The cooling of the Selfoss geothermal reservoir in southern Iceland

Abstract

A study of the temperature distribution in the Selfoss geothermal area is presented in this report. This geothermal area supplies the town of Selfoss with hot water and has experienced a gradual cooling during its 40 years of exploitation. By estimation of the average initial temperature, two geothermal systems were identified in this area, the upper geothermal system in the depth interval between 50 and 1000 m with a temperature range of 80-96 C, and the lower geothermal system below 1200 m depth with a temperature range between 130 to 150 C. The magnitude of cooling has been determined at several levels by comparing the initial temperature with the present temperature distribution. The direction of cold water inflow was established on the basis of constructed temperature maps at several depths, and cross-sections. It has an east - west direction from the farm Thorleifskot towards the middle part of the field. Cold water flow was specified in the upper geothermal system at two levels with the strongest influence at 150 m depth and cooling of more than 55 C. The temperature distribution in the southwest part of the field is different to the other parts of the field. These conclusions were partially confirmed through chemical analysis of water samples, mainly by differing chlorine content in the thermal water.