Casing　
Casing is a large-diameter pipe that serves as the structural retainer for thewalls of oil and gas wells, or wellbore. It is inserted into a well bore andcemented in place to protect both subsurface formations and the wellbore fromcollapsing and to allow drilling fluid to circulate and extraction to takeplace. The following diagram illustrates different types of casing used in theextraction process.

● Surface casing
Protect the well from contamination in shallow water and gas layers.
Support the wellhead equipment and sustains the weight of the other layers ofcasing.

● Intermediate casing
Isolates different layers of pressure to facilitate normal circulation ofdrilling fluid and protect the production casing.
Intermediate casing facilitates the installation of blow-out preventers,anti-leakage devices and tailpipes in the well.

● Production casing (oil string)
It is the conduit, through which oil and gas pass from a below surfacereservoir.
It protects the well and separates fluids into various layers.
For the manufacture of tubing, green pipes undergo an "upsetting"process to thicken the walls of the pipes. Apart from this step, themanufacturing of tubing and casing is identical. First, green pipes arethreaded on each end. The threading is then inspected and, if necessary,rethreaded to ensure that the thread accurately connects to the coupling. Thetubing and casing are strengthened by surface treatment and are subject tovarious inspections, including drifting and hydrostatic tests before they arecoated and packaged into tubing and casing.