Quantitative Characterization of Mineral Properties and Microstructures in Deep Coal-Rock

Abstract In this study, a comprehensive and practical experimental method was employed to quantitatively characterize mineral properties and microstructure characteristics of deep coal-rock (DCR). Experimentally, core samples were collected from two study areas in a DCR reservoir, and the petrological and mineral characteristics of the DCR were determined by performing proximate analysis, vitrinite reflectance, maceral components, scanning electron microscopy (SEM) and X-ray diffraction (XRD) experiments. Additionally, other tests such as physisorption and high-pressure mercury injection (HPMI) were conducted to quantitatively characterize the nano- to micro-microstructures. The microscopic components of the DCR are primarily composed of vitrinite, followed by inertinite, whereas liptinite and mineral matter occur in comparatively lesser quantities. The storage space within the DCR primarily consists of cellular pores and fractures, with the majority of these pores being occupied by clay minerals, and the crystalline materials present in the DCR contribute to an increase in the specific surface area of the coal rocks to a certain extent. Although the difference in total pore volume (PV) between the micropores and mesopores of Samples Q and J is relatively minor, such a modest increase in total PV contributes to a substantial rise in specific surface area (SSA), leading to a pronounced discrepancy in SSA between these two types of samples. Despite the presence of microcracks in Sample Q, micropores remain the predominant pore type, contributing over 99% to the SSA, thereby serving as the primary space for adsorbed gas. However, macropores and microcracks significantly contribute to the total PV, providing essential flow channels for desorption, diffusion, and seepage of adsorbed gas. Such quantitatively characterized mineral properties and microstructures of DCR will provide valuable insights for assessing the presence of free gas and adsorbed gas.