Page 265 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 241

Drilling Fluids Program [ ]
2.5.10

their molecule. Each end aligns itself to be in either the oil phase or the
water phase. They bridge the oil-water interphase. The emulsifier, cou-
pled with shear, forms the minute emulsion droplets.
Emulsifiers that consist of soaps will have their cationic ends
aligned in the water phase. Monovalent cations such as sodium will
form soaps that formulate straight emulsions. A typical example is
sodium oleate.
However, divalent cations such as calcium can form soaps with two
large organic groups attached that are organophylic. This “imbalance”
between the hydrophylic (calcium) ends and the organophylic ends
will create an invert emulsion.
The type of soaps used in oil muds are commonly formed from tall
oil fatty acids. Tall oil is derived from pine trees. It is an oil synthesized
by the tree and its quality is dependent on the climate and weather
when it is produced. The quality of tall oil can be significant in the
effectiveness of the emulsifier. “Tall” is the Norwegian word for “pine.”
Not all soaps are generated from tall oil fatty acids. An example of
another type of soap used as an oil mud emulsifier would be soap
derived from fish oil fatty acid.
Not all emulsifiers used in oil muds are soaps. Various amine based
surfactants such as polyamines or polyamidoamines are also used.
They also will have hydrophylic and organophylic ends to their mole-
cules. In some cases these may be mixed in with the soaps in their
packaging or may be supplied separately.
Initially, in early oil mud systems, the term “primary emulsifier” was
applied to the soap type emulsifiers and “secondary emulsifier” to other
types. This terminology is now very loose because many invert systems
are based upon non-soap type emulsifiers with no soaps present.
Alkalinity control. Alkalinity control in most invert emulsion
muds is obtained by the addition of common calcium hydroxide
(lime), or in some cases calcium oxide. The use of a calcium base is
particularly relevant when tall oil fatty acid soaps are in use.
Oil-base muds do not suffer from the problems of hydrogen sulfide
or carbon dioxide contamination that can occur in a water-base fluid.
However, such contamination in an oil-base mud will show itself as the
increased addition rate of lime to maintain alkalinity.
Viscosifiers—organophylic clays. Hydrophilic clays (e.g., ben-
tonite) that do not normally disperse in a nonpolar fluid such as an

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