# Analysis and Characterization of Micro-Nano Pores in Coal Reservoirs of Different Coal Ranks
## 1 Introduction  
Coal, as a complex porous medium, exhibits a dual pore system consisting of both pores and fractures. The distribution of pore sizes ranges from millimeters to nanometers [1; 2], spanning multiple scales, directly impacting the adsorption and fluidity of coalbed methane [3]. The complex pore networks within coal play a crucial role in controlling the occurrence and migration of coalbed methane, with factors such as development degree, porosity, pore number, distribution, and connectivity influencing both abnormal methane enrichment and mine safety [4; 5]. Furthermore, studying the coal pore structure is of great significance for understanding the occurrence and migration of coalbed methane and optimizing its exploitation. The genetic diversity and wide size distribution of coal pores contribute to its intricate pore network, which encompasses various types of pores interconnected through micro-scale cracks [6]. These characteristics serve as direct indicators of the gas generation, storage capacity, and permeability of coal reservoirs. Advanced technologies, such as mercury intrusion porosimetry (MIP), low-pressure gas adsorption, and various imaging techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), micro-CT, and small-angle X-ray scattering (SAXS), among others, have been increasingly employed for coal pore structure analysis [7; 8]. While these methods offer insights into the complex pore structure, each excels within specific measurement ranges. For instance, [PERSON] et al. [9] utilized MATLAB algorithms to analyze SEM images of coal samples, extracting parameters such as porosity, pore radius, pore throat radius, pore coordination number, pore throat ratio, and specific surface area. To determine the accuracy of the analysis results, constant-rate mercury injection and low-temperature N\({}_{2}\) adsorption experiments were conducted. To study the pore structure characteristics of the medium- and high-rank coal in the Qinshui Basin, [PERSON] et al. [10] measured the specific surface area and pore size distribution data of different coal samples using liquid nitrogen adsorption. [PERSON] et al. [11] conducted a study on ten groups of coal samples sourced from different regions, focusing on saturated water, centrifugation, and a low-field NMR system. They conducted low-field NMR tests on coal in different states to investigate the pore structure characteristics across various coal ranks. Furthermore, while NMR, gas adsorption, and mercury intrusion experiments offer wide measurement ranges and high precision, they are limited by the test principles, each targeting different pore size ranges. For instance, low-temperature N\({}_{2}\) experiments are suitable for studying pore sizes ranging from 2 nm to 50 nm, whereas low-temperature CO\({}_{2}\) experiments are more adept at analyzing pore sizes of \(\sim\)2 nm. Therefore, these methods may not fully reflect the three-dimensional (3D) pore structure characteristics of coal [12]. The nano-scale pores within coal serve as the primary adsorption space for gas, forming a complex pore network that complicates the gas migration channels [13; 14]. Recently, computed tomography (CT) has emerged as a non-destructive technique capable of achieving micro- and nanometer resolutions, enabling the visualization of high-resolution 3D pore structures without sample damage. This technology enables the intuitive study of the pore throat morphology in unconventional reservoirs [15]. Digital core models and constructed CT scanning offer the direct and accurate quantitative characterization of microscopic pore structures and seepage simulations. These models can be tailored to different pore structures and types, facilitating detailed investigation into microscopic pore structures and seepage mechanisms [16; 17].