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Casing and Casing Design: Introduction
Casing seat selection determines the total no. Of casings required in a well
Casing seat selection also determines the depth of each casing.
Bore-hole geometry determines the hole size and their corresponding diameters of casings
Casing and Casing Design: Objective
Depth and diameter of a casing is known from previous exercises
The selected casing must withstand various loads which might impose on casings during operations/ entire life
The designed casing must be as economical as possible
Casing design determines the grade(s), nominal weight(s), and types of thread required for a particular well considering safety & cost-effectiveness
Casing and Casing Design: Influencing Factors
Loading conditions during drilling and production
Formation strength Thermal effects Corrosive environment Hole irregularities Availability of casings
Casing And Casing Design: Input Data
Formation & Fracture pressure profile Location of lost circulation and permeable zones Location of salt zone Type of well (vertical/ directional/ horizontal) Temperature profile Presence of H2S, CO2 & Nacl
Minimum hole size required Type of completion Sp. Gravity of packer fluid Worst case loads that may occur during
completion/production/ work over operations
Availability of casings / inventory Regulatory requirements
Casing And Casing Design: Design Criteria
Burst Collapse Axial tension / compression Biaxial Bending Buckling Corrosion
Casing And Casing Design: Loading Conditions – Burst
BURST CONDITION WHEN pi > pe
Pi
Pe
INTERNAL PRESSURE(LOAD) = piEXTERMAL PRESSURE(BACK-UP) = pe
Net stress imposed on casing or ‘resultant’ =load – back up = pi - pe
Casing And Casing DesignConditions–collapse
Pi
Pe
EXTERNALPRESSURE(LOAD) = peINTERNAL PRESSURE(BACK-UP) = pi
COLLAPSE CONDITION WHEN pe > pi
NET STRESS IMPOSED ON CASING OR ‘RESULTANT’ =LOAD – BACK UP = Pe - Pi
Casing And Casing Design: Loading Condition-tension
Most axial tension arises from weight of casing itself.
Other tension loadings can arise due to bending, drag, shock loading and during pressure testing.
Increase in temperature and pressure can impose tension loadings in casing
Casing and casing designcasing design: safety factors
Because of uncertainties in determining actual loadings and as well as casing properties a factor is used to allow for such uncertainties and to ensure that casing properties always remain greater than loadings.
This factor is called ‘design factor’ or ‘safety factor’
Safety factor is defined as the ratio of rating of casing and resultant loadings
Casing And Casing Design: Safety factors (Contd.)
For example, safety factor in burst
=Burst resistance of casing
Resultant burst loading
Casing And Casing Design: Safety Factors(contd.)
Oil industry has no uniform policy on safety factors of casing design
Safety factors are normally decided by the individual company in accordance of their company policy.
Following safety factors are used in ongc(I) BURST – 1.1 to 1.125(Ii) COLLAPSE – 0.85 (cemented portion)
-1.125 (uncemented portion)(Iii) TENSION – 1.8 (without buoancy)
- 1.6 ( with buoyancy)
Casing and casing design: Design approaches
In oil industry, various approaches to design casing are followed.
However, two most widely used approaches are ;
(i) Conventional(Ii) Maximum load concept
Approaches are different from one another due to different assumptions in loads and back ups
Casing And Casing Design: Load Determination
(CONV-BURST)
(SURFACE-INTER-PROD)
CEMENT
CSD
SURFACE
NEXT SHOE
OPEN HOLE
Assumptions:-A kick generates at next shoe depth
-Mud inside casing & open hole is thrown out by gas and casing is full of gas inside
Casing And Casing Design: Assumptions
Back-up:- Barytes in mud behind casing would be settled at bottom in course of time and thereby saline water column would remain in annulus
CSD
SURFACE
NEXT SHOE
OPEN HOLE
SALINEWATER
Casing And Casing Design: Computation
(SURFACE-INTER-PROD)
CSD
SURFACE
NEXT SHOE
OPEN HOLE
SALINEWATER
Load at surface -1 = Formation pressure at next shoe depth
e .0001138 x 0.65 x depth (metres))
Load at surface - 2 =
Formation pressure at next shoe depth - Hydrostatic pressure of gas column
Casing And Casing Design: Computation
(SURFACE-INTER-PROD)
SALINEWATER
CSD
SURFACE
NEXT SHOE
OPEN HOLE
Load at surface – 3 = Fracture pressure at casing shoe depth
e .0001138 x 0.65 x depth (metres)
Use greater of the three above in case of exploratory well for safety and minimum one for development well
Casing And Casing Design: Computation
(SURFACE-INTER-PROD)LOAD AT CASING SHOE
USING EQUILATERAL TRIANGLEBCDE
Next shoe depth
Dep
th
CSD
0
Pressure
Load at surface
Load at CSD
0
A BC
D E
F
= FB
FD
LOAD AT CASING SHOE = GD + DE
G
Casing And Casing Design: Computation
(SURFACE-INTER-PROD)
BACK UP AT SURFACE = 0BACK UP AT CSD =HYDROSTATIC PRESSURE OF SALT WATER= 0.052x CSD X SP. GRAVITY OF SALT WATER
(PSI)= CSD X SP. GRAVITY OF SALT WATER ÷ 10 (KG/ CM2 )
Casing And Casing Design: Computation
(SURFACE-INTER-PROD)RESULTANT
RESULTANT AT SURFACE = SURFACE PRESSURE – 0
RESULTANT AT CSD = LOAD AT CSD – HYDROSTATIC PR. OF
SALT WATER
Casing And Casing Design: Load Lines
GRAPHICAL REPRESENTATION
Load at surface
Load at CSD
Load line
Back up line
Resultant
Dep
th
Pressure
Casing And Casing Design
COLLAPSE
Casing And Casing Design: Load Determination
(CONV- COLLAPSE) (SURFACE-INTER-PROD)
CSD
Active mud in annulus
CasingLoad at surface = 0
Load at CSD ( in kg/cm2) = hydrostatic Pr. Of mud used during casing lowering
Load at CSD ( in kg/cm2) = depth(m) X mudweight(gm/cc)/10
Casing And Casing Design: Load Determination BACK-UP (CONV- COLLAPSE) (SURFACE-
INTER-PROD)
CSD
Active mud in annulus
Casing
Assumption:
Casing is totally empty Inside due to mud loss During drilling next phaseIn case of surface & Intermediate casing.
In case of production Casing, assumption is Same but due to artificial Lift & plugged formation
Casing And Casing Design: Load Lines
GRAPHICAL REPRESENTATION (COLLAPSE)Load at surface = 0Back up at surface = 0Resultant at surface = 0
Load at CSD = Hyd Pr. Active mudBack up at CSD = 0Resultant at CSD = Hyd. Pr of Active mud
Load line = resultantCSD
Pressure
Dep
th
Load line
Back up line
0
Casing And Casing Design
TENSION
Casing And Casing Design: Determination
Tension load is primarily due to the casing’s own weight
Tension load increases during pressure testing of casing.
Tension load also increases due to increase in temperature
Increase of sp. Gravity of mud both outside and inside of casing increases tension in casing.
Casing And Casing Design: Computation
Tension load = weight of casing in air/ unit length x depth
= Kg/ m x depth in metre (kgs)
= PPF(Lbs /ft) X Depth (metre) x1.489 (Kgs)
Casing And Casing Design: Computation
Other axial loads – shock load Shock loading is often expressed as
F shock = 3200 wn lbs where, wn = ppf
= 1450 wn kgs where, wn = kg/m Considering average running speed of
185 ft/min or 56 metre/min
Casing And Casing Design: Computation
Other axial loads – bending force
Bending force fb = 63 dwn lbf
Where, D = OD in inch
Wn = nominal weight, ppf
= rate of angle change/ 100ft.
Casing And Casing Design: Computation
Other axial loads – temperature CHANGE IN AXIAL FORCE DUE TO
TEMPERATURE CHANGE = - E t
Where, E = young’ modulus of steel
= 30 X 106 psi for steel
= Thermal coefficient of expansion
= 6.9 x 10-6 0F-1
T = average change in temperature( 0F)
Casing And Casing Design: Computation
Normally, shock load & bending loads are not considered unless specific conditions are expected in well.
Also, in general, temperature will typically have a secondary effect on tubular design
These loads are not generally considered in casing design
Casing And Casing Design
BIAXIAL
Casing And Casing Design: Bi-axial
All pipe strengths are based on uniaxial stress state.
Pipe in the well bore, however, is always subjected to combined loading conditions.
Fundamental basis of casing design is that if stress in pipe wall exceed yield strength of material, a failure condition exists
Hence, yield strength is a measure of maximum allowable stress.
Casing And Casing Design: Bi-axial
Published collapse resistances of casings are under zero axial load.
Axial tension reduces the yield strength of material.
In three modes out of four modes of collapse resistances equations, except elastic collapse, collapse strength is directly proportional to the yield strength of material.
It follows that tension decreases both yield strength and collapse resistance of casing.
Casing And Casing Design: Computation
Graphical representation of hoop stress-axial stress on % of yield biaxial ellipse is available.
For easy application, a table comprising factors ‘x’ and ‘y’ is calculated from the above ellipse and readily available
Reduced collapse resistance of casing under axial loading can be determined from this table
Casing And Casing Design: Computation
Determination of reduced collapse resistance of casing under axial loading using ‘x’&‘y’ factor
X = axial load / pipe body yield strength
Obtain value of ‘y’ from table corresponding to ‘x’
Reduced collapse strength = published collapse strength x ‘y’
Casing And Casing Design: Comments On Biaxial Stress
Neither approach is rigorous treatment of the topic
Depending on the type of load, burst & collapse rating of zero axial stress increases or decreases
Tensile loads increases burst rating but decreases collapse rating
Compressive loads increases collapse rating but decreases burst rating
Casing and casing design: example
Design the casing using conventional approach with the following input data:
(a) Casing size : 9-5/8”(b) Casing shoe depth : 3000 m(c) Next casing shoe depth : 4200 m(d) Formation pressure at 3000m : 1.32 mwe(e) Formation pressure at 4200 m : 1.6 mwe(f) Sp. Gravity of mud during lowering : 1.36(g) Sp. Gravity of mud in next phase : 1.65
Casing And Casing Design: Example
(h) Fracture pressure at 3000m : 1.8 mwe(i) Type of well : vertical/ exploratory(j) Following casing are available:
N-80, 53.5 ppf, BTC– 2000 mN-80, 47 ppf, BTC – 1500mN-80, 43.5 ppf,BTC – 2000mCONSIDER FOLLOWING SAFETY FACTORS :BURST – 1.1, collapse – 1.125Tension – 1.8 (neglecting buoyancy) biaxial effects are to be considered
Casing And Casing Design: Burst
SOLUTION Inside Pressure
FORMATION PR. AT 4200M = 1.6 X 4200
10= 672 Kg/ Cm2
SURFACE PR. = 672
e .0001138 X .65 X 4200= 492 Kg/ Cm2
LOAD AT 3000 M
X=180 X 3000
4200= 128
= 492 + 128
= 620
3000x
492
4924200672
180
1200Kg/ Cm2
Casing And Casing Design: Burst
0
492 - =
10
BACK UP AT SURFACE =
BACK UP AT CSD =
RESULTANT AT SURFACE = 0 492 Kg/cm2
RESULTANT AT CSD = 620 - 321 = 299 Kg/ cm2
1.07 X 3000 = 321Kg/ cm2
Outside Pressure
Casing And Casing Design: Collapse
COLLAPSE LOAD AT SURFACE = 0
COLLAPSE LOAD AT CSD =1.36 X 3000
10 = 408 Kg/ cm2
COLLAPSE BACK UP AT SURFACE = 0
COLLAPSE BACK UP AT CSD = 0
RESULTANT AT SURFACE = 0
RESULTANT AT CSD = 408 - 0 = 408 Kg/ cm2
Outside Pressure
Inside Pressure
Casing And Casing Design: Graphical Representation
DE
PT
H
PRESSURE
3000
0 492
299
Resultantburst
408
Collapse loadline
Collapse - backup
Collapse load line= Collapse resultant
620321
Burst load line
Burst back up
Casing And Casing Design: Graphical Representation
DE
PT
H
PRESSURE
3000
0 492
299
Resultantburst
408
Collapse loadline
Collapse - backup
Burst back up
Equation ofresultant line is
y = 15.54x - 7645
CASING AND CASING DESIGN: Selection Of Casing
Bottoms Up Casing Selection is Preferable. As such minimum collapse pressure required for casing
= 408 x 1.125 = 459 Kg/ cm2
From Data Table, available casing with this collapse resistance is N-80, 53.5 #
Next lower grade available casing is N-80, 47 # and collapse rating of this casing is 334 Kg/ cm2. From graph or calculation shown below, this casing can be lowered up to 334 x 10
1.125x1.36= 2183 2180 M
So, 2180 – 3000 : N-80. 53,5 #
Casing And Casing Design: Biaxial Effects
Depth of N-80, 47# needs correction for Bi-axial effect Maximum collapse effect is at 2180 M. P.B.Y.S OF 47# CASING = 492 X 103 Kgs
TENSILE LOAD AT 2180 M= (3000-2180) x 53.5 x 1.488 = 65.28 x 103 Kgs
FACTOR ‘X’ = 65.28x 103 Kgs
492 x 103 Kgs= 0.132
CORRESPONDING ‘Y’ VALUE = 0.958COLLASE RATING AT ZERO AXIAL STRESS = 334 Kgs/ cm2
COLLAPSE RATING UNDER TENSILE LOAD = 0.958 x 334 = 320 Kgs / cm2
REVISED COLLAPSE DESIGN FACTOR UNDER TENSILE LOAD
= 320 / 296 = 1.08 NOT SAFE
Casing And Casing Design: Biaxial Effects
FACTOR ‘X’ = 72.44x 103 Kgs492 x 103 Kgs = 0.147; ‘Y’ VALUE = 0.951
REDUCED COLLAPSE RATING = 0.951 x 334 = 317 Kgs / cm2Casing could be lowered to:
From graph or calculation shown below, this casing can be lowered up to
320 x 101.125x1.36 = 2091 2090
M
317 x 101.125x1.36 = 2071 2070 MTaking L = 2050 M
Net Collapse Pressure at 2050 M = 2050 x 1.36 = 279 Kgs/ cm2
10
Again, length and hence weight has increased. It is an iterative process. It needs to be done once or twice.
Casing And Casing Design: Biaxial Effects
Reduced Collapse Resistance due To Biaxial Load at 2050 M
X = 75.62x 103 Kgs
492 x 103 Kgs= 0.153; ‘Y’ VALUE = 0.950
Reduced collapse rating = 0.950 x 334 = 317 Kgs / cm2
Revised collapse design factor = 317 / 279 = 1.136 Hence safe
Burst and tensile S.F. are much higher than desiredSo, 2050 – 3000 : N-80. 53,5 #
Casing And Casing Design: Burst
Next depth to which N-80, 47 # could be used for Burst and Tension need to be checked. Burst rating = 483kg/cm2
Considering S.F.burst 1.1 the resultant burst load to which the casing can be subjected to 483/ 1.1 = 439kg/cm2
From the similar triangle ADE and ABCAC/AE= BC/DEAC = 3000 (492-439)/ (492-299)Depth at which resultant burst press 439kg/cm2 exists =823M or 820M
O
D
OA492
B C
321299
439
E3000
4200Press kg/cm2
Depth M
672
So, N-80, 47 # can be used below 820M, i.e 2050- 820M
Thereafter N-80, 53.5# having Burst resistance = 558 kg/cm2 can be used up to surface as it is > the required pressure of 492x 1.1 =541 kg/cm2
So, 0 –820M: N-80, 53.5#
Casing And Casing Design: Burst
Casing And Casing Design: Selection
Casings which are selected are as follows :
0 to 820 M N-80. 53.5#820 to 2050 M N-80 47 #2050 to 3000M N-80 53.5#
Casing And Casing Design: Burst & collapse
DEPTH Safety factors
(burst)
Safety factors
(collapse)
53.5# 47# 53.5# 47#
0 558/ 492
= 1.13
- - -
820 558/439 =1.27
483/439
=1.1
412/111
=3.71
288/111
=2.59
2050 558/360
= 1.55
483/360
=1.34
445/279
=1.59
317/279
= 1.136
3000 558/299
=1.86
- 465/408
=1.139
-
Casing And Casing Design: Tension
TOTAL WEIGHT OF CASING IN AIR
= WEIGHT OF 53.5# (820M) + WEIGHT OF 47# (1230M) + WEIGHT OF 53.5# (950M)
= (53.5 x 820 + 47 x 1230 + 950 x 53.5) x 1.488 Kgs
= 226 927 Kgs
= 227 Tonne
Casing And Casing Design: Tension
PIPE BODY YIELD, JOINT STRENGTH AND TENSION SAFETY FACTORS
DEPTH TENSION LOAD
( x103)
RATINGS SAFETY FACTORSNom. Wt.
(# ppf)
P.B.Y.S
( x103)
Jt. Strength
(x103) BTC
0 227 53.5 563 601 2.48
820 162 53.5 563 601 3.47
820 162 47 492 526 3.03
2050 76 47 492 526 6.47
Casing And Casing Design: Summary
Depth
(Mts)
Casing
(9 5/8” )
Collapse S.F. (Min)
Burst S.F. (Min)
Tensile S.F. (Min)
0 - 820 N-80, 53.5 # 3.71 1.13 2.48
820 - 2050
N-80, 47 # 1.136 1.1 3.03
2050 - 3000
N-80, 53.5 # 1.139 1.55 High
Reduced collapse resistance of N-80, 53.5# at 2050 M
X =
Casing And Casing Design: Annexure I
75.62x 103 Kgs
563 x 103 Kgs= 0.134; ‘Y’ VALUE = 0.957
REDUCED COLLAPSE RATING = 0.957 x 465 = 445 Kgs / cm2
563 x 103 Kgs
Reduced collapse resistance of N-80, 53.5# at 820 M
X = (950 x 53.5 + 1230 x 47) x 1.488
= 0.287; ‘Y’ VALUE = 0.886
REDUCED COLLAPSE RATING = 0.886 x 465 = 412 Kgs / cm2
Reduced collapse resistance of N-80, 47# at 820 M
X = (950 x 53.5 + 1230 x 47) x 1.488
= 0.328; ‘Y’ VALUE = 0.863
REDUCED COLLAPSE RATINGCollapse Pressure at 820 M = 820 x 1.36 / 10 = 111Kgs / cm2
= 0.863 x 334 = 288 Kgs / cm2
492 x 103 Kgs
ANNEXURE II
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