Baisch, Stefan and Weidler, R. and Vörös, Robert and Wyborn, D. and de Graaf, L. (2006) Induced Seismicity during the Stimulation of a Geothermal HFR Reservoir in the Cooper Basin, Australia. Bulletin of the Seismological Society of America, 96 (6). pp. 2242-2256. DOI: https://doi.org/10.1785/0120050255
Full text not available from this repository.Abstract
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 loca- tions 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-permeable 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.