Pore-scale investigation on coupled diffusion mechanisms of free and adsorbed gases in nanoporous organic matter

The long-term productivity of shale reservoirs is determined by shale gas diffusion properties in tight mudrocks, particularly in nanoporous organic matter (OM), within which the shale gas is mainly stored as free gas in the centers of pores and as adsorbed gas on pore surfaces. Although the multiple diffusion of free gas in porous media has been previously studied, the coupled diffusion mechanisms that involve both free and adsorbed gases as well as the contribution of surface diffusion induced by adsorbed gas, remain elusive. In this paper, a novel local coupled diffusivity lattice Boltzmann model (LCD-LBM), which considers both multiple diffusions of free gas and surface diffusion of adsorbed gas, is developed. The developed model allows prediction of the coupled diffusion mechanisms of free and adsorbed gases in the reconstructed nanoporous OM. The proposed model is validated by a molecular dynamic simulation result for shale gas diffusion in the nanoscale slit pore. The results show that OM diffusivity is in the magnitude order of 10⁻⁸ m²/s for an effective pore diameter of the OM ranging from 1 nm to 10 nm. The contribution of the adsorbed gas is prominent in the nanopores whose effective pore diameters are less than 2 nm. Furthermore, OM diffusivity is nearly independent of the gases diffusion direction and adsorbed gas molecules distribution pattern. The present work is conducted at the pore-scale, and thus it can be easily extended to evaluate the gas coupled diffusion mechanisms in other nanoporous materials.