Experimental investigation of anisotropy ratio evolution in coal permeability: Implications for underground compressed air energy and CO₂ storage

Abstract

Reliable forecasting of coal seam gas production and gas injectivity (e.g., CO₂ or air) requires an accurate understanding of coal’s anisotropic permeability, which governs the directional flow of gas. Although the anisotropic nature of coal permeability is well recognized, little attention has been paid to how this ratio evolves with changes in effective stress or with the injection of gases that have different affinities to coal. In this work, more than 600 permeability tests were conducted on eight cubic Australian coal samples using He, N₂ and CO₂ gases under varying effective stresses, providing a comprehensive dataset that allows the combined effects of effective stress and gas adsorption on permeability anisotropy to be robustly assessed on the same samples. The results demonstrated that all coal samples exhibited evident permeability anisotropy, with ratios ranging from 1.11 to 6.55. For the first time, quantitative relationships between the anisotropy ratio, effective stress, and initial permeability were established for each of the three injection gases, highlighting how gas adsorption and effective stress changes both anisotropic permeability magnitude and ratio. These findings provide new insights into the directional flow behavior of gases in coal seams, with implications for underground compressed air energy storage and CO₂ sequestration.

Keywords

Anisotropy / Permeability / Gas adsorption / Effective stress / Cleat compressibility / Compressed air energy storage

Acknowledgments

The project was partially funded by industry members APLNG, Arrow Energy, and Santos through The Gas and Energy Transition Research Centre in The University of Queensland. The information, opinions and views expressed in this presentation do not necessar-ily represent those of The University of Queensland, The UQ Gas and Energy Transition Research Centre or its constituent members or associated companies.

References

[1] Liu XD, Sang SX, Zhou XZ, Wang ZL. Coupled adsorption-hydro-thermo-mechanical-chemical modeling for CO₂ sequestration and well production during CO₂-ECBM. Energy 2023; 262:125306.

[2] Schmidt F, Menéndez J, Konietzky H, Pascual-Muñoz P, Castro J, Loredo J, Bernardo Sánchez A. Converting closed mines into giant batteries: Effects of cyclic loading on the geomechanical performance of underground compressed air energy storage systems. J Energy Storage 2020; 32:101882.

[3] Menéndez J, Ordóñez A, Rodrigo Á, Loredo J. Energy from closed mines: Underground energy storage and geothermal applications. Renew Sustain Energy Rev 2019; 108:498-512.

[4] Wu QF, Yu MG. Effect of barrier hole shape on gas explosion behavior under non-uniform conditions. Fuel 2026; 405:136464.

[5] Ran QC, Liang YP, Ye CF, Ning YH, Ma TF, Kong FJ. Failure analysis of overlying strata during inclined coal seam mining: Insights from acoustic emission monitoring. Eng Fail Anal 2025; 182:110023.

[6] Li JH, Li BB, Cheng QY, Gao Z. Characterization of anisotropic coal permeability with the effect of sorption-induced deformation and stress. Fuel 2022; 309: 122089.

[7] Gao XY, Liu YJ, Zhang LM, Deng CB, Song LN, Zhang Y. Regulatory mechanism of microscopic pore structure on anisotropy of gas multimodal seepage in original coals. Energy 2024; 300:131611.

[8] Ran QC, Zhao WT, Liang YP, Ye CF, Ning YH. Deciphering confining pressure effects on coal failure mechanisms using acoustic emission approaches. Eng Fract Mech 2025; 329:111592.

[9] Chen M, Masum S, Sadasivam S, Thomas H. Modelling anisotropic adsorption-induced coal swelling and stress-dependent anisotropic permeability. Int J Rock Mech Min Sci 2022; 153:105107.

[10] Yue GW, Liu H, Yue JW, Li MM, Liang WM. Influence radius of gas extraction borehole in an anisotropic coal seam: Underground in situ measurement and modeling. Energy Sci Eng 2019; 7(3):694-709.

[11] Yan TH, Xu XM, Liu JF, Zhang YH, Arif M, Xu XW, Wang Q. Data-driven predictive model of coal permeability based on microscopic fracture structure characterization. J Rock Mech Geotech Eng 2025; 17(7):4476-89.

[12] Liu A, Liu SM, Wang G, Sang GJ. Modeling of coal matrix apparent strains for sorbing gases using a transversely isotropic approach. Rock Mech Rock Eng 2020; 53(9):4163-81.

[13] Zhang TC, Li JX, Zhu YR, Rudolph V, Chen ZW. Impact of gas adsorption on coal relative permeability: A laboratory study. Int J Rock Mech Min Sci 2025; 194:106191.

[14] Ge ZL, Li CT, Zhou Z, Zhang XY, Guan YR, Sheng MY. Expansion deformation and permeability characteristics of bituminous coal with different moisture content after CO₂ adsorption. Geoenergy Sci Eng 2023; 227:211828.

[15] Xiao ZY, Wang G, Wang CS, Jiang YJ, Jiang F, Zheng CC. Permeability evolution and gas flow in wet coal under non-equilibrium state: Considering both water swelling and process-based gas swelling. Int J Min Sci Technol 2023; 33 (5):585-99.

[16] Palmer I, Mansoori J. How permeability depends on stress and pore pressure in coalbeds: A new model. SPE Reserv Eval Eng 1998; 1(6):539-44.

[17] Pan ZJ, Connell LD. A theoretical model for gas adsorption-induced coal swelling. Int J Coal Geol 2007; 69(4):243-52.

[18] Shi JQ, Durucan S. Drawdown induced changes in permeability of coalbeds: A new interpretation of the reservoir response to primary recovery. Transp Porous Medium 2004; 56(1):1-16.

[19] Liu SM, Wang Y, Harpalani S. Anisotropy characteristics of coal shrinkage/swelling and its impact on coal permeability evolution with CO2 injection. Greenh Gases Sci Technol 2016; 6(5):615-32.

[20] Espinoza DN, Vandamme M, Dangla P, Pereira JM, Vidal-Gilbert S. A transverse isotropic model for microporous solids: Application to coal matrix adsorption and swelling. J Geophys Res Solid Earth 2013; 118(12):6113-23.

[21] Wu SW, Wang L, Liu C, Su S, Lu Z, He XX, Yang QL, Guo LW. Experimental study of deformation induced by high-pressure methane adsorption and desorption: Insights into anisotropy and hysteresis characteristics. Int J Rock Mech Min Sci 2025; 186:106035.

[22] Zhang XY, Wu CF, Wang ZW, Zhou AT. Experimental study of the effective stress coefficient for coal permeability with different water saturations. J Petrol Sci Eng 2019; 182:106282.

[23] Li JH, Li BB, Wang ZH, Ren CH, Yang K, Gao Z. A permeability model for anisotropic coal masses under different stress conditions. J Petrol Sci Eng 2021; 198:108197.

[24] Snow DT. Anisotropic permeability of fractured media. Water Resour Res 1969; 5(6):1273-89.

[25] Xiao K, Zhang ZT, Zhang R, Gao MZ, Xie J, Zhang AL, Liu Y. Anisotropy of the effective porosity and stress sensitivity of coal permeability considering natural fractures. Energy Rep 2021; 7:3898-910.

[26] Lou Z, Wang K, Zang J, Zhao W, Qin BB, Kan T. Effects of permeability anisotropy on coal mine methane drainage performance. J Nat Gas Sci Eng 2021; 86:103733.

[27] Yan Z, Wang K, Zang J, Wang C, Liu A. Anisotropic coal permeability and its stress sensitivity. Int J Min Sci Tech 2019; 29(3):507-11.

[28] Duan MK, Jiang CB, Gan Q, Zhao HB, Yang Y, Li ZK. Study on permeability anisotropy of bedded coal under true triaxial stress and its application. Transp Porous Medium 2020; 131(3):1007-35.

[29] Liu C, Yin GZ, Li MH, Shang DL, Deng BZ, Song ZL. Deformation and permeability evolution of coals considering the effect of beddings. Int J Rock Mech Min Sci 2019; 117:49-62.

[30] Tan YL, Pan ZJ, Liu JS, Zhou FB, Connell LD, Sun WJ, Haque A. Experimental study of impact of anisotropy and heterogeneity on gas flow in coal. Part ii: Permeability. Fuel 2018; 230:397-409.

[31] Clavaud JB, Maineult A, Zamora M, Rasolofosaon P, Schlitter C. Permeability anisotropy and its relations with porous medium structure. J Geophys Res Solid Earth 2008; 113(B1):B01202.

[32] Armitage PJ, Faulkner DR, Worden RH, Aplin AC, Butcher AR, Iliffe J. Experimental measurement of, and controls on, permeability and permeability anisotropy of caprocks from the CO₂ storage project at the Krechba Field, Algeria. J Geophys Res Solid Earth 2011; 116(B12):B12208.

[33] Li M, Ma YX, Kong XW, Xia ZH, Zhu HQ. Seismic application in Australia Bowen basin CBM well drilling and development well placement. In: Proceedings of the International Field Exploration and Development Conference 2018. Singapore: Springer Singapore; 2019.p.485-94.

[34] Kelloway SJ, Ward CR, Marjo CE, Wainwright IE, Cohen DR. Quantitative chemical profiling of coal using core-scanning X-ray fluorescence techniques. Int J Coal Geol 2014; 128:55-67.

[35] Sander R, Pan ZJ, Connell LD. Laboratory measurement of low permeability unconventional gas reservoir rocks: A review of experimental methods. J Nat Gas Sci Eng 2017; 37:248-79.

[36] Liu C, Zhang JH, Wu SW, Qi JH, Yu BC, Wang L. Experimental study on permeability evolution of deep high-stressed coal under major horizontal stress unloading paths. Int J Min Sci Technol 2024; 34(11):1495-508.

[37] Zhang C, Zhang L, Wang W. The axial and radial permeability testing of coal under cyclic loading and unloading. Arab J Geosci 2019; 12(11):371.

[38] Raza SS, Ge L, Rufford TE, Chen ZW, Rudolph V. Anisotropic coal permeability estimation by determining cleat compressibility using mercury intrusion porosimetry and stress-strain measurements. Int J Coal Geol 2019; 205:75-86.

[39] Peng S. Evaluating the accuracy of liquid permeability measurements in shale and tight rocks using transient flow method and comparison with gas permeability. Mar Petrol Geol 2023; 157:106491.

[40] Hildenbrand A, Schlömer S, Krooss BM. Gas breakthrough experiments on fine-grained sedimentary rocks. Geofluids 2002; 2(1):3-23.

[41] Chen ZW, Pan ZJ, Liu JS, Connell LD, Elsworth D. Effect of the effective stress coefficient and sorption-induced strain on the evolution of coal permeability: Experimental observations. Int J Greenh Gas Contr 2011; 5(5):1284-93.

[42] Zhao YY, Zhao YX, Wei MY, Liu JS, Cui DX. A new experimental system and method for periodically measuring permeability and strains of coal. Gas Sci Eng 2023; 110:204909.

[43] Zhang K, Zhou H, Hu DW, Zhao Y, Feng XT. Theoretical model of effective stress coefficient for rock/soil-like porous materials. Acta Mech Solida Sin 2009; 22 (3):251-60.

[44] Lv A, Ali Aghighi M, Masoumi H, Roshan H. The effective stress coefficient of coal: A theoretical and experimental investigation. Rock Mech Rock Eng 2021; 54(8):3891-907.

[45] Zhang XY, Wu CF, Wang ZW. Experimental study of the effective stress coefficient for coal permeability with different water saturations. J Petrol Sci Eng 2019; 182:106282.

[46] Chen ZW, Liu JS, Pan ZJ, Connell LD, Elsworth D. Influence of the effective stress coefficient and sorption-induced strain on the evolution of coal permeability: Model development and analysis. Int J Greenh Gas Contr 2012; 8:101-10.

[47] Niu QH, Cao LW, Sang SX, Zhou XZ, Wang ZZ. Anisotropic adsorption swelling and permeability characteristics with injecting CO₂ in coal. Energy Fuels 2018; 32(2):1979-91.

[48] Evans JP, Forster CB, Goddard JV. Permeability of fault-related rocks, and implications for hydraulic structure of fault zones. J Struct Geol 1997; 19 (11):1393-404.

[49] Jasinge D, Ranjith PG, Choi SK. Effects of effective stress changes on permeability of Latrobe valley brown coal. Fuel 2011; 90(3):1292-300.

[50] Perera MSA, Ranjith PG, Viete DR. Effects of gaseous and super-critical carbon dioxide saturation on the mechanical properties of bituminous coal from the Southern Sydney Basin. Appl Energy 2013; 110:73-81.

[51] Pan ZJ, Connell LD, Camilleri M. Laboratory characterisation of coal reservoir permeability for primary and enhanced coalbed methane recovery. Int J Coal Geol 2010; 82(3-4):252-61.

[52] Zhang XG, Ranjith PG, Ranathunga AS, Li DY. Variation of mechanical properties of bituminous coal under CO₂ and H 2O saturation. J Nat Gas Sci Eng 2019; 61:158-68.