 20 and 19
partly 1, 2, 3, 4 and 3,6 7 7, 12 16, 12 22 22
- 4, 6,7, 9, 10 9 and 11 and 1 and 1
- 10 and 11
Chemical character- sitics11
TCO, =Tmeas >Tmeas Variable =Tmeas =Tmeas =Tmeas Low
C1 ca 30 10-20 <10 >40 <10 >100 <10
Na/Cl 5-10 5-15 5-10 1-2 3-6 ca 1 calO
co2/h2s <10 >30 Variable 20-30 10-20 <10
F/ClxlO3 10-50 >100 10-50 10-50 >100 <10 >100
11 Cl: mg/kg. Ratios: mg/mg.
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