Determination of Diffusion Coefficient for Alkane Solvent–CO₂ Mixtures in Heavy Oil with Consideration of Swelling Effect

Abstract

A generalized methodology has been developed and successfully applied to determine diffusion coefficient of alkane solvent–CO2–heavy oil systems with consideration of swelling effect. Theoretically, a one-dimensional and one-way mass transfer model incorporating the volume translated Peng–Robinson equation of state (PR EOS) has been developed to describe the mass transfer from alkane solvent–CO2 mixture to heavy oil, which accounts for the oil swelling effect resulted from gas dissolution. The heavy oil sample has been characterized as three pseudocomponents, while the binary interaction parameter (BIP) correlations are tuned with the experimentally measured saturation pressures. Both apparent diffusion coefficients for gas mixtures and individual diffusion coefficient of each component of a mixture are determined once the discrepancy between the measured and calculated dynamic swelling factors of heavy oil has been minimized. The volume translated PR EOS with the three characterized pseudocomponents and the tuned BIP correlations is able to accurately predict the phase behavior of alkane solvent–CO2–heavy oil systems. Compared to the apparent diffusion coefficient, better agreements between the measured and calculated dynamic swelling factors have been obtained by use of the individual diffusion coefficients. Addition of C3H8 into CO2 stream is found to not only diffuse faster into heavy oil than CO2 but also contribute to a larger degree of oil swelling, leading to a faster and enhanced swelling effect of C3H8–CO2–heavy oil system in comparison with the CO2–heavy oil system.

The Effect of Oil Properties on the Supercritical CO2 Diffusion Coefficient under Tight Reservoir Conditions

“…The effect of oil viscosity on the CO2 diffusion coefficient is depicted in Figure 10. The figure shows that a lower viscosity can facilitate the CO2 diffusion process, which agrees with the negative relationship between the two parameters that are proposed in the literature [59,[74][75][76]. Moreover, Figures 9 and 10 also show the obvious tendency of the diffusion coefficients at different stages.…”

“…In this study, several pseudo-components are introduced as substitutes for all hydrocarbon components in the oil sample, which can efficiently decrease the calculation load of PR EOS without influencing the computing accuracy [56][57][58]. Specifically, in this work, hydrocarbons with a single carbon number (SCN) are lumped into three pseudo-components, which have been tested as having a good result on the oil/CO 2 system [59,60]. The parameters of the pseudo-components were determined with a series of empirical models [61][62][63][64][65][66][67][68], and the specific data are listed in the results section.…”

“…The viscosity-temperature curves of the oil samples are presented in Figure 4, and the viscosity of each oil sample at the experimental temperature is marked in the figure (the pentagram symbols). The carbon distributions of the oil samples that were determined with the method in the literatures [55,59] are illustrated in Figure 5. It is notable that the carbon distributions of kerosene and crude oil are determined with the above method, and the carbon distribution of mixed oil is determined by combining the carbon distribution of the two oils according to their mixing ratio.…”

“…where zi, and are the mole friction of component i in the mixed oil, crude oil and kerosene samples, respectively, and n c and n k are the mole numbers of crude oil and kerosene, respectively. Figure 5 shows that the components lighter than C 9 account for a tiny part of those oil samples (<1%), which means that there is no obvious extraction effect during the mass transfer process [56,59,60]. The components of the CO 2 phase were analyzed by a GC method after the diffusion experiments, and no hydrocarbons were found in the CO 2 phase.…”

“…Their conclusion is summarized as equation 10, where μ is the viscosity of the solvent or pure liquid phase: (1) The increase in the viscosity of the oil samples. Numerous scholars have indicated that the viscosity of the liquid phase has an obvious influence on the diffusion coefficient [59,[72][73][74][75]. The effect of oil viscosity on the CO 2 diffusion coefficient is depicted in Figure 10.…”