In this paper a numerical fracture model based on the displacement discontinuity boundary element method and a finite difference method is presented for solving the problem of coupled rock deformation, fluid transport and interface slip associated with hydraulic fracture propagation across frictional interfaces. The rocks on both sides of the interface are assumed to be impermeable, isotropic, and elastic materials. A Newtonian fluid is injected at a constant rate into the hydraulic fracture. The propagating fracture may induce a tensile stress in excess of the rock strength on the intact side of the interface. In addition, if the interface fails in shear, frictional sliding can also induce tensile stress that may result in fracture initiation in the intact rock. A simple criterion based on the critical tensile strength of the intact rock is used to predict whether a new fracture can initiate at any position along the interface. In the model, only one new fracture is permitted to be introduced. The crossing interaction gives rise to step-over or straight fracture paths depending on the rock strength. The newly-created fracture can be propagated further by fluid entering and pressurizing it, which finally provides the hydraulic fracture a way to cross or escape the interface. Numerical results are presented as a function of rock strength for the time-dependent variations of fluid pressure, crack opening and fluid lag, fracture paths with and without offsets on the interface. Special attention is devoted to the problem of fluid flow associated with different opening profiles and fluid loss from the main fracture into secondary interface fractures during fracture crossing.
A propagating hydraulic fracture often intersects and interacts with existing frictional interfaces in layered sedimentary rocks.