Influence of pH and water content on the type and stability of acidic crude oil emulsions

Dependent upon the conditions of pH and water content, an acidic crude oil may form different type of emulsions with different stability. These oils generally contain large amounts of naphthenic acids, RCOOH, which result from crude oil biodegradation. They also contain heavier compounds such as asphaltenes and resins. All of these amphiphiles may contribute to the formation and stability of emulsions. In this work, an acidic crude oil (total acid number = 1.25) was distilled in three fractions to separate naphthenic acids from resins and asphaltenes. The influence of pH and water content on the type and stability of emulsions prepared with the crude oil and its fractions was investigated. The role of the light, intermediate, and heavy amphiphiles present in the crude oil on its emulsifying properties has been discussed by comparing the emulsion type and stability diagrams obtained for the different oil phases. It has been found that the type of emulsion is governed by the acidic amphiphiles contained in the intermediate fraction. The stability of oil-in-water emulsions is ensured by electrostatic repulsion between the naphthenates, RCOO-, present at the interface, whereas that of water-in-oil emulsions is due to the amphiphiles contained in the heavy fraction, i.e., asphaltenes and resins.