Anisotropic strength and failure behaviors of transversely isotropic shales: An experimental investigation

Abstract

The deformation and fracture of anisotropic shale rocks are of great interest to geoengineers concerned with assessing the stability of boreholes and underground excavations. However, some mechanic properties associated with anisotropic rock fracture remain ambiguous. We have purposely reported triaxial failure tests on two sets of transversely isotropic shale plugs (i.e., Marcellus and Eagle Ford shales) cutting parallel and perpendicular to the bedding planes. The experimental results suggest that the bedding planes give rise to considerable differences in the elastic properties, failure strength, and failure modes in the bedding-normal and bedding-parallel directions. In general, the static Young’s modulus is higher parallel to bedding than normal to bedding, whereas the static Poisson’s ratios measured from the vertical and horizontal shale plugs do not exhibit a certain relationship. The ratio between the bedding-parallel and bedding-normal failure strength is approximately 1.9 for Marcellus shales, whereas it is approximately 1.3 for Eagle Ford shales. At the failure point, the axial strains are appreciably larger than the radial strains. In contrast, the two orthogonal radial strains exhibit different characteristics in the bedding-normal and bedding-parallel directions, giving rise to complex failure modes. The extended cracks in the vertical plugs lead to fracturing in a shearing manner, with a single shear plane in the Eagle Ford shale and two conjugate shear planes in the Marcellus shale. In the horizontal plugs, the Eagle Ford shale exhibits shear banding in the bedding-normal direction and a combination of shear and extension in the bedding-normal direction. In contrast, the Marcellus shale exhibits three conjugated shear planes.