Research On The Design Theory Of Spherical Single-cone-PDC Composite Drill Bit

The sharp rise in the demand of oil and gas resources,driven by China’s economic boom,is increasingly contributing to the imbalance of supply and demand.The year 2015 saw a record high in terms of reliance on exported crude oil(over 60%).Accordingly,our commitment to the national strategic plan on energy calls for exploration of new oil and gas resources.Since drilling represents the only optimum choice prevailing worldwide for the exploration of oil and gas resources,and drill bit,as an important tool,is used to directly crush rocks for boreholes in rotary drilling as a necessary tool,whose service life and performance directly determines drilling speed thus affecting exploration and development efficiency.As a type of cutting drill bit between tri-cone bit and PDC bit,single-cone drill bit has gained popularity in drilling community for its special way of rock crushing and higher mechanical drilling speed,and serves as the top choice in small-hole side drilling and deep well drilling.It is indicated by the field application tracking survey and statistical analysis that earlier excess abrasion and dead spot of cutting makes up the major failure forms of single-cone bit.The currently popular hard alloy single-cone bit,is only applicable to softer stratum,and shows severe early abrasion and shorter service life in harder stratum featuring high abrasiveness.That’s why it is of realistic and future significance for the designing and development of single-cone bit characterized by higher abrasion resistance and broader lithologic adaptability.This paper,aimed at the problems in single-cone bit,provides a design of a new-type spherical single-cone PDC composite drill bit by adopting the design concept of spherical single-cone bit and PDC bit as well as the principle of slow alternating operation of PDC cutting teeth;and the theory on the systematic design of new-type spherical single-cone PDC composite drill bit is established via systematic theoretical and simulation research.(1)Built on the brief elaboration on the current study and prospect of single-cone bit home and abroad,its application at present and main problems existing,this paper brings forward the background and purpose or significance of selected topic as well as the way of thinking and content of the study.(2)Scheme design of three types of spherical single-cone PDC composite drill bits is in place based on the proposed design concept of spherical single-cone PDC composite drill bit and comparative analysis on the rock crushing mechanism of single-cone drill bit,tri-cone bit and PDC bit,analysis and su,marization of failure mechanism of single-cone drill bit via field survey.(3)Considering the top rake angle and side-rotation angle installed on the roller cone as well as the form of PDC teeth,geometric and kinematic equation is established on the PDC teeth of spherical single-cone PDC composite drill bit,together with the theories on the calculation of radial,tangent and longitudinal component velocities and accelerations at any point on the cutting edge.In this way,quantitative analysis and study can be done on the structural parameter of single-cone drill bit,location parameter of PDC teeth on roller cone as well as how rock properties affect these velocities and accelerations which provides theoretical basis for the confirmation of the optimum structural parameters of drill bits.(4)With the establishment of equation of moving track of PDC teeth of single-cone drill bit at the shaft bottom,simulations are carried out of the tracks at different axial inclinations,penetration depths and times for three designs of spherical single-cone PDC composite drill bits.Evaluation criteria are proposed based on the shaft-bottom coverage rate and the maximum uncrushed boss and get the result of how the two aspects are shaped by axial inclination of drill bits in all designs.Besides,scope of the optimum axial inclination design is confirmed in all schemes,which provides theoretical basis for the optimization of axial inclination,and evaluation and upgrading of structural design of tooth face of spherical single-cone PDC composite drill bit.(5)Contact mechanics-based theoretical calculation formula is established for the interaction generated by the vertical indentation and horizontal rock shaving and cutting of PDC teeth of single roller cones(axial,radial and tangent forces).The relation formula is established for the interaction between roller palm and cone,and between roller cone and PDC teeth,which leads to the theoretical formula for the resultant force and its torque of the interaction between PDC teeth and rock on roller cone and drill bit;besides,researches are carried out on the correlation of factors including the contact force and indentation depth,location,height,top rake angle and side rotation angle of PDC teeth on roller cone in the case of indentation and rock shaving and cutting of PDC teeth.In this way,simulation and analysis is finished on numerical values about full-teeth rock crushing by spherical single-cone PDC composite drill bit,leading to the curve of response of velocity and acceleration with time as well as the interactive cutting forces between PDC teeth and rock.(6)Based on the above-mentioned content of study,the theory of spherical single-cone PDC composite drill bit designing is established systematically.And it is followed accordingly by the structural designing of Φ115mm spherical single-cone PDC composite drill bit,along with the analysis on the strength of major components.This brings an international pioneering achievement-real object of spherical single-cone PDC composite drill bit,which provides hardware foundation for laboratory and field test.The research work of this paper has improved the theory of design of single-cone-PDC composite drill bit in small-hole side drilling and deep-well drilling.It has great theoretical and engineering application value.