Steel pipe cemented in place in an oil or gas well as the drilling progresses or when the well is completed. The casing serves several purposes:

(1) to prevent the [[formation]] walls from caving or squeezing into the hole during drilling;

(2) to protect one formation from damage when heavy [[mud]] weights are required in drilling another;

(3) to provide a means to isolate porous and [[permeability|permeable]] formations from one another at the wellbore;

(4) to prevent contamination of shallow [[aquifer|aquifers]] by drilling [[mud|muds]]. See also [[casing string]].