Nuclear magnetic resonance study of changes in unfrozen water and pore characteristics in frozen coal

QIN Lei; WANG Hui; LI Shugang; LIU Pengfei; LI Jiawei

doi:10.16511/j.cnki.qhdxxb.2025.26.005

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Journal of Tsinghua University(Science and Technology) ›› 2025, Vol. 65 ›› Issue (3) : 601-613. DOI: 10.16511/j.cnki.qhdxxb.2025.26.005

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Nuclear magnetic resonance study of changes in unfrozen water and pore characteristics in frozen coal

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Abstract

[Objective] Coalbed methane (CBM) outbursts pose a threat to coal mining safety, particularly in China, where many coal seams exhibit low permeability, making CBM extraction inefficient. Liquid nitrogen fracturing technology is an important anhydrous method for enhancing coal seam permeability. The phase transition of coal pore water under low temperature conditions profoundly affects the coal pore structure. Studying the pore-water phase transition and pore changes during low-temperature liquid nitrogen freeze-thaw cycles is crucial for evaluating the effectiveness of this technology and improving CBM extraction efficiency. [Methods] This experiment involves four groups of water-saturated bituminous coal samples, each subjected to different numbers of liquid nitrogen freeze-thaw cycles. The transverse relaxation time T₂ curve, cumulative porosity, and cumulative pore throat distribution of coal samples in the initial, frozen and thawed states were evaluated by using low-field nuclear magnetic resonance (NMR). [Results] These findings showed the following: (1) Even at -196 ℃, trace amounts of unfrozen water remained in the coal pores. As the temperature increased, the ice in the smaller pores first melted, producing micropore unfrozen water. Smaller pore diameters corresponded to lower pore melting pints of pore ice. (2) As the pore ice gradually melted, the cumulative porosity increased in three stages: exponential function type rapid growth, primary function type uniform growth, and quadratic function type growth. (3) By integrating the T₂ curve, it is observed that the unfrozen water content changed exponentially with increasing temperature in the negative temperature intervals. Closer to 0 ℃, the unfrozen water content growth rate increased. At temperatures between -196 ℃ and -34 ℃, micropore unfrozen water constituted nearly 100%, whereas temperatures between -20 ℃ and 0 ℃ saw mesopore and macropore unfrozen water dominating. (4) NMR tests on raw and fully melted coal samples revealed that the permeability of coal samples increased with the number of freeze-thaw cycles. The effective porosity and permeability growth rates increased from 20.68% to 24.15% and 109.73% to 122.20% respectively, as the number of cycles increased from 5 to 30. However, the marginal utility of permeability enhancement of coal samples became increasingly pronounced. [Conclusions] Liquid nitrogen cyclic freezing of coal effectively increases the effective porosity and permeability of water-saturated coal. Liquid nitrogen fracturing can be used as an anhydrous fracturing technology to increase the coal seam permeability in water-scarce coal mining areas, thus improving the CBM extraction efficiency. The results of this study can provide valuable insights into the evolution of seepage pore structure during low-temperature medium cyclic freezing and thawing of coal and guide field operations related to liquid nitrogen fracturing and permeability enhancement.

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liquid nitrogen fracturing / unfrozen water / pore structure / low-field nuclear magnetic resonance

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QIN Lei, WANG Hui, LI Shugang, LIU Pengfei, LI Jiawei. Nuclear magnetic resonance study of changes in unfrozen water and pore characteristics in frozen coal[J]. Journal of Tsinghua University(Science and Technology). 2025, 65(3): 601-613 https://doi.org/10.16511/j.cnki.qhdxxb.2025.26.005

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