Reserach On Formation And Characteristics Of Rock Breaking By PDC Cutter

PDC bit is the most used bit for oil and gas drilling at home and abroad.The adaptability of bit in different geology will have a direct impact on the construction period,cost and quality of drilling operation.With the exploitation of shallow oil and gas resources,today’s exploitation target has gradually changed to deep or complex formation oil resources.In order to achieve good drilling effect,it is necessary to reflect the rock breaking efficiency from the side through easily observable rock cuttings and cracks,and then make reference to the optimization of a series of conditions such as drill head design,cutting unit selection,cutting unit parameter setting and so on.Therefore,this paper makes relevant theoretical,numerical simulation and Experimental Research on the process of rock debris generation of PDC single tooth rock breaking.The specific research contents are as follows:（1）Set up a test-bed for PDC single tooth rock breaking experiment.During the experiment,the real-time cutting force and axial force in the cutting process are collected through the data acquisition system,and the linear relationship between them is explored.Experiments were carried out under the conditions of changing cutting depth,PDC tooth caster angle,PDC tooth diameter and confining pressure,and comparative force data were collected to explore the effects of various factors on cutting force,axial force and mechanical specific work without drilling fluid pressure.Then the bad arc length is calculated by cutting depth,back inclination angle and tooth diameter.The specific relationship between bad arc length and cutting force is explored through fitting curve.（2）By observing the cutting force data and rock cutting marks,combined with the capture of high-speed camera,the general law of rock debris generation is obtained.By collecting and screening and weighing the rock cuttings produced by cutting rock under different conditions,this paper explores the particle size distribution characteristics of rock cuttings under the influence of cutting depth,caster angle,tooth diameter and confining pressure and the influence relationship between the actual quality of rock cuttings and theoretical quality.It is found that the four parameters do not affect the general distribution law of rock cuttings,but only the size of characteristic particle size.Then the roughness index of rock cuttings is calculated,and the general law of characteristic particle size and roughness index at different depths is explored.Finally,a new method to obtain the rock breaking efficiency of PDC teeth without cutting force is proposed.（3）After analyzing the feasibility,the numerical simulation of PDC single tooth cutting experiment is carried out by using PFC^2D discrete element software,and combined with the pictures produced by high-speed camera in the experimental process,the general law of PDC tooth cutting rock crack is obtained.Combined with the simulated images under different cutting depth,PDC tooth caster angle and cutting speed,the corresponding crack propagation law is obtained,and it is found that the crack is formed because the external force on the rock exceeds the shear strength of the rock.By analyzing the number of cracks and point force data,it is found that the cracks are ladder shaped,and most of the failure forms of cracks are tensile failure.The research conclusion of crack propagation echoes with the characteristic law of rock debris generation,which is of great significance to improve the rock breaking efficiency of PDC bit.