BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Filtrate Ph Range
Freshwater System
Filtrate pH is normally controlled in the 8.0 to 10.0 range with caustic soda additions being made
slowly using a chemical barrel. The hopper should never be used to mix caustic soda (NaOH) or
caustic potash (KOH). Ideally, pH values of 8.0 to 9.0 are preferred for freshwater systems.
Limit the pH to a maximum of 9.5 when UNI-CAL (lignosulfonate) is used. The pH should be
reported on the filtrate, and not on the whole fluid.
Seawater System
A pH of 9.5 to 10.0 should be maintained if hardness reduction is necessary for API filtrate
control.
Range for Filtrate Hardness (Ca++, Mg++)
Freshwater System
Maintain hardness levels below 400 mg/L. A concentration of 200 to 300 mg/L calcium tends to
show the best stability.
Seawater System
If low viscosity and API filtrate values are not required, seawater systems may be run at natural
pH and hardness. This is especially true when the objective of the system is to control gumbo
shale.
Filter cake quality and API filtrate control are adversely affected by high hardness. Therefore,
when sand sections are drilled, the pH of the system may be increased to chemically suppress the
hardness level. This aids in attaining the maximum hydration of both bentonite and polymers to
improve the control both the cake and filtrate quality. In seawater, the pH should be raised
initially with caustic or potassium hydroxide to a maximum value of 9.5 to 9.7. This will
precipitate most of the magnesium. Caustic soda is the preferred product for pH increases
because the potassium ion will have a negative effect on filtration control. Additions of soda ash
and/or sodium bicarbonate should then be used to precipitate out calcium to the desired hardness
level.
Note:
Calcium and magnesium concentrations should be measured and reported, not total
hardness.
API Filtrate
A variety of products are available for API filtrate control. Laboratory and field tests have
indicated that LIGCO ® is an effective filtrate control agent at moderate pH levels. In addition,
LIGCO will act as a mild deflocculant. MIL-PAC is also effective for filtrate control; however, it
may increase the viscosity to an undesirable range. MIL-PAC and MIL-PAC LV are effective in
seawater NEW-DRILL systems.
The filter cake quality of the NEW-DRILL system makes API filtrate values of 10 to 20 cc/30
minutes sufficient in “most” situations. To determine cake compressibility, filtrate values should
be measured and reported at 100, 200, or 500 psi, and at 7½ and 30-minute intervals.
UNI-CAL Additions
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-61