Design of a hydrothermal spallation drilling tool – Flame impingement experiments

Abstract

Despite the great potential of geothermal electricity production, its exploitation is hindered from the high investment risk of geothermal projects. Exploration, test and production drilling account for 40-70% of the costs for a geothermal power plant. Half of this amount is invested without knowing whether an energy source will be found, or if it will be productive in an economically viable way. These facts call for a reduction of the drilling costs, in order to make geothermal electricity production competitive. Spallation drilling is a technology that could achieve this goal. It takes advantage of the properties of crystalline rocks, to break them into small disk-like fragments (spalls) due to thermal stresses. For the deeper boreholes of geothermal power plants and Enhanced Geothermal Systems (EGS), flames in supercritical water (hydrothermal flames) should be implemented to heat up the rock and induce these thermal stresses. This paper presents the first steps towards the development of a hydrothermal spallation drilling tool in a novel high pressure pilot plant. The drilling nozzles are characterized based on their heat transfer capabilities, by measuring the two-dimensional impingement temperature profiles of the flames that are produced from each nozzle. The optimized design of the drilling tool has been successfully implemented in drilling experiments on Iragna granite rocks.