Summary
Resolution of water-and-oil emulsions is critical to the oilfield industry. A wide variety of undesirable emulsions are formed during the production, handling, and processing of crude oil. Although various methods are used, dehydration of crude oils is achieved mostly by gravitational sedimentation, normally at elevated temperatures and with the addition of chemical demulsifiers.
The quantitative evaluation of emulsion stability by a critical-electric-field (CEF) technique was developed to play a significant role in chemical-demulsifier research. It was found that the CEF technique is useful not only in the evaluation of water-in-oil-emulsion stability, but also in studying the mechanisms of stabilization and demulsification.
A method was developed to study the mechanism of emulsion stabilization in terms of flocculation and coalescence behavior of a crude-oil emulsion. The effect of chemical demulsifiers on emulsion stability was evaluated in terms of the method developed in this study. By following this approach, it is possible to determine the relative amount of energy required for both flocculation and coalescence in the presence of a chemical demulsifier.
Introduction
The inevitable creation and subsequent resolution of water-in-oil emulsions during the production and processing of crude oils are of significant importance in the oilfield industry. These emulsions, which typically could be any combination of water-in-oil, oil-in-water, or complex emulsions, are diverse in their nature and stability. The majority of oilfield emulsions are resolved by the application of chemical demulsifiers in special processes under specific conditions. The stability of crude-oil emulsions is influenced by many variables; therefore, and chemical demulsifiers are developed specifically for each application to achieve optimum economic efficiency.
Emulsion stability of water-in-oil emulsions encountered in the oilfield industry can be evaluated with various methods (e.g., determining droplet size and distribution, determining the amount of water resolved as a second phase, analyzing moisture of the oil phase, and more-sophisticated methods such as interfacial rheology). Sullivan et al. (2004) suggested the use of CEF as a method to provide information for stability-correlation development.
Commercial separation of a dispersed aqueous phase from typical crude oil by electrostatic methods is well-known and dates to the early 20th century (Cottrell 1911; Cottrell and Speed 1911). Electrostatic dehydration technology is still being developed and refined to play an important role in challenging oilfield applications (Warren 2002).
The use of CEF, as a method to evaluate water-in-oil-emulsion stability, has been developed recently by Kilpatrick et al. (2001). In their CEF technique, a sample of water-in-oil emulsion is injected between two parallel electrode plates. A direct-current voltage is applied between the two electrodes and is increased in incremental steps, with continuous monitoring of the conductivity or the amount of electrical current through the oil sample. Fig. 1 shows a simple diagram of the CEF technique.
In response to the increasing applied electric field, the water droplets tend to align themselves to form agglomerated columns of droplets, which form a conducting bridge once a critical voltage (or electric field) has been reached. The strength of the electric field at which the sample shows a sharp increase in conductivity (increase in current through sample, between the two electrode plates) is recorded as the CEF. By this method, relative emulsion stability is compared quantitatively in terms of the CEF value and expressed in units of kV cm-1. In contrast to the method of Sjöblom, we have used alternating current with parallel-plate electrodes at the tip of a probe, which was submerged in the hydrocarbon medium.
Comparison of crude-oil emulsions by CEF techniques is well-documented (Sullivan et al. 2004; Aske et al. 2002), but no reference to the use of CEF in chemical-demulsifier development could be found. It is the purpose of this study to develop the CEF technique for application in chemical-demulsifier research.
The Rheology of Light Crude Oil and Water-In-Oil-Emulsion
