| Conventional PDC bit scraping theory greatly limits the cutting efficiency and working life of the bit in difficult drilling formation.Composite rotary cutting PDC bit is a kind of new idea which uses fixed wings with mixed cutters and rotary cutting wheel with conical cutters to form reticular cross cutting marks at the bottom of the well so as to realize the principle of cross cutting and breaking,which is of great significance for improving the adaptability and rock breaking efficiency of the bit.For this reason,theoretical,numerical simulation and experimental research are carried out on the cutting structure of compound rotary PDC drill bit.The main research contents are as follows:(1)Based on the equal cutting principle,the contour curve equation of the flat-bottomed crown of the compound rotary PDC drill bit is established,and a mixed arrangement of conical and conventional PDC cutters was designed for the cutter wing.The force analysis is carried out on the cutters on the rotary-cutting wheel,the optimal cutter type and the arrangement method of cutter are determined,and the geometric position parameter equation of the rotary-cutting wheel is established.The motion characteristics of the planetary motion mechanism of the cutting structure are analyzed,and the critical transmission ratio expression of the wheel series and its reasonable range are obtained.A cutting trajectory equation of the rotary cutting wheel in the cutting process is established,and the cutting marks are generated by using the MATLAB,and the influencing factors are analyzed,which lays a foundation for the design of the cutting structure of the bit.(2)Based on the theoretical research,the numerical simulation of the rock-breaking of the cone cutters and the conventional PDC cutters on the fixed blade wing of the composite rotary-cutting PDC bit is carried out by means of explicit finite element method.Through the study of rock breaking of two types of cutters combinations in the same rail,the influence of pre-breaking depth of conical cutters on cutting force,axial force and mechanical specific energy of conventional PDC cutters were obtained,And it was determined that the conical cutter critical pre-breaking depth is 1.2mm;Through the study of rock breaking of two types of cutters combinations in different rails,The influence law of the cutting force,axial force and mechanical specific energy of conventional PDC cutters were obtained,and the optimum height difference and spacing of the two kinds of cutters were determined,which are 0.4mm and 9mm,respectively.The rock breaking mechanism of the two kinds of cutters combined cutting process was revealed by dynamic damage cloud diagram and crack analysis.(3)According to the numerical simulation results,the experimental study on the cutting of conical cutters and conventional PDC cutters mixed is carried out.The design and calibration of the mixed cutters cutting test device are carried out,and the conventional PDC cutters and conical cutters mixed cutting rock test is carried out to verify the reliability of the numerical simulation results;The rock breaking mechanism of the mixed cutters is further revealed from the force signal and cuttings of the cutting process.As a result,the pre-breaking of the conical cutters during the same rail cutting makes the subsequent rock breaking mechanism of the conventional PDC cutters become brittle,the difficulty of rock breaking is reduced and the impact is reduced,which is helpful to improve the service life of cutters;During different-rail cutting,the reasonable mixed cutters parameters are helpful to the formation and expansion of cracks inside the rock,which makes it less difficult conventional PDC cutters to break rock,and the rock is more prone to volume breakage,and then improve the efficiency of rock breaking,which lays a foundation for the optimal design of composite rotary cutting PDC bit. |
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