This paper presents a framework for a strain-based design of tubular strings in extreme-temperature or high-pressure/ hightemperature (HP/HT) wells. The relevant concepts are illustrated by examples from analytical and experimental investigation of a casing material considered for use in thermally stimulated wells operated by Shell Canada Limited (Shell) in western Canada. Much of this framework is also relevant to other applications where deformation-driven loading mechanisms are present.Strain-based design uses material capacity beyond its elastic range to overcome a number of economic and technical hurdles encountered in conventional load-based designs. It has been used successfully in field applications where plastic deformations occur (e.g., thermal wells and pipelines). However, current industry standards for material selection have their origins in load-based design. More sophisticated material-characterization tools are required for strain-based designs, in which post-yield material properties govern much of the system response.This paper describes the application of strain-based concepts to the design of casing strings under combined loading where some load components are deformation controlled. The paper emphasizes the need to address strain localization, high-strain cyclic plastic loading, strain-rate-dependent strength, and associated stress-relaxation effects.Strain-based design is most effective if relevant and reliable post-yield material properties are available. Experimental investigation of a candidate material considered for Shell Canada's thermal wells consisted of a series of custom-designed coupon-scale tests. The tests were conducted to acquire data describing the post-yield material response to monotonic and cyclic loading at temperatures ranging from 20 to 350°C.Conclusions of this paper summarize findings of the executed material-evaluation program, outline options to minimize strainlocalization impacts, and provide recommendations for strainbased designs of well-completion tubulars. Following these recommendations should result in higher reliability and more costeffective wells in completion programs using strain-based strategies for designs of extreme service wells.

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