Study On PDC Bit With Rock Breaking Mechanism And Model Of Composite Impact

With the development of the society and the economy,more and more oil resources are demanded.The exploration and development are ranging from shallow wells to deep wells,and even ultra-deep wells.In these areas,the rock formation is characterized by high hardness,high abrasivity and poor drill ability,which hinders the improvement of the rate of penetration in deep and hard formation.Therefore,to solve the problems such as quick failure of drill bit,low rate of peneration and high drilling cost in the process of drilling in hard formation,it is necessary to develop a new type of rock-breaking tool to improve the rate of peneration.In view of the above phenomenon,research institutions at home and abroad have researches on the drilling technology.Although the traditional axial drilling technology and torsional drilling technology both can improve the rate of penetration,each of them still has shortcomings.The impact energy of the axial impact is relatively small,and when the impact frequency exceeds a certain value,it will affect the life of the impactor,which decreases the amplitude of the improvement of the rate of drilling;the rock-breaking efficiency of the torsional impact drilling is relatively high,but the demands of the matching level between the drill bit and the impactor is higher when drill in high abrasive formation.So combine their advantages,the new drilling technology-composite impact drilling technology,is created.It can inhibit or even eliminate the stick-slip vibration of the drill bit in the process of drilling,and improve the rate of peneration of the complex formation.Based on the wave theory,the dynamics model of drill string is established.The causes of bit bounce and stick-slip vibration are analyzed.The composite impact model of PDC bit is established and the effect that the composite impact drilling have on the stick-slip vibration is analyzed detailedly.The rock-breaking effect of the composite impactor is analyzed by field test.The following conclusions are obtained:(1)Based on the wave theory,the dynamics model of drill string is established.At the same time,the boundary conditions of the model are given,and the model is solved.The neutral time delay model describing the drilling behavior is obtained.(2)The bit bounce and stick-slip vibration are analyzed.The nonlinear function describing the stick-slip vibration of the drill bit is established,and the calculation formula of the dry friction coefficient is obtained.Then the change curve of the friction torque of the drill bit vary with the angular velocity is made according to the formula,which can simulate the coulomb friction characteristics at the low angular velocity,and pointed out that the causes of stick-slip self-excited vibration is the anti-damping characteristics genetated from the nonlinear friction torque between the drill bit and the rock;(3)The composite impact model of PDC bit is established.It is concluded that the composite impact can reduce the stick-slip vibration of PDC bit and reduce the occurrence of the bit bounce.(4)Based on the rock breaking theory of the composite impact,the drilling tool was developed.The field tests in Jinyue 204 well and Qigu 1 well show that the composite impactor can effectively protect the PDC bit and reduce the torque fluctuation,so as to inhibit the stick-slip vibration of the drill bit,reduce the wear degree of the drill bit,improve the footage of the single drill bit and improve the rate of peneration.The research can inhibit the stick-slip vibration of the drill bit,improve the rate of peneration greatly,so have the value to popularize,and the application prospect is broad.