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What Is a Casing String?

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  • Written By: Alex Newth
  • Edited By: Angela B.
  • Last Modified Date: 03 September 2016
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A casing string is a long section of male and female pipes used in an oilfield. To form a seal between the pipes and the land, the casing string often is cemented in place to ensure there is no damage when oil is going through the pipes. This string of pipes is used to avoid any drilling problems, because there may be issues with the land if the drill goes from surface to target depth in one shot. Most casing strings are made from steel, and there are different grades for different land and drilling conditions.

Casing string formations are long lengths of pipe that help oilfield workers for many different reasons. While it is possible to have this string made from a single pipe, this is rare. Instead, many different pipes normally are used, and they are separated into long pipes and short pipes — compared to household pipes, the short ones still are very long. Each of the long pipes has male connectors, the short pipes are female, and they make a seal around the male pipes.

After the casing string is dropped into an oilfield wellbore, cement commonly is used to seal up the wellbore. There are several reasons for this. If the string is not cemented, then there is a chance it will move around during operation, which increases the chances of it bursting. Another reason is because this forms a pressure seal around the pipe, which also can help protect the string’s integrity.

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The purpose for using a casing string is to keep the land safe for oilfield workers and the environment. For example, if someone drilled from the surface down to the target depth without using these pipes, then the land may severely change or the oil may spill into fresh water. Instead, the drill is lowered through the pipe, which makes it easier to keep the land safe and it ensures the oil does not cross into the water or dirt.

Nearly all casing string pipes are made from steel, because steel is non-corrosive and can stand up to the force of gushing oil. There are different grades of steel used in forming these pipes, and these grades can suit different purposes. For example, lighter grades are easier to use and work well for easy drilling conditions, while heavier grades may be needed for harder drilling conditions.

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