Abstract: This paper studies the anisotropic properties of shale formations in the shale gas blocks of Pengshui in Chongqing city. Uniaxial and triaxial compression tests on specimens of Longmaxi formation in Shizhu county were carried out. The anisotropy of mechanical properties, strength and failure modes were analyzed. The anisotropic failure mechanisms were revealed. The results show that: (1) Significant anisotropy is observed on the specimens. The elastic modulus parallel to bedding planes is the maximum, and the modulus perpendicular to bedding planes is the minimum. The increasing rate of elastic modulus decreases gradually as the confining pressure increases, while the variation trend of Poisson's ratio with bedding orientations of β=0°, 30°, 60° and 90° is opposite. The well-developed pore and microcrack in the bedding planes maybe contribute to these results. (2) The compressive strength of the specimen for β=0° is the maximum, the value for β=90° is a little less, and the minimum is in the orientation of β=30° at the same confining pressure. As the bedding orientation changes, the compressive strength curves present U-shape. The anisotropic compressive strength variations can be better described with the Hoek-Brown failure criterion of different orientations. (3) The anisotropic failure modes are mainly resulted from the anisotropy of failure mechanisms. The strength anisotropy is governed by the anisotropy of failure mechanisms. Under the condition of uniaxial compression, tensile splitting along bedding planes occurs for β=0°, sliding failure along bedding planes occurs for β=30°, composite shear failure across and along bedding planes occurs for β=60°, tensile splitting across bedding planes occurs for β=90°. But under the condition of triaxial compression, conjugate shear failure across bedding planes occurs for β=0°, sliding failure along bedding planes occurs for β=30°, shear failure across bedding planes occurs for β=60° and 90°. The layered sedimentary structure and weak cementing strength of bedding planes are the causes of the anisotropy of failure mechanisms. The results can provide references for the wellbore stability analysis of horizontal wells and hydraulic fracturing design in the shale gas exploitation.

Key words: shale, anisotropy, bedding planes, Hoek-Brown failure criterion, failure modes, failure mechanism