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Abstract

AAPG Bulletin, V. 103, No. 6 (June 2019), P. 1297-1319.

Copyright ©2019. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/11151817226

Fractal characteristics of the anisotropic microstructure and pore distribution of low-rank coal

Junpeng Zou,1 Weizhong Chen,2 Diansen Yang,3 Jingqiang Yuan,4 and Yu-Yong Jiao5

1Faculty of Engineering, China University of Geosciences, Wuhan, Hubei, China; [email protected]
2State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei, China; Geotechnical and Structural Engineering Research Center, Shandong University, Jinan, Shandong, China; [email protected]
3State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei, China; [email protected]
4State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei, China; [email protected]
5Faculty of Engineering, China University of Geosciences, Wuhan, Hubei, China; [email protected]

ABSTRACT

Low-rank coal reserves account for nearly 55% of the national coal reserves in China, and it is extremely important to understand their anisotropic microstructure and coalbed methane storage and sorption characteristics. In this study, microstructures that develop parallel to and perpendicular to bedding of low-rank coals are studied through scanning electron microscopy. The micromorphology and microfracture and mineral distributions are examined. Based on fractal theory and digital image processing technology, the fractal box-counting dimensions of microfracture distributions that form parallel as well as perpendicular to the coal bedding planes are calculated, and the anisotropic fractal characteristics of these microfracture distributions are characterized. The pore-size distribution and the fractal characteristics of the microscopic pore structure are systematically examined by means of nitrogen adsorption and mercury intrusion tests. The results of the fractal box-counting dimension calculations indicate that microfractures distributed perpendicular to coal bedding are rougher and more irregular than those distributed along a bedding plane, with the former exhibiting higher space filling and methane adsorption capacity. The fractal dimension of the microscopic pore structure reveals two inflection points measuring approximately 5 and 80 nm, thereby indicating that the low-rank coal exhibits a microscopic pore structure with a three-level fractal characteristic.

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