| Title | Hydrothermal Spallation Drilling as a Possible Alternative Drilling Method for Deep Geothermal Energy Harvesting |
|---|---|
| Authors | Schuler, Rothenfluh, Stathopoulos, Brkic, Meier, Rohr |
| Year | 2013 |
| Conference | European Geothermal Conference |
| Keywords | hydrothermal spallation drilling, supercritical water jet, CFD, entrainment, heat transfer, turbulent heat flux, variable turbulent Prandtl number. |
| Abstract | Hydrothermal spallation drilling (HSD) is a promising alternative drilling technology that could prove to be economically advantageous over conventional drilling techniques for drilling deep wells in hard rock formations. Therefore, this drilling method can be seen as a possible approach to make deep geothermal systems more competitive with respect to other sources of energy. This HSD technique uses the properties of certain rock types to disintegrate into small fragments when heated up rapidly to high surface temperatures by a highly energetic impinging jet. The heat and momentum flux of the hot impinging jet towards rock and the rock surface temperature are identified as crucial parameters. In water (resp. water-based drilling fluid) filled boreholes below 2 kilometres depth, water exceeds its critical pressure and thus hydrothermal flames or water jets in the supercritical state (.221bar, .374‹C) are favoured to provide the required heat for thermal rock-fragmentation. Such a hot supercritical water jet operated downhole in a dense, aqueous environment and has to be adapted in such a way to get as much of the energy contained in the jet transferred to the rock. This can be achieved by minimizing radial heat losses to the environment and maximizing heat transfer between hot jet and rock. The radial heat transfer to the environment is crucial for the efficiency of the overall HSD process. Radial heat transfer mechanisms due to entrainment and turbulent mixing were investigated experimentally in a high-pressure vessel simulating the harsh conditions in terms of pressure and temperature found downhole. Additionally, a numerical model based on the commercial CFD-tool ANSYS FLUENT was developed to describe the cool down of supercritical water jets. The challenges in simulating supercritical water system due to the strongly varying thermophysical properties were also highlighted. Finally all numerical results were validated with experimental data. The major finding of this investigation was the fact, that for almost all operating conditions applied in the experiments and simulations, the supercritical penetration length of the jet was always roughly in the range of the nozzle diameter. Thus increasing the energy input of the jet at the nozzle does not elongate the supercritical penetration length due to enhanced entrainment and turbulent mixing with the cold subcritical environment. The developed model was able to predict the experimentally detected trends and showed an acceptable agreement with measurements. For the design of a possible HSD head, the proposed model can be used as a useful tool to predict the important radial heat losses. |