CH4 and CO2 Adsorption Characteristics of Low-Rank Coals Containing Water: An Experimental and Comparative Study

Adsorption characteristics of CO2-CH4-H2O-coal matrix systems are extremely important for investigating geological CO2-storage and enhanced coalbed methane recovery. To study the adsorption mechanism of CH4 and CO2 under the equilibrium water state, this study conducted high-pressure mercury intrusion porosimetry, low-temperature N-2 adsorption, and high-pressure methane isothermal adsorption of CH4 and CO2 on coal samples under equilibrium water condition. From these experimental results, the parameters including Gibbs free energy change (Delta G), entropy change (Delta S), maximum adsorption volume, selectivity coefficient (alpha) and Henry's constant (K-H) were extracted to discuss the influence of moisture on the CH4 and CO2 adsorption behavior of low-rank coals. The results show that water in low-rank coal had an "inhibitory effect" on the adsorption of CH4 and CO2. The maximum theoretical adsorption capacity of CO2 (a(2)) in each coal sample was greater than that of CH4 (a(1)), and the absolute value of the CO2 entropy change (Delta S-2) was greater than that of the CH4 entropy change (Delta S-1), indicating that low coal-rank coals had higher adsorption capacity of CO2 than that of CH4. For the equilibrated-water isothermal adsorption results, the differential adsorption of CH4 and CO2 was due to differences in coal maceral compositions and the development of micro- and meso-pores in different coals. It was concluded that CO2 enhanced coalbed methane technology has better application potential in low-rank coal reservoirs than in medium-to-high rank coal reservoirs.