Self-Potential Variation at the Yanaizu-Nishiyama Geothermal Field and Its Interpretation by the Numerical Simulation

Abstract

The Yanaizu-Nishiyama geothermal power plant commenced operating in May 1995. A positive self-potential (SP) anomaly was observed over the high temperature zones before the commencement of the fluid production. The positive SP anomaly disappeared on the profile in 1996, possibly owing to the development of the downward geothermal flow into the reservoir. Further surveys were carried out between May and September after 2 monthsí shut-in of all the production wells in 1998. The positive SP anomaly was again observed on the profile in May, but decreased with time after the re-start of production. Numerical simulation was performed using a conceptual reservoir model and the SP variation was calculated. The model shows a positive SP anomaly above the high conductive vapour zone that is produced by the upward fluid flows for the natural-state condition. The production-induced flow becomes dominant during exploitation and the current sinks replace the positive sources of conduction current, resulting in the negative SP anomalies on the surface. The SP recovers substantially during the shut-in of the production wells. This is caused by the reduction in fluid flows from the vapour zone to the production zones. The SP decreases sharply after the re-start of production. The simulation reproduces the observed changes in SP at the Yanaizu- Nishiyama geothermal field.