Page 30 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

• Conductor casing, which is usually no more than 20 to 50 feet (7-17
meter) long, installed before main drilling to prevent the top of the
well from caving in and to help in the process of circulating the
drilling fluid up from the bottom of the well.
• Surface casing is the next type of casing to be installed. It can be
anywhere from 100 to 400 meters long, and is smaller in diameter to
fit inside the conductor casing. Its primary purpose is to protect fresh
water deposits near the surface of the well from being contaminated
by leaking hydrocarbons or salt water from deeper underground. It
also serves as a conduit for drilling mud returning 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. Its primary purpose is to minimize the hazards associated
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 are 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 date. For example, if it is
expected that the well will be deepened later, then the production
casing must be wide enough to allow the passage of a drill bit later
on. It is also instrumental in preventing blow-outs, 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 inches 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.

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