Pipe installed in thermally stimulated wells is subject to high axial loads arising from the combination of axial constraint and temperature changes. These axial loads are manifested as applied strains, which are a function of the material thermal expansion behaviour, rather than applied loads. Variations in tubular strength and stiffness act to redistribute these strains and, in some cases, cause local strains to be significantly higher than the global average. This ‘strain localization’ can be particularly pronounced when the temperature change is sufficient to yield the pipe body and when the system is subject to variations in tubular constraint.

The energy industry has identified that a robust design must account for strain localization (DACC IRP3, 2012). However, at present there is no industry accepted practice for characterizing the potential for strain localization and selecting appropriate design margins to protect casing from damage.

This paper provides an overview of strain localization in thermal well tubulars including discussion of the sources of constraint in both casing and liner, and discussion of the mechanisms that can trigger and exacerbate strain localization. The authors propose an analytical solution to calculate strain localization magnitude under basic conditions and demonstrate finite element methods for handling complex string designs and configurations. Finally, a case study provides an example of how the methods can be used to optimize string design to reduce the likelihood of problematic strain localization.

The discussion and finite element results provide an enhanced understanding of the mechanisms that will lead to strain localization and, in turn, provide guidance on what conditions to consider in the design of thermally stimulated wells. The analytical approach provides a practical approach to rapidly assess potential strain localization scenarios and ensure that those posing particular risk can be either characterized more rigorously or re-designed to minimize failure potential.

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