Utilization of methane and carbon dioxide as cushion gas in underground hydrogen storage: An insight from molecular simulation on competitive adsorption and diffusion in shale organic nanopore

KC Zeng and PX Jiang and RN Xu, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 185, 151949 (2025).

DOI: 10.1016/j.ijhydene.2025.151949

Underground hydrogen (H2) storage is crucial for establishing a hydrogen economy due to its large scale and long-term capabilities. Using carbon dioxide (CO2) or methane (CH4) as cushion gases in depleted shale gas reservoirs is expected to reduce H2 loss and enhance recovery. However, research on this topic is limited due to the reservoir's low porosity and permeability. This study employed realistic shale organic nanopore models and the Grand Canonical Monte Carlo method to simulate pure H2 adsorption and competitive adsorption of H2/CO2 and H2/CH4 mixtures. Results showed that cushion gas molecules accumulate near pore surfaces, while H2 molecules concentrate in the nanopore center. This nonuniform distribution reduces H2 adsorption layer density, aiding production. Both CO2 and CH4 have higher adsorption affinity than H2, making them suitable as cushion gases. Molecular dynamics simulations revealed that H2 exhibits significantly higher self-diffusion coefficients compared to CO2 and CH4, potentially improving H2 purity. The study also established a relationship between nondimensional self-diffusion coefficients and Knudsen numbers. CO2, with stronger adsorption affinity and lower diffusivity, is more suitable as a cushion gas than CH4. These findings demonstrate the feasibility of using CO2 and CH4 as cushion gases in depleted shale gas reservoirs, supporting the establishment of a hydrogen economy.

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