Research On The Wear Mechanism And The Evaluation Criteria Of Abrasiveness Between Formation Rock And Drill Bit

During the drilling process,the drill bit is wearing out while knapping rocks.The wear of the drill bit teeth reduces the rock-knapping efficiency,and that of the outer diameter reduces the wellbore diameter,which will cause drilling efficiency greatly reduced.The rock's ability to wear a drill bit is called rock abrasiveness.There has been no uniform evaluation method and grading standard yet for the abrasiveness of rock,which is mostly evaluated with curve fitting based on some experimental data and empirical models from mathematical statistics analysis.This method is not only complicated in operation,but also time-consuming and shows lack of research on the wear mechanism.In addition,for the evaluation of the adaptability of the drill bit,only indicators like rock drillability are considered,neglecting the influence of the rock on the wear degree of the drill bit,so that the selected drill bit is not completely matched with the formation.In view of the above problems,and based on the tribological principles,the wear mechanism between different types of drill bit and the formation is analyzed in depth,and the evaluation index and grading method of rock abrasiveness are established,which provides a basis for the selection and design of the drill bit.According to the topography of rock and drill bit surface,the fractal representation method of surface roughness is established.Combined with the surface morphology and energy spectrum analysis results of the drill bit,the wear mechanism of the PDC composite piece,the roller bit teeth and the gauge teeth of drill bit is determined.According to the rock-knapping characteristics of roller bit and PDC bit,the contact force models of single tooth pressing in rocks and single cutting tooth cutting rocks are established respectively.Combined with the rock knapping conditions,the reaction force of the stratum on the bit teeth and the gauge protection is deduced.Then the surface contact topography and stress condition of the two-body friction pair formation-drill bit are combined to obtain the calculation model for the real contact area,and according to the characteristics of the PDC bit's sliding friction and roller bit's rolling friction,the friction coefficients under sliding and rolling conditions are calculated.Since a wealth of heat is generated at the sliding friction between the drill bit and the rock,the formation-drill bit's temperature rising process and temperature distribution under the sliding friction condition are revealed according to the friction surface contact area formula and the heat conduction theory,yielding the change of actual contact area between the drill bit and the formation within a certain temperature range.Due to the presence of drilling fluid during the drilling process,the friction coefficient and temperature of the contact surface between the formation and the drill bit are much smaller than those under dry friction conditions,and based on the study of the lubrication characteristics of the drilling fluid surface,the friction calculation model between the formation-drill bit friction pairs under boundary lubrication conditions is established.The actual heat transfer process of the drilling fluid circulation is analyzed by the law of conservation of energy,obtaining the temperature field of the bottom hole under the cooling condition of the drilling fluid.According to the wear mechanism of the drill bit,the influencing factors of wear rate under different wear patterns are analyzed and,combined with the actual friction force and the temperature field distribution of the bottom hole,the bit wear equation under the drilling fluid circulation condition is established.In experimental research,the basic properties of rock contained in the wear equation are particularly tested.In the experiment,twenty groups of rock cores from different regions with different properties are selected,then their elastic modulus,Poisson's ratios,cohesions and internal friction angles are measured in uniaxial and triaxial compression experiments;their mineral components are analyzed by X-ray diffraction experiment;and the fractal dimension of the surface roughness of the cores is obtained through scanning electron microscopy.Then the equation is corrected by comparing the results of the indoor micro-bit wear test with the calculation results of the bit wear equation.Taking the wear amount of the drill bit as the evaluation index,the numerical analysis on the rock properties affecting the wear of the drill bit is conducted and the grade difference is calculated,thus the main control factors and control mechanism of the rock wear on the drill bit are obtained.The research shows that the influencing order of rock factors on bit wear is: quartz content > internal friction angle > surface fractal dimension > cohesion > elastic modulus > Poisson's ratio.Finally,the drill bit related parameters and working condition parameters in the bit wear equation are standardized.Then,the wear weightlessness of the rock cutting tool per unit volume is used as a measure of the rock's abrasiveness,and the drillability grade parameters of the rock are introduced,thus the evaluation indexes of rock abrasiveness under sliding conditions and rolling conditions are established,and the grading criteria of rock abrasiveness are given based on the calculation results of different lithological characters.The method has good applicability and is of great significance for enriching the tribology theory and guiding the design and optimization of the drill bit.