Diffusion tortuosity in complex porous media from pore-scale numerical simulations

Abstract

Diffusive transport is an important mechanism of mass transfer in gas or liquid phases confined in porous media. The continuous presence of solids in porous media causes diffusion paths to deviate from straight lines, leading to hindered diffusion characterized by the tortuosity of the media. In this study, the correlation between porosity and tortuosity in complex porous media is studied numerically using a random walk particle tracking method. Porous media studied include synthetic geometries based on three-dimensional Voronoi tessellations taking the conformations of networks of tubes, fissures, and fibers. Simulation results show that homogeneous networks of fissures are the least tortuous, homogeneous networks of tubes are intermediate, and fibrous geometries are the most tortuous. The effect of heterogeneities on the tortuosity is also investigated using both synthetic geometries and geometries of real rocks. It is found that vuggy porosity increases the tortuosity. Specifically, the larger the vug size relative to the inter-vug connections, the higher the tortuosity. Digitalized rock samples have higher tortuosities than the synthetic geometries because of the additional impact of imperfect pore connectivity.