Micro-CT characterization and fractal study on the fracture structure of coal under the liquid nitrogen cold soaking

Abstract

The development of coalbed methane in China is constrained by complex geological conditions characterized by low permeability, low saturation, low reservoir pressure, and high adsorption (“three lows and one high”), posing significant challenges to its efficient development. The liquid nitrogen-induced fracturing and permeability enhancement technology can effectively promote the expansion and connection of macroscopic and microscopic fractures, thereby improving the permeability of coal seams. In this study, industrial micro-CT scanning technology, the VRA-UNet method, and fractal dimension calculation methods are employed to conduct an in-depth analysis of the action mechanism of liquid nitrogen cold soaking on the fracture structure of coal bodies with different metamorphism degrees. The results indicate that liquid nitrogen cold soaking promotes the generation, expansion, and connection of new fractures inside coal bodies to form fracture networks. Via Matlab programming and VG Studio MAX image analysis software, fracture extraction and calculation are performed on CT-scanned coal samples; it is statistically found that the quantitative fracture indices of coal increase after liquid nitrogen cold soaking. Compared with the fracture spectrum peak proportions of raw coal samples, the fracture spectrum peak proportions of anthracite, bituminous coal, and lignite increase by 8.375%, 12.680%, and 79.939% respectively after liquid nitrogen cold soaking. By combining the VRA-UNet method for coal fracture identification, the box-counting method is used to calculate that the fractal dimension of coal fractures after liquid nitrogen cold soaking is larger than that of raw coal samples. The research findings of this paper will provide theoretical and technical support for the efficient development of coalbed methane and the improvement of coal seam gas extraction rates.

Keywords

liquid nitrogen cold soaking / crack morphology / micro-CT / fractal measurement

References

[1] Zhang H, Nie B-s, Liu X-fet al. . The availability of industrial purge gas: Experimental and kinetic study of explosion characteristics of low-concentration H2/CH₄ mixtures [J]. Energy. 2024, 313: 133910.

[2] Wang X-z, Zhou J-p, Sun Xet al. . The influences of composition and pore structure on the adsorption behavior of CH₄ and CO₂ on shale [J]. Frontiers of Earth Science. 2021, 15(2): 283-300.

[3] Liu P, Nie B-s, Zhao Z-det al. . Permeability of micro-scale structure in coal: Insights from μ-CT image and pore network modelling [J]. Gas Science and Engineering. 2023, 111: 204931.

[4] Wang T, Luo Z-m, Wen Het al. . The explosion enhancement of methane-air mixtures by ethylene in a confined chamber [J]. Energy. 2021, 214119042.

[5] Sun C-w, Liu S-m, Li S-det al. . Methane hydrate formation characteristics under different initial conditions and their impact on coal seam properties [J]. Advances in Geo-Energy Research. 2025, 163229-243.

[6] LI Xue-long, CHEN Shao-jie, WANG Sheng, et al. Study on in situ stress distribution law of the deep mine: Taking Linyi mining area as an example [J]. Advances in Materials Science and Engineering, 2021(1): 5594181. DOI: https://doi.org/10.1155/2021/5594181.

[7] Wu M-y, Jiang C-b, Deng B-zet al. . Characterization and reconstruction of rough fractures based on vector statistics [J]. Geoenergy Science and Engineering. 2024, 234: 212664.

[8] Kong X-g, He D, Liu X-fet al. . Strain characteristics and energy dissipation laws of gas-bearing coal during impact fracture process [J]. Energy. 2022, 242123028.

[9] Zhu Q-zhong. Theoretical basis of dredging and efficient development of high-rank coalbed methane in China: A case study of the Qinshui Basin [J]. Coal Geology & Exploration. 2022, 50382-91(in Chinese)

[10] Kang S-l, Lü Y-m, Wang C-wet al. . Control of sedimentary environments on gas contents of coal seams: A case study of No. 15 coals bed of the Taiyuan Formation in Shouyang area, Qinshui Basin [J]. Journal of Palaeogeography (Chinese Edition). 2024, 262416-430(in Chinese)

[11] Zou Q-l, Zhang T-c, Ma T-fet al. . Effect of water-based SiO₂ nanofluid on surface wettability of raw coal [J]. Energy. 2022, 254: 124228.

[12] Zhou Y-b, Zhang R-l, Huang J-let al. . Influence of alkaline solution injection for wettability and permeability of coal with CO₂ injection [J]. Energy. 2020, 202117799.

[13] Zhou Y-b, Li Z-h, Zhang R-let al. . CO₂ injection in coal: Advantages and influences of temperature and pressure [J]. Fuel. 2019, 236493-500.

[14] Su E-l, Wei J-q, Chen H-det al. . Effect of CO₂ injection on coalbed permeability based on a thermal-hydraulic-mechanical coupling model [J]. Energy & Fuels. 2024, 38(12): 11078-11092.

[15] Qin L, Wang P, Lin H-fet al. . Quantitative characterization of the pore volume fractal dimensions for three kinds of liquid nitrogen frozen coal and its enlightenment to coalbed methane exploitation [J]. Energy. 2023, 263: 125741. Part A

[16] Liu S-m, Sun H-t, Zhang D-met al. . Experimental study of effect of liquid nitrogen cold soaking on coal pore structure and fractal characteristics [J]. Energy. 2023, 275127470.

[17] Liu J-l, Jin Y, Zhu Y-jet al. . Study on deterioration characteristics and fracturing mechanism of concrete under liquid nitrogen cold shock [J]. International Journal of Concrete Structures and Materials. 2021, 15(1): 49.

[18] Wang P, Li B, Wang K-jet al. . Critical drilling and completion techniques for deep coalbed methane in the Shenfu block and their applications [J]. Coal Geology & Exploration. 2024, 52844-56(in Chinese)

[19] Wang H, Yang Z-z, Li Yet al. . Study on drilling and fracturing technology for deep complex structure coal reservior in Qinshui Basin [J]. Coal Science and Technology. 2019, 479105-111(in Chinese)

[20] Ren F-p, Han C-s, Wang L-xet al. . Microbially enhanced CBM well production rate technology and its application [J]. Oil Drilling & Production Technology. 2016, 38(3): 395-399(in Chinese)

[21] Ritter D, Vinson D, Barnhart Eet al. . Enhanced microbial coalbed methane generation: A review of research, commercial activity, and remaining challenges [J]. International Journal of Coal Geology. 2015, 146: 28-41.

[22] Li B-t, Lin H-f, Wei J-pet al. . Investigation on coal damage and fracture extension law of liquid nitrogen injection pre-cooling and fracturing under true triaxial stress [J]. International Journal of Mining Science and Technology. 2025, 35(2): 213-229.

[23] Han S-c, Gao Q, Yan X-cet al. . Thermally-induced cracking behaviors of coal reservoirs subjected to cryogenic liquid nitrogen shock [J]. Journal of Rock Mechanics and Geotechnical Engineering. 2024, 16(8): 2894-2908.

[24] Liu S-m, Wang D-k, Yin G-zet al. . Experimental study on the microstructure evolution laws in coal seam affected by temperature impact [J]. Rock Mechanics and Rock Engineering. 2020, 53(3): 1359-1374.

[25] Qin L, Ma C, Li S-get al. . Mechanical damage mechanism of frozen coal subjected to liquid nitrogen freezing [J]. Fuel. 2022, 309: 122124.

[26] Lin H-f, Li B-t, Li S-get al. . Development and application of true triaxial experimental system for the whole process of coal fracturing induced by liquid nitrogen injection [J]. Chinese Journal of Rock Mechanics and Engineering. 2025, 44(2): 276-291. in Chinese)

[27] Lin H-f, Li B-t, Li S-get al. . Enhancing coalbed methane recovery using liquid nitrogen as a fracturing fluid: A coupled thermal-hydro-mechanical modeling and evaluation in water-bearing coal seam [J]. Energy. 2024, 291: 130445.

[28] Shen W-l, Chen Z-q, Wang Met al. . Fracture characteristics and fracture interface buckling mechanism of cantilever rock mass under non-uniformly distributed load [J]. International Journal of Mining Science and Technology. 2026, 362375-397.

[29] Li X-l, Chen D-y, Liu S-met al. . Enhanced permeability mechanism in coal seams through liquid nitrogen immersion: Multi-scale pore structure analysis [J]. Journal of Central South University. 2025, 3272732-2749.

[30] Kong X-g, Li S-g, Wang E-yet al. . Experimental and numerical investigations on dynamic mechanical responses and failure process of gas-bearing coal under impact load[J]. Soil Dynamics and Earthquake Engineering. 2021, 142: 106579.

[31] Kong X-g, Li S-g, Wang E-yet al. . Experimental and numerical investigations on dynamic mechanical responses and failure process of gas-bearing coal under impact load [J]. Soil Dynamics and Earthquake Engineering. 2021, 142106579.

[32] Yu Z-f, Bai J-p, Yang J-det al. . Influence of pore structure and basic coal properties on gas efficient desorption capacity in coal [J]. ACS Omega. 2025, 10(17): 17504-17514.

[33] Liu S-m, Li X-l, Wang D-ke. Advanced characterization of coal microstructure before and after liquid nitrogen cold soaking [J]. Coal Science and Technology. 2023, 51593-102(in Chinese)

[34] Qin L, Lv S-y, Lin H-fet al. . Microscopic fracturing and unfrozen water hysteresis effects analysis of lignite and anthracite [J]. Physics of Fluids. 2025, 37: 013311.

[35] Zhang H-w, Shen J, Wang Get al. . Experimental study on the effect of high-temperature nitrogen immersion on the nanoscale pore structure of different lithotypes of coal [J]. Energy. 2023, 284128596.

[36] Liu S-m, Sun H-t, Zhang D-met al. . Nuclear magnetic resonance study on the influence of liquid nitrogen cold soaking on the pore structure of different coals [J]. Physics of Fluids. 2023, 35012009.

[37] Yang R-y, Cong R-c, Liu Het al. . Pore-scale analysis of coal structure and mechanical properties evolution through liquid nitrogen thermal shock [J]. Natural Gas Industry. 2021, 41(7): 82-92(in Chinese)

[38] Wang D-k, Zhang P, Liu S-met al. . Experimental study on evolutionary characteristics of porefissure structure in coal seam under temperature impact [J]. Journal of China Coal Society. 2018, 43123395-3403(in Chinese)

[39] Wang D-k, Liu S-m, Wei J-pet al. . A research study of the intra-nanopore methane flow law [J]. International Journal of Hydrogen Energy. 2017, 42(29): 18607-18613.

[40] Zhang P, Wang D-k, Yu Cet al. . Digital coal core construction process and crack characterization based on industrial CT scanning [J]. Journal of Henan Polytechnic University (Natural Science). 2019, 38(6): 10-16(in Chinese)

[41] Li H-w, Wang L-g, Zhang C-het al. . Structural damage mechanism for cold loaded coal samples under different confining pressures [J]. Chinese Journal of Applied Mechanics. 2017, 34(4): 788-794823. (in Chinese)

[42] Zhang C-h, Li W-l, Wang X-zet al. . Research of fracturing mechanism of coal subjected to liquid nitrogen cooling [J]. Journal of Hebei University of Science and Technology. 2015, 36(4): 425-430(in Chinese)

[43] Liu X. Experimental study on the freeze-thaw cycle characteristics of rocks and numerical simulation of low-temperature response [D]. 2013, Beijing, China University of Geosciences(in Chinese)

[44] Liu S-q, Sang S-x, Ma J-set al. . Three-dimensional digitalization modeling characterization of pores in high-rank coal in the southern Qinshui basin [J]. Geosciences Journal. 2019, 23(1): 175-188.

[45] Wang D-k, Tian X-r, Wei J-pet al. . Fracture evolution and nonlinear seepage characteristic of gas-bearing coal using X-ray computed tomography and the lattice Boltzmann method [J]. Journal of Petroleum Science and Engineering. 2022, 211: 110144.

[46] Wang D-k, Zeng F-c, Wang J-get al. . Dynamic evolution characteristics and fractal law of loaded coal fractures by micro industrial CT [J]. Chinese Journal of Rock Mechanics and Engineering. 2020, 39(6): 1165-1174(in Chinese)

[47] Wang D-k, Zhang P, Wei J-pet al. . Experimental study on dynamic evolution of three-dimensional fracture structure of loaded coal based on CT visualization [J]. Journal of China Coal Society. 2019, 44S2574-584(in Chinese)

[48] Xue K-s, Zhang Z-y, Zhong C-let al. . A fast numerical method and optimization of 3D discrete fracture network considering fracture aperture heterogeneity [J]. Advances in Water Resources. 2022, 162: 104164.

[49] Li G, Qin Y, Yao H-pet al. . 3D pore structure characterization and permeability anisotropy visualization simulation of fusain [J]. Energy. 2024, 291130422.

[50] Song R, Tian J, Wu M-yet al. . Experimental and numerical modeling of deformation-cracking mechanics of 3D-printed rock samples with single fracture [J]. Advances in Geo-Energy Research. 2023, 8(2): 126-135.

[51] Yang G-s, Zhang Q-s, Ren J-xet al. . Study on the effect of freezing rate on the damage ct values of Tongchuan sandstone [J]. Chinese Journal of Rock Mechanics and Engineering. 2004, 23244099-4104(in Chinese)

[52] Ren J-xi. Real time CT testing on damage evolution characteristics of frozen cracked-sandstone under uniaxial compression [J]. Chinese Journal of Rock Mechanics and Engineering. 2005, 24(Suppl.1): 4670-4673(in Chinese)

[53] Liu S-m, Yang K, Sun H-tet al. . Adsorption and deformation characteristics of coal under liquid nitrogen cold soaking [J]. Fuel. 2022, 316: 123026.

[54] Liu S-min. Experimental study on the effects of liquid nitrogen cold soaking on the microstructure evolution and adsorption deformation characteristics of coal [D]. 2021, Chongqing, Chongqing University(in Chinese)

[55] Wang Q, Zhao D, Feng Z-cet al. . Experimental study on fracturing of coal by injection liquid nitrogen in drill based on CT scanning [J]. Coal Science and Technology. 2017, 45(4): 149-154. in Chinese)

[56] Wang D-k, Zhang P, Pu Het al. . Experimental research on cracking process of coal under temperature variation with industrial micro-CT [J]. Chinese Journal of Rock Mechanics and Engineering. 2018, 37102243-2252(in Chinese)

[57] Farrokhrouz M, Akhondzadeh H, Yang Y-fet al. . Coal cleat network stimulation through a combination of acidizing and liquid nitrogen fracturing [J]. Gas Science and Engineering. 2024, 125: 205308.

[58] Wang L-m, Cheng L, Yin S-het al. . Effect of liquid nitrogen cold soaking on the pore structure and fractal characteristics during in situ leaching of sandstone uranium ores [J]. Advanced Powder Technology. 2025, 36(7): 104943.

[59] Shu C, Wang H-t, Li X-let al. . A thermo – hydro – mechanical model: Capturing the effects of initial permeability and gas pressure on outburst-prone indicators [J]. Natural Resources Research. 2020, 2931897-1914.

[60] Qin L, Wang P, Li S-get al. . Gas adsorption capacity changes in coals of different ranks after liquid nitrogen freezing [J]. Fuel. 2021, 292: 120404.

[61] Wang H-m, Liu Y, Song Y-cet al. . Fractal analysis and its impact factors on pore structure of artificial cores based on the images obtained using magnetic resonance imaging [J]. Journal of Applied Geophysics. 2012, 8670-81.

[62] Mandelbrot B. How long is the coast of Britain? statistical self-similarity and fractional dimension [J]. Science. 1967, 1563775636-638.

[63] Yao Y-b, Liu D-m, Tang D-zet al. . Fractal characterization of seepage-pores of coals from China: An investigation on permeability of coals [J]. Computers & Geosciences. 2009, 35(6): 1159-1166.

[64] Lee G J, Pyun S I, Rhee C K. Characterisation of geometric and structural properties of pore surfaces of reactivated microporous carbons based upon image analysis and gas adsorption [J]. Microporous and Mesoporous Materials. 2006, 93(1–3): 217-225.

[65] Russell D A, Hanson J D, Ott E. Dimension of strange attractors [J]. Physical Review Letters. 1980, 45(14): 1175-1178.

[66] Wang D-k, Wang L-h, Qin Y-get al. . Fracture identification and 3D reconstruction of coal-rock combinations based on VRA-UNet network [J]. Coal Science and Technology. 2025, 53(2): 96-108(in Chinese)

RIGHTS & PERMISSIONS

Central South University