Study on adsorption properties of coal for methane and deuterated methane

Coal seam water injection is a comprehensive disaster outburst prevention measure, which is often applied to the prevention and control of gas, dust and rockburst. Therefore, obtaining the law of water distribution in coal-water-methane three-phase system is of great theoretical significance to reveal the mechanism of outburst prevention, rock burst prevention and dust prevention for coal seam water injection. For the measurement of water distribution in the coal, water and methane three-phase system, nondestructive testing is a preferred method. Nuclear magnetic resonance testing technology has its unique advantages for the measurement of water distribution. However, when NMR is used to measure the law of water distribution in the coal-water-methane three-phase system, the hydrogen atom in the methane molecule and the hydrogen atom in the water molecule interfere with each other, which makes it impossible to accurately obtain the law of water distribution. In this paper, it is proposed to use deuterated methane (CD4) instead of methane (CH4) to study the law of water distribution in gas-bearing coal. It is necessary to first obtain the adsorption characteristics of coal for deuterated methane and methane. The adsorption capacity and the isosteric adsorption heat of coal for deuterated methane and methane were studied by experimental method and molecular dynamics simulation in this paper. The results show that the difference of deuterated methane and methane adsorbed by coal is very small, and the adsorption capacity and adsorption equilibrium pressure meet the Langmuir functional relationship. The isosteric adsorption heat of coal for deuterated methane and methane decreases with the increase of adsorption equilibrium pressure, and the isosteric adsorption heat and gas pressure meet the functional relationship of negative index. At the same temperature, there is little difference in the adsorption heat of coal to deuterated methane and methane. The isosteric adsorption heat of coal for deuterated methane and methane increases with the decrease of temperature, and the isosteric adsorption heat has a limit value. The adsorption of deuterated methane and methane in coal is physical adsorption. Deuterated methane can be used instead of methane to test law of water distribution in gas-bearing coal by nuclear magnetic resonance. The results lay a theoretical foundation for quantitative study of water distribution in coal, methane and water three-phase system by NMR.