Thermal Shock Effect on Acoustic Emission Response During Laboratory Hydraulic Fracturing in Laizhou Granite

To investigate the thermal shock effect on microseismic response during hydraulic fracturing in hot dry rock, laboratory hydraulic fracturing experiments combined with acoustic emission (AE) monitoring were performed on granite after heating and rapid water-cooling treatments. Thereafter, the influence of thermal treatment level and the number of cycles on hydraulic fracture geometry, injection pressure curve, and the spatial distribution and focal mechanism was analyzed. Besides, the maximum AE amplitude and the localization results of large AE events with amplitudes larger than 7.0 mV were further investigated to discuss the thermal shock effect on reducing breakdown-induced seismicity. Experimental results show that the thermal shock effect was beneficial for reducing the maximum amplitude of AE events during laboratory fracturing experiments on Laizhou granite. After single-cycle thermal treatment, large AE events tended to disperse far away from rather than located around the open-hole section only when the thermal treatment level exceeded the threshold temperature (300 °C). At the thermal treatment level of 300 °C, increasing the number of cycles had a little influence on reducing the breakdown-induced seismicity due to the limited reduction of breakdown pressure. Most of the large AE events are still mainly detected during the occurrence of breakdown and located around the open-hole section. At the thermal treatment level of 400 °C, shear events were dominant. Even though a complex fracture network was created, no obvious cluster of large AE events was detected around the open-hole section.