Study On The Breaking Law Of High Temperature Rock Under Ultrasonic Vibration

At present,drilling in hard rock is a major problem in deep scientific exploration and resource exploitation.The problems of low drilling efficiency,short bit life,long cycle and high cost exist in conventional drilling technology.As the drilling depth increases,the formation temperature gradually increases.For every 1km increase in the conventional formation depth,the temperature increases by 30?on average.The increase of temperature has great impact on the conventional drilling.As a new assistant rock crushing technology,ultrasonic vibration has the characteristics of low axial pressure,concentrated energy and fast energy transmission,and has been widely used in the fields of material processing,oil and gas infiltration,space drilling,etc to provide valuable experience for using ultrasonic vibration in the field of crushing hard rock.Ultrasonic vibration breaking rock technology has attracted the attention of scholars at home and abroad,and a wealth of theoretical and experimental studies have been carried out,but there has not been a study on the influence law of rock temperature on ultrasonic vibration rock breaking.Therefore,the representative hard rock-granite which is widely existed is selected as the research object in this paper,and the crushing effect of high-temperature granite under ultrasonic vibration is studied from the macroscopic perspective to the microscopic perspective to obtain the crushing rule of high-temperature granite under ultrasonic vibration.It provides theoretical support for the application of ultrasonic vibration technology in deep hard rock and geothermal drilling and has practical significance.This article take the combination of theoretical analysis,numerical simulation and indoor test,and carry out a series of experiments and numerical simulation research with the method of digital image processing in Matlab software,PFC2D?Particle Flow Code 2D?numerical simulation software,nuclear magnetic resonance?NMR?detection,uniaxial compressive strength test,penetration detection test to obtain high temperature granite micro crushing effect,macro crack distribution characteristics,physical and mechanical properties change rule and so on under ultrasonic vibration,the main achievements are as follows:?1?Based on the fatigue damage theory,the process of rock fragmentation by ultrasonic vibration is analyzed,and the equation of rock vibration response to motion stress is given.Based on the resonance theory,the relationship between rock response amplitude and ultrasonic vibration frequency is given,and the optimal vibration frequency is obtained.The relationship between the energy density of stress wave and the stress on rock during the process of ultrasonic vibration is analyzed from the perspective of energy.Finally,the damage evolution equation of rock under the influence of temperature is given by introducing the damage variables in damage mechanics combining the thermal damage of rock.?2?Through the discrete element software PFC2D the numerical simulation solution of the model of ultrasonic vibration under different temperature granite is finished,the numerical simulation results show that the model of rock under heat treatment,the internal thermal stress appear random distribution within the model,and when applying ultrasonic vibration load,stress chain of the rock model changed from disordered state to an orderly transfer of top-down transfer,with the high frequency conversion between the stress state of tension and compression.Under the ultrasonic vibration load,the cracks in the rock model are mainly concentrated between rock and the loading interface area,and there are three types,including smooth joint tensile type,smooth joint shear type and parallel bond tensile type.Among them,the ratio of smooth joint tensile crack is the highest.It can be seen that the tensile failure between crystals under ultrasonic vibration is the cause of internal crack in rock.The number of cracks increased gradually with the increase of temperature.When the temperature of the rock model reached more than 150?,the smooth joint tensile crack and parallel bond tensile crack in the rock model grew rapidly under ultrasonic vibration.The azimuth angle of cracks in the rock model is concentrated in the direction of 90°,which is parallel to the direction of ultrasonic vibration loading.With the increase of temperature,all except the cracks in the direction of 90°,the proportion of other angle cracks increases together.?3?According to the macroscopic crushing volumes of granite samples in different temperatures under ultrasonic vibration within the same loading time,the result shows when the temperature of rock samples is lower than 100?,the crushing forms of granite are mainly fatigue crushing and the crushing volume is small.When temperature is higher than 150?,the accumulated thermal damage promotes a large number of internal cracks to expand and penetrate,and promotes the macroscopic fracture of the rock samples,resulting in a significant increase in the fracture volume.?4?According to the change of porosity increase of granite samples in different temperatures under ultrasonic vibration,the porosity increase of rock samples under ultrasonic vibration load is characterized by periodic characteristics under the influence of temperature.The increase of rock samples is small when the temperature is lower than 100?,significantly increases when the temperature is higher than 150?,and increases steadily with the increase of temperature.According to the T₂ spectral change curve of granite samples,the change histogram of pore radius distribution,and the evolution bar charts of various pore volume ratios,when the temperature of rock samples reaches 150?,the pore size distribution volume ratio of rock samples decreases with the increase of temperature,while the pore size ratio of large pores increases.It indicates that the stress concentration gradually occurs in the intercrystalline pores of the granite sample as the temperature increases,which is more easily to lead to the expansion of the pores in the granite under ultrasonic vibration.?5?The distribution characteristics of cracks in the rock profile obtained by penetration detection method correspond to the results of numerical simulation.As the temperature increases,the crack area in the rock profile increases gradually,and the crack area increases significantly when the temperature reaches 150?.When the temperature of rock sample reaches 150?,the uniaxial compressive resistance of rock sample deteriorates rapidly under the action of ultrasonic vibration.In addition,in addition to the porosity damage variable D_p,the crack damage variable D_c at the pixel scale can represent the change of uniaxial compressive strength of rock,and the relationship is exponential.