Fracture Toughness Anisotropy in Shale Under Deep In Situ Stress Conditions

Insight into the anisotropic fracture properties of shale under a high in situ stress is important for the design and optimization of hydraulic fracturing treatment for deep shale formations. In this work, a series of hydraulic fracturing tests were conducted on hollow double wing crack specimens of Longmaxi shale with three principal fracture orientations: arrester, divider, and short transverse orientations. To reveal the influence of in situ stress on shale fractures, various confining pressures, i.e., 0, 20, 40, 60, and 80 MPa, were applied. The 3D anisotropic fracture properties of shale under deep in situ stress conditions were then investigated and highlighted. The breakdown pressure, mode I fracture toughness, and fracture energy were found to increase significantly with the confining pressure, indicating that the fracture resistance of the studied shale increases remarkably under high in situ stresses. Furthermore, as expected, both the fracture toughness and fracture energy exhibited remarkable anisotropies under each confining pressure condition, and the minimum fracture toughness and fracture energy were always found in the short transverse orientation. Interestingly, the fracture toughness and fracture energy in the arrester orientation were found to be smaller than those in the divider orientation under confining pressures ranging from 0 to 60 MPa, while the opposite was true under a confining pressure of 80 MPa, demonstrating that shale fracture anisotropy is influenced by in situ stress that should be carefully considered in hydraulic fracturing design. For fracture morphology, a higher confining pressure leads to a smoother fracture surface and more transgranular cracking.