Complex Fluid-Driven Fractures Caused by Crack-Parallel Stress

Managing fluid-driven fracture networks is crucial for subsurface resource utilization, yet the current understanding of the key controlling factors remains insufficient. While geologic discontinuities have been shown to significantly influence fracture network complexity, this study identifies another major contributor. We conducted a new set of experiments using a transparent true triaxial cell, which enabled video recording of the temporal evolution of fluid-driven fracture paths. Using pseudo-2D samples without macroscale structural discontinuities, we observed multiple occurrences of hydraulic fracture curving and branching under anisotropic boundary stresses. We proposed a theoretical model demonstrating that the stress parallel to the crack line in the solid matrix near the crack tip (i.e., the T-stress) accounts for the observed fracture curving behavior. This finding suggests that T-stress is an additional mechanism contributing to the complexity of fluid-driven fracture networks in the subsurface, besides the geologic discontinuities.