Effect of Fabric on Fracture Toughness and Crack Propagation Behavior of Gneiss

The anisotropy of fracture behavior of gneiss is a key constraint to the fracturing of reservoir fracture network. The differences of fracture toughness, fracture energy and fracture morphology of gneiss in different directions are studied by selecting gneiss in Daqingshan of Inner Mongolia. The results show that: The fracture process of gneiss specimens under three-points bending loading includes four stages: pore compression, elastic compression deformation, fracture and rapid unloading. The brittleness of gneiss is significant, which is beneficial for volumetric fracturing. The fracture toughness of gneiss ranges from 0.93 to 1.28 MPa·m^0.5, and the fracture performance of gneiss varies greatly in different orientations. The vertical bedding/vertical lineation > parallel bedding/vertical lineation > vertical bedding/parallel lineation > parallel bedding/parallel lineation has certain mechanical constraints on the selection of fracture propagation direction. In the vertical schistosity direction, the fracture propagation of gneiss is seriously affected by the weak plane. The shear and displacement of cracks occur at the schistosity face and form bedding-parallel fractures. In the later stage, the fractures turn to extend along the loading direction. In the parallel schistosity direction, the fractures of gneiss extend parallel to the maximum principal stress orientation. The fracture surface is flat, and the fracture tortuosity is small, which is not conducive to fracture network formation. The breaking energy consumption of samples in vertical bedding/vertical lineation, parallel bedding/vertical lineation, vertical bedding/parallel lineation, parallel bedding/parallel lineation direction is 0.47, 0.43, 0.37 and 0.25 J, respectively, and the energy consumption of fracture in each direction of gneiss varies greatly. In order to expand the reservoir volume fracturing coverage, it is recommended to drill horizontal wells parallel to gneiss lineation.