Numerical Analyses of the Granite Fragmentation in Rotary-Percussive Drilling with the Consideration of Pre-Existing Fracture

Rotary-Percussive drilling technique is an efficient rock breaking method and well-suited for geothermal drilling. Taking typical granite as research object, the fragmentation mechanism of rock was investigated using the distinct element method with the consideration of pre-existing fractures. The investigation begins by establishing a cutter-rock interaction model to simulate the rotarypercussion drilling into the rock. In the model, discrete fracture network (DFN) was used to better reflect the complex behavior of rock materials and its parameters, such as fracture size, distribution and intensity, are obtained by statistics and calculation of fractures in outcrops. Bonded Block Model (BBM) was used to simulate the initiation of cracks that can coalesce and propagate to fracture the rock. The macroscopic physical parameters of rocks and fractures were obtained through a series of trial calculations by continuously adjusting the microscopic contact parameters. As a dynamic problem, the damping effect of the rock was introduced into our model by fitting it to the laboratory tests. Finally, two cases were studied: 1. Rock fragmentation only under impact load; 2. Rock fragmentation under the coupling of impact load and lateral movement. The results represented the fragmentation process of rock under the cutter indentation and the coupling of impact load and lateral movement. Both typical fracture system under the indentation of cutter and the skip-slip phenomenon are obtained. When the dip angle of cutter takes the value of 45 degrees and the impact amplitude take the value of 280 kN, the specific energy gets the minimum value which shows a high rock breaking efficiency. With the increase of dip angle of cutter, the intensity of skip-slip effect is increased, which is bad for drilling. The impact amplitude has no effect on the skip-slip effect, but the particle size is higher when the impact amplitude is higher. The research done by this paper is of great significance to guide the application of Rotary-Percussive drilling and to reduce the cost of the development of geothermal resources.