Numerical analysis of hydro-mechanical coupling behaviour during shearing of rock fractures based on an improved friction factor model

Both the complex geometrical morphology of rough-walled rock fractures and the nonlinearity of fluid flow contribute to resistance in fluid flow through rock fractures. The interactions of the shear-flow process further complicate the characterisation of flow behaviours in rock fractures. In this study, an improved friction factor model involving both the effects of viscous and inertial forces is presented based on the Forchheimer equation. The model incorporates two key variables, i.e. Reynolds number and relative roughness, which reflect the effects of flow regimes and fracture roughness, respectively. The changes in geometrical parameters induced by shearing are considered, with the peak asperity height predicted through a correlation with post-peak roughness degradation. The hydraulic aperture during shearing is estimated using a suggested equation that accounts for the mobilised contact area ratio and variable aperture distribution. The parametric sensitivity analysis reveals that shear-induced changes in fracture geometry enhance the flow nonlinearity in rock fractures. The model performs well in predicting the friction factor based on two validation criteria. Then, the proposed friction factor model is incorporated into the three-dimensional distinct element code (3DEC) in the form of the Darcy-Weisbach equation. Coupled with the numerically implemented mechanical model and hydraulic aperture prediction model, numerical simulations of coupled shear-flow processes in single rock fractures are conducted. The simulation outcomes are validated through comparison with the experimental results, showing acceptable agreement and demonstrating that the numerical model is capable of accurately evaluating the hydro-mechanical coupling behaviour during the shearing of rock fractures.