When thinking about emulsion stability, it may be helpful to realize that in a pure oil and pure water mixture, without an emulsifying agent, no amount of agitation will create an emulsion. If the pure oil and water are mixed and placed in a container, they quickly separate. The natural state is for the immiscible liquids to establish the least contact or smallest surface area. The water dispersed in the oil forms spherical drops. Smaller drops will coalesce into larger drops and this will create a smaller interface area for a given volume. If no emulsifier is present, the droplets will eventually settle to the bottom causing the smallest interface area. This type of mixture is a true “dispersion.”
An emulsifying agent has a surface active behavior. Some element in the emulsifier has a preference for the oil, and other elements are more attracted to the water. An emulsifier tends to be insoluble in one of the liquid phases. It thus concentrates at the interface. There are several ways an emulsifier changes a dispersion into an emulsion. The action of the emulsifier can be visualized as one or more of the following:
1. It decreases the interfacial tension of the water droplet, thus causing smaller droplets to form. The smaller droplets take longer to coalesce into larger droplets, which can settle quickly.
2. If forms a viscous coating on the droplets that keeps them from coalescing into larger droplets when they collide. Since coalescence is prevented, it takes longer for the small droplets created by agitation to settle out.
3. The emulsifiers may be polar molecules, which align themselves in such a manner as to cause an electrical charge on the surface of the droplets. Since like electrical charges repel, two droplets must collide with sufficient force to overcome this repulsion before coalescence can occur.
Naturally-occurring surface active materials normally found in crude oil serve as emulsifiers. Paraffins, resins, organic acids, metallic salts, colloidal silts and clay, and asphaltenes (a general term for material with chemical compositions containing sulfur, nitrogen, and oxygen) are common emulsifiers in oil fields. Workover fluids and drilling mud are also sources of emulsifying agents. The type and amount of emulsifying agent has an immediate effect on the emulsion’s stability. It has been shown that the temperature history ofthe emulsion is also important as it effects the formation of paraffins and asphaltenes. The speed of migration of the emulsifying agent to the oil/water interface and the behavior in terms of the strength of the interface bond are important factors. An emulsion treated soon after agitation or the creation of paraffins and asphaltenes can be less stable and easier to process if the migration of the emulsifler is incomplete. An aged emulsion may become more difficult to treat. Normally, the lower the crude viscosity and lighter the crude the more rapid the aging process. Therefore, early treatment may be a lesser factor in treating low-viscosity, high-API-gravity crudes.