Molecular dynamic simulations on the hydrogen wettability of caprock: Considering effects of mineralogy, pressure, temperature and salinity

Abstract

Underground hydrogen storage (UHS) is increasingly recognized as a promising solution for large-scale energy storage. The hydrogen (H₂) wettability of caprock plays a critical role in the sealing behavior of UHS. Despite various research efforts, a systematic understanding of H₂ wettability remains limited. This study investigates the H₂ wettability of caprock, with particular emphasis on the effects of mineralogy, pressure, temperature, and salinity. Molecular dynamics (MD) simulations were conducted to predict H₂ density, H₂-brine interfacial tension, and H₂-brine-mineral contact angles for halite, calcite, quartz, and Na-montmorillonite. By integrating theoretical analysis, MD simulations, and comparisons with previous simulation and experimental results, we demonstrate that increasing pressure and salinity, as well as decreasing temperature, enhances H₂ wettability. The mechanisms underlying these effects were elucidated through intermediate variables such as H₂ density, H₂-brine interfacial tension (IFT), and interaction energy between water and the mineral. The water wettability of the four minerals follows the order: montmorillonite > calcite > quartz > halite, further supported by the total interaction energy per unit area. Pressure exhibits a linear negative correlation with the cosine of the contact angle, attributed to changes in H₂ density. The opposing effects of the three intermediate variables obscure the impact of temperature on the H₂ wettability of caprock. The combined influence of IFT and water-rock interaction energy results in an increase in contact angle with rising salinity. For montmorillonite, the electrical double layer at the water-rock interface significantly reduces H₂ wettability, but it has a negligible effect on the changes in H₂ wettability under varying temperature and salinity conditions.