Influence of Process Parameters on Thermal Rock Fracturing under Ambient Conditions

In order to make geothermal energy more competitive on the market and therefore to reduce the drilling cost of deep boreholes, during the last few decades, researchers are trying to develop new drilling techniques. A potential solution is a contact-free drilling method by means of a flame jet. Exposing a rock surface to the heat of a flame, which penetrates through a rock sample and increases the internal stresses, small disk-like fragments are formed and removed from the surface creating a hole. At the Institute of the Process Engineering, ETH Zurich, the research is focused on giving an explanation of rock fracturing, called spallation drilling and to link the gained results to the required process parameters. An existing high pressure drilling pilot plant, which models experimentally the conditions in a borehole, has a small optical access, limitation in rock probe size and limited number of suitable measurement techniques. Therefore an ambient spallation drilling setup has been constructed, to perform more convenient measurements. Performing the preliminary spallation experiments with the ambient spallation setup, the presented work gives the resulting relation between the required operating conditions for successful thermal spallation and the rock fracturing. Maximum used heat of combustion was up to 45 kW, reached penetration rate was 1.97 m/h and removal rate around 3 m 3 /h. Different rock types have been also tested in order to find the one with the best spallation results and hence, it will be used for the further research. Additionally, an infrared camera is used for monitoring the thermal spallation drilling process and the hole creation. 1. INTRODUCTION A method that is still under development to drill deep geothermal wells is thermal spallation drilling where heat from a flame jet or a laser beam induces stresses in the rock causing its failure. Results are more successful with the flame jet than with the laser beam, because the jet also removes created disc-like fragments called spalls. This impinging flame jet introduces both the heat transfer and the spall removing as important factors for successful thermal spallation drilling.