Numerical investigation into mechanical responses of frictional discontinuities with hydraulic fractures

Understanding the mechanical responses of frictional discontinuities (FDs) with hydraulic fractures (HFs) is essential to interpret microseismic events and fracture propagation for geomechanics and geophysics. However, the mechanical analysis is not clearly studied using a robust numerical method because of its complexity with nonlinear constraints. In the study, we present a 2D boundary element model with a complementarity algorithm to investigate this nonlinear contact problem in HF-FD interaction. The new model enforces appropriate contact boundary conditions of the FD through a complementarity algorithm. All the contact modes of the FD, including sticking, opening, and slipping, can be precisely described and efficiently determined. The new model avoids the drawbacks of using trial calculations or fracture stiffness as in previous studies to prevent the interpenetration of fracture surfaces. The model has been validated against analytical solutions of frictional inclined fractures and previously published numerical solutions of HF-FD interaction. Parametric studies show that the FD's opening or slipping responses generally exist in the HF-FD contact position neighborhood. FD slippage is the dominant mechanical response induced by the approaching pressurized HF. Such slippage can be reduced by increasing in its friction coefficient or friction strength, whereas FD opening can be increased under higher friction coefficient and strength. The higher the fluid pressure in the HF, the more likely the HF crosses the FD; thus, using viscous fluid or high pumping rate is conducive to HF crossing FDs. HFs tend to cross FDs with a slight shift, especially in formations with variable friction coefficients or cohesions along the FD. This is a new mechanism used to interpret fracture offset observed in geomechanics and geophysics. The study provides a systematical analysis of the mechanism for HF-FD interaction and can help investigate the mechanism of microseismic events and complex HF propagation.