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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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Selection of Casing Setting
Depths
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Selection of Casing Setting
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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Selection of Weight, Grade, and
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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Selection of Weight, Grade, and
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