Page 179 - Drilling Technology in Nontechnical Language

170 Drilling Technology in Nontechnical Language Second Edition

Mud chemical properties

The chemical characteristics of the mud are mostly determined by
wellbore stability considerations of the formations drilled through in
a particular hole section. In addition, the mud should not damage the
reservoir (reduce permeability), or at least damage done to reservoir
permeability should be capable of being repaired (e.g., by using acid to
remove plugging solids) or bypassed (explosive perforations penetrating
through the damaged zone).
A brief description of different problems and required chemical
characteristics follows.

Reactive shales. Many hole problems are caused by incompatibility
between water and shales (the reverse of diagenesis, discussed in
chapter 1). This may be solved by using oil/water emulsion muds (with
oil as the continuous phase) or 100% oil muds. This isolates water from
the shales and so prevents hydration. Oil muds are becoming increasingly
difficult to use in some areas due to environmental concerns and resulting

government regulation changes. These muds are also expensive.
Water-based muds may use various chemical inhibitors to control
reactive shales, such as potassium chloride (KCl). KCl works by swapping
places with sodium ions in the clay structure. As the potassium ion is
smaller than the sodium ion, this causes the clay structure to shrink rather
than expand.
Other useful chemicals include polymers, as described previously in
this chapter. Clay crystals have electrostatic charges on their faces and
edges. If a polymer molecule also has opposing charges along its length,
the polymer sticks to the clay crystal and prevents water from reaching it.
Another development is the use of soluble silicates in clay stabilization.
These are soluble at high pH (alkaline; above pH 10) but precipitate out of
solution as solids if the pH drops. As tiny amounts enter the pore spaces
between crystals, the pH drops, and silicate precipitates and forms a
barrier to further water penetration. Use of silicates seems to cause the
clay to harden over time. As silicates are cheap, readily available, and
environmentally friendly, their use will no doubt increase into the future.

Salts. A nonsalt-saturated, water-based mud will leach out salt
formations, causing extreme hole enlargement and possible cementing

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