Hydrothermal Spallation Drilling - Design and Experiments for a Novel Drilling Technology

Abstract

Despite the great potential of geothermal electricity production, its exploitation is hindered from the cost particularities and the high initial investment risks of the respective projects. In most geothermal power production projects, nearly 15% of the total investment costs are spent in a high risk environment. Exploration, test drilling and the actual production drilling account between 40-70% of the total costs of a geothermal power plant, and 50% of it is spent during the high risk period of the projects. These facts call for alternatives and cost reduction in the drilling technologies, in order to make geothermal electricity production competitive. Spallation drilling is a technique that can reduce the drilling costs for geothermal electricity production. It takes advantage of the properties of certain rock types to spall them into small disk-like fragments due to the thermal stresses. Hydrothermal flames can be used in water-filled boreholes 2-3 kilometers deep to provide the required heat to spall the rock. The work presented in this paper focuses on the development of the drilling tool in a novel high pressure (300 bar) pilot plant. The pilot plant consists of 4 main fluid networks leading to the pressure vessel. Two of them transport the cooling media as the two other ones carry the fuel and the oxidizing agent. The pressure vessel has a volume of 5.8 L and can sustain a pressure of 600 bar at a wall temperature of 500°C. The design of the drilling nozzles and the initial flame entrainment studies are presented. Test drilling experiments in rock probes (8 cm diameter - 15 cm length) follow. For the successful ones, the flame impingement heat transfer is measured with heat flux sensors custom-made for the harsh conditions in hydrothermal flames. This procedure is repeated until the drilling tool is optimized for a certain rock type. During the preliminary experiments, flames with a thermal power of 78 kW were used and surface temperatures of 420°C were measured. Rock probes were successfully spalled, thus proving the potential of hydrothermal flames.