Wettability of water-H₂-quartz and water-H₂-calcite experiment and molecular dynamics simulations: Critical assessment

In sandstone and carbonate, H₂-wettability is an essential factor that defines structural and residual trapping capacities and significantly influences multi-phase fluid dynamics in the rock. Several studies have assessed H₂ wettability using contact angle measurements on quartz and calcite at different pressure and temperature conditions; however, the reported data is fraught with a high degree of uncertainty. In this study, atomistic molecular dynamics (MD) simulation was used to calculate the water-H₂-silica and water-H₂-calcite contact angles under different conditions (pressure up to 20 MPa; 300K and 323K temperatures; and zero, 60 000 and 213 000 ppm salinities) in order to figure out the source of discrepancies in the laboratory contact angle measurement due to surface roughness and possible surface contamination. Further, we have re-performed the experimental measurements of the contact angles for both minerals and confronted them with our MD results. This study observed that in the presence of hydrogen, the water contact angles on clean quartz and calcite substrates are zero in all conditions (i.e., different pressures, temperatures, and salinities) using the sessile drop method and Molecular Dynamics (MD). This means H₂ gas is a completely non-wetting phase and can be easily withdrawn after underground storage; however, cushion gas is essential to overcome snap-off and H₂ residual trapping. These results highlight the importance of adopting a suitable cleaning method before contact angle measurement.