The use of a post-yield design basis for thermal service tubulars (such as in cyclic steam stimulation, continuous steam injection, and geothermal wells) is common in the industry. However, very few case studies have appeared in the literature on the application of these methods and the quantitative impact of their use on the well design. In this paper, we present the use of a post-yield design approach to design thermal service wells that is based on work presented by Suryanarayana and Krishnamurthy (2015).

The design approach was applied in two recent thermal service well designs. Working Stress Design is used to confirm the suitability of the tubular for life cycle loads except for those load cases that are dominated by stresses arising from temperature change (i.e., thermal loads). For thermal loads, post-yield design approaches based on Modified Holliday and Low Cycle Fatigue (LCF) are used. The work flow is presented and discussed in terms of practical application of the approach. Two different case studies are presented - a cyclic steam stimulation case, and a geothermal well case. The method is applied to all the tubulars in the well, but this paper will focus on the production casing. Threaded connections are treated as strain localization points, and are included in the LCF design approach. Practical design issues such as impact of cement shortfall, directional control, and parameter uncertainties are discussed. The design implications of applying a post-yield design basis are shown to be significant in some cases. The design optimization and alternative design choices that resulted from the application of this approach are discussed. Finally, ongoing work to enhance the design approach is discussed. The authors believe this work provides a practical basis for design of thermal service wells.

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