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Hydraulic fracturing can be recognized as an emerging method used in the mining of heat in Enhanced Geothermal Systems as well as in the extraction of oil and gas entrapped within shale formations. While there are several experimental studies focusing on the initiation and propagation of hydraulically-induced fractures under uniaxial and biaxial loading conditions, a very limited number of experimental studies investigate the effect of triaxial loading conditions on fracture initiation and propagation. This study describes an experimental setup, which was designed to allow one to independently apply and control three orthogonal stresses in prismatic granite specimens while simultaneously applying a hydraulic pressure inside pre-fabricated flaws. Moreover, the test setup allows one to observe and subsequently interpret the fracturing processes through visual and acoustic emission (AE) monitoring. The observations obtained in the current study using a triaxial state of stress were interpreted and compared with existing experimental studies that used other states of stress. It was observed that whitening of some grains and high-amplitude AE events occurred where visible cracks eventually developed for the triaxial state of stress investigated. Comparison with previous studies, in which only vertical loads (uniaxial) were applied, shows that the aperture of the hydraulically-induced fractures for the triaxial condition is significantly smaller than for the uniaxial loadings and that the coalescence patterns are, in general, stress-state-dependent. In terms of AE data, the total number of AE events in the specimens subject to triaxial stresses were significantly higher than in the tests using uniaxial stresses, even though most of the events (65%) had a relatively low-amplitude (<50dB) in contrast to the uniaxial tests, in which low-amplitude events were typically less than 50%. 

The finite element code ABAQUS is used to model a typical granite specimen subjected to either uniaxial or biaxial compression, with two pre-fabricated flaws with the geometry 2a-30-30 in which only one flaw is pressurized. The maximum and minimum principal stresses as well as the maximum shear stresses are analyzed around the flaw tips and along the bridge between the inner flaw tips of the pressurized and non-pressurized flaws. When the specimen is loaded uniaxially, the maximum principal stresses in the bridge between inner flaw tips are tensile near the pressurized flaw and decrease significantly as one moves towards the non-pressurized flaw. For the biaxial loading, mainly compressive principal stresses are observed for low hydraulic pressures; tensile stresses start to develop for larger hydraulic pressures, but only near the pressurized flaw. For both uniaxial and biaxial cases, tensile and shear cracks may occur near the pressurized flaw but are not theoretically possible near the non-pressurized flaw.