Casing string

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Casing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented. The pipe 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.

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. 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 with cement, before drilling the subsequent hole section. This casing string is made up of joints of pipe, of approximately 40ft (12m) in length, with threaded connections. Depending on the conditions encountered, three or four casing strings may be required to reach the target depth. The cost of the casing can therefore 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.