| The high-temperature effect generated by underground coal oxidation(coal fire)can cause damage to surrounding rocks,resulting in changes in their physical and mechanical properties and microstructure.It may also lead to the evolution of poorly permeable formations into water-bearing layers,causing mining water hazards.This thesis aims to study the evolution law of permeability of natural weak permeable formations under the high-temperature effect of a coal fire.The original rock of the overlying silty sandstone in Zhangjiamao Mine is selected as the research object,and high-temperature experiments are conducted on it at different temperatures(25-1000℃).Multiple testing methods and means are used to test and analyze the physical parameters,microstructure,and permeability characteristics of the silty sandstone after different temperature treatments,focusing on the influence of high temperature on the permeability evolution of the silty sandstone and explaining the microstructure mechanism of permeability evolution.The main achievements of this thesis include:(1)Physical parameter tests are carried out on silty sandstone samples treated at different temperatures.It is found that the mass,density,wave velocity,and dielectric constant of the silty sandstone gradually decrease as the treatment temperature increases,while the volume and gamma radiation dose rate increase.Further study on the average difference in mass change rate between different samples under the same temperature conditions shows that the higher the pre-treatment temperature,the greater the average difference in mass change rate,indicating that the discreteness of mass change rate increases with increasing pre-treatment temperature.A study on the damage factor of wave velocity reveals that the average value of the damage factor at 600℃ is 0.6,indicating that high temperature has a significant impact on rock damage.(2)The microstructure of silty sandstone samples after being treated at different temperatures is studied by scanning electron microscopy,X-ray diffraction,nuclear magnetic resonance,CT scanning,and other means.It is found that before the treatment temperature reaches 400℃,the main changes in the silty sandstone are the volatilization of various components of water inside and the initiation of microcracks.At 400-500℃,kaolinite dehydrates and decomposes,leading to an increase in the large pore and connected porosity,and the total porosity also increases significantly.At 500-600℃,quartz phase transition causes expansion,resulting in some pores being squeezed,the number of large pores decreases,and the total porosity also decreases,with isolated pores increasing.At 600-1000℃,the total porosity begins to increase,and the proportion of large pores increases.Especially when the treatment temperature reaches1000℃,the proportion of large pores becomes extremely high,and isolated pores almost disappear,and the porosity increases significantly.Further study on the influence of microstructural changes on macro physical parameters reveals that when macro physical parameters change abruptly,there are often microscale phenomena such as microdamage,mineral decomposition,and mineral phase transition.Mass,density,volume,and wave velocity respond better to microstructural characteristics,while the sensitivity of dielectric constant and gamma radiation dose rate to microstructural changes is relatively poor.(3)Permeability tests are conducted on silty sandstone samples treated at different temperatures,and it is found that the permeability of silty sandstone varies differently in different temperature ranges.Kaolinite dehydrates and decomposes between 400-500℃,which results in increased porosity,thereby significantly increasing the permeability of silty sandstone.At 600℃,there is a significant decrease in permeability,mainly caused by quartz phase transformation.In addition,confining pressure has an impact on the permeability of silty sandstone.During the application of confining pressure,some pores are already compacted.Therefore,the higher the confining pressure,the smaller the permeability of silty sandstone.The changes in the permeability of silty sandstone during the entire stress-strain process can be divided into three stages: slow decline,slow rise,and rapid rise.Further analyzing the microscopic changes affecting permeability,it is found that porosity is the most closely related factor to permeability.In practical applications,the permeability of silty sandstone can be predicted by measuring its porosity. |
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