9 - What Is A Special Drift Casing?
Updated: Aug 22, 2018
Casing is an essential part of an oil or gas well. since It:
secures weak formations from collapsing,
provides a mean to contain pressure in the case of well control, and
serves as a conduit for tools and equipment used to execute the next phase of the well (whether it is drilling or completion).
This last function (of being a conduit) is the main reason why we drift the casing. So let us now examine few concepts:
What is drifting?
It is the process of passing a cylinder through the casing joints (before running them in the hole) to confirm that the casing roundness and eccentricity are not below an allowable threshold (in other words, to make sure that the casing's inner diameter is large enough to allow tools and equipment to pass through it without obstruction).
Isn't Casing Manufactured According To A Specified ID and OD?
It is true that casing is manufactured according to specific dimensions. However; the manufacturing process isn't perfect enough to guarantee size-consistency (even within the same joint). Therefore; tolerances are allowed in the manufacturing process.
Then What Is "Special Drift" Casing?
As mentioned earlier, a drift is used to ensure that the casing inner diameter doesn't fall below a certain size. But since the allowable tolerance might differ from an application to another, the Oil & Gas industry identified three drift categories:
Alternative Drift: is a size of drift (more conservative in some casing sizes) that is listed in the API 5CT specifications as an alternative to the API drift.
Special drift: it refers to any drift specifications other than the ones listed in the API 5CT specifications. They tend to be the most conservative (this is what large operators request usually).
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