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

The growing interest\nin large-scale underground hydrogen\n(H<sub>2</sub>) storage (UHS) emphasizes the need for a comprehensive\nunderstanding\nof the fundamental characteristics of subsurface environments. The\nwetting preference of subsurface rock is a crucial parameter influencing\nthe H<sub>2</sub> flow behavior during storage and withdrawal processes.\nIn this study, we utilized molecular dynamics simulation to evaluate\nthe wetting preference of the silica surface in subsurface hydrogen\nsystems, with the aim of addressing disparities observed in experimental\nresults. We conducted an initial comprehensive assessment of potential\nmodels, comparing the wettability of five common silica surfaces with\ndifferent surface morphologies and hydroxyl densities in CO<sub>2</sub>–H<sub>2</sub>/water/silica systems against experimental data.\nAfter introducing the INTERFACE force field as the most accurate potential\nmodel for the silica surface, we evaluated the wetting behavior of\nthe α–quartz (101) surface with a hydroxyl density of\n5.9 number/nm<sup>2</sup> under the impact of actual geological storage\nconditions (333–413 K and 10–30 MPa), the coexistence\nof cushion gases (i.e., CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>) at various mole fractions, and pH levels ranging from 2\nto 11 characterized through considering the negative charges of 0\nto −0.12 C/m<sup>2</sup> via deprotonation of silanol on the\nsilica surface. Our results indicate that neither pressure nor temperature\nhas a significant impact on the wetness of the silica in the case\nof pure H<sub>2</sub> (single component UHS operations). However,\nwhen CO<sub>2</sub> coexists with H<sub>2</sub>, especially at higher\nmole fractions, an increase in pressure and a decrease in temperature\nlead to higher contact angles. Moreover, when the mole fraction of\ncushion gas ranges from 0 to 1, the contact angle increases 20, 9.5,\nand 4.5° for CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>, respectively, on the neutral silica substrate. Interestingly, at\nhigher pH conditions where the silica surface carries a negative charge,\nthe contact angle considerably reduces where surface charges of −0.03\nand −0.06 C/m<sup>2</sup> result in an average reduction of\n20 and 80% in the contact angle, respectively. More importantly, at\na pH of ∼11 (−0.12 C/m<sup>2</sup>), a 0° contact\nangle is observed for the silica surface under all temperatures, pressures,\ntypes of cushion gases, and varying mole fractions.