Numerical investigation of gas diffusion properties in nanoporous organic matter

Gas diffusion properties in nanoporous organic matter is vitally important for assessing the productivity of shale gas reservoirs. To reveal the gas diffusion behavior in organic matter accurately, a local diffusivity lattice Boltzmann (LD-LB) model is developed on the pore-scale, in which the local gas diffusion property and organic matter structural feature are considered. To reflect the morphology features of organic matter effectively, the pore network and random placing spheres method are employed to reconstruct the nanoporous structure of organic matter. The structural feature is characterized by a modified thirteen directions average method. The effects of pressure, temperature, Knudsen diffusion, and geometric parameters of organic structure on gas diffusion ability are investigated. Results show that the diffusivity of organic matter increases with the temperature while decreases with the increase of pressure. The Knudsen diffusion takes effect on the low pressure and can be neglected on the high pressure (>20MPa). The geometric parameters of organic structure have significant influence on gas diffusion ability. The regular porous structure overestimates the gas diffusion ability. The modified Mezedur equation can effectively predict the organic matter diffusion ability.