Apparent and phase diffusion coefficients for H2 geological storage scenarios using a modified Maxwell-Stefan method

Geological storage of hydrogen (H₂) is a promising strategy for large-scale energy storage, crucial for integrating intermittent renewable energy sources. Diffusion is a key mechanism governing the long-term containment security and potential migration of stored H₂ within subsurface formations. This study presents analytical assessment and its validation with experimentally measured values of H₂ diffusion coefficient in porous media under conditions relevant to geological storage, specifically using parameters inspired by the Ketzin pilot site for town-gas and CO₂ storage. We employ an enhanced diffusion model based on the Maxwell-Stefan (MS) formulation, coupled with correlations for fluid and rock properties (Peng-Robinson EOS, Chung viscosity, extended Henry's law, Sharqawy brine density, Meyer-Elshahaby brine viscosity). The model calculates the apparent diffusion coefficient (D_app) considering bulk gas, Knudsen diffusion, dissolved phase, and potential minor sorptive transport, including fracture contributions. D_app(H₂) was calculated for representative Ketzin reservoir (sandstone) and caprock (claystone) conditions under varying gas saturations, yielding values consistent with expected experimental ranges (e.g., 10^-9 to 10^-8 m²/s for wet sandstone, 10^-10 to 10^-9m²/s for wet caprock). Phase diffusion analysis revealed significant composition dependence of Fickian coefficients. Results consistently identified water saturation (S_w), tortuosity (τ), constrictivity (χ), and gas phase accessibility (α) as the most influential parameters controlling D_app(H₂). Monte Carlo simulations, incorporating parameter uncertainty based on plausible Ketzin parameters, indicated a potential variability in D_app(H₂) spanning approximately two orders of magnitude, underscoring the need for accurate site characterization. Spearman rank correlation analysis confirmed the high impact of S_w, τ, χ, and α on D_app(H₂) uncertainty.