Enhanced permeability mechanism in coal seams through liquid nitrogen immersion: multi-scale pore structure analysis

Abstract

The geological structure of coal seams in China is remarkably varied and complex, with coalbed methane reservoirs marked by significant heterogeneity and low permeability, creating substantial technical challenges for efficient extraction. This study systematically investigates the impact of Liquid Nitrogen Immersion (LNI) on the coal’s pore structure and its mechanism of enhancing permeability with a combination of quantitative Nuclear Magnetic Resonance (NMR) analysis, nitrogen adsorption experiments, and fractal dimension calculations. The results demonstrate that LNI can damage the coal’s pore structure and promote fracture expansion through thermal stress induction and moisture phase transformation, thereby enhancing the permeability of coal seams. The T₂ peak area in the NMR experiments on coal samples subjected to LNI treatment increases by an average of 15%, the BET specific surface area decreases to 6.02 m²/g, and the BJH total pore volume increases to 14.99 mm³/g. Furthermore, changes in fractal dimensions (D₁ rising from 2.804 to 2.837, and D₂ falling from 2.757 to 2.594) indicate a notable enhancement in the complexity of the pore structure. With increasing LNI cycles, the adsorption capacity of the coal samples diminishes, suggesting a significant optimization of the pore structure. This optimization is particularly evident in the reconstruction of the micropore structure, which in turn greatly enhances the complexity and connectivity of the sample’s pore network. In summary, the study concludes that LNI technology can effectively improve the permeability of coal seams and the extraction efficiency of coalbed methane by optimizing the micropore structure and enhancing pore connectivity, which offers a potential method for enhancing the permeability of gas-bearing coal seams and facilitating the development and utilization of coalbed methane.

Keywords

liquid nitrogen immersion / coal seam pore structure / permeability / NMR / fractal dimension

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