Hydraulic fracturing in high-temperature crystalline geothermal reservoirs requires reliable prediction of fracture initiation and growth under coupled thermal damage and microstructural heterogeneity. Most existing studies assume homogeneous rocks or ambient conditions, which limits mechanistic understanding of how thermal damage and grain-scale heterogeneity jointly govern fracture initiation, propagation and connectivity in high-temperature crystalline geothermal reservoirs. We develop a coupled fluid-solid discrete element framework (Hydra-Hete-GBM) that explicitly represents grains and grain boundaries. Using this framework, we systematically investigate the effects of temperature (150–600 \(^{\circ }\text{C}\) ), differential stress (2.5–10 MPa), and injection rate (2–5 \(\times\) 10 \(^{-5}~{\text {m}}^{2}/{\text {s}}\) ). Model predictions are validated against published laboratory hydraulic fracturing experiments reported in the literature, showing consistent trends in pressure evolution and fracture morphology. Results indicate that the maximum principal stress controls the macroscopic propagation path, whereas grain-scale heterogeneity regulates local deflection and branching. Uniform microstructures promote stable planar growth, while structurally heterogeneous domains trigger stress concentrations that increase branching and tortuosity. Fine-grained media are dominated by tensile failure and yield smoother paths; coarse-grained media exhibit stronger grain boundary guidance and more mixed-mode failure. Increasing injection rate and stress differential both enhance directionality and suppress heterogeneity-driven instability. Elevated temperature promotes distributed thermal cracking. Above \(450~^\circ \text{C}\) , thermally induced microcracking increases total crack length, but reduces the coherence of the hydraulically effective fracture path, with network connectivity decreasing further as heterogeneity increases. These findings provide a mechanistic basis for tailoring injection strategy and stress management to improve fracture directionality, connectivity, and stimulation efficiency in high-temperature crystalline geothermal reservoirs.
