Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Structural Analysis of Casings in High Temperature Geothermal Wells in Iceland Gunnar Skúlason Kaldal, Magnús Þ. Jónsson, Halldór Pálsson, Sigrún Nanna Karlsdóttir Faculty of Industrial Engineering, Mechanical Engineering and Computer Science, University of Iceland, Hjardarhagi 2-6, 107 Reykjavík, Iceland gunnarsk@hi.is Keywords: Casing, structural analysis, concrete, finite element, thermal loading, thermal expansion ABSTRACT Large temperature changes are a central design concern in a diverse range of structures. Large and quick wellbore temperature changes in high temperature geothermal wells, e.g. during discharge and quenching of wells, produce large thermal stresses in the production casing which can cause casing failures. The wellbore temperature change during discharge causes the wellhead to rise due to thermal expansion of the casings, since the wells are constructed of several concentric steel casings which are fully cemented to the top. The structural integrity of such casings is essential for the utilization of high temperature geothermal wells. The casings in connection to the wellhead form a structural system which involves nonlinear interaction of the contacting surfaces. Therefore, the structural system is analyzed numerically with the use of the nonlinear finite element method (FEM). Three FEM models are presented here with the purpose of evaluating the structural integrity of high temperature geothermal well casings. A load history is used in the analysis, consisting of transient wellbore temperature and pressure changes. 1. INTRODUCTION Energy of deep geothermal heat sources is extracted from geothermal reservoirs through geothermal wells. The energy rich water turns to steam as the pressure drops while it flows up the well. High temperature geothermal wells are often constructed of three concentric casings; a surface casing, an anchor casing and a production casing where the geothermal fluid flows. The casing components that form the casing are either connected with threaded couplings or welded together. Each casing is cemented externally all the way to the top for structural support and leakage prevention. The purpose of the casings is multifold; to prevent collapse of the borehole, to prevent flow from unwanted aquifers, for blow out prevention during drilling and to be a conductor for the geothermal fluid to flow up the well (Björnsson et al., 1978). The anchor casing is connected to an expansion spool below the master valve, allowing for axial displacement for the production casing inside the wellhead when it expands thermally. Numerous casing load cases arise during different stages of geothermal wells, see Figure 1, the main ones being; casing weight (A), differential pressure between outer and inner surface of the casing (B) and temperature changes (C and D). Figure 1: Main load cases of casings; (A) Self weight, (B) Differential pressure and (C and D) Temperature changes. In general, casing strength is calculated in terms of axial tensile strength, collapse and burst pressures. The most important design loads for oil and gas are casing weight, tensile loading and fluid pressure, in geothermal wells however, high temperature loading is the most severe (Hole, 2008). The temperature change from the cementing temperature conditions to production temperature conditions is typically around 200-300°C uppermost in the well, but the temperature distribution of the casings during cementing provides the initial conditions for thermal stress calculations. Thermal expansion generates thermal stress in the casings and concrete because of the thermal gradient in between the layers. Assuming completely constrained casing, the thermal stress is about 2.5 MPa/°C, which means that a K55 steel casing reaches its yield point (f ym = 379 MPa) at a temperature change of approximately 150°C. Fortunately, K55 casing steel is very ductile and can therefore generate large strain before problems occur. A well composed of concentric steel casings, concrete and surrounding rock formation forms a structural system which involves a number