casing string

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1. n. [Drilling]
An assembled length of steel pipe configured to suit a specific wellbore. The sections of pipe are connected and lowered into a wellbore, then cemented in place. The pipe joints are typically approximately 40 ft [12 m] in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings. Long casing strings may require higher strength materials on the upper portion of the string to withstand the string load. Lower portions of the string may be assembled with casing of a greater wall thickness to withstand the extreme pressures likely at depth. Casing is run to protect or isolate formations adjacent to the wellbore. The following are the most common reasons for running casing in a well: 1) protect fresh-water aquifers (surface casing) 2) provide strength for installation of wellhead equipment, including BOPs 3) provide pressure integrity so that wellhead equipment, including BOPs, may be closed 4) seal off leaky or fractured formations into which drilling fluids are lost 5) seal off low-strength formations so that higher strength (and generally higher pressure) formations may be penetrated safely 6) seal off high-pressure zones so that lower pressure formations may be drilled with lower drilling fluid densities 7) seal off troublesome formations, such as flowing salt 8) comply with regulatory requirements (usually related to one of the factors listed above).