Effects of SC-CO₂ and Brine on the Anisotropic Mechanical Properties of Shale

The process of CO₂ waterless fracturing and enhanced production of shale gas involves a physical and chemical reaction between supercritical carbon dioxide (SC-CO₂), brine, and shale rock, and this reaction can alter the physical and mechanical properties of shale rock. However, there has been limited investigation into the evolution of anisotropic mechanical characteristics. Therefore, uniaxial compression experiments were conducted on shale samples with varying bedding angles, soaked in SC-CO₂ + brine for different durations. The evolution of shale strength, elasticity, failure mode, acoustic emission (AE) signal, and fractal dimension under varying bedding angles and soaking times was analyzed, and the evolution characteristics of shale anisotropy were defined. The results indicate that: An increase in soaking time results in a notable softening of the complete stress-strain curve, accompanied by a gradual decline in uniaxial strength and elastic modulus, a gradual increase in peak strain and Poisson's ratio, and a gradual intensification of the shale dilatation phenomenon. The AE signals in the compaction and yield failure stages are significantly enhanced following soaking. The AE fractal dimension is observed to increase with the increase of bedding inclination and soaking time. This indicates that the complexity and irregularity of shale deformation and failure are stronger after soaking. The shale failure mode is closely related to the bedding inclination and soaking time. For bedding inclinations of 45° and 60°, shear failure along the bedding plane is the predominant mode of failure, and the shale failure that occurs after soaking is prone to produce more secondary cracks, resulting in a more thorough degree of shale failure. Following prolonged immersion of SC-CO₂ + brine, the anisotropy of shale strength, elasticity, acoustic emission signal, and acoustic emission fractal dimension is markedly augmented. This phenomenon can be attributed to the ease with which SC-CO₂ + brine invades along the bedding plane, thereby continuously weakening interbedding cementation. This, in turn, results in a gradual increase in the difference between the bedding plane and the matrix, which significantly enhances the anisotropy.