This document outlines the key considerations for developing an effective casing program. It discusses the different types of casings used, including conductor, surface, intermediate, production casing, and liners. The functions of each casing type are described. Important factors to determine casing setting depths are mentioned, such as formation pressures and stability, as well as factors for the production casing like completion method and expected production. The advantages of smaller hole sizes for cost reduction are balanced with the need for sufficient diameter for completion and production.

1. Drilling Engineering 2 Course (1st Ed.)

2. 1.
2.
3.
4.
5.
6.
Laminar Flow in Pipes and Annuli
Turbulent Flow in Pipes and Annuli
Pressure Drop Across Surface Connections
Pressure Drop Across Bit
Optimization of Bit Hydraulics
Particle Slip Velocity

3. Casing functions
Casing costs compromise
one of the largest cost items of the drilling project.
Therefore proper planning of
casing setting depths and casing selection is vital
to realize a cost effective and safe well.
The casings themselves fulfill multiple functions
that can be summarized as:
Isolate porous formations with different fluid-pressure
regimes from contaminating the pay zone,
Prevent near surface fresh water zones
from contamination with drilling mud,
Protect the hole from caving in,
Provide a connection and support of the wellhead equipment,
Provide exact dimensions for running
testing, completion and production subsurface equipment.
Fall 13 H. AlamiNia
Drilling Engineering 2 Course (1st Ed.)
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4. Casing Types
According to the different
functions,
the total casing program
consists of
different casings strings.
Conductor Casing
Surface Casing
Intermediate Casing
Production Casing
Liners
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Drilling Engineering 2 Course (1st Ed.)
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5. Conductor Casing
The function of the conductor is
to enable circulation of the drilling fluid to
the shale shakers without eroding
the surface sediments directly below the rig foundation.
The conductor prevents the subsequent casings from
corrosion and may partly support the wellhead weight.
Commonly a diverter is installed on top of the
conductor casing to divert an unexpected inflow
of formation fluids into the wellbore
away from the rig-site and the personal.
Conductor setting depths are
in the range of 150 to 600 [ft] (46-183 m),
their seizes range from 36 to 20 [in] (0.91-0.5 m).
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Drilling Engineering 2 Course (1st Ed.)
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6. Surface Casing
The function of the surface casing is
 to prevent cave in of unconsolidated, weak near-surface formations
as well as protect the shallow, freshwater sands
from contamination with drilling mud.
 As the conductors,
surface casing protects the subsequent casings from corrosion.
Before the surface casing is set,
 no blow out preventers (BOP) are installed.
 After setting the surface casing and installing the wellhead, a BOP is
available to handle kicks when drilling the intermediate hole section.
Surface casing setting depths are in the range
from 300 to 5,000 [ft] (91-1524 m),
their diameters range from 24 to 17-1/2 [in].
 the surface casing setting depth is often determined by government
or company policy and not selected due to technical reasoning.
Fall 13 H. AlamiNia
Drilling Engineering 2 Course (1st Ed.)
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7. Intermediate Casing
The intermediate casing string
is a purely technical casing.
One or more may be necessary to handle abnormal
formation pressures, unstable shale formations, lost
circulation or cave-in zones.
An intermediate casing may also be necessary to realize
the planned mud weight profile.
When for example an abnormally pressured formation is
encountered, it may have to be protected by an intermediate
casing so when formation pressure of the formations below is
normal, a lower mud weight can be applied.
Intermediate casing diameters
range from 17-1/2 to 9- 5/8 [in].
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Drilling Engineering 2 Course (1st Ed.)
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8. Production Casing
The production casing is set through the
prospective production zone(s).
This casing string protects the environment
in case of production tubing failure and
permits the tubing string to be maintained or
replaced during the production life.
Commonly production casing and production liners
have gas-tight connections,
their diameters range from 9-5/8 to 5 [in].
A production casing diameter of 7 [in] is encountered often.
Fall 13 H. AlamiNia
Drilling Engineering 2 Course (1st Ed.)
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9. Liners
To save cost, the casing installed
sometimes doesn’t reach until the
surface but finishes within the
previous string.
Such a casing configuration is called
liner.
A liner is mounted on a so called
“liner hanger” to the previous casing
string.
Commonly the liner head is several
hundred feet into the previous
casing to enable a good cement seal.
Liner
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Drilling Engineering 2 Course (1st Ed.)
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10. Various typical casing programs
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Drilling Engineering 2 Course (1st Ed.)
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11. develop a casing program
To develop a casing program, first the various
casing setting depths have to be determined.
Since the primary reason to drill a well is
to produce hydrocarbons out of a reservoir,
the final casing inside diameters
have to be large enough to allow
for the forecasted completion and production schemes.
Factors like completion type
(open hole, cased hole, monobore production, etc.),
expected amount of production (production tubing seize),
expected production forecast (e.g. need of gas-lift, etc.) and
seize of evaluation tools to be run have to be considered.
In general, for production purposes
the well diameters shall be as large as possible.
Fall 13 H. AlamiNia
Drilling Engineering 2 Course (1st Ed.)
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12. Advantages of small hole sizes
On the other hand, as small as possible hole seizes
reduces the total cost of the well since:
drilling times are faster,
less mud has to be used (purchase and disposal of
mud),
smaller mud equipment can be used (cleaning, pumps, etc.),
smaller casings can be used (cheaper, higher strength at same
grade),
smaller rig can be applied (lighter casings, smaller mud
volume),
rig site can be smaller (especially important offshore and
platform types like TLP where weights are limited).
Out of this reason a technology called slim-hole drilling
was developed.
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Drilling Engineering 2 Course (1st Ed.)
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