STABILITY OF WATER IN ACIDIC CRUDE OIL EMULSIONS

摘要

During the oil production process, water-in-crude oil emulsions can be stabilized by the interfacial activity of some natural acidic fractions : a pH-increase can cause the formation of surface active naphthenates. For a given crude oil, it is often difficult to forecast the potential problems that may occur in production due to surface active species. A great deal of work has been devoted to the analytical investigation of these species. Alternatively, we present in this paper a more phenomenological approach based on the physico-chemical study of the phase behavior of the water-crude oil system as a function of pH, sodium and calcium concentration. Emulsion stability is first evaluated for various crudes using the classical bottle-test method. Surface tension measurements are then performed on decanted water, allowing a characterization of the surface active species (especially their surface activity response to pH changes related to their partitioning between oil, water and interface). The results are used to interpret the observed emulsion stability pattern and the conclusions are confronted with analytical data, namely the molecular weight distribution of the naphthenic acids. Typically, for some crudes, a progressive emulsion stabilization, as pH increases, can be observed, suggesting a continuum in the distribution of the surface active species, whereas, for other crudes, abrupt transitions, from unstable to stable, or stable to unstable emulsions, occur at critical pH revealing the coexistence of surface active and non surface-active naphthenates. As a function of calcium and sodium concentration, and depending on pH and crude oil nature, emulsion stabilization or destabilization is also observed, interpreted in terms of naphthenate partitioning and association with metallic cations. From a practical point of view, this approach provides guidelines to design properly the production operations in order to prevent emulsion formation. For example, it may be recommended to achieve phase separation under pressure to avoid a pH increase or to use an appropriate naphthenate inhibition treatment.