Mechanism of adsorption capacity enhancement of coal due to interaction with high-pressure scCO2-water system

Coal interaction with high-pressure scCO2-water may change methane adsorption capacity affecting CO2-ECBM efficiency. In the present work, we experimentally investigated mechanism of adsorption capacity enhancement of coal due to interaction with high-pressure scCO2-water system. Experimental pressure and temperature are 9 MPa and 40 °C, respectively, with reaction time 24 h, 72 h, 120 h, and 240 h. Organic structure, pore structure, methane adsorption capacity were measured to characterize scCO2 reaction with coal samples. It's found that high pressure leads to a more significant compaction of micro-pores comparing with meso-, macro or even larger pores or fracture. Pore compaction existed during the whole reaction time (240 h) during which specific surface areas of different coal samples expose to N2–H2O system decreased by 28∼38%. While SSAs (pores with size below 100 nm) of coal samples expose to scCO2-H2O system increased by 26∼70% from 24 h to 240 h after a decrease of 17∼38% at the initial 24 h, which is mainly due to pore expansion and emerged corrosion pores. However, pore filling caused by chemical reaction decreased pore volume and specific surface area. For pores larger than 100 nm, SSAs show an increasing trend expose to both scCO2-H2O system and N2–H2O system. SSAs of coal samples expose to scCO2-H2O system increased by 14∼51% from 0 h to 240 h. While they increased by 19∼36% after expose to N2–H2O system, which indicating that high pressure can also lead to meso- and macro-pore expansion and new hydro-fractures.