A Thermodynamic Correlation for the Minimum Miscibility Pressure in CO2 Flooding of Petroleum Reservoirs

Abstract

Summary. A graphic equation-of-state (EOS)-generated correlation for the minimum miscibility pressure (MMP) of CO2/crude-oil displacements is presented, along with a brief review of 17 other MMP correlations. presented, along with a brief review of 17 other MMP correlations. The MMP is estimated as the cricondenbar or first-contact miscibility pressure of the pure or impure CO2/crude-oil system in which the C5+ oil pressure of the pure or impure CO2/crude-oil system in which the C5+ oil fraction is modeled as a single normal alkane of equivalent molecular weight. The correlation accounts for all the experimentally observed trends in MMP in a consistent manner and is independent of any CO2/crude- oil data base. A correction is introduced, however, to prevent a predicted increase in the MMP with decreasing temperature at low temperatures for high-molecular-weight oils. The average predicted-MMP/ experimental-MMP ratio for 157 CO2/crude-oil MMP values is 1.09, with a standard deviation of 0.19. Unlike other correlations, specifically those relating MMP with CO2 density, a decrease in MMP with temperature at elevated temperatures (greater than 120C [greater than 248F]) is predicted. Introduction In the evaluation of oils that may be successfully recovered by CO2 flooding, several types of experiments are performed. These include slim-tube displacements (STD), high-pressure volumetric (PVT) and vapor/liquid equilibrium (VLE) studies, core displacements, and continuous multiple-contact experiments. The displacement of recombined reservoir fluid or stock-tank oil from a slim tube packed with sand or glass beads has become a standard experiment used to obtain an estimate of the pressure required to displace the oil with CO2 miscibly. The sandpacked coil provides a medium for the mixing of CO2 and oil in a flowing multiple-contact process that approaches a highly ideal displacement. It is not intended to simulate the reservoir rock. The experiment isolates the effects of phase behavior at various pressures during the multiple contacts of CO2 with oil. The MMP, or minimum dynamic miscibility pressure (MDMP), has been given several formal definitions, includingthe pressure at which the breakover point in the ultimate recovery curve from a series of displacements occurs;the pressure at which 80% of the oil in place (OIP) is recovered when the GOR reaches 7200 std m3 CO2/m3 oil [40,000 scf CO2/bbl oil];the lowest pressure at which injection of 1.2 PV of CO2 yields nearly maximum recovery while exhibiting transition-zone fluids that appear to he the result of multiple-contact miscibility (MCM);the pressure at which the recovery-vs.-pressure curve experiences a sharp change in slope and levels off;the pressure at which not only 85 % of the OIP is recovered by CO2, but also 95 to 98 % of the oil is recovered using 1.2 PV CO2 with no two-phase flow occurring at the outlet; andthe pressure at which 90% of the OIP is recovered at CO2 breakthrough, with no prior two-phase flow observed at the outlet. Despite these different MMP definitions and experimental techniques, STD remains a commonly used, simple method of obtaining useful information relating pressure and displacement efficiency and the effects of small concentrations of such gases as methane or nitrogen in the CO2. To facilitate screening procedures and to gain insight into this miscible displacement process, many correlations relating MMP to the physical properties of the oil and displacing gas have been proposed physical properties of the oil and displacing gas have been proposed (see Table 1 and the Appendix). Ideally, such a correlation should account for each parameter known to affect the MMP, should be independent of an MMP data base so that it will not need revision each time a more extensive set of data is acquired, should be based on thermodynamic or physical principles that affect the miscibility of fluids, and should be directly related to the MCM process. The parameters that have been found to influence the MMP are temperature, parameters that have been found to influence the MMP are temperature, oil composition, and the contaminants present in the CO2 (displacing-fluid composition). Note that none of the correlations proposed, including our contribution, possess all these attributes. proposed, including our contribution, possess all these attributes. In our opinion, the greatest potential for determination of the MMP lies in calculation by computer simulation of the MCM process that occurs in the slim tube. The Natl. Petroleum Council's empirical correlation of MMP with API gravity and temperature provides rough estimates of MMP, Holm and Josendal correlated CO2 MMP with temperature and the average molecular weight of the C5+ fraction of the crude on the basis of the Benham et al. rich-gas correlation. Mungan extended Holm and Josendal's work to higher molecular weights. Glaso presented a generalized numerical correlation for miscible presented a generalized numerical correlation for miscible displacement that is also based on the Benham data. It should be noted that no CO2 was used in Benham's original work. Furthermore, rich-gas displacement of oil is achieved by establishing condensing- gas-drive miscibility, whereas CO2 flooding attains miscibility with oils through vaporizing gas drive. Johnson and Pollin empirically correlated MMP with the critical temperature, critical pressure, composition, and molecular weight of the injection gas; reservoir temperature; and the API gravity, density, and number average molecular weight of oil. This correlation, however, may be used only when the injection gas is a binary mixture of CO2 and N2 or CO2 and CH4. Yelling and Metcalfe correlated MMP with temperature alone, making corrections only if the bubblepoint pressure exceeds the predicted MMP. The research of Cronquist yielded empirical correlations of MMP with reservoir temperature and several oil characterization parameters in which temperature, mole percent of methane in oil, and the molecular weight of the pentane-plus fraction of the oil were found to be the most important parameters in the determination of the MMP. Dunyushkin and Namiot predicted the MMP to be the critical pressure at reservoir temperature of a CO2/ normal-alkane binary system, where the alkane represented the crude oil's average molecular weight of the pentane and heavier hydrocarbon, C5+, fraction. SPERE P. 81