Multifractal Analysis in Characterizing Adsorption Pore Heterogeneity of Middle- and High-Rank Coal Reservoirs

Junjian Zhang; Chongtao Wei; Xuanxuan Chu; Veerle Vandeginste; Wei Ju

doi:10.1021/acsomega.0c01115

Multifractal Analysis in Characterizing Adsorption Pore Heterogeneity of Middle- and High-Rank Coal Reservoirs

ACS Omega. 2020 Aug 3;5(31):19385-19401. doi: 10.1021/acsomega.0c01115. eCollection 2020 Aug 11.

Authors

Junjian Zhang^{1

2

3}, Chongtao Wei^{2

3}, Xuanxuan Chu⁴, Veerle Vandeginste⁵, Wei Ju^{2

3}

Affiliations

¹ College of Earth Sciences & Engineering, Shandong University of Science and Technology, Qingdao 266590, China.
² Key Laboratory of Coalbed Methane Resource & Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China.
³ School of Resources and Earth Science, China University of Mining and Technology, Xuzhou 221116, China.
⁴ Nottingham Transportation Engineering Centre, University of Nottingham, Nottingham NG7 2RD, U.K.
⁵ GeoEnergy Research Centre and School of Chemistry, University of Nottingham, Nottingham NG7 2RD, U.K.

Abstract

Nanopore heterogeneity has a significant effect on adsorption, desorption, and diffusion processes of coalbed methane. The adsorption pore size distribution heterogeneity was calculated by combining N₂ with CO₂ adsorption data, and factors affecting multifractal and single-fractal dimensions were studied. The results indicate that pore size distribution of micropores (with pore diameters smaller than 2 nm) and meso-macro-pores (with pore diameters between 2 and 100 nm) in coal samples exhibit typical multifractal behavior. The overall heterogeneity of micropores in high-rank coal samples is higher than that in the middle-rank coal samples. The low-probability measure areas control the overall heterogeneity of pores with diameters of 0.40-1.50 nm. The high-probability measure area heterogeneity and spectral width ratio have a higher linear correlation with coal rank and pore structure parameters than those of low-probability measure areas. Heterogeneity of high-probability measure areas and overall pore size distribution are controlled by pores with diameters of 0.72-0.94 nm. Multifractal parameters of meso-macro-pores have no clear relationship with coal rank. The pore volume of 2-10 nm diameter shows a good linear correlation with heterogeneity of low-probability measure areas, and pores of this diameter range are the key interval that affected pore size distribution heterogeneity. The single-fractal dimension obtained using the Frenkel-Halsey-Hill (FHH) model shows a positive linear correlation with heterogeneity of the low-probability measure areas. It indicates that this parameter can effectively characterize the pore size distribution heterogeneity of low-probability measure areas in meso-macro-pores.