PETE 203DRILLING ENGINEERING

CASING DESIGN

Objectives Understand primary functions of casing Recognize the various types of casing

strings used. Understand the procedures used in the

design of casing strings.

Types of Strings of Casing

1. Drive pipe or structural pile {Gulf Coast and offshore only} 150’-300’ below mud-line.

2. Conductor string. 100’ - 1,600’ (BML)

3. Surface pipe. 2,000’ - 4,000’

(BML)

Diameter Example

16”-60” 30”

16”-48” 20”

8 5/8”-20” 13 3/8”

Types of Strings of Casing

4. Intermediate String

5. Production String (Csg.)

6. Liner(s)

7. Tubing String(s)

7 5/8”-13 3/8” 9 5/8”

Diameter Example

4 1/2”-9 5/8” 7”

Example Hole and String Sizes (in)

Structural casing Conductor string

Surface pipe

IntermediateString

Production Liner

Hole Size

30”20”

13 3/8

9 5/8

7

Pipe Size

36”26”

17 1/2

12 1/4

8 3/4

Example Hole and String Sizes (in)

Structural casing Conductor string

Surface pipe

IntermediateString

Production Liner

Hole Size

30”20”

13 3/8

9 5/8

7

Pipe Size

36”26”

17 1/2

12 1/4

8 3/4

Example Hole and String Sizes (in)

Structural casing

Conductor string

Surface pipeIntermediate StringProduction Liner

250’

1,000’

4,000’

Mudline

Example Casing ProgramsExample Casing Programs

CONDUCTOR (1 or 2) (40’ – 300’) Prevent eroding surface sediments and

rig foundation by circulating the drilling fluid to the shale shaker.

Protects subsequent casing strings from corrosion.

Install director system on it.

SURFACE (300' - 5000') Control caving and washing out of poorly

consolidated surface beds. Protect fresh water sands from possible

contamination by drilling fluid mud, oil or gas and or salt water from lower zone.

Install BOP on it.

INTERMEDIATE CSG (1 or 2)

It depends on well depth and geology in specific area.

Primary Purpose: To seal off troublesome zones which:

Contaminate drilling fluid. Jeopardize drilling progress with possible pipe

sticking, excessive hole enlargement. Contain abnormal pressure fluids, protect

formation below the surface casing from higher pressure credited by mud.

PRODUCTION CSG 1. It is set through productive interval to;2. Segregate pay zone.3. Can be used to produce fluid instead of tubing.

DRILLING LINER1. Lower in cost.2. Functions like intermediate.

PRODUCTION LINER1. Lower in cost2. Functions like production.

6.2 Standardization of CasingAPI: American Petroleum Institute Standards.

Range of length Outside diameter Wt/ft. Type of coupling Steel grade

Classification of CSG.

1. Outside diameter of pipe (e.g. 9 5/8”)

2. Wall thickness (e.g. 1/2”)

3. Grade of material (e.g. N-80)

4. Type to threads and couplings (e.g. API LCSG)

5. Length of each joint (RANGE) (e.g. Range 3)

6. Nominal weight (Avg. wt/ft incl. Wt. Coupling) (e.g. 47lb/ft)

Length of Casing Joints

RANGE 1 16-25 ft

RANGE 2 25-34 ft

RANGE 3 > 34 ft.

OUTSIDE DIAMETER (4.5 -20")

Tolerance 0.75%Tolerance 0.75% Usually slightly oversized.Usually slightly oversized. Minimum permissibleMinimum permissible Wall thickness = 87.5% of nominal wall Wall thickness = 87.5% of nominal wall

thickness.thickness. Nominal = approximate averageNominal = approximate average Drift diameter : Check IDDrift diameter : Check ID

Minimum mandrel diameter that must passMinimum mandrel diameter that must pass Unobstructed through the pipe.Unobstructed through the pipe. Insures a bit size less than drift diameter.Insures a bit size less than drift diameter.

WEIGHT PER FOOTWEIGHT PER FOOT

Nominal wt/ftNominal wt/ft : : is not true wt/ft but is useful is not true wt/ft but is useful for for identification purposes as an identification purposes as an approximate average (wt/ft)approximate average (wt/ft)

Plain-end wt/ftPlain-end wt/ft: is the wt/ft of the pipe body : is the wt/ft of the pipe body excluding excluding the threaded portion and coupling the threaded portion and coupling wt.wt.

Average wt/ft:Average wt/ft: total wt. of avg. joint of total wt. of avg. joint of threaded pipe threaded pipe with a coupling attached power with a coupling attached power tight tight of one of avg. joint.of one of avg. joint.

For design calculation nominal wt/ft. is often used.For design calculation nominal wt/ft. is often used.

6.3 Type of Coupling A coupling is a casing connector which is

made of casing material. Most common types of casing. Joints are

externally threaded from each end. API specification the coupling should be

of the same grade as the pipe body.

Casing Threads and Couplings

API round threads - short { CSG }

API round thread - long { LCSG }

Buttress { BCSG }

Extreme line { XCSG }

Other …

CSG & LCSGThese connectors have the same basic design:These connectors have the same basic design:

Threads are round shaped and are spaced to give Threads are round shaped and are spaced to give eight threads/inch.eight threads/inch.

Sometimes they are called API 8-round threads.Sometimes they are called API 8-round threads. Threads are cut with a taper of 3/4 in/ft.Threads are cut with a taper of 3/4 in/ft. These are commonly used connectors because of their These are commonly used connectors because of their

proven reliability, ease of manufacture and low cost.proven reliability, ease of manufacture and low cost. Cut with a 60 angle, and has round peaks and roots.Cut with a 60 angle, and has round peaks and roots. Thread compound must be used to fill the voids and Thread compound must be used to fill the voids and

obtain a sealobtain a seal.. Tensile strength of the jointTensile strength of the joint

Joint Efficiency = Joint Efficiency = < < 100%100% Tensile strength of the pipe bodyTensile strength of the pipe body

API BCSG CONNECTORAPI BCSG CONNECTOR

Joint efficiency is 100% in most cases. It is tapered but longer thread run out ¾

inch/ft. for upto 75/8 inch. Thread shape is square to reduce

unzipping tendency. 5 threads cut to the inch. 1 inch/ft for 16 inch csg.

API XCSG CONNECTOR

It is integral joint. Pipe thicker near the wall. OD is less than other API couplings Sealing mechanism is metal to metal seal

between the metal and the box. Much more expensive.

API ConnectorsAPI Connectors

STRENGTHSTRENGTH

Strength is designated by casing GRADEStrength is designated by casing GRADE Grade code : Letter and number H-40, J-55, C-Grade code : Letter and number H-40, J-55, C-

75, L-8075, L-80 Letter is arbitrary.Letter is arbitrary. Number designates the minimum yield strength Number designates the minimum yield strength

of steel in thousands of psi.of steel in thousands of psi. Yield strength :tensile stress required to Yield strength :tensile stress required to

produce a total elongation per unit length of produce a total elongation per unit length of 0.005 on a0.005 on a standard test specimen.standard test specimen.

Minimum yield strength = 80% of average yield Minimum yield strength = 80% of average yield strength observed.strength observed.

Grades of Casing Recognized by the APIGrades of Casing Recognized by the API

API ConnectorsAPI Connectors

Tensile force balance on pipe body

Example 7.1:Compute the body-yield strength for 20-in., K-55 casing with a nominal wall thickness of 0.635 in. and a nominal weight per foot of 133 lbf/ft.

A*F syieldten

Tensile force balance on pipe body

Solution:

This pipe has a minimum yield strength of 55,000 psi and an ID of:

.in730.18)635.0(200.20d

K55

A*F syieldten

Tensile force balance on pipe body

Thus, the cross-sectional area of steel is

and a minimum pipe-body yield is predicted by Eq. 7.1 at an axial force of:

.in.sq63.38)73.1820(4

A 22s

lbf 000,125,2)63.38(000,55Ften

A*F syieldten

Pipe Body Yield Strength

where

p22

y Y)dD(4

P

in pipe, of diameter inside din pipe, of diameter outside D

psi strength, yieldminimum specified Ylbf strength, yieldbody pipe P

p

y

Pipe Body Yield Strength

ExampleWhat is yield strength of body of 7”, 26 #/ft, P-110 casing?

p22

y Y)dD(4

P

lbf402,830000,110)276.67(4

P 22y

lbf 000,830Py (to the nearest 1,000 lbf).

…agrees with Tables

Internal Yield Pressure for Pipe (Burst)

where

DtY2

875.0P p

in pipe, of O.D.Din thickness, wallnominal t

psi strength, yieldminimum Ypsi pressure, yieldinternal P

p

FP = DLP

FT = 2tLYP

DLP = 2tLYP

Dt2Y

P p

FP

FT

Example

For 7”, 26 #/ft P-110 pipe

9,955 7*2

6.276)-(7*110,000*2*0.875

psi 960,9P (to the nearest 10 psi)

…agrees with Tables.

DtY2

875.0P p

TABLE 7.8 –COMMONLY USED BIT SIZES THAT WILL PASS THROUGH API CASING

CasingSize

(O.D., in)

WeightPer Foot(lbm/ft)

InternalDiameter

(in.)

DriftDiameter

(in.)

Commonly Used

Bit Sizes(in.)

4 9.510.511.613.5

4.094.0524.0003.920

3.9653.9273.8753.795

3 7/83 ¾

5 11.513.015.018.0

4.5604.4944.4084.276

4.4354.3694.2834.151

4 ¾3 7/8

Casing Design - Collapse

Casing Design - Tension

Casing Design - Burst (from internal pressure)

Internal Yield Pressure for pipe Internal Yield Pressure for couplings Internal pressure leak resistance

p pInternal Pressure

Casing Design - Burst

Example 1

Design a 7” Csg. String to 10,000 ft.

Pore pressure gradient = 0.5 psi/ftDesign factor, Ni=1.1

Design for burst only.

Burst Example

1. Calculate probable reservoir pressure.

psi 000,5 ft000,10*ft

psi5.0pres

2. Calculate required pipe internal yield pressure rating

psi 500,51.1 *000,5N *pp iresi

23 lb/ft26 lb/ft

N-80

Example

3. Select the appropriate csg. grade and wt. from the Casing tables:

Burst Pressure required = 5,500 psi

7”, J-55, 26 lb/ft has BURST Rating of 4,980 psi7”, N-80, 23 lb/ft has BURST Rating of 6,340 psi7”, N-80, 26 lb/ft has BURST Rating of 7,249 psi

Use N-80 CSG, 23 lb/ft

API Design Factors (typical)

Collapse 1.125

Tension 1.8

Burst 1.1

Required

10,000 psi

100,000 lbf

10,000 psi

Design

11,250 psi

180,000 lbf

11,000 psi

Table 3

Grade D/t Ratio

H40 16.44 and less

J & K55 14.8 and less

C75 13.67 and

N80 13.38 and less

C95 12.83 and less

P105 12.56 and less

P110 12.42 and less

2//1/2 tDtDYmc (4)