American Petroleum Institute (API) design equations describing burst and collapse limits of tubulars do not address pipe body response when axial stress in the casing exceeds the material yield strength. However, casing yielding commonly occurs in thermal operations in western Canada, where steam-assisted gravitydrainage (SAGD) and cyclic-steam-stimulation (CSS) operating temperatures generally range from 200 to 350 C. Cemented production casing is subject to both passive and active loading conditions during operation: thermally induced strain-based cyclic axial loading occurs in conjunction with net internal or external differential pressure. A sound engineering basis for selecting tubular configurations that considers the combined loading state in this situation and establishes an appropriate design margin does not currently exist.This paper describes numerical analyses for combined postyield loading conditions and provides a starting point for burst and collapse design for thermal casing. Burst analysis of axially constrained casing indicates that, contrary to what might be inferred from elastic-strength calculations, an initial thermally induced axial compressive strain does not substantially reduce the burst (rupture) pressure. By contrast, even low net external pressures can lead to ovalization and loss of wellbore access when combined with thermally induced axial strain if the cement sheath does not offer adequate radial support. Sensitivity studies demonstrate the strong influence of pipe diameter-to-thickness ratio (D/ t) and pressure ratios and pipe material mechanical properties on ovalization response. Analysis results are compared with API burst and collapse predictions, thermal operating experience at Shell Canada's Peace River project, and available physical testing results for similar loading conditions. IntroductionPipe body collapse and burst limit characterizations have been the focus of much effort and standardization. The work is leading to a better understanding of limits: the impact of axial stresses, material properties, defects, and casing/cement/formation interaction; statistical variations and probabilistic formulation; and comparison with physical testing results (primarily for collapse). However, much of the focus of the past work and subsequent standardization has been geared toward elastic designs and the quantification of associated safety factors. Work described in this paper explores deformation responses and sensitivities of cemented thermal casing strings that are axially loaded beyond material yield (by means of thermally induced mechanical straining) but that are generally operated at differential pressures that are substantially lower than those required to satisfy elastic limits associated with burst and collapse design. Rapid acceleration of the use of thermal enhanced-oil-recovery (EOR) techniques and the lack of sound burst and collapse design equations for such wells highlight the need for advancement in this area. Stated simply, one might cast the question this way: "M...

API design equations describing casing burst and collapse limits do not address pipe body response when axial stress in the casing exceeds the material yield strength. However, casing yielding commonly occurs in thermal operations in Western Canada, where SAGD and CSS operating temperatures generally range from 200°C to 350 °C. Cemented production casing is subject to both passive and active loading conditions during operation: thermally-induced strain-based cyclic axial loading occurs in conjunction with net internal or external differential pressure. A sound engineering basis for selecting tubular configurations that considers the combined loading state in this situation and establishes an appropriate design margin does not currently exist. This paper describes numerical analyses for combined post-yield loading conditions and provides a starting point for burst and collapse design for thermal casing. Burst analysis of axially constrained casing indicates that, contrary to what might be inferred from elastic strength calculations, an initial thermally-induced axial compressive strain does not substantially reduce the burst (rupture) pressure. By contrast, even low net external pressures can lead to ovalization and loss of wellbore access when combined with thermally-induced axial strain if the cement sheath does not offer adequate radial support. Sensitivity studies demonstrate the strong influence of pipe D/t and pressure ratios and pipe material mechanical properties on ovalization response. Analysis results are compared to API burst and collapse predictions, thermal operating experience at Shell Canada’s Peace River project, and available physical testing results for similar loading conditions.