Casing and Casing Design 1

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


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



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


(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



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 )


(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


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.


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)


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


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.


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)


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


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.


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


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


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


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.


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 #


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: 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

Casing and Casing Design 1

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