Page 228 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 204

[ ] Well Programming
2.5.1

The daily mud report should include all those properties that were
defined in the mud program. This allows you to ensure that the proper-
ties are maintained as specified. You also need to ensure that perfor-
mance indicators are reported, such as low gravity solids levels, sand
content, etc. The following topics look at some basic principles and then
go on to examine various mud systems from a well planning viewpoint.

2.5.1. Reaction of Clays to Water: General Principles

When a well penetrates a rock, the equilibrium that has developed
over geological time can be immediately disturbed. With clays, the
dominant effects of diagenesis are compaction of clay structure and
dehydration. Some clays will hydrate and expand when they are pene-
trated and supplied with water (from the drilling mud). A process that
has taken millions of years can, in some cases, be reversed in less than
an hour.
The essence of drilling clay formations is to provide a drilling mud
that will inhibit the tendency of the clays to reverse the process of dia-
genesis. The potential reactivity of a clay formation will depend on the
types of clays present and the physical environment. Some clays are
more likely to hydrate and expand than others.
One highly reactive clay mineral in the presence of supplied water
is montmorillonite. The montmorillonite crystal structure comprises
large, flat sheets of alternating octahedral and tetrahedral layers. For
this reason, it is described as a “mixed layer” clay. Other types of mixed
layer clay also occur.
Water that is allowed to enter the crystal structure can cause the
crystal lattice to expand because of changes in electrostatic forces. This
expansion is described as dispersion. The more polar in nature water
can become, the more it will cause clay dispersion. The polar nature of
water can be increased by the addition of alkalis, such as the monova-
lent bases, sodium hydroxide, or potassium hydroxide. The theoretical
surface area of fully dispersed montmorillonite is around 800 m /g.
2
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.
Mechanical forces also affect clay formation stability. For instance,
because the well is exposed to surge/swab pressures while tripping, the

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