Host-rock and caprock wettability during hydrogen drainage: Implications of hydrogen subsurface storage

Underground hydrogen storage (UHS) is an integral part of H₂ economy value chain, essential for industrial-scale actualization of global decarbonization objectives. UHS in depleted hydrocarbon reservoirs is considered a safer and promising storage technique due to presence of large porous formation and impermeable seal, but its effectiveness depends on precise estimation of rock wettability, a crucial parameter in reservoir characterization, which controls pore-scale gas distribution, H₂ containment safety and withdrawal efficiency. Several recent studies have provided contact angles data for host rock-water-H₂ (quartz and sandstone) and caprock-water-H₂ (mica and shale) measured through sessile drop method. However, the contact angle datasets for carbonate rock-water-H₂ measured via captive bubble method, which can reflect the wettability of the rock during imbibition and drainage are largely unknown. The present work characterized the wettability of carbonate-water-H₂ systems for various rock types, prepared from five different lithologies with different mineral assemblages. The contact angle θ was measured using the captive bubble method at two different temperatures of 303 K and 348 K, and three different pressures (3.44, 10.34, and 17.23 MPa). Experimental results showed that all rocks remained intrinsically strongly water-wet (θ ranged between 17° – 30°) at all experimental conditions. Furthermore, no significant change in contact angles occurred with changing temperature and pressure. For instance, at 17.23 MPa, contact angle of H₂/brine on anhydrite (S-4) rock were measured as 19° and 20° at 303 K and 348 K respectively, suggesting that H₂ remains the non-wetting phase with increasing storage depth and at warmer reservoirs. The study suggests that the pore-scale flow and fluid connectivity of H₂ may not be influenced by changing wettability in pure storage rocks/caprocks. Thus, the wettability of carbonates storage rocks and caprocks may not be over-predicted by assuming strongly water-wet conditions for the non-contaminated rock surfaces during UHS.