Reactivation of Natural Fractures Driven by Fluid Pressure and Stress Transfer During Hydraulic Stimulation: A Three-Dimensional Discrete Element Modeling Study of the Bedretto Underground Laboratory

Fluid injection into fractured crystalline rock enhances permeability by opening new fractures and reactivating natural ones, yet the relative roles of fluid pressure and elastic stress transfer remain insufficiently constrained. In this study, we develop a fully coupled hydro-mechanical Particle Flow Code in 3 Dimensions (PFC3D) model calibrated against two mini-frac tests in Rotondo Granite at the Bedretto Underground Laboratory. Applied to intact and naturally fractured intervals, the model reproduces the observed pressure evolution and enables quantitative analysis of fracture slip and stress redistribution, thereby revealing two distinct reactivation mechanisms. The first mechanism arises from effective stress reduction. Elevated pore pressure lowers the effective normal stress and weakens frictional resistance, leading to localized and directionally consistent shear within the high-pressure core. Weaker and more diffuse slip develops outward following the pattern of elastic stress perturbation, and minor shear failure appears at the far edge of the fluid-affected region due to shear stress transfer acting on the compressed faces of the opening fracture. This spatially hierarchical slip structure reflects a transition from deformation dominated by effective stress reduction to deformation dominated by elastic stress transfer. The second mechanism is governed by elastic stress transfer. Deformation of pressurized fractures redistributes surrounding stresses and induces weak, remote shear on neighboring fractures that remain disconnected from the fluid. The resulting stress perturbation resembles that generated by localized volumetric expansion and promotes slip on nearby fractures. An analytical estimate indicates that the radial extent of stress perturbation exceeds the fluid-pressurized region and increases with injected volume while decreasing with rock stiffness. These results establish a unified, field-calibrated framework linking fluid pressure, fracture deformation, and stress redistribution during hydraulic stimulation.