Rock breaking mechanism of composite impact of full-size PDC bit based on finite element analysis

In order to solve the problems of low rock breaking efficiency in hard formation and the decrease of bit life caused by stick-slip vibration of drilling tools, the full-size bit composite impact rock breaking mechanism was studied by finite element method(ANSYS). The effects of impact frequency, WOB(weight on bit) and rotational speed on the efficiency of rock breaking were analyzed. It was found that both tensile and compressive stresses existed in rock during bit rock breaking, and rock failure was manifested in comprehensive failure of “tensile & compressive shear”. It is concluded that under the condition of torsional impact frequency of 25 Hz, the optimal axial impact frequency is 13 Hz, and the drilling rate is the fastest under the combination of these two impact frequencies. The influence of WOB on rate of penetration(ROP) increases linearly, and the relationship between ROP and WOB under impact load or not is regressed. The influences of rotational speed on the ROP is relatively small and tends to increase slightly. It is considered that the hard rock layer causes PDC bits to be cut into abrasive state and causes little change in the mechanical drilling speed. Composite impact could significantly improve the ROP of drill bit in formation drilling. The research on composite impact rock breaking mechanism of full-size PDC bit is of great significance to the improvement of drilling efficiency and the development of composite impact technology.