Effects of Paraffinic Group on Interfacial Tension Behavior of CO₂–Asphaltenic Crude Oil Systems

The interfacial tension (IFT) of a crude oil/CO₂ system is recognized as the main property affecting the efficiency of CO₂ flooding during an enhanced oil recovery (EOR) process. The addition of a paraffin group hydrocarbon to asphaltenic crude oils as an asphaltene precipitant component is aimed to mimic the asphaltene precipitation process during crude oil production and transportation. Asphaltene precipitation would critically affect the interfacial behavior of crude oil/CO₂ systems. In the first part of this study, the equilibrium densities of oil samples which contain n-heptane at different ratios were measured over varying pressures at 323 K. Then, the equilibrium IFT between CO₂ and the crude oil samples was measured using the axisymmetric drop shape analysis (ADSA) technique. This investigation was followed by measuring the minimum miscibility pressure (MMP) of the oil/CO₂ systems using the vanishing interfacial tension (VIT) technique. The results showed that the density of oil sample increases linearly with pressure. Moreover at a constant pressure and temperature, the density was linearly decreased with n-heptane content of the crude oil sample. Linear correlations between density, n-heptane content, and pressure at the temperature of 323 K for each oil sample were also noticed. The results of IFT tests indicated that asphaltenic crude oil samples have two slope IFT-pressure behavior. It was found that for oil samples with high asphaltene content (9 wt %), the higher the n-heptane content of the oil sample is, the lower is the pressure of the IFT’s slope change, while for low asphaltenic oil sample (0.56 wt %) an increase of n-heptane has little effect on the point of the slope change of the IFT. Consequently, it was found that asphaltenes increase the rate and magnitude of the light component extraction in oil/CO₂ systems. The experimental results showed that the MMP of the oil samples decreased linearly with the n-heptane content of the oil sample. The linear relation between the MMP and n-heptane content revealed the crucial role of the paraffinic group as the controlling miscibility component of the crude oils.