The attenuation of ultrasonic waves in coal: the significance in increasing their propagation distance

As a new technology, characterised by an absence of pollution and low energy consumption, ultrasonic waves have been widely used to promote the fracturing of oil-bearing and coal bed methane reservoirs, so as to increase their permeability. The propagation of ultrasonic waves in coals, and its influencing factors, is the key factors determining ultrasonic wave behaviour in promoting the permeability of coal seams. In this research, a device for measuring the attenuation of the ultrasonic waves in coal and rock masses was used to measure the attenuation coefficient of ultrasonic waves in coal briquettes with different properties. The standing wave method was used to fit the attenuation curves of the ultrasonic waves in the coal briquettes with different variables. Meanwhile, a dynamic strain instrument was used to record the energy attenuation experienced by ultrasonic waves in fractured coals with different moisture contents. The experimental results demonstrated that increases in the number of the fractures, and the content and size of the impurities in the coal briquettes, all led to an increased attenuation coefficient of the ultrasonic waves. Meanwhile, the attenuation coefficient was positively correlated with the cube of the number of fractures, and the content and size of the impurities. Besides, it was discovered that with the increase in the moisture content in coal briquettes, the attenuation coefficient of the ultrasonic waves increased firstly, followed by a decrease, and the energy consumed by the ultrasonic waves was increased firstly before a reduction as it penetrated through the fractured coals of the same length and increased moisture content. Additionally, it was found that by injecting water into the coals fractured by ultrasonic waves, the attenuation coefficient of the ultrasonic waves in coals can be reduced. Therefore, the propagation distance of the ultrasonic waves in coals was increased.