关键词: 页岩, 单轴压缩, 各向异性, 应力-应变, 应变能

Abstract: The bedding plane has an important effect on mechanical properties of shale and its accumulation and dissipation of strain energy. In this study, the scanning electron microscope (SEM) tests and uniaxial compression tests were carried out on Longmaxi Formation shale with different bedding plane angles. Then, this study investigated anisotropy characteristics of elastic modulus, Poisson's ratio and the stress-strain of initial cracking, critical dilatancy and peak characteristic points. The evolutions of input strain energy, releasable elastic strain energy and dissipated strain energy during the deformation and failure of shale were analysed, and the relationship among the input strain energy, bedding angles and compressive strength was revealed. The results showed that the brittle mineral content of Longmaxi Formation shale was found to be as high as 72.58%, and its microstructure was distinctly anisotropic. With the increase of bedding plane angle, the stress and strain of initial cracking, critical dilatancy and peak characteristic points were all decreased first and then increased. A minimum value was found at 30°, and the curves generally presented U-shape. With the bedding plane angle increasing, the input strain energy, releasable strain energy and dissipated strain energy of initial cracking, critical dilatancy and peak characteristic points were also decreased first and then increased, reaching a minimum value at 30°. Stress-strain and strain energy of each feature point showed significant anisotropy sensitivity, and the anisotropic sensitivities at 0°≤ ≤30°and 30°≤ ≤60°were greater than that at 60°≤ ≤90°. The peak stress had correlations linearly with both the initial cracking stress and critical dilatancy stress, while there was a corresponding quadratic nonlinear relationship between the input strain energy and compressive strength. This study provides references for shale gas drilling, reservoir fracturing and wellbore stability prediction and warning.

Key words: shale, uniaxial compression, anisotropy, stress-strain, strain energy