Casing string run to isolate producing zones, to provide reservoir fluid control, and to permit selective production in multizone production. It is the string through which the well will be completed.
The choice of tubing diameter will determine the diameter of the production casing. API standards specifies five production tubing diameters [...] accordingly production strings will have diameter of 7" for high-output wells or wells with multiple completion (two or three tubings). More moderate producers will be equipped with 5 1/2 or 5" strings.