Multiple diffusion mechanisms of shale gas in nanoporous organic matter predicted by the local diffusivity lattice Boltzmann model

Shale gas diffusion properties in organic matter (OM) are crucial for estimating gas yield. The gas diffusion process involves multiple diffusion mechanisms namely molecular diffusion, Knudsen diffusion and transition diffusion due to the nanoporous features of OM. Although a single diffusion mechanism in OM has been well-studied, the coexistence of multiple diffusion mechanisms remains unclear. In this work, a local diffusivity lattice Boltzmann model is proposed to consider multiple diffusion mechanisms at the pore-scale. Algorithms of pore network and random-placing sphere are integrated to reconstruct the nanoporous structures of OM. A local Knudsen number is involved to automatically identify local diffusion mechanisms, thus determining the corresponding local diffusivity. The diffusion properties of shale gas in OM are investigated at various shale reservoir pressures and temperatures and OM structural parameters. OM diffusivity is overestimated in the typical pressure range if multiple diffusion mechanisms are ignored. Furthermore, the OM diffusion mechanism gradually transfers from Knudsen diffusion to molecular diffusion with continuously increased pressure. To conveniently predict the diffusion capability of OM, a general empirical formula, which is mainly expressed as a function of porosity and average Knudsen number, is established on the basis of the present simulation results.