Influence law of pore water storage characteristics on the gas adsorption characteristics of coal

Abstract

This study mainly investigates the influence of pore water characteristics on the adsorption properties of coalbed methane through integrated low field nuclear magnetic resonance (LF-NMR), adsorption experiments, and molecular dynamics (MD) simulations. Pore water states in three coal ranks were characterized during progressive hydration. Multi-scale analysis revealed how pore water evolution regulates methane adsorption processes. During the diffusion-dominated stage (M2-M3), adsorbed water penetrates into the micropores. In the highly wettable brown coal (L1), the adsorbed water content reaches 2.12 g while in the anthracite (A1), it is only 0.29 g. During the active water injection stage (M4-M6), non-adsorbed water dominates in anthracite (over 85% of the total water content of 4.01 g), while adsorbed water remains dominant in lignite (over 60% of the total water content of 3.52 g). Water content plays a key role in methane adsorption in coal. During the water addition phase, the influence of methane adsorption on medium-to-low-rank coal is relatively weak, while the methane adsorption capacity of high-rank coal A1 shows a significant decrease during both the water diffusion and water addition phases, corresponding to a reduction in Langmuir volume of 21.22 cm³/g. Molecular dynamics (MD) results further show that the free energy between molecules on the surface of hydroxyl-modified coal increases, with hydroxyl groups driving electrostatic interactions between coal and water molecules. Increased steric hindrance inhibits hydrogen bond formation and reduces the rate of hydrogen bond growth. There is a significant correlation between pore water content and coal-water molecular interaction energy, which cross-scale validates the results of LF-NMR testing and MD simulations.

Keywords

Coal rank / Pore water storage / Gas adsorption / Langmuir-like / Competitive adsorption / Cross-scale

Acknowledgments

This work was financially supported by the National Science Fund for Distinguished Young Scholars (No. 51925404), the National Natural Science Foundation of China (Nos. 52104233, 52104228 and 52404261), and the Fundamental Research Funds for the Central Universities (No. 2023ZDPY05).

References

[1] Gui XH, Xing YW, Cao YJ, Liu JT. Recent advances and thinking in process intensification of low quality coal slime flotation. J China Coal Soc 2021; 46:2715-32. in Chinese.

[2] Li MJ, Li FF, Qiu J, Zhou H, Wang HR, Lu HL, Zhang N, Song ZY. Multi-objective optimization of non-fossil energy structure in China towards the carbon peaking and carbon neutrality goals. Energy 2024; 312:133643.

[3] Xue WH, Wang LY, Yang Z, Xiong ZW, Li XY, Xu QQ, Cai ZX. Can clean heating effectively alleviate air pollution: An empirical study based on the plan for cleaner winter heating in northern China. Appl Energy 2023; 351:121923.

[4] Dong FY, Yin HY, Cheng WJ, Zhang C, Zhang DY, Ding HX, Lu C, Wang Y. Quantitative prediction model and prewarning system of water yield capacity (WYC) from coal seam roof based on deep learning and joint advanced detection. Energy 2024; 290:130200.

[5] Kang JH, Zhu JP, Wang YP, Zhou FB, Liu YK. Dynamical modeling of coupled heat and mass transfer process of coalbed methane desorption in porous coal matrix. Int J Heat Mass Transf 2022; 183:122212.

[6] Rong TL, Liu PJ, Wang JW, Ren XJ, Li XZ, Wang GY.Evolution of permeability and gas seepage in deep coal under 3D stress. Nat Resour Res 2024; 33 (2):765-91.

[7] Karacan CÖ, Ruiz FA, Cotè M, Phipps S. Coal mine methane: A review of capture and utilization practices with benefits to mining safety and to greenhouse gas reduction. Int J Coal Geol 2011; 86(2-3):121-56.

[8] Kiyama T, Nishimoto S, Fujioka M, Xue ZQ, Ishijima Y, Pan ZJ, Connell LD. Coal swelling strain and permeability change with injecting liquid/supercritical CO₂ and N2 at stress-constrained conditions. Int J Coal Geol 2011; 85(1):56-64.

[9] Plaksin MS, Rodin RI. Improvement of degasification efficiency by pulsed injection of water in coal seam. IOP Conf Ser: Earth Environ Sci 2019; 377 (1):012052.

[10] Wei JP, Jiang W, Si LL, Xu XY, Wen ZH. Experimental research of the surfactant effect on seepage law in coal seam water injection. J Nat Gas Sci Eng 2022; 103:104612.

[11] Cai YL, Zhai C, Yu X, Sun Y, Xu JZ, Zheng YF, Cong YZ, Li YJ, Chen AK, Xu HX, Wang S, Wu XZ. Quantitative characterization of water transport and wetting patterns in coal using LF-NMR and FTIR techniques. Fuel 2023; 350:128790.

[12] Shishlyaev VV, Pimenov VP. Assessment of filtration properties of a coal seam and the length of a hydraulic fracturing crack based on the results of quantitative interpretation of injection falloff tests. Bull Kuzbass State Tech Univ 2024:35-44.

[13] Chen AK, Zhai C, Yu X, Xu JZ, Zheng YF, Zhu XY, Xu HX. Quantitative analysis of the effect of NaClO stimulation in anthracite and bituminous coal with FTIR and LF-NMR. Energy Fuels 2024; 38(3):2018-32.

[14] Darcel C, Le Goc R, Lavoine E, Davy P, Mas Ivars D, Sykes E, Kasani HA. Coupling stress and transmissivity to define equivalent directional hydraulic conductivity of fractured rocks. Eng Geol 2024; 342:107739.

[15] Yue JW, Xu JL, Zhang JG, Shi BM, Zhang MY, Li Y, Wang C. Gas displacement characteristics during the water wetting process of gas-bearing coal and microscopic influence mechanism. Sci Total Environ 2024; 949:175034.

[16] Wang G, Li HX, Yan S, Huang QM, Wang SB, Fan JY. Effect of glycerol microemulsion on coal seam wetting and moisturizing performance. J Mol Liq 2022; 367:120405.

[17] Chang YH, Yao YB, Liu DM, Liu Y, Cui C, Wu H. Behavior and mechanism of water imbibition and its influence on gas permeability during hydro-fracturing of a coalbed methane reservoir. J Petrol Sci Eng 2022; 208:109745.

[18] Wang F, Yao YB, Wen ZA, Sun QP, Yuan XH. Effect of water occurrences on methane adsorption capacity of coal: A comparison between bituminous coal and anthracite coal. Fuel 2020; 266:117102.

[19] Zhang JC, Lv DW, Yin DW, Zhang XY, Li XL, Fan KK. Gas recovery and flowback in trans-coal-limestone fracture: An in situ wettability microscale visualization insight. Gas Sci Eng 2025; 142:205707.

[20] Meng JQ, Yin FF, Li SC, Zhong RQ, Sheng ZY, Nie BS. Effect of different concentrations of surfactant on the wettability of coal by molecular dynamics simulation. Int J Min Sci Technol 2019; 29(4):577-84.

[21] Zhou G, Xing MY, Wang KL, Wang Q, Xu Z, Li L, Cheng WM. Study on wetting behavior between CTAC and BS-12 with gas coal based on molecular dynamics simulation. J Mol Liq 2022; 357:118996.

[22] Gao MZ, Li HM, Zhao Y, Liu YT, Zhou WQ, Li LM, Xie J, Deng J. Mechanism of micro-wetting of highly hydrophobic coal dust in underground mining and new wetting agent development. Int J Min Sci Technol 2023; 33(1): 31-46.

[23] Zhou G, Ma YL, Fan T, Wang G. Preparation and characteristics of a multifunctional dust suppressant with agglomeration and wettability performance used in coal mine. Chem Eng Res Des 2018; 132:729-42.

[24] Xu CH, Wang DM, Wang HT, Ma LY, Zhu XL, Zhu YF, Zhang Y, Liu FM. Experimental investigation of coal dust wetting ability of anionic surfactants with different structures. Process Saf Environ Prot 2019; 121:69-76.

[25] Charrière D, Behra P. Water sorption on coals. J Colloid Interface Sci 2010; 344 (2):460-7.

[26] Xie ZX, Liu SY. Interface mechanism of promoting low-rank coal flotation by characteristic groups in hydrophilic moieties of cationic-anionic surfactants. Langmuir 2025; 41(2):1502-18.

[27] Liu ZQ, Zhou G, Li SL, Wang CM, Liu RL, Jiang WJ. Molecular dynamics simulation and experimental characterization of anionic surfactant: Influence on wettability of low-rank coal. Fuel 2020; 279:118323.

[28] Busch A, Han FS, Magill CR. Paleofloral dependence of coal methane sorption capacity. Int J Coal Geol 2019; 211:103232.

[29] Gensterblum Y, Merkel A, Busch A, Krooss BM. High-pressure CH4 and CO₂ sorption isotherms as a function of coal maturity and the influence of moisture. Int J Coal Geol 2013; 118:45-57.

[30] Nie BS, Liu XF, Yuan SF, Ge BQ, Jia WJ, Wang CL, Chen XH. Sorption charateristics of methane among various rank coals: Impact of moisture. Adsorption 2016; 22(3):315-25.

[31] Zhou YN, Sun WJ, Chu W, Liu XQ, Jing FL, Xue Y. Theoretical insight into the enhanced CH4 desorption via H2O adsorption on different rank coal surfaces. J Energy Chem 2016; 25(4):677-82.

[32] Jin ZH, Firoozabadi A. Effect of water on methane and carbon dioxide sorption in clay minerals by Monte Carlo simulations. Fluid Phase Equilib 2014; 382:10-20.

[33] Billemont P, Coasne B, De Weireld G. An experimental and molecular simulation study of the adsorption of carbon dioxide and methane in nanoporous carbons in the presence of water. Langmuir 2011; 27(3):1015-24.

[34] Li ZB, Ren T, Cheng YP, He XQ, Qiao M, Black D, Li K, Nemcik J. Study of methane and carbon dioxide adsorption-desorption hysteresis in coals from Sydney Basin: A theoretical and experimental approach. Int J Min Sci Technol 2024; 34(10):1453-63.

[35] Association CNC. Chinese Classification of Coals. In: General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China; 2009. in Chinese.

[36] Akhlaghi MH, Naderi M, Abdi-Jalebi M. Graphene-loaded aphron microbubbles for enhanced drilling fluid performance and carbon capture and storage. ACS Appl Nano Mater 2024; 7(22):26187-201.

[37] Zhao R, Xie Y, Li ZL, Weng HL, Zhu DR, Mao YF, Wang HM, Zhang QL. Unveiling the promotion effect of ethylenediamine on preparation of Ni/CeO2 catalyst for low-temperature CO₂ methanation. Int J Hydrog Energy 2024; 51:451-63.

[38] Jalilian M, Mahdavi S, Pourafshary P, You Z, Mohammadi AH. Assessing the impact of geochemical mechanism and interpolation factor selection on the precision of low-salinity waterflooding modelling: A comparative study. geosyst Eng 2024; 27(6):304-18.

[39] Kuang YC, Xie WH, Wu HY, Liu XQ, Sher F, Qiu SX, Dang J, Zhang SF. Molecular structure of coal macerals and thermal response behavior of their chemical bonds obtained by structural characterizations and molecular dynamics simulations. Energy 2024; 301:131735.

[40] Wang K, Guo L, Xu C, Wang WJ, Yang T, Lin SS, Cai YB. Multiscale characteristics of pore-fracture structures in coal reservoirs and their influence on coalbed methane (CBM) transport: A review. Geoenergy Sci Eng 2024; 242:213181.

[41] Nie W, Zhang XH, Niu WJ, Bao Q, Tian QF, Li RX, Shi CF, Tong K, Zhang ZH, Lian J. Synergy of chelating agents and surfactants on the wettability and pore structure of coal dust: Experimental study and molecular simulation. Chem Eng J 2024; 502:157954.

[42] Isah UA, Rashid MI, Lee S, Kiman S, Iyodo HM. Correlations of coal rank with the derived Fourier transform infra-red (FTIR) spectroscopy structural parameters: A review. Infrared Phys Technol 2024; 141:105456.

[43] Singh PP, Ranganathan R. Superior impact resistance conferred by hierarchical nacre-inspired nanocomposites: A molecular dynamics study. Carbon 2025; 233:119885.

[44] Yang QM, Zhang XH, Qiao XG, Wang Z, Zhang RX, Pang J, Wang JY. Effect of oxygen functional groups on water-blocking effect in coal through DFT and MD simulation. J Mol Liq 2024; 15:133643.

[45] Tararushkin EV, Smirnov GS, Kondratyuk ND, Kalinichev AG. Structure and properties of thaumasite and its aqueous interfaces revealed by molecular dynamics simulations. Cem Concr Res 2025; 197:107944.

[46] Tang X, Fu XX, Zou J, Zhang DF. Responses of physico-chemical properties and CH4 adsorption/desorption behaviors of diverse rank coal matrices to microwave radiation. Gas Sci Eng 2024; 128:205359.

[47] Wang L, Liu MX, Li J, Sun YW, Lv B, Ma XH, Zhou MQ, Zhang JY. Investigation of methane adsorption mechanism in low-rank bituminous coal considering internal water distribution: A molecular simulation and experimental study. Chem Eng J 2024; 495:153562.