1. n. [Drilling Fluids]

Calcium carbonate, such as limestone, marble or oyster shells, that has a specified minimum and maximum range of particle sizes and may also have a specified distribution of sizes. It is used as a bridging agent in drill-in, workover and completion fluids to positively seal permeable zones by plugging pores at the wellbore face. It has the additional advantage that it can be dissolved by acid treatment to clean up the zone afterwards.

See: bridging material,  calcium carbonate,  drill-in fluid,  mudding off,  particle-size distribution,  sized salt