Investigation on rock-breaking and debris flow of high-pressure CO2 jet drilling using discrete element methods (DEM)

To address the low efficiency and severe thermal wear of drill bits in geothermal drilling, CO2 (carbon dioxide) jet drilling as a new method is proposed. However, the rock-breaking and debris flow of composite rock-breaking of high-pressure CO2&PDC cutter in CO2 jet drilling are crucial problems and remain unclear. Therefore, we established a discrete element model of composite rock-breaking, validated with experimental results. The rock-breaking and debris flow are investigated from the perspectives of the cutting force, rock stress and crack generation. Additionally, many impacting factors, including the stand-off distance, injection angle, nozzle diameter, and jet impact velocity, are investigated. Research results indicate that both PDC cutting and high-pressure CO2 jet impacting would apparently contribute to the rock stress increment and crack propagation. Meanwhile, the CO2 jet obviously contributes to rock debris carrying, consistent with the observation from high-speed photography experimental results. The affecting parameters are investigated using the discrete element method (DEM), and the findings show that the optimal values are, respectively, 90 degrees at jet injection angle, 50 m/s at CO2 jet velocity, 1.5 mm at nozzle diameter, and 20 mm at stand-off distance. Moreover, the jet velocity and nozzle diameter are the most influential parameters among all influencing parameters. These studies clearly illustrate the rock-breaking and debris, which are helpful for the CO2 drilling improvement and drill bits designing for geothermal wells.