High-frequency torsional vibration percussive drilling is considered a promising approach to improve drilling performance in deep hard formations, so studying its rock breaking mechanism and effect is the essential and basic issue. Compared with percussive-rotary drilling and high-pressure jet drilling, it can increase the service life of drilling tools and reduce drilling cost significantly. In this paper, based on elastic mechanics and principles of mechanical vibration, the cutting mechanism model of PDC (polycrystalline diamond compact) drill bit cutter and the amplitude–frequency characteristic model of steady-state vibration response of rock system are established, as well as the response characteristics of the rock system are also pointed out. Based on elastic–plastic mechanics and rock mechanics, Drucker–Prager criterion is adopted as the constitutive equation of rock, and plastic strain is used as the criterion of rock failure. Moreover, the model of rock damage failure is also established. The effect of exciting frequency on ROP (rate of penetration) has been studied by carrying out rock breaking test using high-frequency torsional vibration percussive drilling of PDC drill bit along with analyzing and handling test data. The study results show that when torsional vibration percussive frequency is approximately equal to inherent frequency of the rock, rock resonates with cutter, the plastic strain and ROP reach maximum, and drilling efficiency is the highest. The research results can help to deepen the understanding of the percussive drilling with high-frequency torsion vibration and provide useful insight into the design, application, and optimization of this new type of downhole tools in drilling engineering.

In view of the limitations of the existing Newton fluid effects on the vortex flow mechanism study, numerical analysis of non Newton fluid effects was presented. Using Reynolds stress turbulence model (RSM) and mixed multiphase flow model (Mixture) of FLUENT (fluid calculation software) and combined with the constitutive equation of apparent viscosity of non-Newtonian fluid, the typical non-Newtonian fluid (drilling fluid, polymer flooding sewage and crude oil as medium) and Newton flow field (water as medium) were compared by quantitative analysis. Based on the research results of water, the effects of non-Newtonian rheology on the key parameters including the combined vortex motion index n and tangential velocity were analyzed. The study shows that: non-Newtonian rheology has a great effect on tangential velocity and n value, and tangential velocity decreases with non-Newtonian increasing. The three kinds of n values (constant segment) are: 0.564(water), 0.769(polymer flooding sewage), 0.708(drilling fluid) and their variation amplitudes are larger than Newtonian fluid. The same time, non-Newtonian rheology will lead to the phenomenon of turbulent drag reduction in the vortex flow field. Compared with the existing formula calculation results shown, the calculation result of non-Newtonian rheology is most consistent with the simulation result, and the original theory has large deviations. The study provides reference for theory research of non-Newtonian cyclone separation flow field

Torsional stick–slip vibrations easily occur when the drill bit encounters a hard or a hard-soft staggered formation during drilling process. Moreover, serious stick–slip vibrations of the drill string is the main factor leading to low drilling efficiency or even causing the downhole tools failure. Therefore, establishing the stick–slip theoretical model, which is more consistent with the actual field conditions, is the key point for new drilling technology. Based on this, a new torsional vibration tool is proposed in this paper, then the multidegree-of-freedom torsional vibrations model and nonlinear dynamic model of the drill string are established. Combined with the actual working conditions in the drilling process, the stick–slip reduction mechanism of the drill string is studied. The research results show that the higher rotational speed of the top drive, smaller viscous damping of the drill bit, and smaller WOB (weight on bit) will prevent the stick–slip vibration to happen. Moreover, the new torsional vibration tool has excellent stick–slip reduction effect. The research results and the model established in this paper can provide important references for reducing the stick–slip vibrations of the drill string and improving the rock-breaking efficiency.