Coal Pores: Methods, Types, and Characteristics

Abstract

Coal pores are the locations of coalbed methane occurrence and enrichment and the main storage space targeted in CO2 sequestration. The systematic investigation of the coal pore structure and its development is of great significance to provide insights into coalbed methane generation, coal mine gas outburst mechanisms, and coal seam CO2 sequestration. In this study, coal pore testing technologies and methods, pore classification methods, and coal pore structure characteristics and their main control factors were systematically investigated and summarized. The results show that direct test methods, indirect fluid injection test methods, and X-ray and spectroscopic methods have been used for the quantitative characterization of the pore structure. However, each testing method has limitations. Therefore, a comprehensive method for the quantitative characterization of the full-scale pore structure must be developed, especially for the accurate quantitative characterization of closed pores in coal. The in situ measurement of pores in coal is one of the future research trends. Classification methods of pores in coal mainly include the classification of the genetic pore type, pore size, and pore morphology. Metamorphism, tectonic deformation, and macerals are the main internal factors affecting the pore distribution in coal. The effect of tectonic deformation on the macromolecular structure and micro- and ultramicropores of coal must be further studied. In addition, molecular mechanics simulations, molecular dynamics simulations, and quantum chemistry calculations of the dynamic response of the macromolecular structure and micro- and ultramicropores during gas adsorption and desorption must be carried out.

The Influence Mechanism of Pore Structure of Tectonically Deformed Coal on the Adsorption and Desorption Hysteresis

“…At present, a series of approaches to sort the pore structures in the coal reservoir have been put forward based on the physical adsorption property and capillary condensation theory (Song et al, 2020b;Mou et al, 2021). To better understand the effects of pore structure on gas adsorption capacity and flow capability, this work employs a combined classification for coal pore size from the International Union of Pure and Applied Chemistry (IUPAC) (Thommes et al, 2015;Wang et al, 2021a) and Hodot (Li et al, 2015;Li et al, 2020): supermicropores (<2 nm), micropores (2-10 nm), mesopores (10-50 nm), and macropores (50-100 nm).…”

“…According to the IUPAC classification standard, the hysteresis loop in the gas adsorption isotherm can be divided into six categories (H1-H5) (Thommes et al, 2015). The nano-scale PS in coal can be predicted by the hysteresis loop pattern in the gas adsorption isotherm, which is also a widely used practice at present (Li et al, 2019b;Song et al, 2020b;Mou et al, 2021). As shown in Figure 1, after the adsorption process ends, desorption begins and the adsorption loop of each coal sample produces a certain "hysteresis loop".…”

“…Pore volume (PV) is mainly contributed by macropores and mesopores. Specific surface area (SSA) is mainly contributed by micropores, where mylonitized coal has the biggest PV and SSA at different pore size stages (Meng et al, 2015;Li et al, 2019a;Mou et al, 2021). In the brittle deformation stage, structural deformation causes the complex micropore structure of coal and poor pore connectivity, and a simple mesopore structure is due to the loss of functional groups and the shrinkage of some mesopore structures under stress.…”

The Characteristics of Closed Pores in Coals With Different Ranks

“…Since it is difficult for probes to contact closed pores directly, direct detection of closed pores in coal is not easy (Mou et al, 2021). There are many techniques for investigating the pore structure of coal.…”

“…However, these injecting fluids methods can only detect the open pores, which are difficult to detect closed pores in coal. (Mou et al, 2021). Other techniques used to characterize microscopic morphology mainly include scanning electron microscopy (SEM) (Li et al, 2019), transmission electron microscopy (TEM) (Zhao et al, 2014), high-resolution transmission electron microscopy (HRTEM) (Wang et al, 2021) and micro-computed tomography (CT) (Li et al, 2017).…”