If analysis of the microseismicity accompanying fluid injections is to be of maximum use in predicting hot dry rock (HDR) reservoir performance, i t should lead to the determination of both the rock volume and active flowing surface area of the reservoir. In the granitic rock at the HDR geothermal site at Fenton Hill, New Mexico, the microearthquakes located during hydraulic fracturing occur in large three-dimensional volumes called seismic clouds. Cores cut from the region prior to fracturing show numerous planar fractures, some mineral -filled, at virtually random orientations. Evidence supports the hypothesis that only a few of these planes make up the flow path between wells for most of the injected fluid. If this is indeed the case, then i t is necessary to be able to distinguish between fractures that accept flow from those which do not. We accomplish this by defining "flow-probable'' planes to be those which have seismicity located relatively farther away from lines where other planes intersect. We show that these flow probable planes intercept wellbores at locations where other data confirm the presence of hydraulically active fractures.

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