Jökull - 01.12.1987, Blaðsíða 33

Gas chemistry of the Krísuvík geothermal field,
Iceland, with special reference to evaluation of
steam condensation in upflow zones
STEFÁN ARNÓRSSON
Science Institute, University of Iceland,
Dunhagi 3, 107 Reykjavík.
Abstract
Subsurface temperatures in the Krísuvík geothermal
field, SW-Iceland, have been estimated from fumarole
data using gas geothermometry. In the eastern part ofthe
field, the Sveifluháls area, temperatures of 280°C are
indicated, but in the western part ofthefield, the Vestur-
háls area, they seem to be around 260°C. Evidence exists
for an upflow of undegassed water in the northeastern
part of the Sveifluháls area around the southern shore of
Lake Kleifarvatn. A method has been developed to esti-
mate the degree of steam condensation in the upflow of
geothermal systems using C02 and N2 chemistry. Con-
densation is envisaged to occur by either or both of two
processes: 1) Conductive heat loss and 2) mixing of the
rising steam with colder water. In the Krísuvík field steam
condensation is seen to be relatively limited, 0-30%.
Evaluation of steam condensation in the upflów helps
interpret those geothermometers that use gas concentra-
tions in the steam. The N2 concentrations in the fumarole
steam, together with the gas geothermometry results, are
taken to indicate extensive boiling in the upflow, i.e.
boiling over a large temperature interval (>50°Cj without
the water separating from the steam.
INTRODUCTION
The Krísuvík geothermal field is one of five high-
temperature fields on the Reykjanes Peninsula in south-
west Iceland. Distribution of thermal manifestations
and resistivity surveys indicate an areal extent of some
40 km2 (Arnórsson et al., 1975a). During 1970-72 explo-
ration surveys and exploratory drillings were carried out
m the area to assess the characteristics of the geothermal
reservoir (Arnórsson, 1971; Arnórsson et al., 1975a,
1975b). Four exploration wells were drilled during this
period ranging in depth from 800 to 1000 m. Another
two wells, 1200-1300 m deep, had been drilled around
1960. Temperature reversals were observed in all these
wells and the maximum temperature in each well was
fairly close to the boiling point curve for depth, the
highest recorded temperature being 262°C in a well by
Trölladyngja (Fig. 1). The data from the drillings and the
exploration surveys were not sufficiently diagnostic to
explain these reversals and to locate upflow zones.
A part of the exploration work in 1970-72 involved the
study of the chemistry of gas from hot pools and fuma-
roles. In interpreting the data it was attempted to estab-
lish how the gas composition might reflect subsurface
temperatures (Arnórsson, 1971, Arnórsson et al.,
1975a). Since that time extensive research has been car-
ried out on gas chemistry in geothermal systems in-
volving assessment of equilibrium between gases and
minerals and geothermometry (Arnórsson etal., 1983a;
Arnórsson and Gunnlaugsson, 1983, 1985; D’Amore
and Celati 1983; D’Amore and Nuti, 1977; D’Amore and
Panichi, 1980; D’Amore andTruesdell, 1979,1980,1985;
D’Amore et al., 1983a, 1983b; Giggenbach, 1980; Nehr-
ing and D’Amore, 1984).
In 1981-1985 a total of 58 samples of gas from fuma-
roles and hot pools were collected in the Krísuvík field.
These data, together with older data (Tables 1 and 2),
have been used to evaluate boiling processes, to predict
subsurface temperatures and to assess gas reactions in
the upflow. A new method, using C02 and N2 chemistry,
has also been developed to estimate the amount of
steam condensation in the upflow, thus improving ge-
othermometry interpretation. Together with geological
and hydrological information the gas data have been
used to delineate a conceptual model for the Krísuvík
reservoir.
GEOLOGICAL FEATURES AND
GEOTHERMAL MANIFESTATIONS
The geothermal manifestations in the Krísuvík field
consist mostly of fumaroles, hot altered ground and
steam heated pools of acid water. The activity, present
JÖKULL
No. 37,1987