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2022
Conference Paper
Titel
Full-scale thermal cycle testing of innovative high-temperature casing connections ("flexible couplings")
Abstract
Well integrity is key for efficient and sustainable utilization of deep geothermal resources. Over the lifetime of a geothermal well, the downhole construction must withstand thermal load changes caused by successive periods of operation and maintenance. Previous studies have shown that the single most occurring failure mechanism for high-temperature geothermal wells is mechanical overload of the casing string due to constrained thermal expansion. Mechanical overload can result in casing failures, e.g. collapse or tensile failure. In the internationally co-funded GEOTHERMICA project GeConnect, the function of an innovative casing connection, the Flexible Coupling, was demonstrated in a real working geothermal environment. Flexible couplings allow for a differential axial displacement between casing and cement which significantly lowers thermal straining of the casing material. The differential movement minimizes plastic deformation of the casing in the temperature range from 150-500 °C. An expert judgement exercise was performed to identify and evaluate potential risks and failures related to the functionality of the flexible coupling. It was found that the most crucial risk of failure relates to the manufacturing process and the construction of the well. Within the H2020 research projects GeoWell and DEEPEGS, full-scale prototypes of flexible couplings have previously been tested in laboratories at ambient temperatures. In a subsequent development phase, the flexible coupling was tested in a full-scale experiment in real working geothermal environment. The full-scale experiment comprised of two concentric casing joints cemented together while the inner casing was equipped with a flexible coupling. The setup was connected to a bypass of a production well in a high-temperature geothermal field in Iceland (up to 280°C). To simulate the extreme working conditions, the experimental construction was heated and cooled in several thermal cycles at moderate- (120°C) to high-temperature (280°C). An extensive analysis was made on the performance of the flexible coupling, the casing sliding behaviour, the cement sheath integrity and the cement-metal boundary. A custom-made monitoring suite was implemented to obtain high-precision real-time distributed temperature and strain information along the sliding part of the experimental setup. Additionally, piezoelectric pressure and acoustic sensors were used to characterize the dynamic sliding process in detail. The results of the field experiment serve as a foundation for the analysis of future flexible coupling field installations.
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