Experimental Phase Densities and Interfacial Tensions for a CO2/Synthetic-Oil and a CO2/Reservoir-Oil System

Abstract

Summary Experimental data are presented for equilibrium vapor and liquid densities and interfacial tensions (IFT's) for two multicomponent mixtures. Data are presented at 120 and 150°F for a CO2/synthetic-oil (containing the n-paraffins, methane to tetradecane) and at 130°F for a CO2/recombined-reservoir-oil system. In both systems, measurements include the near-critical region, where IFT's become very low. These data should be useful in developing and testing models to predict phase behavior and IFT's for CO2 EOR operations. Introduction Various studies1–3 have suggested that low IFT's between an injected phase and a reservoir oil may lead to improved displacement efficiencies and reduced residual oil saturations in EOR operations. In gas EOR processes, such as CO2 injection, sufficiently low IFT levels (less than approximately 0.04 dynes/cm) occur only when the CO2/oil mixtures that develop during the displacement process approach the mixture critical composition at reservoir temperature and pressure conditions. When CO2 injection processes began to receive widespread attention, we undertook a systematic study of IFT behavior in CO2/hydrocarbon systems because of the lack of such CO2 data. At that time, the only available data were for CO2/n-butane4 and CO2/recombined-reservoir-oil systems.5 In previous studies, 6–10 we presented experimental results for CO 2 with hydrocarbons, including n-butane, n-decane, n-tetradecane, benzene, cyclohexane, and n-butane/n-decane. In addition, we evaluated predictive techniques to describe IFT's in such systems.11 These studies concentrated on "building-block" binary and ternary systems that were designed as a basis for developing and testing models to predict IFT behavior. The systems presented in this paper can provide useful data for ultimate testing of such models in multicomponent systems. Experimental Method and Procedure Apparatus. The apparatus used in this work has been described previously6,8,10, for measurements in this study, the apparatus was essentially in the same configuration as in the CO2/n-tetradecane study.8 Procedures. Experimental procedures were identical to those used for the CO2/ n-butane/n-decane ternary system study.10 Recombined CO2/oil mixtures were prepared in separate high-pressure transfer cylinders and stored over mercury at ambient temperature. Pressures were maintained at sufficiently high levels to ensure that each mixture was in the single-phase liquid state. The mixture was charged to the equilibrium cell by injecting a volume of mercury into the transfer cell and displacing an equal volume of mixture to the equilibrium cell. The pressure in the equilibrium cell (at the operating temperature of 120, 130, or 150°F) was increased in successive steps by injecting additional amounts of material from the transfer cell into the equilibrium cell. Equilibrium was established at each pressure, then phase densities and IFT's were measured; this allowed data at all pressures to be taken on a mixture of constant overall composition. Measurements were taken until the pressure was reached where the mixture passed from a two-phase into a single-phase state. In contrast to our studies6–10 on binary and ternary mixtures, phase compositions were not measured for these multicomponent oil systems. CO2/Synthetic Oil. The synthetic oil used in this study is similar to that used by Turek et al.12 in their study of phase compositions for several CO2/synthetic-oil mixtures. Amoco Production Co.'s Tulsa Research Center prepared the two CO 2/oil mixturesa and supplied ˜/750 cm3 of each mixture for this study. The CO2 compositions were selected to produce overall mixtures near their critical compositions at the temperatures of interest (˜92% CO2 at 120°F for one mixture and ˜85% at 150°F for the other). Table 1 shows mixture compositions determined by chromatographic analyses. a The compositions are very similar to those Turek et al. reported for the fluids they studied. CO2/Recombined Oil. The CO2/recombined-reservoir-oil mixture was prepared and supplied by Chevron Production Technology Co. as cm 3 of a liquid-phase CO2/oil mixture.b The CO2 content was selected to produce a near-critical mixture at 130°F (˜56% CO 2). Tables 2 and 3 show the compositions of the mixture and the base oil.c Table 2 shows the results of an extended analysis (through C25+) of the base oil and an analysis of the CO2/oil mixture (55.55 mol% CO2). For comparison, Table 2 also includes analyses done on the same base oil in 1975. Table 3 presents a detailed paraffin/olefin/naphthene/aromatic (PONA) analysis of the base stock-tank oil. Experimental Results CO2/Synthetic Oil. Measurements were taken at 120 and 150°F for mixtures with the compositions shown in Table 1. Measurements taken until a pressure was reached where the mixture passed from a two-phase into a single-phase state showed that the mixtures reached the single-phase state by passing through a bubblepoint condition at both temperatures. However, visual observation of the fluid in the equilibrium cell as the bubblepoint pressure was approached showed that each mixture was very near its critical point. p. 170–174 Apparatus. The apparatus used in this work has been described previously6,8,10, for measurements in this study, the apparatus was essentially in the same configuration as in the CO2/n-tetradecane study.8 Procedures. Experimental procedures were identical to those used for the CO2/ n-butane/n-decane ternary system study.10 Recombined CO2/oil mixtures were prepared in separate high-pressure transfer cylinders and stored over mercury at ambient temperature. Pressures were maintained at sufficiently high levels to ensure that each mixture was in the single-phase liquid state. The mixture was charged to the equilibrium cell by injecting a volume of mercury into the transfer cell and displacing an equal volume of mixture to the equilibrium cell. The pressure in the equilibrium cell (at the operating temperature of 120, 130, or 150°F) was increased in successive steps by injecting additional amounts of material from the transfer cell into the equilibrium cell. Equilibrium was established at each pressure, then phase densities and IFT's were measured; this allowed data at all pressures to be taken on a mixture of constant overall composition. Measurements were taken until the pressure was reached where the mixture passed from a two-phase into a single-phase state. In contrast to our studies6–10 on binary and ternary mixtures, phase compositions were not measured for these multicomponent oil systems. CO2/Synthetic Oil. The synthetic oil used in this study is similar to that used by Turek et al.12 in their study of phase compositions for several CO2/synthetic-oil mixtures. Amoco Production Co.'s Tulsa Research Center prepared the two CO 2/oil mixturesa and supplied ˜/750 cm3 of each mixture for this study. The CO2 compositions were selected to produce overall mixtures near their critical compositions at the temperatures of interest (˜92% CO2 at 120°F for one mixture and ˜85% at 150°F for the other). Table 1 shows mixture compositions determined by chromatographic analyses. a The compositions are very similar to those Turek et al. reported for the fluids they studied. CO2/Recombined Oil. The CO2/recombined-reservoir-oil mixture was prepared and supplied by Chevron Production Technology Co. as cm 3 of a liquid-phase CO2/oil mixture.b The CO2 content was selected to produce a near-critical mixture at 130°F (˜56% CO 2). Tables 2 and 3 show the compositions of the mixture and the base oil.c Table 2 shows the results of an extended analysis (through C25+) of the base oil and an analysis of the CO2/oil mixture (55.55 mol% CO2). For comparison, Table 2 also includes analyses done on the same base oil in 1975. Table 3 presents a detailed paraffin/olefin/naphthene/aromatic (PONA) analysis of the base stock-tank oil. CO2/Synthetic Oil. Measurements were taken at 120 and 150°F for mixtures with the compositions shown in Table 1. Measurements taken until a pressure was reached where the mixture passed from a two-phase into a single-phase state showed that the mixtures reached the single-phase state by passing through a bubblepoint condition at both temperatures. However, visual observation of the fluid in the equilibrium cell as the bubblepoint pressure was approached showed that each mixture was very near its critical point. p. 170–174