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Frontiers of Earth Science

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Front. Earth Sci.    2022, Vol. 16 Issue (4) : 916-933    https://doi.org/10.1007/s11707-022-0969-2
RESEARCH ARTICLE
Fractal characterization of pore structure and its influence on CH4 adsorption and seepage capacity of low-rank coals
Guangyuan MU1, Haihai HOU2, Jiaqiang ZHANG3, Yue TANG3, Ya-nan LI1, Bin SUN4, Yong LI1, Tim JONES5, Yuan YUAN3, Longyi SHAO1()
1. State Key Laboratory of Coal Resources and Safe Mining and College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China
2. College of Mining, Liaoning Technical University, Fuxin 123000, China
3. Oil & Gas Resource Survey Center, China Geological Survey, Ministry of Land and Resource, Beijing 100029, China
4. Department of Coalbed Methane, Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
5. School of Earth and Environmental Sciences, Cardiff University, Cardiff CF103YE, UK
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Abstract

The pore structures of coal can directly affect the adsorption and seepage capacity of coalbed methane (CBM), which therefore is an important influence on CBM exploration and development. In this study, the pore structures of low-rank coals from the Middle Jurassic Xishanyao Formation in the southern Junggar Basin were analyzed, and the fractal dimensions (D1, D2, D3 and D4 corresponding to pore sizes of 0−5 nm, 5−100 nm, 100−1000 nm and 1000−20000 nm, respectively) were calculated to quantitatively describe these coal pore structures. The results show that Xishanyao coal is characterized by open pore morphology, good pore connectivity and well-developed seepage pores and microfractures, which is beneficial to CBM seepage. The D1 and D2 can be used to characterize the pore surface and structure of adsorption pores respectively. The D3 and D4 can be used to represent the pore structure of seepage pores. Compared with adsorption pores, the structure of seepage pores is more affected by the change of coal rank. The D1 is better than D2 in characterizing the methane adsorption capacity. When D1 > 2.2, D1 is positively correlated with Langmuir volume (VL) and negatively correlated with Langmuir pressure (PL), while D2 shows a weak opposite trend. The coals with the higher D1 and lower D2 are associated with a higher VL, indicating the coal reservoir with more complex pore surfaces and simpler pore structures has stronger methane adsorption capacity. D4 is better than D3 in characterizing the methane seepage capacity. The porosity and permeability of coal reservoirs increases with the increase of D4, while D3 displays an opposite trend, which is mainly related to the well-developed microfractures. The well-developed fracture system enhances the seepage capacity of the Xishanyao coal reservoir. This study reveals the fractal characteristics of pore structure and its significant influence on adsorption and seepage capacity of low-rank coal.
Keywords southern Junggar Basin      Middle Jurassic      low-rank coal      coalbed methane      pore structure      fractal dimensions     
Corresponding Author(s): Longyi SHAO   
Online First Date: 27 June 2022    Issue Date: 11 January 2023
 Cite this article:   
Guangyuan MU,Haihai HOU,Jiaqiang ZHANG, et al. Fractal characterization of pore structure and its influence on CH4 adsorption and seepage capacity of low-rank coals[J]. Front. Earth Sci., 2022, 16(4): 916-933.
 URL:  
https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-0969-2
https://academic.hep.com.cn/fesci/EN/Y2022/V16/I4/916
Fig.1  (a) Position of the study area in Junggar Basin (modified from Fu et al., 2016b). (b) Distribution of sampling points in the southern Junggar Basin. (c) Stratigraphic column for the coal-bearing strata in southern Junggar Basin.
Sample No. Coal lithotype Ro,max/% Proximate analysis/wt% Coal macerals/% CH4 isothermal adsorption
Mad Aad Vad FCad V I L VL/(m3·t−1) PL /MPa
NS-1 semi-bright 0.62 4.74 13.87 31.10 50.29 45.61 48.92 5.47 9.76 5.05
NS-2 semi-bright 0.59 4.54 10.92 32.98 51.56 54.61 38.32 7.07 n n
MM2-1 bright 0.58 2.29 16.38 37.73 43.60 77.23 7.67 10.89 n n
MM2-2 bright 0.62 2.26 11.69 - - 74.00 11.48 10.54 n n
MM2-3 semi-bright 0.72 1.75 26.45 - - 81.16 7.73 0.97 n n
MM2-4 semi-bright 0.64 2.08 13.69 37.73 46.50 90.32 4.15 4.61 n n
MM2-5 bright 0.65 2.27 8.07 38.00 51.66 89.43 4.07 4.88 n n
MM2-6 semi-dull 0.66 2.10 7.00 34.15 56.75 78.28 17.21 4.51 n n
MM2-7 semi-bright 0.65 1.84 12.63 38.32 47.21 79.10 8.96 11.94 n n
TX-1 bright 0.68 3.59 6.94 30.02 59.45 56.93 40.67 2.40 15.43 5.11
TX-2 semi-bright 0.69 3.02 4.03 27.91 65.04 59.60 37.80 2.60 n n
XGG-1 bright 0.65 3.82 3.97 30.52 61.69 62.50 34.05 3.45 n n
KG-1-1 semi-dull 0.59 3.35 5.74 35.23 55.68 58.85 36.21 3.29 9.85 4.12
KG-1-2 semi-bright 0.64 3.49 3.03 34.19 59.29 53.38 42.62 1.48 11.22 4.74
KG-1-3 semi-bright 0.64 3.79 2.57 32.62 61.02 49.80 44.80 2.80 12.94 5.11
KG-1-4 semi-bright 0.60 3.69 2.80 32.95 60.56 59.50 38.39 0.58 14.12 5.82
KG-1-5 semi-dull 0.58 3.78 5.48 32.79 57.95 53.80 43.41 1.30 10.62 4.57
CXY-2 bright 0.57 2.24 5.08 34.15 58.53 65.62 32.08 2.31 18.16 3.79
Tab.1  Results of lithotype, maximum vitrinite reflectance, proximate analysis, maceral and CH4 isothermal adsorption of the coal samples from southern Junggar Basin
Sample No. PD1 /nm SBET /(m2·g−1) VBJH /(10−3cm3·g−1) Pore content /volume (%) P/P0: 0−0.5 (0−5 nm) P/P0: 0.5−1 (5−100 nm) Loop type
VN1 VN2 VN3 A1 D1 = 3 + A1 R2 A2 D2 = 3 + A2 R2
NS-1 7.81 0.756 1.477 43.96 49.83 6.20 − 0.8435 2.1565 0.9745 − 0.2478 2.7522 0.9539 C
NS-2 10.49 0.731 1.917 43.03 45.76 11.21 − 0.8133 2.1867 0.9680 − 0.2935 2.7065 0.9468 C
MM2-1 7.24 1.922 3.477 79.77 17.92 2.31 − 0.5951 2.4049 0.9861 − 0.2419 2.7581 0.9490 A
MM2-2 13.04 0.283 0.922 50.85 40.23 8.92 − 0.7480 2.2520 0.9839 − 0.3572 2.6428 0.9784 B
MM2-3 11.58 0.264 0.763 41.22 46.67 12.11 − 0.8443 2.1557 0.9727 − 0.3227 2.6773 0.9544 C
MM2-4 15.86 0.116 0.461 27.49 58.20 14.31 − 0.8758 2.1242 0.9801 − 0.4224 2.5776 0.9775 C
MM2-5 7.72 0.283 0.546 60.02 33.25 6.73 − 1.0758 1.9242 0.9712 − 0.2437 2.7563 0.9195 B
MM2-6 9.44 0.284 0.671 39.86 50.97 9.17 − 0.8195 2.1805 0.9703 − 0.2800 2.7200 0.9479 C
MM2-7 8.24 0.177 0.365 45.81 42.56 11.63 − 1.0417 1.9583 0.9782 − 0.2445 2.7555 0.9318 C
TX-1 8.46 1.417 2.995 51.62 40.36 8.02 − 0.7038 2.2962 0.9701 − 0.2571 2.7429 0.9297 B
TX-2 7.59 2.287 4.339 52.64 39.82 7.54 − 0.6647 2.3353 0.9640 − 0.2332 2.7668 0.9301 B
XGG-1 6.32 1.811 2.860 69.79 29.11 1.10 − 0.7029 2.2971 0.9604 − 0.2036 2.7964 0.8749 B
KG-1-1 7.59 1.114 2.113 56.55 39.84 3.61 − 0.7110 2.2890 0.9647 − 0.2454 2.7546 0.9121 B
KG-1-2 7.13 1.399 2.493 63.11 34.15 2.74 − 0.7068 2.2932 0.9567 − 0.2370 2.7630 0.8930 B
KG-1-3 6.85 1.950 3.340 64.96 30.82 4.22 − 0.7161 2.2839 0.9630 − 0.1824 2.8176 0.8258 B
KG-1-4 6.80 1.665 2.829 66.13 31.58 2.29 − 0.7587 2.2413 0.9550 − 0.2218 2.7782 0.8704 B
KG-1-5 6.41 1.342 2.151 64.25 32.28 3.48 − 0.7259 2.2741 0.9554 − 0.1978 2.8022 0.8681 B
CXY-2 9.26 0.305 0.705 41.50 46.15 12.35 − 0.8796 2.1204 0.9787 − 0.2647 2.7353 0.9256 C
Tab.2  Results of low-pressure nitrogen adsorption, fractal dimension and loop type of coal samples from southern Junggar Basin
Fig.2  Three types of nitrogen adsorption/desorption results and pore size distribution based on LTNA for typical Xishanyao coal samples. (a) Nitrogen adsorption/desorption curves. (b) BJH pore volume distribution. (c) BET specific surface area distribution.
Sample No. Φ/% Kair/mD PTD/nm PD2/nm Vin/mL MMS/% Eex/% Pore content/volume (%) 100−1000 nm 1000−20000 nm Curve type
VM1 VM2 VM3 VM4 D3 R2 D4 R2
NS-1 7.55 2.193 70 1238 0.77 91.71 48.85 13.27 37.43 27.69 21.61 2.9333 0.9951 3.8779 0.1259 B
NS-2 5.43 151.660 24 1378 0.55 88.49 67.76 18.24 45.39 13.81 22.56 3.1496 0.9872 3.4407 0.7197 C
MM2-4 4.48 0.010 30 1992 0.43 90.92 64.69 16.69 41.62 9.47 32.22 3.1272 0.9298 3.4048 0.4309 D
TX-1 6.97 2.448 54 680 0.75 91.55 50.39 12.77 42.47 27.75 17.01 3.3120 0.9730 3.7063 0.7350 B
TX-2 5.59 0.181 32 266 0.63 93.95 58.34 16.11 51.54 23.05 9.31 3.4470 0.9791 3.6623 0.6047 B
XGG-1 4.77 0.051 20 666 0.45 90.21 77.89 21.83 53.59 9.83 14.76 3.5192 0.9297 3.5303 0.7076 A
KG-1-1 3.17 0.130 24 666 0.31 85.54 82.87 10.53 63.04 10.62 15.81 3.6084 0.8771 3.4573 0.6852 C
KG-1-2 4.34 1.080 20 48 0.50 94.45 73.77 20.33 58.33 14.18 7.16 3.5868 0.9790 3.3311 0.8567 A
KG-1-3 4.28 0.001 18 26 0.46 90.05 86.23 24.16 60.52 8.67 6.65 3.6904 0.7986 3.2492 0.7861 A
KG-1-4 4.31 0.005 18 24 0.49 93.07 82.48 23.79 56.57 11.08 8.56 3.8566 0.5926 3.4618 0.5742 A
KG-1-5 4.48 0.002 18 44 0.47 93.95 81.67 23.23 59.56 10.38 6.83 3.6208 0.6909 3.5492 0.8517 A
CXY-2 5.08 36.482 24 52 0.57 92.99 73.99 19.51 57.14 15.73 7.61 3.6773 0.8852 3.5471 0.6786 A
Tab.3  Results of mercury intrusion porosimetry, fractal dimension and curve type of coal samples from southern Junggar Basin
Fig.3  Four types of mercury intrusion/extrusion curves based on MIP for typical Xishanyao coal samples from the southern Junggar Basin.
Fig.4  SEM images show pore and maceral characteristics of typical coal samples. (a) widely developed primary pores and sparse gas pores in fusinite; (b) many gas pores were developed in telocollinite; (c) primary pores and gas pores in the corpocollinite; (d) microfractures were widely developed in vitrinite; (e) primary pores infilled with clay minerals in deformed fusinite; (f) structural fractures in the vitrinite.
Fig.5  Diagrams showing representative examples of the relationship between ln(lnP0/P) and lnV for typical low-rank coal samples.
Fig.6  Relationship between fractal dimensions D1 and D2.
Fig.7  Diagrams showing representative examples of the relationship between lnP and ln(dV/dP) for typical low-rank coal samples.
Fig.8  Relationship between fractal dimensions D3 and D4.
Fig.9  Mercury intrusion/extrusion curves show coal matrix compression.
Fig.10  Comparison of the pore size distribution by (a) LTNA and MIP joint analysis and (b) NMR relaxation.
Sample No. Pore content/volume (%)
V1 V2 V3 V4
NS-1 1.92 2.29 53.88 41.91
NS-2 4.57 5.41 34.22 55.81
MM2-4 0.79 1.65 22.22 75.34
TX-1 5.37 4.28 55.83 34.52
TX-2 11.12 8.80 57.00 23.07
XGG-1 15.55 7.96 30.71 45.77
KG-1-1 13.25 11.15 30.47 45.13
KG-1-2 15.24 9.64 49.35 25.77
KG-1-3 24.23 14.01 34.71 27.05
KG-1-4 18.21 10.29 39.89 31.61
KG-1-5 16.05 9.71 44.66 29.58
CXY-2 2.41 3.17 63.63 30.78
Tab.4  Pore size distribution estimated by the combination of LTPA and MIP for coal samples from southern Junggar Basin
Fig.11  Relationship between fractal dimensions D1 and Ro,max, Mad, Vad, FCad and BET surface area (a, c, e, g, and i); and relationship between fractal dimensions D2 and Ro,max, Mad, vitrinite, Aad and average pore diameter (b, d, f, h, and j).
Fig.12  Relationship between fractal dimensions D1, D2 and Langmuir volume, Langmuir pressure.
Fig.13  Relationship between fractal dimension D3 and Ro, max, Vad, vitrinite and Aad (a, c, e, and g); and relationship between fractal dimension D4 and Ro,max, Vad, vitrinite and macropore content (b, d, f, and h).
Fig.14  Relationship between fractal dimension D3 and porosity (a) and permeability (b) and relationship between fractal dimension D4 and porosity (c) and permeability (d).
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