Estimating reservoir fluid interfacial tension: an insight into the role of polar species of crude oil

Interfacial tension (IFT) is important in developing oilfield exploration and production strategies, enhanced oil recovery, and oil-water separation. Tests using reservoir simulators such as Eclipse show that the selected IFT has significant impact on prediction of water coning and production. In petroleum engineering, IFT is often assessed through correlations based on bulk properties excluding the impact of surface-active components. Crude oil contains many different types of surface-active components among which naphthenic acids and asphaltene-like molecules play an important role in IFT between oil and water. It is desirable to be able to link the IFT to the concentration of the natural surfactants to predict variations across a petroleum reservoir. To develop this relation, the equilibrium IFT was investigated in various systems of varying acid content. As expected, naphthenic acids reduce IFT of oil and water significantly. The IFT reduction is however dependent on the structure and composition of naphthenic acids as well as the nature of the solvent. To include the natural surfactancy in the model a Gibbs-Langmuir approach is used to describe the impact of naphthenic acids content in crude oil on IFT. This is compared to two models currently used to estimate IFT in the petroleum industry. The current models are not equipped to capture the role of natural surfactants and hence results may lead to erroneous predictions on crude oil production. We therefore propose an approach where bulk properties that estimate the bulk IFT is used in combination of a term based on the Gibbs-Langmuir model to capture the perturbation by the naphthenic acids to yield more accurate estimates of IFT. This require fitting to at least two experimental points to yield accurate results.