For the studies on hydraulic fracturing (HF), reservoir formations with natural fractures and their interaction with hydraulic fractures are more concerned. Stochastic Microcracks Rock Model is the microscopic numerical geo-model, which applies the Monte Carlo simulation of stochastic microcracks using Distinct Element Method (DEM). This model compiled in dynamic modeling codes presents more realistic behavior of the rock formations. For characterizing rock mass, the microseismic events are wildly regarded as the indicators of shear slip, and for unconventional reservoir, the induced microseismicity (MS) by HF can be applied in creation more flowing paths to increase very low permeability. Therefore, it is essential to investigate the MS source mechanisms in the rock mass, and with the effect of pre-existing fractures. In this paper, a modified analytical model of microseismicity is derived and verified with numerical simulations coupling Discrete Element Method by Particle Flow Code (PFC3D). The results show that there are the correlation between shearing microseismic event and interaction between hydraulic fractures and natural fractures; and fracture fluid could also contribute to the shear failure during stimulation. This combined numerical modeling provides a unique alternative method for understanding the controlling factors of MS gen-eration and its effect on stimulation. In addition, microseismic events possess the potential to characterize the fracture geometry qualitatively, to evaluate the fluid behavior in fractured formation, to predict fracture propagation, and ultimately to optimize stimulation design.

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