Optimization of stimulation is key to successful development of unconventional reservoirs. Microseismic monitoring is the most powerful tool to help us understand where and what is happening during and after the stimulation. Yet very little is understood about the relationship between microseismicity and hydraulic fractures: some believe microseismic events are part of the hydraulic fractures, some believe they are resulting from stress changes and fluid leak-off. Microseismic datasets with accurate event locations complemented with source mechanisms lead us to a new level of understanding of the interaction between hydraulic fracturing and seismic response. There are at least four geomechanical models to explain observed failure mechanisms and the opening (or closing) of hydraulic fractures (seismic tensile opening, leak-off cloud of seismicity around the hydraulic fracture, shearing between aseismic tensile opening and horizontal fractures shearing on vertical planes). Unfortunately none of these models is consistent with observations presented in this study. Hence we developed a new geomechanical model of bedding plane slippage and vertical shearing induced by hydraulic fractures in shale reservoirs.

We present a case study including detailed source mechanism inversion for a microseismic dataset from hydraulic fracturing of a shale gas play in the North America. We observe source mechanisms dominated by shear failure with dip-slip and strike-slip sense of motion. The dip-slip mechanisms are prevailingly oriented with shear planes along the maximum horizontal stress. This can be explained as slippage on beddings planes caused by aseismic opening of hydraulic fractures. The strike-slip mechanism show small but real components of non-shear deformation. This can be also explained as slippage on vertical plane perpendicular to maximum horizontal stress with slight opening as these events are direct part of the hydraulic fracture. This model explains large energy difference between seismic and hydraulic energy, and prevailing orientation of the shear planes of the induced microseismic events. In addition, the bedding planes are weak planes in the shale formation likely to fail. The model can better constrain fundamental parameters of induced hydraulic fractures and describe hydraulic fractures and their interaction with the shale plays.

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