Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds
- F.B. Growcock (Amoco Production Co.) | C.F. Ellis (Chevron U.S.A. Inc.) | D.D. Schmidt (Amoco Production Co.) | J.J. Azar (U. of Tulsa)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1994
- Document Type
- Journal Paper
- 39 - 46
- 1994. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 2.2.3 Fluid Loss Control, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 2.5.2 Fracturing Materials (Fluids, Proppant), 1.6 Drilling Operations, 2.7.1 Completion Fluids, 1.11 Drilling Fluids and Materials
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It is shown that the electrical breakdown induced during electrical stability (ES) tests of oil-based muds involves formation of a conductive bridge in the mud between the electrodes. Observations with an optical microscope demonstrate that this bridge is composed of aqueous fluid and particulate solids. Water is the key component, whereas particulate solids. Water is the key component, whereas the solids appear to be involved only as carriers of the water. Hematite is an exception since it, like water, can serve as a conductor. The absence of visible chemical changes during ES tests supports the contention that the rapid rise in the current is a conduction phenomenon, rather than true dielectric breakdown of the base oil.
The ES voltage, the voltage at which 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 qualitatively consistent with a theoretical expression for breakdown of particulate-contaminated dielectric fluids, which suggests particulate-contaminated dielectric fluids, which suggests that the ES of a mud is a measure of its emulsion stability but not necessarily its oil wetness or oil-wetting tendency. Trends in ES are also consistent in most cases with other field indicators of emulsion stability, such as high-temperature highpressure fluid loss. Anomalies in ES trends are explained in terms of a physicochemical model for electrical breakdown.
A field test based on the effect of incremental additions of barite was developed that serves to indicate the need for additional emulsifier, regardless of the type of weighting agent in the mud. A similar test is proposed that utilizes incremental additions of emulsifier. When coupled with the barite test, the emulsifier test can help to define the oil-wetting tendency of the mud, as well as its emulsion stability.
Invert oil-based muds are water-in-oil (W/0) emulsions which typically contain an organophilic clay (OPC) and a weighting material, e.g., barite or hematite. The water phase is usually a solution of a salt (CaC12 is the most common) whose concentration is adjusted to match the water activity of the formation; this minimizes transfer of water to or from the water-sensitive zones and maintains a stable wellbore." The W/O emulsion itself is usually stabilized with a "primary emulsifier" (often a fatty acid salt), while the weighting material, along with drill solids which the mud acquires in use, is made oil-wet and dispersed in the mud with a "secondary emulsifier" (typically a strong wetting agent, such as a polyamide 2).
Electrical stability (ES) of an oil-based mud is considered a measure of its emulsion stability. In the laboratory, a mud with a high degree of emulsion stability is generally smooth, shiny and does not adhere to the stirring spindle of a mixer. By contrast, a mud with a low degree of emulsion stability is dull, grainy and shows a marked tendency to adhere to the spindle. The oil wetness or oilwetting tendency of an invert emulsion mud is defined here as the ability of the mud to incorporate foreign materials into the external, or oil, phase. A mud with high emulsion stability is phase. A mud with high emulsion stability is oilwet, by definition, but may not necessarily be oilwetting. In his patent application for the first ES meter, Crittendon hypothesized that ES is related to the stability of W/O emulsions and that higher ES voltages correspond to more stable, or "tighter", emulsions. This result was extrapolated to invert emulsion drilling fluids, which are more complex by virtue of their solids content. Thus, a mud with a high ES voltage was considered to be stable.
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