Tugas Casing Seat Selection

8/11/2019 Tugas Casing Seat Selection

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Casing Seat Selection


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FUNCTIONSOFCASING

To keep the hole open and to provide a support for weak,

or fractured formations.

To isolate porous media with different fluid/pressure

regimes from contaminating the pay zone

To provide a passage for hydrocarbon fluids; most

production operations are carried

To provide a suitable connection for the wellhead

connection

Introduction


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2. Conductor pipeRun from surface to some shallow depth to protect nearsurfaceunconsolidated formation

Provide a circulation for the drilling mud to protect foundation of theplatform

May be connecting of BOP or cut at surface or diverter connection

Sizes from 18 5/8 in to 20 in

3. Surface casing (13 3/8 in.)Run to prevent caving of week formation encountered at shallow depthsShould be set in competent rock like limestone: to ensure that the

formation will not fractured at the casing shoe by high mud weight usedlater in the next holeProtect against shallow blow-out, thus BOPs are connected to top

TYPESOFCASING

Introduction


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TYPESOFCASING

Introduction

4. Intermediate casing (9 5/8 in.)Usually set in the transition zone below or above

pressured formation (salt and/or caving shale)

Need good cementing o prevent communication

behind the casing between zones; multistage

cementing may be used for long strings

5. Production casing (7 in.)

Isolate production zones

Provide reservoir fluid control

Permit selective production in multi zones production

6. Liner casing

A string of casing that does not reach to the surface

Hang on the intermediate casing, by use of suitable

packer and slips called liner hanger


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Introduction

To obtain the most economical design, casing strings often consist of

multiple sections of different steel grade, casing depths, wall

thickness, and coupling types. Such a casing string is called a

combination string. Additional cost savings sometimes can beachieved by the use of liner combination strings instead of full strings

running from the surface to the bottom of the hole. However, the

potential savings must be weighted against the additional risks and

costs of a successful, leak-free tieback operation as well as theadditional casing wear that results from a longer exposure of the

upper casing to rotation and translation of the drill string.


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Selection of Casing Setting

Depths

The selection of the number of casing strings and their setting

depths generally is based on a consideration of the pore pressure

gradients and fracture gradients of the formations to be penetrated.

The pore pressure and fracture pressure are expressed as an

equivalent density and are plotted vs. depth. A line representing the

planned-mud-density program also is plotted. The mud densities are

chosen to provide an acceptable trip margin above the anticipated

formation pore pressure to allow for reductions in mud weight

caused by upward pipe movement during tripping operation. A

commonly used trip margin is 0.5 lbm/gal or one that will provide 200-

500 psi of excess bottomhole pressure over the formation pore

pressure.


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Depths


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Depths

Point a: to prevent the formation fluid into the well

and to reach the desired depth.

Point b: to prevent the fracture of formation -->

intermediate casing need to run at this depth.

Point c: Fluid density is reduced until it reaches to

margin of the curve

Point d: casing shoe of the surface casing


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Example

A well is being planned for a location in Jefferson Parish, LA. The

intended well completion requires the use of 7 production casing set

at 15,000 ft. Determine the number of casing strings needed to reach

this depth objective safely, and select the casing setting depth of each

string. Pore pressure and fracture gradient, and lithology data from

logs of nearby wells are given in Fig 7.21. allow a 0.5 lbm/gal trip

margin, and a 0.5 lbm/gal kick margin when making the casing seat

selections. The minimum length of surface casing required to protect

the freshwater aquifers is 2000ft. Approximately 180 ft of conductor

casing generally is required to prevent washout on the outside of theconductor. It is general practice in this are to cement the casing in

shale rather than in sandstone.


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Example


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Selection of Casing Sizes

To enable the production casing to be placed in the well,

the bit size used to drill the last interval of the well must be

slightly larger than the OD of the casing connectors.

The selected bit size should provide sufficient clearance

beyond the OD of the coupling to allow for mud cake on

the borehole wall and for casing appliances, such as

centralizers and scratchers. The bit used to drill the lower

portion of the well also must fit inside the casing string

above.


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Selection of Casing Sizes


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CASINGSPESIFICATION

Casing is specified by: grade, weight per unitlength, outside diameter and wall thickness, type

of coupling, and length of joint.

API defines three types of casing weight

Nominal weight: normally based on thecalculation, not exact, use for design and given in

tables.

Plain end weight: the weight of casing joint

without inclusion of threads and couplingsThreads and coupled weight


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Selection of Weight, Grade, and

Couplings

The high-internal pressure loading condition used for theburst design is based on a well control condition assumed

to occur while circulating out a large kick.

The high-external pressure loading condition used for the

collapse design is based on a severe lost-circulation

problem.

The high-axial tension loading condition is based on an

assumption of stuck casing while the casing is run into the

hole before cementing operations.


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Couplings


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Selection of Weight, Grade, and Couplings

The burst design should ensure that formation fracture pressure at the

casing seat will be exceed before the burst pressure is reached. Thus,

this design uses formation facture as a safety pressure release

mechanism to ensure that casing rupture will not occur at the surface.

The pressure with the casing is calculated assuming that only

formation gas is in the casing.

The external pressure outside the casing that helps resist burst is

assumed to be equal to the normal formation pore pressure for the

area.

Burst Design


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Couplings

The collapse design is based either on the most

severe lost-circulation problem that is felt to be

possible or on the most severe collapse loading

anticipated when the casing is run. For both cases, the

maximum possible external pressure that tends to

cause casing collapse results from the drilling fluid that

is in the hole when the casing is placed and cemented.

Collapse Design


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Selection of Weight, Grade, and Couplings

If a severe lost circulation zone is encountered near the bottom of the next

interval of hole and no other permeable formations are present above the lost

circulation zone, the fluid level in the well can fall until the BHP is equal to the

pore pressure of the lost circulation zone.

lcpmlc DDD 052.0052.0 max

where Dlc is the depth of the lost circulation zone; gp is the pore-

pressure gradient of the lost circulatio zone; rmaxis the maximum muddensity anticipated in drilling to Dlc; and Dmis the depth to which the

mud level will fall.

Collapse Design

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Tugas Casing Seat Selection