Study On Evolution Of Gas Permeability And Pore Structure Characteristics Of Sandstone After Thermal Damage

At present,geotechnical engineering is gradually shifting from shallow layer to deep layer,which makes many geotechnical engineering involve high temperature and high pressure environment.Therefore,various characteristics of rocks under high temperature and high pressure conditions have become the research hotspot,especially the permeability characteristics and pore structure characteristics of rocks.In order to investigate the effects of thermal damage on various properties of sandstone,a series of physical parameters and gas permeability tests were carried out for sandstone pretreated at different temperatures(20?-800?).The changes of pore structure characteristics,porosity and pore size distribution of sandstone after thermal damage were analyzed by mercury injection method and scanning electron microscopy.The gas permeability characteristics and pore structure evolution of sandstone after thermal damage were studied by means of macroscopic and microscopic tests.Specific research results are as follows:(1)The basic physical properties of sandstone treated at different temperatures were tested.The mass,volume,density and wave velocity of sandstone were measured,and the mineral composition of sandstone treated at different temperatures was analyzed.It is found that the mass and density of sandstone decrease gradually with the increase of pretreatment temperature,while the volume of sandstone does not change significantly.The wave velocity increases slightly at first and then decreases gradually with the increase of pretreatment temperature.The main mineral composition of sandstone has not changed,but its clay mineral composition has changed,and kaolinite has gradually transformed into illite/smectite mixed bed.(2)Gas permeability tests of sandstone pretreated at different temperatures under fixed confining pressure were carried out.It is found that the variation trend of gas permeability with temperature is relatively complex.When the pretreatment temperature is between 20? and400?,the gas permeability increases with the increase of temperature,when the pretreatment temperature reaches 500?,the permeability of sandstone gas decreases,and when the temperature continues to rise,the permeability of gas further increases,and the variation range increases.The gas permeability of sandstone heated at 800? is 21 times that of unheated sandstone.(3)The permeability tests of sandstone treated at different temperatures under different confining pressures and gas injection pressures were carried out.It is found that the gas permeability of sandstone decreases exponentially with the confining pressure,and the temperature has a certain influence on the sensitivity of confining pressure and the hysteresis effect of confining pressure.The gas permeability of sandstone decreases exponentially with the increase of gas injection pressure,and the pressure sensitivity of sandstone will change after treatment with different temperatures.When the gas injection pressure is less than 2MPa,the influence of temperature on the pressure sensitivity of sandstone is obvious.The slippage effect appears in the gas permeability tests of sandstone treated with different temperatures,and different factors(temperature,confining pressure,etc.)have certain influences on the slippage effect.(4)The pore structure of sandstone treated with different temperatures was analyzed by mercury intrusion porosimetry and scanning electron microscope.It is found that the influence of temperature on the pore structure of sandstone is complex,which is mainly caused by thermal cracking,thermal expansion and spalling of mineral particles.The analysis results of porosity and pore size distribution show that the porosity increases exponentially with temperature,and the small-size pores develop into large-size pores.Based on CPSD algorithm,the permeability prediction of sandstone digital image is studied,and it is found that the higher multiples of digital image can be more accurate for permeability prediction,and the predicted value of permeability gradually increases with the increase of pre-processing temperature.There are 54 figures,20 tables and 129 references in the thesis.