Effect of CO2 on the dynamic and equilibrium interfacial tension between crude oil and formation brine for a deepwater Pre-salt field

Abstract

Interfacial tension (IFT) between oil and brine is key to determine the Capillary Number, which governs capillary forces in the porous medium. As a result, changes in IFT affect the displacement efficiency and the residual oil saturation having direct impact on the Recovery Factor. Fluids in Pre-salt fields (offshore Brazil) have particular characteristics, such as high salinity in the brines, elevated content of asphaltenes and resins in the oils and high CO2 content in the produced gas for some fields. The effect of CO2 on the IFT depends on complex electrostatic and acid-base equilibria, and cannot be predicted by a single fluid property. This work presents the experimental study of the effect of CO2 dissolution on the IFT between a Pre-salt oil with a high total base number (TBN) and non-negligible total acid number (TAN) and a high salinity brine using the pendant drop technique. The oil and its asphaltenic fraction were characterized by Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and elemental analysis identifying characteristic functional groups and indicating both acidic and basic polar functional groups. CO2 dissolution resulted in an IFT increase of 56% evidencing that, despite the high TBN of this oil, basic groups have low surface-activity. Moreover, a study of IFT as a function of pH showed that the maximum IFT was found in strongly acidic environments, but constant IFT values were obtained at neutral and basic pH values. Thus, the interfacial activity is governed by asphaltenes and resins with polar groups of varying acid/base behavior. The presence of CO2 also accelerated the dynamic behavior of IFT reducing the equilibrium time by 70%. This is caused by the reduction of oil viscosity, which impacts the relaxation time and enhances the diffusion of surface-active species towards the interface. The results in this work evidence that the CO2 content should be considered when modeling fluid properties for this type of reservoirs, as it affects the oil-brine IFT. In addition, for this oil with high content of surface-active compounds and non-negligible TAN and TBN, CO2 flooding does not contribute to reduce the IFT. Therefore, the efficiency of this Enhanced Oil Recovery technique in this type of field relies on viscosity reduction, miscible displacement and wettability alteration rather than IFT reduction.