Mass transfer from a bypassed region to a flowing region is a very strong function of the solvent phase behavior. Diffusion, dispersion, and capillarity-driven crossflow can contribute to this mass transfer in addition to pressure-and buoyancy-driven crossflow. Our experiments indicated that the mass transfer rate increased with enrichment. Liquid phase diffusion played a significant role and capillary pumping did not contribute to mass transfer in the cases studied. IntroductionInjection of carbon dioxide and/or enriched hydrocarbon gases is still a popular enhanced oil recovery technique despite the low oil prices of the recent years. The cost of solvent injected is the key to the economics of such a gasflood project. In 1D displacements, recovery decreases sharply with pressure (or enrichment) below the minimum miscibility pressure (MMP), or the minimum miscibility enrichment (MME). 1,2 However, reservoirs are 3D. Recent laboratory work of Shyeh-Yung 3 shows that oil recovery does not fall sharply below MMP in tertiary CO 2 floods; near-miscible solvents can be effective at core-scale in vertical floods. Near-miscible solvents are defined as solvents with composition near the MME composition; enrichment can be slightly lower than the MME. Chang et al. 4 have shown that recovery in gravity-dominated secondary gasfloods does not increase monotonically with enrichment in corescale hydrocarbon displacements. Burger et al. 5 showed that improvement in overall recovery in the near-miscible region comes from improvement in sweep over the more enriched solvents in laboratory-scale vertical corefloods using computed tomography (CT). Pande 6 has shown that, at the field scale, the optimum enrichment for a water-alternating-gas flood depends on the heterogeneity of the reservoir. Reservoir rocks are heterogeneous. The viscosity and density of injectants are significantly smaller than those of oils to be displaced. Rock heterogeneity, gravity override, and viscous fingering conspire to bypass oil in gasfloods. Recovery of oil depends on the amount of bypassing and the amount of crossflow/mass transfer from the bypassed regions to the invaded regions.The objective of this paper is to understand the amount of crossflow and mass transfer from bypassed regions to the invaded regions in gasfloods. There are four contributing factors to this crossflow/ mass transfer: pressure-driven, 7 gravity-driven, dispersion/diffusion-driven and capillarity-driven. 8 The pressure-driven and gravity-driven crossflows are accounted for directly in flow equations. The accuracy of these two crossflow terms, however, depends on the resolution of the gridblocks and the accuracy of the pressure fields. The other two factors need to be incorporated into the flow equations. Dispersion and diffusion are often not represented mechanistically at the field scale. The capillarity-driven crossflow is often ignored at the field scale. When the bypassed fluid and displacement fluid are first contact miscible (FCM), there is no capillary crossflow. When thes...