Numerical Investigation on Grain-Scale Crack Evolution During Hydraulic Fracture Propagation in Sandstone: Insights from Gradient Pore Water Pressure

The incomplete mechanism of grain-scale hydraulic fracture (HF) propagation induced by gradient pore pressure limited the understanding of multi-scale HF propagation. This paper studied the grain-scale evolution of cracks under the disturbance of gradient pore water pressure. A sandstone meso-structure construction method based on the K-means clustering algorithm was proposed. A fluid–solid force transfer principle of fluid–solid coupling was improved to achieve the loading of pore water pressure in the form of body forces. The results show that the continuous intensified disturbance of gradient pore water pressure serves as the dominant factor controlling the initiation and coalescence of grain-scale microcracks within the tensile force-chain region, the formation of microscopic HFs, and final opening into macroscopic HFs. Influenced by the grain-scale meso-heterogeneity in permeability, the originally isolated pore clusters become seepage channels non-collinear with HFs when partially connected with HFs, inducing the bifurcation of the pore water pressure field. Influenced by the meso-heterogeneity in tensile strength, discrete micro-cracks initiate in the bifurcated region of the pore pressure field, further inducing more complex HF propagation modes such as grain-scale deflection and discontinuous propagation. Grain-scale evolved cracks interconnect isolated pore clusters in rock to improve connectivity.