Assessment of wettability and rock-fluid interfacial tension of caprock: Implications for hydrogen and carbon dioxide geo-storage

Underground hydrogen (H₂) storage (UHS) and carbon dioxide (CO₂) geo-storage (CGS) are prominent methods of meeting global energy needs and enabling a low-carbon global economy. The pore-scale distribution, reservoir-scale storage capacity, and containment security of H₂ and CO₂ are significantly influenced by interfacial properties, including the equilibrium contact angle (θ_E) and solid-liquid and solid-gas interfacial tensions (γ_SL and γ_SG). However, due to the technical constraints of experimentally determining these parameters, they are often calculated based on advancing and receding contact angle values. There is a scarcity of θ_E, γ_SL, and γ_SG data, particularly related to the hydrogen structural sealing potential of caprock, which is unavailable in the literature. Young's equation and Neumann's equation of state were combined in this study to theoretically compute these three parameters (θ_E, γ_SL, and γ_SG) at reservoir conditions for the H₂ and CO₂ geo-storage potential. Pure mica, organic-aged mica, and alumina nano-aged mica substrates were investigated to explore the conditions for rock wetting phenomena and the sealing potential of caprock. The results reveal that θ_E increases while γ_SG decreases with increasing pressure, organic acid concentration, and alkyl chain length. However, γ_SG decreases with increasing temperatures for H₂ gas, and vice versa for CO₂. In addition, θ_E and γ_SL decrease, whereas γ_SG increases with increasing alumina nanofluid concentration from 0.05 to 0.25 wt%. Conversely, θ_E and γ_SL increase, whereas γ_SG decreases with increasing alumina nanofluid concentration from 0.25 to 0.75 wt%. The hydrogen wettability of mica (a proxy of caprock) was generally less than the CO₂ wettability of mica at similar physio-thermal conditions. The interfacial data reported in this study are crucial for predicting caprock wettability alterations and the resulting structural sealing capacity for UHS and CGS.