In drilling technology, casing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented.
[1] This and related API documents also promulgate standards for the threaded end finish, the wall thickness (several are available in each size to satisfy various design parameters, and in fact are indirectly specified by standardized nominal weights per linear foot; thicker pipe obviously being heavier), and the strength and certain chemical characteristics of the steel material.
Then drilling commences on the subsequent hole section, necessarily with a smaller bit diameter that will pass through the newly installed casing.
Other than the obvious cost savings, the liner installation allows larger drill pipe or production tubing to be used in the upper portions of the well.
Depending on the conditions encountered (e.g., zones of differing formation pressure gradients), three or four casing strings may be required to reach the target depth.