Temperature distribution productivity and utilization schemes for the Efri-Reykir geothermal field, S-Iceland

Abstract

The Efri-Reykir field is a low-temperature geothermal field located in the upper low-lands of South Iceland. Twenty-two shallow exploration wells and one production well (ER-23) have been drilled in the field. The formation temperature distribution of the geothermal system has been estimated on the basis of 80 temperature logs measured in the wells. The conceptual model of the Efri-Reykir geothermal field is based on the formation temperature distribution. It includes up-flow in a near-vertical fracture located between wells ER-23 and ER-21 intersected in well ER-23 at 700 m depth. This fracture is connected to a near-horizontal aquifer which is found at about 20 m depth below the now extinct hot springs, but dips towards the west and is found at a depth of 80 m in well ER-23. Analysis of production test data from well ER-23 shows that the well is highly productive. The reservoir appears to be single-phase liquid- dominated with a reservoir temperature of 145°C and enthalpy of about 610 kJ/kg. The maximum wellhead pressure of ER-23 is 4.3 bar-g and the maximum total flow 73.6 kg/s. If the wellhead pressure is lower than 3.1 bar-g some steam will be produced, the maximum being 6.3 kg/s. In spite of approximately 600,000 tons per year production during the last 8 years, very little pressure drop appears to have taken place in the reservoir. The permeability-thickness of the system is estimated to equal 200 Dm, which indicates a very high permeability. The geothermal energy at Efri-Reykir is currently used for space heating, greenhouses and a swimming pool. Some electricity production is possible. This would preferably be done with a binary cycle, which is efficient at the temperatures found at Efri-Reykir.