A relatively thin-walled, large diameter, steel pipe that is used to seal parts of a formation. Its main function is to isolate troublesome or unstable formations and to protect fresh water horizons especially domestic water bearing sandstones in land wells.
Casing is graded by the API for (1) outer diameter and wall thickness, (2) weight per unit length, (3) type of coupling, (4) length, and (5) grade of steel.