A Model for Interfacial Activity of Acidic Crude Oil/Caustic Systems for Alkaline Flooding

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Abstract
The interaction of the alkali in floodwater and the acids in reservoir crude results in the in-situ formation of surfactants, which are responsible for the lowering of interfacial tension (IFT) in caustic flooding. The extent to which IFT is lowered depends on the specific properties of the crude oil and the injection water. Therefore, it is important to establish the relationship between IFT and the essential chemical properties of the acidic oil and the floodwater. This paper presents such a relationship. In this discussion, the adsorption and the desorption of the active species at the interface are modeled as ionic processes using the Gouy-Chapman theory of the diffuse double layer. The interfacial potentials calculated using this model show a fair agreement with the experimentally measured trend of elcctrophoretic mobility. Also, the model rationalizes the experimentally observed effects of alkali concentration, salinity, and the oleic- to aqueous-phase ratio on IFT. We conclude that the acid number of the crude oil may not correlate directly with interfacial activity. Even in cases of low-acid-number crudes, significant interfacial activity could be obtained because of highly hydrophobic active species in the crude.

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