Nanoscale simulation of shale transport properties using the lattice Boltzmann method: permeability and diffusivity

TL;DR

This study uses nanoscale simulations with the lattice Boltzmann method to analyze shale transport properties, revealing the significant role of Knudsen diffusion and providing correction factors for permeability estimates.

Contribution

It introduces a nanoscale simulation approach combining SEM-based reconstruction and LBM models to evaluate shale permeability and diffusivity, highlighting the impact of Knudsen diffusion.

Findings

Knudsen diffusion significantly affects shale transport mechanisms.

Correction factors for permeability are greater than 1 across pressure ranges.

Most transport occurs in the transition regime, with no Darcy flow observed.

Abstract

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.