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

Adsorption characteristics of CO 2 –CH 4 –H 2 O-coal matrix systems are extremely important for investigating geological CO 2 -storage and enhanced coalbed methane recovery. To study the adsorption mechanism of CH 4 and CO 2 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 CH 4 and CO 2 on coal samples under equilibrium water condition. From these experimental results, the parameters including Gibbs free energy change ( ΔG ), entropy change ( ΔS ), maximum adsorption volume, selectivity coefficient ( α ) and Henry’s constant ( K H ) were extracted to discuss the influence of moisture on the CH 4 and CO 2 adsorption behavior of low-rank coals. The results show that water in low-rank coal had an “inhibitory effect” on the adsorption of CH 4 and CO 2 . The maximum theoretical adsorption capacity of CO 2 ( a 2 ) in each coal sample was greater than that of CH 4 ( a 1 ), and the absolute value of the CO 2 entropy change ( ΔS 2 ) was greater than that of the CH 4 entropy change ( ΔS 1 ), indicating that low coal-rank coals had higher adsorption capacity of CO 2 than that of CH 4 . For the equilibrated-water isothermal adsorption results, the differential adsorption of CH 4 and CO 2 was due to differences in coal maceral compositions and the development of micro- and meso-pores in different coals. It was concluded that CO 2 enhanced coalbed methane technology has better application potential in low-rank coal reservoirs than in medium-to-high rank coal reservoirs.