During hydraulic stimulation treatment in an EGS reservoir, it is suggested that new fractures are initiated from stimulated preexisting fractures and propagate through a reservoir. This phenomenon has been observed in laboratory-scale experiments and field-scale observations. This stimulation mechanism which includes both creating new fractures and stimulating preexisting natural fractures is called “mixed-mechanism stimulation”. In the mixed-mechanism stimulation, a fracture propagation from a preexisting fracture and the interaction of newly formed fractures and preexisting fractures play an important role in a complex fracture network creation. Especially in an EGS reservoir, preexisting fractures are large and dominate in a fracture network. Hydraulic stimulation is performed in a less permeable geothermal reservoir, which could mean that the large number of preexisting fractures are poorly connected. To better understand the mechanical interaction between fractures, how newly formed fractures propagate in a reservoir, and how fluid flows in a fracture network, a numerical model based on fracture mechanics and fluid mechanics is needed. In this study, we developed a physics-based numerical model that couples fluid flow between fracture surfaces, fracture deformation, and fracture propagation driven by fluid injection. We modeled propagation of a hydraulic fracture, shear stimulation of a preexisting fracture, and propagation of a wing crack driven by fluid injection. The objective was to investigate the effect of mechanical interaction between newly formed fractures and preexisting fractures on a fracture network creation during hydraulic stimulation treatment in an EGS reservoir. The numerical experiment results show that there are three fracture network patterns observed; 1) a hydraulic fracture crosses the preexisting fracture, and continues propagating, 2) a hydraulic fracture follows the preexisting fracture, and the preexisting fracture initiates only one wing crack, and 3) a hydraulic fracture follows the preexisting fracture, and the preexisting fracture initiates two wing cracks. Those results imply that higher injection pressure would create a more complex fracture network with the existence of well-oriented fractures and lower injection pressure would create a less complex fracture network with less flow path branching. The fracture network complexity is affected by the fracture intersection angle and the magnitude of stress shadow effects as well. Understanding the difference between the cases where only one wing crack propagates and two wing cracks propagates will be necessary to proceed this study. Also, preexisting fractures are expected to be distributed in a complex arrangement with varied orientations, size distributions, and heterogeneous connectivity in an actual EGS reservoir. Further study will be done on modeling a realistic reservoir containing numbers of fractures with a reasonable variation, and confirm the implications of this study and understand fracture network creation at a reservoir scale.

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