Well Design

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• Well Design
• PE 413
• Casing Design

2
Casing Design
Introduction

• The casing design process involves three distinct
  operations
• The selection of the casing sizes and setting
  depths
• The definition of the operational scenarios which
  will result in burst, collapse and axial loads
• The calculation of the magnitude of these loads
  and selection of an appropriate weight and grade
  of casing.

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Casing Design
Calculate Loads on the Casing Axial Load
The axial load on the casing can be either
tensile or compressive, depending on the
operating conditions.
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Casing Design
Calculate Loads on the Casing Axial Load
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Example 1
Compute the body-yield strength for 20, K-55
casing with a nominal wall thickness of 0.635
and a nominal weight per foot of 133 lbf/ft.
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Example 1
Solution d 20.00 2(0.635) 18.73
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Casing Design
Calculate Loads on the Casing Burst Pressure
The casing will experience a net burst loading if
the internal radial load exceeds the external
radial load.
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Casing Design
Calculate Loads on the Casing Burst Pressure
9
Casing Design
Calculate Loads on the Casing Burst Pressure
10
Casing Design
Calculate Loads on the Casing Burst Pressure
If casing is subjected to internal pressure
higher than external, it is said that casing is
exposed to burst pressure. Burst pressure
conditions occur during well control operation or
squeeze cementing. Equation (4) is used to
calculate the internal pressure at which the
tangential stress at the inner wall of the pipe
reaches the yield strength of the material. The
factor 0.875 represents the allowable
manufactruing tolerance of -12.5 on wall
thickness. Because a burst pressure failure will
not occur until after the stress exceeds the
ultimate tensile strength, using a yield strength
criterion as a measure of burst strength is an
inherently conservative assumption.
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Example 2
Compute the burst-pressure rating for 20, K-55
casing with a nominal wall thickness of 0.635
and a nominal weight per foot of 133
lbf/ft Solution
Rounded to the nearest 10 psi
12
Casing Design
Calculate Loads on the Casing Collapse Pressure
The casing will experience a net collapse loading
if the external radial load exceeds the internal
radial load. The greatest collapse load on the
casing will occur if the casing is evacuated
(empty) for any reason.
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Casing Design
Calculate Loads on the Casing Collapse Pressure
If external pressure exceeds internal pressure,
the casing is subjected to collapse. Such
conditions may exist during cementing operations
or well evacuation. Collapse strength is
primarily function of the materials yield
strength and its slenderness ratio, dn/t.
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Casing Design
Calculate Loads on the Casing Collapse Pressure
pe, pi external and internal pressure sr, st
radial and tangential stresses Note equations
(5) and (6) are used under no axial tension or
axial compression. Data in Table 7.6 apply only
for zero axial tension and no pipe bending.
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Example 3
Consider a drillpipe of E-75 4 ½ outer diameter
with a unit weight of 20 lb/ft inside a wellbore
filled with 9.5 ppg mud. At a location of 3800 ft
from the surface, pressure inside the pipe is
2000 psi, and pressure outside the pipe is 1700
psi. Determine the tangential and radial stresses
at r ro.
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Example 3
E-75 4 ½ and 20 lb/ft drillpipe has an inner
diameter of 3.64 in. Considering r is equal to
ro 2.25
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Casing Design
Collapse Pressure Regimes
The collapse strength criteria consist of four
collapse regimes determined by yield strength and
dn/t. Each criterion is discussed next in order
of increasing dn/t. Yield strength collapse
Yield strength collapse is based on yield at the
inner wall. This criterion does not represent a
collapse pressure at all. For thick wall pipes
(dn/t lt 15), the tangential stress exceeds the
yield strength of the material before a collapse
instability failure occurs. Assumed that the
pipe is subjected only to an external pressure
pe. From eq. (6), the absolute value of
tangential stress st is always greatest at the
inner wall of the pipe and that for burst and
collapse loads. Hence, the yield strength
collapse occurs at the inner wall r ri then
equation (6) becomes
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Casing Design
Collapse Pressure Regimes
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Casing Design
Collapse Pressure Regimes
Plastic collapse Plastic collapse is based on
empirical data from 2,488 tests of K-55, N-80 and
P-110 seamless casing. No analytic expression has
been derived that accurately models collapse
behavior in this regime. The minimum collapse
pressure for the plastic range of collapse is
calculated by equation (10).
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Casing Design
Collapse Pressure Regimes
Transition Collapse Transition collapse is
obtained by a numerical curve fitting between the
plastic and elastic regimes. The minimum collapse
pressure for the plastic-to-elastic transition
zone is calculated by equation (11)
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Casing Design
Collapse Pressure Regimes
Elastic Collapse Elastic collapse is based on
theoretical elastic instability failure this
criterion is independent of yield strength and
applicable to thin-wall pipe (dn/t gt 25). The
minimum collapse pressure for the elastic range
of collapse is calculated by using equation
(12) Most oilfield tubulars experience collapse
in the plastic and transition regimes.
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Casing Design
Collapse Pressure Regimes
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Casing Design
Collapse Pressure Regimes
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Casing Design
Collapse Pressure Regimes
Apply only when axial stress is zero and no
internal pressure
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Example
Compute the collapse pressure rating for 20,
K-55 casing with a nominal wall thickness of
0.635 and a nominal weight per foot of 133
lbf/ft. Solution dn/t 20/0.635 31.49 This is
the transition collapse
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Casing Design
Combined Stress Effects
All the pipe strength equations previously given
are based on a zero axial stress state. This
idealized situation never occurs in oilfield
applications because pipe in a wellbore is always
subjected to combined loading conditions. The
fundamental basis of casing design is that if
stresses in the pipe wall exceed the yield
strength of the material, a failure condition
exists. Hence the yield strength is a measure of
the maximum allowable stress. To evaluate the
pipe strength under combined loading conditions,
the uniaxial yield strength is compared to the
yielding condition.
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Casing Design
Combined Stress Effects
The most widely accepted yielding criterion is
based on the maximum distortion energy theory,
which is known as the Huber-Von-Mises Theory.
This theory states that if the triaxial stress
exceeds the yield strength, a yield failure is
indicated. Note that the triaxial stress is not a
true stress. It is a theoretical value that
allows a generalized three-dimensional stress
state to be compared with a uniaxial failure
criterion (the yield strength).
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Casing Design
Combined Stress Effects
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Casing Design
Combined Stress Effects
Setting the triaxial stress equal to the yield
strength and solving equation (13) give the
results Equation (14) is for the ellipse of
plasticity. Combining Eq. (14) and eq. (6)
together and let r ri, will give the
combinations of internal pressure, external
pressure and axial stress that will result in a
yield strength mode of failure.
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Casing Design
Combined Stress Effects
As axial tension increases, the critical
burst-pressure increases and the critical
collapse-pressure decreases. In contrast, as the
axial compression increases, the critical
burst-pressure decreases and the critical
collapse-pressure increases.
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Example
Compute the nominal collapse pressure rating for
5.5, N-80 casing with a nominal wall thickness
of 0.476 and a nominal weight per foot of 26
lbf/ft. In addition, determine the collapse
pressure for in-service conditions in which the
pipe is subjected to a 40,000 psi axial tension
stress and a 10,000 psi internal pressure. Assume
a yield strength mode of failure.
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Example
For collapse pressure rating, r ri then eq.
(6) becomes
33
Example
From eq. (14) with we have
34
Example
For in-service conditions of sz 40,000 psi and
pi 10,000 psi
Solving eq. (14) gives