| Citation: |
QIN Lei，ZHANG Xian，LIN Haifei，et al. Distribution of unfrozen water and evolution of temperature-stress field during freeze-thaw of liquid nitrogen in heterogeneous coal[J]. Coal Science and Technology，2025，53（S1）：76−86. DOI: 10.12438/cst.2023-2021
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The content and distribution of unfrozen water in the pores of coal during freeze-thaw directly affect the microscopic pore structure and macroscopic mechanical properties of coal, and determine the anti-reflection effect of coal seam and the extraction efficiency of coalbed methane. Taking anthracite as the research object, and studies the characteristics of unfrozen water in the freezing and melting process of coal samples based on two-dimensional nuclear magnetic resonance technology. By comprehensively measuring T1−T2 spectrum, T2 curve and 3D peak map, the pore structure and spatial evolution law of unfrozen water distribution in the freezing and thawing process of coal samples are quantitatively analyzed. The experimental results show that the freezing time has different effects on the spatial evolution of different fluid distribution in the pores during the freezing process of liquid nitrogen. The distribution space of free water decreases exponentially with freezing time, and is affected by freezing time, followed by bound water. The distribution space of structural water and adsorbed water is almost not affected by the freezing time, and its distribution space only depends on the pore structure characteristics of the coal sample. According to the spatial distribution of different fluids, it is divided into two stages: accelerated freezing stage I (0～20 s) and stable stage II (20～60 s). The content of unfrozen water in pores is affected by temperature and thermal stress during the melting of frozen water-saturated coal samples. The melting of coal samples starts from the small holes until the temperature rises to a certain extent, and the large holes begin to melt. According to the above analysis, the generation and evolution model of thermal stress caused by the change of temperature field in fracture is established. The pore expansion and contraction analysis system and pore fluid analysis system based on liquid nitrogen frozen coal sample during melting process are summarized, involving T2 atlas analysis and 3D peak map calculation, and then the change rule of unfrozen water content during melting process is analyzed. It is calculated that the pore structure is gradually complicated during the melting process of coal sample, and the pore water content increases as a function of temperature. The free water content increased from 24% to 44%, the bound water content increased by 11%, the adsorbed water and structural water content increased by 6%, and the pore water content was determined by the pore size and the total pore volume.
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