Effect and mechanism of ultrasonic mechanical vibration on methane adsorption

Ultrasonic stimulation technology with mechanical vibration effect and thermal effect offers unique advantages for promoting coalbed methane desorption and improving reservoir permeability. Recently extensive studies have been conducted on the effects of ultrasound on reservoir physical characteristics and methane adsorption behavior. However, most studies have neglected the influence of each effect as an independent variable on methane adsorption. Particularly, there has been inadequate attention given to the adsorption behavior of methane by coal under ultrasonic mechanical vibration. The coupling relationship among coal-water-methane molecules under ultrasonic mechanical vibration has yet to be revealed. Here, anthracite and bituminous coal were used as research objects, and the variable separation of the ultrasonic mechanical vibration was carried out using a constant temperature device. The effect and mechanism of ultrasonic mechanical vibration on methane adsorption were revealed by isothermal adsorption experiments. The results showed that ultrasonic mechanical vibration had a certain promotion effect on methane adsorption, which was opposite to the comprehensive effect. However, this promotion effect was influenced by the coal rank, moisture condition, and pressure. The effect of ultrasonic mechanical vibration on the molecular structure of the coal was an internal mechanism that enhanced methane adsorption. Among them, the methyl structure of the aliphatic side chain, the hydroxyl structure, the aromatic hydrogen structure, and the oxygen-containing heteroatom structure were sensitive to ultrasonic mechanical vibration. This influence improved the orientation degree of the coal molecular structure and increased the coal's surface free energy and adsorption potential, resulting in the ability of the coal to adsorb more methane molecules under the same pressure conditions. The content of sensitive functional groups was the main reason for the difference in promotion effect. There were more sensitive functional groups in bituminous coal, leading to a more significant promotion of methane adsorption by ultrasonic mechanical vibration. Furthermore, the moisture content would also affect the promotion effect. Moisture in coal strengthened the effect of ultrasonic mechanical vibration on sensitive functional groups by facilitating ultrasonic propagation through the coal. Therefore, in moisture-equilibrated samples, ultrasonic mechanical vibration had a greater effect on methane adsorption.