Page 241 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 217

Drilling Fluids Program [ ]
2.5.5

top and intermediate hole sections containing long sand intervals
where there is not much clay present. The formation will not con-
tribute much clay to the mud.
In drilling these kinds of sand formations, there is not a lot of reac-
tivity with the formation. The mud should provide viscosity and filtra-
tion control. Buildup of thick filter cake should be avoided. The com-
monest problems in drilling these formations are a tendency for solids
buildup, substantial losses over the solids control system and, if the
mud weight is not controlled, there is a potential for downhole losses.
Substantial dilution is normally required.
A gel CMC mud should meet the requirements for viscosity and fil-
tration control but at the same time be relatively inexpensive in view
of the high dilution rate required. Normally, a gel CMC mud will con-
tain at least 8 ppb of prehydrated bentonite in order to provide some
viscosity and a filter cake for the CMC to act upon. CMC is used for
filtration control and usually, as viscosity is required, it will be in the
form of high viscosity or extra viscosity CMC. Additional viscosity, if
required, can be obtained from xanthan gum.
The usage of anionic polymers to provide encapsulation is unnec-
essary, but PAC might be used to give viscosity and filtration control
dependent on its price against the high viscosity forms of CMC.
Gypsum polymer mud. The principle behind this type of polymer
mud is to create inhibition by providing an environment with as high
a concentration of calcium ion as possible. See also “Gypsum lignosul-
fonate mud” in Section 2.5.4, “Dispersed Water-Based Muds.”
Gypsum is the source of the calcium ion but, unlike a gypsum lig-
nosulfonate mud, the pH is maintained at a neutral level. This allows
substantial calcium to be soluble in the system (in excess of 2500 ppm
calcium ion). PHPA cannot be used in this mud but PAC can. The sta-
bility of PAC to calcium ion is greatly increased due to the reduced pH.
Lime polymer mud. The principle means of inhibition of this sys-
tem is the presence of soluble calcium and the use of the anionic poly-
mer, PAC. This principle was discussed previously in “Gypsum poly-
mer mud” and Gypsum lignosulfonate mud.”
Lime (calcium hydroxide) will normally come to a base level at a
pH of 12.4 when added to water. However, if a monovalent base such
as sodium hydroxide is added to the calcium hydroxide, it is possible
to control the amount of calcium ion that will go into the solution. For

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