Experimental study on molecular structure and multifractal characteristics of coal modified by anaerobic microorganisms

Microbial degradation could effectively change the microstructure of coal and improve the permeability. The effects of microbial degradation on the surface functional groups, crystal structure, and nanopore structure of coal were investigated by various test methods. Multiscale pore heterogeneity parameters under biogenic influence were calculated based on multifractal theory. Meanwhile, the correlations between different microstructure parameters were analyzed using Spearman correlation analysis. The results show that the C O group and aromatic -CH group content decrease following microbial degradation. The aromaticity I of samples S2 and S3 increases from 1.57 to 2.56 and 1.77, respectively, while the aromaticity I of Y1 and Y2 decreases. The DOC value of the samples becomes smaller, the length of fatty chains increases, and the skeletal structure of coal becomes looser. Microbial activity can lead to a reduction in ink-bottle pores while increasing the proportion of open and semi-closed pores, improving pore connectivity. Microorganisms convert micropores and transitional pores into meso- and macropores by consuming small-molecule organic matter in coal, significantly reducing the content of micropores and increasing the average pore size. After biotransformation, the connectivity H value of coal samples increases, and the heterogeneity ∆ α value of meso-macropores decreases. Microorganisms enhance the uniformity and connectivity of meso-macropore spaces by digesting organic matter and enlarging pores, simplifying the pore structure. Relevant results are of guiding importance for elucidating the evolution of coal matrix microstructure during biotransformation and the mechanism of efficient coalbed methane extraction.