Mechanisms of shale water wettability alteration with chemical groups after CO2 injection: Implication for shale gas recovery and CO2 geo-storage

Abstract

Supercritical carbon dioxide (Sc-CO2) has been used in shale gas exploitation for efficient fracturing and the added benefit of CO2 geo-storage. Many studies have suggested that the water wettability of shale significantly influences the success of shale gas recovery. The aim of this work was to investigate the effect of CO2 treatment on shale water wettability. X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were used to analyze the raw and CO2-treated shale samples using peak fitting and grey relational (GR) analyses. The results showed that the CO2-treated shale surface showed weaker hydrophilicity than the raw shale sample. The GR analysis results indicated that Si–OH contributed most to shale water wettability because Si–OH formed hydroxyl bonds with the water molecules on the shale surface, significantly enhancing shale hydrophilicity. The proportion of Si–OH in the shale chemical groups decreased, and fewer hydroxyl bonds were formed after the CO2 treatment, weakening shale hydrophilicity. The content of hydrophilic –OH groups was lower in the CO2-treated shale samples than the raw samples, and increases and decrease were observed in the content of the C–O groups. A decrease in the contents of hydrophilic calcite, dolomite, and clay minerals also contributed to shale hydrophilicity weakening. These results imply that the injection of CO2 into shale reservoirs weakens shale hydrophilicity, improving shale gas recovery because the water blocking effect is minimized. The findings in this research can be used for shale wettability reversal by chemical reagents, which is of great significance for improving gas productivity and CO2 geological storage.