Casing Failures in High Temperature Geothermal Wells: An Analytical Study

In geothermal wells, temperature effect plays a very important role because of its high and cyclic temperature operation, usually in the range of 150 °C to 450 °C. Casing designed for traditional oil and gas production cannot be simply used in geothermal wells because of their different working conditions in these two scenarios. Thus, a robust design methodology for the geothermal casing is important from both economic as well as safety perspective. Various studies have reported that the single most common failure in high-temperature geothermal wells is the mechanical overload of casing string due to constrained thermal expansion. In this research work, the classical Holliday approach for the post-yield design is reviewed and modified to include the effects of thermal yield strength deration effect and Bauschinger effect in the design process of high temperature cyclic loading encountered in case of geothermal wells scenarios. In this study, the influence of thermal deration of yield and the Bauschinger effect on the allowable stress window is investigated. Empirical relations are used to estimate the reduction in the allowable stress window. In this research work, it has been found that the thermal derations effect and Bauschinger effect cause significant reduction in the allowable stress range. A reduction of 30% in the allowable stress window is observed due to these two factors from a case study. So, these two effects should be properly considered when doing a casing design for geothermal well. Holliday approach ignored several effects due to the high temperature cyclic loading occurring in case of geothermal wells. This research work has addressed the limitations of the Holliday's classical approach to include these effects to make the design more robust for the geothermal well tubular design.