Different Effect Mechanisms of Supercritical CO2on the Shale Microscopic Structure

Abstract

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.