%A Stefan Baisch
%A R. Weidler
%A Robert Vörös
%A D. Wyborn
%A L. de Graaf
%J Bulletin of the Seismological Society of America
%T Induced Seismicity during the Stimulation of a Geothermal HFR Reservoir in the Cooper Basin, Australia
%X 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.
%N 6
%P 2242-2256
%V 96
%D 2006
%I Seismological Society of America
%R doi:10.1785/0120050255
%L epos1375