Impact of CO₂ impurity in hydrogen gas on wetting characteristics of carbonate minerals; new insights and implications for hydrogen geo-storage in saline aquifers

The effectiveness of Underground Hydrogen Storage (UHS) as a long-term solution for sustainable green energy relies on secure containment in geological formations and optimized storage and retrieval processes, where fluid-rock interactions, particularly the wettability of the rock, play a crucial role. Additionally, the pre-injection of a cushion gas, such as CO₂, to maintain sufficient pressure for hydrogen (H₂) withdrawal can influence wettability. This study employed the tilted plate method to examine contact angle hysteresis of the wetting phase (water) on a carbonate rock substrate, measuring advancing and receding contact angles in the presence of various H₂-CO₂ mixtures ([0.30 CO₂ + 0.70 H₂], [0.50 CO₂ + 0.50 H₂, 0.70 CO₂ + 0.30 H₂]) at pressures (500, 1200, 2000, and 3000 psi) and temperatures (50 °C and 80 °C). Further analyses using AFM (Atomic Force Microscopy) and EDS (Energy Dispersive X-ray Spectroscopy) were conducted to assess the effects of CO₂ impurity on the carbonate rock surface. Our findings indicate that while pressure has minimal effect on the wetting properties of the carbonate substrate, higher temperatures make the surface more water-wet. More importantly, CO₂ concentration plays a critical role in system wettability, as increasing the CO₂ mole fraction from 30 % to 70 % significantly reduces the water-wetness of the carbonate surface. Specifically, the rock remains water-wet under reservoir conditions when CO₂ is 50 % or less, with contact angles between 42° and 65°, whereas at higher CO₂ levels, it shifts toward neutral wettability, with contact angles ranging from 80° to 100°. AFM and EDS analyses indicate that changes in surface roughness and elemental concentration due to CO₂ exposure contribute to these wettability variations. As a result, at lower CO₂ concentrations because of more water-wet state of the surface and higher IFT, a higher gas column height and storage capacity is acheivable, whilst stronger snap-off effect and hence more trapped gas during water imbibition process, impairs the hydrogen recovery efficiency. The opposite applies at higher CO₂ levels. Thus, optimizing CO₂ concentration is a key factor in balancing the storage capacity and the recovery efficiency. The findings of this work enhance our understanding of hydrogen geo-storage mechanisms in carbonate reservoirs with CO₂ as a cushion gas, supporting more reliable predictions for underground hydrogen storage projects.