Dynamic Mohr–Coulomb evaluation of natural fracture stability in deep shale under hydraulic fracturing induced stress superposition

Abstract

Natural fractures influence hydraulic fracture propagation, pressure diffusion, casing integrity and stimulation performance in deep unconventional reservoirs. Conventional Mohr-Coulomb stability analysis often assumes a fixed or weakly updated stress field, which cannot capture evolving geomechanical perturbations caused by hydraulic fracturing. This study develops a dynamic Mohr-Coulomb stability evaluation framework based on hydraulic fracture induced stress superposition. The framework integrates laboratory constrained rock mechanical parameters, three-dimensional geomechanical modeling, induced stress redistribution and dynamic updating of the Mohr stress circle during fracture propagation. Continental shale and marine shale cases are used to test its applicability under different tectonic and sedimentary conditions. The simulations show that hydraulic fracturing increases three principal stresses, with the minimum horizontal principal stress showing the strongest response. The effective stress-superposition range is mainly confined to adjacent fracturing stages, indicating a localized but mechanically significant interaction zone around active hydraulic fractures. Static analysis shows that the critical pore pressure increment required for natural fracture slip varies nonlinearly with fracture approach angle, defining a friction controlled stability window. Dynamic analysis further incorporates changes in principal stress, pore pressure, maximum horizontal stress orientation and fracture cohesion. Comparison with treatment pressure responses and microseismic observations shows that the predicted activation risk zones are consistent with field scale fracture activation. Pumping rate optimization reduces these zones by weakening induced stress and pore pressure perturbations near critically oriented natural fractures. The proposed framework provides a quantitative basis for diagnosing natural fracture activation risk, mitigating casing deformation and optimizing fracturing parameters in deep fractured shale reservoirs.