Experimental study on methane diffusion kinetics in three typical metamorphic coals

Abstract

During coalbed methane (CBM) depletion, the diffusion property of methane is one of the key factors affecting the production of CBM recovery. In this work, the pore structure of three typical coals with different metamorphic degrees are tested by scanning electron microscopy and low field nuclear magnetic resonance. The high precision diffusion experiment system is developed to study the diffusion property of methane under different pore structure, gas adsorption pressure and coal sample size. The effects of tortuosity and pore length on methane diffusion is quantitatively analyzed, and the multistage clustering effect of coal particle microscopic composition is discussed. The results show that the pore structure leads to different diffusion resistance, which affects the initial diffusion rate of methane. Coals with high metamorphic degree correspond to complex pore structure and low diffusion rate of methane, while coals with low metamorphic degree correspond to simple pore structure and high diffusion rate of methane. With the increase of methane adsorption equilibrium pressure, the maximum diffusion ratio increases gradually, but the growth rate decreases gradually. The diffusion behavior of methane is closely related to the size of coal particles, and this change has an obvious size effect. As the particle size gradually increases, the effective diffusion coefficient of coal samples changes inversely, but when the coal particle reaches the critical size range, the change of diffusion coefficient begins to stop. The structure model of multistage clustering of coal particles is established and the causes of coal particle size effect are revealed. The research results can effectively guide the development of CBM, gas migration during CO2 enhanced coalbed methane and CO2 geological storage.