Sustainable Geothermal Production of the Seltjarnarnes Geothermal Field, Iceland

Abstract

The Seltjarnarnes geothermal field has been in production for a local heating system for the last 40 years. The field is located on a peninsula within the town Seltjarnarnes, a suburb of Reykjavík, which is the capital city of Iceland. The maximum thermal gradient in shallow research wells within the field is 380 °C/km. About 50 L/s of 95-120 °C hot water is produced from three main feed zones. A fourth feed zone of temperature exceeding 150 °C, probably exists below 2500 m depth. Part of the return water is collected and mixed with the produced water to keep an appropriate temperature for distribution. The geothermal field is possibly associated with a NNW-SSE trending fault zone, with the highest permeability and up flow zone just outside the northern shoreline and the main area of production. When the field was first produced the water was found to be somewhat saline, with about 1‰ salinity, and the salinity increased with time and production rate, causing severe corrosion problems. The best production well yields the most saline water, probably due to increased circulation in the middle of the up flow zone. In spite of calcite super saturation insignificant scaling has been detected so far. There are no indications of reservoir cooling due to seawater inflow, but the production characteristics of the water have changed during the 40 years of production. The regional draw down in the field has not increased during the last 20 years even though the production rate has increased slightly. Due to plans of building quite a few new buildings in the town the demand of energy for heating will increase in near future. Several options for future production are being considered, along with drilling of a new production well. A tracer test has been run in the field during the last two years in the order to estimate the possible rate of reinjection of effluent water into the field. Preliminary results are promising, indicating that up to 10 L/s of return water can be reinjected on a long time basis with nominal cooling of the production water. As the geothermal field is far from being fully exploited the future possibilities are manifold, even though sustainable goals would be continuously retained.