Reservoir Monitoring in the Okuaizu Geothermal Field Using Multi-Geophysical Survey Techniques

Abstract

To obtain a better understanding and more precise forecast of the geothermal reservoir, reservoir monitoring using multigeophysical survey techniques is an effective way to build a more realistic model than conventional methods. We have carried out gravity and self-potential (SP) monitoring and introduced these data to constrain reservoir simulations in the Okuaizu geothermal field in northeastern Japan, where a 65MWe geothermal power station has been operated since 1995.
Gravity changes have been monitored since 1992. Although a rapid reduction in gravity was observed for the first three years of operation, change have become smaller since 1998. SP monitoring has been carried out by repeated surveys and by continuous measurements. As a result of the 1996 survey, the dominant SP anomaly of positive polarity over the high-temperature zone in the natural state was found to disappear. Continuous monitoring afterwards revealed changes in SP associated with the field-wide shut-in of about one month in 1998 and 2002.
History-matching studies are underway using these data from geophysical monitoring as constraints on reservoir models.