Evolution of the Pore-Fracture System of Coal: A Case Study of Zhaozhuang Colliery, Qinshui Basin

Abstract

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.