A mud is said to be contaminated when a foreign material enters the mud system and causes
undesirable
changes in mud properties, such as density,
viscosity, and filtration. Generally,
water-based mud systems are the most susceptible to contamination.
Mud contamination can
result from overtreatment of the mud system with additives or from material entering the mud
during drilling.
The most common contaminants to water-based mud systems are solids (added, drilled, active,
inert); gypsum/anhydrite (Ca ++ ); cement/lime (Ca++ ); makeup water (Ca++ , Mg++ ); soluble
bicarbonates and carbonates (HCO
3
−
, CO
3
—
); soluble sulfides (HS
−
, S
—
); and salt/salt water flow
(Na+ , Cl
−
).
Solids Contamination. Solids are materials that are added to make up a mud system (bentonite,
barite) and materials that are drilled (active and inert). Excess solids
of any type are the most
undesirable contaminant to drilling fluids. They affect all mud properties. It has been shown that
fine solids, micron and submicron sized, are the most detrimental
to the overall drilling
efficiency and must be removed if they are not a necessary part of the mud makeup. The
removal of drilled solids is achieved through the use of mechanical separating equipment
(shakers, desanders, desilters, and centrifuges). Shakers remove solids
in the size of cuttings
(approximately 140μ or larger). Desanders remove solids in the size of sand (down to 50μ).
Desilters remove solids in the size of silt (down to 20μ). When solids become smaller than the
cutoff
point of desilters, centrifuges may have to be used. Chemical flocculants are sometimes
used to flocculate fine solids into a bigger size so that they can
be removed by solids-removal
equipment. Total flocculants do not discriminate between various types of solids, while selective
flocculants will flocculate drilled solids but not the added barite solids. As a last resort, dilution is
sometimes used to lower solids concentration.
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