Summary
Recent research on four-component 1D displacements has shown that enriching the gas above the minimum miscibility enrichment (MME) can increase oil recovery substantially for certain systems. Research has shown further that the oil-recovery increase can be very sensitive to the level of dispersion at the enrichment chosen. The main focus of this paper is to extend the research on four-component systems to displacements of multicomponent oils and gases in which the recovery is affected by dispersion and enrichment. We consider here a 12-component oil displaced by solvents enriched above the MME. For this case, the increase in recovery (displacement efficiency) above the MME can be as large as 15% original oil in place (OOIP), depending on the level of mixing. The methodology outlined can be used as a screening tool to determine whether a significant benefit may exist and whether further 2D and 3D studies are warranted.
A secondary focus of the paper is to examine in detail how dispersion affects recoveries and displacement mechanisms for the 4- and 12-component systems. We show that for the case of the four-component model, the displacement mechanism changes from a combined condensing/vaporizing (CV) displacement to a strictly condensing one as enrichment increases above the MME. We also show how to quantify the percentage of the CV displacement that is vaporizing or condensing by calculating the compositional distances between key tie lines identified from "dispersion- free" theory.
Introduction
The objective of enriched-gas floods is to achieve a multicontact miscible (MCM) displacement by a sufficient enrichment of the gas with intermediate components. If a near-MCM process occurs, then a highly efficient local displacement can be achieved. Because the local displacement efficiency is one of the primary factors that govern ultimate recovery, it is very important to quantify how mixing the oil and gas in reservoirs can adversely impact the efficiency of the MCM process.
One of the key variables in enriched-gas floods, therefore, is the optimum enrichment for a highly efficient displacement. Slimtube experiments are often used to help determine the optimum enrichment. Because these experiments typically show that the oil-recovery increase beyond the MME is minimal, the optimum enrichment is often taken to be the MME. Other factors, such as the availability of solvents in the field and surface facility considerations, also can impact the choice of enrichment.
Recent results by Johns et al.1 show that the slimtube results may be misleading because of the scaleup of dispersion from the laboratory to the field scale. Mixing by dispersion and other mechanisms is likely much greater in the field than the level of dispersion found in laboratory cores. Oil and gas mixing in a reservoir can be caused by mechanisms such as molecular diffusion, mechanical dispersion, gravity crossflow, viscous crossflow, and capillary crossflow.2
Several authors have examined the effect of mixing and enrichment above the MME on oil recovery. Johns et al.1 considered the effect of dispersion on recovery in 1D displacements. They showed that the "knee" in the recovery curve from slimtube experiments depends on the level of dispersion. For small dispersivities typical of slimtubes, the knee occurs at the MME. For greater levels of mixing, they showed that the knee could occur at enrichments much greater than the MME.
Chang3 matched coreflood displacements with reservoir simulations at different enrichments. He showed that recovery increased sharply for enrichments above the MME. Chang concluded that the increased recoveries were caused by higher displacement and sweep efficiencies as the enrichment level increased. The better sweep efficiency was attributed to increased gas density with enrichment. Jerauld4 also observed an increase in recovery above the MME. Giraud et al.5 observed that the highest recovery occurred at pressures above the minimum miscibility pressure (MMP).
Stalkup2 showed that significant additional recovery might be obtained by injecting enriched gases above the MME. A significant increase in recovery occurred for longitudinal dispersivities as low as 0.3 ft, when the solvent and water were injected in slugs. He also concluded that mixing of the solvent and the oil by viscous crossflow during water alternating gas (WAG) might dominate other mixing mechanisms in the reservoir (i.e., dispersion). Numerous other papers also have examined the effect of viscous crossflow, capillary pressure, diffusion, gravity, heterogeneities, and numerical grids on recovery.6–19
The main focus of this paper is to extend the work for four-component systems to displacements of a 12-component oil by solvents enriched above the MME. The effects of realistic levels of dispersive mixing on the displacement efficiency of the floods are examined. The slope in the recovery curves is used to quantify the effect of dispersion on the displacement efficiency. We also show how dispersion and enrichment affect the CV displacement mechanisms. The displacement mechanisms of the miscible process are quantified exactly for the first time using dispersion-free theory. We use numerical dispersion in this research to mimic physical dispersion.
Analytical and Numerical Models
The numerical solutions for 1D flow are calculated with the U. of Texas at Austin Compositional Simulator (UTCOMP), a compositional simulator that includes volume change on mixing.20 Analytical solutions to dispersion-free flow in one dimension are solved using hyperbolic conservation equations with the assumptions stated by Helfferich.21 The analytical solutions are used to find the MME and the key tie lines in the displacement.22–26
Experimental Investigation on CO2 Injection in Block M
