Interpretation of Pressure-Composition Phase Diagrams for CO2/Crude-Oil Systems

Abstract

Abstract Results of single-contact phase behavior studies for CO2/crude-oil mixtures often are presented as pressure-composition (P-X) phase diagrams. In such diagrams, regions of pressure and CO2 mole fraction for which more than one phase forms can be identified easily. Phase diagrams for CO2/crude-oil systems can be quite complex, however, since under some conditions such mixtures can form a liquid and a vapor, two liquid phases, or two liquids and a vapor in equilibrium. This paper examines P-X diagrams for two ternary systems, CO2/propane/hexadecane and CO2/methane/hexadecane, and describes transitions from one diagram to another that occur with changes in system temperature or changes in oil composition. Nine experimentally determined P-X diagrams are presented for mixtures of Wasson crude oil with CO2. Three different oils, stock-tank oil, stock-tank oil plus 312 scf/bbl [560 std m3/m3] solution gas, and stock-tank oil plus 602 scf/bbl [ 1084 std m3/m3 ] solution gas, were studied at three temperatures, 90, 105, and 120F [32, 41, and 49C]. Comparison of the resulting phase diagrams with those discussed for the simpler ternary systems indicates that the principal features of the crude oil phase diagrams are qualitatively consistent with those of the ternary systems. The results of the CO2/crude-oil experiments indicate that for low-temperature systems (below about 120F [49C]), the extrapolated vapor pressure (EVP) Of CO2 is a good estimate of the pressure required to produce a dense, relatively incompressible CO2-rich phase that can extract hydrocarbons efficiently from a crude oil. Hence, in the absence of other experimental evidence, the EVP curve can be used as a rough estimate of the minimum miscibility pressure (MMP) for low-temperature reservoirs. Introduction Investigations Of CO2/crude-oil phase behavior commonly include visual observations of the volumetric behavior of binary CO2/crude-oil mixtures. In such studies, metered volumes of CO2 and oil are placed in a windowed cell, and then the volume of the resulting mixture is changed, usually by injecting or removing mercury from the cell. The volume change alters the pressure of the cell contents and also changes the volumes of whatever phases are present as components redistribute between phases. The volumes of the phases are obtained by measuring the positions of interfaces within the cell, and the pressures at which phases appear or disappear are noted. The results of such studies are conveniently plotted on a pressure-composition (P-X) phase diagram (Fig. 1). On such a diagram, saturation pressures and the fraction of the cell volume occupied by one of the phases can be represented. In Fig. 1, bubble-point pressures, at which the visual cell is filled with a single phase, but below which a second phase appears at the top of the cell, are labeled "B." Dewpoint pressures (D) are those at which a second phase appears at the bottom of the cell. The dew- and bubble point curves meet at a critical point (C). Contours of constant volume fraction of one of the phases also meet at the critical point. P-X diagrams have been reported for a number of CO2/crude-oil systems. Those reported for temperatures below about 50C [122F] show liquid/liquid (L1/L2) and liquid/liquid/vapor (L1/L2/V) phase behavior while those at higher temperatures show only liquid/vapor (L/V) phase separations. The phase diagrams reported, however, show distinct differences in the shapes and locations of two- and three-phase regions. In this paper we examine the reasons for those differences and attempt to relate the appearance of a P-X diagram to phase behavior of mixtures that in reality contain many components rather than the two shown on a P-X diagram. To determine what qualitative features should appear on a P-X diagram, we examine the behavior of two ternary CO2/hydrocarbon systems at temperatures above and below the critical temperature of CO2. In addition, we report the results of a series of experiments to determine the effects of changing system temperature and the amount of solution gas on the phase behavior of a CO2/crude-oil system. Finally, we compare the behavior of the CO2/crude-oil systems with that of the simpler hydrocarbon systems. Construction of a P-X Diagram from Binary and Ternary Phase Data A P-X diagram represents only a portion of the phase behavior of a system that contains more than two components. The relationship between the representation of phase behavior on a P-X diagram with the behavior of the more complex system can be seen in Figs. 2 and 3. SPEJ P. 485^