Calcium Carbonate is 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.

CaCO3 is used as a temporary plug formulated with graded granules or flakes of calcium carbonate that are generally circulated into place as slurry and allowed to settle out. Calcium carbonate plugs commonly are used to isolate lower production zones, either to enable a column of well control fluid to be placed, or to provide some protection for a lower zone while treating upper zones. Because of their high reaction rate with hydrochloric acid, calcium carbonate plugs are easily removed using common acidizing materials and equipment.