Modeling Interactions between Hydraulic and Closed Natural Fractures in Brittle Crystalline Rocks: A Fluid–Solid Coupling Grain-Based Approach for Characterizing Microcracking Behaviors

Activating closed natural fractures (NFs) in hot dry rock (HDR) reservoirs is a critical way to improve fluid conductivity and stimulate production. However, the current hydraulic fracturing simulation technology is limited in its ability to investigate the interaction mechanism between hydraulic fractures (HFs) and NFs under the interference of mineral heterogeneity in HDR. In this study, we combine a modified hydro-grain-based model (hydro-GBM) and the smooth joint model (SJM) to establish a discrete element model of granite with closed NFs, investigating the effects of in-situ stress, approach angle, and physico-mechanical properties of NFs on interaction modes. Our results show that mineral heterogeneity induces HFs on both sides of the borehole to propagate asymmetrically along low-strength minerals and mineral boundaries, enhancing the complexity of HFs. As the approach angle, NF interface friction coefficient, or NF bond strengths decrease and differential in-situ stress or NF permeability increases, the offset of HFs propagating along NFs increases, thus promoting the activation degree of NFs and resulting in a decline in the average activity level of acoustic emission (AE) events and the proportion of large events in NFs. Furthermore, numerical simulations reveal the evolution laws of fracturing results, such as breakdown pressure, fracture propagation pressure, spatial distribution of microcracks, fluid pressure field, and normal stresses on NFs, which provide valuable insights for constructing complex and efficient fracture networks in HDR reservoirs.