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

Understanding the desorption and seepage behaviors of CO2 gas and CO2–CH4 gas mixture in coal is important for CO2 geological sequestration and enhanced coalbed methane recovery. An experimental study of desorption and seepage in coal under the action of a single gas (CO2) and a gas mixture (50% CH4, 50% CO2) was conducted under the influence of the pore pressure and stress environment. The results indicated that the CO2 permeability of coal increased with increasing pore pressure and decreased with increasing σ1 and σ3. CO2 desorption increased with increasing pore pressure, and the Dubinin–Astakhov model suitably captured the CO2 desorption behavior. Under the same hydrostatic and pore pressure conditions, the seepage volume of CO2 was significantly larger than the desorption volume. Regarding the considered CO2–CH4 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 CH4 concentration in the CO2–CH4 gas mixture at the outlet was always higher than that of CO2, 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 CH4 for vitrinite, and due to the relatively low solubility of CO2 under low pressures, preferential CH4 desorption occurred. As the test proceeded, the concentration of desorbed or seeped CH4 decreased gradually, the concentration of CO2 continued to increase, and finally, the relative concentrations of these two gases equalized eventually.