Study On The Effect Of Temperature And Stress On The Thermal Fracture Of Granite

With the decreasing of non renewable energy,the development of new energy has become a hot topic in the scientific community.High temperature geothermal energy,as a new type of sustainable development energy,is rich in resources in China.Studying the influence mechanism of temperature and stress on the thermal cracking of granite has great guiding function for the development of high temperature geothermal energy.The thermal stress of granite will produce small expansion deformation.When the thermal stress is greater than the bond strength between the granite,it will produce thermal cracking and crack.Granite,as a reservoir rock with high temperature geothermal energy,is in a high temperature and high pressure state at the bottom of the ground.It is very difficult to study the law of thermal fracture caused by the factors such as temperature and stress in the rock under actual ground conditions.It is difficult to explore the change of the microstructure of rock under the coupling condition of the experimental temperature stress.Many scholars have done a lot of work on the numerical simulation,but the accuracy of the conclusion still needs to be verified by the physical experiment.In this paper,the influence of temperature and stress on the thermal rupture of granite is investigated by the three axis test machine,which is developed by Taiyuan University of Technology.The test machine can exert three unequal stresses and the maximum temperature is heated to 450 C in the experiment.The permeability of granite under different stress states is between 100 and ~450 C.The law of the acoustic emission is collected and analyzed with the acoustic emission equipment during the heating process.After the experiment,a high precision micro CT system is used to scan the granite specimen,and the information of the specimen before the experiment is analyzed and compared,and the change of the microstructure of the rock is compared.The influence of temperature and stress on rock thermal cracking is deduced through the above experimental means.This paper draws the following conclusions:Through on-line measurement of permeability,it is found that:(1)The permeability of granite increased slowly and rapidly increased at two degrees 100~400,and the threshold temperature was about 340 degrees Celsius.And in the ultimate stress strength of rock,the size of axial compression of granite specimen affects the increase of permeability of granite before and after the threshold temperature,that is,the greater the axial pressure,the greater the increase of the temperature threshold before and after the permeability of granite.(2)The three axis stress of rock can induce the development direction of the crack in the rock,and the crack in the rock is easy to grow along the high direction of stress.Therefore,the permeability will have a larger scale in the same confining pressure,and the two specimens with the same three-dimensional stress and the different stress direction are in the horizontal direction.The smaller the permeability of specimens is because the lateral development of fractures takes a large proportion along the direction of confining pressure,which is not conducive to the breakthrough of gas flow network in axial compression direction.Through acoustic emission test,it is found that:The energy rate of acoustic emission of granite at 100~400 C shows a periodic change,and the energy rate of the active acoustic emission increases greatly.After the active phase of acoustic emission,the rate of acoustic emission energy will decrease,and the propagation rate of the wave in the rock decreases with the increase of temperature(100~340 C)and slow down(3).40~430 C)two stages.CT control before and after heating was found:The porosity of granite specimens heated at 430 C is about 30%,while the porosity of granite specimens heated to 220 and 370 C increases by 6% and 20%,respectively,indicating that the thermal fracture degree of granite at 370~430 C is more intense.After heating the granite specimen,the proportion of the material with smaller density increased.