Abstract:

Anisotropy shear behaviors of shale are obvious as many bedding planes exist in shale. Determining the shear failure behavior of shale is essential for predicting fracture growth geometry and shear fracture permeability. This paper investigates the mechanical properties of bedding planes and the anisotropy of shear behaviors for shale. A series of shear tests are conducted using an open-box-style inclined device on shale samples (60 mm × 60 mm × 60 mm) from Lucaogou formation in China. After the shear test, the shear fracture surfaces and stress-strain curves are obtained. Shear properties of shale under various angles between the shear stress and bedding planes are analyzed. The results show that when the shear stress is parallel to shale bedding planes, the internal friction angle and cohesion are significantly smaller than those with shear stress perpendicular to bedding planes. The peak shear strength increases with the shear angle between the shear stress and bedding planes. The shear failure mechanism of shale can be divided into three types: shearing along the bedding planes, shearing across the shale matrix together with sliding along the bedding planes, and shearing across the shale matrix. Shear fracture surfaces are planar for shearing along bedding planes and shear fracture surfaces are serrated for the shearing across shale matrix. The obtained results contribute to the understanding of anisotropy shear failure in shale formations and can also be used to model hydraulic fracture growth in shale formations.

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