| Abstract |
Lifetime well integrity of geothermal wells is an important consideration in the development of geothermal resources. Despite its importance, coverage of this topic in the literature and standards is limited. Indeed, Working Stress Design (WSD) remains the basis of tubular design in geothermal wells, driving designers to select higher yield strength materials to satisfy the large cyclic thermal loads. Materials selection is usually driven by corrosion considerations, sometimes leading to the selection of expensive corrosion resistant alloys. Unfortunately, wells continue to fail even with high strength or CRA materials. Use of lower yield strength materials is desirable because of the higher ductility and increased fatigue life. However, this implies acceptance of yielding. In a previous work (Suryanarayana and Krishnamurthy, 2018), the authors have demonstrated the use of a post-yield, low cycle fatigue based design approach to improve lifetime well integrity of geothermal wells. Post-yield design addresses some of the major threats to well integrity in geothermal wells: low cycle fatigue, brittle failure, and connection failures. However, there are other concerns in well integrity, often manifesting in the early life of a well, that should be addressed in a complete design. These include tension-collapse (or “cold collapseâ€), inelastic buckling of short unsupported sections, and collapse induced by trapped annular pressure, resulting in integrity issues such as tubular failures and leaks. Tension-collapse or cold collapse conditions occur due to the combination of high tension and a collapse load. This mode of loading occurs in the cold half cycle. A design check to cover this load scenario is described in the paper. Short unsupported sections are sites for inelastic buckling. It is shown in this work that inelastic buckling during the heat half cycle, followed by tensile parting during cool down, can potentially cause kinks or tensile failures in such locations, even though the tubular may satisfy working stress or post-yield design criteria. A mathematical approach to calculating the maximum allowable temperature to avoid such failures is presented. Collapse may also occur due to trapped fluid pressure during the hot half cycle. This failure mode is more difficult to mitigate by design. Therefore, carefully planned cementing strategies to avoid the creation of trapped fluid pockets may be the only rational approach. Well architecture modifications that can mitigate this failure mode are also discussed. Finally, despite appropriate design, tubulars may succumb to various corrosion mechanisms. The paper discusses typical corrosion mechanisms in geothermal wells, and presents an approach to analyze corrosion and consider it in the design of tubulars, and in the selection of materials. The use of thermodynamic and thermohydraulic simulation tools to establish the phase behavior and estimate the rate of corrosion (and hence loss of wall) is discussed. The methods described are illustrated with practical examples. The information provided in this paper offers an additional basis for design of geothermal wells to improve lifetime well integrity. |