1. n. [Drilling Fluids]
The chemical CaSO4, which occurs naturally as the mineral anhydrite. Gypsum is the dihydrate mineral form, CaSO4·2H2O. Anhydrite and gypsum (commonly called gyp) are found in the subsurface and drilling even small stringers of these minerals can upset a freshwater or seawater mud. Gyp muds, lime muds, and oil muds tolerate these salts best.
CaSO4 is used as a mud treatment when no pH increase is needed to remove carbonate ion contamination in freshwater and seawater muds. (Lime increases pH when added for this purpose.) Gypsum and lime treatments are often used together to keep pH in the proper range. The test for determining the dissolved and undissolved calcium sulfate in a gyp mud requires two titrations with the strong EDTA reagent and Calver II indicator when performed to API standards. It also requires a retort analysis for water content in the mud in order to calculate CaSO4 content, lbm/bbl.