A 3D Analysis of Rock Failure around Hydraulic Fractures with Reference to Induced Seismicity

Abstract

Three-dimensional stress and pore pressure distributions around a hydraulic fracture are numerically calculated to analyze the potential for rock failure resulting from pressurization of the hydraulic fracture. The three-dimensional numerical model combines the finite element and the poroelastic displacement discontinuity methods. The model is applied to the problem of constant water injection into a fracture in Westerly granite. Rock failure is assessed using the Mohr-Coulomb failure criterion with tension cut-off. Results show that rock failure can occur in the vicinity of the fracture, especially near the fracture tips. The dominant failure mode is tension in the close vicinity of the fracture where the pore pressure attains its highest values. Shear failure potential exists near the tip area and away from the fracture walls where shear stresses are sufficiently high. For the relatively strong intact rock considered, the extent of shear failure zone is limited. This can change by considering the rock mass strength, and the possibility of slip on pre-existing joints and cracks, increasing the injection rate, and considering rock heterogeneity. Simulation of injection into multiple fractures shows the development of a much larger critically stressed area with potential for micro-seismicity.