Experimental Study of Desorption and Seepage Characteristics of Single Gas and CO2–CH4 Gas Mixture in Coal

Understanding the desorption and seepage behaviors of CO 2 gas and CO 2 –CH 4 gas mixture in coal is important for CO 2 geological sequestration and enhanced coalbed methane recovery. An experimental study of desorption and seepage in coal under the action of a single gas (CO 2 ) and a gas mixture (50% CH 4 , 50% CO 2 ) was conducted under the influence of the pore pressure and stress environment. The results indicated that the CO 2 permeability of coal increased with increasing pore pressure and decreased with increasing σ 1 and σ 3 . CO 2 desorption increased with increasing pore pressure, and the Dubinin–Astakhov model suitably captured the CO 2 desorption behavior. Under the same hydrostatic and pore pressure conditions, the seepage volume of CO 2 was significantly larger than the desorption volume. Regarding the considered CO 2 –CH 4 gas mixture, the seepage volume decreased exponentially as a function of time and the desorption volume increased exponentially over time. Regardless of seepage or desorption in the tests, the CH 4 concentration in the CO 2 –CH 4 gas mixture at the outlet was always higher than that of CO 2 , which is closely related to the gas pressure, microscopic composition of coal, and coal rank. An increase in coal rank could increase vitrinite reflectance, due to the higher affinity of CH 4 for vitrinite, and due to the relatively low solubility of CO 2 under low pressures, preferential CH 4 desorption occurred. As the test proceeded, the concentration of desorbed or seeped CH 4 decreased gradually, the concentration of CO 2 continued to increase, and finally, the relative concentrations of these two gases equalized eventually.