Casing string

Casing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented. The pipe segments (called "joints") are typically about 40 ft [12 m] in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings. (Some specialty casing is manufactured in one piece with a female thread machined directly into one end.)

Specification 5CT of the American Petroleum Institute standardizes 14 casing sizes from 4.5 inches (11.43 cm) to 20 inches (50.80 cm) outside diameter. Related API documents also promulgate standards for the threaded end connections, the wall thickness (several are available in each size to satisfy various design parameters) and the strength and certain chemical characteristics of the steel material. A range of material strengths -- termed "Grades" and ranging from 40,000 pounds per square inch (276 MPa) to 150,000 pounds per square inch (1,034 MPa) yield strength -- is available for most combinations of OD and wall thickness to meet various design needs. Finally, the API publications provide performance minimums for longitudinal strength ("joint strength") as well as resistance to internal (bursting) and external (collapsing) pressure differentials.

Casing is run to protect or isolate formations adjacent to the wellbore. It is generally not possible to drill a well through all of the formations from surface (or the seabed) to the target depth in one hole section. For example, fresh-water-bearing zones (usually found only near the surface) must be protected soon after being penetrated. The well is therefore drilled in sections, with each section of the well being sealed off by lining the inside of the borehole with steel pipe, known as casing, and filling the annular space (or at least the lower portion) between this casing string and the borehole with cement. Then drilling commences on the subsequent hole section, necessarily with a smaller bit diameter that will pass through the newly installed casing.

A liner is a casing string that does not extend to the surface, being hung instead from a liner hanger set inside of the previous casing string but usually within about 300 feet (91 m) of its bottom. 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. (A disadvantage is the occasional difficulty in effecting a pressure seal by squeeze cementing the casing-liner overlap zone.)

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. The cost of the casing can constitute 20-30% of the total cost of the well. Great care must therefore be taken when designing a casing programme which will meet the requirements of the well.