A fracture network created by a hydraulic stimulation treatment in an EGS reservoir is often complex, including both newly formed fractures and preexisting natural fractures. One of the possible mechanisms of the reservoir permeability enhancement is a wing crack propagation from a sheared preexisting natural fracture. Wing cracks are initiated by the stress concentration at the sheared fracture tips. Well oriented fractures could slip at an injection pressure lower than the least principal stress, therefore it is not unrealistic to assume that those sheared fractures initiate wing cracks from their edges. To investigate the effect of wing cracks on the reservoir permeability enhancement during hydraulic stimulation treatment in an EGS reservoir, we modeled the wing crack propagation driven by fluid injection in a setting of hydraulic stimulation by injecting water into a preexisting natural fracture at varied fracture angles and injection pressure. Our results suggest that wing cracks are likely to have high transmissivity and large fluid storage volume after hydraulic stimulation treatment.

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