Well Design

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• Well Design
• PE 413

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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 be achieved 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 the additional 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 --gt
  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 the conductor.
  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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Selection of Weight, Grade, and Couplings
In general, each casing string is designed to
withstand the most severe loading conditions
anticipated during casing placement and the life
of the well. The loading conditions that are
always considered are burst, collapse, and
tension. Because the loading conditions in a well
tend to vary with depth, it is often possible to
obtain a less expensive casing design with
several different weights, grades, and couplings.
The casing design usually is based on an
assumed loading condition. the assumed design
load must be severe enough that there is a very
low probability of a more severe situation
actually occurring and causing casing failure.
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Selection of Weight, Grade, and Couplings
The high-internal pressure loading condition used
for the burst 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
Burst Design
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.
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Selection of Weight, Grade, and Couplings
Collapse Design
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.
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Selection of Weight, Grade, and Couplings
Collapse Design
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.
where Dlc is the depth of the lost circulation
zone gp is the pore-pressure gradient of the
lost circulatio zone rmax is the maximum mud
density anticipated in drilling to Dlc and Dm is
the depth to which the mud level will fall.