Jökull


Jökull - 01.12.1987, Síða 33

Jökull - 01.12.1987, Síða 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
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