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Chapter 9 – CASING AND CEMENTING 225

abandon it), for instance if the well will be completed later on by another
rig. In this case, the well will be reentered and the cement plugs drilled out
before continuing with completion operations.

If for some reason the lower part of the well is to be redrilled along
a separate path, a cement plug may be used both to abandon the lowest
section of the original wellbore and to allow the drill bit to depart from
the original bore and drill a new one. The main criterion for this kickoff
cement plug is that the set cement must have a higher compressive
strength than the surrounding rock, or else the bit will drill back along the
original hole.
Sometimes, formations exposed in the wellbore can be very unstable.
They may be fractured (either naturally or as a result of the drilling

operation), or they may react with the drilling fluid in some way. This
results in material becoming detached from the wellbore wall and falling
into the well. The hole enlarges. This might be cured by setting cement
across it to fill the enlarged hole and isolate the troublesome formation; the

cement is then drilled through to form a cement-lined hole.
Cement plugs are set by running tubing into the well and pumping
cement down the tubing and into the well. The tubing is then withdrawn,
leaving behind a column of cement. Setting a successful cement plug
the first time is more difficult than achieving a good cement job outside

the casing. The annular capacity is larger (because tubing or drillpipe
is smaller than casing), so annular velocities are lower. Thus complete
removal of the mud is harder to achieve. The slurry must be designed so
that once in place, it does not move as the tubing is removed or afterwards
while it is still fluid. Contamination of the cement by mud in the well is a

real problem, which can be solved by using a special tool on the bottom of

the tubing to direct the flow of cement outwards and upwards, instead of
straight down as would occur with just plain-ended tubing. Cement plugs
require good planning and careful execution to meet the objectives without
having to be repeated.

Summary

In this chapter, the major elements of casing design were discussed.
The criteria by which a casing design is judged is whether it meets the
requirements of the well design at the lowest cost. Casing costs generally

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