Study on the coal damage effects caused by the CO2-alkaline water two-phase displacing gas and wetting coal

Abstract The technique of CO2-alkaline water two-phase displacing gas and wetting coal can effectively prevent the risk of coal and gas outburst, while it will damage coal under the erosion of both acid and alkali. To explore the influence of this measure on the physical and chemical properties of coal, the variations of minerals and micro-structures in different coal samples were tested with several research methods including the XRD, XRF and FTIR. The results showed that, after adopting this method, the content of kaolinite and AI, Si and S decreased to different extent, and the content of carbonate, C and Na increased clearly. In the immersed water of DL, ZX and XT coal samples, the HCO3– disappeared completely, the Ca2+ and Mg2+ decreased markedly, while the content of CO32– increased observably, with the increment of 88.7%, 85.3%, 77.7%. The micro-structure in coal was more developed, there were more granular minerals distributed on the surface of coal, and the number and diameter of pore structure all rose. The total volume and the average diameter of pore structure in three coal samples increased distinctly, with the increment of 87.5%, 62.5%, 37.5% and 121.8%, 8.5%, 20.3%, respectively. In addition, the proportion of oxygen-containing functional groups in three coal samples increased significantly, with the increment of 11.4%, 41.4% and 4.5%. Therefore, this measure can obviously improve the effects of water injection, the hydrophily, and the water-holding capacity of coal, more effectively prevent the danger of coal and gas outburst.