Page 85 - Drilling Technology in Nontechnical Language

76 Drilling Technology in Nontechnical Language Second Edition

hole. Increasing the flow rate also increases the pressure in the annulus,

and eventually losses to a formation down hole might start to occur. The
annulus between the casing and the hole is much smaller than it was with
the drillstring, and this gives higher pressures while circulating. As soon
as mud losses are detected, the pump is slowed down a little. This flow rate

will be the maximum flow rate during the cement job. Circulating continues
until about 120% of the casing inside volume is pumped. This ensures that
there is no debris (such as rags, brushes, or someone’s hard hat) inside the

casing that can plug the float valves, which would be disastrous if it were
to happen while cementing. At the same time, this helps to improve the
cement job for reasons discussed in chapter 9, “Casing and Cementing.”

Once satisfied that all is well, cement is pumped down the casing. In
front of the cement is a rubber or plastic plug that seals in the casing. This
moves down ahead of the cement, wipes all the mud off the inside of the
casing, and separates the mud and cement to avoid contamination. This is
called the bottom plug. The bottom plug has a plastic diaphragm on the
top, which will break when the plug hits the float collar, allowing cement

to flow through the plug and down through the float valves.

Behind the cement is used another plug that is similar to the bottom
plug. This second plug, called the top plug, differs in that it is solid and
does not rupture when it lands on the bottom plug. The purpose of the top
plug is to wipe cement from the inside of the casing and to separate the
cement from the mud behind it. When the top plug lands on the bottom

plug, the plugs seal and prevent further movement of fluid down the casing.
This allows the driller to see exactly when the cement is in place because
the pumping pressure will increase. At this time, the driller can pressure
test the inside of the casing to make sure there are no leaks in it.
After pressure testing above the casing hanger to ensure that the hanger
seals work, the blowout preventer can be removed. Then a housing called
a casing spool is added to the wellhead, in which the next string of casing
will land.

The assembly of casings, hangers, and spools is called the wellhead.
The status of the wellhead is illustrated in figure 3–22.

The inside of the casing spool has a similar profile to the casinghead
housing, only smaller. Each string of casing requires a casing spool to land
in (with this particular type of wellhead).

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