Coupled Hydro-Mechanical Investigation of Fracture Propagation and Seismicity of Hydrofracturing in Naturally Fractured Rock

Hydraulic fracturing in naturally fractured rock is governed by complex interactions between fluid flow, rock deformation, fracture propagation, and induced seismicity. In this study, a fully coupled hydro-mechanical framework based on the FDEM is developed to investigate fracture evolution and seismic responses during fluid injection in fractured rock masses. Three representative horizontal stress ratios (R = 1.0, 1.5, and 2.0) were considered to investigate the influence of stress anisotropy on fracture propagation and induced seismicity. The results demonstrate that stress anisotropy exerts a dominant control on fracture propagation patterns, fluid pressure diffusion, and induced seismicity. Under low stress ratios, fracture propagation is diffuse and strongly influenced by pre-existing fractures, whereas higher stress ratios promote localized, directional fracture growth controlled primarily by the stress field. Fluid pressure becomes increasingly concentrated with increasing stress ratio, leading to higher injection pressures and more pronounced pressure fluctuations. The spatial and temporal evolution of mean stress and volumetric strain closely follows that of fluid pressure, indicating that fluid pressurization directly controls effective stress reduction and associated deformation. Seismic analysis reveals a systematic decrease in the Gutenberg–Richter b-value with increasing stress ratio, indicating a transition from distributed micro-fracturing to more coherent fracture reactivation and larger seismic events. Under quasi-steady injection pressure conditions, fracture propagation is found to be episodic and unstable, as evidenced by pronounced positive and negative spikes in the fracture volume change rate and associated pressure fluctuations; these are accompanied by intermittent fracture opening and closure, stress redistribution, and temporary reductions in cumulative seismic moment. These findings provide new insights into the coupled mechanisms governing hydrofracturing-induced seismicity and have important implications for the assessment and mitigation of seismic risks in subsurface engineering applications.