Fractal evolution of lignite microstructure by biosurfactant treatment: Implications for pore-fracture network reconstruction

To explore the influence of biosurfactants on the microstructure fractal of low-rank coal, lignite was taken as the research object. Coal samples were treated with rhamnolipid solutions of different concentrations. Combined with multi-scale characterization techniques such as computed tomography (CT) three-dimensional reconstruction, scanning electron microscopy (SEM), low-temperature liquid nitrogen adsorption, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR), we quantitatively analyzed the complexity and heterogeneity evolution laws of pore structure based on fractal theory. The results show that rhamnolipid has a significant concentration dependence on the regulation of the fractal dimension of lignite, and the fractal dimension of each scale reaches a peak at the optimal concentration of 0.5%. Quantitative analysis of structural parameters shows that the 0.5% concentration treatment significantly increased the average surface porosity of lignite from 6.91% to 11.88%, with an increase of 72.2%. The contribution rate of fracture porosity increased from 2.16% to 4.88%, forming a continuous dominant seepage channel. Mechanism studies have shown that rhamnolipid achieves multi-scale pore-fracture reconstruction through the synergy of physical dissolution and chemical modification. SEM observations have revealed that the 0.5% concentration treatment forms a typical “honeycomb-like” porous structure on the surface of the coal body, and the irregularity of the pore edges is significantly enhanced. XRD analysis shows that the spacing of the aromatic layers increased from 0.359 nm to 0.366 nm, and the stacking height of microcrystals decreased from 8.467 nm to 3.721 nm, providing favorable conditions for pore development. FTIR analysis confirmed that at this concentration, the content of oxygen-containing functional groups on the coal surface reached its peak, significantly enhancing the hydrophilicity of the coal body. This study constructed a multi-scale fractal characterization system of “molecular – microcrystalline – microscopic – macroscopic”, confirming that the fractal dimension is a key quantitative indicator for evaluating the modification effect of biosurfactants and revealing its synergistic mechanism of “dissolution – pore expansion – functional group introduction – structural complication”. It provides a theoretical basis for the application of green biosurfactants in the permeability enhancement of low permeability coal seams and the structural modification of coal reservoirs. • Multi-scale system tracks fractal evolution in rhamnolipid-modified lignite. • Fractal dimensions show concentration dependence, peaking across all scales at 0.5%. • Fractal analysis reveals cross-scale synergistic mechanisms of pore evolution.