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Numerical simulation of miscible EOR processes requires calculation of the phase equilibria that exist between solvent and oil over the entire solvent/oil composition range. One calculation approach is to tabulate K-values from experimental data and allow the simulator to access the table when necessary. This approach can lead to erroneous conclusions for CO2 miscible processes if there are insufficient tabulated phase equilibrium data to cover all possible compositions, in particular those cases where three phases exist at some point in the displacement. This paper presents a different approach to the problem. If a generalized equation of state (EOS) can match experimental data, then it can be used in a reservoir simulator to calculate the phase equilibria necessary for the prediction of fluid compositions, densities, and viscosities during a displacement process. Previous work has shown how a generalized Redlich-Kwong equation can be used to calculate typical hydrocarbon phase behavior relationships that exist in condensate and black oil reservoir systems. The equation parameters have been modified further for use in hydrocarbon/CO2 calculations over a wide range of CO2 concentrations. Appropriate mixing rules for description of the phase equilibria in CO2/hydrocarbon systems have been developed. Experimental binary vapor/liquid equilibrium (VLE) data have been used to evaluate the constants in the EOS for pure CO2 as well as interaction terms used within the mixing rules. Predictions of phase equilibria then have been made and compared with experimental data for a synthetic multicomponent hydrocarbon/CO2 system and a crude oil/CO2 system.

Abstract Carbon dioxide flooding under miscible conditions is being developed as a major process for enhanced oil recovery. This paper presents results of research studies to increase our understanding of the multiple-contact miscible displacement mechanism for CO2 flooding. Carbon dioxide displacements of three synthetic oils of increasing complexity (increasing number of hydrocarbon components) are described. The paper concentrates on results of laboratory flow studies, but uses results of phase-equilibria and numerical studies to support the conclusions.Results from studies with synthetic oils show that at least two multiple-contact miscible mechanisms, vaporization and condensation, can be identified and that the phase-equilibria data can be used as a basis for describing the mechanism. The phase-equilibria change with varying reservoir conditions, and the flow studies show that the miscible mechanism depends on the phase-equilibria behavior. Qualitative predictions with mathematical models support our conclusions.Phase-equilibria data with naturally occurring oils suggest the two mechanisms (vaporization and condensation) are relevant to CO2 displacements at reservoir conditions and are a basis for specifying the controlling mechanisms. Introduction Miscible-displacement processes, which rely on multiple contacts of injected gas and reservoir oil to develop an in-situ solvent, generally have been recognized by the petroleum industry as an important enhanced oil-recovery method. More recently, CO2 flooding has advanced to the position (in the U.S.) of being the most economically attractive of the multiple-contact miscibility (MCM) processes. Several projects have been or are currently being conducted either to study or use CO2 as an enhanced oil-recovery method. It has been demonstrated convincingly by Holm and others that CO2 can recover oil from laboratory systems and therefore from the swept zone of petroleum reservoirs using miscible displacement. However, several contradictions seem to exist in published results.. These authors attempt to establish the mechanism(s) through which CO2 and oil form a miscible solvent in situ. (The solvent thus produced is capable of performing as though the two fluids were miscible when performing as though the two fluids were miscible when injected.) In addition, little experimental work has been published to provide support for the mechanisms of multiple-contact miscibility, as originally discussed by Hutchinson and Braun.One can reasonably assume that the miscible CO2 process will be related directly to phase equilibria process will be related directly to phase equilibria because it involves intimate contact of gases and liquids. However, no data have been published to indicate that the mechanism for miscibility development may differ for varying phase-equilibria conditions.This paper presents the results of both flow and phase-equilibria studies performed to determine the phase-equilibria studies performed to determine the mechanism(s) of CO2 multiple-contact miscibility. These flow studies used CO2 to displace three multicomponent hydrocarbon mixtures under first-contact miscible, multiple-contact miscible, and immiscible conditions. Results are presented to support the vaporization mechanism as described by Hutchinson and Braun, and also to show that more than one mechanism is possible with CO2 displacements. The reason for the latter is found in the results of phase-equilibria studies. SPEJ P. 242

The isothermal effects of pressure and composition on the mixture and partial enthalpy differences of the methane-propane binary were derived from PVTy data. A graphical and numerical technique was used to derive the enthalpy differences from 100° to 460°F. and 200 to 2,000 lb./sq. in. abs. for the complete composition range of the binary. The isothermal effects of pressure and composition on the mixture enthalpy data of the methane-propane binary were also calculated with the Benedict-Webb-Rubin and the Redlich-Kwong equations of state. The results of the three methods of calculation are compared. The comparison showed that the two-constant Redlich-Kwong equation of state is as good as the eight-constant Benedict equation of state in the superheated vapor region. The data derived in this work were combined with other methane binary enthalpy data from the literature to construct a general correlation of the effect of pressure and composition on the enthalpy of methane-light hydrocarbon binaries. The correlating parameters are the pseudo reduced temperature and pressure and a mole-average acentric factor.

Abstract This paper presents results of a laboratory study of retrograde condensate recovery by revaporization into dry injection gas. Flow tests were performed in 10.6-ft long sandpacks at 100F and 1,500 psi. In three runs methane revaporized the liquid from a n-heptane-methane mixture in the presence of immobile water. Two of these tests were water-wet, and the third was totally oil-wet. In the three runs n-heptane recovery was complete after 2.5 hydrocarbon PV of injection. There was no significant performance difference between the two wettability extremes. In a fourth experiment, a methane-hydrogen sulfide mixture revaporized a synthetic light, sour condensate. No water saturation was present. Equilibrium compositions and volumetric data were obtained for the four-component condensate. The heavy component, n-heptane, was removed alter 6 PV production. Comparison of the effluent fluid compositions with known equilibrium data shows that the flowing fluid was equilibrium vapor and that the mixing zone between equilibrium vapor and dry injection gas was short. Data indicated that complete recovery of retrograde liquid occurred after it was contacted by a sufficient quantity of dry gas. Introduction When pressure declines below the fluid dew point in a gas condensate reservoir, a liquid phase forms. In this process, referred to as retrograde condensation, the quantity of liquid formed is frequently small enough that the liquid is not a flowing phase. To prevent loss of valuable retrograde liquids, the process of dry gas cycling has been employed for several years as a more or less standard practice. In this procedure the reservoir pressure is maintained above the fluid dew point so that the liquid components may be produced as vapor and then separated at the surface. Although full pressure maintenance by gas cycling seems ideal in terms of preventing liquids loss, several factors can reduce the attractiveness of such an operation. From a study of a condensate reservoir in Alberta, Canada, Havlena et al. concluded that cycling under conditions of declining pressure leads to economic advantages and to a high recovery of hydrocarbon liquids. This study considered effects of volumetric sweep efficiency, retrograde behavior of the original wet gas and revaporization characteristics of the retrograde liquid when contacted by dry gas. The first major work concerning revaporization of liquid in a gas condensate system is that of Standing et al. Calculations based upon the PVT behavior of a recombined gas condensate fluid indicated that all retrograde liquid can be recovered if it is contacted by a sufficient quantity of dry gas. The paper considered the effect of variable permeability upon the recovery of retrograde liquid. Standing et al. concluded that recovery of heavier components in the retrograde liquid is greatest if reservoir pressure is allowed to decline below the dew point prior to dry gas injection. Since the work of Standing et al., several laboratory studies have been reported which show that recovery of hydrocarbon liquids by vaporization into dry injected gas can contribute to increased recovery above that obtained by ordinary production practices. Vaporization from retrograde condensate, conventional oil and volatile oils reservoirs has been considered. There is little work that deals with revaporization recovery from condensate reservoirs. SPEJ P. 87ˆ

An isothermal throttling calorimeter system has been built for obtaining the effects of pressure on the enthalpy of liquids and vapors. The propane-benzene system has been studied at 200°0, 300°, and 400°F. for pressures from 200 to 1,000 Ib./sq.in.abs. The experimental results are evaluated with respect to errors, and the data on the pure components are compared with values from the literature. Calculation methods are employed, and the calculated results are compared with the experimental data in an effort to evaluate the calculation methods.

Enthalpies of components in ideal gas mixtures can be computed from existing data. However the isothermal effects of pressure and composition on partial enthalpies of mixture components have not been completely characterized. An investigation of the application of the Redlich-Kwong equation of state to the calculation of partial enthalpies is reported here. Methods are developed for convenient hand calculations and for computations on the IBM-650. Calculated partial enthalpies are compared with values derived from experimental P-V-T-C data for binary hydrocarbon mixtures. The calculated values agree fairly well with the derived data for pressures up to 2,500 lb./sq. in. abs. Best agreement is generally obtained when the mixture is well above the critical temperature of the more abundant component. No consistent bias was indicated however.

Abstract Experimental data were used for determining miscibility in liquefied petroleum gas (LPG) slug flooding and enriched gas drive of crude oils. The miscibility data for LPG slug flooding includes cases where the driving gas contains large amounts of nitrogen and when low pressure miscible displacement is desired. The results of flow tests for enriched gases miscibly displacing crude oil are given. These data cover a wide range of reservoir oil properties and miscibility pressures. Methods for predicting compositional requirements for both miscible slug displacement processes are recommended and should be useful for preliminary engineering evaluation of miscible slug displacement for a reservoir. Introduction The two most frequently applied hydrocarbon solvent processes for miscible displacement of crude oil are liquefied petroleum gas (LPG) slug flooding and enriched gas drive. A slug of the LPG or enriched gas is injected and followed by dry gas or gas-water displacement. In both cases the injected material forms a miscible slug in the reservoir. Generally, there are two fluid contact zones in which the establishment of miscibility must be considered as related to the fluid compositions and the reservoir temperature and pressure. The first zone is the solvent-reservoir oil pressure. The first zone is the solvent-reservoir oil contact zone; the second zone is where the lean scavenging gas and solvent come together. For successful miscible displacement, there must be single-hydrocarbon-phase condition throughout both contact zones. Aside from possible repressuring procedures which may be undertaken prior to solvent procedures which may be undertaken prior to solvent injection, the primary engineering control for achieving miscibility is the proper specification of the solvent and driving gas compositions. This paper discusses the compositional requirements for paper discusses the compositional requirements for miscibility to be achieved in both contact zones and considers cases where the reservoir pressure is very low or the driving gas contains a large amount of nitrogen. LPG SLUG FLOODING FOR MISCIBLE DISPLACEMENT OF CRUDE OIL In LPG slug flooding there is no problem in achieving miscibility with the crude oil under conditions where the solvent remains liquid. Miscibility between the LPG slug and the driving gas may be the limiting factor. At pressures below 1,100 to 1,200 psia, miscibility often cannot be achieved between the LPG and driving gas, and even higher pressures may be required if the available driving gas contains an appreciable concentration of nitrogen. Another area of increasing interest is LPG slug flooding in reservoirs where the pressure is 1,000 psia or below. At these pressures the methane-LPG transition cannot be pressures the methane-LPG transition cannot be single phase at temperatures below 160 degrees F. The only practicable approach to achieving miscible displacement under these conditions is to inject an ethane-rich buffer slug between the LPG and the driving gas. To determine the allowable nitrogen concentration for gases driving LPG, the phase behavior of nitrogen-methane-propane mixtures was experimentally studied at 105 degrees and 120 degrees F. Similarly, equilibrium-phase behavior data were obtained for the methane-ethane-propane system at 105 degrees F. The latter results allow estimates to be made of the buffer-slug composition necessary for miscible displacement at low pressures. Also, the effects of small amounts of butane and pentane on the phase behavior of the nitrogen-methane-propane and the nitrogen-methane-ethane-propane system were studied. SPEJ p. 278