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III. Cementing

The main purpose of cement within a well is to achieve zonal isolation.
Cement reinforces the casing and prevents the flow of hydrocarbons through the
annular space. This section discusses the cementing of the final production
casing in the Macondo well.

A. Cementing Process

Prior to cementing a well, the rig crew conditions the wellbore by
circulating mud through it. This conditioning cleans out any cuttings or other
debris in the casing, drill pipe and wellbore that could interfere with cement
placement. When mud is circulated completely through the casing (so that the
mud on the bottom of the well returns to the surface), operators have achieved a
“complete bottoms‐up.” By performing a complete bottoms‐up, the crew not
only cleans out the wellbore, but can also analyze the mud that had been on the
bottom of the well to determine whether hydrocarbons are present before
cementing.

After circulating mud, the crew pumps the volume of cement modeled for
the job down the well, followed by the drilling mud that is pumped behind the
cement to push the cement to its planned location. Darts and wiper plugs
separate the cement from the mud to prevent the oil‐based mud from mixing
with, and possibly contaminating, the cement. At Macondo, the crew first
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pumped base oil (synthetic oil with no additives) ahead of a water‐based spacer
(a spacer is a fluid mixture that keeps the mud and cement separated). The
spacer was followed by a bottom dart, the cement, the top dart and more spacer.
After the second spacer, the crew pumped drilling mud to push the materials in
front of it down the drill pipe.

When the darts reach the end of the drill pipe, the darts launch bottom
and top wiper plugs that separate the cement from the other materials traveling

down the well. When the bottom plug reaches the top of the float collar,
pressure causes the plug to rupture, and cement passes through the plug into the
shoe track. After all of the cement has moved through the bottom plug and into
the track, the top wiper plug lands on the float valves. The top plug is not

88 A “dart” is a device pumped through a tubing string to activate downhole equipment and
tools. A “wiper plug” is a plastic component that travels down the well to separate the spacer
and the cement.

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