Strength softening pattern of shale lamina under CO<sub>2</sub>-water interaction based on scratch technique

Due to the small thickness of the laminae，traditional experimental methods such as single triaxial based on standard cylindrical samples are difficult to respond to the effect of strong inhomogeneity of the laminated shale on the mechanical properties. In this paper，the laminated shale of the Qingshankou Formation was selected to conduct scratch and CO₂soaking experiments，and the softening pattern of different laminated strengths under different immersion times was analyzed. The results reveal that as the soaking time increases，induced cracks form around the scratched area，with the shape of a“Y”. Supercritical CO₂-water had the most significant effect on calcite-rich lamina. Compared with the original shale specimens，the compressive strength decreased by 48.1%，57.3%，and 60.3%，and the elasticity modulus decreased by 39.3%，49.2% and 50.5%，respectively，with the increase of the soaking time(1，2，3 d). The compressive strength and elasticity modulus of the clay-rich lamina first showed a rapid decreasing trend with increasing soaking time and then remained constant. The compressive strength and elastic modulus of the quartz/calcite lamina first decreased and then slightly increased. This study provides a new perspective on the softening behavior of shale under supercritical CO₂-water interaction.