CO2 Minimum Miscibility Pressure: A Correlation for Impure CO2 Streams and Live Oil Systems

Abstract

Abstract This paper presents an empirically derived correlation for estimating the minimum pressure required for multicontact miscible (MCM) displacement of live oil systems by pure or impure CO2 streams. Minimum miscibility pressure (MMP) has been correlated with temperature, oil C5+ molecular weight, volatile oil fraction, intermediate oil fraction, and composition of the CO2 stream. The effects of temperature and oil C5+ molecular weight on pure CO2 MMP have been well documented. However, CO2 sources are rarely pure, and solution gas usually is present in reservoir oils. The correlation presented in this paper accounts for the additional effects on MMP caused by the presence of volatile components (methane, C1; and N2) and intermediate components (ethane, C2; propane, C3; butane, C4; hydrogen sulfide, H2S; and CO2) in the reservoir oil. This correlation also is capable of estimating MMP for a contaminated or enriched CO2 stream on the basis of the pure CO2 MMP. Introduction Miscible displacements using hydrocarbon solvents have been described in the literature by many authors.1–6 The use of a slim tube apparatus for the establishment of MMP requirements for enriched or vaporizing gas drives was presented by Deffrenne et al.5 and Yarborough and Smith.7 Rutherford8 referred to these systems as conditionally miscible processes. The initial work of Rathmell et al.9 and Ballard and Smith10 illustrated that the mechanisms of CO2 displacements are similar to those of high-pressure MCM vaporizing gas drives. Since the high solubility of CO2 in reservoir oils diminishes the pressure required for miscibility to occur, a CO2 vaporizing gas drive can operate in the same manner as a lean-gas injection process, but at significantly lower pressures. Correlations for the prediction of MMP requirements for CO2 flooding are extremely helpful in the screening of candidate reservoirs for CO2 floods. Holm and Josendal11 were the first to introduce a method for estimating the MMP required for CO2 displacing oil. Other correlations for CO2 MMP have been introduced by the Natl. Petroleum Council,12 Yellig and Metcalfe,13 and Johnson and Pollin,14 as well as a new correlation from Holm and Josendal.15,16 This paper presents an empirical approach to MMP estimation. Included in this study are the effects of solution gas (live oil systems) and the effects of impure CO2 sources. Concepts of Miscible Displacement Miscible displacement is represented most easily by a ternary diagram. A pseudoternary diagram for a hypothetical hydrocarbon system is shown in Fig. 1. This is a pseudoternary representation since the apexes do not consist of pure components, but it can be used to qualitatively describe the process of miscible displacement. Fig. 1 has been divided into three areas: Zone 1, Zone 2, and Zone 3. Zone 1 represents the area of first-contact miscibility. Any solvent falling within this region can be mixed with the reservoir oil shown, such that any and all mixtures will fall outside of the two-phase region. Zone 2 represents the region of multicontact miscibility. Solvents within this area, while not initially miscible in all proportions with the reservoir oil, eventually will achieve miscibility through the repeated contacts of the reservoir oil and equilibrium fluids. There are two types of MCM processes: vaporizing gas drive, where the solvent is enriched by components vaporized from the reservoir oil, and condensing or enriched gas drive, where the solvent contributes to the enrichment of the reservoir fluid.17 Commercially viable CO2-miscible EOR processes are usually of the MCM type, because reservoir pressure requirements for this process are significantly lower than required for first contact miscibility. Zone 3 represents the area of immiscible displacement. Displacement of the reservoir oil by an fluid falling within Zone 3 will result in multiphase flow. The mass transfer between the oil and displacing fluid is such that miscibility cannot be achieved.