BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
The most common types of emulsions are oil in water and water in oil. An example of oil-in-water
emulsion is milk. Examples of water in oil emulsions include grease, margarine, and oil-base fluids.
The first and most common emulsions studied were oil in water. Water-in-oil emulsions were studied
later and sometimes are called invert emulsions.
Figure 5-8
Emulsions
Dispersing one liquid as small droplets in another liquid increases the surface area between the two
liquids. The surface energy/tension that exists at the interface between two liquids is called the
interfacial energy or interfacial tension. This interfacial energy/tension tends to minimize the surface
area and work must be performed (energy expended) to increase interfacial surface area and overcome
these forces. A mixer can form a mechanical emulsion between two pure liquids, such as water and
hydrocarbon, but the emulsion breaks and the two phases separate when the agitation ceases. This deemulsification
is due to interfacial energy/tension minimizing interfacial surface area. In order to form
a stable emulsion, a third component is necessary.
Surfactants
Surface active compounds are compounds that orient at interfaces or surfaces and lower surface or
interfacial energy/tension. Surface active compounds are often called surfactants. The most common
example is soap, as indicated in Figure 5-9.
Figure 5-9
Surfactant Structure (Soap)
This compound has both a hydrophilic “head,” which is polar and ionized, and an organophilic “tail,”
which is nonpolar (see Figure5-10). The structure is called amphiphilic to indicate the dual character
of these compounds. Surfactants are usually elongated and only partially soluble in either oil or water.
The degree to which surfactants are soluble in oil or water can be denoted by the hydrophiliclipophilic
(organophilic) balance (HLB). Surfactants which are more water soluble (hydrophilic) tend
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 5-13