Effect of CO2 Injection on the Interfacial Tension for a Brazilian Pre-Salt Field

Interfacial tension (IFT) between oil and brine plays a key role in determining the capillary forces in the porous medium. When studying Enhanced Oil Recovery (EOR) methods, it is of great relevance to characterize the IFT. In the case of Pre-Salt reservoirs, CO2 and water alternated with gas injections are being considered as EOR techniques. For paraffinic oils, such as alkanes, the presence of CO2 decreases the IFT between oil and brine. However, for Pre-Salt oils with high concentrations of asphaltenes and resins, the effect of CO2 injection on the oil-brine IFT has not been reported. This work uses the drop shape analysis technique to measure the IFT between a Pre-Salt crude oil and synthetic brine with the composition of formation water in the presence and absence of CO2. The results were compared to those obtained for synthetic oil consisting of alkane and aromatic molecules. For the crude oil, CO2 dissolution, which decreases brine pH, increased the IFT between oil and brine. Oil characterization retrieved high concentration of asphaltenes and resins and considerable acid and basic numbers. In addition, infrared spectroscopy and nuclear magnetic resonance of the asphaltene fractions of the crude oil reported acid functional groups in these polar compounds. Therefore, the surface activity of the polar compounds in the oil may be reduced at lower pH. On the other hand, for the synthetic oil, CO2 decreased the IFT as previously reported for alkane molecules. Therefore, this work shows the difference in the effect of CO2 on IFT, which depends on the composition of the oil and aqueous phases. Furthermore, the acid/base characterization of the polar compounds is relevant to understand the effect of CO2 dissolution on the resulting IFT.