| In recent years,with the development of new types of resources in China,high temperature has become more and more common in various projects.The thermal stability of rock is of great significance to the safety and efficiency of the projects such as the selection of nuclear waste repository sites,development of hot and dry rock resources,and underground coal gasification technologies.Therefore,the influence of temperature on rock properties is important one of research subjects in rock mechanics.One of the important topics.In the existing research on the influence of temperature on rocks,only one or some physical quantities are usually measured,and the analysis of the nature of these changes is ignored.The method of direct observation by scanning electron microscopy also has defects,such as it can only observe the changes in the appearance of the rock and cannot observe the confining pressure.In this paper,a variety of experimental methods are used to obtain the change of quality,size,wave velocity,coefficient of variation,porosity and permeability of different types of rocks with temperature.Combined with the direct observation results of scanning electron microscopy,the evolution of the internal structure of rock after heating was analyzed comprehensively.Moreover,according to the internal structural characteristics of rocks,some wave velocity-permeability models suitable for different types of rocks have been established to provide a theoretical basis for achieving high-temperature ultrasonic technology to measure permeability.The main research contents and achievements of the paper are as follows:1.The change law of the physical and mechanical properties of different types of rocks affected by temperature were obtained.According to the internal micro-structural characteristics of rock,the rocks were classified(fissure-type rocks,pore-type rocks and fissure-pore rocks),and five typical rocks(Maluanshan granite,Bershan granite,Tongxin sandstone,Pengshui shale and Longkou oil shale)were selected for high-temperature experiments.The physical quantities such as rock mass,size,wave velocity,anisotropy of wave velocity,porosity,and permeability are measured in the experiment.Based on the characteristics of each parameter test,the test flow is properly planned and designed.The physical and mechanical properties of rock with temperature change as follows:(1)The rock mass gradually decreases with increasing temperature.The main reasons for the loss of mass are: a.Escape of adsorbed water,interlaminar water,zeolite water,and mineral bound water inside the rock;b.Decomposition and volatilization of organic matter in some rocks.Because of the large porosity and the high content of mineral-bound water,the loss rate of pore-type rocks is much higher than that of fissure-type rocks.(2)The size of the rock(height,diameter,and volume)gradually increases with increasing temperature.Oil shale is special.The dimensional parameters of oil shale along the bedding direction will decrease with increasing temperature.This is mainly related to the volume shrinkage of dehydration of minerals such as montmorillonite and analcite.The rocks other than oil shale have more intense volume change at 550~600 ?,which is mainly related to the volume expansion caused by quartz phase transition.(3)The axial wave velocity of rock decreases with increasing temperature.There is a linear relationship between temperature and wave velocity of fissure-type rocks.There are two wave-speed dips at 300 ? and 550 ? in the Maluanshan granite and Beishan granite(indicating the generation and expansion of a large number of cracks),and a slight change in wave speed at about 350 ?(indicating that the change in the crack amount is small);there is a significant turning point(quartz phase transition temperature)in the temperature-wave velocity relationship of the pore-type rocks,the wave velocity changes slowly before the turning point,and decreases sharply after the turning point.(4)The porosity of rock gradually increases with increasing temperature.Porous rocks have much higher porosity than fractured rocks.The fractured rock has a low water content,and the change of porosity is mainly caused by thermal stress.While the porosity rock has a high moisture content.The main reason for the porosity change is caused by the combined effect of thermal stress and water escaping.As with the wave velocity,there are also two sudden increase phases and one micro-variation phase in the porosity of fissure-type rocks.There is also a sharp turning point in the porosity of pore-type rocks.(5)The permeability of rock increases with temperature.The permeability of fractured rock increases approximately exponentially with increasing temperature.The permeability of pore-type rock has a significant decline with increasing temperature.The reason for this phase is that the particle size decreases rapidly with the effect of the large particle cracking and mineral dehydrogenation,the pore structure is dispersed and refined,the effect of the decrease of particle size on the reduction of permeability is greater than the effect of porosity increase on the rise of permeability.2.The micromorphological evolution mechanism of different types of rock after heat treatment is obtained.Scanning electron microscopy(SEM)was used to directly observe the surface of the heat-treated specimens.The effects of temperature and magnification on the crack statistics were analyzed,and the regularity of particle size changes was analyzed.The results are as follows:(1)The crack densities of granite and oil shale treated at different temperatures were counted.It was found that both the temperature and the amplification factor will cause crack density changes: the higher the temperature,the greater the crack density;the larger the magnification,the greater the crack density.The increase of temperature will lead to the occurrence of cracks which can cause a substantial increase in crack density.The increase in the magnification of the field of view will account for the cracks that cannot be observed by the naked eye at a small magnification which can increase the crack density from a statistical point of view.(2)The grain sizes of granite and shale after heat treatment at 50 and 500 ? were statistically calculated.It was found that the grain size of the rock after heat treatment was significantly reduced,and the distribution was more concentrated.The particle size of fissure-type rocks is reduced due to the formation of cracks in thermal stress;The particle size of pore-type rocks is reduced due to the dehydration of clay and dehydrogenation of the mineral.(3)In observations of sandstone,shale,and oil shale,it was found that large pore-crack structures are easily formed around large particles that differ greatly from the surrounding particles in the size.Due to thermal expansion,large particles squeeze the surrounding small particle structure,and cause some irreversible deformation.When the temperature drops,large particles shrink in volume,creating a pronounced pore fracture structure around the particles.This is one of the mechanisms of generation of pores and cracks with high temperature.3.The wave velocity-permeability models suitable for different rock structures were established.The analysis of the physical and mechanical properties of rock with temperature changes reveals that the properties of the rocks are interrelated,and the nature of each property change is the evolution of the internal structure of the rock.Further,the relationship between wave velocity and permeability were analyzed in theory.The results are as follows:(1)Based on the Hudson's fractured media model,a wave velocity-porosity model of fractured media was established.In combination with the generalized Kozeny-Carman equation,a wave velocity-permeability model suitable for fissure-type rock was established;the rock material was regarded as a combination of background material and pore part.Based on the Reuss boundary theory and the equivalent compressibility of porous media,the wave-porosity model of dry porous media was established.In combination with the generalized Kozeny-Carman equation,a wave velocity-permeability model suitable for pore-type rock was established.(2)The new model can invert the variation law of the particle size of the rock with temperature: in the low temperature stage,the particle size slowly decreases;when the temperature reaches the threshold,the particle size decreases rapidly;in the high temperature stage,the particle size is stable and almost unchanged.The rapid decrease of the particle size of the granite particle corresponds to the first sudden drop of the wave velocity,and the rapid decrease of the particle size of the sandstone corresponds to the dehydrogenation temperature of kaolinite.(3)Based on the self-similarity of rock pore fracture structure,it is assumed that the aspect ratio,pore shape factor,and particle shape factor do not change before and after heat treatment.The models were simplified and compared with the experimental results.The results show that the new models can better reflect the relationship between rock speed and permeability,providing a theoretical basis for achieving ultrasonic technology measurement of permeability.4.The evolution of internal structure after heat treatment of different types of rock is summarized.Combining the inversion results of the thermal variation laws of the physical and mechanical properties of rocks and the direct observation results of scanning electron microscopy,the comprehensive evolution rules of internal structures of rocks after heating were analyzed.The results are as follows:(1)The thermal evolution process of internal structure of fractured rock can be divided into four stages: Stage I,primary microcracks develop steadily,and new microcracks generate gradually;Phase II,thermal stress rises to the threshold,original microcracks develop rapidly,and a large number of microcracks generate and extend.Stage III: Microcrack density reaches the threshold,some of microcracks merge into large cracks,stress is released in the vicinity,and some microcracks are closed;Stage IV,new cracks are rarely generated,closed microcracks expand again,and existing cracks continue to develop.(2)The thermal evolution process of the internal structure of porous rock can be divided into three stages: Stage I,adsorption water and interlayer water escape,pores increase,thermal stress causes structure fine-tuning;Stage II,after the threshold temperature is reached,cement particle cluster dehydrate and dissociate,the mineral particles are destruct by dehydrogenation crystal lattice,which cause a sharp decrease in particle size;Phase III,particle size is stable,high thermal stress caused by the phase transformation of quartz caused structural changes,a large number of pore fracture structure,rapid decline in wave velocity,porosity and The penetration rate increases rapidly.5.The following observations have also been made in the study of the influence of temperature on rocks:(1)The anisotropy of rock wave velocity before and after heat treatment was compared.It was found that the directionality of rock wave anisotropy did not change before and after heating,indicating that the thermal crack propagation direction is the same as that of primary cracks.If the anisotropy is not obvious and the direction of the geostress can't be determined by the anisotropy method of wave velocity,heat treatment can solve this problem.(2)The coefficient of variation was introduced as a measure of the degree of anisotropy.The coefficient of variation eliminates the effect of different units and averages on the degree of anisotropy.The coefficient of variation of fissured rock continues to increase in stages I and II where new cracks are generated.It is almost unchanged in stages III and IV where new cracks are rarely generated;the coefficient of variation of pore-type rocks increases with temperature.This shows that the coefficient of variation can reflect the occurrence of new cracks.(3)The observation of large particles around sandstone,shale,and oil shale tends to produce larger pore fractures.This phenomenon reveals the mechanical mechanism of the thermal Kaiser effect: The thermal Kaiser effect is the memory of the rock that redistributes the rock structure to the heat treatment.The rock is heated and the internal structure is readjusted,resulting in plastic deformation that cannot be recovered by cooling.When the temperature is increased again and does not reach the maximum temperature that was previously received,it is difficult for the rock to generate large thermal stress and acoustic emission is difficult to occur.(4)The experiment found that the different forms of water in the rock escaped,the dehydrogenation reaction of mineral components and the decomposition and volatilization of organic matter,and had a great influence on the rock structure and various physical and mechanical properties.Analyzing the rock composition and understanding the thermal stability of each component is of great significance for studying the evolution of physical and mechanical properties and the internal structure of the rock. |
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