Casing Design - Jimmy Wang

7/30/2019 Casing Design - Jimmy Wang

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Casing Design

University of Petroleum,China

By Jimmy Wang


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Content

section 1 functions of casing

section 2 casing types

section 3 strength properties

section 4 casing specification

section 5 casing design

section 6 other considerations


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The general picture of casing

After this topic, we should know the

following:

1 the functions of oil well casing

2 the various types of casing strings

3 the procedure used in the design of

casing strings

Purpose


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Casing

Casing string

Surface casing

Intermediate casing

Production casing

Liner

Drilling liner

Tube

Formation

Tensile force

Collapse strength

Collapse pressure

Collapse resistance

Burst strength

Burst pressure

Burst resistance

Compression load

Pressure coefficient

Technical words


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section 1 Functions of Casing

1 to keep the hole open and to provide a

support for weak, or fractured formations;

2 to isolate porous media with different

fluid/pressure regimes from contaminating the

pay zone;

3 to prevent contamination of near-surface

freshwater zones;

Can you give

some functions?


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4 to provide a passage for hydrocarbon

fluids;

5 to provide a suitable connection for

the wellhead equipment ;

6 to provide a hole of known diameter

and depth to facilitate the running of

testing and completion equipment.


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section 2 Casing Types

1 the necessity of classification

(1) the presence of high-pressured zones at

different depths;

(2) the presence of weak, unconsolidated

formations or sloughing shaly zones


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2 the classification of casing

(1) Stove pipeA. Functions

a. To prevent washout of near-surface

unconsolidated formations ;

b. To provide a circulation system for the

drilling mud ;

c. To ensure the stability of the ground surface

upon which the rig is sited.


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B. Sizefrom 26 in to 42 in

C. feature

a. cannot carry any wellhead equipment

b. can be driven into the ground with a pile

driver.


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(2) Conductor pipe

A. Functions

a to protect nearsurface unconsolidated

formations;

b to seal off shallow-water zones;c to provide a circuit for the drilling mud ;

d to protect the foundations of the

platform(offshore).B. Size

from 18 5/8 in to 30 in


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C. Setting depth

from the surface to some shallow depth

D.Features

a one or more BOPs may be mounted on this

casing;

b always cemented to the surface;

c either to support the subsequent casing

strings and wellhead , or simply cut at the

surface after setting the surface casing.


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(3) Surface casing

A. Functions

a to prevent caving of weak formations that

are encountered at shallow depths.

b to ensure that the formations at the casing

shoe will not fracture at high hydrostaticpressures which maybe used later.

c to prevent shallow blowouts as drilling

process.

B. Size

13 3/8 in(in the Middle East)

18 5/8 in or 20 in (in North Sea)


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C. Setting depth

chosen to protect troublesome formations, thief

zones, water sands, shallow hydrocarbon zones

and build-up sections of deviated wells.

D. Feature

BOPs are connected to the top of the string


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(4) Intermediate casing

A. Functions

a to seal off a severe-loss zones;

b to protect problem formations,such assalt sections or caving shales;

c to prevent communication behind the

casing between the lower hydrocarbon

zones and upper water formations.

B. Size

the most common size is 9 5/8 in


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C. Setting depth

usually set in the transition zone below or above an

over-pressured zone

D. Feature

a good cementation of the casing must be ensured

b multistage cementing may be used to cement long

strings of intermediate casing


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(4) Production casing

A. Functions

a to isolate producing zones;

b to provide reservoir fluid control

c to permit selective production in mutizone

production.

B. Size

the normal size is 7 in

C. Feature

a the last casing string;

b the well will be completed through the

string.


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(4) Liner casing

A. Introduction of liner casing

a not to reach the surface;

b hung on the intermediate casing

B. Setting depths

set at the bottom and hung from the intermediate casing


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C. Advantages:

a total costs of the production string is reduced;

b running and cementing times is reduced ;

c the length of reduced diameter is reduced.

D Disadvantages:

a possible leak across a liner hanger;

b difficulty in obtaining a good primary cementation due

to the annulus between the liner and the hole.


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section 3 Strength properties

Casing strength properties are normally

specified as:

(1) yield strength

(2) collapse strength

(3) burst strength

Strength

properties


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1 yield strength

Load-elongation graph

A

B

C

Yield strength

Ultimate strength

Fracture strength

Load

O Elongation

(1) O-A-B

This part is a straight line and can be called as the

elastic range.

Strength

properties


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Hookes law is only applicable to this portion:

=E (3-1)

Where =applied stress=load/cross-section area

E = Youngs modulus

=deformation=elongation/original length

A. to result in no damage to the internal structure;

removal of the load will resume its original shape and

length.B. Point B is defined as yield strength.

Strength

properties


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Note:

when quoting the strength of the casing, it iscustomary to use the yield strength of casing.

API define the yield strength as the tensile stressrequired to produce a total elongation of 0.5% of the

gauge length.

Strength

properties

(2) B-C

to result in a change in the internal structure and in a

loss of strength;

removal of the load will not resume its original shape

and length.


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(3) Axial tension

Ften= yieldAs (3-2)

Ften=(/4) yield(dn2-d2) (3-3)

Strength

properties


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Compute the body-yield strength for 20-in, K-55 casing with

a nominal wall thickness of 0.635 in and a nominal weightper foot of 133 lb/ft.

Solution. This pipe has a minimum yield strength of 55,000

psi and an ID of

d=20.00-2(0.635) = 18.730 in.

Thus, the cross-sectional area of steel is

As=(/4)(202-18.732)=38.63 sq in.

and minimum pipe-body yield is predicted by at an axial

load of

Ften =55,000(38.63)=2,125,000 lbf.

Example

Strength

properties


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2 collapse strength

(1) Concept

Collapse strength is defined as the

maximum external pressure to

collapse a specimen of casing.

(2) Types

A. Elastic collapse: specimen fails

before it deforms.

B. Plastic collapse: specimen

deforms before it fails.

Strength

properties


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Strength

properties


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(3) Elastic collapse

The elastic collapse pressure Pc

, can be determined

from the following formula:

Where E: Youngs modulus of steel;

: Poissons ratio;

t: casing thickness;

D: the outside diameter of casing

(3-4)Strength

properties

bar

t

D

t

D

EPc 22

1

1

1

2


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In Imperial units, E=30106psi and =0.3; hence theequation (3-4) simplifies to

psi

t

D

t

DPc 2

6

1

1095.46

In metric units, the equation (3-4) becomes

bar

t

D

t

DPc 2

6

1

10198.2

(3-5)

(3-6)

Strength

properties


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(4) Plastic collapse

The minimum collapse pressure Pp in the plastic range maybe determined from the following equation :

Where A,B and C are constants depending on the grade of

steel used and Y is yield strength.

(3-7)CBtD

AYPp

/

Strength

properties


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(5) Transition collapse pressure

The collapse behavior PT, in the transition zone between

elastic and plastic failure is described by the following

formula:

(3-8)

Where F and G are constants can be given by A,B and C.

psiGtD

FYPT

/

Strength

properties


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3 Burst strength

(1) Concept

defined as the maximum value of internal

pressure required to cause the steel to yield.

(2) The minimum burst pressure for a casing can begot by the following Barlows formula:

D

YtP

br

2875.0

The coefficient 0.875 can be deduced if the imperial

units are used in the above equation.

(3-9)

Strength

properties

Example


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ExampleCompute the burst-pressure rating for 20-in,K-55

casing with a normal wall thickness of 0.635 in

and a normal weight per foot of 133 lb/ft.

Solution

The burst-pressure rating is computed by use of the

above equation.

Pbr=0.8752550000.63520=3056 psi

Rounded to the nearest 10psi, this value becomes

3060psi. This burst-pressure rating corresponds to the

minimum expected internal pressure at which permanent

pipe deformation could take place, if the pipe is subjected

to no external pressure or axial loads.

Strength

properties


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E. The types of liner

a drilling liners: to isolate lost circulate or abnormally

pressured zones to permit deeper drilling

b production liners: to replace a full casing to provide

isolation across the producing or injection zones

c tie-back liner: a section of casing extending upwards

from the top of an existing liner to the surface or

wellhead

d scab liner: used to repair existing damaged casing

e scab tie-back liner:a section of casing extending from

the top o fan existing liner.


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section 4 Casing specification

Casing specification is referred to the

following parameters:

a. Outside diameter and wall thickness;

b. Weight per unit length;c. Type of coupling and thread;

d. Length of joints;

e. Grade of steel.

Casing

specification


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1 Outside diameter and wall thickness

Different depths

Different pressure

Different diameter

and wall thickness

Economy

Casing

specification


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2 Weight per unit length

API defines three types of casing weight :

(1) nominal weight;

(2) plain end weight;

(3) threaded and coupleded weight.

Casing

specification

(1) i l i ht


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(1) nominal weight

Used for the purpose of identification of

casing types during ordering.

Expressed in lb/ft or kg/m.

Not exact weights and normally based on

the calculated, theoretical weight per

foot for a 20ft length of threaded or

coupled joint.

Casing

specification


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Nominal weight,Wn , is calculated from

the following formula:Wn=10.68(D-t)t+0.0722D

2 lb/ft

whereD: outside diameter, in;

t: wall thickness, in.

(4-1)

Casing

specification


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(2) plain end weight

The plain end weight is the weight isthe weight without the inclusion of

threads and couplings.

The plain end weight can be calculatedby use of the following formula,taken

from API Standards:

Wpe=10.68(D-t)t lb/f twhereD: outside diameter, in;

t: wall thickness, in.

(4-2)

Casing

specification

(3) threaded and coupleded weight


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(3) threaded and coupleded weight

The threaded and coupleded weight is the

average weight of a joint including the threadsat both ends and a coupling at one end when

power-tight.

It can be calculated by use of the following formula:

W=(20-(NL+2J)/24)Wpe+weight of coupl ing-weight r emoved in threading two pipe end

20

Where W=threaded and coupled weight (lb/ft);

NL=coupling length (in);

J=distance from end of pipe to center of coupling in the

power-tight position (in);

Wpe=plain end weight.

(4-3)

Casing

specification

3 T f li d th d


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3 Types of coupling and thread

A coupling is a short section of casing and isused to connect two casing joints.

A casing joint is externally threaded at both

ends. The most common type of coupling is

internally threaded from each end.

API specifies that a coupling should be of the

same grade as the pipe body.

Casing

specification


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In general, the casing and coupling are specified by the type

of threads (or connection) cut in the pipe or coupling.

API defines three principal elements of thread:

(1) thread height or depth, defined as the distance between

the thread crest and the thread root measured normal to

the axis of the thread;

(2) lead, defined as the distance from one point on a threadto a corresponding point on the adjacent thread, as

measured parallel to the thread axis;

(3) taper, defined as the change in diameter of a thread

expressed in inches per foot of thread length;

(4) thread form-- most casing threads are squared or V-

shaped.

Casing

specification


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The following are the most widely used connections.

(a) API 8 round thread;

(b) Buttress thread;

(c) VAM thread;

(d) Extreme line threaded coupling;

(e) Buttress double seal (BDS) thread.

Casing

specification

Casing


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Casing

specification


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VAM thread configuration

Casing

specification

4 Length of joint


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4 Length of joint

API has the specified three ranges in which a

pipe length must lie.These are as follows:

Range Length (ft) Average

length (ft)1 16-25 22

2 25-34 31

3 Over 34 42

Casing

specification

5 Grade of steel


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5 Grade of steel

API lists eight different grades of casing,as

follows:

Casing

specification

6 The failure modes of casingCasing


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6 The failure modes of casing specification


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section 5 Casing design

Casing design is influenced by the following factors:

a. The loading conditions during drilling and

production;

b. The strength properties of the casing seat and ofavailable casing;

c. The degree of deterioration to which the pipe will be

subjected during the entire life of the well;

d. The requirements of completion and production;e. Safety;

f. Economy;

g. The availability of casing.

Casing

design

1 Design criteria


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1 Design criteria

A tensile force

(a) originate from casing-own-weight,bending

forces, and shock loading.

(b) the weakest point is located at the uppermost

joint of the string.

Casing

design


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B Collapse pressure

(a)Originate from the column of mud

(b) Collapse pressure

C=m gh

Where m : mud density

h: depth; g: acceleration due to gravity.

(c) the collapse is zero at the top;

the collapse is the highest at the bottom

(d) The collapse pressure never exceeds the

collapse resistance.

(5-1)

Casing

design

Casing


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C Burst pressure

(a) normally based on the maximum formation pressure

that can be encountered during the drilling of next holesection.

(b) At the top it is the highest.

At the bottom it is the least.

D Compression load

(a) Originate when casings carry inner strings.

(b) Since production casings don not carry inner strings,

they dont develop any compression load.

Casing

design

E Other loadings


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E Other loadings

(a) bending with tongs during make-up;

(b) pull-up of the joint and slip crushing

(c) corrosion and fatigue failure of the body and

threads;

(d) pipe wear due to running wire line tools andstring assembly;

(e) additional loadings strings treatment operations

such as squeeze-cementing ,acidising and hydraulicfractures.

Casing

design


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F Conclusions

(a) Only tensile force, collapse pressure,burst pressure

and compression load will be considered in the design.

(b) Other loadings,with the exception of (e) cannot bedetermined directly and be accounted for through the

use of safety factors.

Casing

design

2 S f t f tCasing


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2 Safety factors

Casing design is not an exact technique, because of the

uncertainties in the determining the actual loadings and

also because of the change in casing properties withtime,resulting from corrosion and wear.

A safety factor is used to allow for such uncertainties in

the casing design and to ensure that the rated

performance of the casing is always greater than anyexpected loading.

Usual safety factors are:

collapse: 0.85---1.125

burst: 1---1.1

tension: 1.61.8

design

3 combination stringsCasing


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3 combination strings

From the above table, the requirements for burst andtension criteria are different from the requirement for

collapse .

Hence a compromise must be reached when designing

for casing.

How to reach the compromise?

Maximum

tension

Maximum

Burst pressure

Maximum

collapse pressure

At the top

High grade or

heavy casing

At the top

High grade or

heavy casing

At the bottom

High grade or

heavy casing

design

Thi i i hi d i th f f bi ti


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This compromise is achieved in the form of a combination

string. In other words,casing of various grades or different

weights are used at different depths of a hole.

top

middle

bottom

Strong and heavy casing

Light yet heavy casing

Heavy casing

The combination string method represents the most

economical way of selecting casing consistent with safety.

Well hole

Casing

design

4 Biaxial effects C i


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4 Biaxial effects

(1) Concept

The combination of stress due to the weight of thecasing and external pressure are referred to the

Biaxial stresses.

(2) The ellipse of plasticity

Holmquist and Nadia in 1939 give the equationabout the relationship for the effect of axial stress on

collapse or burst .

yieldrztrrt 2)()()(

222

(5-2)

Where r ,t ,and z are the radial, tangential ,and verticalstresses, respectively.

Casing

design

h b i d h i d


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From the above equation and other equation and we can

deduce the following equation :

yield

iz

yield

iz

yield

it ppp

2

1

4

31

2

(5-3)

Casing

design


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Conclusion

Axial tension has a detrimental effect on

collapse-pressure rating and a beneficial

effect on burst-pressure rating.

Axial compression has a detrimental effect

on burst-pressure rating and and a beneficial

effect on collapse-pressure .

Casing

design

5 Graphic method for casing design


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5 Graphic method for casing design

The method is first described in 1965 in a series of articles by

Goins et al and has been adopted by many oil companies.

(1) Collapse line

A determined as follows:

(a) calculate the external load due to the mud column,H;

(b) calculate the internal load due to the mud inside the

casing, H1;

(c) calculate the collapse pressure C, as the difference

between H and H1,

C=H-H1

B Join the zero coordinates at the surface with the

value of C at the casing shoe to get thepressure---

depth graph.

(5-4)

Casing

design

(2) Burst line C i


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(2) Burst line


(a) calculate the external load due to an assumed mud

column of 0.465psi/ft.

(b) calculate the internal load to the formation pressure.

(c) calculate the burst pressure as the difference

between (a) and (b).

Casing

design

B


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a at the shoe

external pressure=CSDGm

internal pressure=Pf(TDCSD)G

burst = internal pressure external pressure

= Pf

(TDCSD)G CSDGm

b at the surface

external pressure=0

internal pressure=Pf

TD

Gburst = PfTD G

(5-5)

(5-6)

Casing

design

Where G=gradient of formation fluid;

Gm=the gradient of mud.

(3) Tensile forces


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(3) Tensile forces


(a) calculate the weight of casing in air;

(b) calculate the buoyancy force;

(b) to calculate total tensile loads and compare them

with the joint or pipe body yield values when the casing

is finally chosen.

(c) calculate the bending force in deviated wells;

(d) calculate shock loads due to arresting casing.

B considerations in selection of casing

(a) to check that the casing can carry its own weightin mud in the initial selection;

(b) to calculate total tensile loads and compare them

with the joint or pipe body yield values in the final

selection.

Casing

design

Example 1


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Example 1

Question:The following three grades of 133/8 in (340 mm)casings are available in a company store. It is required to run a

combination string based on collapse and tension only. The casing is

run in 67 pcf(1.0734 kg/l) mud to 6200 ft (1890 m). Safety factors

are 1.8 for tension and a minimum of 0.85 for collapse.

Grade Weight Collapse Yield strength

lbm/ft psi 1000 lb

body coupling

K55 54.5 1130 853 636

K55 68 1950 1069 1300L80 72 2670 1661 1693

Joint type: LTC for K55, 54.5 lb/ft and BTS for remaining grades.

Casing

design

SolutionC i


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(1) Collapse

Collapse pressure=676200/144=2884.7 psi

On a graph of depth against pressure draw a collapse

pressure line between zero at surface and 2885 psi at 6200

ft. Draw the collapse resistances of the three grades as

vertical lines, as shown in the next Figure .

From the Figure, selection based on collapse is as shown

in the next page table. (Note: Minimum safety factor in

collapse=collapse resistance of casing divided by collapse

pressure of mud column.)

Casing

design

Note that the last grade was only suitable down to a depth of 5400 ft for


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a safety factor of 1.However, since a minimum safety factor of 0.85 is to

be used, this grade is suitable down to 6200 ft,with the lowest safety

factor being 0.93 at TD. Above 6200 ft the safety factor value in

collapse increases and assumes a maximum value of

=1.7

Depth Grade and weight Length of section Minimum safety factor

0-2500 ft K55, 54.5 Ibm/ft 2500 ft 1

2500-4200 ft K55, 68 Ibm/ft 1700 ft 1

4200-6200 ft L-80, 72 lb/ft 2000 ft 184/(1840*0.1053)=0.93

1950

(2500*67/144)

Casing

design


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design

(2) TensionCasing

design


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( )

Casing-carrying capacity must be checked from the bottom joint to the

surface. Two values of yield strength are given in the table of strength

properties. One specifies the yield strength of pipe body and the other the

yield strength of the coupling. The lower of these two values is used for the

calculation of the safety factor in tension. Therefore, starting from the

bottom, see table below.

design

Si i i f f f 1 8 i b d i i


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Since a minimum safety factor of 1.8 is to be used in tension,

the K55, 54.5 lbm/ft (81.2 kg/m) may be used if it is designed

to carry a maximum weight, W, given by:

1.8=6.36*1000/W

W=353.33 lb

Hence, usable weight of section of 54.5# = (Total weight which

can be carried)(weight of lower casing grades)

weight of section of 54.5#=353 333259 600=93 733 lb

and

length of usable section of K55,

54.5#=93733lb/(54.5lbm/ft)=1720ft

Casing

design

Casing


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Remaining top length = 2500 - 1720 = 780 ft

A heavy casing must be used for the top 780 ft. Try K55,68#(next heavy casing).

Total weight that can be carried by the top joint of K55 is:

= 353 333 + 78068 = 406 373 lb

g

design

SF in tension for K55, 68# at top joint Casing


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SF in tension for K55, 68# at top joint

=1069*1000/406373=2.6

Hence, the final casing selection, based on collapse and tension,is as follows:

Casing

design

In exploration wells the designer often discards grades which give a


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marginal safety factor. In fact, the above selection could well be simplified

further to obtain added safety factors and to eliminate the risk of using the

wrong joint in a critical section of the well. In this example grade K55,

54.5# (81.2 kg/m), is the weakest grade and can therefore be eliminated

from our selection. Hence, final selection can be made as follows:

Casing

design

(4) Buoyancy


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( ) y y

Consider a cylinder of 1m (or 1ft) in length, of density

s ,which is fully immersed in a fluid of density ofm, of

outside diameterdo and inside diameter ofdi.

A

Air weight of cylinder=

or Wa=Ass g

B

Buoyancy force of cylinder=

or Wm=Asm g

gdd mio

14

22

gdd mio

14

22

(5-7)

(5-8)

Casing

design

C Casing


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the effective or buoyant weight of the casing

WB=Wa

Wm

=Wa(1m/ s)=WaBF

where BF = (1m/ s) and is called buoyancy factor.(5-9)

C s g

design

Example 2 Casingdesign


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pQuestion:7 in (177.8 mm) casing, 26#(38.7 kg/m), is

to be set at 17 000 ft (5182 m). If the internal diameter

is 6.276 in (159.4 mm), determine the buoyancy forceand buoyancy factor assuming that the mud density is

93.5 lbm/ft3(1.498 kg/l).

Solution

Weight of casing in air = 26 17 000 = 442 000 lb

Buoyancy factor =(1m/ s)=(193.5/489.5)=0.895

where density of steel = 489.5 lb/ft3 (7.85 kg/l)

Buoyant weight of casing= 0.809 442 000 = 357 578 lb

Buoyancy force = 442 000 - 357 578 = 84 422 lb

design

(5) Bending forceCasing


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(5) e d g o ce

Arise when casing is run in highly deviated wells or in

wells with severe dog-leg problems.

Assume:

(1) a beam subjected to pure bending;

(2) plane transverse sections will remain plane after bending;

(3) the radius is large in comparison with the transverse dimensions;

During the pure bending, the upper surface stretches and is in tension,

while the lower surface shortens and is in compression .

NA (neutral axis): a surface exists between the compressed and

stretched surfaces and has no longitudinal deformation.

design

HJ at a distance y from NA and has the same length as KL at the NA.

Aft b di th f HJ d f t f di R d i l d d


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After bending the surface HJ deforms to an arc of radius R and included

angle d.

Thus the longitudinal strain,e, in the H`J` is

R

y

Rd

RddyR

HJ

HJJHe

((5-10)

Casing

design

From Hooks law, we can get

(5 11)


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=Ee=Ey/R

If the original length of the beam is L and the total deformation angle is

, then

NA=R=L

(5-11)

(5-12)

From the above 2 equations, we can get

=Ey/(L/)=E y/L (5-13)

The maximum tensile stress occurs at the upper extreme

end of the beam at y=D/2, where D is the diameter of thebeam. Thus,

=E D/(2L) (5-14)

Casing

design

Also bending force (FB)= A Casing


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Also, bending force (FB)= A

where A is the cross-sectional area.

Hence, FB=EDA /(2L) (5-15)

When is expressed in degrees, while the above formula becomes

FB=EDA /(2L) *(/180) (5-16)

Casing

design

Equation (3-19) in field units Casing


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q ( )

A imperial units

E=modulus of elasticity of steel:30106

psi;

D=in; A=in2; L=ft; =degrees

Therefore, FB=218.17102DA /L (5-17)

In practice, the rate of change per 100ft is used to indicate the

degree of dog-leg severity. Hence , replacing L by 100in equation(3-22)

gives FB=218DA (5-18)

FB=63DWN lb (5-19)

Casing

design

metric unitsFB=63DWN lb (5-0)

(6) Shock loads Casing


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(6) Shock loads

Significant shocks loading can develop if a casing string

is suddenly stopped.Axial stresses result from sudden velocity changes

changes in a manner analogous to water-hammer in a

pipe caused by a sudden value closure.

Elastic theory leads to the following equation for axialshock loads resulting from instantaneously stopping the

casing:

sz Ev (5-19)

Wherezis the change in axial stress caused by the shock

load, vis the change in pipe velocity, E is Youngsmodulus, ands is the density of steel.

design

After average values for Youngs modulus and steel density


86/160

are substituted ,this equation becomes:

vz

1780 (5-20)

Wherez : psi

v: ft/sec.

Casing

design

Casing Design Example Casingdesign


87/160

An exploration well is to be drilled to a total depthof13 900 ft (4327 m). Relevant data are as follows.

Drilling program:0-350 ft (107 m), 26 in (660.4 mm) hole

350-6200 ft (1890 m), 171/2in (444.5 mm) hole

6200-10 400 ft (3170 m), 121/4in (3l 1.2 mm) hole

10 400-13 900 ft (4237 m), 8

1

/2in (215.9 mm) hole

Casing program:20 in(508 mm) casing to be set at 350 ft (107 m)

133/8in(339.7 mm) casing to be set at 6200 ft (1890 m)

91/8in(244.5 mm) casing to be set at 10 400 ft (3170 m)

7 in (177.8 mm) casing to be set at 13 900 ft (4237 m)

The casing head housing will be installed on the 20 incasing. The 7 in casing will be run to the surface.

Mud programme: Casingdesign


88/160

Down to 350 ft (107 m),

mud weight is 65 pcf (1.041 kg/l)

Down to 6200 ft (1890 m),


Down to 10400 ft (3170 m),


Down to 13 900 ft (4237 m),mud weight is 87 pcf (1.394 kg/l)

Safety factors:Burst = 1.1

Collapse = 0.85

Tension = 1.8

Formation fluid gradient:


89/160

0-6200 ft (1890 m), Pf= 0.465 psi/ft (0.105 bar/m)

6200-10 400 ft (3170 m),Pf= 0.48 psi/ft (0.1086 bar/m)

10 400-13 900 ft (4237 m), Pf= 0.57 psi/ft (0.1289 bar/m)

The 12 in hole experiences a maximum dog-legseverity of3o/100 ft. Other sections of the well experiencenegligible deviation. Shock loads are to be included inthe design of 9 5/8 in and 7 in casing strings.For collapse, burst and yield strength values referto sometables.Design suitable casing strings for the given hole sizes,taking into consideration the available casinggrades andthe maximum expected pressures.

Casing

design

solutionCasing

design


90/160

1. Conductor pipe (20 in casing)This pipe is set at 350 ft (107 m) and will besubjected to formation pressure from the nexthole drilled to a depth of 6200 ft (1890 m). Itwill be assumed that no gas exists at thisshallow depth and kick calculations will bebased on a water kick situation in whichformation gradient is 0.465 psi/ft (0.105bar/m). Note that if gas is known to exist atshallow depths, it must be included in thecalculations.

Collapse Casingdesign


91/160

Collapse pressure at surface = 0

Collapse pressure at 350 ft =

where mud weight is lbm/ft3.Therefore,collapse pressure at 350 ft

65350/144= 158 psi (11 bar)

mud weight depth144

This pressure acts on the outside of the casingand for the worst possible situation assume thatthe casing is 100% evacuated (as is the case ina complete-loss circulation situation).

Burst

Casing

design


92/160

Burst pressure = internal pressureexternal pressurea) Burst at shoeFrom Figure 10.11,

formation pressure at next TD = 62000.465or

Pf = 2883 psi (199 bar)Internal pressure = Pf - (TD - CSD)G

= 2883 - (6200 - 350)0.465= 163 psi (11 bar)

where G = gradient of invading fluid= 0.465 psi/ft.External pressure=casing settingdepthmud gradient

external pressure = (35065)/144= 158 psi (11 bar)

Figure 10.11

Burst at shoe = internal pressureexternal pressure


93/160

= 163 - 158= 5 psi (0.4 bar)

(b) Burst at surfaceBurst at surface = Pf TDG

= 2883 62000.465= 0

It should be noted that the zero values wereobtained as a result of the fact that a salt-water kickis considered. If instead a gas kick is considered, theburst pressure values at the shoe and surface will be2135 psi and 2140 psi, respectively.

Casing

design

Selection


94/160

A graph is not normally required and selection is determined bycomparing the strength properties of available casing withexisting pressures.From Table 10.4 it can be seen that all the available gradeshave collapse and burst values above those calculated above.Hence, select grade K555, 94#,having collapse pressure= 520psi (36 bar), burst pressure=2110 psi (145 bar) and yieldstrength= 1 479 000 lb (6579 kN). It should be noted that gradeK55, 94# is the lightest and the cheapest of the three availablegrades.Since the casing head housing is installed on the 20 in casing,the latter will be subjected to compression forces resultingfrom the weights of subsequent casing strings.This casing will be checked later to determine whether it iscapable of carrying other casing strings.

Casing

design

2. 133/8 in casing Casingdesign


95/160

This string is set at 6200 ft and will be subjected, in the event

of a kick, to formation pressures from the next hole drilled to a

TD of 10 400 ft.

CollapseCollapse pressure at surface = 0

Collapse pressure at 6200 ft(1890m)=676200/144

=285psi(199 bar)

The collapse line is drawn between 0 at the

surface and 2885 psi at 6200 ft, as shown in Figure

10.12.


96/160

Casing

design

From Table 10.5 the collapse resistances of the


97/160

available grades as adjusted for a safety factor of0.85 are as follows:The collapse resistance values are plotted asvertical lines, as shown in Figure 10.12

Casing

design

Burst Casingdesign


98/160

Formation pressure from next TD= 10 4000.48

= 4992 psi (344 bar)

(see Figure 10.13).

Burst at shoe = internal pressureexternal pressureInternal pressure = Pf - (TD - CSD)G

= 4992 - (10 400 - 6200)0.1

= 4572 psi (315 bar)

( where G = gradient of invadingfluid, assumed to be gas having a0.1 psi/ft gradient)

External pressure = CSD x 0.465


99/160

where 0.465 psi/ft is the gradient of mud outside thecasing. Therefore,

external pressure = 6200 x 0.465= 2883 psi (199 bar)

Thus,Burst at shoe = 4572 - 2883

= 1689 psi (116 bar)Burst at surface = internal pressure

- external pressureExternal pressure = 0Internal pressure = Pf - (TD) G

Casing

design

Therefore,burst at surface = Pf - (TD)G


100/160

= 4992 - 10 4000.1= 3952 psi (273 bar)

The burst line can now be drawn between 1689 psiatthe shoe and 3952 psi at the surface; see Figure10.12.

From Table 10.5, of casing properties, the burstresistances of the available grades are givenbelow,together with adjustment for SF = 1.1.

Casing

design

Selection


101/160

Selection should consider the lightest weightsfirst, as these grades are the cheapest. On the

basis of collapse only, Figure 10.12 indicatesthat the given grades are suitable for thefollowing depths:

0-3050 ft K55, 54.5#

3050-4950 ft K55,68#

4950-6200 ft L80, 72#

On the basis of burst only, Figure 10.12 gives

the following selection:

0-2400 ft L80, 72#

2400-4200 ft K55, 68#

4200-6200 ft K55, 54.5#

Casing

design

When selection is based on both collapse and


102/160

burst,Figure 10.12 indicates that grade K55,54.5#does not satisfy the burst requirement from 0

to 4200 ft. Also,grade K55, 68# does not satisfyburst from 0 to 2400 ft.Hence, selection from 0 to2400 ft is limited to grade L80, 72#.

Below 2400 ft, grade K55, 68# is suitable for

collapse from 0 to 4950 ft and for burst from 2400ft to 4200 ft. Hence, the middle section consistsof K55,68#from 2400 to 4200 ft.

The last section of hole can only be satisfied bygrade LB0, 72# in both collapse and burst; seeFigure10.12. Hence, selection based on collapseand burst is(see table below):

Casing

design


103/160

Note that grade K55, 54.5# has been rejected,since it does not satisfy both collapse and

burst at once along any section of the hole.

Tension If bending and shock forces are ignored, thesuitability of selected grades in tension can be checked bycomparing the weight in air carried by each section with its

yield strength. For the 93 in and 7 in casing,effects ofbending and shock loading will be included and buoyantweight will be considered to reduce the possibility of over-designing. Hence, starting from the bottom, see table at thetop of next page.

Casing

design

Weight of section grade and weight cumulative weight safety factor


104/160

(1000 lb) (1000 lb) =yieldstrengthcumul

ative weight

144.0 L80,72# 144.0 1650/144=11.5

122.4 K55,68# 266.4 835/266.4=3.13

172.8 L80,72# 439.2 1650/439.2=3.8

Note that yield strength values are obtained from the givenTable as the lowest value of either the body or couplingyield strength.

The safety factor must, at least, be equal to the requiredvalue of 1.8 if any of the selected grades is to satisfy thecriterion of tension. The table overleaf produces values ofSF of greater than 1.8, which indicates that the gradessatisfy collapse, burst and tension.

Casing

design

Pressure testing After the casing is landed and cemented, itis the practice to test the casing prior to drilling thecasing shoe. The testing pressure employed by some operating


105/160

casing shoe. The testing pressure employed by some operatingcompanies is 60% of the burst rating of the weakest grade ofcasing in the string.Hence,testing pressure of 133 in

= 60% x burst pressure of K55, 68//

= 60% x 3450

= 2070 psi (143 bars)

During pressure testing an extra tensile force is exerted onthe casing and the SF should, again, be > 1.8 for the topjoint (or the joint of weakest grade). Hence,total tensileforce during pressure testing at top joint

= buoyant weight of casing

+ tensile force due to pressure testing

=weightin airBF + (/4) (ID)2testing pressure

Casing

design

BF=(1 / )=1 67/489 5=0 863


106/160

BF=(1m/ s)=1-67/489.5=0.863

From the given table we can get the inside diameter of L80,72# as 12.347 in (313.6 mm).

Therefore,

total tensile force = (439.2 x 0.863) 1000

+ (/4) (12.347)2 2070

= 379 030 + 247 847

= 626 877 lb

SF in tension for top joint = 1 661 000/ 626 877

=2.65

Casing

design

Biaxial effects Check the weakest grade of selectedcasing for biaxial effects as follows.


107/160

g

Tensile ratio = weight carried by weakest joint

yield strength of body (or coupling)Weakest grade selected is the K55, 68#, having a body yieldstrength of 1 069 000 lb and a coupling strength(LTC) of835000 lb.

Hence,

tensile ratio =266.41000/835000=0.319

For a tensile ratio of 0.319, Table 10.8 showsthat the collapse resistance of the casing is

reduced to approximately 80% of its original(under zero load) value.Hence, collapse resistanceof K55, 68# = 0.81950

under biaxial loading = 1560 psi(108 bars)

Casing

design

Collapse pressure due to mud at 2400 ft (i.e. topjoint of grade of the K55 68#)


108/160

joint of grade of the K55, 68#)

=672400/144=1117 psi(77bars)

Therefore,

SF in collapse for top joint of K55, 68#

=collapse resistance collapse pressure

=1560/1117

=1.4

Final selection

Depth Grade and weight

0-2400 ft (732 m) L80, 72#(107 kg/m)

24004200 ft (1280 m) K55, 68#(101 kg/m)

4200-6200 ft (1890 m) L80, 72#(107 kg/m)

Casing

design

3. 9 5/8incasing


109/160

The 95/8in casing is set at 10400 ft and willbe subjected, in the event of a kick, to

formation pressures from the next hole drilledto a TD of 13 900 ft.

Collapse

At surface

collapse pressure = 0

At shoe

collapse pressure =7310400/144

= 5272 psi (363.5 bars)

Draw a line between 0 and 5272 psi as shown inFigure 10.14.

Casing

design


110/160

From Table 10.6 collapse properties ofavailablecasing are as follows:


111/160

Grade Weight (lbm/ft) Collapse pressure

SF = 1 SF = 1.1C75 43.5 3750 3750/1.1=4412

L80 47.0 4750 4750/1.1=5888

C95 53.5 7330 7330/1.1=8624

The above collapse resistances can be drawn as vertical lines,

as shown in Figure 10.14.

Casing

design

Burst

The 95/8in casing will be subjected in the event of a kick


112/160

The 9 /8in casing will be subjected in the event of a kick,to a formation pressure of:

0.57 psi/ft13 900 ft = 7923 psi (546 bar)

Burst at shoe = internal pressure

- external pressure

Burst at shoe = [Pf - (TD - CSD) x G]

- CSD 0.465

A gas kick is considered for this string; thus, G = 0.1psi/ft.

Therefore,

burst at shoe = 7923 - (13 900 - 10 400)

0.1 - 10 4000.465

= 2737 psi (189 bars)

(where TD = next hole depth = 13 900 ft).

Casing

design

Burst at surface = Pf TDG

Therefore,


113/160

Therefore,

Burst at surface = 7923 - 13 9000.1

= 6533 psi (450.4 bar)

The burst line can now be plotted between 6533 psi at thesurface (i.e. at zero depth) and 2737 psi at 10 400 ft,asshown in Figure 10.14

From Table 10.6 burst pressures of available gradesof 9~ incasing as adjusted for an SF = 1.1 are:

Grade Weight (lbm/ft) Collapse pressure

SF = 1 SF = 1.1

C75 43.5 5930 5390/1.1=5391L80 47.0 6870 6870/1.1=6245

C95 53.5 7330 9410/1.1=8555

Burst resistance lines are plotted, as shown in Figure10.14.

Casing

design

Selection based on collapse and burstF Fi 10 14 l ti b d ll d


114/160

From Figure10.14, selection based on collapse andburst is as shown at the table.

Buoyant weight of casing = 474.75BF

BF= 173/489.5 =0.851

Buoyant weight of casing = 474.75 0.851

= 404.012 1000 lbCasing

design

Tension The suitability of the selected gradesintension will be investigated by considering the


115/160

g y gtotal tensile forces resulting from casing buoyantweight,bending force and shock load. Starting from

the bottom, the weight carried by each section canbe calculated, as follows:

Depth (ft) Weight of each section Weight in air carried( 1000 lb) by top joint of each

section

10 400--8700 79.90 79.90

8700-3200 239.25 79.90 + 239.25 = 319.153200-800 112.80 319.15 + 112.8 = 431.95

800-0 42.80 431.95 + 42.8 = 474.75

Casing

design

By use of the equations

bending force = 63 D W


116/160

bending force = 63 D WN

drag force = 3200 WN

where WN is the weight per unit length, Table10.10 can be constructed. Table 10.10 shows thatall the selected grades satisfy the tensionrequirement.

Casing

design


117/160

Pressure testingTesting pressure


118/160

= 60% of burst pressure of lowest grade (C75, 43.5#)

= 0.6 5930

= 3558 psi (245 bar)

During pressure testing, an extra tensile force is generatedand selected grades with marginal SF should be checked. At800 ft grade L80, 47# has the

lowest SF of 1.8 (see Table

10.10); hence, this gradeshould be checked.

During pressure testing,

total tensile force

= buoyant load+ tensile force due to pressure testing

From table 10.10,

buoyant force at 800 ft = 361.21(1000 lb)

Casing

design

Total tensile load at 800 ft

= 361 21 100+ (/4)(8 681)2 3558


119/160

= 361.21 100+ (/4)(8.681)2 3558

580.623 100

SF in tension = 1086/580.623=1.87

Biaxial effectsCheck the weakest grade selected.Grade C75, 43.5#is the weakest grade, carrying a total buoyantload of 248.41 1000 lb, as shown in Table 10.10.

Tensile ratio =weight carried / yield strength

=248.41/942

=0.264

Casing

design

From Table 10.8 it can be seen that, for a tensile ratio of0.264, the collapse resistance reduces to 84% of its

i i l l H ll i t f C75 43 5#


120/160

original value. Hence,collapse resistance of C75, 43.5#under biaxial loading

= 0.84 x 3750

= 3150 psi (217.6 bar)

SF in collapse= collapse resistance under biaxialloadingcollapse pressure at 3200 ft

=3150 (733200/144)=1.94

Final selection

Depth Grade and weight

0-800 ft (244 m) C95, 53.5# (79.7 kg/m)

800-3200 ft (976 m) L80, 47# (70 kg/m)

3200-9700 ft (2957 m) C75, 43.5# (64.8 kg/m)

9700-10 400 ft (3170 m) L80, 47# (70 kg/m)

Casing

design


121/160

Casing

design

4. 7in casing


122/160

This string is set at 13 900 fi (4237 m).Collapse

Collapse pressure at surface = 0Collapse pressure at 13 900 ft=8713900/14

= 8398 psi (579 bar)

This pressure acts on the outside of the casing,and for the worst possible situation assume thatthere is zero pressure inside the casing. Draw thecollapse pressure line, as shown in Figure 10.15,between 0 psi at the surface and 8398 psi at 13900 ft.

Casing

design

Collapse resistances, from Table 10.7, are as follows:

Grade Weight (lbm/ft) Collapse resistance


123/160

SF = 1 SF = 0.85

K55 26.0 4320 4320/0.85=5082

L80 29.0 7020 7020/0.85=8259

C95 29.0 7820 7820/0.85=9200

Collapse resistances can now be drawn as verticallines in Figure 10.15.

Casing

design


124/160

Casing

design

BurstBurst pressure = internal pressure


125/160

p p

- external pressure

(a) Burst at shoe

Internal pressure = Pf = 0.57 13 900

= 7923 psi (546.5 bar)

External pressure = Gmud CSD

For added safety, the external pressure resistinginternal pressure is assumed to be that of a mud columnoutside the casing, even though the casing is cemented.Also, the mud is assumed to deteriorate so that itsgradient decreases to that of salt water, largely becauseof settlement of solids. Hence,

G = 0.465 psi/ft (0.1052 bar/m)

Burst at shoe 7923 - 13 900 0.465

= 1460 psi (100 bars)

Casing

design

(b) Burst pressure at surface

= internal pressure - external pressure


126/160

internal pressure external pressure

7923 - 13 900

gradient of invading fluid (assumed gas)

= 7923 - 13 900 0.1

6533 psi (450.4 bars)

Worst conditions In practice, hydrocarbonproduction is carried out through a tubing (single

or dual) sealed in a packer, as shown in Figure10.16. Thus, under ideal conditions only thecasing shoe will be subjected to burst effects.

Casing

design

However, a situation may arise inpractice when

the production tubing leaks gas to the7 in casing.


127/160

In this case, the surface pressure (6533 psi) isnow acting on the column of packer fluid betweenthe casing and the tubing; see Figure 10.16.

Casing

design

Hence, burst calculations for production casing should bemodified as follows.


128/160

(a) Burst pressure at shoe = surface pressure + hydrostaticpressure of packer fluid -external pressure.

Normally

packer fluid - drilling mud = 87 pcf (1.394 kg/l)

Burst at shoe = 6533 +8713900/144- 13 900 x 0.465

= 8467 psi (584 bar)

(b) Burst at surface = 6533 psi (450 bar)

Note: All these calculations assume that there is no cementoutside the casing.

Casing

design

The burst line is drawn between 6533 psi at thesurface and 8467 psi at 13 900 ft, as shown in


129/160

Figure10.15. From Table 10.7 burst resistances asadjusted for an SF of 1.1 for the available gradesare:

Grade Weight (lbm/ft) Burst resistance

SF = 1 SF = 1.1

K55 26.0 4980 4980/1.1=4527

LB0 29.0 8160 8160/1.1=7418

C95 29.0 9690 9690/1.1=8809

Adjusted burst lines (for SF= 1.1) can now bedrawn as vertical lines in Figure 10.15.

Casing

design

Selection based on burst and collapseFrom Figure10 15 selection based on burst and


130/160

From Figure10.15 selection based on burst andcollapse is as follows:

Depth (ft) Grade and weight

0-6100 LB0, 29#

6100-13 900 C95, 29#

TensionThe suitability of selected grades in tensioncan be checked by considering the cumulative

weight carried by each section. Hence, startingfrom the bottom, see the table in next page.

Casing

design


131/160

The above table considered the weight of casingsections in air only and a marginal safetyfactor of 1.68was obtained. This value is belowthe required SF of1.8 and it is instructive tocheck the suitability of this grade by addingthe effects of shock loading (bending effectsare assumed to be negligible) as shown inTable10.11.

From Table 10.11 it is evident that grade L8029# is not suitable as a top joint. Furtherrefinement can be made when pressure testing isconsidered.

Casing

design


132/160

Pressure testingNormally, casing is tested to 60% of its mill burst pressure.


133/160

Taking the lowest grade (L80),therefore,

test pressure = 0.6 x 8160 = 4896 psi (338 bar)

The weakest joint is the top joint of the lowergrade.Therefore, load at top joint=buoyant weight of casing+ tensile force, resulting from extra pressure on inside ofcasing

= Wair BF + /4(di)2 test pressure

where di is the internal diameter of the casing = 6.184 inand BF = 1 -(87/489.5)= 0.822.

Therefore,

total load at surface

= 0.822 x 403.1 1000 /4(6.184)2 4896

= 478 400 lb

Casing

design

Therefore,

SF in tension during pressure test


134/160

g p=676000/478400= 1.41

Thus, the top joint of L80 must be replaced by ahigher grade casing if the SF in tension of 1.8is to be maintained throughout the running andtesting of the casing. Hence, the maximum load, W,

that L80 can carry and still produce an SF = 1.8is given by

1.8=676 000/W

Therefore,

W= 375 556 lb

Casing

design

Hence,

weight of usable L80 section


135/160

weight of usable L80 section

= total weight carried (W) - air weight of C95, 29#

= 375 556 - air weight of C95, 29#

= 375 556 - 226 200

= 149 356 lb

and

usable length of L80, 29# = 149 356 lb/ (29 lb/ft )

=55150ft

Casing

design

From Table 10.11 the total tensile load at topjoint is still 424.15 1000 lb, since the two


136/160

different grades have the'same weight per foot of

29. If the weights were different, thenanother table should be constructed.

If grade C95 is used as the top section (950 ftlong),then

SF at top joint = 803000424150 1.89

(Note: Yield strength of C95, 29= 803 000 lb.)

During pressure tests,

SF803000/478400=1.68

Casing

design

Thus, even with the higher grade, the SF duringpressure testing is still below 1.8. To maintain

SF f1 8 d th t t b l 4896


137/160

an SF of1.8, decrease the test pressure below 4896psi. Hence,

1.8=803000/(buoyant weight + tensile force due to pressure test )=803000/(331.348+(/4)(6.184)2P)

where P is the required pressure test.Therefore, P= 3821 psi (263 bar).Hence, the new selection is:

Depth (ft) Grade0-950 C95, 29#950-6100 L80, 29#

6100-13 400 C95, 29#

Casing

design

Biaxial effectsBi i l l di d ll i f


138/160

Biaxial loading reduces collapse resistance ofcasing and is most critical at the joints of the

weakest grade. Two positions will be investigated.

(a) At 950 ft

tensile ratio

=buoyant weight carried by top joint of L80/ yield strength

tensile ratio (TR)

=(13 900-950)29BF/676000

= 0.46

Casing

design

Table 10.8 shows that for a TR =0.46, collapseresistance reduces to 69% of its original collapsealue(i e under ero load) Therefore actual


139/160

value(i.e. under zero load). Therefore, actualcollapse resistance of LB0 = 0.69 x 7020 = 4844 psi

SF in collapseat 950 ft

=collapse resistance of casing collapse pressure of mud

=4844 (87950/144)

=8.4

(b) At 6100 ft

tensile ratio =154450/676000 = 0.23

Table 10.8 shows that for a TR=0.23, collapseresistance reduces to 86% of its original value.Therefore, adjusted collapse resistance of L80 =0.86 7020= 6037 psi at 6100 ft

Casing

design

SF in collapse at 6100 ft

6037/(876100/144)


140/160

6037/(876100/144)

=1.6

Final selection

Depth (ft) Grade and weight

0-950 C95, 29#950-6100 LB0, 29#

6100-13 900 C95, 29#

Casing

design


1 Effect of hydrogen sulfide on casing


141/160

1 Effect of hydrogen sulfide on casingA Hydrogen embrittlement

When hydrogen sulfide is present, the rate at which the

hydrogen atoms (H) combine to form hydrogen gas (H2) is

reduced. As a result, atomic hydrogen (H) may enter the

metal at a significant rate before recombining. The presenceof this molecular hydrogen within the steel reduces its

ductility and causes it to break in brittle manner rather than

yield. This phenomenon known as hydrogen embrittlement.

The resulting failure is called sulfide cracking.

Water must be present for corrosion reaction to occur, whichgenerates hydrogen atoms. Dry hydrogen sulfide does not

cause embrittlement.

Other

considerations

B temperature effect on hydrogen embtittlement

Other

considerations


142/160

Hydrogen embrittlement is especially significant high-strength steels at low temperature. Common carbon steels with

yield strengths below 90,000psi generally not fail by sulfide

cracking for temperature above 100F.

There is evidence that, as temperature increases, casing with a

higher minimum yield strength than 90,000 psi can be used

safely in wells that contain hydrogen sulfide in the produced

fluids. In deep, abnormally pressured walls, a practical casing

design is difficult to obtain without the use of some high-strength steel.

Kant and Greer have presented the results of experimental laboratory and

field tests of several steel grades that were exposed to hydrogen sulfide in

varying concentrations, at various temperatures, and at various stress levels.


143/160

Shown in Fig. 7. i6 and 7.17 arc the maximum salt stress levels observed

(expressed as percent of minimum yield strength) for various steel grades,hydrogen sulfide concentrations, and exposure temperatures.

C incubation time

Failure resulting from hydrogen

b ittl t ft d t


144/160

embrittlement often do not occur

immediately after exposure to

hydrogen sulfide. A time period duringwhich no damage is evident is

followed by a sudden failure. During

the time period before failure,called

incubation period, hydrogen is

diffusing to points of high stress.Fig.7.18 shows test results of the time

to failure for different RHNs

(Rockwell hardness number) and

different applied stresses. Fig. 7.19

shows the effect of hydrogen sulfide

concentration.

Other

considerations

2 Effect of field handling on casing

A Performance properties that a given joint of casing


145/160

A Performance propertiesthat a given joint of casing

will exhibit in the field can be affected adversely by

several field operations. For example, burst strength is

affected significantly by the procedure and equipment

used to make up the pipe.

Tests have shown that burst strength can be reducedby as much as 70% by combinations of tong marks

that penetrate 17% of the wall thickness and 4% out-

ofroundness caused by excessive torque.

Other

considerations

B Mechanical deformationscan also occur while the

i i t t d t l ti hil it i i t th


146/160

casing is transported to location or while it is run into the

hole. Any mechanical deformity in the pipe normally results

in considerable reduction in its collapse resistance. This is

especially true for casing with high dn/t ratios. A thinwall

tube that is deformed by 1% out-of-round will have its

collapse resistance lowered by 25%. Thus, the slightest

crushing by tongs, slips, or downhole conditions diminishesthe collapse resistance by a significant amount.Some of the

special hydrogen-sulfide-resistant casings,such as C90,

can be stress-hardened by careless handling.If this occurs,

the resistance to hydrogen embrittlement can be lost.

Other

considerations

3 compression in conductor pipe


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Since the conductor pipe carries the weight of

other strings, it must be checked for compressionloading.The procedure is to determine the total

buoyant weight of strings carried and then

compare this with the yield strength of the

conductor pipe. A minimum safety factor of 1.1

should be obtained.

In this analysis it is assumed that the tensile

strength of casing is equal to its compressivestrength.

Other

considerations

Example

One submerged weight of 133/8 in, 95/8 in and 7 in strings

Other

considerations


148/160

One submerged weight of 13 /8 in, 9 /8 in and 7 in strings

in a mud weight of 0.465 psi/ft, so that the worst casing is

taken in account.

Hence, BF=(10.465/3.39)=0.863

where 3.39psi.ft is the pressure gradient of steel.

Casing Air weight 1000lb

13

3

/8 in 439.295/8 in 481.25

7in 403.1

Total air weight carried by conductor pipe

=1317050 lb


149/160

3 7050 b

Total buoyant weight carried by conductor pipe

= 13170500.863

=1136614 lb

Yield strength of coupling of top joint of K55,94#=1479000

Hence ,

SF in compression= =1.3

1479000

1136614

Other

considerations


150/160


1 Effect of hydrogen sulfide on casing


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1 Effect of hydrogen sulfide on casingA Hydrogen embrittlement

When hydrogen sulfide is present, the rate at which the

hydrogen atoms (H) combine to form hydrogen gas (H2) is

reduced. As a result, atomic hydrogen (H) may enter the

metal at a significant rate before recombining. The presenceof this molecular hydrogen within the steel reduces its

ductility and causes it to break in brittle manner rather than

yield. This phenomenon known as hydrogen embrittlement.

The resulting failure is called sulfide cracking.

Water must be present for corrosion reaction to occur, whichgenerates hydrogen atoms. Dry hydrogen sulfide does not

cause embrittlement.

Other

considerations

B temperature effect on hydrogen embtittlement

Other

considerations


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Hydrogen embrittlement is especially significant high-strength steels at low temperature. Common carbon steels with

yield strengths below 90,000psi generally not fail by sulfide

cracking for temperature above 100F.

There is evidence that, as temperature increases, casing with ahigher minimum yield strength than 90,000 psi can be used

safely in wells that contain hydrogen sulfide in the produced

fluids. In deep, abnormally pressured walls, a practical casing

design is difficult to obtain without the use of some high-strength steel.

Kant and Greer have presented the results of experimental laboratory and

field tests of several steel grades that were exposed to hydrogen sulfide in

varying concentrations, at various temperatures, and at various stress levels.


153/160

Shown in Fig. 7. i6 and 7.17 arc the maximum salt stress levels observed

(expressed as percent of minimum yield strength) for various steel grades,hydrogen sulfide concentrations, and exposure temperatures.

C incubation time

Failure resulting from hydrogen



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immediately after exposure to

hydrogen sulfide. A time period duringwhich no damage is evident is

followed by a sudden failure. During

the time period before failure,called

incubation period, hydrogen is

diffusing to points of high stress.Fig.7.18 shows test results of the time

to failure for different RHNs

(Rockwell hardness number) and

different applied stresses. Fig. 7.19

shows the effect of hydrogen sulfide

concentration.

Other

considerations

2 Effect of field handling on casing

A Performance properties that a given joint of casing


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A Performance propertiesthat a given joint of casing

will exhibit in the field can be affected adversely by

several field operations. For example, burst strength is

affected significantly by the procedure and equipment

used to make up the pipe.

Tests have shown that burst strength can be reducedby as much as 70% by combinations of tong marks

that penetrate 17% of the wall thickness and 4% out-

ofroundness caused by excessive torque.

Other

considerations

B Mechanical deformationscan also occur while the



156/160


hole. Any mechanical deformity in the pipe normally results

in considerable reduction in its collapse resistance. This is

especially true for casing with high dn/t ratios. A thinwall

tube that is deformed by 1% out-of-round will have its

collapse resistance lowered by 25%. Thus, the slightest

crushing by tongs, slips, or downhole conditions diminishesthe collapse resistance by a significant amount.Some of the

special hydrogen-sulfide-resistant casings,such as C90,

can be stress-hardened by careless handling.If this occurs,

the resistance to hydrogen embrittlement can be lost.

Other

considerations

3 compression in conductor pipe


157/160

Since the conductor pipe carries the weight of

other strings, it must be checked for compressionloading.The procedure is to determine the total

buoyant weight of strings carried and then

compare this with the yield strength of the

conductor pipe. A minimum safety factor of 1.1

should be obtained.

In this analysis it is assumed that the tensile

strength of casing is equal to its compressivestrength.

Other

considerations

Example

One submerged weight of 133/8 in, 95/8 in and 7 in strings

Other

considerations


158/160

8 8

in a mud weight of 0.465 psi/ft, so that the worst casing is

taken in account.

Hence, BF=(10.465/3.39)=0.863

where 3.39psi.ft is the pressure gradient of steel.

Casing Air weight 1000lb

133/8in

439.295/8 in 481.25

7in 403.1

Total air weight carried by conductor pipe

=1317050 lb


159/160

Total buoyant weight carried by conductor pipe

= 13170500.863

=1136614 lb

Yield strength of coupling of top joint of K55,94#=1479000

Hence ,

SF in compression= =1.3

1479000

1136614

Other

considerations


160/160

Casing Design - Jimmy Wang

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Transcript of Casing Design - Jimmy Wang