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Dri ll ing Eng ineering
Lesson 5
Casing Design
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Casing Design
Why Run Casing?
Types of Casing Strings
Classification of CasingWellheads
Burst, Collapse and Tension
Example Effect of Axial Tension on Collapse Strength
Example
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Casing Design
Why run casing?
1. To prevent the hole from caving in
2. Onshore - to prevent contamination of
fresh water sands
3. To prevent water migration to
producing formation
What is casing? Casing
Cement
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Casing Design
4. To confine production to the wellbore
5. To control pressures during drilling
6. To provide an acceptable environment forsubsurface equipment in producing wells
7. To enhance the probability of drilling to total
depth (TD)e.g., you need 14 ppg mud to control a lower zone,
but an upper zone will fracture at 12 lb/gal.
What do you do?
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Types of Strings of Casing
1. Drive pipe or structural pile
{Gulf Coast and offshore only}
150-300 below mudline.
2. Conductor string. 100 - 1,600(BML)
3. Surface pipe. 2,000 - 4,000(BML)
Diameter Example
16-60 30
16-48 20
8 5/8-20 13 3/8
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Types of Strings of Casing
4. Intermediate String
5. Production String (Csg.)
6. Liner(s)
7. Tubing String(s)
7 5/8-13 3/8 9 5/8
Diameter Example
4 1/2-9 5/8 7
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Example Hole and String Sizes (in)
Structural casing
Conductor string
Surface pipe
IntermediateString
Production Liner
Hole Size
30
20
13 3/8
9 5/8
7
Pipe Size
36
26
17 1/2
12 1/4
8 3/4
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Example Hole and String Sizes (in)
Structural casing
Conductor string
Surface pipe
IntermediateString
Production Liner
250
1,000
4,000
Mudline
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Functions of Casing
IndividuallyConductor pipe
Provides a mud return path
Prevents erosion of ground below rig
Same as Drive pipe
Supports the weight of subsequent casing
strings Isolates very weak formations
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Surface casing
Provides a means of nippling up BOP
Provides a casing seat strong enough tosafely close in a well after a kick.
Provides protection of fresh water sands
Provides wellbore stabilization
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Intermediate or protective casing
Usually set in the first abnormally
pressured zone Provides isolation of potentially
troublesome zones
Provides integrity to withstand the highmud weights necessary to reach TD or
next csg seat
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Production casing
Provides zonal isolation (prevents
migration of water to producing zones,isolates different production zones)
Confines production to wellbore
Provides the environment to installsubsurface completion equipment
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Liners
Drilling liners
Same as Intermediate or protective casing
Production liners
Same as production casing
Tieback liners Tie back drilling or production liner to the
surface. Converts liner to full string of casing
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Classification of CSG.
1. Outside diameter of pipe (e.g. 9 5/8)
2. Wall thickness (e.g. 1/2)
3. Grade of material (e.g. N-80)
4. Type to threads and couplings (e.g. API LCSG)
5. Length of each joint (RANGE) (e.g. Range 3)
6. Nominal weight (Avg. wt/ft incl. Wt. Coupling)
(e.g. 47 lb/ft)
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se
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Length of Casing Joints
RANGE 1 16-25 ft
RANGE 2 25-34 ft
RANGE 3 > 34 ft.
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Casing Threads and Couplings
API round threads - short { CSG }
API round thread - long { LCSG }
Buttress { BCSG }
Extreme line { XCSG }
Other
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API Design Factors (typical)
Collapse 1.125
Tension 1.8
Burst 1.1
Required
10,000 psi
100,000 lbf
10,000 psi
Design
11,250 psi
180,000 lbf
11,000 psi
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Burst
Design for maximum pressure on the
inside of the casing. API design
recommendations call for the worstcase scenario, which is the casing is
empty, and no external pressure. The
pressure to design for is the estimatedformation pressure at TD for production
casing, or estimated formation pressure
at the next casing depth.
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Collapse
API design recommendations call for
worst case, where there is no pressure
inside the casing, and we design for themaximum mud weight at the casing
depth. We also allow for the reduction
of the collapse rating from the weight ofthe casing hanging below the depth of
interest.
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Tension
API recommendations call for worst
case, where there is no buoyancy
effect. Design is based on the weight ofthe entire casing string.
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Normal Pore Pressure Abnormal Pore Pressure
0.433 - 0.465 psi/ft gp
> normal
Abnormal
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P G
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X-mas TreeWing Valve
Choke Box
Master
Valves
Wellhead
Hang Csg. Strings
Provide Seals
Control Productionfrom Well
Press. Gauge
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Wellhead
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Wellhead
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Casing Design
Burst: Assume full reservoir pressure all along the wellbore.
Collapse: Hydrostatic pressure increases with depth
Tension: Tensile stress due to weight of string is highest at top
STRESS
Tension
Burst
Collapse
Collapse
Tension
Depth
Burst
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Casing Design - Collapse
Collapse pressure is affected by axial stress
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Casing Design - Tension
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Casing Design - Burst
(from internal pressure)
Internal Yield Pressure for pipe
Internal Yield Pressure for couplings
Internal pressure leak resistance
p pInternalPressure
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Casing Design - Burst
Example 1
Design a 7 Csg. String to 10,000 ft.
Pore pressure gradient = 0.5 psi/ft
Design factor, Ni=1.1
Design forburst only.
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Example
3. Select the appropriate csg. grade and wt.from the Halliburton Cementing tables:
Burst Pressure required = 5,500 psi
7, J-55, 26 lb/ft has BURST Rating of4,980 psi
7, N-80, 23 lb/ft has BURST Rating of6,340 psi
7, N-80, 26 lb/ft has BURST Rating of7,249 psi
Use N-80 Csg., 23 lb/ft
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23 lb/ft
26 lb/ft
N-80
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Collapse Pressure
The following factors are important:
The collapse pressure resistance of a pipe
depends on the axial stress
The API Design Factor
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Casing Design
Collapse pressure - with axial stress
1.
P
A
2/12
P
A
PPA
Y
S5.0
Y
S75.01YY
YPA= yield strength of axial stress
equivalent grade, psiYP= minimum yield strength of pipe, psi
SA= Axial stress, psi (tension is positive)
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Example 3
Determine the collapse strength for a 5 1/2 O.D.,
14.00 #/ft, J-55 casing under axial load of100,000 lbf
The axial tension will reduce the collapse pressure
as follows:
P
p
A
2
p
APA Y
Y
S5.0
Y
S75.01Y
psi
Area
FS AA 820,24
012.55.54
000,100
22
2
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Example 3 contd
The axial tension will reduce the collapse
pressure rating to:
psi216,38
000,55000,55
820,24
5.0000,55
820,24
75.01Y
2
PA
Here the axial load decreased the J-55
rating to an equivalent J-38.2 rating
P
p
A
p
APA Y
Y
S
Y
SY
5.075.01
2
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Example 3 - contd
We shall be using API Tables to correct for the
effect of axial tension on collapse strength of
casing.
The Halliburton Cementing Tables list the
collapse resistance of 5 -in, 14.00 lb/ft J-55
casing at 3,120 psi.
The axial tension in this case would derate the
collapse strength to about 2,550 psi.
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Casing Design Example
Example Problem
API Design Factors
Worst Possible Conditions
Effect of Axial Tension on Collapse Strength
Iteration and Interpolation
Design for Burst, Collapse and Tension
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Casing Design Example
Design a 9 5/8-in., 8,000-ft combinationcasing string for a well where the mud wt.
will be 12.5 ppg and the formation pore
pressure is expected to be 6,000 psi.
Only the grades and weights shown are
available (N-80, all weights). Use API
design factors.
Design for worst possible conditions.
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Casing Design - Solution
Before solving this problem is it necessary tounderstand what we mean by Design Factors
and worst possible conditions.
API Design FactorsDesign factors are essentially safety factors
that allow us to design safe, reliable casing
strings. Each operator may have his own setof design factors, based on his experience,
and the condition of the pipe.
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Casing Design
well use the design factors recommended by the
API unless otherwise specified.
These are the API design Factors:
Tension and Joint Strength: NT = 1.8
Collapse (from external pressure): Nc= 1.125Burst (from internal pressure): Ni = 1.1
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Casing Design
What this means is that, for example, if we
need to design a string where the maximum
tensile force is expected to be 100,000 lbf,
we select pipe that can handle 100,000 * 1.8= 180,000 lbfin tension.
Note that the Halliburton Cementing Tableslist actual pipe strengths, without safety
factors built in.
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Casing Design
Unless otherwise specified in a particularproblem, we shall also assume the following:
Worst Possible Conditions1. ForCollapse design, assume that the
casing is empty on the inside (p = 0 psig)
2. ForBurstdesign, assume no backup
fluid on the outside of the casing (p = 0 psig)
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Casing Design
Worst Possible Conditions, contd
3. ForTension design,
assume no buoyancy effect
4. ForCollapse design,
assume no buoyancy effect
The casing string must be designed to stand up to the
expected conditions in burst, collapse and tension.
Above conditions are quite conservative. They are also
simplified for easier understanding of the basic concepts.
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Casing Design - Solution
Burst Requirements(based on the expected porepressure)
The whole casing string must be capable of
withstanding this internal pressure without failing in
burst.
psi600,6P
1.1*psi000,6
FactorDesign*pressureporeP
B
B
Dep
th
Pressure
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Casing Design - Solution
Collapse Requirements
For collapse design, we start at the bottom of
the string and work our way up.
Our design criteria will be based on
hydrostatic pressure resulting from the 12.5
ppg mud that will be in the hole when thecasing string is run, prior to cementing.
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Casing Design
Collapse Requirements, contd
severelessare
tsrequiremencollapsetheholetheupFurther
.bottomtheatd'reqpsi850,5P
125.1*000,8*5.12*052.0
factordesign*depth*weightmud*052.0P
c
c
Depth
Pressure
C i D i
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Casing Design
Reqd: Burst: 6,600 psi Collapse: 5,850 psi
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Casing Design
Note that two of the weights ofN-80 casingmeet the burst requirements, but only the
53.5 #/ft pipe can handle the collapse
requirement at the bottom of the hole (5,850psi).
The 53.5 #/ft pipe could probably run all the
way to the surface (would still have to checktension), but there may be a lower cost
alternative.
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C i D i
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Casing Design
First Iteration
At what depth do we see this pressure (4,231
psig) in a column of 12.5 #/gal mud?
ft509,65.12*052.0
231,4
5.12*052.0
Ph
h*5.12*052.0P
c1
1c
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Casing Design
This is the depth to which the pipecould be run if there were
no axial stress in the pipe
But at 6,509 we have (8,000 - 6,509) =
1,491 of53.5 #/ft pipe below us.
The weight of this pipe will reduce the
collapse resistance of the 47.0 #/ft pipe!
8,000
6,509
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C i D i
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Casing Design
Interpolation between these values showsthat the collapse resistance at 5,877 psi
axial stress is:
psi148,4125.1
666,4P
psi666,4)600,4680,4(*)000,5000,10(
)000,5877,5(680,4P
cc1
1c
With the design factor,
2112
11c1P PP
SS
SSP
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Casing Design
This (4,148 psig) is the pressure at a
depth
Which differs considerably from theinitial depth of6,509 ft, so a second
iteration is required.
ft382,65.12*052.0
148,4h2
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Casing Design
Second Iteration
Now consider running the 47 #/ft
pipe to the new depth of6,382 ft.
psi378,6in572.13
lbf563,86S
lbf563,865.53*)382,6000,8(W
22
2
Casing Design
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Casing Design
Interpolating again,
This is the pressure at a depth of
psipcc 140,4600,4680,4*5000
5000378,6680,4
125.1
12
ft369,65.12*052.0
140,4h3
21
12
11c1
D.F.
1P PP
SS
SSP
Casing Design
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Casing Design
This is within 13 ft of the assumed value. If
more accuracy is desired (generally not
needed), proceed with the:
Third Iteration
psi429,6572.13
259,87S
lbf259,875.53*)369,6000,8(W
'369,6h
3
3
3
Pcc3 = ?
C
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Casing Design
Third Iteration, contd
2
3
140,4
)600,4680,4(*000,5
000,5429,6680,4125.11
cc
cc
Ppsi
Pthus
C i D i
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Casing Design
Third Iteration, contdThis is the answer we are looking for, i.e.,
we can run 47 #/ft N-80 pipe to a depth of
6,369 ft, and 53.5 #/ft pipe between 6,369and 8,000 ft.
Perhaps this string will run all the way to the
surface (check tension), or perhaps an evenmore economical string would include some
43.5 #/ft pipe?
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Casing Design
At some depth the 43.5 #/ft pipe would be
able to handle the collapse requirements,
but we have already determined that it willnot meet burst requirements.
!NO
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N-8053.5 #/ft
N-8047.0 #/ft
N-8043.5 #/ft?
Depth = 5,057?5,066?5,210?
Depth = 6,3696,369
6,3826,509
8,000
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Tension Check
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Tension Check
The Halliburton cementing tables give a
yield strength of1,086,000 lbffor the pipe
body and a joint strength of905,000 lbffor
LT & C.
surfacetoOKisft/#0.47