| Title | Flexible Couplings for Improved Casing Design in high-temperature Geothermal Wells |
|---|---|
| Authors | I. THORBJORNSSON, G.S.KALDAL |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Casing design, Casing collapse, Thermal expansion, Flexible couplings, GeoWell, DEEPEGS. |
| Abstract | A new approach, Flexible Coupling, aimed at reducing the risk of plastic deformation and failures of casings has been developed and laboratory tested in the EU funded project GeoWell (www.geowell-h2020.eu). This approach allows for thermal expansion of casing segments, which is one of the major concerns in the operation of high-temperature geothermal wells and relates to structural damages during warm up and discharge after drilling. Wells are constructed of several casings with different outer diameters and depths. The innermost casing is subject to high stresses due to thermal expansion that can lead to plastic deformation when used in constrained conditions, that increases chances of casing failures. The casings are made of steel of grades chosen according to standards API 5CT and ANSI/NACE MR0175/ISO 15156. Carbon steel commonly used for geothermal casings has a thermal expansion coefficient around 0.012 mm/m°C. Casings, cemented in at lower temperatures, are subject to high stresses during warm up and production due to thermal expansion. Casing connections have larger outside diameters than the casings, which means that the connections are anchored in the cement and as therefore prevent free thermal expansion of the casings. The new concept of Flexible Couplings that enable axial elongation of casing segments has been designed and built and pressure tested at 165 bars and tensile strength verified against numerical models. This new concept also allows for cooling of production wells for workovers as the risk for coupling rupture is considerably reduced. No extra tools, training nor extra work is needed at the drill rig. Running in casings made up with the new connections is therefore carried out with the same procedures as common casing jobs. This paper focus is on the testing results and the current status of this emerging technology |