Functional Molecules and the Stability of Water-in-Crude Oil Emulsions

In the production and transportation of crude oil, undesirable water-in-oil emulsions may form. Undesirable water-in-oil emulsions may also form in hydraulic fracking when acid is added to the injection water. We investigate the role of acid–base interactions in the stability of water-in-oil emulsions. Asphaltenes and resins, referred to as functional molecules, constitute the polar fractions of crude oil. They contain heteroatoms such as N, O, and S, which lead to acidic and basic functionalities in petroleum fluids and contribute to the stability of water-in-oil emulsions. We have quantified the acidity and the basicity of nine petroleum fluids (light, medium, heavy, and extra-heavy crude oils) by measuring their total acid number (TAN), total base number (TBN), and hydroxyl number (HN). Except in extra heavy oils where the TAN and TBN are close, all the other oils are basic (i.e., TBN > TAN). We observe that all the light crude oils form stable emulsions with deionized (DI) water, while medium, heavy, and extra-heavy oils do not. Addition of a strong acid (HCl, 4–37 wt %) to the aqueous phase enhances emulsion stability in light oils, induces the formation of water-in-oil emulsions in medium oils, and does not lead to emulsions in heavy and extra-heavy oils. Addition of a strong base (NaOH, 4–20 wt %) to the aqueous phase destabilizes emulsions in light oils, but leads to emulsions in a medium-density oil. There is no effect on heavy and extra heavy oils. The effect of salts on the stability of water-in-oil emulsions is also studied. At the salt concentrations studied (4–30 wt %), we observe that salts destabilize emulsion; multivalent salt solutions are more potent at weakening emulsions, compared with mono- and divalent salt solutions. These results are attributed to the preferential hydration of ions by water molecules, as compared to interactions of asphaltenes with water.