Interactions of dynamic supercritical CO_(2) fluid with different rank moisture-equilibrated coals: Implications for CO_(2) sequestration in coal seams

The interactions between CO_(2) and coals during CO_(2)-ECBM(CO_(2) 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 CO_(2) and CH_(4) within coal reservoirs.To simulate CO_(2)-ECBM process more practically,the dynamic interactions of supercritical CO_(2)(sc CO_(2))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 sc CO_(2)/H_(2)O 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 sc CO_(2)/H_(2)O exposure decrease.The multi-fractal analysis verifies that the sc CO_(2) exposure can enhance the pore connectivity of various rank coals.Apart from the pore morphology,the exposure of sc CO_(2)/H_(2)O also affects the oxygenic functional groups on coal surface.Particularly,the exposure of sc CO_(2)/H_(2)O reduces the content of CAO and C@O 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^(-1)1#coal and Kaiyuan-9#coal witness a growth in COOH.The content of total oxygenic functional groups of all coals after interaction with sc CO_(2)/H_(2)O decreases;on the contrary,that of CAC/CAH of all coals after sc CO_(2)/H_(2)O exposure increases.In summary,the interaction with sc CO_(2)/H_(2)O significantly changes the pore system and oxygenic functional groups of various rank coals,which needs further attention regarding CO_(2)-ECBM.