Page 233 - Drilling Technology in Nontechnical Language

224 Drilling Technology in Nontechnical Language Second Edition

refers to the first time that the casing was cemented) is called secondary

cementing. Unfortunately, secondary cementing is quite tricky and has a
low success rate.

The general procedure is that holes are made in the casing using a
perforating gun. Drillpipe is run in the hole down to the depth of the
perforations. Cement is pumped down the drillstring, where it exits
around the perforations. Pressure is then applied to try to force cement
to enter the perforations. With a little luck, the cement might plug the
channel and the perforations. Forcing cement in under pressure is called
squeeze cementing.
A better chance of success comes if the top of cement is just too low,
and it is then possible to perforate above the top of cement. Circulation
can be established, hopefully, by opening the annulus outside the casing

and pumping fluid into the casing. If circulation is possible, cement can be
placed by circulation rather than squeezing. It is also possible that bits of
formation (cuttings and cavings) have settled around the casing, and this
can prevent circulation.

This technique is also used when wells have to be finally abandoned,
if for any reason (such as government regulations) some or all of the annuli

are required to be filled with cement.

Curing lost circulation

Lost circulation can occur from a variety of causes. This will be
discussed in more detail in chapter 13.
If serious lost circulation occurs, cement can be used to cure it. The
difficulty is in getting the cement in the right place and keeping it there

while it sets.

Cement plugs

A cement plug is a column of cement that is set at some point in the
well. Cement plugs serve a variety of purposes.
During well abandonment, cement plugs are set at various points

inside the casing to prevent downhole fluids from reaching the surface
in the future. Cement plugs are also used to suspend a well (temporarily

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