Page 237 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 213

Drilling Fluids Program [ ]
2.5.5

described as pseudo-plastic or shear thinning. The best known poly-
mer viscosifier is xanthan gum, which will provide substantial viscosi-
ty at low shear but is shear thinning. Other examples are various poly-
acrylamides, guars and semi-synthetic guars, polyanionic, or hydroxy
ethyl celluloses (PAC and HEC).
Xanthan gums are exceedingly long molecules with molecular
weights in many millions. The digestion of cellulose by the bacteria,
xanthomonas campestris, synthesizes them. They are also commonly
known as XC polymers, after the name of the bacteria.
Filtration control additives. These act in a number of ways. They
may impart filtration control by improving the nature of the filter cake
of mud by their deflocculating action. They may increase the viscosity
of the filtrate and make it more difficult for it to pass through the filter
cake. They may also provide colloidal-size particles, which will bridge
off in the filter cake if the polymers are not completely water-soluble.
Examples are carboxy methyl cellulose (CMC), which in their low vis-
cosity forms will provide a deflocculating effect. High viscosity forms
will provide viscosity to the filtrate. Some starches will provide col-
loidal particles. Examples of other filtration polymers are asphalts, lig-
nites, resins, and some polyacrylates
Other uses of polymers are as surfactants, shale stabilizers such as
glycols, emulsifiers, de-emulsifiers, foaming agents, lubricants, and
corrosion inhibitors.
Anionic polymers: PHPA and PAC. Two types of anionic polymers
are available to provide encapsulation. These are partially hydrolyzed
polyacrylamide (PHPA) and polyanionic cellulose (PAC).
PHPA provides far more anionic charge than PAC because its mol-
ecular weight is much greater and its charge density is a lot higher. If
encapsulation is required, PHPA is the best polymer to use if the
chemistry of the drilling mud allows it to be used. It is only stable at
a pH maximum of 10.0 and a filtrate calcium of 300 to 400 ppm.
Sometimes it cannot be used, such as where the mud needs a high pH
and the presence of high-filtrate calcium. An example would be if sub-
stantial and persistent CO contamination was experienced that
2
would require a high pH and the addition of some form of calcium.
However, encapsulation can also be achieved by using PAC, but the
concentration of PAC required will be a lot higher than that of PHPA.
Normally with PHPA, 1 ppb is kept in excess in the system, which

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