Supercritical CO₂ (Sc-CO₂), a supercritical solvent, can extract small organic molecules (fluid) from coal, changing pore structures to affect gases storage and migration in the coal matrix. Five undeformed coals before and after the second coalification jump were collected to simulate Sc-CO₂ extraction performed with supercritical extraction equipment. Pore structures of the samples before and after Sc-CO₂ extraction were characterized using mercury porosimetry. The results show that there are significant changes in pore size distribution of samples. ΔV_Ma and ΔV_Me of coal samples are positive, ΔV_Tr and ΔV_Mi are positive for most coals, and ΔV_Mi of higher coals are negative; the ΔS_Ma and ΔS_Me are positive with small values, the ΔS_Tr and ΔS_Mi are positive and negative before and after the second coalification jump; thus, the pore connectivity is improved. These results indicate that Sc-CO₂ extraction not only increases the numbers of micropores, but also enlarges the pore diameter size; these changes in the pore structure are influenced by the second coalification. The changes in the pore structure by Sc-CO₂ extraction provide more spaces for gas storage and may improve the pore throats for gas migration.

Biogenic coalbed methane (CBM) is an important alternative energy that can help achieve carbon neutrality. Accordingly, its exploration and development have become a research hotspot in the field of fossil energy. In this review, the latest detection technologies for and experimental research on biogenic CBM in China in recent decades are summarized. The factors influencing the generation of biogenic CBM and the identification method of biogenic CBM are systematically analyzed. The technologies to detect biogas and the research methods to study microbial diversity are summarized. The literature shows that biogenic CBM is easily produced in the presence of highly abundant organic matter of low maturity, and the organic matter reaching a certain thickness can compensate for the limitation of biogenic CBM gas production due to the small abundance of organic matter to a certain extent. Biogenic CBM production could be increased in an environment with low salinity, medium alkalinity, and rich Fe²⁺ and Ni²⁺ sources. Furthermore, biogenic CBM can be identified by considering three aspects: (1) the presence of gas composition indicators; (2) the content of heavy hydrocarbon; and (3) variation in the abundance of biomarkers. In recent years, research methods to study the microbial community and diversity of CBM-producing environments in China have mainly included 16S rRNA gene library, fluorescence in situ hybridization, and high-throughput sequencing, and the dominant microorganisms have been determined in various basins in China. The results of numerous studies show that the dominant bacterial phyla are commonly Firmicutes and Proteobacteria, while the archaeal fraction mainly includes Methanoculleus, Methanobacterium, Methanocorpusculum, and Methanothrix. This review summarizes and discusses the advances in biogenic CBM production and the associated microbial community in order to promote further development of coal biotransformation and CO₂ bio-utilization to meet energy demands under carbon neutrality.

Knowledge of the interactions among different microorganisms is important to understand how ecological function transformation is affected by elevated CO₂ levels in CO₂-enhanced oil recovery (CO₂-EOR) sites. Molecular ecological networks were established to reveal the interactions among different microbes of the soil bacterial community with the high-throughput sequencing data of 16S rRNA genes. The results showed that these networks are a powerful tool to identify and explain the interactions and keystone species in the communities under elevated CO₂ pressure. The structures of networks under different CO₂ leakage concentrations were different as a result of the networks’ topology properties, such as node numbers, topological roles of individual nodes, and network hubs. These indicators imply that the interactions among different groups were obviously changed. Moreover, changes in the network structure were significantly correlated with soil pH value, which might suggest that the large CO₂ leakage affected the soil ecosystem functions by changing the network interactions. Additionally, the key microbial populations such as Bacteroidetes and Proteobacteria were distinguished based on network topology to reveal community structure and ecosystem functioning. The work developed in this study could help microbiologists to address some research questions that could not be approached previously, and, hence, might represent a new area of research for microbial ecology.

The pore structure and gas adsorption are two key issues that affect the coal bed methane recovery process significantly. To change pore structure and gas adsorption, 5 coals with different ranks were treated by CS₂ for 3 h using a Soxhlet extractor under ultrasonic oscillation conditions; the evolutions of pore structure and methane adsorption were examined using a high-pressure mercury intrusion porosimeter (MIP) with an AutoPore IV 9310 series mercury instrument. The results show that the cumulative pore volume and specific surface area (SSA) were increased after CS₂ treatment, and the incremental micropore volume and SSA were increased and decreased before and after Ro,max=1.3%, respectively; the incremental big pore (greater than 10 nm in diameter) volumes were increased and SSA was decreased for all coals, and pore connectivity was improved. Methane adsorption capacity on coal before and after Ro,max=1.3% also was increased and decreased, respectively. There is a positive correlation between the changes in the micropore SSA and the Langmuir volume. It confirms that the changes in pore structure and methane adsorption capacity due to CS₂ treatment are controlled by the rank, and the change in methane adsorption is impacted by the change of micropore SSA and suggests that the changes in pore structure are better for gas migration; the alteration in methane adsorption capacity is worse and better for methane recovery before and after Ro,max=1.3%. A conceptual mechanism of pore structure is proposed to explain methane adsorption capacity on CS₂ treated coal around the Ro,max=1.3%.

The dissolution of supercritical carbon dioxide (ScCO₂) in water forms a ScCO₂–H₂O system, which exerts a transformative influence on the physicochemical characteristics of coal and significantly impacts the CO₂-driven enhanced coalbed methane (CO₂-ECBM) recovery process. Herein, the effect of ScCO₂–H₂O treatment on the physicochemical properties of coal was simulated in a high-pressure reactor. The migration of major elements, change in the pore structure, and change in the CH₄ adsorption capacity of coal after the ScCO₂–H₂O treatment were detected using plasma emission spectroscopy, the low-temperature liquid nitrogen adsorption method, and the CH₄ adsorption method, respectively. The results show that (1) the ScCO₂–H₂O treatment led to mineral reactions causing a significant migration of constant elements in the coal. The migration of Ca ions was the most significant, with an increase in their concentration in treated water from 0 to 16–970 mg·L⁻¹, followed by Na, Mg, and K. Al migrated the least, from 0 to 0.004–2.555 mg·L⁻¹. (2) The ScCO₂–H₂O treatment increased the pore volume and pore-specific surface area (SSA) of the coal via the dissolution and precipitation of minerals in the coal pores. The total pore volume increased from 0.000795–0.011543 to 0.001274–0.014644 cm³·g⁻¹, and the total pore SSA increased from 0.084–3.332 to 0.400–6.061 m²·g⁻¹. (3) Changes in the CH₄ adsorption capacity were affected by the combined effects of a mineral reaction and pore structure change. The dissolved precipitates of the minerals in the coal pores after the ScCO₂–H₂O treatment caused elemental migration, which not only decreased the mineral content in the coal pores but also increased the total pore volume and total pore SSA, thus improving the CH₄ adsorption capacity of the coal. This study provides theoretical support for CO₂ sequestration and ECBM recovery.