Experimental and theoretical study on the differential rock-failure characteristics of cascade nozzles

Efficient development of unconventional natural gas using self-rotating cascade-nozzle bits will help accelerate the construction of a sustainable energy system. However, the reasons for the differences in the rock-failure characteristics of the cascade nozzles are still unclear. Experiments were conducted on sandstone and coal using the developed cascade-nozzle bit under different pressures. The rock-breaking characteristics of cascade nozzles are influenced by nozzle arrangement parameters, jet pressure, and rock lithology. The nozzle arrangement and pressure affect the force exerted by the jet on the rock and the rock-breaking performance. The central nozzle, which vertically impacts the rock, has the maximum shock pressure and rotating stagnation pressure, resulting in the deepest drilled erosion hole and, therefore, the strongest rock-breaking capacity, serving as the axial drilling during the drilling process. The depth and width of the distinct rock-failure characteristics of the different cascade nozzles increase rapidly and linearly, respectively, with increasing pressure. Rock lithology influences the failure mode of the rock under the forces. Sandstone mainly undergoes erosion failure due to the scouring of the cementing materials and particles, while coal can experience either erosion failure (pressure less than 10 MPa) or splitting failure (pressure greater than 15 MPa), or both simultaneously (pressure of 15 MPa). The research findings help improve the rationality of the cascade-nozzle arrangement.