Surface Casing (16-in.)
Surface casing is set to a depth of 5,000 ft and cemented back to the surface. Principal loads to be considered in the design of surface casing are: collapse, burst, tension and biaxial effects. Inasmuch as the casing is cemented back to the surface, the effect of buckling is ignored.
Collapse pressure arises from the differential pressure between the hydrostatic heads of fluid in the annulus and the casing, it is a maximum at the casing shoe and zero at the surface. The most severe collapse pressures occur if the casing is run empty or if a lost circulation zone is encountered during the drilling of the next interval.
At shallow depths, lost circulation zones are quite common. If a severe lost — circulation zone is encountered near the bottom of the next interval and no other permeable formations are present above the lost-circulation zone, it is likely that the fluid level could fall below the casing shoe, in which case the internal pressure at the casing shoe falls to zero (complete evacuation). Similarly, if the pipe is run empty, the internal pressure at the casing shoe will also be zero.
At greater depths, complete evacuation of the casing due to lost-circulation is never achieved. Fluid level usually drops to a point where the hydrostatic pressure of the drilling fluid inside the casing is balanced by the pore pressure of the lost circulation zone.
Surface casing is usually cemented to the surface for several reasons, the most important of which is to support weak formations located at shallow depths. The presence of a cement sheath behind the casing improves the collapse resistance by up to 23% (Evans and Herriman, 1972) though no improvement is observed if the cement sheath has voids. In practice it is almost impossible to obtain a void-free cement-sheath behind the casing and, therefore, a saturated salt-water gradient is assumed to exist behind the cemented casing to compensate for the effect of voids on collapse strength. Some designers ignore the beneficial effect of cement and instead assume that drilling fluid is present in the annulus in order to provide a built-in safety factor in the design. In summary, the following assumptions are made in the design of collapse load for surface casing (see Fig. 3.6(a)):
1. The pressure gradient equivalent to the specific weight of the fluid outside the pipe is that of the drilling fluid in the well when the pipe was run.
|
(a) COLLAPSE (b) BURST |
Fig. 3.6: Collapse and burst load on surface casing.
‘2. Casing is completely empty.
3. Safety factor for collapse is 0.85.
Collapse pressure at the surface = 0 psi
Collapse pressure at the casing shoe:
Collapse pressure = external pressure — internal pressure = GPm x 5,000 — 0 = 9.5 x 0.052 x 5.000 — 0 = 2,470 psi
In Fig. 3.7, the collapse line is drawn between 0 psi at the surface and 2.470 psi at
5,0 ft. The collapse resistances of suitable grades from Table 3.3 are presented below.
Collapse resistances for the above grades are plotted as vertical lines in Fig. 3.7. The points at which these lines intersect the collapse load line are the maximal depths for which the individual casing grade would be suitable. Hence, based on collapse load, the grades of steel that are suitable for surface casing are given in Table 3.5.
|
Pressure in ds! |
Selection bosed on |
|||||
|
1000 |
2000 3000 4000 5000 |
Collopse |
Burst |
Collopse Burst |
||
|
K55 75# -1000 |
1 K55 109# |
/1 I1 ‘ I 1 /II |
— in»*: о tn О * Г"- |
00 ft K55 109# |
00 ft |
|
|
— 2000 |
/ / L80 / 84# j |
1 I ! j L80 1 j 84# I 1 |
2450 ft |
L80 84# |
L80 84# |
|
|
-3000 |
I / / i—— /I / I |
I i 1 i _ l__l___________ 1 1 I 1 |
L80 -84#-] 3500 ft |
3000 _ft_ |
3500 ft |
|
|
-4000 -5000 — |
f 1 VK55 A75# I i i V 4 |
I > 1 1 1 j K55 J 109# I i i |
K55 109# 5000 ft |
K55 75# 5000 ft |
K55 109# 5000 ft |
|
CASING SHOE ■ ‘ Collapse lood line |
————————— Burst load line
Fig. 3.7 : Selection of steel grade and weight based on the collapse and burst load for 16-in. surface casing.
The design for burst load assumes a maximal formation pressure results from a kick during the drilling of the next hole section. A gas-kick is usually considered to simulate the worst possible burst load. At shallow depths it is assumed that the influx of gas displaces the entire column of drilling fluid and thereby subjects the casing to the kick-imposed pressure. At the surface, the annular pressure is zero and consequently burst pressure is a maximum at the surface and a minimum at the shoe.
For a long section, it is most unlikely that the inflowing gas will displace the entire Table 3.4: Collapse resistance of grades suitable for surface casing.
|
Grade |
Weight |
Coupling |
Collapse resistance |
|
|
(lb/ft) |
(psi) |
|||
|
SF = 1 |
SF = 0.85 |
|||
|
K-55 |
75 |
STC |
1.020 |
1.200 |
|
L-80 |
84 |
STC/BTC |
1.480 |
1.741 |
|
K-55 |
109 |
BTC |
2.560 |
3,012 |
|
Section |
Interval |
Grade and |
Length |
|
(ft) |
Weight (lb/ft) |
(ft) |
|
|
1 |
0 — 2.450 |
K-55. 75 |
2,450 |
|
2 |
2,450 — 3.550 |
L-80. 84 |
1,100 |
|
3 |
3,550 — 5.000 |
K-55, 109 |
1,450 |
column of drilling fluid. According to Bourgoyne et al. (1985), burst design for a long section of casing should be such as to ensure that the kick-imposed pressure exceeds the formation fracture pressure at the casing seat before the burst rating of the casing is reached. In this approach, formation fracture pressure is used as a safety pressure release mechanism so that casing rupture and consequent loss of human lives and property are prevented. The design pressure at the casing seat is assumed to be equal to the fracture pressure plus a safety margin to allow for an injection pressure: the pressure required to inject the influx fluid into the fracture.
Burst pressure inside the casing is calculated assuming that all the drilling fluid inside the casing is lost to the fracture below the casing seat leaving the influx — fluid in the casing. The external pressure on the casing due to the annular drilling fluid helps to resist the burst pressure: however, with time, drilling fluid deteriorates and its specific weight drops to that of saturated salt-water. Thus, the beneficial effects of drilling fluid and the cement sheath behind the casing are ignored and a normal formation pressure gradient is assumed when calculating the external pressure or back-up pressure outside the casing.
The following assumptions are made in the design of strings to resist burst loading (see Fig. 3.6(b)):
1. Burst pressure at the casing seat is equal to the injection pressure.
2. Casing is filled with influx gas.
3. Saturated salt water is present outside the casing.
4. Safety factor for burst is 1.1.
Burst pressure at the casing seat = injection pressure — external pressure, p0, at
5,0 ft.
Injection pressure = (fracture pressure + safety factor) x 5,000
Again, it is customary to assume a safety factor of 0.026 psi/ft (or equivalent drilling fluid specific weight of 0.5 ppg).
Injection pressure = (14.76 + 0.5) 0.05’2 x 5,000
= 3,976.6 psi
External pressure at 5,000 ft = saturated salt water gradient x 5,000
= 0.465 x 5,000 = 2,325 psi
Burst pressure at 5,000 ft = 3.976.6 — 2.325
= 1,651.6 psi
Burst pressure at the surface = internal pressure — external pressure
Internal pressure = injection pressure — Gp x 5,000
= 3,976.6 — 500 = 3,476.6 psi
where:
GPg = O. lpsi/ft
Burst pressure at the surface = 3,476.6 — 0
= 3,476.6 psi
In Fig. 3.7, the burst load line is drawn between 3,476.6 psi at the surface and 1,651.6 psi at a depth of 5,000 ft. The burst resistances of suitable grades are presented in Table 3.6.
Table 3.6: Burst resistance of grades suitable for surface casing.
Grade Weight Coupling Burst resistance
(lb/ft) (psi)
TOC o "1-5" h z, SF = 1 SF — 1.1 K-55 75 STC 2,630 2,391
L-80 84 STC/BTC 4,330 3,936
K-55 109 BTC 3,950 3,591
The burst resistances of the above grades are also plotted as vertical lines in Fig.
3.7. The point of intersection of the load line and the resistance line represents the maximal depth for which the individual grades would be most suitable. According to their burst resistances, the steel grades that can be selected for surface casing are shown in Table 3.7.
|
Section |
Depth |
Grade and |
Length |
|
(ft) |
Weight (lb/ft) |
(ft) |
|
|
1 |
3,000 — 5.000 |
K-55. 75 |
2.000 |
|
2 |
0 — 3.000 |
L-80. 84 |
3,000 |
|
3 |
0 — 3.000 |
K-55. 109 |
3.000 |
Selection Based on Both Collapse and Burst Pressures
When the selection of casing is based on both collapse and burst pressures (see Fig. 3.7), one observes that:
1. Grade K-55 (75 lb/ft) satisfies the collapse requirement to a depth of 2.450 ft, but does not satisfy the burst requirement.
2. Grade L-80 (84 lb/ft) satisfies burst requirements from 0 to 5.000 ft but only satisfies the collapse requirement from 0 to 3.550 ft.
3. Grade K-55 (109 lb/ft) satisfies both collapse and burst requirements from 0 to 5,000 ft.
4. Steel grade K-55 (75 lb/ft) can be rejected because it does not simultaneously satisfy collapse and burst resistance criteria across any section of the hole.
For economic reasons, it is customary to initially select the lightest steel grade because weight constitutes a major part of the cost of casing. Thus, the selection of casing grades based on the triple requirements of collapse, burst, and cost is summarised in Table 3.8.
|
Table 3.8: Most economical surface casing based on collapse and burst loading.
|
As discussed in Chapter 2, the principal tensile forces originate from pipe weight, bending load, shock loads and pressure testing. For surface casing, tension due to bending of the pipe is usually ignored.
In calculating the buoyant weight of the casing, the beneficial effects of the buoyancy force acting at the bottom of the string have been ignored. Thus, the neutral point is effectively considered to be at the shoe until buckling effects are considered.
The tensile loads to which the two sections of the surface casing are subjected are presented in Table 3.9. The value of Yp — 1.861 x 103 lbf (Column (7)) is the joint yield strength which is lower than the pipe body yield strength of 1.929 x 103 lbf. ~ ”
|
Table 3.9: Total tensile loads on surface casing string.
|
|
(5) |
(6) |
(7) |
|
Shock load carried |
Total tension |
|
|
by each section |
(1.000 lbf) |
qjp ^P " Total tension |
|
(1,000 lbf) |
(4) + (5) |
|
|
3.200 И’, |
||
|
348.8 |
484.022 |
1.738/484.022 = 3.59 |
|
268.8 |
659.152 |
1,861/659.152 = 2.82 |
It is evident from the above that both sections satisfy the design requirements for tensional load arising from cumulative buoyant weight and shock load.
Pressure Testing and Shock Loading
During pressure testing, extra tensional load is exerted on each section. Thus, sections with marginal safety factors should be checked for pressure testing conditions.
Tensional load due to pressure testing
= burst resistance of weakest grade (L-80, 84) x 0.6 x As = 4,330 x 0.6 x 24.1 = 62,611.8 lbf
Total tensional load during pressure testing
= cumulative buoyant load + load due to pressure testing
Shock loading occurs during the running of casing, whereas pressure testing occurs after the casing is in place; thus, the affects of these additional tensional forces are considered separately. The larger of the two forces is added to the buoyant and bending forces which remain the same irrespective of whether the pipe is in motion or static.
Hence,
Y
SF = ———— — L.——-
Total tension load
1,861,0 ( „
“ 62,611.8 + 390,352 ~ 4-11 This indicates that the top joint also satisfies the requirement for pressure testing.
It was shown previously that the tensional load has a beneficial effect on burst pressure and a detrimental effect on collapse pressure. It is, therefore, important to check the collapse resistance of the top joint of the weakest grade of the selected casing and compare it to the existing collapse pressure. In this case, L-80 (84 lb/ft) is the weakest grade. Reduced collapse resistance of this grade can be calculated as follows:
Buoyant weight carried by L-80 (84 lb/ft) = 135,222 lbf.
(1) Axial stress due to the buoyant weight is equal to:
_ 135,222
°a 7r(d2 — dj)/4
135,222 “ тг(162 — 152)/4 = 5,608 psi
(2) Yield stress is equal to:
1,929,000 ay ~ 7r(162 — 152)/4
= 80,000 psi
(3) From Eq. 2.163, the effective yield stress is given by:
0.5
|
-0.5 | — |
|
и e о,, |
1 — 0.75 I —
|
5,608 80,000 |
|
5,608 80,000 |
|
0.5 |
|
= 80,000 = 77,048 psi |
|
1 — 0.75 |
(4) djt = 16/0.495 = 32.32
(5) The values of A, B,C, F and G are calculated using equations in Table 2.1 and the value of at (as determined above, i. e., 77,048 psi) as:
TOC o "1-5" h z A = 3.061
В = 0.065
С = 1,867
G = 0.0425
|
= 13.510 = 22.724 = 31.615 |
(6) Collapse failure mode ranges can be calculated as follows ( Table 2.1) [(Л-2)2 + 8(Д + СЛге)]а5 + (Л-2)
2 (В + С/сте)
С + ае(В — G) 2 + В/А
Inasmuch as the value of d0/t is greater than 31.615, the failure mode of collapse is in the elastic region. For elastic collapse, collapse resistance is not a function of yield strength and, therefore, the collapse resistance remains unchanged in the presence of imposed axial load.
Both Section 1 and Section 2 satisfy the requirements for the collapse, burst and tensional load. Thus, the final selection is shown in Table 3.10.