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

The geological storage of hydrogen (H 2 ) requires reliable long-term caprock sealing, yet the nanoscale interactions between H 2 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 H 2 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 H 2 contents. Results show that H 2 accelerates hydration-state transitions, narrows the stability window of crystalline swelling, and promotes the asymmetric plume formation in confined interlayers. H 2 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 H 2, 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 H 2 further promotes Mt sheet separation by disrupting nanoscale liquid bridges. Collectively, these results provide the first atomistic-scale evidence that intercalated H 2 reshapes swelling energetics, elastic anisotropy, and failure pathways in Mt, highlighting critical nanoscale mechanisms that may compromise caprock integrity during underground H 2 storage.