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• The large investments required to drill for oil
and gas are made primarily by oil companies.
• Small oil companies invest mostly in shallow,
less expensive wells drilled on land.
• Investments in expensive offshore wells can
be afforded only by large oil companies.
• A well is classified as a wildcat well if its
purpose is to discover a new petroleum
• The development well is to exploit a known
• Usually the geological group recommends
wildcat well location, while the reservoir
engineering group recommends development
• The drilling engineering groups make
preliminary well designs & cost estimates for
the proposed well.
• The legal group secures the necessary
drilling and production rights.
• Surveyors establish & stake well location.
• Usually the drilling is done by drilling
Contracting the Well to be Drilled
• When a contract to drill an oil well is signed, many
conditions and agreements are included. The
drilling rig contractor will have agreed to drill a well
to a specified depth. The conditions for payment are
included. The contract payment may be based on
the time on the location, a set sum of money, or by
• Most wells are contracted by the foot, but the
contract is likely to make allowances for unforeseen
problems and non drilling activities, such as time
spent allowing the casing cement to set.
Types of Casing
• Conductor Casing
• Surface Casing
• Intermediate Casing
• Production Casing
Figure 1: Typical Well Diagram
• A conductor string is a short length of large
diameter pipe run where the ground is soft. It
acts as a conduit to return drilling fluid to mud
pits and prevents "cave in," the sloughing of
ground around the rig base. It also protects
shallow water sands.
• ODs: 16, 18-5/8, 20, 24, 30 and 36 inches.
• Setting depths: 50 to 250 feet.
• Surface casing is the first string of casing
used after the conductor pipe. It is required in
some instances by law (to protect ground
water) and is normally cemented full length.
• Surface casing supports the BOP stack and
subsequent casing and tubing strings, and is
normally the only string designed to carry
• ODs: 13-3/8, 16, 18-5/8, 20, and 24 inches.
• Setting depths: 200 to 5,000 feet.
• Intermediate casing is any string between the
surface and production string. Intermediate
casing may or may not be cemented full
• Intermediate casing may be used to seal off
weaker zones from higher pressure drilling
fluids required in subsequent drilling. It may
also protect previous casing strings from
higher burst pressures, and provide support
for liner casing.
• ODs: 4-1/2 to 13-3/8 in.
• Setting Depths: 2000 - 20,000 ft.
• The production string is the primary pressure
containment string It spans the total well
depth and is not normally cemented full
• Allows segregation of producing formations
and undesirable fluids from contaminating
• ODs: 4-1/2, 5, 5-1/2, 7 and 7-5/8 in.
• Setting Depths: 2000 - 30,000 ft.
• Liners are short casing strings run to isolate
specific formations. As they are normally
short and relatively deep in the well, burst and
collapse pressures control the design.
• Also, it is normal to have only one weight,
grade and joint type in the string.
• Liners extend downward from the shoe of the
previous casing string, and are normally
cemented full length. Many liners use flush
joint connections for improved clearances.
• ODs: 4-1/2 to 9-5/8 in.
• Setting Depths: 5000 - 20,000 ft.
• Tubing conveys the oil or gas to the surface.
In some wells dual or triple strings of tubing
are used to isolate and produce different
zones. In other circumstances, a dual string
may be used where one produces and the
other carries fluid pumped in to kill the well or
to chemically inhibit the production string.
• Tubing is usually replaced on a regular basis
because of the wear caused by producing
fluids or gases.
• ODs: 1 to 4-1/2 in.
• Setting Depths: 1000 - 25,000 ft.
• MWD – after one stand
• EMS – taken every stand
• GSS – used when there
is magnetic interference
from casing string / fish.
• GMS – used when critical
depth measurement is
required for perforation,
Figure 2: Casing Diagram
Example of Well Survey
Figure 3: Well Survey