Fractal characterization of pore structure and its influence on CH4 adsorption and seepage capacity of low-rank coals

Mu, Guangyuan, Hou, Haihai, Zhang, Jiaqiang, Tang, Yue, Li, Ya-nan, Sun, Bin, Li, Yong, Jones, Tim ORCID: https://orcid.org/0000-0002-4466-1260, Yuan, Yuan and Shao, Longyi 2022. Fractal characterization of pore structure and its influence on CH4 adsorption and seepage capacity of low-rank coals. Frontiers of Earth Science 16 , pp. 916-933. 10.1007/s11707-022-0969-2

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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 increase 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.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Earth and Environmental Sciences
Publisher:	Springer
ISSN:	2095-0195
Date of First Compliant Deposit:	4 August 2022
Date of Acceptance:	31 December 2021
Last Modified:	07 Nov 2023 09:02
URI:	https://orca.cardiff.ac.uk/id/eprint/151681

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