Methane Adsorption/Desorption–Diffusion Behaviors in Water-Bearing Coals Based on Nuclear Magnetic Resonance

The methane adsorption/desorption–diffusion performance in coal is a key factor in determining the occurrence and development potential of coalbed methane. The relationship between methane adsorption/desorption–diffusion behaviors in coal and water was studied by a nuclear magnetic resonance experiment on middle- and high-rank coal samples from the Qinshui Basin, China. Based on low-field nuclear magnetic resonance theory and methods, the variation law of methane adsorption/desorption capacity with time was analyzed, the adsorption/desorption–diffusion properties of methane under different water pressures and moisture contents were studied, and the influence mechanism of water on the methane desorption–diffusion behavior in coal was revealed. The research findings indicate that, with the passage of time, the methane adsorption/desorption capacity exhibits a trend of rapid increase and then tends to be stable, and the methane desorption process in all samples showed varying degrees of hysteresis; moisture content will reduce the value of extreme methane adsorption/desorption capacity; the maximum desorption rate and average effective diffusion coefficient of methane decrease with the increase of moisture content. This research is of great significance to the development and production of coalbed methane.