Title:

Monitoring Fluid Depletion at the Geysers by the Gravity Change/Subsidence Ratio

Authors:

R.G. Allis

Geo Location:

The Geysers, California

Conference:

Stanford Geothermal Workshop

Year:

1982

Session:

Geophysics and Well Logging

Language:

English

File Size:

554KB

View File:

Abstract:

The ratio of gravity change to subsidence may be a useful indicator of the rate of fluid depletion in vapour-dominated reservoirs. less than about lo%, this ratio is close to -40 pgals/cm if there is no recharge or reinjection to the reservoir. endent of reservoir thickness, pressure drop and porosity. mass (assumed to be e100 C), the ratio is reduced by a factor 1 - R . The rate of injection of condensate into The Geysers reservoir would reduce this ratio to between -24 to -28 pgals/cm. available data at The Geysers (1974-1977) shows the most heavily produced part of the reservoir to have a ratio of -14 ugals/cm. discrepancy may be significant, and could be due to a component of natural liquid recharge, or to decompression effects at the boundaries of the vapour-dominated zone. negative value for the ratio is implied by the few data points in the smaller pressure sink around power plants 9-10: suggesting little recharge or reinjection is occurring in this area. unproduced part of the reservoir could be caused by tectonic subsidence, or could be due to decompression of a preddnantly liquiddominated reservoir.


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