This document provides an overview of casing design and casing point selection. It defines different types of casing strings, describes the casing point selection process and maximum load casing design method. Key points covered include mechanical properties of steel, criteria for selecting casing depths, kick tolerance calculation, and examples of kick tolerance calculations. The objective is for engineers to understand casing design principles and be able to select appropriate casing points.

1. Schlumberger
Private
Basic Casing Design and
Casing Point Selection
UTC Instructor

2. Schlumberger
Private
Objectives
On completion of this module the engineer should
be able to…
• Define the different types of casings.
• Describe the casing point selection process
• Describe the maximum load casing design
method.
• Demonstrate the bottom up method of casing
setting depth initial estimation.

3. Schlumberger
Private
3 UTC Instructor
Introduction
• Design Considerations
• Cost, (usually up to 17% of the total well cost)
• Bottomhole Pressure
• Service Conditions (casing handling)
• Material Properties
• Internal Yield
• Collapse
• Tension

4. Schlumberger
Private
Statement of Standard
All casing and tubing shall be designed to
withstand all loads that can be imposed on them
during installation and the lifetime of the well.
No well construction program shall be
commenced without an approved casing and
tubing design.

5. Schlumberger
Private
5 UTC Instructor
Mechanical Properties Of Steel
• API Standards
– Specification; API, 5A.
– Bulletins; 5C2, properties of casing, tubing, drill pipe.
– Recommended; API. RP7G, care and use of tubular.
• H2S & CO2
• Exposure to more than 0.05 psia of H2S pressure
and CO2 corrosion can lead to failure,
– Common practice is the use chromium alloy (casing type
L80 or Stainless steel)

6. Schlumberger
Private
Types of Casing Strings
• Conductor
• Surface
• Intermediate
• Drilling Liner
• Production

7. Schlumberger
Private
Types of Casing Strings
Conductor Casing
 Purpose
1. Provides mud returns to tanks.
2. Divert flow in case of emergency.
3. Support subsequent casing loads.
 Installation
1. Driven
2. Jetted
3. Drilled and cemented

8. Schlumberger
Private
Types of Casing Strings
Surface Casing
 Purpose
1. Protect Fresh water aquifers.
2. Provide wellbore integrity,
- Provides a BOP seal
- Allows drilling into abnormal pressure safely by
isolating shallow hazards.
Definition: Casing set at or above 6500’ or in sub-normal
pressure.
Setting depth is based on mechanical and regulatory
considerations

9. Schlumberger
Private
Casing Point Selection Criteria
Mechanical Considerations
• Ability of the weakest exposed formation beneath the
casing (usually shoe FG) to withstand the load imposed
by well control operation.
• Likelihood of differential sticking occurring while running
casing.

10. Schlumberger
Private
Types of Casing Strings
Intermediate Casing
 Purpose
1. Provide mechanical integrity.
- Case off problem zones.
- Allows higher blowout protection as it is set in more
competent formations than surface casing (higher shoe
strength)
Definition: Casing set to 6500’ or deeper in abnormal pressure.
Setting depth is based on mechanical considerations

11. Schlumberger
Private
Types of Casing Strings
Drilling Liners
 Purpose
1. Provide mechanical integrity as intermediate casing but at
lower cost.
Definition: Partial string of casing hung in previously set
casing string and is set to a depth greater than 6500’ or in
abnormal pressure.
Setting Depth is based on mechanical considerations

12. Schlumberger
Private
Types of Casing Strings
Production Casing/Liners
 Purpose
1. Provide isolation of the producing zone from other zones.
2. Withstand the anticipated loads during production and
testing operations for the wells life time.
Setting Depth is the last string across the production zone.
This may be a casing to surface or a liner that is hung.

13. Schlumberger
Private
13 UTC Instructor
Casing Point Selection
• Why is Casing set in the Hole?
• Casing is set for two Drilling reasons;
• To consolidate the hole already drilled, (steel filter cake)
• To provide the pressure control integrity to drill ahead.

14. Schlumberger
Private
14 UTC Instructor
Criteria For Selecting Casing
Depths
Each casing string is run as deep as possible based
on kick tolerance, unless other reasons dictate it
to be run higher.

15. Schlumberger
Private
15 UTC Instructor
Other Restrictions On Casing
Shoe Depth
• Wellbore Stability
Within the limit allowed by kick tolerance, we may restrict
the length of OH sections, to minimize deterioration of
wellbore with time
• Mud Requirements
Formations may affect casing depth (reactive shale)
• Directional Requirements.
Directional Problems may alter casing points (drag, torque)

16. Schlumberger
Private
Casing Point Selection Criteria
Other factors affecting casing setting depth.
1. Underground fresh water zones
2. Shallow hazards
3. Directional profile
4. Sidetrack requirement
5. ECD at shoe

17. Schlumberger
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17 UTC Instructor
Special Criteria for Conductor
The Conductor pipe needs to be set deep enough
to carry
subsequent axial loads from all other casing strings.
• It also must withstand the bending loads from
environmental conditions.

18. Schlumberger
Private
18 UTC Instructor
Special Criteria for Surface Casing
The Surface Casing is usually set in the first
competent
formation which is strong enough to close in on a
kick.

19. Schlumberger
Private
19 UTC Instructor
Special Criteria for Intermediate
Casing
The Intermediate Casing is set as deep as possible
to
allow sufficient shoe strength for drilling ahead.

20. Schlumberger
Private
20 UTC Instructor
Intermediate Casing - other
considerations
Intermediate casing may also be set for directional
or wellbore stability reasons
• Reduce torques and drags in an extended reach
hole.
• Case off possible differential sticking zones and
perform directional work below casing.
• Case off some problem zones prior to drilling
ahead.

21. Schlumberger
Private
21 UTC Instructor
Special Criteria for Production
Casing
The Production Casing is set through or just above
the
reservoir, depending on the type of completion to
be
used.

22. Schlumberger
Private
Casing Point Selection Criteria
Information gathered prior to casing design.
• Estimated pore pressure and rock strength using offset
data.
• The minimum and maximum casing sizes to be run at TD
that would allow for logging testing and a completion
program.
• The effects of geological uncertainties on casing setting
depths and the ability to safely circulate out the maximum
anticipated kick volume

23. Schlumberger
Private
Casing Point Selection Criteria
After gathering Information
• Develop a pore pressure and fracture gradient versus
depth plot.
• Plan the well from TD up.
- Determine the maximum formation pressure at TD.
- Add a Trip margin and determine minimum weight at TD.

24. Schlumberger
Private
Bottom Up Method
• Plot the pore pressure gradient curve
• Plot the Mud weight curve. The mud weight should
balance the highest PP in the OH with a TM of 0.5 ppg.
• Plot the estimated actual FG curve, and the designed FG
curve, which is FG less allowance for Well control, surge
or ECD.
• Start on the bottom on the mud weight curve and draw a
vertical line up to the designed FG curve. This is the initial
estimated production casing or liner.
• Cont…..

25. Schlumberger
Private
Casing Point Selection Criteria

26. Schlumberger
Private
26 UTC Instructor
Kick Tolerance

27. Schlumberger
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27 UTC Instructor
Kick Tolerance
Kick tolerance is the maximum
volume gas kick which can be
safely circulated out without
causing formation failure at the
weakest point of the open hole
(usually deemed to be the
casing shoe).
(SICP)
(SIDPP)

28. Schlumberger
Private
28 UTC Instructor
Kick Tolerance
.
Kick tolerance is a measure of the size of
a gas kick that can be handled.
The following assumptions are made…
 A gas influx comes from TD up to the casing point or stays at TD.
The kicking formation has a pore pressure equal to or greater than
the mud hydrostatic.
 Shut in casing pressure (SICP) = MAASP when the top of gas is at
the casing shoe, using the drillers method.
 Based on these assumptions, we calculate the volume of a gas kick.
This is the maximum size of gas influx, which is what we call KICK
TOLERANCE

29. Schlumberger
Private
29 UTC Instructor
Kick Tolerance
More assumptions
 For an Exploration or Appraisal well, we can assume that
the kicking formation may have a pore pressure gradient of
10% higher than the mud gradient.
 i.e. A planned mud gradient of 0.5 psi/ft will assume a pore
pressure gradient at the kick depth of 0.55 psi/ft.
 For a development well in a known area, assume that the
kicking formation may have a pore pressure which is equal
to the mud gradient.
 In this case any kick taken will be a swabbed kick.

30. Schlumberger
Private
30 UTC Instructor
The Well as a ‘U’-Tube - Static
The Pressure at Point 1 = Pressure at Point 2
P1 = BHP = SIDPP + HPDS
P2 = BHP = SICP + HPA
P1

31. Schlumberger
Private
31 UTC Instructor
The Well as a ‘U’-Tube - Static
Therefore: P1 = P2
SIDPP@surf + HPDS = SICP@surf + HPA =>
SIDPP@surf = SICP@surf - HPDS + HPA =>
SIDPP@surf = MAASP@surf – MgxTVDTD +(TVDTD – Hi)Mg + GgxHi
1st Step: Determine what the maximum height of influx can be when it
reaches the casing shoe*.
*Assumed that weakest formation is at the shoe
g
g
@surf
@surf
i
G
M
SIDPP
MAASP
H




32. Schlumberger
Private
32 UTC Instructor
The Well as a ‘U’-Tube - Static
• To find MAASP@surf & SIDPP @surf:
– P@Shoe = MAASP@surf + Mg x TVDshoe = >
MAASP@surf = P@Shoe - Mg x TVDshoe
– P@TD = SIDPP @Surf + Mg x TVDTD = >
SIDPP @Surf = P@TD - Mg x TVDTD

33. Schlumberger
Private
33 UTC Instructor
The Well as a ‘U’-Tube - Static
• Where:
When kick at the shoe : P@Shoe = MAASP @ shoe = Fg x
TVDshoe
When kick is at TD: P@TD = SIDPP@TD = {Mg+(10%Mg)} x
TVD TD Kick Gradient

34. Schlumberger
Private
34 UTC Instructor
Hydrostatic Pressure
Hydrostatic Pressure
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0 1000 2000 3000 4000 5000 6000
Pressure (psi)
TVD
(psi)
Hydrostatic Pressure

35. Schlumberger
Private
35 UTC Instructor
Hydrostatic Pressure & SIDPP
SIDPP
SIDPP@surf
Hydrostatic Pressure
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0 1000 2000 3000 4000 5000 6000
Pressure (psi)
TVD
(psi)
SIDPP
Hydrostatic Pressure

36. Schlumberger
Private
36 UTC Instructor
Hydrostatic Pressure & SIDPP &
MAASP
MAASP@surf
MAASP
SIDPP
SIDPP@surf
Hydrostatic Pressure
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0 1000 2000 3000 4000 5000 6000
Pressure (psi)
TVD
(psi)
MAASP
SIDPP
Hydrostatic Pressure

37. Schlumberger
Private
37 UTC Instructor
Hydrostatic Pressure & SIDPP &
MAASP
MAASP@Shoe
MAASP@surf
SIDPP@surf
SIDPP@TD
Mud Hydrostatic Pressure
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0 1000 2000 3000 4000 5000 6000
Pressure (psi)
TVD
(psi)
MAASP
SIDPP
Hydrostatic Pressure
MAASP@Surf = MAASP@Shoe - Hydro. P@Shoe
SIDPP@Surf = SIDPP@TD - Hydro. P@TD

38. Schlumberger
Private
38 UTC Instructor
MAASP@Shoe
MAASP@surf
MAASP
SIDPP
Hydrostatic Pressure
MAASP@Surf = MAASP@Shoe - Hydro. P@Shoe

39. Schlumberger
Private
39 UTC Instructor
Kick tolerance
2nd Step: The height of influx represents a volume:
V@shoe = Hi@shoe x Annular Capacity@shoe
So this is the influx volume which will cause the
pressure at the shoe to reach the maximum
allowable value when the kick reaches the shoe.

40. Schlumberger
Private
40 UTC Instructor
Kick tolerance
• However, we also have to consider the possibility
that the pressure at the shoe can reach the
Maximum allowable value when the kick enters
the wellbore!!!
• Therefore, we have to calculate the second kick
tolerance at TD.

41. Schlumberger
Private
41 UTC Instructor
Annular Gas Expansion
• Pressure reduces in gas as depth of
gas in well decreases.
• Gas expands as it rises and
pressure is reduced (Boyle’s Law).
• Gas height increases, mud height
decreases.
So: P1V1 = P2V2

42. Schlumberger
Private
42 UTC Instructor
Kick Tolerance
3rd Step: Therefore, we can calculate V@TD & the
height of the influx at TD at that V@TD :
P@Shoe x V@Shoe = P@TD x V@TD
And
@TD
@TD
@TD
Capacity
Annular
V
H 
@TD
@Shoe
@Shoe
@TD
P
V
x
P
V 

43. Schlumberger
Private
43 UTC Instructor
Kick Tolerance
The final kick tolerance is determined by which
of either of the two kick tolerances is the
smallest:
 If V@Shoe> V@TD , and if H@shoe >H@TD,then V@TD
is the kick tolerance
 If V@Shoe> V@TD , and if H@shoe < H@TD,then
recalculate V@TD for critical height H@Shoe at TD.
 If V@Shoe< V@TD, then we need to calculate a new
volume using the critical height (H@Shoe) as if it
were around the BHA and DS.

44. Schlumberger
Private
44 UTC Instructor
Kick Tolerance Rule of Thumb
• If MAASP psi  target Vertical Depth in feet < 0.1,
kick tolerance is going to be low.
• If MAASP psi  target Vertical Depth in feet > 0.1,
kick tolerance is probably going to be OK.

45. Schlumberger
Private
45 UTC Instructor
Kick Tolerance
Example 1:
Calculate the Kick Tolerance for the following scenario.
Casing shoe at 6000’ with a FG of 0.72 psi/ft, plan to drill to
the next casing point at 8500’ with a mud gradient of 0.62
psi/ft in a vertical exploration well. (Kick gradient 10%more
than mud gradient.)
Assume a gas gradient of 0.12 psi/ft at the casing shoe,
and 12 ¼” hole with 5” DP and 300’ of 8” drill collars.

46. Schlumberger
Private
48 UTC Instructor
Kick Tolerance
Example2:
Calculate the Kick Tolerance for the following scenario.
Casing shoe at 5000’ with a FG of 0.65 psi/ft, plan to drill to
the next casing point at 7500’ with a mud gradient of 0.55
psi/ft in a vertical exploration well. (Kick gradient 10%more
than mud gradient.)
Assume a gas gradient of 0.12 psi/ft at the casing shoe,
and 12 ¼” hole with 5 7/8” DP and 300’ of 8” drill collars.

47. Schlumberger
Private
Maximum Load casing
Design
• This is a method for selecting specific casing based on
operational conditions and resulting stresses.
• Concept: Design for most severe realistic load to minimize
risk of failure.
• Maximum load cases are based on geographical region,
geologic section and organizational philosophy.

48. Schlumberger
Private
Maximum Load casing
Design
• Design Cases:
– Burst
– Collapse
– Tension
– Special considerations:
– H2S/CO2, Temperature, Multi-axial stress correction,
Stability.

49. Schlumberger
Private
Steps for Basic Casing
Design
Develop Maximum
Load case condition
for burst
Calculate resulting
loads, (Design Line)
Select casing strings
with load capacity
>/= Load line
Burst Design Collapse Design
Develop Maximum
Load case condition
for Collapse
For casing selected
in burst design,
check that load
capacity >/= Load
line
Develop Maximum
Load case condition
for Tension
Tension Design
Calculate resulting
loads, (Design Line)
Calculate resulting
loads, (Design Line)
Multiply design line
by safety factor,
(Load Line)
Multiply design line
by safety factor,
(Load Line)
Multiply design line
by safety factor,
(Load Line)
For casing selected
in collapse design,
check that load
capacity >/= Load
line

50. Schlumberger
Private
52 UTC Instructor
Minimum Design Factors
DESIGN LOADS Surface & intermediate
casings, drilling liners
Production
casings/ liners
Tubing
Collapse 1.0 1.1 1.125
Burst
- normal service
- critical service
1.1
1.25
1.1
1.25
1.1
1.25
Tension
pipe body
connection
1.3
1.5
1.3
1.5
1.3
1.3
Compression 1.3 1.3 1.3
Triaxial 1.25 1.25 1.25

51. Schlumberger
Private
Casing Burst Design
Maximum pressure at casing shoe,dependent on FG. (Internal Pre
s
fg
inj D
SF
EMW
P )
(
052
.
0 

The maximum internal burst loading pressure at
surface is a function of the injection pressure, and is
given as:
s
g
shoe
inj
isurf D
G
P
P 
 @
Load Line
External pressure due to annular drilling fluid….0.465psi/ft
D
G
P f
e 
Worst case scenario is gas filled above FG+SF

52. Schlumberger
Private
Casing Burst Design
Pressure with burst design factor of 1.1
Design Line
)
( @
@ e
shoe
inj
shoe
br P
P
P 

@ shoe
@ surface )
( e
isurf
brsurf P
P
P 

@ shoe
@ surface
b
br
b DF
P
P 
b
brsurf
bsurf DF
P
P 
From the casing Tables choose appropriate casing

53. Schlumberger
Private
Casing Collapse Design
Load Line
Worst case scenario is full evacuation with Mud on the outside
D
P m
e )
(
052
.
0 

Maximum collapse pressure at shoe, (external pressure)
The casing is empty and open to atmospheric
pressure. This will render the surface pressure to be 0
psi
Design Line
1
.
1
)
( e
C P
P 
From the casing Tables check casing collapse rating

54. Schlumberger
Private
Casing Tension Design
Worst case scenario is stuck casing with an over pull 100,000 lbs a
As per SLB casing manual, TVD should be used for gravity relate
Tensile load calculation. If Biaxial stress calculations are not bein
made then a safety factor of 1.6 shall be used.
Load Line
lbs
wL
F TVD
wt 000
,
100


Design Line
SF
F
F wt
wtd )
(

From the casing Tables check casing tensile rating
Tensile load will change if pumps are on while pulling on casing

55. Schlumberger
Private
57 UTC Instructor
Casing Sizes Decision Tree
Tubing size, in
Casing & liners
size, in
Bit & hole
size, in
Casing size, in
Casing & liners
size, in
Bit & hole
size, in
Casing & liners
size, in
Bit & hole
size, in
Casing size, in
Bit & hole
size, in
16
14-3/4
11-3/4
11-7/8
10-5/8
8-5/8
7-7/8
5-1/2
5-3/4
4-1/2
3-1/2
1.9
20
17-1/2
13-3/8
14
12-1/4
8-3/4
6-1/8
4-1/2
2-3/8
10-3/4
9-1/2
7-5/8
7-3/4
6-1/2
5
24
20
16
14-3/4
11-3/4
11-7/8
10-5/8
8-5/8
7-7/8
5-1/2
2-7/8
30
26
20
17-1/2
13-3/8
14
12-1/4
9-5/8
8-1/2
8-3/4
7
3-1/2
18-5/8
9-5/8
9-7/8
8-1/2
7
5-7/8
6-5/8
4-3/4
4
2-1/16

56. Schlumberger
Private
58 UTC Instructor
API CASING SPECIFICATIONS
• API Specifications (5A, 5AC and 5AX)
• Weight of Casing
• Nominal Weight Expressed in lb/ft or kg/mt ( Weight of CSG plus tool joint ).
• Plain End Weight Expressed in lb/ft or kg/mt ( Weight of CSG w/out tool joint) .
• Length of Casing
Range Length (ft) Average length (ft)
1 16 -25 22
2 25 -34 31
3 Over 34 42

57. Schlumberger
Private
59 UTC Instructor
API CASING SPECIFICATIONS
• Colour Grade Identifications

58. Schlumberger
Private
60 UTC Instructor

59. Schlumberger
Private
61 UTC Instructor
Maximum Load Case - Example
• Scenario for burst- Gas filled at shoe fracture pressure and normal
pressure gradient outside.
• Scenario for collapse- Total evacuation, drilling mud in the annulus.
• Scenario for tension- no biaxial and triaxial calculations applied.
Exploration Well
9 5/8" Casing 47 ppf N-80 setting depth 9500' TVD
Mud weight casing will be set in is 12.5 ppg
Normal pressure gradient is 0.465 psi/ft
Gas gradient is 0.12 psi/ft
Fracture gradient at shoe 16 ppg
Burst design factor 1.1
Collapse design factor 1.0
Tension design factor 1.6

60. Schlumberger
Private
62 UTC Instructor
Maximum Load Case – Example 2
• Scenario for burst- Gas filled at shoe fracture pressure and normal
pressure gradient outside.
• Scenario for collapse- Total evacuation, drilling mud in the annulus.
• Scenario for tension- No biaxial and triaxial calculations applied.
Development well
-13 3/8" Casing /Setting depth 5000' MD/3500’TVD
-Mud weight casing will be set in is 10 ppg
-Normal pressure gradient is 0.465 psi/ft
-Gas gradient is 0.12 psi/ft
-Fracture gradient at shoe 14.5 ppg
-Burst design factor 1.1
-Collapse design factor 1.0
-Tension design factor 1.6

61. Schlumberger
Private
63 UTC Instructor
Kick Tolerance
Example:
Calculate the Kick Tolerance for the following scenario.
Casing shoe at 3500’ with a FG of 0.72 psi/ft, plan to drill to
the next casing point at 8500’ with a mud gradient of 0.56
psi/ft in a vertical exploration well. (Kick gradient 10%more
than mud gradient.)
Assume a gas gradient of 0.12 psi/ft at the casing shoe,
and 12 ¼” hole with 5 1/2” DP and 300’ of 8 1/4” drill
collars.