Casing is a large diameter pipe that is assembled and inserted into a recently drilled section of a borehole. Similar to the bones of a spine protecting the spinal cord, casing is set inside the drilled borehole to protect and support the wellstream. The lower portion (and sometimes the entirety) is typically held in place with cement. 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.

Casing that is cemented in place aids the drilling process in several ways:

• Prevents contamination of fresh water well zones.
• Prevents unstable upper formations from caving in and sticking the drill string or forming large caverns.
• Provides a strong upper foundation to allow use of high-density drilling fluid to continue drilling deeper.
• Isolates various zones, which may have different pressures or fluids, in the drilled formations from one another.
• Seals off high pressure zones from the surface, minimizing potential for a blowout
• Prevents fluid loss into or contamination of production zones.
• Provides a smooth internal bore for installing production equipment.

Optimum design of the casing program decreases the well construction costs, enhances the efficiency of operations and also diminishes the environmental impacts.

A slightly different metal string, called production tubing, is often used without cement inside the final casing string of a well to contain production fluids and convey them to the surface from an underground reservoir.