Molecular Insights into Caprock Integrity of Subsurface Hydrogen Storage: Perspective on Hydrogen-Induced Swelling and Mechanical Response

The geological storage of hydrogen (H2) requires reliable long-term caprock sealing, yet the nanoscale interactions between H2 and clay minerals remain critically underexplored, despite their importance for storage security. This lack of understanding has limited our ability to predict mechanical stability and leakage risks in H2 storage formations. Using molecular simulations, this study investigates the swelling behavior and mechanical properties of sodium montmorillonite (Mt), a common smectite clay, under varying hydration states and interlayer H2 contents. Results show that H2 accelerates hydration-state transitions, narrows the stability window of crystalline swelling, and promotes the asymmetric plume formation in confined interlayers. H2 alters cation and water coordination, thereby weakening Na+–Mt electrostatic interactions and modulating H-bond networks at the interface and in the bulk. Mechanical analysis reveals pronounced anisotropy in Mt. In-plane stiffness is mainly governed by basal spacing expansion, whereas out-of-plane stiffness is highly sensitive to the initial presence of water or H2, which weakens interlayer cohesion. Tensile and compressive strengths in the in-plane directions follow in-plane stiffness trends, while the out-of-plane tensile strength is governed by Mt–water H-bonds. The presence of H2 further promotes Mt sheet separation by disrupting nanoscale liquid bridges. Collectively, these results provide the first atomistic-scale evidence that intercalated H2 reshapes swelling energetics, elastic anisotropy, and failure pathways in Mt, highlighting critical nanoscale mechanisms that may compromise caprock integrity during underground H2 storage.