Pore/fracture structure and gas permeability alterations induced by ultrasound treatment in coal and its application to enhanced coalbed methane recovery

Abstract: Ultrasonic treatment is a highly potential method for reservoir permeability modification in enhanced coalbed methane recovery. Quantitative evaluations of ultrasonic induced changes in structure and permeability in coal are of great interest for potential field applications of ultrasound technology. In this study the scanning electron microscope (SEM) and low-temperature liquid nitrogen (LTLN) were used to examine the ultrasound induced alterations in coal pore/fracture, and further measured gas permeability in coal by coupling the ultrasound field and external stress. The SEM images show that ultrasound treatment can extend the original pore/fracture, increase the connectivity and remove barriers in gas seepage channel in coal. The LTLN data reveals that with the ultrasonic treatment (25 kHz, 18 kW, 2 hours), the maximum N2adsorption quantity of the tested samples increased by 106.5 % ∼ 258.4 %, and the average pore diameter increased by 37.06 % ∼ 81.40 %; the specific surface area increased by 94.80 % ∼118.15 %, and the total pore volume increased by 87.12 %∼ 252.55 %. It implies that coal pores in size ranging from 1 nm to 100 nm can be remarkably improved by ultrasonic treatment, which is significant beneficial to promote gas desorption/diffusion within coal matrix. Permeability tests demonstrate that gas permeability in stressed coal increased by 29.3%∼57.1% with the implement of ultrasound field (25 kHz, 18 kW, 30 minutes). Ultrasonic performance on permeability improvement is proportional to the treatment time, but the function of ultrasonic fracturing has time threshold effect, and after a certain period of treatment, the ultrasonic fracturing performance becomes very limited in stressed coal.