PRECISION GRAVITY CHANGES AT THE GEYSERS GEOTHERMAL RESERVOIR, 1975 - 2000

Abstract

During two field trips to The Geysers reservoir in July and September 2000, precision gravity and GPS measurements were made at 32 benchmarks previously occupied between 1975 and 1979. Comparisons with the late-1970s measurements indicate gravity changes of between +60 and -440 mGal. When corrections are made for the gravity effect of ground movement based on the elevation measurements of Mossop et al. (1997), the resultant gravity changes attributable to subsurface mass changes range between -100 and -700 mGal. The greatest gravity decreases are in the central Geysers region between Units 14 and 18-13. The southwestern limit of this area is not delineated by these measurements. Assuming most of the mass to have been withdrawn from a reservoir thickness of about 1 km, these changes imply the mass loss to have occurred due to dry-out of trapped water with an equivalent 2% pore volume (i.e. original undisturbed liquid-saturated porosity was 0.02). In the old production area (Units 1 - 8), the gravity change since 1974 is only about -450 mGal. However consideration of the net mass extracted from the early steam field prior to the first precision gravity measurements in 1974 suggests that a gravity decrease of around 200 mGal had probably already occurred in this area. There has also been significant injection in this area. An additional 30 new benchmarks have been established during the 2000 surveys to enable observation of the amplitude of seasonal gravity during 2001. The new network will then be expanded to around 150 benchmarks to allow annual precision gravity and elevation monitoring to help delineate future saturation changes in the reservoir as a result of an enhanced injection program planned for The Geysers.