Characteristics of AE observed during hydraulic stimulation operations at Ogachi, Hijiori and Cooper Basin sites.

Many AE (Acoustic Emission) events were observed during hydraulic stimulation operations to create Hot Dry Rock (HDR) geothermal reservoirs at different depths from around 1,000 m to 4,200 m at three different sites of Ogachi and Hijiori in Japan, and Cooper Basin in Australia. Comparing the water injection operations with observed AE hypocenter locations and the rate of events, some differences and similarities were found among the three sites. Seismic activities at the beginning of the water injection phase were low in the Ogachi and Hijiori until the injected water volume reached that of the previously injected volume, but the activity was high from the beginning of the injection at the Cooper Basin site. It was seen in the Cooper Basin that the area previously activated was quiet in the successive stimulation operations. The relationship between the injected water volume and the seismically activated regions showed that the seismic activated region extended almost linearly with the injected water volume. But the inclination of the relationship is site dependent. The seismic activated region in Cooper Basin extended much further than at the other sites. These differences are thought to be strongly dependent on the regional natural joint system.