For an emulsion to exist there must be two mutually immiscible liquids, an emulsifying agent, and sufficient agitation to disperse the discontinuous phase into the continuous phase. In oil production, oil and water are the two mutually immiscible liquids. An emulsifying agent in the form of small solid particles, paraffins, asphaltenes, etc., is almost always present in the formation fluids, and sufficient agitation always occurs as fluid makes its way into the well bore, up the tubing, and through the surface choke.
The difficulty of separating the emulsified water from the oil depends on the “stability” of the emulsion. The stability of an emulsion is dependent on several factors:
1. The difference in density between the water and oil phases.
2. The size of dispersed water particles.
4. Interfacial tension.
5. The presence and concentration of emulsifying agents.
The difference in density is one of the factors that determines the rate at which water droplets drop through the continuous oil phase. The greater the difference in density, the more quickly water droplets will settle from the oil phase. The water particle size also affects the rate at which water particles move through the oil phase. The larger the particle, the faster it will settle out of the oil phase. The water particle size in an emulsion is dependent upon the degree of agitation that the emulsion is subject to before treating. Flow through pumps, chokes, valves, and other surface equipment will decrease water particle sizes.
Viscosity plays two primary roles. First, as viscosity increases, more agitation is required to shear water particles down to a smaller average size in the oil phase. Therefore, the size of water particles that must be removed to meet water cut specifications for a given treating system increases as viscosity increases. Second, as viscosity increases, the rate at which water particles move through the oil phase decreases, resulting in less coalescence and increased difficulty in treating.
When no emulsifier is present, the interfacial tension between oil and water is high. When interfacial tension is high, water particles coalesce easily upon contact. When emulsifying agents are present, however, they decrease the interfacial tension and obstruct the coalescence of water particles.
The above factors determine the “stability” of emulsions. Some stable emulsions may take weeks or months to separate if left alone in a tank with no treating. Other unstable emulsions may separate into relatively clean oil and water phases in just a matter of minutes.
Normal oil field emulsions consist of an oil continuous or external phase, and a water dispersed or internal phase. In some isolated cases, where there are high water cuts, it is possible to form reverse emulsion with water as the continuous phase and oil droplets the internal phase. Complex emulsions have been reported in low gravity, viscous crudes. These mixed emulsions contain a water external phase and have an internal water phase in the dispersed oil. The vast majority of oil treating systems deal with normal emulsions and that is what is discussed in this chapter.
Figure 6-1 shows a normal emulsion. The small water droplets exist within the oil continuous phase. Figure 6-2 shows a close up of a “skin” of emulsifying agent surrounding a water drop, and Figure 6-3 shows two drops touching, but being prevented from coalescing due to the film of emulsifying agent around each drop.