Page 235 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 211

Drilling Fluids Program [ ]
2.5.5

ionic polymers. A nondispersed system should have sufficient anionic
polymer present for encapsulation of clays and other low gravity solids
and their removal by the solids control system.
If the amount of clay in the mud, the cation exchange capacity
(MBT), or the amount of low gravity solids are not controlled, the rhe-
ology of the fluid can become uncontrollable in the nondispersed state.
The polymer makeup should be able to provide encapsulation and
good low shear rate rheology but, most importantly, there should be
sufficient encapsulating polymer in the system at all times. If there is
not, the MBT will go out of control first and this will be followed by a
lack of control of the rheology. It should be remembered that the
encapsulating polymer concentration will deplete all the time in the
system as it is removed with the clays over the shakers.
In some cases, the use of nondispersed polymer muds is attempt-
ed and fails. The muds fail because the low gravity solids are not kept
under control. This may be due to poor solids control systems. They
may fail because insufficient encapsulating polymer is maintained in
the system or because filtrate chemistry is not correctly controlled.
Failure of a nondispersed polymer system will invariably result in an
inability to control viscosity. The resultant action is to disperse the sys-
tem. However, when the system is dispersed it usually, and at least tem-
porarily, acts as a dispersed mud without any substantial inhibition. In
planning a mud program, it is better to evaluate the ability to maintain
a nondispersed polymer system. If the conclusion is that this cannot be
successfully achieved, it is better to consider the application of a gyp
lignosulfonate mud.
Types of polymer. A polymer consists of molecules (monomers)
that have been processed to be chemically joined (polymerized) to
form a chemical chain (polymer). The monomers are usually organic
(based on carbon chemistry), but they may also be based on the chem-
istry of other atoms, e.g., silicon or phosphorous.
Polymers may be specified in a number of ways. Their molecular
weight will give an idea of how long their polymer chains are. Their
degree of polymerization (or number of repeated units) will also
demonstrate this. Their degree of substitution (of charged side chains)
will give an idea of the net charge on the polymer and their charge den-
sity (charge per mole) will also show this.

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