BAKER HUGHES - Drilling Fluids Reference Manual

Oil / Synthetic Fluids
to form oil in water emulsions and have high HLB values. For example, Drilling Mud Emulsifier
(DME) has an HLB of 7.7. Surfactants which are more oil soluble (organophilic) and form water in
oil emulsions, have low HLB values. In addition to the HLB characteristic is the type salt or valence
of the hydrophilic portion of the surfactant. Monovalent cations promote oil in water emulsions and
Divalent cations promote water in oil emulsions.
Figure 5-10
Orientation of Surfactants at an Interface
Surfactants orient at interfaces and lower surface energy/tension. This reduces the forces necessary to
form new interfacial area and reduces the interfacial area after the emulsion is formed. Surfactants
oriented at the interface also form a chemical skin around the emulsified, dispersed phase and
mechanically stabilize the interface, thereby helping to prevent droplets from coalescing and breaking
when colliding. Small insoluble particles will also do this by forming a mechanical emulsion.
Examples of mechanical emulsifiers are lignite and bentonite.
Temperature increases the frequency of droplet collisions and decreases the stability of emulsions.
Imposing an electrical field across an emulsion will also tend to break the emulsion, which is the basis
of the emulsion stability test for oil-base fluids. Increasing the viscosity of the external phase will
decrease the frequency of collisions and stabilize the emulsion.
Dispersions
Powdered solid suspended in a liquid are called a dispersed system, or simply a dispersion. The
internal or dispersed phase is the solid and the continuous phase is the liquid (see Figure 5-11).
Figure 5-11
Dispersion of a Mineral Powder
Baker Hughes Drilling Fluids
Reference Manual
5-14 Revised 2006