||David LENTSCH, Klaus DORSCH, Natascha SONNLEITNER, Achim SCHUBERT
||ultra-deep geothermal wells, casing failures, casing collapse, Germany, Molasse Basin, casing design, cementing techniques, advanced drilling technologies
||World Geothermal Congress
||Drilling and Completion Technology
||About 40 deep geothermal wells have been drilled successfully in the last 10 years in the Molasse Basin of southern Germany. To date, this is the deepest low-mid enthalpy hydrothermal exploration in a sedimentary basin worldwide. Some of the wells are 6000 m (20,000 ft) deep. In the coming years, even deeper and more challenging wells are planned. Most of the wells are highly productive, and the exploration and development efforts to date have been successful. However, drilling and well completion in this geological setting close to the Alps with high tectonic stresses and overpressured zones is challenging. Moreover, high temperature fluids that are produced with high flow rates through the casing string induce additional stresses. Consequently, one of the major problems in the past was casing failures. Therefore, a study was performed to identify the mechanisms and reasons for casing failures. A catalogue of these measures was then developed that serves as a guideline in the planning and construction phase of a drilling project. These measures primarily involve adaptions of former guidelines used in the design of casings, the utilization of advanced drilling and completion technologies, and guidelines for the selection, placement, and quality control of cement. This paper provides a summary of the findings in our study and describes the measures that are necessary to avoid casing failures in deep geothermal wells in the Molasse Basin in southern Germany or in comparable deep sedimentary basins.