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Reservoir conditions related to induced seismicity at the Geysers steam reservoir, northern California

U.S. GEOLOGICAL SURVEY WATER RESOURCES DIVISION, MAIL STOP 404, BOX 25046DENVER FEDERAL CENTER, DENVER, COLORADO 80225
U.S. GEOLOGICAL SURVEY, 908 FEDERAL CENTER, RESTON, VIRGINIA 22092

Abstract

Seismicity at The Geysers appears to be induced by steam production, in which water boils to steam within a pervasively fractured reservoir subject to high levels of tectonic shear stresses and strains.

The steam reservoir is composed of fractured Franciscan graywacke with a very heterogeneous, multiple permeability and porosity distribution. Permeability is controlled by the larger fracture systems of the reservoir, which act as a very permeable conduit to less permeable rock containing liquid water. The reservoir fluid remains close to the liquidus conditions during production, implying both a temperature and pressure decline in the rock matrix bounding the fracture systems as the reservoir is depleted.

Reservoir earthquake focal mechanisms are nearly aligned with the regional tectonic strain field, and reservoir earthquakes are indistinguishable from tectonic earthquakes elsewhere in the region. Within the reservoir, the volume defined by the earthquakes has not changed within the resolution of the seismic network from 1975 to 1979. The log moment per earthquake also has not changed, suggesting a steady-state process of stress buildup and release. Added to the zero correlation of the fluid injection history with the earthquake activity, the above observations suggest that elevated pore pressures and fluid injection are not likely causes for The Geysers induced seismicity.

Instead, all available evidence points to the association of induced seismicity with pore pressure and temperature declines. The two most likely mechanisms are either an increase in local shear stress levels with fissure deflation, or a transition from stable to unstable sliding ("stick-slip"). No other documented mechanism for induced seismicity are consistent with in situ reservoir conditions at The Geysers.

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