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.
Injection 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 gas flood project. In ID displacements, recovery decreases sharply with pressure (or enrichment) below the minimum miscibility pressure (MMP), or the minimum miscibility enrichment (MME). However, reservoirs are 3D. Recent laboratory work of Shyeh-Yung shows that oil recovery does not fall sharply below MMP in tertiary CO2 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. have shown that recovery in gravity-dominated secondary gasfloods does not increase monotonically with enrichment in corescale hydrocarbon displacements. Burger et al. 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 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.