When the thermal stresses induced in a geothermal well’s casing string exceed the yield strength of the casing material, the fatigue behavior of the latter can be defined as low cycle fatigue (LCF). The connection threads in the casing body amplify the local stress distribution and lower LCF resistance.
A theoretical approach is presented to evaluate LCF resistance and compared with preliminary results from experiments on a large diameter buttress connection. The latter is commonly used in geothermal well completions. This paper shows that, under extreme loads, the LCF resistance of the buttress thread connection can be as low as 10 cycles.

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