全尺寸PDC钻头复合冲击破岩机理的有限元分析

针对坚硬地层破岩效率低、钻具粘滑振动造成钻头寿命降低等问题，通过ANSYS有限元分析软件对全尺寸钻头复合冲击破岩机理进行研究。分析了冲击频率、钻压和转速对钻头破岩效率的影响，通过分析发现钻头破岩过程中，岩石内部既存在拉应力，也存在压应力，岩石破坏表现形式为“拉伸+压剪”的综合作用破坏。研究得出在扭转冲击频率为25 Hz条件下，此时轴向冲击的最佳频率为13 Hz，在这2种冲击频率的配合下机械钻速最快；钻压对机械钻速影响较大，两者呈线性正相关关系，并回归出有无冲击载荷条件下钻头机械钻速与钻压的变化关系式；转速对机械钻速的影响较小，两者呈微增趋势，分析认为是坚硬岩层使PDC钻头变切削为研磨状态导致机械钻速变化不大；复合冲击在地层钻进中能明显提高机械钻速。全尺寸PDC钻头复合冲击破岩机理研究对于钻进效率的提高和复合冲击技术的发展具有重要意义。 

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.