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Frontiers of Earth Science

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Front. Earth Sci.    2019, Vol. 13 Issue (1) : 75-91    https://doi.org/10.1007/s11707-018-0712-1
RESEARCH ARTICLE
Effects of nano-pore system characteristics on CH4 adsorption capacity in anthracite
Chang’an SHAN1,2,3(), Tingshan ZHANG4, Xing LIANG5, Dongchu SHU5, Zhao ZHANG5, Xiangfeng WEI6, Kun ZHANG7, Xuliang FENG1, Haihua ZHU4, Shengtao WANG8, Yue CHEN8
1. School of Earth Sciences and Engineering, Xi’an Shiyou University, Xi’an 710065, China
2. Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China
3. Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan 430074, China
4. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
5. Exploration?and?Development?Department, Zhejiang Oilfield Company, CNPC, Hangzhou 310023, China
6. SINOPEC Exploration Company, Chengdu 610014, China
7. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China
8. Shaanxi Yanchang Petroleum International Exploration and Development Engineering Co. Ltd, Xi’an 710075, China
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Abstract

This study aims to determine the effects of nanoscale pores system characteristics on CH4 adsorption capacity in anthracite. A total of 24 coal samples from the southern Sichuan Basin, China, were examined systemically using coal maceral analysis, vitrinite reflectance tests, proximate analysis, ultimate analysis, low-temperature N2 adsorption–desorption experiments, nuclear magnetic resonance (NMR) analysis, and CH4 isotherm adsorption experiments. Results show that nano-pores are divided into four types on the basis of pore size ranges: super micropores (<4 nm), micropores (4–10 nm), mesopores (10–100 nm), and macropores (>100 nm). Super micropores, micropores, and mesopores make up the bulk of coal porosity, providing extremely large adsorption space with large internal surface area. This leads us to the conclusion that the threshold of pore diameter between adsorption pores and seepage pores is 100 nm. The “ink bottle” pores have the largest CH4 adsorption capacity, followed by semi-opened pores, whereas opened pores have the smallest CH4 adsorption capacity which indicates that anthracite pores with more irregular shapes possess higher CH4 adsorption capacity. CH4 adsorption capacity increased with the increase in NMR porosity and the bound water saturation. Moreover, CH4 adsorption capacity is positively correlated with NMR permeability when NMR permeability is less than 8×10−3 md. By contrast, the two factors are negatively correlated when NMR permeability is greater than 8×10−3 md.
Keywords CH4 adsorption capacity      anthracite      nano-pore structure      NMR physical properties     
Corresponding Author(s): Chang’an SHAN   
Just Accepted Date: 25 July 2018   Online First Date: 06 September 2018    Issue Date: 25 January 2019
 Cite this article:   
Chang’an SHAN,Tingshan ZHANG,Xing LIANG, et al. Effects of nano-pore system characteristics on CH4 adsorption capacity in anthracite[J]. Front. Earth Sci., 2019, 13(1): 75-91.
 URL:  
https://academic.hep.com.cn/fesci/EN/10.1007/s11707-018-0712-1
https://academic.hep.com.cn/fesci/EN/Y2019/V13/I1/75
Fig.1  General tectonic setting, study area location, and study well distribution.
Sample Vitrinite
/vol.% a)		 Inertinite
/vol.% a)		 MMb)
/vol.% a)		 Ro, max
/%		 Proximate analysis/wt.% c), ad d) Ultimate analysis/wt.% c), ad d)
Made) Aade) Vadg) FCadh) Carbon Hydrogen Nitrogen
L1 92.7 5.3 2 3.12 0.84 31.05 10.1 58.02 57.75 1.77 0.61
L2 65 33.5 1.5 3.13 0.72 26.13 10.83 62.33 63.98 2.09 0.62
L3 18.4 69.6 12 3.31 0.75 21.21 8.01 70.04 67.81 2.38 0.71
L4 70.1 24.1 5.8 2.91 0.47 34.58 13.41 51.54 53.84 1.63 0.38
L5 44.8 51.2 4 3.09 0.55 29.9 7.97 61.58 61.52 2.03 0.55
L6 84.8 8.5 6.7 3.05 0.78 30.11 9.28 59.83 60.69 2.07 0.59
L7 22.9 75.7 1.4 3.01 0.75 19.3 7.7 72.26 71.7 2.56 0.86
L8 68.9 30.1 1 2.92 1.1 14.72 8.88 75.31 75.72 2.67 0.82
L9 20.4 78.8 0.8 2.89 0.58 33.12 6.9 59.41 60.3 1.91 0.49
L10 67.9 14.3 17.8 2.85 0.83 32.98 8.29 57.91 57.32 1.95 0.59
L11 81.2 6.5 12.3 2.94 0.72 29.11 8.65 61.53 62.28 2.05 0.56
L12 90 8.7 1.3 3.12 0.79 20.2 9.05 69.97 71.59 2.34 0.76
L13 87.8 7 5.2 3.09 0.83 19.53 8.33 71.32 71.39 2.38 0.84
L14 56.6 39.7 3.7 3.18 0.93 26.12 7.97 64.99 64.21 2.23 0.75
L15 73.5 20.5 6 3.29 0.69 24.36 11.17 63.79 65.06 2.05 0.61
L16 84.5 5.8 9.7 2.73 0.77 23 9.16 67.07 67.65 2.47 0.7
L17 65.7 29.7 4.6 2.64 0.69 28.38 9.17 61.76 62.53 2.29 0.74
L18 79.2 19.5 1.3 2.84 0.83 10.6 7.31 81.27 80.39 3.08 1.15
L19 60.5 38 1.5 2.87 0.58 31.82 8.71 58.9 59.83 2.15 0.62
L20 21 78 1 2.96 0.59 18.7 8.19 72.53 72.15 2.75 0.92
L21 87.6 7.8 4.6 3.16 0.48 43.5 7.06 48.96 48.88 1.46 0.4
L22 45.7 51 3.3 3.26 0.59 29.25 11.44 58.73 60.94 1.74 0.53
L23 25.1 71.9 3 3.09 0.73 37.74 7.91 53.64 51.09 1.61 0.56
L24 82.2 11.8 6 3.11 0.67 34.81 9.81 54.71 53.01 1.43 0.48
Tab.1  Complete list and properties of 24 coal core samples
Fig.2  Relationships between N2 adsorption amount and SBET, VBJH: (a) relationship between N2 adsorption amount and SBET; (b) relationship between N2 adsorption amount and VBJH. SBET = total specific surface area calculated using the BET equation; VBJH = pore volume calculated using the BJH model.
Sample name dBJH
/(nm)		 SBET /(m2·g?1) N2
adsorbed amount/
(cm3·g?1)		 SBJH/(m2·g?1) VBJH/(×10?3 cm3·g?1)
Stotal super micropores Micropores Mesopores Macropores Vtotal super micropores Micropores Mesopores Macropores
(<4 nm) (4?10 nm) (10?100 nm) (>100 nm) (<4 nm) (4?10 nm) (10?100 nm) (>100 nm)
Ss-mic Percentage /% Smic Percentage /% Smes Percentage /% Smac Percentage /% Vs-mic Percentage /% Vmic Percentage /% Vmes Percentage /% Vmac Percentage /%
L1 8.654 2.233 2.8553 2.021 1.026 50.8 0.663 32.8 0.323 16 0.009 0.4 4.364 0.745 17.1 1.068 24.5 2.157 49.4 0.394 9
L2 8.074 2.042 2.3287 1.761 0.988 56.1 0.546 31 0.205 11.6 0.022 1.2 3.554 0.773 21.8 0.876 24.6 1.287 36.2 0.618 17.4
L3 8.38 1.26 1.6346 1.249 0.705 56.4 0.378 30.3 0.16 12.8 0.006 0.5 2.616 0.521 19.9 0.596 22.8 1.188 45.4 0.311 11.9
L4 7.161 1.411 1.3031 1.082 0.599 55.4 0.366 33.8 0.114 10.5 0.003 0.3 1.933 0.462 23.9 0.593 30.7 0.751 38.9 0.127 6.6
L5 11.039 1.268 2.1651 1.229 0.634 51.6 0.344 28 0.234 19 0.017 1.4 3.394 0.474 14 0.57 16.8 1.674 49.3 0.676 19.9
L6 8.258 1.322 1.6838 1.294 0.592 45.7 0.499 38.6 0.191 14.8 0.012 0.9 2.671 0.452 16.9 0.797 29.8 0.995 37.3 0.427 16
L7 7.276 1.104 1.2754 1.125 0.655 58.2 0.345 30.7 0.116 10.3 0.009 0.8 2.048 0.481 23.5 0.551 26.9 0.73 35.6 0.286 14
L8 6.535 1.21 1.3463 1.377 0.813 59 0.449 32.6 0.11 8 0.005 0.4 2.25 0.59 26.2 0.726 32.3 0.67 29.8 0.264 11.7
L9 13.49 1.549 3.3532 1.556 0.671 43.1 0.486 31.2 0.363 23.3 0.036 2.3 5.251 0.487 9.3 0.8 15.2 2.739 52.2 1.225 23.3
L10 8.051 0.918 1.0942 0.87 0.456 52.4 0.308 35.4 0.103 11.8 0.003 0.3 1.751 0.348 19.9 0.472 27 0.808 46.1 0.123 7
L11 6.742 1.06 1.0679 1.053 0.681 64.7 0.28 26.6 0.084 8 0.008 0.7 1.776 0.485 27.3 0.427 24 0.577 32.5 0.287 16.2
L12 6.135 1.788 1.2238 1.417 0.87 61.4 0.431 30.4 0.107 7.5 0.009 0.7 2.173 0.629 28.9 0.659 30.3 0.62 28.5 0.265 12.2
L13 7.829 1.473 1.1884 1.002 0.613 61.2 0.283 28.2 0.1 10 0.006 0.6 1.962 0.451 23 0.434 22.1 0.783 39.9 0.294 15
L14 10.199 1.035 1.6357 0.998 0.471 47.2 0.328 32.9 0.187 18.7 0.012 1.2 2.544 0.365 14.3 0.537 21.1 1.245 48.9 0.397 15.6
L15 17.842 0.611 1.5834 0.551 0.221 40.1 0.163 29.6 0.156 28.3 0.011 2 2.46 0.171 7 0.272 11.1 1.497 60.9 0.52 21.1
L16 8.73 1.564 2.2196 1.629 0.972 59.7 0.452 27.7 0.193 11.8 0.012 0.7 3.555 0.696 19.6 0.701 19.7 1.632 45.9 0.526 14.8
L17 8.68 1.129 1.4761 1.122 0.602 53.7 0.374 33.3 0.139 12.4 0.007 0.6 2.436 0.465 19.1 0.577 23.7 1.052 43.2 0.342 14
L18 8.984 1.281 1.9808 1.439 0.812 56.4 0.434 30.2 0.183 12.7 0.01 0.7 3.231 0.596 18.4 0.68 21 1.494 46.2 0.461 14.3
L19 6.471 1.192 1.1145 1.187 0.699 58.9 0.359 30.2 0.127 10.7 0.002 0.2 1.921 0.515 26.8 0.558 29 0.789 41.1 0.059 3.1
L20 7.236 1.416 1.3255 1.167 0.655 56.1 0.387 33.2 0.118 10.1 0.007 0.6 2.111 0.506 24 0.604 28.6 0.74 35.1 0.262 12.4
L21 6.947 1.536 1.7399 1.65 0.959 58.1 0.517 31.3 0.16 9.7 0.014 0.8 2.866 0.701 24.5 0.812 28.3 0.972 33.9 0.381 13.3
L22 9.045 1.297 1.5318 1.074 0.585 54.5 0.361 33.6 0.12 11.2 0.008 0.8 2.428 0.452 18.6 0.555 22.9 1.032 42.5 0.389 16
L23 9.295 0.797 1.2053 0.839 0.432 51.5 0.288 34.3 0.112 13.4 0.007 0.8 1.949 0.328 16.8 0.44 22.6 0.878 45 0.304 15.5
L24 8.961 0.98 1.3346 0.961 0.533 55.5 0.303 31.5 0.119 12.4 0.006 0.6 2.153 0.407 18.9 0.475 22.1 0.963 44.7 0.308 14.3
Tab.2  Coal structure analysis results from low-temperature N2 adsorption- desorption analyses of 24 coal core samples
Fig.3  (a) The relationship between pore-size distribution and the specific surface area; (b) the relationship between pore-size distribution and pore volume. SBJH = the specific surface area calculated using the BJH equation; VBJH = the pore volume calculated using the BJH model.
Fig.4  (a) D1 type of the hysteresis loops; (b) low-temperature N2 adsorption–desorption isotherms of 8 samples; (c) shapes of nanoscale coal pores.
Fig.5  (a) D2 type of the hysteresis loops; (b) low-temperature N2 adsorption–desorption isotherms of 10 samples; (c) shapes of nanoscale coal pores.
Fig.6  (a) D3 type of the hysteresis loops; (b) low-temperature N2 adsorption–desorption isotherms of 6 samples (c) shapes of nano-pores.
Sample Sample volume
/cm3		 Calibration coefficient
a		 Calibration coefficient
b		 Permeability coefficient
C		 NMR
porosity
/%		 NMR
permeability
/(×10?3 md)		 Bound water
saturation
/%		 Movable water
saturation
/%		 T2
cutoff value
/ms
L1 5.31 0.04595 ?0.370168 10 6.7 8.1148 91.74 8.26 33
L2 7.55 0.04595 ?0.370168 10 5.58 1.7149 88.26 11.74 33
L3 7.36 0.04595 ?0.370168 10 4.08 0.1607 92.93 7.07 33
L4 13.91 0.04595 ?0.370168 10 4.49 0.5833 89.29 10.71 33
L5 8.67 0.04595 ?0.370168 10 8.67 8.6344 88.99 11.01 33
L6 11.85 0.04595 ?0.370168 10 3.72 0.1072 93.03 6.97 33
L7 10.93 0.04595 ?0.370168 10 3.57 0.3471 87.24 12.76 33
L8 12.62 0.04595 ?0.370168 10 4.17 0.5985 87.66 12.34 33
L9 7.86 0.04595 ?0.370168 10 4.65 1.0377 87.02 12.98 33
L10 15.15 0.04595 ?0.370168 10 5.7 0.4399 93.94 6.06 33
L11 11.54 0.04595 ?0.370168 10 6.02 2.5442 87.77 12.23 33
L12 7.31 0.04595 ?0.370168 10 7.28 20.4739 78.76 21.24 33
L13 9.33 0.04595 ?0.370168 10 6.29 4.1861 85.95 14.05 33
L14 7.73 0.04595 ?0.370168 10 5.41 1.5718 88.06 11.94 33
L15 11.37 0.04595 ?0.370168 10 6.76 11.3663 81.07 18.93 33
L16 12.28 0.04595 ?0.370168 10 3.17 0.887 77.2 22.8 33
L17 4.62 0.04595 ?0.370168 10 2.5 0.279 78.86 21.14 33
L18 5.06 0.04595 ?0.370168 10 5.24 9.1846 74.12 25.88 33
L19 5.58 0.04595 ?0.370168 10 5.61 2.6269 86.01 13.99 33
L20 6.7 0.04595 ?0.370168 10 5.18 2.0754 85.51 14.49 33
L21 7.1 0.04595 ?0.370168 10 4.58 0.8206 87.97 12.03 33
L22 12.09 0.04595 ?0.370168 10 6.04 1.4583 90.52 9.48 33
L23 15.55 0.04595 ?0.370168 10 3.68 0.032 96 4 33
L24 17.41 0.04595 ?0.370168 10 4.01 0.0098 98.09 1.91 33
Tab.3  Results?of 24 coal core samples from nuclear magnetic resonance (NMR) experiments
Fig.7  NMR T2 relaxation?time spectrum of 24 coal core samples.
Sample CH4 isothermal adsorption analysis
VLa)/(m3·t?1) PLb)/Mpa Correlation
coefficient
VLadc) VLdafd) VLeme) PLadf)/PLdafg)/PLemh)
L1 32.01 47 30 2.45 0.9998
L2 25.81 35.28 24.95 2.24 0.9994
L3 27.44 35.16 25.83 2.27 0.9991
L4 23.97 36.9 22.53 2.52 0.9997
L5 30.65 44.07 28.61 3.13 0.9991
L6 21.71 31.42 20.73 2.04 1
L7 29.04 36.32 28.04 2.07 0.9997
L8 29.87 35.49 28.96 2.06 0.9999
L9 28.59 43.11 27.13 2.81 0.9966
L10 22.82 34.48 21.2 1.97 0.9999
L11 26.12 37.22 24.43 2.11 1
L12 28.83 35.58 26.82 2.06 1
L13 33.63 42.22 32.37 2.52 0.9988
L14 28.33 38.83 26.94 2.24 0.9997
L15 26.23 34.99 24.66 2 0.9999
L16 30.26 39.69 28.86 2.58 0.9992
L17 22.62 31.89 21.43 2.57 0.9986
L18 32.06 36.2 30.47 1.79 0.9999
L19 26.3 38.9 25.12 2.45 0.9981
L20 28.75 35.62 27.82 2.08 0.9999
L21 23.55 42.05 22.47 2.24 0.9995
L22 27.56 39.28 25.73 2.62 1
L23 25.13 40.84 23.94 2.67 1
L24 25.53 39.58 24.06 2.91 0.9997
Tab.4  CH4 isothermal adsorption analyses of 24 coal core samples
Fig.8  (a) The relationship between CH4 adsorption capacity and SBET; (b) The relationship between CH4 adsorption capacity and VBJH.. SBET = the total specific surface area calculated using the BET equation; VBJH = the pore volume calculated using the BJH model; VLdaf = Langmuir volume (dry ash-free basis).
Fig.9  Pore specific surface area contribution of super micropores, micropores, mesopores, and macropores of 24 coal core samples. (a) Histogram of pore specific surface area contribution percentage of super micropores, micropores, mesopores, and macropores of 24 coal core samples; (b) histogram of the average pore specific surface area contribution percentage of super micropores, micropores, mesopores, and macropores of 24 coal core samples.
Fig.10  Pore volume contribution of super micropores, micropores, mesopores, and macropores of 24 coal core samples. (a) Histogram of pore volume contribution percentage of super micropores, micropores, mesopores, and macropores of 24 coal core samples; (b) histogram of the average pore volume contribution percentage of super micropores, micropores, mesopores, and macropores of 24 coal core samples.
Fig.11  (a) The relationship between CH4 adsorption capacity and NMR porosity; (b) The relationship between CH4 adsorption capacity and NMR permeability; (c) The relationship between CH4 adsorption capacity and the bound water saturation. VLdaf = Langmuir volume (dry ash-free basis).
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