The measurement of static physico-chemical properties of oil-water interfaces provides a useful method of indicating where problem emulsion stability areas arise for water-in-crude oil emulsions and the effectiveness of chemical demulsifiers in resolving these problems. The paper includes a discussion of interfacial tension, interfacial shear viscosity and interfacial film compressibility, particularly as applied to complex crude oil systems, and deals with the modification of these properties on the addition of a chemical demulsifier.
Stable water-in-crude oil emulsions occur at many stages during the production and processing of much of the world's oil. The breaking of these emulsions, frequently by the addition of a chemical demulsifier, is an essential step. Factors that affect the stability of such emulsions, and the role played by the demulsifier in destabilizing them, are of paramount importance.
Little experimental information exists for complex crude oil systems. It has been suggested(1) that the structural mechanical properties of the natural crude oil emulsifiers in the interfacial layer surrounding the drop are important. This layer provides a resistance to coalescence in the final stage of emulsion breaking. The chemical demulsifier is thought to destabilize the emulsion by causing a disintegration of this layer at the interface. This work supports that viewpoint.
Static physico-chemical properties of crude oil/water interfaces have been evaluated over a variety of conditions and correlations with emulsion stability, particularly those encountered during oil production and processes, have been sought.
Thus, the relevance of oil/water interfacial tension to emulsion formation and stability has been investigated. The influence of interfacial shear viscosity (and viscoelasticity) on the coagulation stage of emulsion resolution has been examined. Also, the resistance of interfacial films to expansion/compression has been studied and the effect on coalescence elucidated. Finally, modification of these physico Mchemical interfacial properties on the addition of a chemical demulsifier has been investigated, with a view to determining the additive's ability to destabilize such emulsions.
Oil/water interfacial tension changes were measured by the method outlined in reference (2). Previous work(3) has shown that interfacial tension lowering alone is not an emulsion stabilizing factor. Table 1 shows the influence of various dissolved salts on γ o/w of Ninian crude oil/ water interfaces. These systems have widely different emulsion stabilities at 25 °C, yet the interfacial tension values are hardly different.
As well as adsorption of surfactants at the interface, mass transport of water-soluble crude oil components across the oil/water interface will also influence the measured γo/w lowering. Figure 1 shows the effect obtained by pre-equilibrating the crude oil/water phases. This results in a more rapid lowering of γ o/w with time, but with the same final value of interfacial tension being achieved. Where the illustrated effect of pre-equilibration is not observed, the anomalous behaviour can usually be attributed to bulk aqueous-phase pH drifts (in non-equilibrated systems) due to transport of acidic material (predominantly across the mid pH range). Any field-added chemical demulsifier in the crude oil will also cause anomalous effects.