| As China's oil and gas exploration and development gradually drills into hard formations such as deep wells and ultra-deep wells and complex difficult-drilling formations,conventional PDC bits and roller bits have low drilling efficiency short life and other issues during rock breaking.It has been unable to meet the requirements of speeding up and increasing efficiency of the current drilling.Therefore,improving the mechanical drilling speed of deep hard formations and prolonging the life of underground drilling tools have become the primary problems that need to be solved urgently in the exploitation of oil and gas resources at home and abroad.The new drill bit combines the impact and scraping action of the rotary drill bit and the PDC drill bit on the rock,and adopts the method of graded rock breaking to provide a new idea for improving the drilling efficiency of the hard formation.Therefore,based on the theory of rock mechanics,elastoplastic mechanics and finite element method,combined with the method of simulation and analysis,the drilling process and rock breaking law of the impact-cutting step drill bit are studied,to provide a theoretical basis for mastering the rock breaking mechanism of the stepped impact-cutting step drill bit and developing and promoting its technology.The main research contents and achievements are as follows:(1)The process of breaking the rock by the impact-rotation cutting structure and the PDC blade cutting structure in the impact and scraping step drill is analyzed,on this basis,the rock breaking mechanism under the combined action of two kinds of cutting structures is studied,and the mechanical mechanism of the bit increasing the drilling rate is obtained,which lays a theoretical foundation for the simulation analysis in the following chapters;(2)The research on the stress field of the ladder bottom hole was carried out.The stepped bottom force model was established by finite element simulation method.The influence of different collar diameters on the stress state of the stepped bottom was compared and analyzed.The results show that:The reaming bottom hole and the collar hole wall of the stepped wellbore have obvious rock stress release,good drillability,and help to ream the PDC blade to break the rock;However,the stress release of the wellbore wall is not obvious,and the wellbore wall is relatively stable.The stress state of the stepped wellbore with different pilot diameters is compared and analyzed,which is used as the reference basis for the diameter of the preferred lead bit;(3)The comparative analysis of the drilling and rock breaking simulation results of the impact-cutting step drill bit and the conventional PDC bit and the two-stage PDC bit in the hard formation was carried out.The results show that:The impact-cutting step drill bit mainly breaks the hard formation rock by tensile stress;The impact-cutting step drill bit can break the rock quickly in hard formation drilling,and the drilling speed is faster;The axial vibration of-the impact-cutting step drill bit is more gradual than that of the other two drill bits,indicating that the drilling process is more stable and the damage to the drill bit is smaller;The average torque and fluctuation range of the impact-cutting step drill bit is about 26.30%and 45.2%of the conventional PDC bit,which is about 28.3%and 39.8%of the two-stage PDC bit;It shows that the impact-cutting step drill bit has good continuity during rock breaking,low torsional vibration and higher energy utilization,which is beneficial to reduce the influence of stick-slip vibration and prolong the life of the drill;The experiment of PDC bit rock breaking bench was carried out,and the finite element simulation model of PDC bit rock breaking was established.The model parameter loading was the same as the experimental loading parameter;Comparing the experimental results obtained by the experimental test and the simulation,the error is below 8%,and the trend is consistent with the change of the mechanical drilling speed,which verifies the reliability of the finite element model;(4)The influence of the size of the lead bit and the relative height of the rock-breaking structure on the rock-breaking characteristics of the hard-drilling step drill bit during the hard formation drilling process was carried out;By changing the peak value of the impact load applied to the lead bit and the impact frequency,the impact of the impact parameter change on the rock breaking of the stepped drill bit is analyzed.The results show that:The smaller the diameter of the lead bit,the greater the counter torque that the bit body is subjected to,and the torsional vibration and axial vibration are more severe;The larger the diameter of the lead bit,the greater the impact energy is transmitted to the lead bit,so the lead bit should be used as an auxiliary rock breaker,but not as the main force to break the rock;The relative height change of the rock-breaking structure has little effect on the counter-torque and axial vibration of the bit body,but the greater the relative height,the more easily the loss of impact energy is caused;Preferred structural size of the impact-cutting step drill bit,the diameter of the reaming structure is 311.2mm,the diameter of the lead bit is 120mm,and the relative height is 40mm.When the peak value of the impact load is lOkN and the impact frequency is 12Hz,the punching the impact-cutting step drill bit has the highest rock breaking efficiency. |
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