Numerical Simulation Study On Rock Breaking Of Two-stage Dual-speed PDC Bit

Although the numerous scholars have carried out extensive research regarding the increase of the rate of penetration (ROP), most of these studies have focused on the bit type, well construction, drilling fluid performance, structural design and strength of the cutting teeth, which does not make any changes in the method of breaking rock, continuing the use of traditional means.In 2006, Shen Zhonghou academician and Peng Ye proposed the double-stage PDC bit, and the new type of PDC bit has changed the traditional way of cutting rock.This paper further researchs the double-stage PDC bit on the basis of the predecessors, and comes out a new type of bit——the double-RPM PDC bit.In this paper, the double-stage PDC bit and double-RPM PDC bit were researched by the finite element simulation software, based on the rock breaking mechanism of PDC bits and the PDC bit design theory, involving elastic-plastic mechanics and rock mechanics, using the Drucker-Prager criterion as the rock constitutive relation. The main conclusions of the paper are as following:(1) The accuracy and reliability of the simulation model is verified by using the DengJingen's stress distribution theory.(2) The study shows that the double-stage PDC bit has very significant effects on acceleration and vibration reduction, and revealed the reason of the speed-increasing.(3) The influence of the main drilling parameters on acceleration and vibration reduction effects of the double-stage PDC bit was researched.The research results show that the acceleration effect of the double-stage PDC bit will gradually diminish with the improvement of the bit weight, and it will firstly increase and then decrease with the improvement of the rotate speed.(4) Optimized the diameter combination of the reaming bit and pilot bit on the double-stage PDC bit. The research results show that the acceleration effect of the double-stage PDC bit will firstly increase and then decrease as the pilot bit diameter increases. The lateral vibration strength will firstly decrease and then increase as the pilot bit diameter increases, and the axial vibration strength will increase with the pilot bit diameter increases. The optimum double-stage PDC bit can superlatively speed up 40.7%, and superlatively decrease the vibration intensity 24.4%.(5) Based on the double-stage PDC bit, we researched the double-RPM PDC bit of the acceleration and vibration reduction effects. The study shows:the ROP of the double-RPM PDC bit will increase with the pilot bit RPM increases. And its acceleration effect will gradually decrease with the pilot bit RPM increases. Through the comparative study found that the double-RPM PDC bit in terms of acceleration and vibration-reduction has more advantages than the double-stage PDC bit, and the ROP of the double-RPM PDC bit can superlatively increase 271.7%, and the vibration intensity superlatively decreases 72.8%.