DETAILED JOINT STRUCTURE IN A GEOTHERMAL RESERVOIR FROM STUDIES OF INDUCED MICROEARTHQUAKE CLUSTERS

Microearthquake clusters form distinct, planar patterns within five st udy regions of a geothermal reservoir undergoing hydraulic fracturing at Fenton Hill, New Mexico. The patterns define individual, slipping j oint surfaces of dimension 40-120 m, containing 80-150 events each. Sh arp, straight edges truncate the clusters; we interpret these as marki ng intersections with aseismic joints. Each edge orientation is consis tent with an intersection between the active joint and a plane oriente d parallel to one of the other clusters we identify. Therefore it appe ars that cluster shapes constrain the geometry of seismic and aseismic joints; both could be important components of the fluid-flow network. The distribution of inferred slip plane orientations is consistent wi th but fails to provide sufficient constraint to differentiate conclus ively between two, very different, stress field estimates, one measure d using pressurization and wellbore breakouts, the other using focal m echanisms of the largest microearthquakes. An impermeable joint model, requiring pore pressure in excess of the normal stress on a joint bef ore slip can occur, was inconsistent with many of the inferred slip pl ane orientations. The high-quality locations were possible because eve nts from the same cluster generated nearly similar waveforms, permitti ng the precise determination of relative arrival times. Standard devia tions of arrival-time residuals fall between 0.1 and 1.1 ms for these clusters. Major axes and aspect ratios of the 90% confidence ellipsoid s range from 6 to 28 m and 1.5 to 8, respectively. Small events domina te the seismic energy release and thoroughly populate the identified, active joints, allowing the hypocenters to reflect details of the join t structure. To further investigate the reservoir structure, we applie d a source-array, slant-stack technique to waveforms from the well-loc ated clusters, yielding directions that scattered energy left each clu ster. By studying paths of scattered waves we expected to pinpoint imp edance contrasts that might have indicated concentrations of fluid-fil led joints. However, results show that scattered energy in the S wave coda left the source region in the same direction as the direct S wave . Direct waves may have excited borehole tube waves that became trappe d in the vicinity of the geophone tool, overwhelming any energy scatte red from the reservoir.