Influence of Carbon Dioxide on the Adsorption of Methane by Coal Using Low-Field Nuclear Magnetic Resonance

The adsorption capacity of coal with respect to carbon dioxide (CO 2 ) is greater than that of methane (CH 4 ). Studying the CO 2 replacement of CH 4 in coal is essential to understanding the mechanism behind CO 2 -enhanced coalbed methane. In this paper, scanning electron microscopy was used to qualitatively study the influence of CO 2 injection pressure on the fracture characteristics of coal. The time effect of the CH 4 /CO 2 adsorption and the effect of the CO 2 injection on the CH 4 adsorption were quantitatively studied using low-field nuclear magnetic resonance. The results suggest three different CH 4 states in the coal samples: CH 4 adsorbed on the pore surface, free CH 4 in the pore center, and free CH 4 between the coal particles. Indeed, the greater the CO 2 injection pressure, the more developed the fracturing and network, which, in turn, increases connectivity. As the adsorption time of CH 4 /CO 2 increases, the adsorption rate of CO 2 /CH 4 gradually decreases. In essence, CO 2 preferentially replaces the CH 4 of the minipores. As the CO 2 injection pressure increases, the difference between the CO 2 equilibrium pressure and the initial injection pressure increases. Moreover, the CH 4 production rate increases, the transverse relaxation time ( T 2 ) spectrum of the adsorption-state area decreases, the T 2 spectrum of the free-state area increases, and the number of adsorption holes gradually decreases, whereas the number of seepage holes increases.