Page 229 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 205

2.5.2
Drilling Fluids Program [ ]

clays can become unstable much faster than they would otherwise.
Starting and stopping the mud pumps too fast can exert destabilizing
forces. Good drilling practices help minimize or avoid such problems.
This is covered in more detail in Section 3.3.7.

2.5.2. Dispersion and Flocculation of Clays in Water

The ability of montmorillonite to expand due to hydration is
exploited in muds. When expanded, it provides a large ionic charge
and in a polar medium such as water, ionic forces will cause the clay
platelets to repel each other. The resultant slurry will have developed
viscosity. As it is commonly known, bentonite (montmorillonite) is
widely used as a viscosifier in certain fluids. The platelet nature of the
clay also produces filtration control.
In some cases, clay platelet dispersion is disturbed by an ionic
charge imbalance at certain parts of the clay structure. The clay
platelets will not evenly separate from each other but they may have
some random and loose attachment. The edge of one clay platelet may
attach itself to the edge or the middle of another clay platelet. A clay in
such a condition is said to be flocculated. The presence of divalent
cations, metal ions such as calcium or magnesium, or anions such as
sulfate, chloride, or carbonate/bicarbonate can flocculate a clay. These
ions can come from the formation drilled, the makeup water (especial-
ly if it is hard water or seawater), or the addition of a base such as cal-
cium hydroxide or magnesium oxide.
If a clay has not been dispersed in water but is put in such a water
environment where it would have flocculated if it were dispersed, the
clay remains in a nonexpanded or aggregated state. This is one method
of stabilizing formation clays by providing such an environment in the
mud. This would not be conducive to dispersing bentonite in order to
obtain viscosity. Consequently, such an environment is avoided when
mixing bentonite by treating out ions that would cause flocculation,
e.g., calcium.
Various chemicals can negate the imbalance of ionic charges that
cause flocculation. These are known as deflocculants. The more pow-
erful deflocculants will put the clay in such a condition that it will fully
disperse. Hence these deflocculants are sometimes described as dis-
persing agents.

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