Numerical simulation study on rock-breaking process and mechanism of compound impact drilling

Improving the rock-breaking efficiency of hard rock formation has always been of concern in drilling. Compound impact drilling technology is a new method to achieve efficient crushing of hard rock by exerting axial and torsional impacts on the drilling bit. In this study, to investigate the dynamic rock-breaking process and mechanism under compound impact, a numerical model was developed based on cohesive elements. Dynamic impact tests on sandstone were conducted to verify the effectiveness of the method. Then, the dynamic rock-breaking process under compound impact was simulated using the proposed model. Finally, the effects of impact parameters and cyclic loading on the rock-breaking performance were systematically studied. The results indicate that the stereoscopic crushing effect of rock can be achieved under compound impact. The amount of broken rock increases with the increase in the impact load amplitude and impact loading times, whereas both the impact angle and duration have an optimal range. When the impact angle is 30°, the impact duration is 2.5 ms, the amount of broken rock is maximum. A novel compound impact drilling tool was designed based on the analysis of the rock-breaking mechanism, the laboratory and field tests were carried out and the parameters were optimized.