Production Casing (7-in. pipe)
Production casing is set to a depth of 19,000 ft and partially cemented at the casing seat. The design load calculations for collapse and burst are presented in
Fig. 3.13.
The collapse design is based on the premise that the well is in its last phase of production and the reservoir has been depleted to a very low abandonment
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Table 3.24: Most economical drilling liner based on collapse and burst loads.
|
|
(1) |
(2) |
(3) |
(4) |
|
Depth |
Grade and |
Buoyant weight |
Cumulative buoyant |
|
(ft) |
Weight |
of section |
weight carried |
|
(lb/ft) |
joint (1.000 lbf) |
by the top joint |
|
|
(1) x Wnx BF |
(1,000 lbf) |
||
|
BF = 0.743 |
|||
|
14,000 — 12,500 |
L-80, 58.4 |
65.105 |
65.105 |
|
12,500 — 10,500 |
P-110, 47 |
69.861 |
134.966 |
|
(5) |
(6) |
(?) |
|
Shock load |
Total tension |
|
|
carried by each |
(1.000 lbf) |
9F — ■ Total tension |
|
section (1,000 lbf) |
(4) + (5) |
|
|
(3,200 Wn) |
||
|
186.88 |
251.985 |
1.151/251.985 = 4.57 |
|
150.40 |
285.366 |
1.213/285.366 = 4.25 |
pressure (Bourgoyne et al.. 1985). During this phase of production, any leak in the tubing may lead to a partial or complete loss of packer fluid from the annulus between the tubing and the casing. Thus, for the purpose of collapse design the following assumptions are made:
1. Casing is considered empty.
2. Fluid specific weight outside the pipe is the specific weight of the drilling fluid inside the well when the pipe was run.
3. Beneficial effect of cement is ignored.
Based on the above assumptions, the design load for collapse can be calculated as follows:
Collapse pressure at surface = 0 psi
Collapse pressure at casing seat
= external pressure — internal pressure
|
(a) COLLAPSE (ь) BURST Fig. 3.13: Collapse and burst loads on the production casing. |
In Fig. 3.13, the collapse line is constructed between 0 psi at the surface and 13,031 psi at 19,000 ft. Collapse resistance of the suitable grades from Table
3.3 are presented in Table 3.26 and all these grades satisfy the requirement for maximum collapse design load.
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Table 3.26: Collapse resistance of grades suitable for production casing.
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In most cases, production of hydrocarbons is via tubing sealed by a packer, as shown in Fig. 3.13. Under ideal conditions, only the casing section above the shoe will be subjected to burst pressure. The production casing, however, must be able to withstand the burst pressure if the production tubing fails. Thus, the design load for burst should be based on the worst possible scenario.
For the purpose of the design of burst load the following assumptions are made:
1. Producing well has a bottomhole pressure equal to the formation pore pressure and the producing fluid is gas.
2. Production tubing leaks gas.
3. Specific weight of the fluid inside the annulus between the tubing and casing is that of the drilling fluid inside the well when the pipe was run.
4. Specific weight of the fluid outside the casing is that of the deteriorated drilling fluid, i. e., the specific weight of saturated salt water.
Based on the above assumptions, the design for burst load proceeds as follows: Burst load at surface = internal pressure — external pressure
Internal pressure at surface = shut-in bottomhole pressure
— hydrostatic head of the gas column = 17.45 x 0.052 x 19,000 — 0.1 x 19.000 = 15,340.6 psi
Burst pressure at surface = 15.340.6 — 0
= 15.340.6 psi
Burst pressure at casing shoe = internal pressure — external pressure
Internal pressure at casing shoe = hydrostatic pressure of the fluid column
+ surface pressure due to gas leak at top of tubing = 17.9 x 0.052 x 19,000 + 15,340.6 = 33.025.8 psi
External pressure at shoe = 0.4bo x 19.000
= 8.835 psi
rst pressure at casing shoe = 33,025.8 — 8.83"
= 9J 1 Qfl Я
In Fig. 3.13, the burst line is drawn between 15.350.6 psi at the surface and 24,190.8 psi at 19,000 ft. The burst resistances of the suitable grades from Table
3.3 are shown in Table 3.27 and are plotted as vertical lines in Fig 3.14.
|
CASING SHOE 19000 ft Collapse load line Burst load line |
|
Depth in feet |
Fig. 3.14: Selection of casing grades and weights based on ihe collapse and burst loads for the production casing.
|
Grade |
Weight (lb/ft) |
Coupling |
Burst resistance (psi) SF = 1 .ST =1.1 |
|
V-150 |
38 |
PTC |
18.900 17.182 |
|
MW-155 |
38 |
Extreme — line |
20.930 19.028 |
|
V-150 |
46 |
PTC |
25.070 22.790 |
|
SOO-155 |
46 |
PTC |
25.910 23.550 |
Selection based on collapse and burst
From Fig. 3.14, it is evident that grade SOO-155. which has the highest burst resistance properties, satisfies the design requirement up to 17.200 ft. It will also satisfy the design requirement up to 16.000 ft if the safety factor is ignored. Thus, grade SOO-155 can be safely used only if it satisfies the other design requirements. The top of cement must also reach a depth of 17.200 ft to provide additional strength to this pipe section. Hence, the selection based on collapse and burst is shown in Table 3.28.
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Table 3.28: Most economical production casing based on collapse and burst loads.
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The suitability of the selected grades under tension is checked by considering cumulative buoyant weight, shock load, and pressure testing. Thus, starting from the bottom, Table 3.29 is produced which shows that all the sections satisfy the requirement for tensional load based on buoyant weight and shock load.
Grade V-150 (38 lb/ft) has the lowest safety factor and should, therefore, be checked for pressure testing. Tensional load carried by this section due to the
|
(1) Depth (ft) |
(2) Grade and Weight (lb/ft) |
(3) . Buoyant weight of each section joint (1.000 lbf) (1) xVF„ x BF BF = 0.726 |
, (4) Cumulative buoyant weight carried by the top joint (1.000 lbf) |
|
19,000 — 16,000 |
SOO-155, 46 |
100.25 |
100.25 |
|
16,000 — 8,000 |
V-150, 46 |
267.34 |
367.59 |
|
8,000 — 3,000 |
MW-155, 38 |
138.03 |
505.62 |
|
3,000 — 0 |
V-150, 38 |
82.82 |
588.43 |
TOC o "1-5" h z (5) (6) (7)
Shock load Total load
Y
carried by carried by the SF = Total load
each joint (1,000 lbf) top joint (1.000 lbf)
|
1.344/247.51 = 5.43 1.344/514.79 = 2.61 1.592/627.21 = 2.56 1,430/710.03 = 2.01 |
3,200 x Wn
147.20 247.51
147.20 514.79
121.60 627.21
121.60 710.03
pressure testing is equal to:
Ft = 18,900 x 0.6 x (A, = 10.95)
= 124,173 lbf
Total tension load carried by V-150 (38 lb/ft)
= buoyant weight carried by the top joint = + tensional load due to the pressure testing
= 588,430 + 124,173 = 712,603 lbf
( Yv _ 1,430,000
SF = -—£—- = 1 1 = 2.01
VTotal load/ 712,603
Inasmuch as this value is greater than the design safety factor of 1.8. grade V-150 (38 lb/ft) satisfies tensional load requirements.
Axial tension reduces the collapse resistance and is most critical at the joint of the weakest grade. All the grades selected for production casing have significantly higher collapse resistance than required. Casing sections from the intermediate position, however, can be checked for reduced collapse resistance (V-150, 46 lb/ft) at 8,000 ft.
As illustrated previously, the modified collapse resistance of grade V-150 (46 lb/ft) under an axial load of 367,356 lbf can be calculated to be ‘23,250 psi. Hence,
|
SF for collapse = |
Reduced collapse resistance Collapse pressure at 8.000 ft 23, ‘250
5,600 4.15
The final selection is summarized in Table 3.30.
|
Table 3.30: Final casing selection for production casing string.
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Usually, buckling is prevented by cementing up to the neutral point where no potential buckling exists. As discussed previously (p. l 16). the depth of the neutral point, x, can be determined by using the following equation:
D(Wn — A0GP + AtGp) + (1 — 2v)(A0ApSo — AtAps )
x = W Т7~~г’———- ‘———————- continue -*■
— (A0GPo — A, GPt)
+ (AUP1 AloVJl ) X [ D^,4; (G’p, + AGPt) + Лp5l] — ASETT + Fas
|
Pc |
-(1 — u)(A0AGPo — A, AGPt) + Ad(GPo — G
where:
Wn = average weight
= (Wni x h) + (Wni x l2)
D
38 x 8,000 + 46 x 11,000 “ 19,000
= 42.63 lb/ft
Ai = average internal area of the pipe
_ (Ai)i x lx + (Aj)-2 x l2
~ D
27.51 x 8,000 + 25.14 x 11,000
“ 19,000
= 26.16 in.2
As = average cross-sectional area of the steel
As! x l + AS2 x /2
~ D
10.95 x 8,000 + 13.32 x 11,000
~ 19,000
= 12.33 in.2
7. = lo = 17.9 ppg
A-ti = A’fo = 0 ppg
AGVi — AGPo = 0 psi/ft Лр51 = Др50 = 0 psi
(A*, — Aiow 1) = average change in internal diameter
= 27.53 — 26.16 = 2.37 in.2
It is also assumed that ~fcma — 18.5 ppg, ДT = 45°F and Fas = 0. Hence (see Eq. 2.212):
19,0 (42.63 — 38.46 x 18.5 x 0.052 + 26.14 x 0.931) + 0 .
Drnr = ————————————————————————- —i* continue
iUC 42.63 — (38.46 x 17.9 x 0.052)
+ 2.37 x 8,000 x 0.931 — 12.32 x 6.9 x 10~6 x 30 x 106 x 45 + 0 ^ +26.14 x 17.9 x 0.052 — 0 + 38.46(0.931 — 0.962)
a~tcm is the specific weight of the cement slurry, lb/gal.
472.280
~ 29.42
= 16.053 ft.
Thus, the casing between 16,053 ft and 19.000 ft is under compressive load and is liable to buckle. To prevent buckling of the pipe it must be cemented to 16.053 ft from the surface.
Alternatively, an overpull, Fas. equal in magnitude to the difference between the axial stress and the average of radial and tangential stresses can be applied at the surface after landing of the pipe. If. for example, the maximal depth of the cement top is set at 18.000 ft. the magnitude of the over-pull required to prevent buckling of the pipe can be obtained as follows:
29.42 and solving for Fas:
Fas = 57,280 lbf