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External pressure 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. External pressure loads are produced by cement and fluids outside the casing, which can be modeled by pressure distributions.
Pressure distributions are typically used to model the external pressures in cemented intervals. These pressure distributions are discussed next.
Fluid pressure is given by the mud gradient above the top-of-cement (TOC), and by the cement gradient below TOC.
Permeable zones: good cement
Again, fluid pressure is given by the mud gradient above TOC, and by the cement gradient below TOC. The exception is that formation pore pressure is imposed over the permeable zone interval. This pressure profile is discontinuous.
Permeable zones: poor cement, high pressure
In this case, the formation pore pressure is felt at the surface through the poor cement. This pressure profile is continuous with depth.
Permeable zones: poor cement, low pressure
In this case, the mud surface drops so that the mud pressure equals the formation pressure. This pressure profile is continuous with depth.
Openhole pore pressure: TOC inside previous shoe
In this case, fluid pressure is given by mud gradient above TOC, cement gradient to the shoe, and the minimum equivalent mud weight gradient of the openhole below the shoe. This pressure profile is not continuous with depth. It is discontinuous at the previous shoe.
Openhole pore pressure: TOC below previous shoe, without mud drop
In this case, fluid pressure is given by the mud gradient above TOC, and by the minimum equivalent mud weight gradient of the openhole below the shoe. This pressure profile is not continuous with depth but is discontinuous at TOC.
TOC below previous show, with mud drop
In this case, the mud surface drops so that the mud pressure equals the minimum equivalent mud weight gradient of the openhole at the TOC. This pressure profile is continuous with depth.
Above/below TOC external pressure profile
In this case, fluid pressure is given by mud gradient above TOC, cement gradient to the shoe, and a specified pressure profile below a specified depth. This external pressure distribution may be discontinuous at the specified depth. If a pressure gradient is specified, the pressure profile may also be continuous at the specified depth.
Noteworthy papers in OnePetro
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.
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.
Manual for Steel Construction, Load and Resistance Factor Design. 1986. Chicago: American Institute of Steel Construction.
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.
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.
Rackvitz, R. and Fiessler, B. 1978. Structural Reliability Under Combined Random Load Processes. Computers and Structures 9: 489.