Study of the rock-breaking and drilling performance of a self-rotatory water-jet bit in water-jet drilling and its influential factors

Water-jet drilling (WJD) is an effective method for stimulating, exploring, and developing oil and gas resources. The self-rotatory water-jet bit (SRWJB) is a type of potentially efficient bit applied in WJD. In this study, the three-dimension drilling model of the SRWJB is established, and the numerical simulation of this model is conducted based on the sliding mesh model combined with RNG turbulence model. The numerical simulation method was validated using experimental results. To improve the drilling performance, the effects of the standoff distance, bit rotation speed, and nozzle layout parameters on the SRWJB drilling capability were investigated. The results show that there exists an optimal standoff distance for the single water jet under the condition of maximum removal volume, that is 15 mm in this experiment. A high rotation speed will make the streamline bend and decrease the jet convergence, which weakens the jet rock-breaking capacity. To achieve a high rock-breaking performance, the design considerations of the SRWJB are as follows: (1) There is an optimal rock-breaking rotation speed for SRWJB by comprehensively considering the jet deflection, moving speed and impact times, such as 200 rpm for the 12 MPa; and the optimal rotation speed increases with the increase of jet pressure. (2) The nozzle arrangement angles should be near 10° to weaken the water cushioning effect. (3) There is an optimal combination of distribution radii for the middle nozzles and lateral nozzles for effectively drilling a large and round borehole. This work will provide a reference for the optimal design of an SRWJB and its application in drilling.