Page 5 - Treating Oilfield Emulsions

Segment 1
Chapter 2

The Theory of Emulsions
Petroleum Extension-The University of Texas at Austin

A n emulsion is a combination of two liquids
fi that do not mix under normal conditions.
Such liquids are said to be immiscible, orincapable
of mixing. In an emulsion, one of the liquids is
spread out, or dispersed, throughout the other in
the form of droplets. These droplets can be of all
sizes, from fairly large to very small. Sometimes
droplets are so small that more than fifty of them
could be placed on the head of a pin. A stable
emulsion is an emulsion that will not break down
without some form of treating. Three conditions
are necessary for the formation of a stable emul­
sion: (1) the liquids must be immiscible; (2) suffi­
cient agitation must occur to disperse one liquid
as droplets in the other; and (3) an emulsifying
agent, or emulsifier, must be present. Figure 4. A photomicrograph of a water-in-oil emul­
sion
Many emulsions, such as in insecticides and
medicines, are prepared for commercial use. They
are made up of two or more liquids that do not however, water is dispersed as droplets in oil
normally mix, plus the emulsifying agent. A com­ (fig. 4). An oil-water emulsion may contain from
mon household emulsion is mayonnaise. Basi­ a trace to 90 percent or more water. An emulsion
cally, it is made of vegetable oil and vinegar with may also be tight (difficult to break) or loose (easy
eggs used as the emulsifying agent. This combi­ to break). Whether an emulsion is tight or loose
nation would not remain mixed if the eggs, or depends on a number of factors, four ofwhich are
some other emulsifying agent, were not present. (1) the properties of the oil and water; (2) the
The oil and vinegar could be mixed by violent amount of agitation, orshear, it undergoes; (3) the
agitation, but they would soon separate after percentage of oil and water found in the emul­
agitation was stopped. Similarly, to form a stable sion; and (4) the types and amount of emulsifiers
emulsion of crude oil and water, an emulsifying present.
agent must be present. Emulsifying agents com­ Occasionally, emulsions produced from some
monly found in petroleum emulsions include fields are reverse emulsions, in which oil is the
asphaltenes, resinous substances, oil-soluble internal phase and is dispersed as droplets in
organic acids, and other finely divided materials water. Very rarely, oil is produced in a dual emul­
that are more soluble, wettable, or dispersable in sion, in which the dispersed phase is droplets of
oil than in water, for example, iron, zinc and oil-in-water emulsion and the external phase is
aluminum sulfates, calcium carbonate, silica, and oil. As stated before, however, most oilfield
iron sulfide. Each of these emulsifiers usually emulsions are normal emulsions in which water
occurs as a film on the surface of the dispersed is dispersed throughout the oil.
droplets. In a water-in-oil (normal) emulsion, two forces
In an emulsion, the liquid that is broken up into are in direct opposition. One force is the film of
droplets is the discontinuous, dispersed, or internal emulsifying agent that surrounds the water drop­
phase. The liquid that surrounds the droplets is lets. This force tends to prevent the droplets from
the continuous, or external, phase. An emulsion of merging to form larger drops, even when the
oil and water may have either oil or water as the droplets collide. The other force is the opposite
dispersed phase, depending on the characteris­ tendency, that of water droplets to join to form
tics of the emulsifying agent. In most cases, larger drops. Larger drops tend to yield to the

7