A new analysis of pressure dependence of the equilibrium interfacial tensions of different light crude oil–CO2 systems

Abstract

In this paper, the equilibrium two-phase compositions are predicted and analyzed to elucidate the pressure dependence of the equilibrium interfacial tensions (IFTs) of three different light crude oil–CO2 systems. First, three series of the dynamic IFT tests for a dead light crude oil–pure CO2 system, a live light crude oil–pure CO2 system, and a dead light crude oil–impure CO2 system at different equilibrium pressures from the literature are used. Second, the modified Peng–Robinson equation of state (PR-EOS) is tuned by using measured pressure–volume–temperature (PVT) data to predict the equilibrium two-phase compositions of the three light crude oil–CO2 systems. Such tuned PR-EOS together with the parachor model is applied to predict the equilibrium IFTs, which are compared with and validated by the measured IFT data. Third, the pressure dependence of the equilibrium IFTs, the initial oil and gas composition effects, and the initial gas fraction effect are examined. The density difference between the light crude oil and gas phase is found to be a key factor in the parachor model for the IFT predictions. The equilibrium IFT vs. pressure curve is found to have three different pressure ranges, which correspond well to those for the density difference. Moreover, the initial oil and gas compositions affect the equilibrium two-phase compositions and IFTs to different extents. The live light crude oil–pure CO2 equilibrium IFT is reduced with an increased initial gas fraction. For the dead light crude oil–pure/impure CO2 system, the miscibility with zero IFT can be achieved only if the initial gas phase has more than 0.70 mol fraction. Otherwise, it is the complete gas dissolution into the light crude oil that leads to zero IFT.