1. . 
A compound with formula CaCO3 that occurs naturally as limestone. Ground and sized calcium carbonate is used to increase mud density to about 12 lbm/galUS [1.44 kg/m3], and is preferable to barite because it is acid-soluble and can be dissolved with hydrochloric acid to clean up production zones. Its primary use today is as a bridging material in drill-in, completion, and workover fluids. Sized calcium carbonate particles, along with polymers, control fluid loss in brines or drill-in, completion, and workover fluids. Insoluble calcium carbonate is the precipitated by-product of mud treatments used for removal of either Ca+2 or CO3–2 by addition of the other ion.
See: alkalinity, bicarbonate, carbonate ion, carboxymethylcellulose, completion fluid, drill-in fluid, Garrett Gas Train, hardness ion, hydroxyethylcellulose, polyanionic cellulose, polymer, sized calcium carbonate, weighting material, workover fluid