The opening and propagation of a hydraulic fracture introduces stress and pressure changes around it that could potentially lead to slippages of natural fractures. If detected, the associated microseismicity is commonly used to indirectly measure the geometry of the hydraulic fracture and the stimulated rock volume. One source of microseismicity is the fluid leakoff and pressure increase into the natural fractures. As pressure increases, the effective stress is reduced and shear failure of the natural fractures can be triggered. The natural fractures can also be reactivated in absence of leakoff due to the stress changes induced in the rock due to the opening of the hydraulic fracture itself and due to the inflation of the natural fractures intersected by the main hydraulic fracture. The volume in which stress changes alone can reactivate natural fractures is commonly thought to be small and localized near the hydraulic fracture tips. Yet, this paper shows that when the presence of natural fractures is explicitly taken into account, together with the potential propagation of further stress changes as the natural fractures slip, then such reactivated volume can be substantially larger. The results presented here demonstrate the importance of local stress heterogeneities in the potential generation of microseismicity and highlight how the local stress heterogeneity directly depends on the mechanical properties of the natural fractures.

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