Comparative Diffusion Characteristics of Hydrogen, Methane, and Carbon Dioxide in Different Rock Types

Green-hydrogen geological storage, geological sequestration of CO₂, and the generation, accumulation, and potential reservoir formation of natural hydrogen and methane involve a broad spectrum of lithologies, including sandstone, carbonate rock, shallow mudstone, halite (salt rock), serpentinite, basalt, granite, coal, and deep organic-rich shale. Marked contrasts in rock physical properties and pore-structure attributes can strongly regulate macroscopic gas diffusion. Therefore, elucidating gas diffusion behavior across different rock types is essential for evaluating gas storage capacity and geologic trapping potential.

In this work, representative rock samples were crushed to 0.50–0.841 μm particles and tested using a self-developed experimental setup to characterize the diffusion behaviors of H₂, CH₄, and CO₂ at 0.5 MPa. Depending on the observed diffusion features, diffusion coefficients were quantified using both unipore and dual-porosity (bidisperse) diffusion models. Pore-structure characteristics were independently constrained by N₂ physisorption, mercury intrusion porosimetry, and scanning electron microscopy, enabling a systematic assessment of pore-structure controls on gas diffusion. Additional experiments were performed on selected samples to compare diffusion coefficients under varying conditions of temperatures and pressures.

The results demonstrate pronounced inter-gas and inter-lithology differences in diffusion behavior, arising from contrasts in gas properties and rock pore structures. Overall, H₂ diffuses the fastest, followed by CH₄, whereas CO₂ exhibits the slowest diffusion. In micropores-rich rocks, CO₂ shows a distinct “fast initial–slow late-stage” diffusion pattern. Furthermore, diffusion coefficients increase with increasing temperatures but decrease with increasing gas pressures.

These findings reveal lithology-dependent response mechanisms in governing gas diffusion and provide a scientific basis for understanding gas migration in deep geological environments. The results also deliver key experimental constraints for studies of natural hydrogen and methane accumulation and for the assessment and optimization of geological CO₂ sequestration.

Acknowledgement: This work was supported by the Basic Science Center Program of the National Natural Science Foundation of China (NSFC) (Type A; No. 42302145).