This paper presents the results of an evaluation of the practicality of using self-potential (SP) monitoring to characterize subsurface changes in a geothermal reservoir caused by field operations. The Onikobe geothermal field has been producing electricity for nearly thirty years. A numerical model of the reservoir had been developed previously, and provides adequate matches to both the natural state conditions and the production history of the field. Recently, the STAR SP postprocessor was applied to this existing reservoir model to calculate temporal changes in SP response at the earth surface. The calculated SP changes are comparable, on the whole, with measured differences between SP surveys performed in 1982 and in 2000. Also, continuous SP measurements were recently carried out at Onikobe for several months, during which the field was shut-in for annual maintenance and then re-started, and temporal SP changes were detected. These changes are in accord with calculated results from the STAR SP postprocessor, and suggest that repeated SP surveys (and/or continuous SP monitoring) could provide useful calibration data for numerical model history-matching studies.

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