Continued Geothermal Reservoir Stimulation Experiments in the Cooper Basin (Australia)

Two fluid injection experiments were conducted in the Cooper Basin, Australia, in 2010 and 2012. Following previous stimulations in the Habanero reservoir, approximately 34; 000 m(3) of water were injected into the granitic crust at 4160 m depth through the Habanero 4 well. More than 29,000 induced seismic events were recorded by a local 24-station network. Event magnitudes are in the range between M-L -1.6 and 3.0. Hypocenter locations could be determined for 21,720 events. The spatial hypocenter distribution indicates that the seismicity occurred on the same sub-horizontal layer structure identified previously. Fault-plane solutions determined for 525 of the strongest events are mostly consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution. The fracture zone is interpreted to be of tectonic origin. Large stress accumulations near a reservoir boundary indicate that the hypocenter distribution is bounded by a structural limit of the fracture zone. High-pressure stimulation at similar depth in a nearby well caused only minor seismic activity. During an eight-day stimulation period, less than 80 induced seismic events were recorded followed by another 139 events over the next six months. The maximum event magnitude was M-L 1.6. Hypocenters are located in close vicinity of the injection well and align along steeply dipping fractures consistent with their fault mechanisms. These fractures are not well oriented for shearing in the regional stress field. They were seismically activated at extremely large overpressures and accepted only small amounts of fluid. Our findings indicate that natural flowing fractures play a key role for hydraulic stimulations in the crystalline crust.