Geothermal Reservoir Monitoring by Continuous Self-Potential Measurements

Abstract

Self-potential (SP) monitoring was conducted at two geothermal fields, Mori (Japan) and Mataloko (Indonesia), with the purpose of detecting changes of fluid flow system in reservoirs caused by well operations. Surface SP was monitored at eight observation points across the reservoir during shut-in and draw-down of the wells at Mori, and at sixteen observation points around an exploration well during its flowtests at Mataloko. To solve the problem of reference points for the measured potential, a concept of relative SP, defined as the differential from the average SP of all monitoring points, was introduced. The advantage of relative SP is clarified through numerical simulations. Results of the field observation show that specific changes of relative SP at each monitoring point were detected corresponding to the well operations. Relative SP monitoring can be applied to investigate both the extent of a reservoir and its hydrological conditions.