Evolution Characteristics of Pore Fissure Structure of Lignite under Cyclic Moisture Imbibition and Wetting

In order to determine the evolution characteristics of the pore fissure structure (PFS) caused by cyclic moisture imbibition and wetting (CMIW), a CMIW experiment of lignite was designed to simulate the water-coal interaction process in practical engineering. The T₂ spectrum variation curves and MRI images were obtained by means of low-field nuclear magnetic resonance (NMR) testing. The evolution characteristics of the PFS of the coal under CMIW were studied by X-ray diffraction (XRD). The results show that the CMIW gradually erodes and discharges a large amount of kaolinite, calcite, and quartz in the coal body, with their relative contents decreasing by 93.69%, 92.20%, and 54.32%, respectively. This leads to a reduction in the number of small and large pores and an increase in the number of medium pores in the coal. The connectivity between small and medium pores shows a trend of deterioration first and then optimization, while the connectivity between medium and large pores continues to improve. The improvement of seepage channels has significantly increased the permeability of coal samples, with increases of 1665% and 381% for coal samples of 0° and 45°. MRI imaging shows that the moisture distribution gradually develops from enrichment at the bottom of the coal sample to a uniform distribution. The moisture content of the coal sample showed an increasing trend. The 0° coal sample shows the largest increase (18.89%) and tends to reach saturation after the third imbibition-wetting cycle. With the increase of repeated wetting times, the fractal dimension of the seepage pores continuously decreases and the anisotropy of water distribution weakens. The research results provide new technical ideas for improving the dust suppression effect of coal seam water injection from the perspective of optimizing the PFS and the uniform distribution of moisture.