Experimental Study On The Damage Evolution Of Sandstone Under Thermos-mechanical Coupling

The research on the thermal damage in sandstone is of great significance in the rock engineering of high-temperature environments including underground coal gasification,geothermal resource development,deep geological burial of nuclear waste and deep underground space utilization.This work investigated the damage mechanism of Datong sandstone structure with the increasing temperature through the description and analysis of meso-structures.In combination with the meso-and macroscopic methods,the change of pore-fracture structure of sandstone was analyzed.The influence of temperature and uniaxial stress on the damage characteristics of sandstone was evaluated using the self-designed multi-functional rock testing equipment.The damage rule of sandstone from microscopic to macroscopic after exposure to temperature and uniaxial stress was obtained.The seepage property of sandstone was investigated using a self-developed high-temperature triaxial permeability testing equipment.The macroscopic thermal damage rule of sandstone was revealed by the analysis of the change of seepage property of sandstone.The main results and conclusions are summarized as follows:(1)The meso-structure changes of sandstones after thermal treatment(25-1000°C)were studied by means of X-ray diffraction analysis,microscopic thin-section analysis,and scanning electron microscopy.And the change law of mineral composition,microstructure,and pore-fracture distribution of the sandstone with temperature was analyzed.The damage mechanism of sandstone fabric and structure with temperature was discussed.The results show that high temperature damages the crystal structure in the sandstone.With temperature varying,the temperature response of the sandstone structure occurs.And the thermal reaction and mineral thermal expansion caused by high temperature are the two main factors affecting the structure change of the sandstone.Thermal cracking occurs on the surface of sandstone after subjected to high temperature,which is divided into four stages: initiation,propagation,connection,and shrinkage.(2)Based on low-temperature nitrogen adsorption experiment and low-field NMR experiments,the evolution of pore-fracture structure of sandstone exposed to high temperature(25-1000°C)was studied.The pore parameters such as pore-fracture type,effective porosity and pore structure distribution were quantitatively characterized.The results showed that increasing temperature facilitates the development of pore-fracture structure of sandstone.500°C is the threshold temperature at which the pore-fracture changes significantly.From the results of low-temperature nitrogen adsorption,it can be seen that at temperatures less than 500°C,the pore morphology is dominated by slit-type pores,the pore size distribution curve shows a unimodal distribution,and the micropore content increases with increasing temperature.However,when the temperature is greater than 500°C,the pore morphology is dominated by cylindrical pores.The pore size distribution curve shows a bimodal distribution and the content of transition pores increases significantly.The specific surface area and average pore size reached the highest and lowest values at 500°C,respectively.The pore structure of sandstone displays good statistical fractal characteristics.The pore distribution at 500°C is the most complex.(3)From the results of NMR experiments,it can be seen that the pore-fracture structure of sandstone shows a staging characteristic with temperature changes.The relationship between porosity and temperature accords with the Boltzmann equation.With the limit of 500°C,the porosity and connectivity of the pores did not change much when the temperature was lower than 500°C.However,when the temperature exceeds 500°C,the content of mesopores and macropores of the sandstone increase significantly,as well the connectivity of the pores.This results in an effective fluid migration connectivity network,leading to an increase in permeability.The correlation between the permeability and temperature coincides with the SDR model.(4)Based on the uniaxial compression experiments of sandstone under high temperature,the stress-strain characteristics of sandstone at different temperatures were analyzed.Using the theory of damage mechanics,the damage variables of sandstone under the coupling of temperature and stress were calculated.The temperature damage to sandstone is different at various temperature stages.(5)By systematically studying the evolution of transport property of sandstone under high temperature and tri-axial stress,the damage mechanism of sandstone under tri-axial stress was analyzed and revealed.The damage of sandstone under tri-axial stress condition is affected by many factors such as rock structure,temperature,stress and pore pressure.The damage in sandstone caused by temperature generally shows that the degree of damage increases with temperature increasing.The study found that around 200 °C and 500 °C are the threshold temperature for the thermal cracking in sandstone.The permeability change law of sandstone with temperature is as follows: Before 200°C,the permeability increases slowly;at about 200°C,the permeability steeply rises,the thermal cracking occurs seriously,and the degree of damage increases sharply.Between 200°C and 500°C,the permeability changes with fluctuations.At 500°C,the thermal damage degree of sandstone reached the most severe,and the permeability appeared the minimum.Above 500°C the permeability shows a rapid increase.