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.