Numerical analysis on the impact of the flow field of hydrothermal jet drilling for geothermal wells in a confined cooling environment

Hydrothermal jet technology is a novel drilling method expected to be suitable for the exploitation of deep geothermal resources, in which the rocks are broken coupled by thermal spallation effect with high velocity impact. Because of the high temperature return fluid, the cooling of the drill string and wellbore is one major problem in hydrothermal jet drilling. This paper presents two cooling configurations in downhole conditions: the lateral configuration and downward configuration. The investigation on the impact of the flow field of the hydrothermal jet in a confined cooling environment is estimated using CFD methods. The influences of jet velocity, jet temperature, cooling water velocity and standoff distance are analyzed. For both cooling configurations, the impact of the flow field of a hydrothermal jet in downhole conditions is divided into three zones: the jet zone, the return zone and the eddy zone. The hydrothermal jet impinges on the bottom rock, and then returns from the annulus mixed with the cooling water. The temperature in the jet zone stays constant, while it decreases in the return and the eddy zone. The temperature around the wellbore is higher due to the return flow of the cooling water. There is a high bottomhole temperature region, while the annulus is cooled by the cooling water in the lateral cooling configuration. In addition, the jet velocity, the cooling water velocity and the standoff distance have a large influence on the pressure and temperature fields, while the jet temperature has a small influence. For the downward cooling configuration, the length of the potential core of the hydrothermal jet is positively correlated to the maximum axial velocity. Finally, two cooling configurations are compared. The lateral cooling plan is suitable for reaming operation. The lateral cooling configuration is better in the cuttings carrying capacity due to the higher annular return velocity. Results in this paper could guide for parameters design of hydrothermal jet drilling technology.