This thesis provides a detailed analysis of multilateral technology for geothermal projects targeting the Malm aquifer in the South German Molasse Basin (SGMB). The specific geological, technical, regulatory, and economic prerequisites of projects in Munich's vicinity are considered, and the technical and economic feasibility is assessed.
The technical evaluation shows that Level 2 and Level 3 junction types, as defined by the industry consortium Technology Advancement for Multi-Laterals (TAML), are best suited for a multilateral development in the SGMB. Level 2 is the preferred variant if the junction area is stable, while Level 3 is favored if conditions are unstable. The position of the multilateral junction is restricted to the reservoir’s top and its caprock.
The advantage of multilateral completions results from the reduction of well pressure losses caused by turbulent flow in the near-wellbore region, open hole section, and completion, as well as the linear flow in the aquifer. The economic potential is estimated based on well performance data, analytical methods, and a numerical reservoir simulation. For projects meeting the energy demand with the motherbore capacity, the benefit of a multilateral completion lies in savings in pump power cost. Projects with unlimited energy demand profit from higher flow rates at constant drawdown. However, this benefit is only obtainable if the well design allows for higher flow rates, pump limitations are not exceeded, and the generated savings compensate the construction costs.
In addition to this thesis’ theoretical work, a first multilateral test well in the SGMB was planned, drilled, and evaluated to add experimental findings. One out of six wells of an inner-city geothermal project in Munich was selected. During construction of the junction, no technical problems occurred, the cost was within budget, and authorization was approved. After a third of the planned lateral length, drilling was stopped due to high karstification. The productivity gain of the multilateral test well was still at 16% and the economic analysis showed a positive economic outcome.
For future projects, the well design should be adapted if multilateral wells are considered. The separation of the branches must be maximized to optimize the reservoir drainage. Thus, the well path geometry of the motherbore must be adjusted to create ideal conditions for drilling a lateral. The design and construction of the motherbore should also focus on creating stable conditions at the potential junction depth. To enhance the understanding of the multilateral system’s flow behavior and the pressure drawdown interference between motherbore and lateral, the test program and data acquisition should be optimized in the future.
In contrast to its frequent use in the oil and gas industry, and despite some very positive case studies from geothermal projects worldwide, multilateral technology is still a niche technology in the geothermal industry. The results of this work show that it is time to increase the acceptance of this technology in the South German Molasse Basin and they could also encourage studies and tests of multilateral technology in other geothermal fields.