Experimental Investigation of the Impact of Various Clean Fracturing Fluids on Coal Microstructure Modification

Fracturing fluids play a crucial role in hydraulic fracturing by significantly improving coal seam permeability and promoting coal seam gas desorption. To investigate the impact of clean fracturing fluids with various compositional ratios on coal microstructure, coal samples from the Hongyang Second Mine in China underwent a series of immersion tests with these fluids. The pore modifications of the coal samples were analyzed using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), and the chemical structures were characterized via energy-dispersive spectroscopy (EDS), X-ray diffractometry (XRD), and Fourier-transform infrared spectroscopy (FTIR). The results indicate that clean fracturing fluids can effectively improve the pore structure of coal samples. The fluid containing hexadecyl trimethyl ammonium chloride (CTAC) as the primary agent increased the pore fractal dimension, total pore volume, and coal sample permeability by 6.19%, 14.73%, and 11.85%, respectively, and reduced tortuosity by 12.50%. The chemical interaction between the clean fracturing fluid and the coal matrix dissolved some of the associated minerals and altered the microcrystalline structure of the coal samples. Notably, the clean fracturing fluid with CTAC as the main component enhanced the aromaticity (fa), crystallite height (Lc), crystallite diameter (La), and the length of aliphatic branched chains in the coal samples by 9.63%, 32.69%, 35.02%, and 12.22%, respectively. Among the experimental conditions, the combination of 0.8% by weight CTAC + 0.2% by weight NaSal + 1.0% by weight KCl had the greatest impact on the microstructural modification of coal. These findings provide theoretical guidance for selecting clean fracturing fluids and have significant implications for the efficient extraction and utilization of coalbed methane.