Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds

Abstract

Summary Until now, drilling-fluid specialists measured trends in the electrical stability (ES) test to decide the appropriate emulsifier treatment for invert-emulsion oil-based muds without knowing what the ES test was measuring. This paper describes what happens in a mud during an ES test, how mud composition affects trends in the ES measurements, and how observed trends can be used to deduce mud-emulsion stability and oil wettability. During an ES test, electrical breakdown is induced in the mud between the electrodes. This process was observed, with an optical microscope, to involve formation of a conductive bridge composed of aqueous fluid and particulate matter. Experiments showed that water forms the conductive pathway in the bridge, whereas the solids appear to be involved only as carriers of the water. Eo, the voltage at which the conductive bridge is formed and the current rises abruptly, was found to be a function of the viscosity and the types and concentrations of solids, aqueous fluid, and emulsifiers (surfactants). These trends are consistent with a theoretical expression for breakdown of paniculate-contaminated dielectric fluids, which suggests that the Eo of a mud is an absolute measure of its emulsion stability but only a relative measure of its oil-wetting tendency. Trends in Eo are also consistent in most cases with other field indicators of emulsion stability, such as high-temperature, high-pressure (HTHP) fluid loss. Anomalies in E0 trends are explained in terms of a physicochemical model for electrical breakdown.