Experimental study on triaxial mechanics and failure characteristics of shale in different brine environments

Shale is an ideal caprock for carbon dioxide sequestration projects in deep saline aquifers. Different brine solutions and confining pressures have a significant effect on the mechanical behaviors of caprock shale, which also affects the safety and stability of carbon dioxide geological storage projects in deep saline aquifers. Therefore, the conventional triaxial compression tests are carried out on the shale specimens saturated with different brine solutions, and their mechanical parameters and failure characteristics are analyzed. Compared with those of the dry specimens, the peak strength and elastic modulus of the specimens saturated with different brine solutions all show a downward trend to different degrees under different confining pressures; and the shale specimens saturated with 5% NaCl solution have the most obvious deterioration effects on the mechanical parameters, followed by the shale specimens saturated with 5% K₂SO₄ solution. The failure modes of the shale specimens saturated with different solutions show more secondary cracks, which are mainly along the bedding direction. By conducting microscopic tests to further analyze the failure characteristics of the specimens, it is found that the main reason for the deterioration of the macro-mechanical properties of the shale saturated with brine solution is that the brine solution intrudes into the shale through the bedding plane and dissolves and erodes the minerals in the bedding plane. Finally, based on the energy evolution laws in the whole process of shale stress-strain curve, the brittleness evaluation of the shale specimens saturated with different brine solutions is carried out. It is found that the brittleness index of the shale specimens saturated with brine solution decreases significantly and is sensitive to the confining pressure.