Synergistic mechanism of ultrasound-surfactant physicochemical coupling for multiscale coal wettability enhancement

Abstract

Improving coal wettability is critical for advancing water injection technologies that suppress coal mine gas and respirable dust hazards. This study developed an innovative ultrasonic-surfactant combinatorial treatment (USCT) and established a multimodal characterization platform integrating FTIR, nitrogen physisorption, and low-field NMR to elucidate its multiscale mechanisms of wettability enhancement. The results reveal that the SDBS surfactant treatment induces chemisorption of hydrophilic moieties (e.g., hydroxyl and oxygenated groups) through amphiphilic assembly, and the surfactant-mediated pore-throat blockage decreases the specific surface area and pore volume by 63.32 % and 37.90 %, respectively. Ultrasound-Surfactant physicochemical synergy triggers multiscale pore architecture evolution, the coal specific surface area increases by 58.45 % (from 2.8966 to 4.5897 m²/g) and pore volume expands by 12.50 % (from 0.0136 to 0.0153 cm³/g) through ultrasonic-induced pore structure redistribution. Notably, the content of hydrophilic moieties in coal exhibits negligible variation, demonstrating stable surface chemical characteristics during pore-structure remodeling. More importantly, the steady-state equivalent wetting pore diameter (R_g,48) decreases from 198.31 to 115.90 nm, indicating enhanced transformation of free water to adsorbed phases through confined hydration dynamics. This phenomenon stems from synergistic physicochemical modulation: the surfactant-derived chemical potential gradient reduces the capillary pressure threshold, while ultrasonic excitation restructures nanoscale bottleneck pores (2–50 nm) to overcome surfactant-induced flow “bridging-blockage”. The findings establish a novel coupled chemoacoustic activation mechanism for optimizing coal seam water injection, achieving simultaneous wettability enhancement and permeability recovery via multiscale fluid–solid interface engineering.