Molecular mechanisms of hydrogen leakage through caprock in moisture and residual gas conditions: A molecular dynamics-Monte Carlo study

Underground hydrogen (H₂) storage has become increasingly popular in recent years; however, H₂ leakage is a critical concern. A conventional reservoir is sealed by a dense caprock; the long-chain hydrocarbons cannot escape through the caprock because of the complex molecular structure and large molecular size, but H₂ leakage can still occur, particularly through the nanopores of the caprock. In this study, we investigate the H₂ leakage problem using the molecular dynamics (MD) and MD-Monte Carlo (MDMC) methods. The results of our MDMC algorithm concur with the MD simulation results, indicating that the MDMC algorithm can feasibly predict the H₂ leakage process. Caprock defects are repaired by water (H₂O) clusters owing to the hydrogen bonding and adsorption of H₂O on the caprock surface. Methane (CH₄) forms an absorption layer on the caprock, inhibiting the probability of contact between H₂ and the rock surface. We further explain the spatial distribution of different gas components using their potential energies and interaction forces. The molecular sealing mechanism is also proposed accordingly, and the H₂O cluster and CH₄ adsorption layer form the double barrier for H₂ leakage. The evaporation of H₂O at high temperatures weakens the stability of the H₂O cluster, and smaller pore sizes (<10.0 Å) within the caprock prevent H₂ leakage. H₂ leakage can be further inhibited by increasing the H₂O content in a H₂ storage project. Thus, a specific amount of H₂O and CH₄ gas can alleviate the H₂ leakage problem.