Study On Thermal Damage And Temperature Field And Thermal Stress Field Of PDC Cutter Breaking Rock In High Temperature Formation

With the increasingly difficult of oil and coal resources and the ever-increasing human need for resources,deep formations resources(for example hot dry rock)will be valuable replacement options in the future.Resources research and growth have increasingly shifted from standard formation to deep formation.Inevitably,hard rock formations and PDC bit wear are encountered during drilling.During the rock breaking process,high rotary rate of PDC bits generates cutting temperature.More than 50 % of the energy is transferred to friction heat.The combination of high formation temperature and heat causes the temperature of PDC bits to increase dramatically.High temperature will not only cause temperature gradient inside the PDC cutter,but also result in uneven thermal expansion due to different various expansion coefficients,which will result in larger thermal stress inside the cutter and cause deformation and wear of the cutter,which will reduce the cutting performance of the drill bit and even make the drill bit fail prematurely.Therefore,the failure of PDC bits are linked to the cutting temperature and stress.It is important to study the thermal damage to PDC bits under high temperature conditions,and cutting temperature and thermal stress field of cutter during rock crushing process to better understand the rock breaking mechanism of PDC bits and the wear law.However,it is not easy to precisely gain cutting temperature and stress field distribution during rock breaking in real time during the actual drilling process.In this paper,a combination of theoretical analysis,numerical simulation and experiments was used to study the influence and distribution law of thermal damage,cutting heat,and thermal stress field of PDC core bit during rock breaking.The microstructure and thermal damage mechanism of PDC cutter under high temperature conditions were investigated by XRD and SEM to reveal the nature of thermal failure of PDC bits and cutter after high temperature heat treatment.According to heat transfer,the theoretical analysis of cutting heat during rock crushing of PDC cutter was carried out,and the theoretical analysis of PDC cutter during rock crushing was established.A full-size PDC bit model was established,and simulation of the temperature and stress of PDC cutter in rock breaking by finite element method.The temperature of the PDC bit in the rock breaking test was measured by non-contact and contact mehods,and the temperature rise law of the PDC bit in the rock crushing process were obtained.The main results are as follows:(1)XRD patterns and crack development characteristics of PDC cutter under high temperature heating conditions were observed by X-ray diffractometer(XRD)and scanning electron microscopy(SEM).The macroscopic results showed that when the heat treatment temperature is 600 ?,the PDC surface appears "green oxidation product",and the appearance of cutter is "cross" at 800 ?,and the cutters completely failed.X-ray diffraction analysis shows that when the temperature reaches 800 ?,the amplitude of diamond diffraction peak has been diminished,and the diamond and metallic cobalt will be oxidized when heated in the air.According to the SEM test findings,when the temperature reaches 500 ?,the surface roughness of PDC specimen increases and grain boundary cracks appear.When the temperature reaches 600 ?,there are not only cracks but also microporous holes on PDC surface.When the temperature of heat treatment reaches 700 ?,there are many cracks on PDC surface.The thermal stress caused by the variation in thermal conductivity of metallic cobalt and diamond is the key mechanism of thermal damage of PDC polycrystalline diamond layer,which leads to cracks and diamond particles falling off.(2)Based on the theory of metal cutting temperature,three main sources of heat generation in PDC bit drilling process are analyzed,which are shear heat generated by cutting rock shear deformation of cutter,friction heat generated by friction between cutter and rock cuttings,friction heat generated by friction between bottom of hot cutter and rock.Combined with the theories of thermo elastoplasticity,tribology and heat transfer,the theoretical mathematical model of temperature rise of PDC cutting teeth in the process of rock breaking is established.The results of the theoretical study show that the temperature rise of drilling fluid is proportional to the square root of its flow rate under drilling fluid circulation conditions.The average temperature of the cutter is the average of the temperature of the cutter without considering the drilling fluid and the temperature of the drilling fluid after heat absorption.(3)The three-dimensional finite element model of the full-scale PDC bit and the rock was established by the finite element ABAQUS software,and the rock fragmentation process of the PDC bit was simulated dynamically,and the temperature rise law of the PDC cutter and the temperature deviation law of the high temperature area were obtained.From the simulation results,the specific location of the highest temperature of the cutter is determined.There are three processes in the process of temperature rise of PDC cutter: the period of sharp rise,the period of slow rise and the period of relative stability.The high temperature zone is migrated from the top of cutting crown to the outer end of cutter.The high temperature area of cutter is fan shaped,primarily spread in the place where the cutter comes into contact with the rock and the cutting speed is maximum.As the rotation speed increases,the temperature difference between the outermost end and the crown top of the cutter will be larger and larger.The cutter temperature reduces as the cutting angle increases.(4)The finite element method was used to simulate and analyze the transient thermal stress in the process of rock crushing by PDC cutter.The simulation results show that the maximum stress of PDC cutters appears near the bonding surface of the diamond layer and the cemented carbide matrix,that is,the top position of the cutter crown.The top position of the cutter crown is vulnerable to tension accumulation,which in turns contributes to excessive local stress.The equivalent stress of PDC cutter increases first and then reduces as the formation temperature increases.The maximum stress of PDC cutters at normal temperature is 2.44 GPa,2.47 GPa at 150 ?,and 1.75 GPa at 250 ?.When three different types of rock are broken,the stress change at the top of the cutter crown increases firstly and then fluctuates.Rock strength has an effect on the tension distribution and fluctuation range of the cutter.The higher the rock hardness grade is,the greater the average stress and fluctuation range of the cutter will be.The stress of the cutter in crushing granite is 58.2% higher than that in crushing marble and 134.8% higher than that in crushing sandstone.High concentration of thermal stress and fluctuating impact at the cutter interface are the main reasons for the fracture failure of PDC cutters.(5)The high-temperature drilling test platform was improved to realize the realtime temperature measurement of PDC cutter during drilling by two methods: noncontact thermal infrared radiation and contact thermocouple,and the temperature change law of cutter during PDC rock crushing and the effect law of rotary rate,weight of bit and rock strength on the temperature of cutter were gained.The rotation speed and weight on bit are positively correlated with cutter temperature.The temperature rise of granite cutters is significantly greater than that of marble and sandstone.The temperature of granite cutters is approximately twice that of marble and three times that of sandstone when drilling under the same conditions.In the process of drilling into high-temperature granite formations,when the granite temperature is 150? and 250?,the temperature change trend of the PDC bit is roughly the same.Both increase first and then decrease and finally tend to a dynamic equilibrium state,and the final temperature was stabilized in the range of 72 ??73.5 ?.The rate of penetration(ROP)of PDC bit was 0.06 mm/s for granite broken at 150 ? and 0.052 mm/s for granite broken at 250 ?.The ROP increased and then decreased with the increase of formation temperature,and the ROP of granite at 150 ? was greater than that at normal temperature and 250 ?.