Effect of supercritical CO₂ treatment on physical properties and functional groups of shales

The influence of Supercritical CO₂ (SCCO₂) on geochemical interaction is considered a key factor affecting CO₂ storage capacity in shales. To address this issue, samples from Eagle Ford and Mancos shales were treated with SCCO₂ for 30 days at 70 °C and 18 MPa. Analytical methods including X-ray diffraction (XRD), optical microscope, and Fourier Transform Infrared spectroscopy (FTIR) were used. The alteration in shale/water contact angles was evaluated based on Sessile drop method. The results show that SCCO₂ treatment can alter the mineral composition of shales. Quartz content generally increased, while clay and carbonate minerals’ contents decreased. Evaluating the dissolution of carbonate minerals, in particular, is beneficial to form an effective mineral carbonation trapping for long-term CO₂ storage. The changes in surface morphology suggest that clay-rich shales are more affected by SCCO₂ treatment compared to sandy/quartz-rich shales. The aromatic hydrocarbons showed minor changes after SCCO₂ treatment compared to the aliphatic hydrocarbons. The increase in oxygen-containing groups after SCCO₂ treatment proves the high adsorption capacity of CO₂ in shales. However, hydroxyl functional groups showed various trends after SCCO₂ treatment, depending on the clay content. Eagle Ford shales displayed a possible turn to CO₂-wet behavior, while the surface of Mancos shales remained strongly hydrophilic. In conclusion, quartz-rich shales could be favorable for CO₂ adsorption and providing more storage capacity.