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Casing Design
Vamegh Rasouli
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Casing Design  Introduction
What is casing?
Why run casing?1. To prevent the hole from
caving in,
2. Onshore: to preventcontamination of freshwater sands,
3. To prevent watermigration to producingformation,
Casing
Cement

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Casing Design  Why run casing  contd
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 totaldepth (TD).
e.g., you need 14 ppg to control a lower zone, but anupper zone will fracture at 12 lb/gal.
What to do?
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Typical Sequence of
Csg. Strings

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Functions of Casing Individually
Drive pipe Driven & cemented to
shallow depth in pre
drilled or predug holes
Provides a mud return
path to surface,
Prevents erosion of
ground below rig.
Conductor pipe Same as Drive pipe,
Supports the weight of next
casing strings,
Isolates very weak
formations.
Diverter installed to shale
shaker Corrosion barrier
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Functions of Casing Individually
Surface casing
Provides a means of
nippling up BOP,
Provides a casing seat
strong enough to safely
close in a well after a
kick,
Provides protection of
fresh water sands,
Provides wellbore
stabilization.
Intermediate casing
Usually set in the first
abnormally pressured
zone,
Provides isolation of
potentially troublesome
zones,
Provides integrity to
withstand the high mud
weights necessary to
reach TD or the next csg.
Seat.

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Functions of Casing Individuallycontd
Production casing Provides zonal isolation
(prevents migration ofwater to producing zonesand isolates differentproduction zones)
Confines production towellbore
Provides the environmentto install subsurfacecompletion equipment
Liners Drilling liners
Same as Intermediatecasing
Production liners Same as production casing
Tieback liners Tie back drilling or
production liner to thesurface. Converts liner tofull string of casing
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Types of Strings of Casing
Diameter Example
1. Drive Pipe or Structural Pile
(Gulf Coast and offshoreonly)
150300 BML
1660 30
2. Conductor String
100  1,600 BML1648 20
3. Surface Pipe
2,000  4,000 BML85/820 13
3/8

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Types of Strings of Casingcontd
Diameter Example
4. Intermediate String 75/8133/8 9
5/8
5. Production String 495/8 7
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Casing Programs

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Casing Programscontd
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Casing Selection
Chart

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Hole Size Pipe Size
36 Conductor casing 30
26 Surface string 20
17 Intermediate pipe 133/8
12 Intermediate String 95/8
77/8 Production Liner 51/2
Example Hole and StringSizes (in)
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Classification of CSG.
Outside diameter of pipe (e.g. 95/8)
Wall thickness (e.g. )
Grade of material (e.g. N80) Type to threads and couplings (e.g. API LCSG)
Length of each joint (e.g. Range III)
Nominal weight (e.g. 47 lb/ft)

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Most Common Grades Minimum Yield
Strength (KPSI)
Ultimate Tensile
Strength (KPSI)
H40 40 60
J55 55 75
K55 55 95
C75 75 95
L80 80 95
N80 80 100
C90 90 100
C95 95 105
P110 110 125
V150 150 160
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Length of Casing Joints
RANGELENGTH
(ft)
I 16  25
II 25  34
III > 34

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Casing Threads and Couplings
API round threadsshort ( CSG )
API round thread  long ( LCSG )
Buttress ( BCSG )
Extreme line ( XCSG )
Other
See Halliburton Book...
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Casing Threads and Couplingscontd
Rounded Threads8 threads per inch4 to 20
Square ThreadsLongerStronger 4 to 20
Integral JointSmaller ID, ODCosts moreStrong 5 to 103/4

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Wellhead & Christmas Tree
Wellhead
Hang Casing Strings
Provide Seals
Christmas Tree
Control Production
from Well
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Wellhead & Christmas Treecontd

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Casing Performance  Uniaxial
Loadings
Axial Tension (couplings & body)
Burst Pressure
Collapse Pressure
Bending Buckling
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Casing Performance  Uniaxial
Tension Strength/Failure

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Tension Strength
Tension Strength Couplings: API Tables
for various couplings
Body (perm. deform.)
Fy
= pipe body yield strength
D = external diameter (nominal)
d = internal diameter
YP = yield stress
Fy
=
4
D
2 d2
YP
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Tension StrengthExample 1
Compute the bodyyield strength for a 7, N80,
23 lb/ft casing.
Solution:
From API Table (1& 2)D = 7 in
d = 6:366 in
YP
= 80; 000 p si
Fy
=
4
72 6:366
2 80; 00 0 = 532 ksi

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Tension Strength Formula
Uses Nominal Diameter
API minimum Thickness 87.5% of original
(nominal) thickness
Yield Strength
Rupture much larger
May deform plastically
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Casing Performance  Uniaxial
Burst (Internal Pressure)
Yield the body
Yield the coupling Leak the coupling
P

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Burst (Internal Pressure)
Barlow (API allows 87.5% of thickness)
Thin Wall Assumption
Pbr = pipe body burst pressure
D = nominal diameter
YP = yield stress
Pbr
= 0:8 75
2 Y
Pt
D
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Burst (Internal Pressure)Example 2
Compute the body burst pressure for a 7, N
80, 23 lb/ft casing.
Solution:
From API TableD = 7 in
d = 6:366 in ! t =7 6:366
2= 0:317 in
YP
= 80; 000 psi
Pbr
= 0:875
2 80; 000 0:317
7
= 6; 340 psi

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Collapse (External Pressure)
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Collapse (External Pressure)contd
The following factors are important:
The collapse pressure resistance of a pipe
depends on the axial stress (biaxial stress) There are different regimes of collapse failure
(depends on ratio D/t) Yield Strength Collapse (thick wall)
Plastic Collapse
Transition Collapse
Elastic Collapse
(Empirical Formulation from API)

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Collapse (External Pressure)contd
Yield Stress Collapse Pressure (thick wall)
r(r) =
pi
r2i
r2
o
r2
+ po
r2o
r2 r2
i
r2 (r2o r
2i )
t(r) =
pi
r2i
r2
o+ r2
p
or2
o
r2 + r2i
r2 (r2
o r2
i)
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Collapse (External Pressure)contd
Yield Strength Collapse Pressure
PYp = pipe body collapse pressure
D = nominal diameter
t = wall thickness
YP = yield stress (effective for biaxial stress)
PYP = 2 YP (D =t) 1
(D =t)2

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Collapse (External Pressure)contd
Plastic Collapse Pressure
PP
= pipe body collapse pressure
D = nominal diameter
t = wall thickness
YP= yield stress (effective for biaxial stress)
A, B, C, F, G,Material (and stress) dependent coefficients
PP
= YP
A
D =t B
C
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Coef.s Formulas (API Bull. 5C3)

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Collapse (External Pressure)contd
Transition Collapse Pressure
PP
= pipe body collapse pressure
D = nominal diameter
t = wall thickness
YP= yield stress (effective for biaxial stress)
A, B, C, F, G, Material (and stress) dependent coefficients
PT
= YP
F
D =t G
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Collapse (External Pressure)contd
Elastic Collapse Pressure
PE = pipe body collapse pressure
D = nominal diameter
t = wall thickness
PE =
46:95 106
(D =t) [(D =t) 1]2

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Collapse (External Pressure)contd
A, B, C, F, G
These values are for the
uniaxial stress
Different values for
effective yield stress
For Biaxial calculate the
effective Yield Stress
and interpolate thecoefficients
(Coef.s depend on Yield Stress)
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Collapse (External Pressure)contd
(D=t)Y P
=
p(A 2)2 + 8(B + C = Y
P) + (A 2)
2 (B + C = Y P)

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Collapse (External Pressure)contd
Upper Limit for Plastic Collapse
(D=t)P T
=Y
P(A F )
C + Y P(B G)
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Collapse (External Pressure)contd
Upper Limit for Transition Collapse
(D=t)T E
=2 + B=A
3 B=A

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Collapse (External Pressure)contd
Boundaries for Axial Stress = 0
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Collapse (External Pressure)Example 3
Calculate the Collapse pressure rating for a 7 in, N80, 23
lb/ft casing.
Solution: 7 in, N80, 23 lb/ft t = 0.317 in
Grade A B C F G
N80 3.071 0.0667 1,955 1.988 0.0434
(D= t) =7
0:317= 2 2:08 ! Pl as tic collaps e for N80

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Collapse (External Pressure)Example 3
PP
= YP
A
D= t B
C
PP
= 80; 000
3:071
22:08 0:0667
1; 955 = 3; 836 psi
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Effect of Axial Stress in the Collapse
ResistanceEffective Yield Stress
Von Mises Criteria (Distortion Energy) Material fails (ductileyield failure) when totaldistortion energy equals uniaxial test distortion
energy
Triaxial Collapse
(a
t)2 + (
t
r)2 + (
r
a)2
= 2 Y2
P

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Triaxial
Biaxial
Triaxial Collapsecontd
Ye = effective yield stress
YP = uniaxial stresstsa =axial stress
pi = internal pressure (pi

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Coef.s Formulas (API Bull. 5C3)
A = 2:8762 + 0:10679 105 YP
+ 0:21301 1010 Y 2P 0:5 3132 1016 Y 3
P
B = 0:0 2623 3 + 0:506 09 106 YP
C = 465:93 + 0:030867 YP 0:10483 107 Y 2
P+ 0:36989 1013 Y 3
P
F =
46:95 106 h3 B =A
2 + B =A i3
YP
h3B =A
2 + B =A
B =Ai h
13B =A
2 + B =A
i2
G = F B =A
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Triaxial CollapseExample 4
For the casing of Example 3, calculate the
corrected critical collapse pressure if a section of
2,000 ft, 7 in, N80, 23 lbm/ft casing is
suspended below it (assume linear weight of 23
lbf/ft and empty borehole  no buoyancy effect).
What is the corrected collapse pressure if the
internal pressure is 1,000 psi?

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Triaxial CollapseExample 4
Solution:
Weight of Casing Below Point in Question
Cross Section Area
Axial Stress
F = 2; 000 23 = 46; 000 lbf
Ac
=
4
72 6:3 6 6
2= 6:6 55 5 i n2
a
=46; 000
6:6555= 6; 9 1 2 psi
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Triaxial CollapseExample 4
Effective Yield Stress (biaxial)
(an equivalent N76.32)
Ye
= rY 2P 3
a
2 2 a
2
Ye
=
s80; 0 00
2 3
6; 9 12
2
2 6; 912
2= 76; 3 20 p si

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Triaxial CollapseExample 4
Interpolated Coef.s
API Fs Formulas (MsExcel Spreadsheet)
Grade A B C F G
C75 3.054 0.0642 1,806 1.990 0.0418
N76.32 3.059 0.0649 1,845 1.992 0.0422
N80 3.071 0.0667 1,955 1.998 0.0434
Yp 76320
A B C F G
3.058 0.0649 1845 1.992 0.0422
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Triaxial CollapseExample 4
Collapse Regime
Yield Regime ULimit:
q(3:0 58 2)2 + 8 (0:0649 + 1;8 45
7 6;3 2 0) + (3:058 2 )
2 ( 0:064 9 + 1;8457 6;320
)= 13:5 4 < 22:08
(D = t) =7
0:3 1 7= 2 2:0 8
(D = t)Y P
=
p(A 2)2 + 8(B + C = Y
P) + (A 2)
2 (B + C = YP
)

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Plastic Regime ULimit:
Collapse occurs in the Plastic Regime
Triaxial CollapseExample 4
(D= t)P T
=Y
P(A F )
C + YP
(B G)
22:0 8 8,000 ft)
pp= 11,000 x 0.650 = 7,150 psi(10,4)xDnc= 6,600 ft
p6600= 7,1500.052x10.5x6,600=3,546 psi
pi= 3,546 psi

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Casing Design Example contd
Burst Pressurecontd:
F.S. = 1.1
pab= (pipo)
pab
= 3,5460.52D
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Casing Design Example contd
Collapse Loading
External Pressure:
po= 0.52 x D
Internal pressure = 0 psi
F.S. = 1.0
pac= 0.52 x D

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Casing Design Example contd
Design for Burst Start at bottom (minimum burst pressure)
pab,4000= 3,5460.52 x 4000 = 1,466 psi
Cheapest casing: (p.320321)
K55, 40.50 lb/ft, Burst Strength 3,130 psi
Minimum depth that can go:pab,D= 3,5460.52 x D = 3130 psi / 1.1
Dmin= 1347 ft
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Casing Design Example contd
Continue with next cheapest CasingK55, 45.50 lb/ft, Burst Strength 3,580 psi
Minimum depth that can go:
pab,D= 3,5460.52 x D = 3,580 psi / 1.1
Dmin= 561 ft

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Casing Design Example contd
Continue with next cheapest CasingK55, 51.00 lb/ft, Burst Strength 4,030 psi
Minimum depth that can go:
pab,D= 3,5460.52 x D = 4,030 psi / 1.1
Dmin= 226 ft (above surface)
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Casing Design Example contd
0 ft
561 ft
1347 ft
4000 ft
103/4K55 51.00 lb/ft
103/4K55 45.50 lb/ft
103/4K55 40.50 lb/ft
Burst Diagram

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Casing Design Example contd
Design for Collapse (uniaxial)
Start at top (minimum collapse pressure)
pac= 0.52 x D
Cheapest casing:
K55, 40.50 lb/ft, Collapse Strength 1,580 psi
Maximum depth that can go:pac,D= 0.52 x D = 1,580 psi / 1.0
Dmax= 3,038 ft
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Casing Design Example contd
Continue with next cheapest CasingK55, 45.50 lb/ft, Collapse Strength 2,090 psi
Maximum depth that can go:
pac,D= 0.52 x D = 2,090 psi / 1.0
Dmax= 4.019 ft

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Casing Design Example contd
0 ft
3038 ft
4000 ft
103/4K55 45.50 lb/ft
103/4K55 40.50 lb/ft
Collapse Diagram
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Casing Design Example contd
Combine Two Diagrams
0 ft
561 ft
1347 ft
4000 ft
3038 ft
+ =
103/4K55 51.00 lb/ft
103/4K55 45.50 lb/ft
103/4K55 40.50 lb/ft
103/4K55 45.50 lb/ft
collapse
burst