BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Oven testing of a fluid can be done at the wellsite and, on many wells, is considered standard
operating procedure. Timely test results are the key for making quicker adjustments to the fluid.
MIL-TEMP ® (or ALL-TEMP ® ) has been found to be an excellent product in cement
contaminated drilling fluids to decrease the gel strength and high-temperature gelation properties.
Generally, about 1 to 3 lb m /bbl of MIL-TEMP ® can correct the flow properties of a cement
contaminated system. MIL-TEMP is a higher temperature application product and generally is
not used for temperatures lower than 250°F to 300°F.
Anhydrite-Gypsum Contamination
Anhydrite and gypsum, both calcium sulfate compounds (CaSO 4 ), are sometimes encountered
while drilling. Gypsum is calcium sulfate with water of crystallization (CaSO 4 +2H 2 0), while
anhydrite is the anhydrous form of calcium sulfate (without water). Anhydrite may occur as thin
stringers. In some areas, it occurs as massive beds ranging from 100 to 800+ feet thick. Anhydrite
has also been associated with salt (stringers, inclusions, overhang, massive salt, etc.) and could
signal potential salt contamination.
Anhydrite contamination contributes calcium ions, which could result in flocculation. Unlike
cement anhydrite does not cause a pH increase since it supplies a sulfate radical in lieu of an
hydroxyl radical. The sulfate radical contributes to flocculation of clay solids only marginally.
In lightly treated fluids, small amounts of anhydrite increase the rheological properties of a fluid.
The severity of change depends to a great degree on the bentonite content and solids particle size
distribution. When anhydrite goes into solution and soluble calcium increases above 200 mg/L,
viscosity may fluctuate noticeably and fluid loss could become more difficult to control. As
anhydrite concentrations increase toward a maximum solubility of approximately 600 mg/L Ca ++ ,
a Base Exchange or change in bentonite characteristics occurs. Flow properties tend to decrease,
and fluid loss becomes difficult to control. Conversion to a gypsum-based fluid has occurred. If a
gypsum fluid is acceptable, then treatments should be made specifically for a gypsum fluid.
When anhydrite contamination occurs, there are several methods for handling it. The drilling
fluid can be maintained as a low-calcium fluid by chemically precipitating most of the calcium
from solution, or it can be converted to a gypsum-type system. For smaller amounts of anhydrite
contamination, chemical removal of calcium is best achieved by adding soda ash. Approximately
0.093 lb m /bbl of soda ash is required to precipitate 100 mg/L of Ca ++ ion, but caution should be
exercised to avoid over-treatment. When Ca ++ ions are reduced to 100 to 150 mg/L, consider
stopping soda ash treatment. The reaction between anhydrite and soda ash is as follows.
CaSO 4 + Na 2 CO 3 → CaCO 3 ↓ + Na 2 SO 4
Remember, pilot testing should support treatment specific to anhydrite contamination.
Soluble sodium sulfate is formed from this reaction and could cause flocculation problems with
prolonged treatments. For this reason, it is sometimes necessary to convert to a gypsum-base fluid
when massive anhydrite is to be drilled with a freshwater system. Over treatment with soda ash
can also cause a carbonate problem.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 4-5