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Testing Flexible coupling for Geothermal Wells GRC 2019
Abstract
Flexible Couplings (FC) as a solution to mitigate thermal straining of casings in geothermal wells have been tested, where its function to allow intermittent displacement of the production casing during warm-up has been verified both for straight and deviated wellbore. During warm-up, free thermal expansion of the steel used for casing in geothermal wells is blocked in conventional design, resulting in high stresses that are often above the yield strength of the material. This stress can both damage the casing as it will release the stress by permanently deforming the casing plastically. The proposed solution allows the casing to expand into the FC that replace conventional connections, and by that ensure that the stress level will remain below the yield strength of the material. Chance of generating plastic strains will therefore be lowered reducing the risk of casing failures. Testing in two, third party, laboratories have been performed within the EU funded projects, GeoWell (www.geowell.eu) and DEEPEGS (www.deepegs.eu) and the plans are to make the first full scale testing in a high-temperature geothermal well in the end of 2019.
... The cost of geothermal wells for co-generation of electricity and district heating system is typically 20-50% of the total investment cost of the energy production . Frequent failure mechanisms in high-temperature geothermal wells are casing collapse and mechanical overload of the casing string in the well due to constrained thermal expansion (Lohne, et al., 2017) (Thorbjornsson, et al., 2019). In addition, for medium enthalpy geothermal wells, temperature and pressure cycles above 100°C during construction, operation and shut-in phases have the potential to severely deteriorate the integrity of the cemented annulus (Ter Heege, et al., 2017). ...
... The sliding function of the connection and its structural limits were evaluated, first by finite-element modeling and then in two full-scale testing facilities ( Figure 2). The results from these tests that were conducted at ambient temperatures are described by Thorbjornsson et al. (2019) and Thorbjörnsson & Kaldal (2020). The connection was further developed in the DEEPEGS project and has been incorporated in well designs for the Krafla Magma Testbed (KMT) that aims towards better understanding of magmatic systems by establishing the first magma observatory (Eichelberger, et al., 2018) (Hólmgeirsson, et al., 2018). ...
Casing failures in-high temperature geothermal wells occur if the casing is overstressed. The most common failures occur because of axial forces and/or differential pressures. For medium-enthalpy geothermal wells, thermal cycling has the potential to severely deteriorate the integrity of the cemented annulus and for higher enthalpy wells yielding of casings becomes a structural concern. The recently innovated patent solution, flexible couplings, aims to reduce thermal straining by allowing displacement into the connections of each casing segment at approximately 12 m intervals (API R3). In the GEOTHERMICA internationally co-funded GeConnect project a full-scale experiment is performed at a well site in a high-temperature geothermal field. The experimental setup is composed of two concentric casings cemented together and the inner one equipped with a flexible coupling. The aim is to investigate effects of thermal cycling on well integrity and to test the function of cemented-in flexible couplings using a bypass of a producing well. Cement sheath integrity and cement-casing boundary are evaluated at moderate to high temperatures, e.g. by using fiber optic distributed sensing technology. Structural models are used to analyze casing-cement interactions and to evaluate prospects and potential improvements of well integrity by using the new technology of flexible couplings.
... The cost of geothermal wells for co-generation of electricity and district heating system is typically 20-50% of the total investment cost of the energy production . Frequent failure mechanisms for high-temperature geothermal are casing collapse and mechanical overload of the casing string in the well due to constrained thermal expansion (Lohne, et al., 2017) (Thorbjornsson, et al., 2019). In addition, for medium enthalpy geothermal wells, it is stated that temperature and pressure cycles above 100°C during construction, operation and shut-in phases have the potential to severely deteriorate the integrity of the cemented annulus (Heege, et al., 2017). ...
... Its function and structural limits was evaluated. The prototype in casing size of 9⅝″ was tested pressure tight up to 110 bar (with pressurized water) and has a load capacity of 300 tons (Thorbjornsson, et al., 2019). Its sliding function was confirmed both in straight position and with bending loads. ...
Common failure mechanisms in high-temperature geothermal wells are casing collapse and tensile joint rupture. For medium-to high-enthalpy geothermal wells, thermal cycling has the potential to severely deteriorate the integrity of the cemented annulus. Additionally, for higher enthalpy wells, yielding of casings becomes a structural concern. A recently innovated patented solution, flexible couplings, aim to reduce thermal strains by allowing displacement from thermally expanding casing segments at ~12 m intervals (API Range 3 casings). In the GeConnect project, a full-scale surface experiment composed of a 9⅝″ casing equipped with a flexible coupling cemented into a 13⅜″ casing is constructed and installed on a well pad of an existing high-temperature geothermal well in Iceland. The aim is to investigate effects of thermal cycling on well integrity and to test the function of cemented-in flexible couplings. Along with testing the flexible coupling, cement sheath integrity and the cement-casing boundary behavior will be evaluated at moderate to high temperatures. Structural models are used to analyze casing-cement interactions and to evaluate prospects and potential improvements of well integrity by using flexible couplings on experimental up to field scale.
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