Electrical Resistance Study Between Well Casings at Reykjanes Geothermal Field

Abstract

Geothermal power production is highly dependent on fractures in the subsurface controlling the heat and mass transport towards production wells. This paper focuses on studying the fracture connectivity between geothermal wells at Reykjanes geothermal field in Iceland using a casing-to-casing resistance method. The method consists of connecting a pair of a current and a potential electrode to a steel casing and another pair to another casing for measuring the resistance between well pairs. The resistance measured between casings is considerably lower if the current is traveling from one casing to another through fractures filled with highly saline geothermal brine instead of going through less conductive rock. Hence, the resistance measured can give valuable information about the fracture network. At Reykjanes, six well pairs are studied including well IDDP-2 that has been deepened down to a depth of 4.7 km. Well IDDP-2 is not connected to any other wells via surface pipelines so the low resistance measured between the production casing of well IDDP-2 and surrounding wells indicates that the current is flowing through a channel of saline geothermal fluid. Taking advantage of using the steel casings as electrodes allows the current to travel along the casing deeper into the reservoir than if surface electrodes are used. Additionally, the difference in resistance values measured between different well pairs helps gaining knowledge about the fracture connectivity in the reservoir.