Effects of pore structure changes on the CH4 adsorption capacity of coal during CO2-ECBM

The effects of supercritical CO2 (scCO2) on pores in coal play a critical role in determining the CH4 adsorption and desorption characteristics and CO2 capacity of coal seams during CO2 geological storage-enhanced coalbed methane recovery (CO2-ECBM). To investigate the effects of pore structure changes caused by scCO2 on the CH4 adsorption capacity of coals, six coal samples with different ranks are exposed to scCO2 and water for 240 h at 62.5 °C and 15 MPa using a scCO2 geochemical reactor. Then, gravimetric isothermal adsorption tests, mercury intrusion porosimetry, N2 adsorption tests, and CO2 adsorption tests are performed to determine the CH4 adsorption capacity, pore volume, and pore specific surface area (SSA) of the coal samples with and without scCO2-H2O treatment. Finally, the influences of scCO2 on the pore structure and the effects of pore structure changes on the CH4 adsorption capacity of the coal samples are analyzed. The results show that the CH4 adsorption capacity of coal and its changes after scCO2-H2O treatment are strongly affected by density effects and pore selection effects. The CH4 adsorption capacity of coal is determined by the micropore volume and SSA; however, their changes only affect the CH4 adsorption capacity in the micropore filling stage when the pressure is <5 MPa with weak effects on the total CH4 adsorption capacity, which is related to their weak changes after scCO2-H2O treatment compared with macropores and mesopores. The changes in CH4 adsorption capacity mainly depend on the changes in macropore and mesopore SSAs after scCO2-H2O treatment; scCO2 mainly changes the CH4 adsorption capacity in the transitional stage, when the pressure is 5–10 MPa, and the monolayer and multilayer adsorption stage, when the pressure is >10 MPa. Meanwhile, the effect of pore structure changes on the CH4 adsorption capacity of coal with different ranks is complex. The CH4 adsorption capacity of subbituminous coal and medium-volatile bituminous coal increases after scCO2-H2O treatment due to the increases in macropore SSA, that of high-volatile bituminous coal slightly decreases, and that of semianthracite coal and anthracite coal slightly increases because of the changes in mesopore SSA after scCO2-H2O treatment. However, the CH4 adsorption capacity of low-volatile bituminous coal increases, which is the result of changes in the SSA of the mesopores and macropores due to slight changes in the micropore volume and SSA.