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/gal (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 byproduct of mud treatments used for removal of either Ca⁺² or CO₃^-2 by addition of the other ion.