Page 266 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 242

[ ] Well Programming
2.5.10

oil, can become oleophylic and dispersible (in oil) by treatment with
amine salts. In this case, the cations of the bentonite clay are replaced
by cationic groups from the amines. The amines are also adsorbed
onto the clay surfaces. When this treated clay (organophylic clay) is
present in an oil, it will disperse. The clay surfaces will be displaced
apart because the cationic groups repel each other and because the
amine salt will extend itself in the oil medium. This process is assist-
ed by shear.
Dispersion of the clay can be further increased and the gel can be
stabilized if an organic group such as an aromatic ring can enter
between the clay surfaces. It is often the case that a stronger gel (high-
er yield point) per amount of organo-clay added is obtained in diesel
rather than in a low-aromatic mineral oil mud. To obtain a sufficient
yield point when using base oils that are low in aromatics, it is neces-
sary to either add additional material, provide increased shear, add an
effective oil wetting agent, or use a clay with an amine salt treatment
that is optimum for the base oil in use.
Organophylic clays vary in a number of ways. When used in
drilling fluids, there are three types of clays that are treated to become
oil dispersible. The most common is bentonite and invariably, it will be
a sodium bentonite (Wyoming). Bentone 38 is an example of such a
type of clay but there are many types. Another clay that is converted to
an organo-clay is hectorite. The hectorite clay is a layer clay, similar to
bentonite but with a much larger clay structure. An example of such an
organo-clay is Bentone 64. This type of clay has the advantage of pro-
ducing good stable rheology at very high fluid temperatures.
Attapulgite (seawater gel) can also be treated to make it organophylic.
This type of clay can improve the suspension properties of the mud but
will not assist in filtration control.
Another way that organo-clays will vary is the type of quaternary
amine salt that is adducted to the clay. The performance of these
“quats” will vary depending on the type of base oil in use. Some quats
are more effective than others, and change in quat on a clay may pro-
vide as much as double the yield point.
The process by which the quat is added to a clay can also make a
difference to the performance of the organo-clay. There are two
processes involved. One is a dry process and the other is a wet process
with subsequent drying. Some companies will make proprietary vari-

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