BAKER HUGHES - Drilling Fluids Reference Manual

Contamination of Water Based Fluids
Salt Contamination
Salt contamination may be a result of make-up water, drilling salt stringers, inclusions,
overhangs, massive salt, or saltwater flows. Where solid salt is drilled in large quantities or where
economic considerations call for the use of salt make-up water, salt water-base drilling fluid
systems are used. In these cases, salt is not a problem. However, in freshwater fluids, salt
becomes a contaminant.
Salt and saltwater flow contamination cannot be removed from a drilling fluid by economical
chemical means. The harmful effect of salt on freshwater fluids is not so much the chemical
reaction effect of the ions as it is the mass ion electrolytic effect which changes the charge
distribution at the clay surfaces and promotes flocculation. The resulting flocculation causes an
increase in rheological properties and fluid loss. With higher concentrations of UNI-CAL ® ,
rheological characteristics tend to remain stable if the solids are in range, although fluid loss may
increase.
As high concentrations of salt are encountered, the Na + and Cl¯ ions become more abundantly
attached or congregate near the clay surfaces and, through “mass action,” gradually dehydrate the
reactive solids in the fluid. The shrinkage effect on the clays may then cause decreased viscosity
and continued increase in fluid loss.
Since salt cannot be precipitated by economical chemical means, salt concentration can be
reduced only by dilution with freshwater.
The choice of reconditioning agents for salt contamination in freshwater fluids depends on the
severity of contamination. Lightly treated UNI-CAL ® drilling fluid systems can usually tolerate
up to a concentration of 10,000 mg/L salt as long as the colloidal reactive solids are at a
reasonable level for the fluid weight being used. As the salt content increases above 10,000 mg/L,
rheological and fluid loss properties become increasingly difficult to control. When higher salt
content is expected or accidentally encountered, an increase in UNI-CAL ® is necessary. The
alkalinity of the system should be slightly higher in salt-contaminated drilling fluids to allow
better UNI-CAL ® solubility.
Maintaining a salt system is much easier if all chemicals, i.e., deflocculants, filtration control
additives, viscosifiers, etc. are presolubilized or prehydrated in a premix tank. When a saltwater
flow is encountered, the high concentration of Na + ions tends to replace some of the H + and Ca ++
ions at the clay surfaces which slightly reduces the pH and may increase the soluble Ca ++ .
Fluid loss is often difficult to control with salt contamination. Prehydrated bentonite and polymertype
materials are helpful in this situation but should be added in moderation since they can
increase rheological properties.
Almost all saltwater flows contain some cations other than Na + . Calcium and/or magnesium are
commonly present. Their effects have been previously discussed.
Baker Hughes Drilling Fluids
Reference Manual
4-6 Revised 2006