Page 227 - Drilling Technology in Nontechnical Language

218 Drilling Technology in Nontechnical Language Second Edition

Usually two joints of casing are run above the float shoe. Then another

float valve is screwed in place, called a float collar. One purpose of this is

that if the valve in the float shoe fails, there is a backup valve in place to
stop cement flowing back into the casing. This will happen otherwise, as

the cement slurry in the annulus is heavier than the mud inside the casing.

With the float shoe and float collar made up, more joints of casing
are screwed on top of the string and lowered into the well. Centralizers
are attached as needed due to the hole deviation and condition. Once the
casing is all in the well, a hanger can be made up that is used to suspend
the casing inside the wellhead. This was explained in chapter 3.
With the casing landed in the wellhead, the next step is to prepare for
cementing. A special container is screwed on top of the casing that holds

two plugs with rubber fins that fit inside the casing (fig. 9–10).

Fig. 9–10. Top and bottom plugs

The bottom plug is hollow, and it has a thin rubber diaphragm at
the top. When ready to start pumping cement, the lower releasing pin is
drawn out, and cement is directed to the inlet pipe between the two plugs.
(The wheels on the cement plug container shown in fig. 9–11 operate the

releasing pins.) The bottom plug travels ahead of the cement, separating it
from the mud below to minimize contamination of the cement with mud.
The fins on the bottom plug, as well as sealing against the inside of the

casing, wipe the thin film of mud off the inside of the casing, again to

reduce contamination.

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