Diffusive Transport of CO₂ and Energy Gases in Tight Reservoir Analogs: Implications for Subsurface Storage and Recovery

Understanding diffusion of gases (hydrogen, methane, and carbon dioxide) in a tight reservoir is essential for efficient hydrocarbon production and storage. This study presents a comparative experimental analysis of hydrogen (H₂), methane (CH₄), and carbon dioxide (CO₂) diffusion through a synthetic cement-based tight core sample (ϕ = 0.1485, L = 80.55 mm). Effective diffusion coefficients were determined at 25, 50, and 60 °C using a high-pressure pulse-decay method, where the pressure difference between upstream and downstream was approximately 200 psi. A single synthetic core, with identical length and diameter throughout, was used for all experiments to represent low-permeability reservoir rock. The findings indicate that hydrogen exhibits the highest effective diffusivity of the three gases, with a higher value than methane and carbon dioxide in all test conditions. At 25 °C, measured diffusion coefficients were 4.75 × 10^–8 m²/s (hydrogen), 1.45 × 10^–8 m²/s (methane), and 6.77 × 10^–9 m²/s (carbon dioxide). With increasing temperature, hydrogen and methane showed monotonic increases in diffusivity, reaching 6.93 × 10^–8 and 1.96 × 10^–8 m²/s at 60 °C, respectively, corresponding to increases of 45.80 and 35.68%. Arrhenius analysis yielded apparent activation energies of 9 kJ/mol for hydrogen and 7 kJ/mol for methane, indicating a weak temperature sensitivity and diffusion dominated by molecular/transitional transport mechanisms. The comparative procedure of diffusivities (H₂ > CH₄ ≫ CO₂) was maintained throughout the temperatures, but the temperature increment from 50 to 60 °C slightly lowered the gaps. These results both give empirical values of gas diffusivity in geologic media under reservoir conditions. Furthermore, by applying diffusion-based leakage equations, the study quantifies the relative leakage and retention tendencies of the three gases, demonstrating that hydrogen presents the highest leakage risk, at 60 °C, hydrogen exhibited leakage rates approximately 3.5 times greater than methane and approximately 25 times greater than carbon dioxide, methane exhibits moderate leakage, and carbon dioxide offers the strongest retention under identical conditions. This work demonstrates a unique quantitative methodology to compare multigas diffusion in tight porous media and provides insight for optimizing gas recovery and storage strategies in the subsurface and critical implications of gas leakage for long-term storage security.