Detection and Impact of Carboxylic Acids at the Crude Oil–Water Interface

The impact of surface active indigenous components on interfacial tension (IFT) of crude oil–water systems is an important parameter in many aspects of crude oil production such as emulsion stability, reservoir wettability, and capillary number calculations. These components may affect productivity across the reservoir due to variations in concentrations. In most cases simulation of IFT is not taking interfacial activity into account and is purely based on oil bulk properties. In this paper we examine two crude oils and their subfractions such as maltenes, deacidified crude, and natural acidic components. Films were prepared at the toluene–water interface with crude oil and its various fractions and studied for interfacial activity and chemical compositions. The chemical analysis of the interfacial active material indicated that carboxylic acids are preferentially adsorbed or concentrated at the oil/water interface. Infrared spectroscopic analysis of the interfacial films clearly demonstrates that carboxylic acids species (e.g., fatty acids, resins, or asphaltenes with a −COOH functionality) are concentrated at the interface. The GC-MS analysis of the interfacial film revealed the presence of a homologous series of linear chain carboxylic acids ranging from C₁₀–C₂₅₊. 2D GC-MS analysis showed that heteroacids are also present. Acid free crude was prepared and back mixed with the original crude oil in different proportions confirming the role of the acids in decreasing IFT. The removal of these relatively small amounts of acids leads to a decrease in IFT between 1.3 and 2.2 mN/m. The results also indicate that acids can be preferentially removed by ion exchange resins without affecting the overall composition of the oil as is shown by back-mixing deacidified oil into the original oil.