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 hydraulicallyinduced 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) 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.

1. INTRODUCTION

Engineered geothermal systems (EGS) expand the potential of utilizing geothermal energy by increasing the natural permeability of hot dry rocks using hydraulic fracturing. However, due to the induced-seismicity observed during hydraulic fracturing applications, the public opinion towards the generalized use of this method has been negative. As such, it is of vital importance to understand the fracture initiation and propagation mechanisms leading to the breakdown of formations, including the micro-seismicity generated. A significant contribution has been done by previous researchers employing laboratory-scale hydraulic fracturing tests to understand the fracturing processes at a fundamental level [1–6]. Haimson and Fairhurst [7] were among the first to conclude that higher injection rates result in larger breakdown pressures from tests conducted on hydrostone. Solberg et al. [8] hydraulically-fractured Westerly granite specimens and concluded that injection rates have a dominant effect on the failure mechanism of granite. In recent studies, Morgan et al. [9] tested Opalinus shale with acoustic emission (AE) monitoring noting that highest injection rates resulted in a higher power at lower frequencies and were associated with greater AE activity. A study conducted by Song et al. [10] hydraulically fractured the Tablerock sandstone specimens to investigate the effect of pressurization rate and initial pore pressure on the magnitude of hydrofracturing breakdown pressure. The variation of fluid pressure (Pfluid) was plotted against the pressurization rate, (equation) , to identify the pressure at which this inflection point occurred, concluding also that this point coincided with the onset of the AE activity. The current study focuses on interpreting the visual and acoustic emission observations from hydraulic fracturing testes conducted on granite specimens using different injection rates.

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