Anisotropic characteristics of stress and strain thresholds of Longmaxi shale

Under the combined action of bedding plane and confining pressure, the anisotropic characteristics of the stress thresholds and the corresponding strains are one of the important factors affecting the brittle mechanical behaviors of shale. Taking Longmaxi formation shale as an example, the variation laws of stress-strain thresholds under different confining pressures and different bedding plane dip angles are analyzed. The results show that: (1) The crack initiation stress${\sigma }_{\text{ci}}$, damage stress${\sigma }_{\text{cd}}$ and peak stress${\sigma }_{\text{f}}$ increase approximately linearly with the increase of the confining pressure, while the closure stress${\sigma }_{\text{cc}}$ is not affected by the confining pressure. (2) The dip angle of bedding plane has a significant effect on${\sigma }_{\text{cd}}$ and${\sigma }_{\text{f}}$, while${\sigma }_{\text{cc}}$ and${\sigma }_{\text{ci}}$ are not affected by the dip angle. Under the low confining pressure, the stress ratios${\sigma }_{\text{cd}}$/${\sigma }_{\text{f}}$,${\sigma }_{\text{ci}}$/${\sigma }_{\text{f}}$ and${\sigma }_{\text{cc}}$/${\sigma }_{\text{f}}$ fluctuate with the increase of the dip angle, and tend to be stable with the increase of the confining pressure. (3) The axial strains and radial strains corresponding to the stress thresholds show the same change rules with the increase of the confining pressure: the peak strain increases the fastest, followed by the damage strain, and the crack initiation strain is the smallest. (4) The axial strains and radial strains corresponding to the stress thresholds show the same change laws with the increase of the inclination angle: the peak strain decreases first and then increases with the increase of the inclination angle from 0° to 90°, and the closure strain, crack initiation strain and damage strain are not strongly related to the dip angle.