Hydraulic fracturing processes and micro-seismicity in granite affected by injection rates

A series of hydraulic fracturing experiments were conducted on pre-flawed prismatic granite specimens whose flaws were hydraulically-pressurized using three different injection rates (0.3, 3.0 and 30 ml/min), with simultaneous visual and acoustic emission monitoring. It was observed that the breakdown pressures were injection rate-dependent, since they increased with injection rate. Visually, while the coalescence patterns were direct for the three injection rates used, the extent of white patching (interpreted by several authors as micro-damage) was significantly larger for lower injection rates. In addition, branching of the hydraulically-induced fracture occurred for higher injection rates. In terms of micro-seismicity, the number of events was, in general, comparable for the three injection rates used. Additionally, the source mechanism analyses showed that 42% of the micro-seismic events were predominantly of double-couple (shear) nature while 32% and 26% were tensile and mix-mode, respectively. These percentages did not vary substantially with the injection rates used. Finally, it was noted that the start of the acoustic emission activity matched, in general, the time at which the pressurization rate (equation Presented) stabilized. In conclusion, the results showed that the injection rate has a strong impact on the breakdown pressures, and this is interpreted to be caused by differences in damage mechanisms and in fluid diffusivity depending on the injection rates used.