Wetting Preference of Silica Surfaces in the Context of Underground Hydrogen Storage: A Molecular Dynamics Perspective

The growing interest in large-scale underground hydrogen (H₂) storage (UHS) emphasizes the need for a comprehensive understanding of the fundamental characteristics of subsurface environments. The wetting preference of subsurface rock is a crucial parameter influencing the H₂ flow behavior during storage and withdrawal processes. In this study, we utilized molecular dynamics simulation to evaluate the wetting preference of the silica surface in subsurface hydrogen systems, with the aim of addressing disparities observed in experimental results. We conducted an initial comprehensive assessment of potential models, comparing the wettability of five common silica surfaces with different surface morphologies and hydroxyl densities in CO₂-H₂/water/silica systems against experimental data. After introducing the INTERFACE force field as the most accurate potential model for the silica surface, we evaluated the wetting behavior of the α-quartz (101) surface with a hydroxyl density of 5.9 number/nm² under the impact of actual geological storage conditions (333-413 K and 10-30 MPa), the coexistence of cushion gases (i.e., CO₂, CH₄, and N₂) at various mole fractions, and pH levels ranging from 2 to 11 characterized through considering the negative charges of 0 to -0.12 C/m² via deprotonation of silanol on the silica surface. Our results indicate that neither pressure nor temperature has a significant impact on the wetness of the silica in the case of pure H₂ (single component UHS operations). However, when CO₂ coexists with H₂, especially at higher mole fractions, an increase in pressure and a decrease in temperature lead to higher contact angles. Moreover, when the mole fraction of cushion gas ranges from 0 to 1, the contact angle increases 20, 9.5, and 4.5° for CO₂, CH₄, and N₂, respectively, on the neutral silica substrate. Interestingly, at higher pH conditions where the silica surface carries a negative charge, the contact angle considerably reduces where surface charges of -0.03 and -0.06 C/m² result in an average reduction of 20 and 80% in the contact angle, respectively. More importantly, at a pH of ∼11 (-0.12 C/m²), a 0° contact angle is observed for the silica surface under all temperatures, pressures, types of cushion gases, and varying mole fractions.