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

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Abstract
Results of single-contact phase behavior studies for CO2/crude-oil mixtures often are presented as pressurecomposition (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/hex- adecane, 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 stocktank oil plus 602 scf/bbl [1084 std m3/m3] solution gas, were studied at three temperatures, 90, 105, and 120°F [32, 41, and 49°C]. 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 120°F [49°C]), 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.