Casing/Tubing design manual october 2005 Chevron
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∆L =
TL
∆
−
α
(10-3
∆
TE
∆
−
=
α
σ
(10-2)
∆
s
AF
σ
∆
=
(10-3)
Figure 10-1 shows an example of a 1,500 ft. of 7-in., 32#, L-80 production casing
string in a vertical steam injection well with an average casing temperature
increase of 400ºF in steam injection period. The thermal expansion of the casing
string can be calculated as:
ε
= 0.0000069*400 = 0.00276
∆
L =
ε
L = 0.00276*1500 = 4.14 ft
L
∆Τ
∆
L
L
Figure 10-1. Casing Expansion in Steam Injection Condition
If the casing thermal expansion is restricted, the thermal axial expansion will be
converted into a thermal axial compressive stress:
∆
σ
= -
ε
E = -0.00276*30000000 = - 82,899 psi
This thermal axial compressive stress exceeds the 7-in., 32#, L-80 grade casing
material yield strength of 80,000 psi. There is a casing material yield stress
reduction at high temperature 204ºC (400ºF), resulting in hot-yield of the casing
under the steam injection condition.
10-2 Casing/Tubing Design Manual
October 2005
10.2.2 Casing Thermal Hot-Yield
Table 10-1 lists the elevated temperatures for different grades of casing in steam
injection wells, where the casing thermal axial expansion is restricted, causing
casing hot-yield. The initial casing temperature is assumed to be 15ºC (60ºF) and
the initial casing axial stress is assumed to be zero. Casing material yield
strength reduction at high temperature is included. It is seen that low-strength
casing of K-55, J-55, N-80, and L-80 will become hot-yielded when the casing is
heated-up to a temperature of 204ºC (400ºF).
Table 10-1. Casing Grade Hot-Yield Temperature
Casing Grade
Nominal Yield Strength
(MPa/psi)
Compressive Yield
Temperature
(ºC/ºF)
J-55, K-55 380/55,000 150/303
L-80, N-80 550/80,000 203/397
C-95, T-95 655/95,000 232/450
P-110 760/110,000 260/500
Q-125 860/125,000 287/548
The casing thermal load and hot-yield development in the steam injection well
can be considered as follows (assume the casing is well-cemented):
• The casing axial compressive load (stress) increases with the start of steam
injection which heats-up the casing (Figure 10-2). As the casing temperature
increases during the injection period, the casing axial thermal load (stress)
increases, until the casing becomes hot-yielded at the temperature listed in
Table 10-1. If the casing is poorly cemented, the casing may fail at hot-yield
because of severe buckling and collapse. When the casing is well-cemented,
it may not fail by buckling and may not collapse at hot-yield because of the
cement support. The casing axial compressive stress then stays almost the
same or reduces slightly as the temperature continues to increase beyond
casing hot-yield temperature.
• Casing tension load (stress) can be developed as the casing temperature
drops in the cooling (soak/production) period and the casing may fail in
tension when the developed casing tension stress reaches the casing
tension yield limit (Figure 10-2) if the casing is not failed by hot-yield in steam
injection period.
Casing/Tubing Design Manual 10-3
October 2005
Figure 10-2. Casing Axial Load (Stress) Curve on Steam Injection Wells (Reference
1)
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October 2005
Figure 10-3. Casing Axial Load vs. Temperature (Lab Test on L-80 Casing),
(Reference 2)
10.2.3 Special Casing Thermal Load Design
Because of the particular thermal axial load condition on the steam injection well,
a casing thermal load design, including thermal axial compression load (in the
steam injection period), needs to be considered for a production casing design in
steam injection wells.
10.2.4 Casing Thermal Compression Design
To properly conduct the casing thermal load design, you must first model the
casing temperature increase (∆T). The increase must be modeled from its initial
temperature (usually the undistributed formation temperature) to the elevated
temperature in the steam injection period. Depending on steam injection
temperature, steam injection duration time, and steam injection method (through
tubing with/without packer), the casing elevated temperature may be modeled by
using computer software WELLCAT
™
software, as shown in Figure 10-4.
Figure 10-4 shows the modeling result of a 7-in. 29# L-80 production casing set
at 2,000-ft depth in an example vertical steam injection well. The steam injection
temperature is 288ºC (550ºF), steam injection rate is 2,500 B/D, and injection
duration time three days, through a 3-1/2-in.tubing (no tubing packer, casing
Casing/Tubing Design Manual 10-5
October 2005
10-6 Casing/Tubing Design Manual
October 2005
perforation at 1,900 ft). The undisturbed formation temperature is 16ºC (60ºF) at
surface with a gradient of 3.2 deg/100 ft. The 7-in. production casing is full-
cemented and tied on the 10-¾-inch surface casing at the wellhead (restricted
from thermal expanding axially).
The modeling result shows that the 7-in. 29# L-80 production casing temperature
will increase up to about 222ºC (430ºF) after a three-day steam injection (the 7-
in. casing is labeled as “Casing 1” in Figure 10-4), and the axial compressive
load reaches about 600,000 lb, causing a casing hot-yield. The reduced casing
body yield strength is 589,000 lb at the elevated temperature of 205ºC (400ºF).
This is also shown in Figure 10-4, where the casing’s load line for “three day
injection” goes slightly out of the von Mises yield ellipse at high thermal axial
compression load indicating the casing thermal hot-yield.
Also shown in Figure 10-4 is the casing buckling on the top 100 ft of the 7-in.
casing under the high thermal axial compression load, due to a lack of cement
support. The casing buckling produces additional casing axial compression that
is shown as a big jump on the casing axial compressive load curve (labeled as
“with bending” because of the buckling bending and makes the casing hot-yield
even worse.
The goal of casing thermal compression design is simply to:
• Design the production casing to avoid hot-yield (compression design factor
1.00), if possible.
• Reduce casing failure risk related to casing hot-yield as the casing hot-yield
makes the casing become unstable and vulnerable to collapse.
Figure 10-4. Casing Temperature and Hot-Yield Modeling for an Example Steam Injection Well, 288ºC (550ºF) Steam Injection
Temperature
Casing/Tubing Design Manual 10-7
October 2005
Casing/Tubing Design Manual 10-8
October 2005
10.2.5 Casing Thermal Tension Design
If casing thermal compression design is met and the casing does not hot-yield in
the steam injection period, then casing thermal tension design is un-needed. This
is because high thermal tension load will not develop in the cooling
(soak/production) period. However, if casing thermal compression design is not
met (casing is thermally hot-yielded in the steam injection period) for some
reason, and assuming the casing will not fail because of the hot-yield casing
thermal tension, design is then needed for the casing thermal tension load
developed during the cooling (soak/production) period.
To determine the casing thermal tension load, casing temperatures at steam
injection and at soak/production need to be modeled. Casing thermal tension
load is calculated by the casing temperature change from steam injection to
soak/production. For the example well (Figure 10-4), if the steam injection
temperature is 343ºC (650ºF), the 7-in. 29# L-80 production casing temperature
will be elevated in a three-day steam injection period about 277ºC (530ºF) and
the casing will be hot-yielded indicating the casing thermal hot-yield. The casing’s
load line for the “injection three days” comes from the von Mises yield ellipse.
The casing temperature dropped at the 15-day production period will be about
54ºC (130ºF). Production rate is 320 B/D, as shown in Figure 10-5.
Figure 10-5. Casing Temperature and Hot-Yield Modeling for an Example Steam Injection Well, 344ºC (650ºF) Steam Injection
Temperature
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October 2005
Because the 7-in. 29# L-80 casing will start hot-yielding at about 203ºC (397ºF)
(Table 10-1), the casing thermal axial load versus casing temperature is roughly
illustrated in Figure 10-6. The casing thermal tension load generated at the
production period (cooling temperature) will be about 182,000 lb giving a casing
tension safety factor of 3.71 = 676,000/182,000 (7-in. 29# L-80 casing tension
yield limit is 676,000 lb).
7" 29# L-80 Production Casing Load
-800000
-600000
-400000
-200000
0
200000
400000
0 100 200 300 400 500 600
Casing Temperature, Deg. F
Casing Load, lb
Steam Injection
Production
Hot-yield
Figure 10-6. Casing Axial Load Curve on an Example Steam Injection Well, 344ºC
(650ºF) Steam Injection Temperature
Casing thermal tension design is complicated and it assumes that the casing will
not fail on hot-yield in steam injection period. The casing thermal tension design
factor is still 1.60. This avoids casing body or connection tension failure in the
cooling (soak/production) period.
10.2.6 Casing Design Practices to Reduce Hot-
Yield
10.2.6.1 High-Strength Casing
High-strength casing (C-95, P-110, or Q-125 grade with T95 grade chemical
requirements) can withstand an elevated temperature of 260ºC (500ºF) (as
shown in Table 10-1) and prevent casing thermal hot-yield in the steam injection
period.
Casing/Tubing Design Manual
October 2005
One concern when using high-strength casing (C-95, P-110, or Q-125) in steam
injection wells, is the sulfide stress cracking risk, with possible H
2
S involved in
the steam injection operation. However, the following should be considered:
• The casing temperature is usually higher than the sour cracking threshold
temperature, 66ºC (150ºF) for C-95, 175ºF for P-110, or 225ºF for Q-125.
• The casing tensile stress level is usually lower than the sour cracking
threshold tensile stress (either axial tensile stress or hoop tensile stress) in
terms of percentage of casing material yield strength (40% for C-95, 20% P-
110 and Q-125, as shown in Table 10-2 and Figure 10-2 for typical steam
injection wells [low injection pressure and shallow depth])
Table 10-2. Test Results and Approximate Threshold Stresses (Reference 3)
SSC Test Results and Approximate Threshold Stresses
Material SMYS
(ksi)
Applied Stress
(% of SMYS)
TTF
(hours)
Approximate
Threshold
20 No Fail
30 No Fail
40 No Fail
40 16
50 160
55 16
60 39
70 12
C-95
Non-Sour
Service
95
80 8
40
70 No Fail
75 No Fail
80 No Fail
95 No Fail
100 142
100 42
T-95
Sour Service
95
110 2
95
10 No Fail
30 8
35 10
40 No Fail
45 No Fail
50 6
55 No Fail
P-110
Non-Sour
Service
110
60 6
20
Figure 10-7 shows the maximum no-failure stress of casing under sour service
conditions at 24ºC (75ºF) in terms of the percent yield strength of the casing
grade. It shows that P-110 or Q-125 casing can be used for any H
2
S
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October 2005