The structure and evolution of coal are intricately linked to its properties at both nanometer and micrometer scales. The refinement of pores and fractures is crucial for assessing outburst risks, evaluating coalbed methane (CBM) reservoirs, and improving the CBM recovery efficiency. This study involved collecting four different coal-body structure samples from the Zhaozhuang colliery in the southern Qinshui Basin. We conducted laboratory analyses to determine the physical properties of the coal and employed X-ray computed tomography (CT) to quantitatively assess the pore size distribution (PSD), volume contribution, morphology, and connectivity across various coal structures. The evolution and comparative characteristics of these coal structures under different tectonic stresses is discussed. Results reveal that the hysteresis loop type of 3# anthracite primarily aligns with the H3 type as per IUPAC classification, featuring predominantly plate-like and wedge-shaped pores at the nanometer scale. A shape factor was introduced to quantitatively categorize the pore types, highlighting its sensitivity to the coal structure. Spherical and tubular pores are mainly present in the aperture range of 0-25 μm, while larger apertures appeared as prolate spheroids and flat fractures. Primary coal contains more spherical or tubular pores, whereas tectonic coal shows a prevalence of slit pores and flat fractures, suggesting that pores and fractures undergo progressive deformation, either breaking or elongating, under tectonic stress. The elastic property is affected by the multifactor of the increased pore volume and changes in pore morphology. Permeability is influenced by PSD and connectivity, demonstrating a quadratic positive correlation with porosity, aperture, and specific surface area. Granular coal exhibited the most favorable permeability characteristics for CBM extraction. The distribution trend of morphology and comprehensive data highlight an evolutionary pattern of different coal structures. The evolution of the coal structure is mainly shaped by brittle and ductile deformation mechanisms. Cataclastic coal is characterized by an increase in new fractures, and granular coal undergoes rapid new fracture formation and enhanced connectivity under strong stress conditions. Mylonitic coal develops under ductile deformation mechanisms. These insights into the properties of various coal structures can significantly enhance our understanding of the CBM recovery efficiency from both microscopic and mesoscopic perspectives.

Well-logging-constrained impedance inversion is an effective process for predicting the thickness and bifurcation of coal seams. Wavelet changes in a complex region achieve the best match between the inverse and source wavelets, affecting the accuracy of the inversion solution and the ability to obtain accurate inverted acoustic impedance (AI) data. We have conducted the joint inversion of wavelet and AI data using iterative methods, which combined the conjugate-gradient (CG) method and particle-swarm-optimization (PSO) algorithm. The Marmousi AI model was used to prove the reliability of the method. The CG-PSO algorithm achieved excellent results compared with the statistical wavelet pickup method. The wavelet obtained by the CG-PSO algorithm is preferred for inversion operations. We applied a new method to invert field data and predict the thickness and bifurcation of coal seams in the karst region. The results find that the wavelet spectrum obtained by the CG-PSO matches the spectrum map of the coal seam in the Yuwang colliery. We determined the distribution of the thickness and bifurcation of the 101 panel, Yuwang Colliery, Yunnan Province. The average error of the predicted coal thickness is 0.17 m (14.4%), which verifies the feasibility and effectiveness of the method. The method provides insights into the AI inversion of constrained waves in complex regions.

The elastic characteristics of different coal structures (primary, cataclastic, granulated, and mylonitized) are important for coalbed problems such as methane exploration and mining. However, due to various factors influencing coal elastic parameters, there is still little understanding about them. In this study, we selected four samples with different coal structures and measured their elastic parameters at a seismic frequency band (4–300 Hz) under varying confining pressure (room pressure up to 20 MPa). The results show that coal elastic wave velocities are frequency-dependent under room-dry conditions. These results contrast with the previous understanding that rock dispersion phenomena are caused by fluids. The dispersion of elastic wave velocities decreases with increasing confining pressure and increases as the coal structure becomes more fragmented. Under room pressure, the P-wave velocity in primary coal and mylonitized coal exhibited 5.91% and 12.04% increases from 4 to 300 Hz, respectively. However, at 20 MPa, these increases were reduced to 2.78% and 4.75%. The fractal viscoelastic model, which is grounded in the staggered interlayer structure of coal matrix, effectively describes the dispersion characteristics of coal. This reveals that the fractal structure of the coal may be used to describe the coal viscoelastic characteristics. We found that frictional dissipation and the viscoelastic components, such as clay and organic matter, caused the dispersion. The difference in compliant pore content is the primary reason for the different dispersion in various coal structures in this study. This research may provide a new basis for studying the elastic wave dispersion of different coal structures.