PETROLEUM ENGINEERING
ASSIGNMENT-02
CASING DESIGN
COMPELETION
NAME: FATIMA BABAR
CLASS: BS 7A (GEOPHYSICS)
DATE: 23-APRIL-2020
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CASING DESIGN
COMPELETION
TABLE OF CONTENTS
ABSTRACT
WHAT IS CASING
CASING DESIGN
o CASING STRING
o CRITERIA TO CHOOSE CASING
o TYPES OF CASING
o CASING POSITION WITH RESPECT TO
FORMATIONS
REFERENCES
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CASING DESIGN
COMPELETION
ABSTRACT
Casing design is an important task in oil and gas well design. Casing design involves evaluation of the
factors that contribute to the failure of the casing and proper selection of the most suitable casing grades
and weights that are both safe and economical for a specific job operation. A good knowledge of stress
calculation is very essential in casing design. During casing design, various modes of casing failure must
be identified and carefully handled such that the selected casing within a well segment is able to
withstand all the failure modes. A safety margin, (also known as factor of safety) is always provided in
casing de sign to allow for the future variations in the casingstrength, loading and other unknown forces
which may be encountered. This article provides key information, education, relevant and technical
details oncasing design for oil and gas well.
WHAT IS CASING
Casing, is a pipe with a large diameter that is assembled, inserted and cemented into a section of a
recently drilled wellbore. It is designed in such a way that it can bear various forces like burst, collapse
and tensile failure and chemically excessive brines as well. The basic data required for casing design is;
Hole size, Casing depth, Casing size, Mud weight, Casing grade, Burst safety factor, Collapse safety
factor, Tension safety factor, Formation fluid gradient, Gradient of invading fluid (gas gradient).
CASING DESIGN
1. Casing String
To design a casing string, one must have knowledge of:
Purpose of casing
o Keeping the hole open by preventing the weak formations from collapsing i.e., caving of
the hole.
o Serving as a high strength flow conduit to surface for both drilling and production fluids.
o Protecting the freshwater-bearing formations from contamination by drilling and
production fluids.
o Providing a suitable support for wellhead equipment and blowout preventers for
controlling subsurface pressure and for the installation of tubing and subsurface
equipment.
o Providing safe passage for running wireline equipment
o Allowing isolated communication with selectively perforated formations of interest.
Geological cross section
o Pore pressure
o Fracture pressure
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o Formation temperature
o Formation type and structure
o Plan for logging programs
o Location of problematic zones as possible loss zones, high permeable zones, unstable
zones, shallow gas hazard, fresh water zones, sour gas (H2S, CO2) zones, etc.
o Production Information
Production information
o Type of packer fluid
o Density of packer fluid
o Type of reservoir fluid
o Expected flow rate, pressure, temperature of the well
o Sour gas (H2S, Co2)
o Maximum production load case
o Completion strategies
Available casing and bit sizes
Cementing and drilling practices
Rig performance
Safety and environmental regulation
2. Criteria to choose casing
Casing damage is always the main problem in the process of oil field development, so the research of its
mechanism and influencing factors plays a great important role in ensuring casing security. In practice,
casing is economically designed against bursting, collapse, tension and bi-axial loads.
Casing Design for Collapse
Casing is usually designed against worst collapse conditions. The worst collapse conditions happen when
the casing inner portion is assumed to be empty and the annulus completely filled with the drilling mud.
Casing collapse stress increases with the mud hydrostatic depth. The casing is usually filled with mud
weight. At the top most segment of the casing where the depth is approximately zero, the collapse stress
is zero.
Casing Design for Burst
Casing is subjected to burst stress. In practice, it is safer to design the casing for unlimited condition.
Under unlimited casing design against burst, it is assumed that the invading fluid from the completely
fills the casing from the bottom to the top. At the top of the hole, the external pressure due to mud is zero
and the internal pressure must be supported entirely by casing body. Burst is the highest at the top and
least at the casing shoe.
Casing Design for Tension
Oil and gas well casings are designed against failure by tension. Tension stress on a casing is due to
own-weight of the casing and the weight of the other casings supported. The topmost casing segment in
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CASING DESIGN
COMPELETION
the well carries the highest tension stress while the last bottom casing in the well carries the least tension
stress. Since the casing is always submerged in the drilling mud, the effect of buoyancy on the casing is
therefore considered in the casing design. Buoyancy Factor can be determined by mud weight and steel
weight in pounds per gallon.
Casing Design Check for Biaxial Loads
Under tensile stress, the casing thickness reduces and the casing collapse strength also reduces. The
remaining collapse strength of the casing should be determined to ensure that casing selected can bear
the collapse stress at the maximum hydrostatic depth of the casing.
3. Types of Casing
Caisson Pipe
On an offshore platform, a caisson pipe, usually' 26 to 42 inches in outside diameter(OD), is driven into
the sea bed to prevent washouts of near surface unconsolidated formations and to ensure the stability of
the ground surface upon which the rig is seated. It also serves as a flow conduit for drilling fluid to the
surface. The caisson pipe is tied back to the conductor or surface casing and usually does not carry any
load.
Conductor Pipe
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CASING DESIGN
COMPELETION
The outermost casing string is the conductor pipe. The main purpose of this casing is to hold back the
unconsolidated surface formations and prevent them from falling into the hole. The conductor pipe is
cemented back to the surface and it is either used to support subsequent casings and wellhead equipment
or the pipe is cut off at the surface after setting the surface casing. Where shallow water or gas flow is
expected, the conductor pipe is fitted with a diverter system above the flow line outlet. This device
permits the diversion of drilling fluid or gas flow away from the rig in the event of a surface blowout.
The conductor pipe is not shut-in in the event of fluid or gas flow, because it is not set in deep enough
to provide any holding force. The conductor pipe, which varies in length from 40 to 500 ft onshore and
up to 1,000 ft offshore, is 7 to 20 inches in diameter. Generally, a 16-inch pipe is used in shallow wells
and a 20-inches in deep wells. On offshore platforms, conductor pipe is usually 20 inches in diameter
and is cemented across its entire length.
Surface Casing
The principal functions of the surface casing string are to: hold back unconsolidated shallow formations
that can slough into the hole and cause problems, isolate the freshwater-bearing formations and prevent
their contamination by fluids from deeper formations and to serve as a base on which to set the blowout
preventers. It is generally set in competent rocks, such as hard limestone or dolomite, so that it can hold
any pressure that may be encountered between the surface casing seat and the next casing seat. Setting
depths of the surface casing vary from a few hundred feet to as much as 5,000 ft. Sizes of the surface
casing vary from 7 to 16 inches in diameter, with 10 and l3 inches being the most common sizes. On
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CASING DESIGN
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land. surface casing is usually cemented to the surface. For offshore wells, the cement column is
frequently limited to the kickoff point.
Intermediate Casing
Intermediate or protective casing is set at a depth between the surface and production casings. The main
reason for setting intermediate casing is to case off the formations that prevent the well from being drilled
to the total depth. Troublesome zones encountered include those with abnormal formation pressures, lost
circulation, unstable shales and salt sections. When abnormal formation pressures are present in a deep
section of the well. intermediate casing is set to protect formations below the surface casing from the
pressures created by the drilling fluid specific weight required to balance the abnormal pore pressure.
Similarly, when normal pore pressures are found below sections having abnormal pore pressure, an
additional intermediate casing may be set to allow for the use of more economical, lower specific weight,
drilling fluids in the subsequent sections. After a troublesome lost circulation, unstable shale or salt
section is penetrated, intermediate casing is required to prevent well problems while drilling below these
sections. Intermediate casing varies in length from 7.000 ft to as much as 15.000 ft and from 7 inches to
1l inches in outside diameter. It is commonly cemented up to 1,000 ft from the casing shoe and hung
onto the surface casing. Longer cement columns are sometimes necessary to prevent casing buckling.
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CASING DESIGN
COMPELETION
Production Casing
Production casing is set through the prospective productive zones except in the case of open-hole
completions. It is usually designed to hold the maximal shut-in pressure of the producing formations and
may be designed to withstand stimulating pressures during completion and workover operations. It also
provides protection for the environment in the event of failure of the tubing string during production
operations and allows for the production tubing to be repaired and replaced. Production casing varies
from 4 51 inches to 9~5 inches in diameter, and is cemented far enough above the producing formations
to provide additional support for subsurface equipment and to prevent casing buckling.
Liners
Liners are the pipes that do not usually reach the surface, but are suspended from the bottom of the next
largest casing string. Usually, they are set to seal off troublesome sections of the well or through the
producing zones for economic reasons. Basic liner system, include:
Drilling liner
Drilling liner is a section of casing that is suspended from the existing casing (surface or intermediate
casing). In most cases, it extends downward into the open hole and overlaps the existing casing by 200
to400 ft. It is used to isolate abnormal formation pressure, lost circulation zones, heaving shales and salt
sections, and to permit drilling below these zones without having well problems.
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CASING DESIGN
COMPELETION
Production liner
Production liner is run instead of full casing to provide isolation across the production or injection zones.
In this case, intermediate casing or drilling liner becomes part of the completion string.
Tie-back liner
Tie-back liner is a section of casing extending upwards from the top of the existing liner to the surface.
This pipe is connected to the top of the liner with a specially designed connector. Production liner with
tie-back liner assembly is most advantageous when exploratory drilling below the productive interval is
planned. It also gives rise to low hanging-weights in the upper part of the well.
Scab liner
Scab liner is a section of casing used to repair existing damaged casing. It may be cemented or sealed
with packers at the top and bottom.
Scab tie-back liner
This is a section of casing extending upwards from the existing liner, but which does not reach the surface
and is normally cemented in place. Scab tie-back liners are commonly used with cemented heavy-wall
casing to isolate salt sections in deeper portions of the well.
Advantages of liners
The major advantages of liners are that the reduced length and smaller diameter of the casing results in
a more economical casing design than would otherwise be possible and they reduce the necessary
suspending capacity of the drilling rig. However, possible leaks across the liner hanger and the difficult)"
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in obtaining a good primary cement job due to the narrow annulus must be taken into consideration in a
combination string with an intermediate casing and a liner.
4. Casing Position with Respect to Formations
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CASING DESIGN
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REFERENCES
Casing Design Theory and Practice book by S.S. RAHMAN and G.V.
CHILINGARIAN
http://www.drillingformulas.com/casing-design-overview-overall-process-of-
how-to-do-casing-design-in-oil-and-gas-industry/
https://www.researchgate.net/publication/329360595_Relevant_Information_
on_Oil_and_Gas_Casing_Design
