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large in diameter – perhaps 20 inches (508 millimetres) or more. Casing is strong
steel pipe. Running casing into the hole is very similar to running drill pipe,
except that the casing diameter is usually much larger and thus requires special
elevators, slips, and tongs to fit it.
Also, the casing crew sometimes installs centralizers and scratchers on the
outside of the casing before they lower it into the hole. The crew attaches centralizers
around the outside of the casing joints and, because the centralizers have bowed
springs, they keep the casing centered in the hole after the crew lowers it in.
Ideally, casing should not come into contact with the walls of the hole. If it does,
cement may not be able to flow into the area between the wall of the hole and the
outside of the casing. Consequently, a void in the cement may occur, which could
allow fluids to flow outside the casing. Fluid flow behind the casing is not
desirable because contamination can occur. For example, salt water from one
formation could flow into another formation containing fresh water and pollute it.
Scratchers also come into play when the casing is cemented. The idea is
that if the driller moves the casing up and down or rotates it (depending on
scratcher design), the scratchers remove the wall cake formed by the drilling mud
and the cement will thus be able to bond better to the hole.
Other casing accessories include a guide shoe, which is a heavy steel-and-concrete
fitting that the casing crew attaches to the bottom of the first joint of casing to go into
the hole. The guide shoe helps guide the casing past small ledges or debris in the hole.
Besides the guide shoe, another accessory is a float collar. The crew installs a float collar
a couple of joints from bottom. The float collar keeps mud in the hole from entering the
casing as the crew lowers the casing into the hole. Just as a ship floats in water, casing
floats in a hole full of mud, if most of the mud is kept out of the casing. The float
collar's valve keeps mud from entering the casing as the crew lowers it into the mud-
filled hole. This buoyant effect helps relieve some of the weight carried by the mast
or derrick.

Cementing
Cement bonds the casing to the hole and prevents fluids in one formation
from migrating to another. Cement also prevents corrosive formation fluids from
damaging the casing. The operator usually hires an oilwell cementing company to
perform the job. Cementing companies stock many kinds of cement and have
special equipment to transport it to the well. At the well, the company mixes the
dry cement with water to form a slurry – a thin, watery mixture that is easy to pump.
Many kinds of mixers are available to blend the water and cement into a uniform
mixture as the cement pumps move it down the casing. Special high-pressure pumps
move the slurry through very strong pipes, or lines, to a cementing head, or plug
container. Earlier, the cementing crew mounted the cementing head on the topmost
joint of casing hanging in the mast or derrick. Just before the slurry arrives at the
head, a crew member releases a rubber plug, a bottom plug, from the cementing head.
The bottom plug separates the cement slurry from any drilling fluid inside the casing
and prevents the mud from contaminating the cement. The slurry moves the bottom
plug down the casing. The plug stops, or seats, in the float collar. Continued pumping

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