BAKER HUGHES - Drilling Fluids Reference Manual

Oil / Synthetic Fluids
As indicated by Figure 5-13, the high-surface-energy (hydrophilic) mineral solid is preferentially wet
by a high-surface-energy/tension polar liquid, such as water. Even if the surface is initially coated or
wet with a low-surface-energy/tension hydrocarbon oil, water will displace the oil and “water-wet” the
hydrophilic surface. Just as “like dissolves like” and “like solvates like,” “like wets like.” Adsorbing
a surfactant, however, alters the mineral surface with the hydrocarbon tails oriented toward the oil.
Figure 5-13
Oil Wetting
The surfactant “mimics” a hydrocarbon surface and lowers the surface energy of the mineral. Water, a
high-surface energy/tension liquid, will not wet a low-energy surface and will tend to “bead up.” The
mineral surface has been rendered “oil-wet.” Examples are waxing the paint on an automobile and
using a wetting agent to “oil-wet” barite during weight-up of oil-base fluid systems.
Dispersions, like emulsions, once formed, are subject to particle/particle collisions. These collisions
can result in destabilization of the dispersion by flocculation, aggregation, and settling of the solids.
Surfactant films help to stabilize dispersions by preventing close approach between particles.
Temperature increases the number of collisions and decreases the viscosity of the continuous phase.
The latter promotes settling due to Stoke's law and is a particular problem in dispersions.
The most important source of instability occurs in combined emulsions/dispersions. Oil-base fluids
can be considered as three-phase oil/water/ionic-mineral solid systems where oil is the continuous
phase and water and mineral solids are dispersed. The mineral solids are hydrophilic and will
preferentially water-wet. In the absence of surfactants, even if the mineral solid is oil-wet initially,
water will tend to displace oil from the surface and water-wet the solids. As the solids water-wet, they
stick together, forming larger diameter agglomerates which settle out. Altering the surface of the ionic
solid with an adsorbed surfactant layer counteracts this tendency and promotes oil wetting.
General Colloidal Systems
Colloidal systems are defined as emulsions or dispersions in which the diameter of the dispersed phase
is of the order of ≤ 1 micron (10 -6 m). The ratio of area to volume in colloidal systems is large (ratios
of 10 6 are not uncommon) and surface effects (surface tension, adsorption, wetting) predominate the
physical characteristics. Emulsification requires the creation of new surface area which interfacial
tension opposes. While surfactants will minimize the problem, it is never eliminated in practical
systems. Common minerals, such as barite, clays, calcite, quartz, etc., are ionic or polar and, as such,
are hydrophilic and preferentially water-wet. Adsorbed surfactants alter the surface, but the inherent
tendency remains. Dispersed phases are constantly tending to collide with each other leading to
Baker Hughes Drilling Fluids
Reference Manual
5-16 Revised 2006