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 degrees C and 80 degrees 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 degrees and 65 degrees, whereas at higher CO2 levels, it shifts toward neutral wettability, with contact angles ranging from 80 degrees to 100 degrees. 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.