Intermediate Casing
Designs for intermediate pipe are different in principle than surface casing designs. Although the designs are computed in a similar manner, the philosophy for selecting burst and collapse design lines is altered. The primary purpose for the differences is to ensure a worst-case approach.
Burst. The maximum burst loading occurs when a kick is taken and the annul us contains both gas and mud. The mud to be considered is the heaviest mud used below the intermediate casing. The casing must be able to withstand 1) kick pressures from the mud and gas, 2) injection pressures at the bottom of the string, and 3) maximum surface pressures at the lop of the string. Similar to the manner used in surface casing, the injection pressure is computed as the fracture gradient at the casing seat in addition to a safety factor (Fig. 12-14).
The maximum surface pressure at the top of the string is worth some consideration. It is often thought the surface pressure of the casing does not need to he greater than the working pressure of the surface equipment (BOPs or wellhead). Another approach is to ensure that the casing can withstand the maximum attainable surface pressure resulting from a kick. For example, if an annulus full of gas would give a surface pressure of 3,900 psi, the casing would not need a burst rating greater than 3,900 psi. Further, a significant overdesign would occur if the casing was selected to equal the 5,000-psi BOP equipment rather than the maximum attainable pressure of 3,900 psi.
The pressure at any point in the casing is at a maximum when both end points, the surface pressure and injection pressure, are satisfied simultaneously. This relationship is expressed in Eq. 12.1:
Where:
P, = maximum surface pressure, psi x — length of mud column, ft y = length of gas column, ft
G,„= gradient of heaviest mud to be used below the intermediate casing, psi/ft
Gf = gas gradient, psi/ft IP = injection pressure, psi
The two unknowns are x and y. A second equation is required to solve for x and y:
Where:
SD = casing setting depth, ft
The backup fluids for the burst design are considered equal In density to formation fluids. The resultant is the difference between the burst load and the backup fluids (see Fig. 12-15a).
The burst design line is the product of the resultant and a design factor. A tentative pipe selection is made to satisfy the burst design line (see Fig. 12-15b).
Collapse. The collapse loading for intermediate pipe is supplied by the mud weight that the casing was set in and the annular cement. The load will be
Fig, 12—14 Kick situation causing maximum burst loading for intermediate pipe
■ Maximum surface
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V pressure | |
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\ Load line | |
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V | |
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Slope equals | |
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gradient of \ | |
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heaviest mud \ | |
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used below \ | |
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casing \ | |
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Backup |
\ Slope equals [\ |
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(saltwater |
\ gas gradient 1 |
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gradient) |
\ \ |
x = length of mud y = length of gas x = length of mud y = length of gas
Injection pressure m a> Q
Resultant
Resultant
- Burst rating for tentative pipe selection
Fig. 12-15 Load and backup lines for intermediate burst designs fa) and (b) tentative string design from the burst design line a cement hydrostatic pressure in areas where cement for intermediate pipe is circulated to the surface.
The collapse backup accounts for some fluid inside the pipe to provide some resistance to collapse. It is usually not practical to consider a complete mud evacuation, or dry pipe, for intermediate casing. As a worst case, the casing seat should be able to support a column of native formation fluids. Therefore, the backup fluid is computed as a column of the heaviest mud used below the intermediate casing that has a hydrostatic pressure equal to a native fluid fracture gradient, i.e., 9.0 Ib/gal or:
Where:
SD = setting depth, ft
G„, = gradient of the heaviest mud to be used below the intermediate pipe, psi/ft L = column length of mud, ft
The collapse load and backup fluids are shown in Fig. 12-16a.
- 12-I6b shows the collapse resultant and design lines. The pipe used for the tentative burst design is evaluated for collapse. Underdesigned sections must be ugraded.
- Occasionally, the biaxial effects of tension oil burst and collapse will allow the use of pipe that appears to be underdesigned during the tentative pipe selection process. However, after the biaxial effects are considered, the pipe is satisfactory. The application of this technique during the design process depends on the knowledge and experience of the design engineer. If a lower-strength section of pipe is used that does not become satisfactory under tension loading, the string must be redesigned with higher-strength pipe. This technique is presented in Example 12.4.
Example 12.4
Design the following intermediate string of casing.
Casing size = 9.625 in. Minimum acceptable drift diameter = 8.55 in. Setting depth = 9,800 ft Minimum section length — 3,000 ft Maximum surface pressure — 6,500 psi Fracture gradient at 9,800 ft = 17.9 Ib/gal
Maximum anticipated mud weight = 13.1 lb/gal Mud weight set in = 10.8 lb/gal Cement top = 7,000 ft Cement weight = 16.4 lb/gal
Solution:
1. The burst injection pressure is computed as:
IP = (0.052) (17.9 + 1.0 lb/gal) (9.800 ft) = 9.631 psi
2. The lengths of the mud and gas columns are computed with Eqs. 12.1 and 12.2:
6.500 psi 4- x(0,052 X ¡3.1 lb/gal) 4- y(0.1 15 psi/ft) = 9,631 psi x + y = SD x I- y 9,800
y = 9.800 - x 6,500 psi + 0,6812 x + (0.115)(9,800 - x) = 9,631
Solving for x:
These values arc plotted on Fig. 12-17,
3. The burst backup is computed with a 9.0-lb/gal fluid. At 9,800 ft, the backup is:
9,800 ft x 0.052 x 9.0 lb/gal = 4.586 psi
- The resultant and design lines are computed and shown in Fig. 12-17,
- A tentative pipe selection is made for the design line. The 43,5-lb/ft, S-95 pipe is underdesigned since 7,510 psi is less than the maximum burst design load of 7,980 psi.
At this point in the design, it is assumed that the tension loading will increase the effective burst rating beyond 7,980 psi. If this assumption proves incorrect, the string must be redesigned. A drilling engineer inexperienced in casing design may not wish to implement this technique since an error will require a redesign.
6. The collapse load on the intermediate casing is created by the 10.8-Ib/gaI mud and the 15.6-ib/gal cement in the annul us (see Fig. 12-18).
Load line
Mud casing was set in
Load line
Mud casing was set in
Cement
Pressure
Cement
Pressure
- Pressure
Fig. 12-16 Load and backup lines for intermediate collapse design (a) and (b) collapse evaluation for the tentative string design
2,000
6,000
8,000
10,000
Design
Resultant
Backup
PSi)
Note that a section of pipe was used that does not exceed the design line. Under a tension (8,911) loa<:i' the pipe's burst strength will exceed the design values.
Setting depth = 9,800 ft
- 9,631)
- 5,045) (5,549) 2,000 4,000 6,000 Pressure, psi
8,000
10,000
- 12-17 Burst design for Example 12.4
- The backup fluid length is calculated from Eq. 12.3:
- 052 x 9.0 lb/gal x 9,800 ft - 0.052 x 13.1 lb/gal x L L = 6,732 ft
- After computing [he resultant and design lines, the tentative pipe design is evaluated. Both weights of S-95 pipe are satisfactory.
- The tension design is computed as previously described. Although not shown in this text, the tension load does increase the burst rating beyond the maximum pressure of 7,980 psi. The final design is:
Length, Weight,
Section ft lb/ft Grade Coupling
2,000
3,060
- 000 ■
- 000
6,319
0 1,000
2,000 3.000 4,000 5,000 6,000 7,000 Pressure, psi
2,000
3,060
- 000 ■
- 000
Setting 9,800 depth_1
6,319
0 1,000
2,000 3.000 4,000 5,000 6,000 7,000 Pressure, psi
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