Thermodynamic Modeling of Phase and Tension Behavior of CO2/Hydrocarbon Systems

Abstract

Abstract The gradient theory of in homogeneous fluid is used to predict phase splits and compositions, interfacial composition profiles, and interfacial tension (IFT) of liquid-liquid, liquid-vapor, and liquid-liquid-vapor equilibria in binary and ternary mixtures of CO2 with propane and decane. The theory's input are the equation of state (EOS) of homogeneous fluid and the influence parameters of inhomogeneous fluid. An efficient computational algorithm is presented for simultaneously generating phase behavior, critical points, interfacial composition profiles, and tension between the phases. Most calculations are made with the Peng-Robinson EOS and the geometric mixing rule for the influence parameters. Use of other EOS and alternative schemes for choosing the influence parameters is explored. Introduction CO2 is a promising agent for enhancing petroleum recovery. Laboratory and field studies have established that CO2 can be an efficient oil-displacing agent. The various mechanisms by which it can act includesolution gas drive,immiscible CO2 drive,hydrocarbon/CO2 miscible drive,hydrocarbon vaporization,direct miscible CO2 drive, andmulticontact dynamic miscible drive. Phase-equilibria a data for CO2-reservoir oils have been reported. The data suggest that two distinct types of equilibria are possible. In one, there are only two phases, liquid and vapor. In the other, there is a region of liquid-vapor equilibrium, but in the phase diagram it exists in conjunction with both liquid-liquid and liquid-liquid-vapor regions. Hutchinson and Braun have shown how a lean gas can develop miscibility with a relatively rich oil. Miscibility is achieved when the lean gas strips intermediates from the liquid until the gas composition is rich enough to be miscible with the original oil. This process is called the high-pressure or vaporizing gas drive. In CO2/crude-oil systems of only one liquid phase and one vapor phase, the miscibility development mechanism can be regarded as vaporization. If the temperature is relatively low, the mechanism is described more accurately as condensation (absorption) of CO2 into the oil phase. In CO2/crude-oil systems that display more than one liquid phase in conjunction with a vapor phase, the mechanism is one of condensation and can account for a phenomenon reported by Shelton and Yarborough, namely that two liquid phases card form either with or without vapor being present. The displacement then has the appearance of a liquid-liquid extraction process. In any case, the miscibility development mechanism is related directly to the phase equilibria of the CO2/reservoir-fluid system. All these mechanisms are characterized by high recoveries in the laboratory. Simon et al. suggested that IFT effects are responsible for high recoveries in a vaporizing situation and might be equally effective in a liquid-liquid extraction situation; consequently, it is desirable to study tension behavior along with the phase behavior of CO2/hydrocarbon systems, as we do here. We make use of a molecular theory, the gradient theory of inhomogeneous fluid, which unifies phase and tension behavior in a practicable way. Such an approach has not been followed before. The CO2/propane (C3) / decane (C1O) system was selected for this study because CO2-C3 and CO2-C1O binary phase equilibria data for wide ranges of temperature and pressure are available. Propane represents the light ends and decane the heavier components. Of course, CO2 and reservoir oils do not have exactly the same phase (and therefore tension) behavior as the simple binary and ternary systems discussed here, but as Hutchinson and Braun demonstrated, these systems can give at least a qualitative description of the phase behavior of CO2/crude-oil systems, although Rathmell et al. indicated that when large amounts of CO2 and methane (C1) are both present, a quaternary diagram is needed to account for the observed behavior. Phase Behavior Calculations The design of a CO2 flooding process requires accurate phase behavior predictions of the equilibrium between the oil in place and the injected CO2. In one approach, the experimental data and extrapolations or interpolations are used in the process simulator. This approach can be quite inaccurate unless a great deal of data are available. Alternatively, an EOS can be postulated and its adjustable parameters fit to a limited amount of data. This is clearly the best approach when a good EOS can be found. As shown in the next section, it is the only feasible approach when IFT are to be predicted along with phase behavior. SPEJ P. 235^