Experimental study on the change of coal structure and microbial community structure during supercritical-CO₂-H₂O-microorganisms-coal interaction process

To study the effects on coal structure and microbial community diversity during CO₂ storage in deep unminable water-bearing coal seams, the lignite in the mining area of Inner Mongolia was selected as the research object. The simulation experiments of supercritical-CO₂ (ScCO₂)-H₂O-microbial-coal interaction under different pressure conditions performed by using an autoclave. Then the solid–liquid phases before and after the reaction were analysed by XPS, low-temperature liquid nitrogen adsorption, 16S rRNA and other testing methods. The results showed that CO₂ injection caused the pH and OD600 values in the system to decrease, and the pH and OD600 values in the system decreased more obviously with the increase of ScCO₂ pressure. Compared with raw coal, the relative content of carbon elements on the surface of the coal after the reaction increased, and the organic carbon (CC/CH) relative content decreased slightly. In contract, the specific surface area, total pore volume, and the pore size of micropores of coal increased, but the average pore size of the mesopores and macropores decreased. At the same time, with the ScCO₂ injection under different pressure conditions, some bacterial cells in the system ruptured and inactivated, resulting in the decrease their relative abundance. In contract, some genera, such as Paraclostridium and Methanoculleus, may have the adaptability to the extreme environment and gradually evolve into the dominant genus. In conclusion, the results provide a theoretical basis for studying the changes of ScCO₂ on coal structure and microbial community structure during CO₂ storage in deep unminable water-bearing coal seams.