Mechanism and application of ultrasonic cavitation in promoting physicochemical modification of coal and gas extraction

To address the technical bottleneck of problematic coalbed methane (CBM) desorption under complex geological conditions, this study proposed water-based ultrasonic cavitation for enhanced coalbed methane recovery (WUC-ECBM). Utilizing self-developed multimodal ultrasonic cavitation reaction system, we employ magnetic levitation adsorption experiments, chemical characterization, scanning electron microscopy, and CT scanning. This is combined with molecular dynamics calculations and finite element analysis. The mechanism by which ultrasonic cavitation modifies coal and promotes CBM extraction is revealed, and engineering application process is concurrently designed. The results indicate that ultrasonic cavitation induced oxidative modification significantly increased the O element content in coal by 30.95 %, leading to enhanced surface polarity. Furthermore, the high temperature and pressure environment generated by cavitation collapse reduce the degree of aromatic condensation (with a minimum XBP of 0.06), thereby weakening the van der Waals forces. In addition, the high energy release from cavitation bubble collapse enhanced the complexity and connectivity of pore structure and formed new pores. Molecular dynamics and finite element calculations show that for treated coal, CH4 adsorption capacity is reduced by a maximum of 25.62 %, the desorption efficiency is improved by 80.73 %, and the desorption time is shortened by 68.42 %. The developed intelligent equipment for focused ultrasonic cavitation targeted promote extraction (IFUCP-30kW) innovatively combines hydraulic fracturing with ultrasonic cavitation. Through “staged advancement and staggered treatment” operational mode, uniform modification of coal reservoirs is ensured. This provides innovative solution for CBM extraction and holds broad prospects for engineering applications.