Page 209 - Drilling Technology in Nontechnical Language

200 Drilling Technology in Nontechnical Language Second Edition

▪ Protect freshwater sources close to the surface from pollution by
the drilling fluids

▪ Isolate loose or weak formations that might fall into the wellbore
and cause problems

Intermediate casing

A shallow well may not need an intermediate casing; a deep well may
need several. The intermediate casings serve as staging posts between the
surface casing and the production casing. The primary purposes of the
intermediate casings are to accomplish the following:

▪ Increase the pressure integrity of the well so that it can be
safely deepened.

▪ Protect any directional work done (e.g., kicking off a directional
well is often done under surface casing and is then protected by

the first intermediate casing).
▪ Consolidate progress already made.
Intermediate casings are usually between 20" and 13⅜" outside
diameter.

Production casing

This is the long-term pressure vessel. The production casing houses
the completion tubing, through which hydrocarbons will flow from

the reservoir to the surface. If the completion tubing were to leak, the
production casing must be able to hold the resulting pressure.
Sometimes the production casing is cemented in place with the casing
shoe above the reservoir and another hole section drilled. This may then be
protected with a liner rather than a string of casing. A liner is effectively a
casing that does not extend to the surface but ends somewhere inside the
production casing. There are pros and cons to liners.
Advantages of liners include the following:
▪ Economics. The cost of the liner and associated equipment is less
than the cost of a full string of casing to the surface.

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