A Cohesive Element-Based Numerical Manifold Method for Hydraulic Fracturing Modelling with Voronoi Grains

In this study, the cohesive element-based numerical manifold method with Voronoi grains is extended by incorporating a coupled hydro-mechanical (HM) model to investigate hydraulic fracturing of rock at micro-scale. The proposed hydraulic solving framework, which explicitly calculates the flow rate and fluid pressure of a compressible viscous fluid based on the cubic law and a linear fluid compressibility model, is first validated against analytical solutions for uncoupled transient and steady flow examples. Then the coupled HM procedure is further verified by two coupled examples, which respectively considers the elastic response of a pressurized fracture and hydraulic fracture (HF) propagation under different perforation inclinations and in situ stresses. Finally, the developed method is adopted to investigate the hydraulic fracture propagation in Augig granite possessing multi-fractures at micro-scale, based on which the effect of friction coefficient of natural fractures (NFs) on hydraulic fracture propagation is examined. The results show that the friction coefficient of the NFs has significant effects on the induced hydraulic fracture pattern. With increasing friction coefficient of the NFs, it becomes more difficult for the NFs to fail, which results in simpler HF patterns. This phenomenon is associated with the change in the type of interaction between HFs and NFs, i.e., from HFs being arrested by NFs to HFs crossing the NFs with offsets and then to HFs directly crossing NFs.