Abstract
•Methane diffusion in realistic kerogen structures follows highly tortuous pathways.•The barrier to methane transport in kerogen is further augmented by sorption forces.•Tortuosity increases by a factor up to 3.6 compared to an adsorption free tortuosity.•Langmuir isotherm can be used to quantify the impact of adsorption on diffusion.•The sorption dependent diffusion is implemented in a larger scale permeability model.
Diffusion tortuosity is an important microstructural parameter for computing effective gas transport coefficients in porous media. In this study, a realistic kerogen structure was built to quantitatively establish a clear understanding of the adsorption impact on diffusion tortuosity in the organic materials of shale. A molecular simulation study was employed to reconstruct a Type II-D kerogen model on a computational platform. A structure of five kerogen macromolecules was obtained through a series of NVT – NPT ensembles. There was a thorough characterization of the porosity and pore size distribution of the final structure. A grand canonical Monte Carlo method was used to compute the adsorption of methane for a pressure range up to 40 MPa. Freundlich and Langmuir models were employed to further parameterize the adsorption capacity. A molecular dynamics diffusion study was performed on the kerogen-adsorbed molecule configurations to obtain the effective diffusion, which was then used to estimate the tortuosity. Methane molecules follow tortuous pathways of diffusion, imposed by the heterogeneity and confinement of kerogen. The tortuosity is further impacted by the adsorption effect. Kerogen-methane interactions increase this tortuosity by a factor as high as 3.6 compared to an adsorption-corrected tortuosity.