BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Carbonate / Bicarbonate Contamination
Under some conditions, large amounts of soluble carbonates or bicarbonates can accumulate in
drilling fluid systems. These ions can adversely affect fluid properties in much the same way as
salt or calcium sulfate. Carbonates may come from over-treatment to remove calcium or cement
contamination. Carbon dioxide gas could be present in certain formations drilled and the
carbonates are formed as a result of the reaction of sodium hydroxide with carbon dioxide.
CO 2 + 2NaOH →Na 2 CO 3 + H 2 ↑
At higher temperatures (≥ 300°F), organics such as lignosulfonate, lignites, etc. yield measurable
carbonates which could create fluid problems if not addressed. Barite can also be a source of
measurable carbonates. The specific ionic radical present in the mud is a function of pH as
presented in Figure 4-1.
Figure 4-1
Solution Species Present in Carbonate Systems as a Function of pH – Calculated for
K 1 =3.5 x 10 -7 and K 2 = 6.0 x 10 -11
Rheological properties are adversely affected in two ways when alkalinity is altered by
bicarbonates and carbonates. First, carbonate and bicarbonate anions in the presence of clays will
cause increases in gel strength (more noticeable with the ten-minute reading) and yield point.
Organic deflocculants such as UNI-CAL ® require hydroxyl ions to function properly. An influx
of CO 2 will react with hydroxyl ions and take them out of solution so they are not available to
react with UNI-CAL ® . Heavy carbonate and bicarbonate contamination creates a condition
where P f , pH, and solids phase could appear to be in the desired range, yet viscosity and filtration
values do not respond to treatment. Typically, an increasing trend in M f (5.0 cc N/50 H 2 SO 4 )
may result in high gel strengths which can be due to increasing carbonates and bicarbonates.
Difficulty in controlling high-temperature/high-pressure (HT/HP) filtration is also quite evident.
Gel strengths will not normally respond to dilution and UNI-CAL ® treatments unless an adequate
amount of caustic soda has been added to maintain the P f .
A second problem is caused by field test procedures in detection and remedial steps for carbonate
and bicarbonate contamination. In water, the P f and M f values can be used to reasonably
determine the quantities of hydroxyl ions and bicarbonate/carbonate anions. However, treated
drilling fluids contain organic acids which function as buffering agents. This can cause the M f test
procedure to show greater amounts of bicarbonate and carbonate than are actually present.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 4-7