Casing Setting Methods
Generally, three techniques are employed to reduce the likelihood of casing failure: use of a high steel grade, prestressing of casing, and allowing casing to expand.
Use of High Steel Grade: When using high steel grade, production casing is cemented in place from the casing shoe to the base of the surface casing or to the surface. This prevents the possibility of casing buckling and improves both burst and collapse resistance. As discussed earlier, inasmuch as pipe body will be subjected to a compressive stress during heating and to a tensile stress during cooling, high steel grade must be selected to ensure that the ultimate yield is not exceeded in either compression or tension. In addition, all joints must be made-up properly during running of the casing.
Prestressing of Casing: The lower 10% of the casing string is first cemented in place using a competent, high-temperature cement. A surface pull is then applied to increase the tension in the upper portion of casing, which is then cemented in place from the top of the high-temperature cement to the bottom of the surface casing or to the surface. This pre-stressing reduces the compressive stresses that occur during heating, because the existing tensile stress must first be reduced to zero before the casing experiences compression. The required casing grade is a minimum when the maximal tensile and compressive stresses achieved during pre-stressing and heating, respectively, are equal. The correct combination of casing grade and prestressing can be determined with a knowledge of the maximal expected casing temperature.
Allowing Casing to Expand: The bottom 109c of the casing string is cemented in place using a competent, high-temperature cement. The casing and joints in this section must be of high strength because they are completely confined. The remainder of the casing string is made up of flush joint casing that has been coated with thermoplastic material. The purpose of the thermoplastic material is to prevent bonding between the cement and casing. This portion of the casing is cemented with low-shear-strength cement. When the cement is set. the upper 90% of the casing is free to expand within the cement sheath. This arrangement permits vertical movement of the casing with reduced buckling and protects it from high temperature effects.
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Table 4.10: Performance analysis of different couplings under cyclic thermal loading. (After Goetzen, 1987; courtesy of ITE-TU Clausthal.)
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ST= standard, TR = thermal resistance
Good success has been reported with this method, but caution must be exercised in selecting the thermoplastic material to ensure that it is not thermosetting. One disadvantage of this method is that a considerable casing rise at the surface can occur during the heating period, especially in deep wells. In a 4.000-ft well, for example, this rise may be as much as 10 — 12 ft. Experience has shown that 7591: of this rise is recovered immediately upon termination of steam injection. Casing can be prestressed, as described earlier, so that the other 25СЛ of elongation is eliminated. In this case, the casing will return to its normal position prior to the time a steam-stimulated well is switched to a pumping operation.