Displacement Behavior of the Condensing/Vaporizing Gas Drive Process

Abstract

SPE Member Abstract There is mounting evidence for some reservoir fluids that phase behavior in condensing-gas drives departs substantially from traditional concepts deduced for three-component fluids and that transition-zone compositions are established by a condensing/vaporizing mechanism rather than by condensation alone. To what extent does displacement behavior depart from traditional concepts as well and what is the significance of any departures? This paper addresses these questions through a series of compositional simulations for displacement in a one-dimensional slim-tube. The simulations show that displacement behavior for a reservoir fluid does not depart substantially from traditional condensing-gas drive concepts for three-components even though the transition-zone building mechanism is one of condensation/vaporization. They show that when injection gas enrichment exceeds a critical value, displacement behavior of the reservoir fluid becomes miscible-like in the following important respects:recovery is essentially 100% after one pore volume of injection in the limit of no dispersion, andfor a realistic level of dispersion, recovery at 1.2 pore volumes injected is insensitive to both relative permeability and relative permeability end-point saturation. Also, as with traditional concepts, the critical enrichment can be determined from slim-tube displacements by finding the point of breakover in a plot of oil recovery at 1.2 PV of injection vs. gas enrichment. However, with the reservoir oil, gases enriched above the critical value show a converging/diverging type of phase behavior on a pseudoternary diagram, and they leave a residual oil saturation, both of which are contrary to traditional concepts. The magnitude of the residual oil saturation depends primarily on mixing caused by diffusion/dispersion and should be relatively small, less than about 5% PV, in reservoir floods and even less in slim-tube displacements. All the simulations were highly sensitive to the number of grid blocks used to model the slim-tube. This effect must be accounted for when simulated slim-tube behavior is compared with experiments, and it must be addressed in reservoir simulations. Simulations of 20% HCPV enriched-gas slugs driven by water show that a very high level of mixing shifts the point of optimum solvent enrichment to higher values. Numerical dispersion will cause this shift, even in simulations with as many as 100 grid blocks representing displacement length. Whether or not physical dispersion in the reservoir is great enough to cause it is an unanswered question. Introduction Traditional condensing-gas drive concepts of phase behavior, miscibility, and displacement behavior derive from the behavior of three-component fluids. This is true also for the interpretation of slim-tube experiments for minimum miscibility pressure or required gas enrichment. P. 171^

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