A Comprehensive Review of Influence of Critical Parameters on Wettability of Rock-hydrogen-brine Systems:  Implications for Underground Hydrogen Storage

The rock wettability is one of the most critical parameters that influences rock storage potential, trapping, and H2 withdrawal rate during Underground hydrogen storage (UHS). However, the existing review articles on wettability of H2-brine-rock systems do not provide detailed information on complexities introduced by reservoir wettability influencing parameters, such as high pressure, temperature, salinity conditions, micro-biotic effects, cushion gases, and organic acids relevant to subsurface environments. Therefore, a comprehensive review of existing research on various parameters influencing rock wettability during UHS and residual trapping of H2 was conducted in this study. Literature that provides insight into molecular-level interaction through machine learning, and molecular dynamic (MD) simulations, and role of surface-active chemicals such as nanoparticles, surfactants, and wastewater chemicals were also reviewed. The review suggested that UHS could be feasible in clean geo-storage formations but the presence of rock surface contaminants at higher storage depth, and microbial effects should be accounted for to prevent over-estimation of the rock storage potentials. The H2 wettability of storage/caprocks and associated risks of UHS projects could be higher in rocks with high proportion of carbonate minerals, organic-rich shale, and basalt with high plagioclase minerals content. However, treatment of rock surfaces with nanofluids, surfactants, methylene blue and methyl orange has proven to alter the rock wettability from H2-wet towards water-wet. Research results on effect of rock wettability on residually trapped hydrogen and snap-off effects during UHS are contradictory, thus further studies would be required in this area. The review generally concludes that rock wettability plays prominent role on H2 storage due to the frequency and cyclic loading of UHS, hence, it is vital to evaluate the effects of all possible wettability influencing parameters for successful designs and implementation of UHS projects.