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.
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.