Design of Composite Casing
As discussed previously, the generalized casing string for use in any situation is one designed to withstand the maximum conceivable load to which it might be subjected during the life of the well. In view of this, for the design of casing adjacent to a salt section, the following loading conditions are assumed:
1. Casing is expected to be evacuated at some point in the drilling operation.
2. Placement of cement opposite the salt section is often difficult and, therefore, any beneficial effect of cement is ignored.
3. Uniform external pressure exerted by the salt is considered to be equal to the vertical depth, i. e., at 1,000 ft pressure is 1,000 psi. A typical abnormal pressure gradient is 1.48 psi/ft.
4. The effect of non-uniform loading is taken into consideration by increasing the usual safety factor by at least 20%.
The intermediate casing string described in Chapter 3 is again considered; however, in this example, a salt section is assumed to extend from 6,400 to 11,100 ft. and the collapse design for P-110 (98 lb/ft) casing is rechecked.
Collapse pressure at 6,400 ft = 12 x 0.052 x 6,400
= 3,993.6 psi.
Collapse pressure at 11,100 ft = 1.48 x 11,100
= 16,428 psi.
Collapse resistance of the current casing grade P-110 (98 lb/ft) = 7,280 psi.
7 280
SF for collapse = — ——— = 0.433
16,428
Alternatively, a liner may be run adjacent to the salt section and the annulus between the two casing cemented. The physical properties of the composite pipe are given in Table 4.7.
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Table 4.7: Physical properties of composite pipe. Property Outer pipe Inner pipe
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Assuming a KT (reinforcement factor) of 1.6. the collapse resistance is calculated as:
Pco — К (pCl + Pc2) = 1.6(7,890 + 7,282) = 24.275.2 psi Thus,
SF for collapse = ——— = 1.47 F 16,428
Generally, it is not possible to obtain a 100% effective cement job in the long annular section of two concentric pipes. A safety factor of 1.5 should, therefore, be used to allow for any uncertainties in the quality of the cement and to ensure that the rated performance is greater than the expected load.