Casing string

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 in length, with threaded connections. Depending on the conditions encountered, 3 or 4 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.