Different Effect Mechanisms of Supercritical CO₂ on the Shale Microscopic Structure

To better understand how supercritical carbon dioxide (CO₂) enhances shale gas production, it is necessary to study the interaction of supercritical CO₂ with shale and its impact on shale microstructure. The different mechanisms by which supercritical CO₂ changes the shale pore structure were studied by X-ray diffraction analyses, scanning electron microscopy (SEM), nuclear magnetic resonance spectroscopy, and low-pressure nitrogen gas adsorption tests on shale samples before and after treatment with different pressures and gases (CO₂ and Ar). The results showed that after treatment with CO₂, the mineral content of shale changed significantly, and in particular, the proportions of calcite and dolomite decreased. The mineral content of shale changed the most after treatment with supercritical CO₂, and the microscopic pores were most observable by SEM. In a gaseous CO₂ environment, the effect of CO₂ adsorption on shale pores is greater than the effects of gas pressure and dissolution reactions. However, in a supercritical CO₂ environment, the changes in shale pore structures are mainly controlled by extraction and dissolution reactions. When shale is exposed to supercritical CO₂, the fractal dimensions of adsorption pores and seepage pores decrease, indicating that the specific surface area and roughness of adsorption pores decrease. This implies that the adsorption capacity decreases, and that the complexity of the seepage pores declines, which is conducive for gas migration.