To investigate the strain behavior of the coal matrix during N2-enhanced coalbed methane (N2-ECBM) under different nitrogen injection pressures, saturated methane samples of four different coal ranks (Ro,ran = 0.41-3.23%) were selected. Nitrogen injection and sealing (N2-IS) was performed under pressures of 1.5-5.5 MPa, followed by desorption with stepwise depressurization (DSD). The induced strain characteristics, strain kinetics, and desorption response were analyzed based on volume strain, residual gas volume, and the stretched exponential equation (SE equation) fitting. The results indicate that the residual gas volume of the coal samples ranged from 3.01 to 15.55 cm3/g after the N2-IS phase and further decreased to 0.30-3.02 cm3/g after the DSD phase. A clear linear relationship (R2 >= 0.88) exists between the adsorption/desorption-induced swelling/ shrinkage strain of the coal matrix and the residual gas volume. The strain response of coal samples of different ranks is jointly governed by pore structure and elastic modulus; coal samples with well-developed micropores exhibited stronger swelling during the N2-IS phase and shrinkage during the DSD phase under higher nitrogen injection pressures. When the nitrogen injection pressure was three times the reservoir pressure, the cumulative volume shrinkage rates of all coal samples increased by more than 10%, including 29.96% for coal C; at 3.67 times the reservoir pressure, all samples increased by more than 15%. For reservoirs with well-developed micropores, high elastic modulus and poor permeability, higher nitrogen injection pressures are more effective in reducing residual gas volume, increasing the cumulative volume shrinkage rate and promoting methane desorption.

Three-way decision method has significant application value in solving interval-valued fuzzy multi-attribute decision-making problems. However, existing methods generally follow minimum risk principle, but neglect the profound impact of decision makers' (DMs) psychological behaviors on decision results. To address this limitation, this study innovatively integrates third-generation prospect theory with interval-valued fuzzy three-way decision method, aiming to handle data uncertainty and describe DMs' psychological behaviors. The proposed method has two main core elements. One is to calculate the dominance degree-based relative outcomes of objects, and further combine it with third generation prospect theory to obtain the relative values of objects. The other is to design an objective conditional probability based on dominance degree, which can accurately reflect the possibility degree of an object being close to the optimal state compared to other objects. Furthermore, we apply the proposed method to select the favorable areas for coalbed methane development in Fukang to verify its effectiveness and practicality. Additionally, we conduct comparative and experimental analyses to further demonstrate the superiority and reliability of our method.

The selection of fracturing fluids represents a pivotal aspect in the exploration of coalbed methane (CBM). The performance of fracturing fluids serves as the linchpin for determining the success of the fracturing process. This paper delves into the influence of fracturing fluids on deep coal seams in the southern Junggar Basin to optimize the CBM fracturing fluid system. Through coal powder expansion, surface tension, permeability damage, and friction resistance experiments, the potential damage mechanisms of coal were identified, and the optimal amounts of various additives were determined. The results showed that the optimal contents of the clay stabilizer, surfactant, and drag reducer were 1 wt%, 0.2 wt%, and 0.1 wt%, respectively. This combination is capable of efficiently minimizing the damage to the coal reservoir resulting from water sensitivity and water blocking. Moreover, the prepared drag reducing agent exhibited an excellent drag reduction effect during its injection into the coal seam. This fracturing fluid formula has well adaptability to coal reservoirs of different coal rank. Liquid-phase damage and solid-phase damage were the primary causes of reservoir damage. Water sensitivity, water blocking, fracturing fluid residues, alkali sensitivity, and stress were the key controlling factors; their contribution rates to coal reservoir damage were 23.2 %, 8.3 %, 52.5 %, 6.2 %, and 9.8 %, respectively. This research significantly contributed to the understanding of the damage mechanisms of coal reservoirs. It further enriches the optimization process of fracturing fluids. Moreover, it provides a solid theoretical foundation for the design of CBM fracturing operations.

To address the dual challenges of activated sludge (AS) disposal from water treatment and weathered coal (WC) valorization, this study innovatively proposed anaerobic co-digestion of AS and WC to enhanced methane production. Comparative experiments showed that a substrate ratio of 1:4 (WC:AS) achieved the highest cumulative methane yield, increasing it by 20.22 +/- 4.99 % (15.9 +/- 3.92 mL). WC biodegradation involved loss of oxygencontaining functional groups and partial cleavage of aromatic structures, with carbon and oxygen mainly present as C-C/C-H and C-O, and notable increases in C-O and C=O bonds. Furthermore, dissolved organic matter derived from substrates, including tryptophan-like substances and soluble microbial degradation products was effectively degraded and utilized. Microbial community analysis revealed increased diversity, showing a notable increase in the abundance of microorganisms with high degradative potential, such as unclassified_Bacteroidota, alongside acetoclastic methanogens such as Methanothrix. Methane was mainly produced via acetoclastic and hydrogenotrophic pathways, with co-digestion increasing the acetoclastic contribution to 54.84 % (13.47 % higher than WC, 31.73 % higher than 40AS). These findings provide a mechanistic basis for transforming water treatment residues and coal waste into clean energy, offering a promising approach for the clean and efficient valorization of waste resources.

Background Hydraulic fracturing serves as a stimulation technique extensively used in the commercial development of coalbed methane (CBM), in which guar gum is commonly used as a fracturing fluid thickener. However, under the low-temperature conditions of coal reservoirs, conventional chemical gel-breaking methods often suffer from incomplete gel breaking and high residue content, resulting in formation damage and a decrease in the production efficiency of CBM wells. Methods Using indigenous bacteria in coal seams as functional strains, this study conducted microbial gel-breaking experiments. It systematically investigated the characteristics of guar gum biodegradation by these bacteria and determined the dominant microbial taxa responsible for guar gum degradation. Results and Conclusions Indigenous bacteria in coal seams achieved the complete degradation of guar gum, meeting the gel-breaking requirement of fracturing fluids (viscosity≤5 mPa·s). Concurrently, they also reduced both the content and particle size of residues, effectively alleviating the potential formation damage induced by insoluble residues during the gel breaking of guar gum fracturing fluids. Guar gum was primarily hydrolyzed by indigenous bacteria into soluble polysaccharides, thereby reducing viscosity and achieving gel breaking. Analysis of microbial community structures revealed that Bacteroidota and Spirochaetota were the dominant functional phyla involved in guar gum degradation. Functional prediction using PICRUSt2 (Phylogenetic Investigation of Communities by Reconstruction of Unobserved States 2) indicated that the guar gum degradation was primarily attributed to the synergistic activities of α-galactosidase (EC 3.2.1.22), β-mannosidase (EC 3.2.1.25), and β-mannanase (EC 3.2.1.78). Among these, β-mannanase exhibited the most pronounced increase in gene abundance, suggesting its central role in gel-breaking of guar gum. Furthermore, environmental factors directly influenced gel-breaking efficiency of guar gum, with the highest degradation efficiency occurring at 45 ℃ and pH 6.0. Despite the inhibitory effect of high salinity on guar gum degradation, the indigenous bacteria retained gel-breaking capability even at a salinity of 40 g/L. This study elucidates the degradation mechanism of guar gum by indigenous bacteria in coal seams and identifies the impact patterns of environmental factors on microbial gel-breaking performance. These findings provide a theoretical basis for the application of indigenous bacteria-based biological gel-breaking technology in CBM extraction.

Accurate identification of the coal body structure (CBS) is critical for efficient coalbed methane (CBM) development. However, the highly imbalanced distribution and complex nonlinearity of well logging data severely hinder precise CBS identification. This study proposes a CBS recognition method for imbalanced logging data by integrating k-means clustering, adaptive synthetic sampling (ADASYN), and random forest (RF) algorithm. First, a hybrid resampling approach combining k-means clustering and ADASYN is applied to rebalance the CBS logging data, achieving effective class equilibrium. Then, a CBS recognition model is constructed using the RF algorithm, with interpretable artificial intelligence (SHapley Additive Explanations, SHAP) employed to identify the key influencing factors. Results show that the proposed k-means-ADASYN-RF model effectively alleviates class imbalance in CBS data while preserving the original data distribution. It achieves excellent identification performance, with precision, recall, and F1 score all above 0.90 on the test set and a macro-average F1 score of 0.95, thus outperforming the RF, ADASYN-RF, and XGBoost models. SHAP analysis indicates that Depth, SP, and CAL logs are the main features affecting CBS identification. This study provides technical support for accurate CBS identification and facilitates efficient CBM exploitation.

Integrated experimental characterization and molecular simulation systematically reveal the molecular structural evolution from low-rank lignite (DM) to medium-rank fat coal (KL) and its regulatory mechanism on CH4/ CO2 adsorption behaviors. Molecular structure models of coal macromolecules, rigorously validated by NMR spectra, were constructed based on industrial analysis, ultimate analysis, 13CNMR , and XPS data. Molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations were employed to elucidate aggregated structure characteristics and gas adsorption mechanisms. Results demonstrate that DM primarily contains single-benzene or small-naphthalene rings with abundant long aliphatic chains and oxygen-containing functional groups (e.g., carboxyl and hydroxyl), forming a loose porous structure dominated by weak van der Waals interactions. In contrast, KL evolves into naphthalene-anthracene condensed aromatic skeletons, exhibiting significantly enhanced aromaticity, shortened aliphatic chains, and removed oxygen-containing groups. This transformation yields compact aromatic stacking, where it-it interactions dominate the cohesive packing, while residual electrostatic interactions primarily arise from polar functional groups and contribute to gas molecule binding. KL exhibits a substantially higher adsorption capacities for both CH4 and CO2 than DM, with pronounced selectivity toward CO2. This selectivity arises primarily from strong interactions between the CO2 quadrupole moment and aromatic it-electron clouds alongside residual polar functional groups. Elevated temperatures reduce the adsorption capacity of both coals, with DM showing a more significant reduction owing to its weaker adsorption strength. This study elucidates how increasing coal rank enhances gas adsorption efficacy through intensified aromatic condensation and pore densification, providing molecular-level theoretical support for coalbed methane exploitation and CO2 geological sequestration.

Anaerobic digestion represents a promising technology for the resource utilization of coal slime (CS). To enhance biomethane conversion efficiency, anaerobic co-digestion experiments of CS and sawdust were conducted. Methane production kinetics were analyzed to assess the promoting effect of co-digestion. Substrate physicochemical transformations were characterized via FTIR and SEM, while metagenomic sequencing coupled with functional annotation using the CAZy and KEGG databases were performed to compare microbial community structure and the expression of key metabolic enzyme genes across different substrate systems. Results showed that adding 10 g sawdust to three types of CS significantly increased cumulative methane yield by 125.44%, 101.39%, and 58.00%, respectively. Methane production lag phases were further shortened, with degradation of lipid structures and hydroxyl groups in CS observed. Microbial communities were optimized, with enrichment of hydrolytically active genera such as Mobilitalea and Sphaerochaeta. Genes encoding hydrolytic enzymes including GHs, GTs, and CBMs, as well as key enzymes involved in cellulose degradation and acetate synthesis, exhibited increased abundance. The relative contribution of acetoclastic methanogenesis was also enhanced. These findings provide theoretical support and practical insight into improving the bioconversion efficiency of CS and sawdust, offering a sustainable pathway for the valorization of solid wastes.

Surrounding rocks are integral components of coal-bearing strata; however, their geochemical effects on biogenic methane formation in coupled coal-rock systems remain poorly understood. In this study, anaerobic digestion simulations combined with chemical, metabolic, and microbial analyses were conducted to elucidate how lithologically distinct coal-associated rocks regulate coal-to-methane bioconversion. The addition of mudstone and sandy mudstone (at mass ratios of 1:1-3:1) enhanced methane yields by up to 37.2% and 104.1%, respectively, compared with those of the corresponding coal-only systems. These coal-rock assemblages exhibited improved pH buffering capacity, higher soluble chemical oxygen demand (SCOD) removal, and greater accumulation of methanogenic precursors (e.g., fatty acids and amines), indicating enhanced substrate biodegradability under coupled conditions. Metagenomic analyses showed enrichment of dominant acidogenic bacteria (Aminobacterium and Proteiniclasticum) and the acetoclastic methanogen Methanothrix, together with increased relative abundances of genes involved in hydrolysis and acidogenesis. These coordinated geochemical and microbial responses were strongly associated with elevated metabolic activity and methane production in the coal-lithology systems. Overall, this work provides experimental evidence for the coupled geochemical-biological mechanisms governing coal bioconversion and offers practical implications for biologically enhanced coalbed methane (BECBM) production.

To investigate the dynamic mechanical behavior of gas-containing coal in deep coal mines, dynamic compression tests were conducted on gas-containing coal using a 50 mm diameter split Hopkinson pressure bar (SHPB) system. The central composite design (CCD) method within response surface methodology (RSM) was employed to explore the effects of impact velocity and gas pressure on the mechanical and energy response characteristics of gas-containing coal. The main findings are as follows: (1) The dynamic compressive strength increases with increasing impact velocity, and the rate of strength attenuation slows down when the gas pressure exceeds 1.2 MPa. The proportion of Stage II in the stress–strain curve is greater than 50%, which dominates the energy evolution process. (2) The energy distribution is jointly governed by impact velocity and gas pressure, which is determined by pore expansion and gas adsorption/desorption. The energy evolution of gas-containing coal can be divided into three stages. As impact velocity increases, the proportion of reflection energy decreases while the proportions of transmission and dissipation energies increase; the opposite trend is observed with increasing gas pressure. (3) The energy consumption density increases significantly with increasing impact velocity, and shows a trend of first increasing and then decreasing with increasing gas pressure. Increasing impact velocity accelerates coal fracture propagation and intensifies inter-particle collisions, leading to an increase in energy consumption density by up to 48%. Increasing gas pressure loosens the coal structure and enhances energy dissipation during transfer, resulting in a decrease in energy consumption density by more than 25%. (4) Based on the RSM-CCD experiments, a two-factor, five-level regression response model with R2 ≥ 0.93 was constructed. The interaction term AB significantly affects both dynamic compressive strength and energy consumption density, while the quadratic term A2 of the single factor has a weak effect on compressive strength. The order of influence on compressive strength is A > B2 > B > AB > A2, and that on energy consumption density is A > A2 > B2 > B > AB. (5) Combining the response surface methodology, the effective stress criterion, and the Drucker–Prager criterion, a statistical damage model for the strength of gas-containing coal was constructed and modified, with a model correlation coefficient R2 ≥ 0.92. The research results can provide a theoretical basis for the prevention and control of dynamic disasters in gas-bearing coal seams in deep coal mines.

Understanding the diffusion behavior of methane under high-temperature and high-stress conditions is important for the exploitation of deep coalbed methane (CBM). In this study, the influencing factors and variation patterns of the methane diffusion coefficient under simulated in situ formation conditions and conventional conditions were investigated using five columnar coal samples (depths: 2744-3017 m) collected from the Yulin area of the Ordos Basin. The results demonstrate that the diffusion coefficient decreases exponentially by up to 78.3% with increasing confining pressure (20.0-50.0 MPa) but increases exponentially by up to 222.6% with increasing gas pressure (15.0-30.0 MPa) and by up to 59% with increasing temperature (45-75 degrees C). The diffusion coefficient parallel to the bedding planes is significantly greater than that perpendicular to the bedding planes by a factor of 25-42 (average of 35). The initial diffusion coefficient fitted by the time-dependent diffusion model under conventional conditions is three orders of magnitude greater than that under in situ conditions. Furthermore, an increase in methane gas pressure leads to a gradual increase in the initial diffusion coefficient, accompanied by an increase in the decay parameter. A higher proportion of macropores within the coal samples correlates with a larger initial diffusion coefficient. These findings contribute to a more comprehensive theoretical framework for the development of deep CBM resources.

To achieve the synergistic application of coal seam permeability enhancement and coalbed gas bioengineering, this study employed an orthogonal design to optimize a guar gum-based fracturing fluid. The effects of the optimized fluid on methane production and coal matrix modification during anaerobic digestion were systematically evaluated. Results showed that methane yield by 19.11 % and the lag phase shortened to 2.40 days. Furthermore, it exhibited improved microbial degradability, meeting viscosity standards for effective gel breaking, while reducing coal wettability and minimizing core damage. Microbial action further degraded the coal structure. Dissolved organic matter during anaerobic digestion was primarily composed of aromatic proteinlike substances, fulvic acid, and soluble microbial metabolites. The optimized composition enhanced the production rate of protein-like substances and substrate degradation capacity. The microbial community was dominated by hydrolytic and acidogenic bacteria such as Proteiniphilum and Lascolabacillus, with acetoclastic methanogen Methanothrix, while hydrogenotrophic Methanoculleus markedly increased. This metabolic shift was functionally supported by the upregulation of key genes involved in glycolysis, the Wood-Ljungdahl pathway, and CO2-reducing methanogenesis pathways. This study offers experimental evidence for the application of guar gum fracturing fluids in biologically driven coalbed methane development.

Coal-associated minerals are important factors influencing biogenic methane production; however, their roles and underlying mechanisms remain incompletely understood. In this study, mine water was used as the inoculum, and coal with its associated minerals served as the substrate. Anaerobic digestion (AD) simulations, combined with X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), electrochemical analyses, and metagenomic sequencing, were employed to systematically investigate the effects of coal-associated minerals on biogenic methane production, with particular emphasis on electrochemical properties and microbial electron transfer-related processes. The results demonstrated that mineral addition was associated with enhanced methane production potential (5.8 %-40.4 %), increased system conductivity and electrochemical capacitance, and elevated electron transfer-related activity (2.9 %-29.4 %). Analyses of extracellular polymeric substances (EPS) and three-dimensional excitation-emission matrix fluorescence spectroscopy indicated that minerals promoted the accumulation of EPS proteins and polysaccharides and contributed to the stabilization of biofilm structures, thereby creating a more favorable microbial metabolic environment. Microbial community analysis further revealed that mineral addition was associated with shifts toward more cooperative interactions between hydrogenotrophic and acetoclastic methanogens and their syntrophic partners. Metagenomic analysis suggested that minerals were linked to enhanced substrate hydrolysis and intermediate metabolite transformation, strengthened coupling among hydrolysis, acidogenesis, and methanogenesis stages, and increased the abundance of genes involved in methanogenesis and microbial electron transfer processes. Collectively, this study highlights the multifunctional roles of coal-associated minerals in shaping the electrochemical environment and microbial metabolic interactions during AD, providing new insights into mineral-mediated enhancement of biogenic methane production and a theoretical basis for the development of biogenic coalbed methane.

Supercritical carbon dioxide (ScCO2) fracturing can not only promote the fracture propagation of coal reservoir, but also cause significant pore transformation. To investigate this transformation effects of fracturing parameters and stress conditions on the multi-scale pores in coal, a true triaxial ScCO(2)fracturing simulation was carried out, and the pore morphology and structure variations of the micropores (<10 nm), transition pores (10-100 nm), mesopores (100-1000 nm), macropores (1000-10000 nm), and megapores (>10000 nm) in coal after ScCO(2)fracturing were analyzed. The results show that ScCO(2)had no obvious effect on the pore shape, but it reduced the complexity and enhanced the connectivity of the pore structure. After ScCO(2)fracturing, the total volume and specific surface area of the pores increased by 114 % and 385 %, respectively, especially regarding the transformation of the megapores and micropores. The transition pores and macropores experienced a general promotive effect, while obvious propagation and merging occurred in the mesopores. With increasing stress difference, the dominant direction of the pore transformation changed from the direction of the maximum horizontal stress to the direction of the vertical stress. With increasing injection flow, the pore transformation scope increased, but the high stress difference and high injection flow were not conducive to increasing the pore volume. The effective transformation of pores induced by ScCO(2)fracturing can enable the smooth desorption and migration of coalbed methane in the low permeability reservoir, which creates the necessary conditions for its long-term and efficient extraction.

The pore–fracture structure of deep coal deposits is highly important for the potential evaluation, investigation, and utilization of deep coalbed methane resources. This study used methods such as low-pressure CO2 adsorption, low-temperature N2 adsorption, high-pressure mercury intrusion porosimetry, scanning electron microscopy, and optical microscopy to describe the pore–fracture structure of deep coal reservoirs at multiple scales and to discuss the development features, complexity, and influence on permeability of the pore–fracture structure of coal reservoirs. The results showed that there were significant differences in the pore volume and specific surface area (SSA) of the coal specimens with respect to the distribution of pore diameters. The micropore volume and SSA accounted for the largest proportions (85.93

ObjectiveThe production characteristics of coalbed methane (CBM) from deep reservoirs differ significantly from those of CBM from shallow reservoirs. Key challenges in deep CBM production include maintaining reservoir permeability or minimizing permeability loss, enhancing CBM (CH4) desorption efficiency, and accurately predicting the laws of CH4 diffusion. There is an urgent need to overcome these challenges through technological innovation and theoretical research. MethodsThis study systematically analyzed the advances in domestic and international research on coal reservoir permeability, CBM desorption, and CBM diffusion. By integrating the classification of production stages of deep coal reservoirs with the dominant CBM migration mechanisms of varying stages, this study summarized the mechanisms and influential factors of deep CBM production. Results and Conclusions The results indicate that deep CBM production can be divided into four stages: rapid production increase, relatively stable production, gradual production decrease, and low production. During the former two stages, reservoir pressure remains high, free gas serves as a primary gas source, and methane migration is dominated by seepage flow. Key influential factors of both stages include coal structure, developmental degrees of pores and fractures, reservoir temperature, in situ stress, and effective stress. At these stages, minimizing permeability loss is crucial, and direct fracturing should be avoided in reservoirs with a high proportion of granulated and mylonite coals. After the relatively stable production phase, an increase in the reservoir permeability caused by reservoir temperature will gradually increase with an enhancement in the slip effect. Controlling pressure drop and slow production can help to slow down the decline of reservoir permeability. In the low-production stage, the permeability loss rate caused by both primary and artificially induced fractures approaches 100%. However, the irreversible permeability loss rate remains significantly lower than that of shallow coal reservoirs, suggesting the feasibility of secondary reservoir stimulation for increased production. From the rapid production increase stage to the relatively stable production stage, the adsorbed gas begins to undergo gradual desorption. In this case, the primary objectives are to expand the desorption range, ensure the opening of seepage channels, and enhance the productivity of CBM wells. Compared to shallow reservoirs, the desorption of adsorbed gas in deep coal reservoirs occurs over a prolonged period, with the critical desorption pressure being challenging to determine accurately. Furthermore, the pathways for gas migration are prone to be compressed and close, leading to a limited desorption range. To achieve precise estimations of CBM recovery rates, it is necessary to adopt a stepwise depressurization desorption method in experimental research. Specifically, achieving a gradual decrease in the reservoir pressure using control measures during CBM production can effectively enhance the desorption rate of adsorbed gas in micropores. In the low-production stage, gas production primarily originates from desorbed gas in remote well areas. In this stage, the production of CBM wells is determined by methane diffusion, with the accurate measurement of the diffusion coefficient and the development of dynamic diffusion models playing a crucial role. Notably, the diffusion coefficient exhibits significant anisotropy, yet current CH4 diffusion models seldom account for the anisotropic characteristics of coal structure. It is necessary to develop a time-varying CH4 diffusion model while considering the CH4 diffusion patterns across multi-scale pores and microfractures in coals. Experiments on the fine-scale characterization of multi-scale pores and fractures, combined with high-temperature with high-pressure nuclear magnetic resonance imaging, allow for the characterization of variations in CH4 density across different pore sizes. This systematic review integrates theories and practice, further laying a theoretical foundation for deep CBM recovery.

The carbon dioxide-enhanced coalbed methane process enables CO2 sequestration and enhances coalbed methane (CBM) extraction efficiency, offering broad application potential in deep CBM development. The coal fracture surface is a significant medium for fluid transport, and its mechanical properties are significant factors for affecting the migration and embedment of proppants. The results show that as CO2 gradually transforms into the supercritical state, the surface indentation depth, creep distance, and residual depth increase gradually. Young's modulus, hardness, and fracture toughness decrease by 39.45%, 36%, and 31.5%, respectively, and are controlled by the indentation depth. The irreversible work ratio effectively reflects the mechanical weakening process and indirectly confirms that the plastic deformation of coal is an irreversible change. The macromolecular structure, surface free energy, and pore and fractures of coal are important factors contributing to the weakening of mechanical properties, mainly manifested in the destruction of the macromolecular structure, the reduction of surface free energy, and the expansion of fractures. In addition, the Burgers model better validates the mechanical properties of coal fracture surfaces during the creep stage. This greatly reduces the resistance to crack propagation in coal, thus decreasing the mechanical properties of coal. These findings provide important insights into the mechanical behavior of coal in the context of CO2 geological sequestration and related applications.

Coal slime (CS) is a major by-product of coal washing that presents environmental risks due to large-scale accumulation, high moisture content, and pollutant release potential. Effective resource utilization of CS is essential for reducing secondary pollution and promoting sustainable coal management. To reflect typical coal ranks and industrial significance, CS samples were collected from Inner Mongolia (lignite), Shanxi (bituminous coal), and a coal preparation plant in Henan (anthracite). The samples were then subjected to anaerobic digestion to investigate microbial and metabolic responses across different coal ranks. Results showed that methane yields were 45.04, 42.92 and 14.67 mL for lignite, bituminous, and anthracite CS, respectively, with lignite and bituminous CS exhibiting higher levels and consumption of chemical oxygen demand and dissolved organic matter. Tryptophan and tyrosine were preferentially consumed by microbial communities. Hydrolytic bacteria, including Sphaerochaeta, Aminobacterium, and Aminivibrio and acetoclastic methanogens (Methanothrix) were predominant. Lignite and bituminous CS exhibited higher abundances of genes and metabolites associated with aromatic amino acid metabolism, which may facilitate microbial substrate utilization and contribute to the observed differences in methane production. KEGG pathway enrichment analysis revealed significant enrichment of the "Biosynthesis of amino acids" pathway, indicating the potential contribution of enhanced amino acid metabolism to CS biodegradation and methane production. This study elucidates the microbial mechanisms of CS bioconversion, providing a scientific basis for its resource utilization.

The development and utilization of coalbed methane(CBM)not only ensures national energy security,but also provides a boost to carbon neutrality.The southern edge of the Junggar Basin is a major CBM resource area and a key de-velopment hotspot in China.However,the region has few high-yield wells,and maintaining stable production is challen-ging.There is an urgent need for a technology that can increase the production of CBM resources within the control range of a single well.Additionally,high volume fractions of CO2 and H2S have become common in the CBM of this region,creating an urgent need for in-situ disposal technology for these gases.Coalbed gas bioengineering offers a promising technology for enhancing both the quality and production of CBM in this region.On-site monitoring and laboratory simu-lation experiments indicate that the CO2 volume fraction is closely linked to reservoir temperature.Acid-producing fer-mentative bacteria and Hydrogen-producing acetic acid bacteria remain active and continue to produce CO2 across a broad range of reservoir temperatures.At lower reservoir temperatures,the metabolism of hydrogenotrophic methanogens is weak and CO2 is difficult to be reduced,which is the main reason for the high CO2 volume fraction in this area.It was also found that the microbial community in the groundwater interacts with organic matter and SO42-during migration.H2S is generated when the groundwater recharge and drainage rates are consistent with the metabolic cycle of methanogens,which is called epigenetic H2S.The presence of these two acidic gases not only compromises production safety but also significantly reduces the quality of CBM.This study introduces a key technology for in-situ microbial-mediated enhance-ment of CBM quality and production,addressing the issue of low-quality CBM in southern Junggar Basin.The necessity and feasibility of this technology are discussed,highlighting its potential to enhance CBM production,facilitate in-situ mi-crobial conversion of CO2,and inhibit H2S generation.The fundamental concept of this technology is to utilize the coal reservoir as an anaerobic fermentation site,with the coal and CO2 present in the reservoir serving as fermentation sub-strates.This approach aims to achieve in-situ suppression of H2S while enabling the biomethanation of CO2.The key chal-lenges of this technology include cultivating efficient microbial communities,especially hydrogenotrophic methanogens that can thrive across a wide temperature range,developing bio-fracturing fluids for in-situ H2S suppression,and establish-ing effective evaluation methods for enhancing CBM quality.Physical simulations of CO2 microbial methanogenesis showed that cumulative methane production by hydrogenotrophic methanogens increased with rising reservoir temperat-ures,reaching a peak of 8.5 m3/t at 55 ℃.At this temperature,the abundance of key enzymes involved in glycolysis,pyr-uvate metabolism,and the TCA cycle was significantly higher compared to other in-situ anaerobic fermentation systems,enhancing both CO2 conversion efficiency.Furthermore,in an anaerobic fermentation system without CO2 and with ad-ded biological inhibitors,biomethane production reached 4.5 m3/t,slightly higher than that of the control group(4.38 m3/t).Notably,the gaseous H2S volume fraction was reduced by 88.8％compared to the control group.And the H2S volume fraction was always zero from the 9th day to the end of gas production during the anaerobic fermentation,achiev-ing in-situ inhibition of H2S.