Page 168 - Drilling Technology in Nontechnical Language

Chapter 7 – DRILLING FLUIDS 159

Montmorillonite is added to the mud to give it certain useful properties.
Commercially supplied montomorillonite is known as bentonite.
Fully dispersed, a clay such as montmorillonite will have its clay platelets
completely separated and held apart by negative charges on the faces of the
platelets. The theoretical surface area of fully dispersed montmorillonite is
around 800 m2/g. It is this huge surface area of dispersed montmorillonite
that causes dispersed mud with added bentonite to become viscous.
A mud designed so that clays are dispersed (either added like bentonite
or from drilled formations) is called a dispersed mud.

Nondispersed muds

A mud in which the hydration and dispersion of a drilled clay is
minimized is called nondispersed. There are a number of ways to achieve
this. The most common is to limit the amount of water that reacts with
the clay by encapsulating the clay with polymer as quickly as possible, to
prevent further access of water to the clay. (A polymer is a repeating chain
of units [called monomers] that are chemically joined together to form a
long chain. Some polymers can be millions of units long.)

The electrical charges on the surface of the clay particle attract sites on
the polymer chain that have an opposite electrical charge. The result is that
the long chain of the polymer can wrap itself around the clay. Very long
chain polymers can hold several clay platelets together (fig. 7–3). Such mud

systems are described as encapsulating polymer muds.

Originally the polymers used as drilling fluid additives were naturally
occurring starches that were easily extracted, such as corn starch, which

was first documented in use in 1937. Other natural polymers were also
tried and entered common use. Now, synthetic polymers are often tailored

to specific drilling situations.
Polymers can perform several different functions, such as the following:

1. Increase the viscosity of the fluid. Viscosity is the degree of

resistance to flow of a fluid. A highly viscous fluid will need

more pressure to pump it through a pipe than a fluid with
lower viscosity.

_Devereux_Book.indb 159 1/16/12 2:09 PM