Oil casing is a type of pipe used to support the walls of oil and gas wells, ensuring the smooth progress of drilling operations and the normal functioning of the well after completion. Depending on the drilling depth and geological conditions, multiple strings of casing are used for each well. Once lowered into the well, the casing is secured via cementation; unlike tubing or drill pipe, it is a single-use consumable that cannot be reused. For ultra-deep and extremely deep wells, oil casing must meet stringent performance requirements, including high strength, high toughness, and resistance to corrosion and collapse. Casing accounts for over 70% of total oil well tubular consumption. Steel grades for oil casing include H40, J55, K55, N80, L80, C90, T95, P110, Q125, and V150, while end connection types include short round threads, long round threads, buttress threads, and special connections.
Based on application, casing is classified into: conductor pipe, surface casing, intermediate casing, and production casing. Conductor pipe is used to isolate seawater or stabilize surface formations near the wellhead; surface casing isolates unstable, soft upper formations and aquifers; intermediate casing isolates complex formations that are difficult to control; and production casing isolates oil, gas, and water zones while establishing flow paths for hydrocarbons. Liners are classified by function into drilling liners, production liners, tie-back liners, and others.
