Enhancing coalbed methane recovery using high power ultrasonic excitation: A nano-micro -to-engineering scale study

High-power ultrasonic excitation can dilate sorptive/diffusive pores in coal, and promote coalbed methane (CBM) deliverability and recoverable reserves. This paper conducted multi-scale research, including nano/micro-scale pore alteration detection, gas permeability evolution tests and engineering-scale ultrasonic-excited CBM extraction. The results show that the maximum nitrogen sorption capacity increased from 2.3140 cm³/g and 8.6910 cm³/g to 3.5424 cm³/g and 12.3527 cm³/g, marking improvements of 53.09% and 42.13% for the two tested coals respectively after ultrasonic excitation. The specific surface area and pore volume within the mesopore size range (2–50 nm) significantly increased, benefiting gas desorption and diffusion. Gas permeability improved by 3.58–7.50 times, enhancing gas transport in fractures and the coal matrix. A miniaturized 18 kW ultrasonic transducer was developed and used for segmented stimulation in long water-filled boreholes. The borehole gas drainage data were monitored to evaluate coalseam remodeling, and it demonstrates that the gas concentration increased by 101.82%–119.90%, and the gas flowrate increased by 89.25%–331.32% in boreholes within 30 days after ultrasonic operation. This work marks the first successful field-scale implementation of ultrasonic technology for CBM recovery, revealing its transformative potential for CBM reservoir stimulation and establishing a foundational framework for advancing this innovative methodology.