Interactions of dynamic supercritical CO2 fluid with different rank moisture-equilibrated coals: Implications for CO2 sequestration in coal seams

Abstract

The interactions between CO2 and coals during CO2-ECBM (CO2 sequestration in deep coal seams with enhanced coal-bed methane recovery) could change pore morphology and chemistry property of coals, thereby affecting adsorption, diffusion and flow capability of CO2 and CH4 within coal reservoirs. To simulate CO2-ECBM process more practically, the dynamic interactions of supercritical CO2 (scCO2) and moisture-equilibrated coals were performed at temperature of 318.15 K, pressure of 12.00 MPa, and duration of 12.00 h. The impacts of the interactions on physicochemical properties of coals were investigated. Results indicate that scCO2/H2O exposure shows minor effect on micropores of coals. However, the exposure significantly decreases the mesopore surface area of bituminous coals, while increases that of anthracites. The mesopore volume and the average mesopore diameter of all the coals after scCO2/H2O exposure decrease. The multi-fractal analysis verifies that the scCO2 exposure can enhance the pore connectivity of various rank coals. Apart from the pore morphology, the exposure of scCO2/H2O also affects the oxygenic functional groups on coal surface. Particularly, the exposure of scCO2/H2O reduces the content of CO and CO of coals. The content of COOH of low rank coals including Hehua-M2# coal, Zhongqiang-4# coal, Buliangou-9# coal and Tashan-5# coal decreases, while the high rank Laochang-11# coal and Kaiyuan-9# coal witness a growth in COOH. The content of total oxygenic functional groups of all coals after interaction with scCO2/H2O decreases; on the contrary, that of CC/CH of all coals after scCO2/H2O exposure increases. In summary, the interaction with scCO2/H2O significantly changes the pore system and oxygenic functional groups of various rank coals, which needs further attention regarding CO2-ECBM.