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3.4 Casing and Cementation 133
and with less mix-up with the mud which has to be displaced by the cement on
the way down.
• Plug cementation (two plugs are used to separate mud and cement): In this
technique a first plug is pumped in front of the cement slurry and a second plug
between the end of the cement and displacement mud. Both plugs are made
of rubber and act as a ‘‘floating’’ seal between the different fluids in order to
avoid mixing of the fluids. The first plug is equipped with a membrane which
bursts at a predetermined differential pressure to allow flow of the cement
slurry after having opened. The second plug seals finally and indicates end of
cementation.
• A schematic of a typical plug cementation (primary cement job of a surface
casing string) is shown in Figure 3.15; details of the two-plug system are shown
in Figure 3.16.
• Liner cementation (with two plugs): A liner cementation is generally similar to a
two-plug cementation; the difference is a drillpipe dart (a small plug which passes
through the drillpipe), latches into the liner plug and seals it while pumped down
as a ‘‘unit.’’
• Cementation ‘‘top-down’’: While all normal cementations are done as described
above (cement rises in annulus from bottom upward) sometimes it may be
necessary to pump cement from top-down in the annulus. Those operations are
often critical and do not allow a separation between mud and cement by plugs.
This technique is sometimes used to fill the upper part of an annulus if the
normal way around the casing shoe is plugged, for example, due to a partial
cementation of the lower part of the annulus. Special care has to be taken not to
collapse the casing because of possible high annular pressure occurrence when
‘‘squeezing’’ the cement from top, particularly if it is applied to large-diameter
casing which have only low collapse pressure resistance!

3.4.5
Cement Slurries, ECD

Standard cement slurries consist of water, cement, and some additives. Relatively
high water content is essential because the cement slurry must always be sufficiently
pumped through. Hence, densities are normally ranging from approximately 1.5
to 2.0 kg l −1 . However, lower as well as higher densities can be mixed, for example,
by adding hollow glass beads to reduce density or barite or iron oxide to increase
density if necessary. Other additives – besides ‘‘density steering’’ additives – are
fluid loss reducer, retarder or accelerator, and ‘‘friction reducer’’ to keep the slurry
flowing even if the solids content is extremely high.
Sometimes two different slurries are used for a single cementation. Most often
the reason is either a limited collapse pressure resistance of large-diameter casing
or a limited fracture gradient of the rock at casing setting depth. In such cases,
normally a lighter ‘‘lead’’ cement slurry (for the upper section) and heavier ‘‘tail’’
slurry (for the lowermost section) is used to seal the annulus.