  Porous structures of shales are reconstructed based on scanning electron
microscopy (SEM) images of shale samples from Sichuan Basin, China.
Characterization analyzes of the nanoscale reconstructed shales are performed,
including porosity, pore size distribution, specific surface area and pore
connectivity. The multiple-relaxation-time (MRT) lattice Boltzmann method (LBM)
fluid flow model and single-relaxation-time (SRT) LBM diffusion model are
adopted to simulate the fluid flow and Knudsen diffusion process within the
reconstructed shales, respectively. Tortuosity, intrinsic permeability and
effective Knudsen diffusivity are numerically predicted. The tortuosity is much
higher than that commonly employed in Bruggeman equation. Correction of the
intrinsic permeability by taking into consideration the contribution of Knudsen
diffusion, which leads to the apparent permeability, is performed. The
correction factor under different Knudsen number and pressure are estimated and
compared with existing corrections reported in the literature. For the wide
pressure range under investigation, the correction factor is always greater
than 1, indicating the Knudsen diffusion always plays a role on the transport
mechanisms of shale gas in shales studied in the present study. Most of the
values of correction factor are located in the transition regime, with no Darcy
flow regime observed.
