Generalized Correlations for Predicting Solubility, Swelling and Viscosity Behavior of CO2 -Crude Oil Systems

Abstract

Abstract This paper presents correlations for predicting the solubility, swelling and viscosity behavior of CO2-crude oil systems. The correlations were developed from experimental data obtained by the authors. These data are also presented. The data were determined by measuring the properties of mixtures of CO2 and nine different oils. Experimental conditions covered a range of 100 to 250 degrees F and pressures up to 2,300 psia. Properties predicted by the correlations have average deviations, expressed as per cent of experimental value, of 2 per cent for solubility, 0.5 per cent for swelling and 12 per cent for viscosity. Introduction Interest in CO2 injection as an oil recovery process has led to the development of performance prediction methods which can be applied to specific reservoirs. To use these performance prediction methods, it is necessary to know the solubility, swelling, and viscosity properties of CO2-crude oil mixtures at reservoir conditions. Some information on these properties has appeared in the literature; however, this information did not cover the range of different oils and conditions needed to prepare generalized correlations for reservoir engineering purposes. Consequently, an experimental program was undertaken to collect the data needed. The data obtained and the correlations developed from the data are described in the following sections of this paper. SOLUBILITY OF CO2 IN CRUDE OILS CO2 solubility data in the literature come from six principal sources. The solubility prediction method of Welker and Dunlop is limited to 80F.The information in Ref. 4 is of two types: the first includes binary and ternary mixtures of CO2 and light hydrocarbons (C1 to C6), and the second gives data for CO2 and heavy hydrocarbons for a temperature range of 40 to 90F.Ref. 5 contains a KCO2 chart for systems whose convergence pressure is 4,000 psia. The KCO2's are based mainly on CO2-natural gas mixtures. Poettmann's work covered CO2 solubility in one condensate and one crude oil. Jacoby and Rzasa measured CO2 solubilities as a function of pressure and temperature for two natural gas-absorber oil mixtures and two natural gas-crude oil mixtures. CO2 concentration in these four systems was fixed at 5 mol per cent. The work reported in this paper extends CO2 solubility data to a variety of different crude oil types in a temperature range from 110 to 250F and pressures up to 2,300 psia. The experimental procedure used by the authors to obtain the solubility data consisted of combining known amounts of pure CO2 and crude oil in a visual cell at a fixed temperature and measuring the bubble point of the mixture. Measurements were made for a total of 40 different CO2-oil mixtures and the results are shown in Table 2. The mixtures included nine different oils (seven crude oils and two refined oils) whose properties are listed in Table 1. All nine oils had vapor pressures less than 1 atm at the experimental temperatures. Consequently, analysis of the bubble-point vapor showed a CO2 concentration over 99 mol per cent. At no time during these experiments was a second, more dense, liquid phase observed. The solubility correlation which was developed from the data in Table 2 is presented in Figs. 1, 2 and 3. In these figures, solubility is expressed as xCO2, the mol fraction of CO2 in the CO2-Oil mixture. Fig. 1 shows solubility as a function of CO2 fugacity and temperature. Fig. 2 shows the same solubility data expressed as a function of saturation pressure and temperature. The solubility shown in Figs. 1 and 2 is for an oil whose UOP characterization factor is 11.7. UOP characterization factors of crude oils can be determined from Ref. 10 if the viscosity and API gravity of the oil are known. Fig. 3 gives the solubility correction factor for oils whose UOP characterization factors differ from 11.7. JPT P. 102ˆ