Fracture Reactivation in Shale: Laboratory Testing and Analytical Modeling

Summary This paper presents a tool for first-order evaluation of fracture/fault reactivation due to fluid injection. It also provides frictional strength parameters of a shale from direct shear testing. The in-house tool, FracStress, was developed based on the analytical Mohr-Coulomb failure criteria and 3D transformation of in-situ stress conditions. The software accounts for the geometry of fracture/fault, 3D stress field, pore pressure and frictional properties. It calculates and visualizes the state of stress for any given fracture orientation. Friction and cohesion of a pre-fractured shale is determined through a direct shear test. The state of stress and frictional strength envelope of material are plotted to estimate the critical injection pressure for the material. The developed tool was tested using the stress condition and test results of Draupne shale in the Ling Depression, the North Sea. The analysis shows that for stress conditions present at the depth of 2582 m, Ling Depression, fractures dipping 60° are close to failure condition. However, fractures dipping 30° and 45° will sustain injection pressure of between 2 to 9 MPa depending on the state of horizontal stresses in the area and the friction coefficient of material.