2 SIMPLE TIPS TO CONSIDER WHILE DRILLING ANHYDRITE / GYPSUM FORMATIONS

Calcium sulfate occurs in nature as Gypsum (CaSO4 l H2O) or Anhydrite (CaSO4 ). It is found in thick sections, stringers, in make-up water, embedded in silts as in evaporite formations and sometimes in the caprock of a salt dome.

Calcium sulfate causes aggregation and flocculation of a fresh water mud, resulting in thickening. The calcium sulfate causes an increase in apparent viscosity, yield point (check also Yield Point In Drilling Mud), gel strength and filtrate. The partially soluble calcium sulfate increases the hardness and sulfate content of the filtrate. If a calcium base mud is in use, the calcium sulfate contamination has little or no effect on the mud properties.

1 A common method of drilling anhydrite or gypsum formations is to pre-treat the mud with thinners that works effectively in the presence of calcium and sulfates and alkali’s. The contamination of the mud by the drilled calcium sulfate is nullified. If it is desired to maintain a fresh water mud after calcium sulfate contamination has occurred, it is necessary to treat out the ions that cause aggregation and flocculation. This may be done by adding soda ash (Na2CO3) or Barium carbonate (BaCO3).

Na2CO3 + CaSO4 = CaCO3 + Na2SO4 (Precipitate)

The calcium is precipitated as insoluble calcium carbonate (limestone). A general rule is to add 0.02 lb/bbl of soda ash for every epm of hardness. After adding the soda ash, a thinner is usually added to reduce the viscosity and gel strength. A difficulty is encountered if large amounts of soda ash are added. The soluble sodium sulfate tends to build up and cause “ash gels” which are indicated by high progressive gel strength.

2 Another method to treat out calcium sulfate contamination is to treat the thickening and filtration increase that has occurred and let the system become an aggregated-deflocculated one. This can be done by using a thinner, adjusting the pH and using a fluid loss controller. If a high pH is maintained, this too may result in “ash gels” due to the formation of sodium sulfate. If high sodium sulfate (Na2SO4 ) occurs, it will require water dilution and lime additions for alkalinity.