Lay summary
This project is aimed at contributing to the ambitious objective of reducing drilling costs by investigating a novel contact-free drilling technology by means of a flame jet in a thermodynamically supercritical water environment. This technology called “hydrothermal spallation drilling”  has a high potential of significantly reducing drilling costs mainly due to the absence of expensive drilling downtimes knownfrom conventional rotary techniques, when worn-out drilling bits have to be replaced. The spallation technology is based on the characteristics of hard polycrystalline rock to disintegrate in small disk-like fragments when rapidly heated up by a flame jet. In all thermal spallation rock drilling experiments reported in literature only shallow holes up to 335m depth were drilled and constantly flushed with air to remove the cuttings. In deep drilling operation, however, the borehole is constantly filled with a water-based drilling fluid, which fulfils a lot of important tasks such as transport of rock cuttings, providing borehole stability or cooling and lubricating the drill bit. Hence, to apply this technology for the drilling of deep wells, a flame under high hydrostatic pressure and in an aqueous environment has to be operated downhole to produce a high-temperature supercritical water jet impinging on the rock’s surface. We have gained a great expertise on the operation of such “hydrothermal flames” burning in a supercritical water environment, since these flames have been extensively studied within the scope of supercritical water oxidation (SCWO) of organic substances. A fundamental scientific investigation of the hydrothermal spallation drilling approach must include predominantly two important mechanisms: The mechanisms of heat transfer from a supercritical flame or water jet to the rock’s surface in an aqueous environment and the specific spallation properties of different rock types. Both mechanisms are going to be comprehensively investigated with this project in theory and with experiments. The result shall allow designing and scaling up suitable burners.