BAKER HUGHES - Drilling Fluids Reference Manual

Contamination of Water-Base Muds
Chapter 4
The composition and treatment of drilling fluids depends on the formation
encountered or material added during drilling operations. Some of these materials
under certain circumstances, along with formation cuttings, can be considered
contaminants. This chapter summarizes the various chemical contaminants and
describes methods of chemical control and removal of water-soluble
contaminants.
Contaminant Overview
If large quantities of contaminants are encountered during drilling, certain factors must be taken
into account depending on the contaminant, and the impact it has on the constituents of the
drilling fluid resulting in a deviation from the desired fluid properties. These factors are
considered individually in the following discussion of each type of contaminant. However, it
should be noted that it is not unusual to experience more than one contaminant at a time, which
requires an analytical approach to identify and rectify the contaminants.
In general, a contaminant is anything that causes undesirable changes in fluid properties. Any of
the liquid, chemical, or solid components of water-base fluids can become a contaminant in some
given situation.
Solids are by far the most prevalent contaminant. Commercial bentonite added in excess, drill
cuttings, or barite may lead to unacceptable rheological and filtration properties and affect the
drilling operation. Water or excess chemical treatment can lead to unacceptable fluid changes
and cause unscheduled viscosifier additions.
The contaminants of interest are those which require chemical treatment. Chemical treatments to
remove these contaminants are possible for some and impossible for others. The important rule is
that treatment must match the contaminant and result in a desired effect on the fluid.
Some contaminants can be predicted and pretreated to control them. The predictable
contaminants are cement, bad make-up water, massive salt, anhydrite formations, or gases such as
hydrogen sulfide and carbon dioxide in areas where documentation shows a probable presence.
These contaminants can be chemically removed in some cases before they can have an overall
negative effect on the clays, organic deflocculants, and/or filtration additives. Pretreatment has
advantages as long as it is not excessive and does not adversely affect fluid properties. Pretreating
a fluid with sodium bicarbonate before drilling cement is one example. By the same
token an over treatment of sodium bicarbonate as a pretreatment can also become a contaminant.
Other contaminants are unpredictable and unexpected, such as those which result from small
feed-ins with a gradual build-up of a contaminant. Predicting potential contamination problems
while drilling an exploration well is difficult. The contaminant shows its effect by altering the
properties of the system and, in part, is determined by the resultant properties. When
deflocculants are slightly depleted, or after a long trip when the fluid is allowed to remain
stagnant and subjected to elevated downhole temperatures, or after additional contaminant enters
the system, changes become evident. It is always necessary to keep complete, accurate records of
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 4-1