Casing string is an assembled length of steel pipe configured to suit a specific wellbore. The sections of pipe are connected and lowered into a wellbore, then cemented in place. The pipe joints are typically approximately 40 ft (12 m) in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings. Long casing strings may require higher strength materials on the upper portion of the string to withstand the string load. Lower portions of the string may be assembled with casing of a greater wall thickness to withstand the extreme pressures likely at depth. Casing is run to protect or isolate formations adjacent to the wellbore.
Once set in place, the casing string is cemented to the surrounding rock to prevent the well from collapsing, isolate different underground formations, and stop fluids such as water, oil, or gas from migrating between layers. It also protects freshwater aquifers from contamination and provides a secure conduit for later drilling, completion, and production operations. Different casing strings, such as conductor, surface, intermediate, and production casing, are used at various depths, each serving a specific structural and safety role in the overall well design.
