Regime-dependent hydraulic fracture propagation in heterogeneous crystalline rocks: a mesostructure-resolved hydromechanical DEM study

Understanding how hydraulic fractures interact with rock heterogeneity across different propagation regimes is essential for interpreting fracture evolution in deep crystalline reservoirs. This study presents an improved mesostructure-resolved hydromechanical DEM model to investigate fracture propagation in granite, explicitly capturing the interaction between realistic mineral grain structures and fluid flow. The numerical model is benchmarked against the KGD analytical solution. The dimensionless toughness (Formula presented) is used to delineate the transition between dominant energy dissipation mechanisms, distinguishing fracture growth controlled primarily by fluid viscosity from that governed by rock fracture energy. The results reveal distinct, regime-dependent propagation behaviors. In the viscosity-dominated regime, a steep pressure gradient along the fracture provides viscous stabilization, resulting in planar and nearly symmetric growth that effectively suppresses the influence of local mineralogical heterogeneity. Conversely, the toughness-dominated regime promotes unstable fracture propagation, manifested by step-wise advance and asymmetric growth due to the increased sensitivity of the fracture tip to local mineral-scale heterogeneity. Furthermore, thermal treatment intensifies mesoscale heterogeneity through differential thermal expansion among constituent minerals, generating heterogeneous internal stresses and inducing intergranular microcracks that act as preferential pathways and thus enhance fracture network complexity. These findings clarify the coupling between fracture propagation regime and mesoscale heterogeneity and provide a mechanistic basis for evaluating injection conditions in geothermal reservoirs. © 2026 The Authors. Published by Elsevier Ltd.