Modeling Hydrogen Permeation Based on Multiflow Regimes in G-Grade Cement Nanoporous Structures for Underground Hydrogen Storage Applications

Accurate prediction of hydrogen permeation in cement-based materials is essential for assessing the integrity and safety of underground hydrogen storage (UHS), a key technology for carbon-neutral energy systems. This study adopts G-grade cement stone as the foundation for both experiments and modeling. By integrating pore size distribution with a Knudsen number based flow regime classification model, the hydrogen permeation behavior in the cement matrix is quantified through weighted contributions of four flow regimes: Darcy flow, slip flow, transition flow, and free molecular flow. A multiregime permeability model is developed to evaluate the pressure-dependent variations in the effective permeability of cement. The proposed model is validated using experimentally measured permeability data of G-grade cement stone. An optimal pore size distribution design strategy is proposed based on the model results. The applicability of the model is extended by evaluating the permeability of various gas species. The results demonstrate that hydrogen permeability in cement is jointly governed by pressure, pore size distribution, and temperature. A nonmonotonic relationship between hydrogen permeability and pressure was observed, exhibiting a pronounced minimum at approximately 2 MPa. The flow regimes─Darcy, slip, transition, and free molecular─dominate hydrogen transport under different conditions, leading to variations in effective permeability. This model provides a framework for predicting the effective permeability of cement in UHS wells and for optimizing cement design parameters, while also offering a theoretical basis for predicting gas permeability in other porous media applications, including hydrogen caprock sealing, shale gas extraction, and geological CO2 sequestration.