Page 228 - Drilling Technology in Nontechnical Language

Chapter 9 – CASING AND CEMENTING 219

Fig. 9–11. Cement plug container in a shipping basket

Once the correct volume of cement is pumped, the upper releasing
pin is withdrawn, and mud is pumped into the upper inlet. Now there is a
column of cement moving down the casing, with the bottom plug below
and the top plug above.
Eventually the bottom plug hits the float collar. It can move no further

down. At this point the rubber membrane ruptures to allow cement to flow
through the plug. Cement now flows out of the float shoe and up around

the casing.

The pumps are slowed down as the top plug approaches the float collar.
Once the top plug lands on top of the bottom plug, it forms a seal. The
surface pumping pressure increases, and this shows that the displacement
is complete.

Figure 9–12 shows a section of casing cut away to show the float
collar valve, with bottom and top plugs in the position they will be in after
completing cement displacement.
Normally at this stage, the casing is pressure tested. If for some reason
the top plug did not land on the float collar after pumping the correct

volume, pumping is stopped, and the casing will have to be pressure tested
once the cement has set (fig. 9–12). This involves a small risk of breaking

the bond between casing and cement; the casing will expand a little under
the test pressure, and when pressure is removed, it will contract again. The
cement is not elastic like steel, and instead of moving back with the casing,
it is possible that the bond will break. This creates a very small annulus
between casing and cement—a microannulus.

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