In drilling technology, casing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented. The purpose of the casing pipe is as follows:
prevent formation fluids from entering the borehole in an uncontrolled way
prevent fluids in the borehole from entering other formations
Background
The pipe segments are typically about in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings. Specification 5C3 of the American Petroleum Institute standardizes 14 casing sizes from to outside diameter. This and related API documents also promulgate standards for the threaded end finish, the wallthickness, and the strength and certain chemical characteristics of the steel material. Several material strengths—termed "Grades" and ranging from to minimum yield strength—are available for most combinations of OD and wall thickness to meet various design needs. Finally, the API publications provide performance minimums for longitudinal strength as well as resistance to internal and external pressure differentials. A typical piece of casing might be described as 9-5/8" 53.5# P-110 LT&C Rg 3: specifying OD, weight per foot, steel strength, end finish, and approximate length. 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 to the target depth in one hole-size section. For example, fresh-water-bearing zones 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 between this casing string and the borehole wall 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 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. Depending on the conditions encountered, 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, and failure can be catastrophic. Great care must therefore be taken when designing an economic casing programme that will meet the requirements of the well.
