There are two basic types of pipes used in oil and gas industry exploration and production standarized by the American petroleum institute(API) and the international standards organization(ISO): casing and tubing .

Casing and tubing strings are the main parts of the well construction. All wells drilled for the purpose of oil or gas production (or injecting materials into underground formations) must be cased with material with sufficient strength and functionality.

Per API, the specification difference between casing and tubing are:

1)Length of the drift mandrel 6 in. or 12 in.for casing and 42 in. for tubing;

2)Joint strength calculation method:minimum tensile strength for casing and minimum yield strength for tubing.

Oil Casing is a series of steel pipes that are run into a drilled oil well to stabilize the well, keep contaminants and water out of the oil stream, and prevent oil from leaching into the groundwater. Casing is installed in layers, in sections of decreasing diameter that are joined together to form casing strings. The five types of casing string are conductor casing, surface casing, intermediate casing, casing liner, and production casing.Steel Casing Pipes have smooth wall & minimum yield strength of 35,000 psi.

Tubing is pipe used for the transportation of crude oil and natural gas from an oil or gas layer to the surface after drilling is complete.Tubing is the conduit through which oil and gas are brought from the producing formations to the field surface facilities for processing. Tubing must be adequately strong to resist loads and deformations associated with production and workovers. Further, tubing must be sized to support the expected rates of production of oil and gas.

According to the API 5 CT, pipe casing steel grade will be divided into H 40,J 55,K 55, C 75,N 80, L 80,C 90, T 95,P 110、Q125.