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Thermal loads on casing and tubing strings
To evaluate a given casing design, a set of loads is necessary. Casing loads result from running the casing, cementing the casing, subsequent drilling operations, production and well workover operations. Temperature changes and resulting thermal expansion loads are induced in casing by drilling, production, and workovers, and these loads might cause buckling (bending stress) loads in uncemented intervals. In shallow normal-pressured wells, temperature will typically have a secondary effect on tubular design. In other situations, loads induced by temperature can be the governing criteria in the design.
Temperature effects on tubular design
Annular fluid expansion pressure
Increases in temperature can cause thermal expansion of fluids in casing and tubing annuli. If an annulus is sealed, the fluid expansion may result in significant burst and collapse pressure loads on the surrounding casings. In many cases, these loads need not be included in the design because the pressure can be bled off via wellhead outlets at surface. However, in subsea wells, the casing annulus cannot be accessed once the casing hanger is landed and in this case, the annulus fluid expansion pressure must be considered during casing design. The pressure increases will also influence the axial load profiles of the casing and tubing strings exposed to the pressures because of pressure ballooning effects.
Tubing thermal expansion
Changes in temperature will increase or decrease tension in the tubing string because of thermal contraction and expansion, respectively. The increased axial tensile load, because of pumping cool fluid into the wellbore during a stimulation job, can be the critical axial design criterion. In contrast, the reduction in tension during production, because of thermal expansion, can increase buckling and possibly result in compression at the wellhead.
Temperature dependent yield
Changes in temperature not only affect loads but also influence the load resistance of casing and tubing strings. The casing material’s yield strength will reduce slightly as temperature increases, which in turn reduces the casing burst, collapse and axial ratings accordingly.
Sour gas well design
Sour gas (H2S) may cause stress corrosion cracking on casing and lead to catastrophic failure of the casing string. So sour gas is a major consideration when selecting the appropriate material for the production casing and tubing. The factors influencing sulphide stress cracking failure include the H2S concentration, pressure, temperature and the fluid pH environment, and in some cases, the casing material with a suitable sour service grade may have to be used to prevent such failure.
Noteworthy papers in OnePetro
Adams, A.J. and MacEachran, A. 1994. Impact on Casing Design of Thermal Expansion of Fluids in Confined Annuli. SPE Drill & Compl 9 (3): 210-216. SPE-21911-PA. http://dx.doi.org/10.2118/21911-PA.
Banon, H., Johnson, D.V., and Hilbert, L.B. 1991. Reliability Considerations in Design of Steel and CRA Production Tubing Strings. Presented at the SPE Health, Safety and Environment in Oil and Gas Exploration and Production Conference, The Hague, Netherlands, 11-14 November. SPE-23483-MS. http://dx.doi.org/10.2118/23483-MS.
Brand, P.R., Whitney, W.S., and Lewis, D.B. 1995. Load and Resistance Factor Design Case Histories. Presented at the Offshore Technology Conference, Houston, 1-4 May. OTC-7937-MS. http://dx.doi.org/10.4043/7937-MS.
Lewis, D.B., Brand, P.R., Whitney, W.S. et al. 1995. Load and Resistance Factor Design for Oil Country Tubular Good. Presented at the Offshore Technology Conference, Houston, 1-4 May. OTC-7936-MS. http://dx.doi.org/10.4043/7936-MS.
Prentice, C.M. 1970. "Maximum Load" Casing Design. J. Pet Tech 22 (7): 805-811. SPE-2560-PA. http://dx.doi.org/10.2118/2560-PA.
Mitchell, R.F. 1996. Forces on Curved Tubulars Caused By Fluid Flow. SPE Prod & Oper 11 (1): 30-34. SPE-25500-PA. http://dx.doi.org/10.2118/25500-PA.
Halal, A.S. and Mitchell, R.F. 1994. Casing Design for Trapped Annular Pressure Buildup. SPE Drill & Compl 9 (2): 107-114. SPE-25694-PA. http://dx.doi.org/10.2118/25694-PA.
Aadnoy, B.S. 1996 Modern Well Design. Rotterdam, The Netherlands: Balkema Publications. WorldCat
Economides, M.J., Waters, L.T., and Dunn-Norman S. 1998. Petroleum Well Construction. New York City: John Wiley & Sons. WorldCat
Manual for Steel Construction, Load and Resistance Factor Design. 1986. Chicago: American Institute of Steel Construction. WorldCat
Mitchell, R.F.: “Casing Design,” in Drilling Engineering, ed. R. F. Mitchell, vol. 2 of Petroleum Engineering Handbook, ed. L. W. Lake. (USA: Society of Petroleum Engineers, 2006). 287-342. SPE Bookstore
Other noteworthy papers
Galambos, T.V., Ellingwood, B., MacGregor, J.G. et al. 1982. Probability-based Load Criteria: Assessment of Current Design Practice. J. of the Structural Division, ASCE, 108 (5): 959-977. ASCE
Rackvitz, R. and Fiessler, B. 1978. Structural Reliability Under Combined Random Load Processes. Computers and Structures 9: 489. ScienceDirect