Experimental Study on Replacing Coal Seam CH₄ with CO₂ Gas

In recent years, many studies have reported the mechanism of CH₄ stimulation by gas injection. However, the studies have focused only on monitoring CH₄ and CO₂ in the tail gas. Thus, it is difficult to distinguish the adsorbed and free gas in the coal and rock and accurately calculate the CO₂/CH₄ replacement ratio in the displacement process. The low-field NMR technology can effectively overcome the drawbacks of the traditional displacement experiments and distinguish the free and adsorbed gas in the coal and rock. In the present study, the NMR technology analyzed the T ₂ spectrum for the CH₄ desorption amount and CO₂/CH₄ displacement efficiency in the replacement of methane with gaseous CO₂. The results suggested the following: (1) the process of CO₂ gas replacing CH₄ can be divided into three stages: the initial stage of competitive adsorption, the dominant stage of competitive adsorption, and the weakening stage of competitive adsorption. (2) The cumulative desorption of CH₄ gas increases with the increase in replacement time. With the increase in temperature, it first increases and then decreases, and the extreme value is obtained at about 40 °C. Additionally, the greater the CO₂ injection pressure is, the greater the cumulative desorption of CH₄ is. (3) The cumulative replacement ratio is positively correlated with the replacement time, and with the increase in replacement time, the increment in the cumulative replacement ratio decreases gradually and the upward trend tends to be stable. Overall, the cumulative displacement ratio would increase with an increase in the CO₂ injection pressure. With the increase in temperature, the maximum value of the cumulative replacement ratio first increases and then decreases, and the extreme value obtained is about 5.49 at 40 °C.