BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Calcium or Magnesium Contamination
Source
Calcium and magnesium are closely related ions that have similar effects on fluid properties.
These ions can originate from make-up water, salt water flows, drilling of gypsum or anhydrite,
drilling cement, and some formations such as the Zechstein, which contains a multitude of
problematic ions.
Effects on Fluid Properties
Calcium and magnesium will replace sodium ions on reactive clays. This effect reduces the
hydration of the clays and promotes flocculation. These actions increase the yield point, gel
strength, and API filtrate of the fluid. In concentrations above 500 mg/L, the NEW-DRILL ®
polymer will also be dehydrated and viscosity and API filtrate may become difficult to control.
The presence of these two cations is measured collectively using the total hardness titration. The
calcium ion can be titrated separately to determine the concentrations of each ion.
Treatment and Control Techniques
Magnesium – The magnesium concentration can be decreased by increasing the pH. Historically,
the solution was to increase the pH of the fluid to 10.5 or above. At this pH level, the magnesium
precipitates as insoluble magnesium hydroxide. A pH level this high is not normally
recommended in the NEW-DRILL ® system. Increasing the pH level to 9.8 should reduce the
magnesium concentration to acceptable levels.
Calcium – Concentrations of this cation above levels of 400 to 500 mg/L are not recommended in
the NEW-DRILL ® system. Reduction of the calcium to this range can be accomplished by
increasing the pH to 9.0 to 9.5 and adding soda ash to remove any calcium above the 500 mg/L
level.
A treatment with 0.1 lbm/bbl of soda ash will remove approximately 100 mg/L of calcium.
Cement Contamination
Source
In most drilling operations, cement contamination may occur when casing is cemented and plugs
are drilled out. The severity of contamination depends on,
• solids content
• concentration of deflocculants
• type of deflocculants
• quantity of cement drilled
• whether the cement is green or cured
• the amount of pH increase.
Effects on Fluid Properties
The problem with cement contamination is a result of the combination of high pH and calcium.
The presence of solids in the fluid causes a viscosity increase, resulting from the flocculation of
these solids due to the high pH and calcium (from drilling cement). The problem may be
aggravated somewhat in a NEW-DRILL system because of the low pHs and low concentration of
thinners typical in this system. If possible, use water to drill cement.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-57