Supercritical Water Jets Penetrating Subcritical Water - Application for Hydrothermal Spallation Drilling

Spallation drilling is a promising alternative drilling technology that could prove to be economically advantageous over rotary techniques for drilling deep wells needed e.g. for geothermal energy production. This drilling technique uses the properties of certain rock types to disintegrate into small disk-like fragments due to thermal stresses when heated up rapidly by a highly energetic jet. In water (resp. water based drilling fluid) filled boreholes at depths below 2 kilometers water exceeds its critical pressure (220.64bar) and hydrothermal flames can be applied to provide the required heat to spall the rock. One such potential spallation drilling head consists of a combustion chamber fed by water, fuel and an oxidant. Fuel and oxidant are preheated and form a supercritical (> 220.64bar, >373.95°C) hydrothermal flame in the aqueous environment of the burning chamber. The water present in the combustion chamber is thus heated up to high, supercritical temperatures and ejected through a nozzle together with the combustion products. This highly energetic supercritical water jet is directed towards the rock surface to induce fragmentation. Thus a hot supercritical water jet is operated downhole in a dense, aqueous environment (drilling fluid) at mostly subcritical conditions. With such a setup, high heat transfer rates from the supercritical water jet towards the cold and dense environment (drilling fluid) are detected as major drawback. The significant velocity, density and enthalpy differences in between the hot jet and the environment result in huge entrainment rates of cold fluid and finally in a rapid temperature decay in the jet. This entrainment effect decreases the overall efficiency of the hydrothermal spallation drilling process significantly. Experiments are conducted in a high pressure reactor with optical access equipped with a preheating and injection system providing submerged supercritical water jets for varying nozzle exit temperatures, mass flow rates and nozzle diameters in order to investigate pseudo critical penetration length (≥375.21°C at 224bar), shape and overall heat transfer for a wide range of operating conditions. In the experiments two different methods were applied and compared with each other. The pseudo critical penetration length was investigated by axial temperature measurements with a mantel thermocouple and a novel optical Schlieren method based on the strong change of the refractive index around the pseudo critical point of water. A good agreement in between both methods was determined. Additionally a numerical model based on the commercial CFD tool (ANSYS FLUENT ® 12.1) was developed to gain deeper inside into the behavior of such jets. Conservation of momentum, mass and energy including the thermo physical properties of water were the basis of the model. In case of a round jet at high Reynolds numbers, the realizable k- turbulence model was used. Finally all numerical results were validated with the experimental measurements and show an acceptable agreement. The major finding of this investigation was the fact, that for almost all operating conditions applied in the experiments, the supercritical penetration length of the jet was always roughly in the range of the nozzle diameter. Thus increasing the jet’s nozzle exit temperature or the mass flow rate of supercritical water does not elongate the jet due to enhanced entrainment and turbulent mixing with cold subcritical environment. The developed model was able to predict the experimentally detected trends and show an acceptable agreement with measurements.