Investigation of a Novel Drilling Technology – Influence of the Surface Temperature for Hydrothermal Spallation Drilling

Abstract

The drilling process accounts for about 60% of the total investment for a deep geothermal power plant. Hydrothermal Spallation Drilling (HSD) is a novel contact-free drilling technology with the potential to significantly reduce the drilling costs and therewith enhance the development of geothermal energy production. Spallation drilling is based on the effect of hard, polycrystalline rock disintegrating into small disc-like fragments, when rapidly heated by a hot fluid jet. In order to enable the spallation technology in bore holes for deep geothermal applications, a flame must be established in an aqueous environment determined by high pressures and high temperatures. Under these conditions, the critical point of water will be exceeded at a certain depth in the bore hole. In this supercritical environment light gases such as methane and oxygen form a homogenous phase with water. This mixture can be ignited, creating a so-called hydrothermal flame. In view of increasing the efficiency of Hydrothermal Spallation Drilling, a profound knowledge of the optimal combination of the flame operating conditions and the rock properties is of utmost importance. Additionally, the spallation process itself must be thoroughly understood to enable the usage of this technology in geothermal drilling processes. Therefore, high-speed surface temperature measurements during the spallation process are reported. Rock probes are heated by two different burners while the local surface temperature during spallation is measured using two pyrometers with a maximal sampling rate of 1000 Hz and a measuring point of 1.5mm in diameter. With this setup the disintegration of single spalls is monitored as well as the induced temperature difference between surface and failure plane during the spallation process. The measured surface temperatures show a significant deviation from existing models and thus contribute to the fundamental understanding of the spallation process.