Sandstone wettability and mixed gas composition: Unraveling the impact of CO₂ in hydrogen geo-storage

Underground hydrogen storage (UHS) is gaining interest as a secure, long-term solution for storing hydrogen in porous geological formations. In UHS, a cushion gas like CO₂ is crucial to maintain the reservoir pressure and optimize recovery. The concept of wettability plays a fundamental role in determining the system's multi-phase displacement characteristics in the porous media. However, there is a gap in the existing literature regarding the wettability of sandstone rocks under geo-storage conditions when H₂ and CO₂ are injected as the bulk and cushion gases, respectively. To address this gap, we conducted a study investigating the wettability hysteresis phenomenon by measuring the advancing and receding contact angles of different mixtures of H₂/CO₂ in contact with brine on a sandstone mineral under different pressures and temperatures, using the tilted plate method. The results show that the contact angle increases with pressure, leading the system to become less water-wet. Conversely, an increase in temperature makes the system more water-wet. Moreover, the measured contact angle remains relatively constant despite changes in the CO₂ concentration. Further analyses utilizing atomic force microscope (AFM) and energy dispersive X-ray spectroscopy (EDS) indicated that the chemical and physical structure of the rock does not change after exposure to CO₂, consistent with the observed lack of wettability change with varying CO₂ fraction. In conclusion, the water-wet state identified under reservoir conditions decreases residual trapping of H₂, facilitating higher recovery but posing a potential risk of leakage. Hence, using a higher fraction of CO₂ as a cushion gas is favorable for reducing the risk by decreasing IFT and, subsequently, the gas column. This study improves our understanding of hydrogen geo-storage mechanisms, aiding more accurate simulations in underground porous systems.