This paper presents the results of a feasibility study to appraise the practicality of using geophysical techniques to detect and characterize subsurface changes induced in geothermal reservoir by field operations. A numerical model of the Onikobe geothermal reservoir (Japan) was developed. The modeling study involved (a) developing a stable solution for the natural state of the field prior to the beginning of field operations in 1975, (b) imposing the known fluid production and injection histories based on field records of well flow rates for the period March 1975 - January 1997, and then (c) assuming that well flow rates remain fixed thereafter, forecasting the future response of the reservoir for an additional ten years.

Then, the STAR microgravity and electrokinetic potential postprocessors were applied to the reservoir model to calculate the probable changes that will be observable at the surface using these geophysical survey techniques. The strength and spatial distribution of the signals were estimated and the practicality of detecting such signals in the field was appraised. The Onikobe field appears to be a marginal prospect for the utilization of microgravity measurements for future reservoir monitoring based on these calculations, since most of the major microgravity changes have already taken place in the now-mature field. It was estimated, on the other hand, that substantial changes in surface SP started taking place in about 1983, and that these changes are continuing to the present day as a result of the ongoing temporal evolution of the brine production-injection scheme.

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