We developed a three-dimensional discrete fracture network simulator that fully implicitly couples fluid flow with the stresses induced by fracture opening and sliding. The simulator describes flow in both newly forming hydraulic fractures and preexisting natural fractures. The boundary element method is used for the mechanical calculations, which makes it possible to perform the simulations without meshing the volume around the fractures. Leakoff is described with a semi-analytical one-dimensional leakoff approximation. To validate the implementation of the boundary element method, we simulated the opening and sliding of a circular crack and compared to analytical solutions. Next, we performed simulations of a single propagating hydraulic fracture (with and without leakoff). The results closely matched analytical solutions for fracture propagation. Finally, we performed simulations of injection induced sliding along a fault. In these simulations, injection was performed at constant pressure and leakoff was neglected. The simulations were performed until the fluid pressure in the fault was uniform and equal to the injection pressure. Consistent with results from the literature, the simulator predicted that the direction of sliding should deviate slightly from the direction of maximum resolved shear stress, as calculated considering only the remote loading conditions. Simulations were performed with the injection point at different locations along the fracture. Even though the final stress state was the same in these simulations, the distribution of sliding direction along the fault was modestly different, an indication of the path-dependence of the problem. Simulations were performed in which the direction of fracture sliding was handled with explicit, rather than implicit, time stepping. The implicit and explicit methods converged to the same answer if sufficiently small timesteps were used, but with larger timesteps, the implicit method was more accurate.
We developed a simulator that fully implicitly couples fluid flow with the stresses induced by fracture deformation (opening and sliding) in three-dimensional discrete fracture networks (DFNs). The code could be used to simulate hydraulic fracturing, induced seismicity, or other processes involving the coupling of fluid flow with fracture deformation.