Pore-scale modeling of transverse dispersion in porous media

A physically based description is provided for the transverse dispersion coefficient in porous media as a function of Peclet number, Pe. We represent the porous medium as lattices of bonds with square cross section whose radius distribution is the same as computed for Berea sandstone and describe flow (Stokes equation) and diffusion (random walk method) at the pore scale (similar to mu m) to compute the transverse dispersion coefficient at a larger scale (similar to cm to similar to m). We show that the transverse dispersion coefficient D-T similar to Pe for all Pe >> 1. A comprehensive comparative study of transverse dispersion with experiment indicates that the model can successfully predict the trends for the asymptotic macroscopic dispersion coefficient over a broad range of Peclet numbers, 0 < Pe < 10(5). We discuss the relation between transverse and longitudinal dispersion coefficient and show that unless one studies solute transport in the advection dominated regime, it is not appropriate to take D-T to be 1 order of magnitude less than D-L.