In this work, the Das and Butler flow model was extended to account for the effect of differential pressure across a laterally and vertically spaced horizontal injection- and production-well pair for solvent vapour extraction (SVX) processes. The extended model provides a more-rigorous dispersion-coefficient correlation using interstitial pore velocity and mean particle size. This was used to history match oil-production data from 3D-scaled physical model experiments and to determine the effective dispersion coefficient of the solvent in heavy oil. The SVX experiments were performed with two model sizes and three different permeabilities spanning than two orders of magnitude. Laterally spaced horizontal wells were used to inject an 86:14 mol% mixture of butane and methane, respectively, as a dense vapour. The average dispersion coefficient of solvent (butane) was found to decrease with injected cumulative solvent and increase with reduction in permeability, and both of these relationships could be approximated with an inverse power function.