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.