Abstract：Ultra-high pressure jet assisted drilling is an affective approach for improving penetration speed；and influential factors and mechanism of rock breakage are key technologies. Experiments of rock breakage by ultra-high pressure water jets under submerged condition are conducted to investigate the effect rules of driven pressure，standoff distance，jet acting time and impact angle on rocks. In the experiments，five kinds of driven pressures from 100 MPa to 200 MPa，and three types of rock samples are selected. Thereafter，numerical simulation with fluid-structure interaction(FSI) method for stress distribution in the coupling system is utilized to analyze rock macro- failure mechanism by jets cutting. The study shows that there exists optimum standoff distance for different driven pressures. Under the experimental condition，the optimum standoff distance is approximately 15 times of the nozzle diameter when the driven pressure is 100 MPa and 20 times for driven pressure of 200 MPa. The optimum standoff distance increases as driven pressure increases. The maximum rock broken volume can be obtained when the jet impact angle is between 12° to 14°. Meanwhile，numerical simulation shows that the micro-mechanism of rock failure due to water jets impingement is a brittle fracture in the condition of tensile and shear stress. The rocks are mainly destructed in a few milliseconds. It constructs a bridge between the micro-failure and macro-breaking mechanism of rocks with water jets impact. The study can offer some rules for optimizing hydraulic parameters and designing drill bit in the ultra-high pressure jet assisted drilling technology
