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to the surface, and helps protect the drill hole from being damaged
during drilling.
 Intermediate casing is usually the longest section of
casing found in a well. The primary purpose of intermediate casing
is to minimize the hazards that come along with subsurface
formations that may affect the well. These include abnormal
underground pressure zones, underground shales, and formations
that might otherwise contaminate the well, such as underground
salt-water deposits. Liner strings are sometimes used instead of
intermediate casing. Liner strings are usually just attached to the
previous casing with 'hangers', instead of being cemented into
place and is thus less permanent
 Production casing, alternatively called the 'oil string'
or 'long string', is installed last and is the deepest section of casing
in a well. This is the casing that provides a conduit from the
surface of the well to the petroleum producing formation. The size
of the production casing depends on a number of considerations,
including the lifting equipment to be used, the number of
completions required, and the possibility of deepening the well at a
later time. For example, if it is expected that the well will be
deepened at a later date, then the production casing must be wide
enough to allow the passage of a drill bit later on. It is also
instrumental in preventing blowouts, allowing the formation to be
'sealed' from the top should dangerous pressure levels be reached.
Once the casing is installed, tubing is inserted inside the
casing, from the opening well at the top, to the formation at the
bottom. The hydrocarbons that are extracted run up this tubing to
the surface. The production casing is typically 5 to 28 cm (2 -11
in) with most production wells being 6 in or more. Production
depends on reservoir, bore, pressure etc. and could be less than
100 barrels a day to several thousand barrels per day. (5000 bpd is
about 555 liters/minute). A packer is used between casing and
tubing at the bottom of the well.

Completion
Well completion commonly refers to the process of
finishing a well so that it is ready to produce oil or natural gas. In

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