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 As drilling progresses, the rig sets additional casings (sections of pipe)
that are slightly smaller in diameter than the hole created by the drill bit.
The combination of casings is referred to as the “casing string.” The
casings are bonded into place using cement. The casing string maintains
the integrity of the wellbore by protecting the sides of the wellbore from:
(1) pressure exerted from the drilling mud; (2) collapse of the hole already
drilled; and (3) influx of fluids from the surrounding formation.

 The outermost casing near the top of the well can be up to four feet in
diameter, and the innermost string of casing near the bottom of the well
can be less than six inches in diameter. The size of the initial and final
casing, the types of casing, and the type of cement used are determined
by the profile of the well being drilled, including factors such as well
depth, temperatures, and well pressures. Once the well is in production,
hydrocarbons are extracted through a tubing string that is run down
through the middle of the production casing string.

During drilling, the rig crew pumps a fluid, called “drilling mud” or
“mud” down the drill pipe and through the drill bit nozzles. Although the fluid
is referred to as “mud,” it is actually a complex system comprised of components
that are designed in light of, and tailored to, a variety of well conditions. The
mud’s primary function is to assist drill crews in maintaining well control.
Drilling mud exerts hydrostatic pressure in the drill pipe and annulus (the space
between the drill pipe and the walls of the casing strings or open hole) that is
equal to or greater than the pressures encountered in the wellbore, thereby
keeping the well balanced and under control. Drilling mud also cools the drill
bit and lifts cuttings to the surface as the mud is circulated during drilling. By
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closely monitoring well pressure, rig crews maintain the wellbore fluid pressure
so that it is equal to or slightly greater than the pressures from the formation.
This type of pressure balance is referred to as an “overbalanced” condition. By
contrast, a well is in an “underbalanced” condition when the formation pressures
exceed the wellbore drilling fluid pressures. Rig crews rely upon a number of
indicators to track fluid pressures.
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15 Operators routinely rely upon drilling mud suppliers to provide assistance with choosing an
appropriate drilling mud. Drilling rig crews include “mudloggers” who monitor drilling mud,
wellbore pressures, and other data.
16 This process of monitoring well pressures is referred to as “measurement while drilling.”
Operators also routinely rely upon drilling mud suppliers to provide assistance with choosing an
appropriate drilling mud and to provide “mudloggers” on the rig to monitor drilling mud,
wellbore pressures, and other data.

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