Casing (borehole)

Casing is a large diameter pipe that is assembled and inserted into a recently drilled section of a borehole.

[1] Deeper strings usually are not cemented all the way to the surface, so the weight of the pipe must be partially supported by a casing hanger in the wellhead.

This decision is often based on subsurface data such as formation pressures and strengths, well integrity,[4] and is balanced against the cost objectives and desired drilling strategy.

The inside diameter of the final casing string (or penultimate one in some instances of a liner completion) must accommodate the production tubing and associated hardware such as packers, gas lift mandrels and subsurface safety valves.

Mechanical properties such as longitudinal tensile strength, and burst and collapse resistance (calculated considering biaxial effects of axial and hoop stresses), must be sufficient at various depths.

[6][5] In order to reduce cost, a liner may be used which extends just above the shoe (bottom) of the previous casing interval and hung off downhole rather than at the surface.

In order to precisely place the cement slurry at a required interval on the outside of the casing, a plug is pumped with a displacement fluid behind the cement slurry column, which "bumps" in the casing shoe and prevents further flow of fluid through the shoe.