Induced seismicity during the stimulation of a geothermal HFR reservoir in the Cooper Basin, Australia

A long-term fluid-injection experiment was performed in the Cooper Basin (Australia) in 2003 to stimulate a geothermal reservoir. More than 20,000 m(3) of water were injected into the granitic crust at 4250 m depth. During reservoir stimulation about 27,000 induced seismic events were detected by a local, eight-station seismic monitoring system deployed in nearby boreholes. Hypocenter locations for 11,068 events were determined by using an averaged velocity model that was calibrated by associating early events with the injection point. The spatial hypocenter distribution forms a nearly subhorizontal structure with a lateral extension of 2 km X 1.5 km and an apparent thickness of approximately 150-200 m, which is in the order of the hypocenter location confidence limits. The hypocenter distribution exhibits a high degree of spatiotemporal ordering with the seismic activity systematically migrating away from the injection well with increasing time. Previously activated regions become seismically quiet indicating relaxation processes. High-resolution relative hypocenter locations determined for clusters of "similar" events locally reduce the apparent thickness of the structure to the level of a few tens of meters indicating that the reservoir is dominated by a single fracture zone only. Consistent with these findings, a subsequently drilled well intersects a dominating, high-penneable fracture within 15 m of the predicted intersection depth. Based on drilling and logging information, the fracture zone is interpreted as a preexisting (possibly tectonically formed) feature that (partly) sheared during stimulation. Triggering of the induced seismicity is found to be predominantly controlled by the increase of fluid pressure implicating a (local) reduction of the effective normal stress resolved on the fracture plane. Additionally, perturbations of the stress field caused by the largest-magnitude events may trigger seismicity ("aftershocks") on a local, short-ranging scale.