Effect of water content on the induced alteration of pore morphology and gas sorption/diffusion kinetics in coal with ultrasound treatment

Abstract Physical excitation using high power ultrasound is an anhydrous fracturing approach for CBM reservoir stimulation. However, the mechanism of ultrasound excitation and its critical influencing factors are still needed to be further investigated. This work focused on the effects of water contents in coals on the alterations of pore structure and gas sorption/diffusion behaviors caused by ultrasound treatment. The results demonstrate that the nitrogen adsorption amount of coal significantly increases with ultrasound treatment. The pores in coals with different water contents experience different degrees of structural alteration. Ultrasound treatment can increase the average pore diameter by up to ~74.98 % in the water-saturated coal, and by up to ~15.59 % in dry coal. The increment of total pore volume is estimated to be ~70.02 %-90.19 % for water-saturated coal, and ~24.67 %-31.02 % for the dry coal. It implies that the pore alteration is more significant in water-saturated coal than that in the low moisture coal with ultrasound stimulation. Because of pore structural alteration, ultrasound stimulation promotes gas sorption/diffusion process as the sorption amount increased by ~7.02 % −47.3 % and the diffusion coefficient increased by ~19.99 %-68.09 %. The effect of ultrasound treatment on the gas sorption and transport in the tested coal varied depending on the water content in coal. The increment of sorption/diffusion amount of the water-saturated coal is pronouncedly greater than that of low water-content coal. The outcome of this study provides justification for ultrasound modification on gas desorption/diffusion excitation and production improvement, especially for high water content CBM reservoirs.