Research On Rock-breaking Mechanism And Design Theory Of The Cross-cutting PDC Bit

Since Improving the drilling speed in deep and hard-to-drill formation is one of the major technical difficulties need to be solved in gas and oil well drilling industry.Among varies research approaches,technology innovation of drill bit is one of the most potential directions.Aiming at overcoming the weaknesses of high wear rate,uneven wear and poor penetration ability in hard-to-drill formation,a novel PDC bit with the rock breaking method of cross-cutting is put forward in this paper.The cross-cutting PDC bit i5 a kind of composite drill bit that configured rotational scraping-wheels with large deviation angle on a conventional fixed-cutter PDC bit.Specifically,each scraping-wheel is mounted with a plurality of cutters and the cutters,as the scraping-wheel rotates,will revolve and cut the rock suceessively so as to form spiral-like cutting tracks.When the spiral-like tracks cross the concentric-circle tracks formed by fixed cutters,a mesh-like bottom-hole pattern will be made.Theoretical analysis and experimental test show that cross-cutting is an efficient rock-breaking method saving both force and power,and that the rotating scraping-wheel does not only even the cutter wear and lower the wear rate,but will also improve the rock-breaking efficiency significantly.Main contents of this paper comprising:1.Basic theory research for the novel PDC bit.Through establishing the compound coordinate system for the PDC bit,basic theories of geometry and kinematics are established and theoretical research on the cross-cutting motion of the cutters are conducted for the PDC bit.Furthermore,the wheel/bit speed ratio model is established and the relationship between the ratio and cross-cutting angle is revealed.2.Numerical simulation and experimental research on single-cutter cross-cutting.First,typical rock samples are selected for testing the mechanical parameters of the rocks,which are furtherly utilized to reveal the constitutive relation for each type of rock.Geometric model of the bottom hole with cross-cutting profile are established.Numerical simulation on the cross-cutting process is conducted to reveal the stress-fields variation in both the cutter and the rock.Shearing slippage of the rock in cross-cutting condition is researched.Single-cutter experiments with different influencing factors are conducted on both cross-cutting and parallel-cutting to test the cutter load.Cutting force and specific volumetric fracture work in cross-cutting and parallel-cutting are contrasted,and the influencing rules of different factors on cutting force and specific volumetric fracture work are respectively researched.3.The calculation of mechanical parameters for the cutters in cross-cutting condition and the establishment of corresponding mechanical models.Based on rock mechanics,rock fracture theory,cutting mechanics and the calculation of cutting angles in cross-cutting condition,the establishing of cutting mechanics model for PDC cutters in cross-cutting condition,as well as the load on the bit,is discussed.4.Research on the cutter-distribution theory and structure design for the novel PDC bit.On basis of the structural characteristics of the novel PDC bit,the design method of deviationangle,journal angle,diameter and other parameters of the scraping-wheel,as well as the cutter-distribution theory for the coupled rotating-fixed cutting structure,is researched.Furthermore,the optimum design method of the wheel-cutter and the journal structure are discussed.5.The development of numerical drilling simulation system for the novel PDC bit.Based on the simulation on both the conventional and novel PDC bits,kinetic and mechanical characteristics of the novel PDC bit are revealed and concluded.6.Indoor experiments on the full-sized bit.First,an experimental bit with both scraping-wheel and fixed-blade are designed and manufactured,where the parameters like the deviation angle and joxumal angle are adjustable.Furtherly,the wheel/bit speed ratio is tested and the results are used to amend the theoretical model.The influence of different paraneters as deviation angle,journal angle,WOB etc.on the load and rock-breaking efficiency is researched.The working load,motion rules,rock-breaking efficiencies of the conventional and novel PDC bit are analyzed and compared,so as to reveal the working characteristics of the novel PDC bit.