Impacts of Reservoir Fluids and Pore Geometry on Hydrogen Adsorption and Diffusion in Organic-Rich Rocks

Summary Hydrogen (used interchangeably with H2) loss through diffusion and its retention via adsorption in reservoir caprocks need to be accurately quantified to evaluate storage security in geological hydrogen storage projects. Reliable quantification of hydrogen adsorption and diffusion in reservoir caprocks requires high safety standards and sensitive experimental measurements. Alternatively, molecular simulations can provide valuable mechanistic insights to complement laboratory data, especially in tight rocks with complex composition such as organic-rich mudrocks. Therefore, the objectives of this paper are (i) quantifying the adsorption and diffusion behavior of hydrogen gas in kerogen molecular structures under different cushion gas concentrations, (ii) quantifying the selectivity of hydrogen gas over the residual methane gas (used interchangeably with CH4) by the rock interface, and (iii) investigating the impact of the rock geochemistry on the hydrogen storage and leak potential in depleted gas reservoirs using molecular dynamics (MD) simulations. We used MD modeling to construct kerogen structures, including cylindrical and slit-like pores. These models were used as inputs to molecular-scale simulations that evaluated the gas adsorption using Monte Carlo simulations. This enabled the construction of the adsorption isotherms for hydrogen and methane gases under varying concentrations of methane at different temperatures and pressures. Moreover, we evaluated the self-diffusion coefficients of hydrogen molecules within the pores of the kerogen structures using MD simulations. The results highlighted the substantial influence of reservoir pressure, temperature, fluid composition, and pore geometry on hydrogen adsorption, loss, and diffusion within the kerogen nanopores. Results showed that the simple Langmuir adsorption model could not accurately capture the H2 adsorption behavior in kerogen nanopores compared with the Dubinin-Astakhov (D-A) model. Methane exhibited higher adsorption selectivity than hydrogen at pressures below 12 MPa, with this selectivity diminishing as pressure and temperature rose. Increasing the temperature from 300 K to 330 K reduced total hydrogen adsorption by 17% at 20 MPa. While transitioning kerogen pore geometry from slit-like to cylindrical had a minimal effect on the competitive adsorption isotherms of H2 and CH4. Meanwhile, pore geometry significantly influenced the magnitude and direction of gas diffusivity, enhancing it predominantly along the axial direction of the main pore channel. The documented results underscore the critical role of pore directionality and type in determining gas diffusivity within kerogen structures. The findings provide valuable insights for optimizing hydrogen storage in depleted gas reservoirs and assessing leakage and loss potential through reservoir caprocks.