Once the cement has set hard, a large spool called a wellhead housing is attached to the top of the 20” casing. This wellhead housing is used to support the weight of subsequent casing strings and the annular valves known as the Blowout prevention (BOP) stack which must be placed on top of the casing before the next hole section is drilled.
Since it is possible that formations containing fluids under high pressure will be encountered whilst drilling the next (17 1/2”) hole section a set of valves, known as a Blowout prevention (BOP) stack, is generally fitted to the wellhead before the 17 1/2” hole section is started. If high pressure fluids are encountered they will displace the drilling mud and, if the BOP stack were not in place, would fl ow in an uncontrolled manner to surface. This uncontrolled flow of hydrocarbons is termed a Blowout and hence the title Blowout Preventers (BOP’s) . The BOP valves are designed to close around the drillpipe, sealing off the annular space between the drillpipe and the casing. These BOPS have a large I.D. so that all of the necessary drilling tools can be run in hole.
When the BOP’s have been installed and pressure tested, a 17 1/2″ hole is drilled down to 6000 ft. Once this depth has been reached the troublesome formations in the 17 1/2″ hole are isolated behind another string of casing (13 5/8″ intermediate casing). This casing is run into the hole in the same way as the 20” casing and is supported by the 20” wellhead housing whilst it is cemented in place.
When the cement has set hard the BOP stack is removed and a wellhead spool is mounted on top of the wellhead housing. The wellhead spool performs the same function as a wellhead housing except that the wellhead spool has a spool connection on its upper and lower end whereas the wellhead housing has a threaded or welded connection on its lower end and a spool connection on its upper end. This wellhead spool supports the weight of the next string of casing and the BOP stack which is required for the next hole section.
Institute of Petroleum Engineering, Heriot-Watt University