Numerical Investigation of Hydraulic Fracturing Induced Seismicity in Fractured Shale Media Using a Fully Coupled HMD–DFN Model

Amid the global transition to low-carbon energy systems, unconventional oil and gas resources play a key role in ensuring energy security. Hydraulic fracturing is a central technology for unconventional resource development, but it may also induce seismicity. This study investigates energy release and induced seismicity during hydraulic fracturing in naturally fractured media. A fully coupled hydro-mechanical-damage (HMD) model combined with a discrete fracture network (DFN) is developed to represent natural fractures in shale reservoirs. By incorporating frictional contact and shear slip, the model simulates tensile propagation of hydraulic fractures and shear slip of natural fractures. Parameter sensitivity analyses are conducted for natural fracture characteristics, differential stress, elastic modulus, and injection rate. The energy release of shear slip and tensile propagation is compared based on total seismic moment. Results show that the seismic moment generated by natural fracture shear slip is several orders of magnitude higher than that from tensile propagation, indicating that shear slip is the dominant energy release mechanism. Increasing the number and length of natural fractures enhances fracture-network connectivity and pressure diffusion, making natural fractures more prone to shear instability. Higher differential stress, elastic modulus, and injection rate may further promote slip-induced energy release and elevate induced seismic risk. These findings provide a theoretical basis for seismic risk assessment and parameter optimization in hydraulic fracturing.