Contamination with Calcium Sulphate (Anhydrite) will result in flocculation of bentonite muds resulting in an increase in yield point, gel strengths and fluid loss.

Polymer muds are relatively insensitive to this type of contamination, unless significant quantities of drilled clay solids are present.

Sources of Contamination

This type of contamination can occur whilst drilling Anhydrite.

Diagnosis

Calcium Sulphate (Anhydrite) contamination will result in:

• an increase in filtrate calcium.
• a decrease in pH
• a decrease in Pt.

Treatment

• Treat out calcium with soda ash at the rate of 0.116 lbs/bbl per 100 ppm calcium.
• Combine treatment with lignosulphonate to regain control of viscosity and fluid loss.

Note: If large amounts of soda ash are added, the soluble sulphate may cause “Ash gels” which are high and progressive.