A study on the brittleness and progressive failure process of anisotropic shale

As the weak part in shale formations, the bedding plane is an important element affecting the stability of boreholes and the formation of hydraulic fracture networks. To study the influence of bedding plane on the progressive failure process of rock, a series of uniaxial compressive tests were conducted on shale samples with bedding plane orientations (θ) of 0–90° with respect to the axial stress directions. The pre-peak failure process of shale observed during the tests is divided into three stages: the elastic deformation stage during which axial strains mainly occur, the stable crack growth stage during which the axial strains equal the lateral strains except for θ = 0° and 15°, and the unstable crack growth stage during which lateral strains are dominant. The peak strengths, crack damage stresses, and the corresponding strains are greatly influenced by the initial anisotropy; however, the crack initiation stresses and corresponding strain thresholds change only slightly. While the normalized lateral strain (ε3cd/ε3ucs) for the crack damage stress is strongly affected by the bedding structure, the other normalized stress and strain quantities can be used to predict the failure process of anisotropic rocks. The newly developed measure of brittleness index m, determined by the peak strength, peak axial strain, and elastic modulus, reflects the whole progressive failure process and was verified in the tests as a reasonable and accurate index.