In addition to electrokinetic coupling, self-potential (SP) anomalies may be generated by various other mechanisms such as thermoelectric coupling, electrochemical diffusion potential, etc. In particular, SP anomalies of negative polarity, which are frequently observed near wells, appear to be caused by an underground electrochemical mechanism similar to a galvanic cell known as geobattery: the metallic well casing acts as a vertical electronic conductor connecting regions of differing redox potential. Electrons flow upward though the casing from a deeper reducing environment to a shallower oxidizing environment, and simultaneously a compensating vertical flow of ions is induced in the surrounding formation to maintain charge neutrality. Ishido and Pritchett (2011) extended the SP postprocessor (Ishido and Pritchett, 1999) to incorporate the above geobattery mechanism in addition to electrokinetic coupling. We will report the results of self-potential monitoring at the Sumikawa field since early 1980s (which started in advance of the startup of the Sumikawa geothermal power station in 1995) and present interpretations of the SP monitoring results based upon reservoir simulations and successive SP postprocessor calculations.

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