To improve the efficiency of coalbed methane and recoverability of reservoirs, enhanced fracturing technology is usually required to improve the low porosity and permeability status of coal reservoirs. As a feasible method for strengthening permeability, microwave–LN₂ freeze–thaw (MLFT) cycles modify the microscopic pore structure of coal through the coupled effect of temperature stress changes, phase change expansion, and fatigue damage. ¹H nuclear magnetic resonance combined with fractal dimension theory was used to characterize quantitatively the pore system and geometric features of coal. The geometric fractal model constructed using the T₂ spectrum indicates that the fractal dimensions D_p and D_e have high fitting accuracy, demonstrating that percolation and effective pores exhibit good fractal characteristics. D_p and D_e are correlated negatively and positively, respectively, with the cyclic parameters. The relevance analysis shows that the NMR fractal method can reflect the pore–fracture heterogeneity of coal, which has a significant effect on the percentage of fluid migration space. This study reveals that MLFT cycles have significant enhancement effects on promoting the extension of multi-type pores structures within the coal matrix, as well as the connectivity and permeability of cracks.

为了提高煤层气开发效率和储层可采性，通常需要采用强化压裂技术来改善煤储层的低孔低渗状况。作为一种可行的强化渗透方法，微波-LN ₂冻融（MLFT）循环通过温度应力变化、相变膨胀和疲劳损伤的耦合效应改变煤的微观孔隙结构。利用¹ H核磁共振结合分形维数理论定量表征煤的孔隙系统和几何特征。利用T _{2谱构建的}几何分形模型表明，分形维数D _p和De具有较高的拟合精度，表明渗流和有效孔隙表现出良好的分形特征。D _p和De_分别与循环参数负相关和正相关。相关性分析表明，核磁共振分形方法可以反映煤的孔隙—裂隙非均质性，对流体运移空间百分比有显着影响。研究表明，MLFT循环对于促进煤基质内多种类型孔隙结构的扩展以及裂缝的连通性和渗透性具有显着的增强作用。