Investigating the Mineralogical and Chemical Effects of CO2 Injection on Shale Wettability at Different Reservoir Temperatures and Pressures

The wettability of shale, which determines the success of carbon dioxide-enhanced shale gas recovery (CO2-ESGR) and the safety of CO2 geo-storage, is influenced due to shale–CO2 chemical interactions. To date, the wettability alteration mechanism of shale by its surficial chemical groups is not well understood. In this study, the variations in the wettability, mineralogy, and infrared spectrum of shale after interaction with CO2 at different temperatures and pressures were studied using the sessile drop method, X-ray diffraction, and Fourier transform infrared spectroscopy analysis. The effect of the contents of minerals and chemical groups on shale wettability was investigated by correlation analysis. It is found that the pressure of CO2 has a larger influence on shale wettability than the temperature, whereas the temperature has a greater effect on the infrared spectra of shale. Quartz has the greatest effect on wettability followed by carbonate and clay minerals. Quartz is originally water-wet, and the negative correlation between the relative quartz content and the wettability of shale implies the deterioration of the quartz hydrophilicity after CO2 treatment, and the same applies to the oxygen-containing groups. These findings indicate the interactions among functional groups and their redistribution on the surface during shale–CO2 interactions. Si–O groups are derived more from the hydrophobic Si–O–Si groups than hydrophilic Si–OH groups after CO2 treatment, reducing the contribution of quartz to shale hydrophilicity. The weakened absorption ability of CO32– ions to water may be caused by the rotation and redistribution of the CO32– groups on the surface after the CO2 treatment. A longer aliphatic hydrocarbon length reduces the shale wettability. Among the hydrophilic −OH groups, free −OH has the largest influence on shale wettability followed by −OH–O, −OH–OH, and −OH−Π. This study has important theoretical significance for understanding the wettability alteration mechanism of shale.