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Casing, Casing Suspension & Casing Design

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Network of Excellence in Training © COPYRIGHT 2002, NExT. All Rights Reserved Casing, Casing Suspension & Casing Design
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Network of Excellence in Training

© COPYRIGHT 2002, NExT. All Rights Reserved

Casing, Casing Suspension& Casing Design

2© COPYRIGHT 2002, All Rights Reserved

Casing and Casing Design• Lecture Contents;

– Objectives,

– Casing String Types and Suspension

– Casing Classification,

– Casing Design Considerations.

3© COPYRIGHT 2002, All Rights Reserved

Casing and Casing DesignObjectives;

• By the end of this lecture YOU will be able to:

– Identify basic functions of casing strings and suspension,

– Relate these functions to: conductor, surface, intermediate and production casing,

– Describe the relative importance of casing grade to ability to withstand various loads,

– Understand basic casing design considerations.

4© COPYRIGHT 2002, All Rights Reserved

Casing and Casing DesignCasing String Types;

• At certain intervals during drilling of a well, casing (steel pipe) is run into the wellbore and cemented in place,– The reasons for running casing:

• Support wellhead equipment and BOPs,

• Prevent mud contamination of fresh water aquifers,

• Isolate zones,

• Seal off lost circulation zones,

• Keep hole open

• Control inflow from the producing zone(s),

5© COPYRIGHT 2002, All Rights Reserved

7” Production casing

Exploration well

5 casings

Development well

4 casings

30”

20”

13-3/8” Intermediate

9-5/8” Intermediate

Reservoir

Conductor

Surface casing

20”

13-3/8”

9-5/8” Intermediate

Casing and Casing Design

6© COPYRIGHT 2002, All Rights Reserved

Casing String Type: Drive Pipe

– This type is commonly pile driven or jetted to a depth of say 100 ft,

– The primary purpose is to protect unconsolidated surface soils from erosion,

– They are typically of large diameter(more than 20 inches),

– Joints are normally welded together

Casing and Casing Design

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Casing String Types: Conductor Pipe– This is the first casing to be run when there is no drive pipe

to prevent washing out under the rig,

– Provides elevation for return flow to Bell nipple,

– It is the string on which Diverter can be set,

– It is cemented to surface or seabed. It supports the wellhead and subsequent casing string and its setting is critical in terms of compressional loading and integrity of the cement,

– Common Sizes and Depths:

• 30” - 20” Welded, 20” - 16” Threaded,

• 30’ - 200’ (< 100’ common).

Casing and Casing Design

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• Casing String Types: Surface Casing

– Protects fresh water aquifers,

– It is the first string on which BOP can be set to provide primary pressure control,

– It is cemented to surface or seabed, to case-off, unconsolidated or lost circulation areas and support subsequent casing strings/ wellhead,

– Common sizes and depth:– 20”- 16” - 13 3/8”- 9 5/8” threaded,

– 100’ - 3000’ (or more).

Casing and Casing Design

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Base Plate

The casing head may be used in conjunction with a base plate for more effective weight distribution.

Conductor pipe Surface casing

Cellar deck

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Bottom Connection

Slip-on weld bottomconnection

Threaded bottomconnection

OR

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• Casing String Types: Intermediate Casing– Separates hole into workable sections,

– The number of intermediate strings set depends on:• Fracture Pressure last shoe,

• Proximity to a potential reservoir. (It is good practice to set intermediate string above reservoir),

• Hole problems (i.e. lost circulation, salt section, differential sticking, caving, overpressurized zones),

– The casing is normally cemented in the previous shoe or to surface. (could be cemented in two stages),

– Common sizes and depths:• 13 3/8”, 10 3/4”, 9 5/8”,

• 3000’ to 10,000’.

Casing and Casing Design

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Casing Hangers

Conductorpipe

Surface casing

Intermediatecasing

Landing area

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Functions of casing hangers

1. Suspends intermediate or production casing string

2. Centres the casing string in the head (or spool)

3. Seals off the casing annulus

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SLIPS

BOWL

ELASTOMERSEAL ELEMENT

Slip type hangers1. Wraparound casing slips with sealing

capability

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SEAL PLATESEAL

SLIP

SLIPBOWL

Slip type hangers1. Wraparound casing slips with sealing

capability

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Annulus sealing with the casing hanger

Isolation of Annulus

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Casing Head SpoolsSecond element of the

wellhead

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Functions of casing head spools1. Seals off (packs off) surface casing string

2. Provides support (landing bowl) for next casing string

provides support for well control equipment (BOP Stack)

Sealing the wellbore from the atmosphere

Controlling access to the wellbore : for pressure control or fluid returns during drilling operations.

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Description of a spool

Seals off (packs off) designed to fit around OD of current casing

Side outlets threaded or studded

Top connection

Landing area forCasing Hanger

Bottom connection compatible with the top connection on the previous casing head or spool

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Isolation Seals and Pack OffPrevents:

• communication between the casing strings

• exposure of the flange seal to annulus pressure.

Next casing spoolto hang-off next

casing.

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• Casing String Types: Production Casing

– Final string to be run for production or testing,

– The primary purpose is to isolate the production zones

thus allowing proper control of the reservoir,

– Cementing is very important to prevent communication

in the annulus,

– Covers worn or damaged intermediate string,

– Common sizes: 7”, 7 5/8”, & 9 5/8”.

Casing and Casing Design

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Casing and Casing Design• Types of Casing: Liners

– Purpose:

• Mostly same as production casing,

• Rig unable to lift long string of casing in Deep Wells,

– Types of Liners

• Production Liners: Most common to save $$,

• Drilling Liners: Cover problem zone or cover worn-out casing in order to be able to continue drilling,

– Common sizes: 4 1/2”, 5”, 7”.

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Casing String Types;

• Production Liner:

Casing and Casing Design

LINER

Last casing

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Casing String Types;

• Production Liner:

– Tie-Back:

• The integration of the liner with casing from top of existing liner to surface,

• To further up casing to cover corroded or damaged zone, (sometimes cemented in place),

• This is often done if production is commercially viable or there is damage to casing above the liner.

Casing and Casing Design

TIE BACK

STINGERWITHSEALS

LINER

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Casing String Types:

• Drive Pipe,

• Conductor Pipe,

• Surface Casing,

• Intermediate Casing,

• Production Casing:

– Liner,

– Liner & Tieback

– Full string

Casing and Casing Design

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Casing and Casing DesignCasing Classifications;

• Outside Diameter:

– This is the normal (nominal) diameter of the casing measured to the outside surface of the casing body,

– API: > 4 1/2” called casing

– API: < 4 1/2” called tubing

• Weight:

– It is the weight of the finished joint including couplings,

– The nominal weight of the casing is expressed in pounds (lbs) per foot.

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Casing Classifications;• Wall Thickness:

– Related to weight,

– The drift diameter is used in sizing the drill bit

• Grade:– The grade of a casing is designated by a letter and number combination (e.g. J55, C75, N80, P110),

– The letter indicates the steel quality (heat treatment)

– The number is the API minimum yield strength in thousands of psi,

– Hence, for example, J55 has a minimum yield strength of 55000 psi.

Casing and Casing Design

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• Casing Design Considerations;– The end product of such design is a pressure vessel

capable of withstanding the expected internal and external pressures and axial loading,

– Hole irregularities further subject the casing to bending forces which must be considered during the selection of casing grades,

– In general, the cost of a given casing grade is proportional to its weight, the heaviest weight being the most expensive,

– And hence the designer must optimize without compromising the safety and quality.

Casing and Casing Design

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Casing Design Considerations;

• Design Criteria;

– Main casing design criteria are:

• Collapse,

• Burst,

• Tension.

Casing and Casing Design

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Casing Design Considerations;• Collapse Pressure:

– This pressure originates from the column of mud used to drill the hole and acts on the outside of the casing,

– Since the hydrostatic pressure of a column of mud increases with depth collapse pressure is highest at the bottom and zero at top.

Casing and Casing Design

Pressure Pressure

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Casing Design Considerations;

• Basic Collapse Assumptions:

– Casing is empty due to lost circulation at shoe or at TD,

– Internal pressure inside casing is zero,

– External pressure caused by:

• mud in which casing was run,

• overburden acting on mobile layers

– No cement outside casing.

Casing and Casing Design

CSD

TD

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Casing Design Considerations;

Basic Burst Assumption:

– Gas to Surface:

• This is an extreme case,

• For added safety it is assumed that the influx fluid displaces the entire drilling mud,

• This will subject the inside casing to bursting effects of formation pressure.

Casing and Casing Design

CSD

TD

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Casing Design Considerations;

Burst Pressure: Gas to Surface

• At the top, the external pressure due to hydrostatic head of mud is zero and the internal pressure must be supported entirely by the casing,

• The burst pressure is highest at the top and lowest at the casing shoe where internal pressure is resisted by the external pressure due to fluids outside the casing,

• In conventional casing design, it is customary to assume a gas kick, thereby anticipating the worst possible type of a kick.

Casing and Casing Design

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Casing Design Considerations;

• Burst Pressure: Gas to Surface:

– The gas gradient is in the order of 0.1 psi/ft.

– Casing seat should be selected so that gas pressure at the casing shoe is less than the formation breakdown at the shoe,

– In exploration wells, where reservoir pressure is not known, formation pressure from the next openhole section is calculated from the maximum mud weight.

Casing and Casing Design

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Casing Design Considerations;

• Tension:

– Most of axial tension arises from the casing weight,

– Other tension loadings can arise due to:

• Bending,

• Drag,

• Shock loading and during pressure testing of casing.

– In casing design, the uppermost joint of the string is considered the weakest in tension as it has to carry the total weight of the casing string.

Casing and Casing Design

36 © COPYRIGHT 2002, All Rights Reserved

Casing and Casing DesignNow YOU should be able to:

• Identify basic functions of casing strings,

• Relate these functions to required characteristics for surface, intermediate and production casing strings,

• Describe the relative importance of casing dimensions and steel grade to ability to withstand various loads,

• Understand basic casing design considerations.

Network of Excellence in Training

© COPYRIGHT 2002, NExT. All Rights Reserved

Casing, Casing Suspension and Casing Design

End of Lecture


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