Geothermal wells undergo large thermal cycles during their life, impacting both design and well integrity. Literature addressing geothermal well integrity is scant, and the primary basis for design remains working stress design, whereby maximum stresses are within the yield strength of the material. This often drives the designer to higher strength options for well tubulars, which can elevate the risk of corrosion and brittle failure. More importantly, the implication of using more ductile, lower strength materials on lifetime well integrity is not well understood. As a result, geothermal wells can fail well before reaching the end of their expected life, compromising the economics of the asset. In this work, we present a post-yield design method for geothermal well tubulars. This method is based on a low cycle fatigue (LCF) approach to assessing well integrity since mechanical failure under high temperature cyclic loading is rooted in fatigue. The approach is based on two key concepts: Critical Strain, a material property, and Ductile Failure Damage Indicator (DFDI), a plastic damage parameter. The new approach accumulates the plastic damage through DFDI, and can handle both cyclic and applied monotonic strains in the plastic region. Since Critical Strain is correlated to microvoid coalescence and incipient fracture due to accumulated plastic damage, it is used as the reference strain in the DFDI damage accumulation. Connection fatigue is included in the LCF design approach via a connection strain localization factor, determined using finite element analyses of the connection under cyclic loading. The paper describes the theoretical and experimental basis of the method, and illustrates its application for geothermal well tubulars. The advantages of this method over the more traditional, Coffin-Manson based LCF models are discussed. The LCF design approach presented in this paper has been used in the past by the authors in the design of cyclic steam injection wells. Since geothermal wells present similar thermal cyclic loading and service requirements, the approach is of interest in geothermal well design. Well integrity must also cover threats such as corrosion and brittle failure. The LCF method has been integrated into materials selection; in addition to corrosion resistance fatigue life is utilized for materials selection, especially for corrosion resistant alloys such as stainless steels, titanium and nickel based alloys. The impact of produced fluid chemistry and temperature on material selection and well design is discussed in the context of corrosion and caustic cracking, two of the more important threats to geothermal well integrity. The paper demonstrates how consideration of these aspects in the design of the well goes hand-in-hand with mechanical considerations, resulting in a well that satisfies its functional requirements while providing integrity over its expected life. The authors believe that a formal, rigorous design approach centered on lifetime well integrity is much needed in the geothermal industry, and hope that this work contributes a basis for such an approach.

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