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Investigation of Fault Mechanisms during Geothermal Reservoir Stimulation Experiments in the Cooper Basin, Australia

Q-con GmbH, Marktstrasse 39, 76887 Bad Bergzabern, Germany baisch{at}q-con.de

Doone Wyborn

Geodynamics Limited, Suite 6, Level 1, 19 Lang Parade, Milton, Queensland 4064, Australia

Subsequent to an initial reservoir stimulation in 2003, a second long-term fluid-injection experiment was conducted in the Cooper Basin, Australia, in 2005 to further enhance and enlarge the existent geothermal reservoir. Approximately 25,000 m³ of water were injected into the granitic crust at 4250 m depth, thereby inducing about 16,000 seismic events, which were recorded by a local eight-station network. Event magnitudes calibrated by recordings of the permanent network of Geosciences Australia are in the range between M_L-1.2 and 2.9 with seismic moments between 10⁹ and 10¹³ N m. Hypocenter locations could be determined for 8886 events. The spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified for the initial reservoir stimulation, which is further extended in the course of restimulation.

Early seismicity is located near the outer rim of the previous zone of seismic activity and subsequently migrates both toward and away from the injection well. The immediate vicinity of the injection well remains seismically quiet during restimulation. These effects can be explained by a simple Kaiser effect model, where the induced seismicity is controlled by the in situ fluid pressure increase.

Compound fault-plane solutions based on P-phase polarities indicate a similar fault mechanism for nearly all events. The dominating fault mechanism is consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution. A small number of events with reverse mechanisms might indicate the existence of conjugated fractures locally intersecting the main fracture zone. Cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly, accumulating up to several centimeters of shear displacement.