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Oilfield Glossary
Oilfield Glossary
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underream

1. vb. [Drilling]
To enlarge a wellbore past its original drilled size. Underreaming is sometimes done for safety or efficiency reasons. Some well planners believe it is safer to drill unknown shallow formations with a small-diameter bit, and if no gas is encountered, to then enlarge the pilot hole. An underreaming operation may also be done if a small additional amount of annular space is desired, as might be the case in running a liner if surge pressures were problematic.