Chemical regulation of coal microstructure and study of water injection displacement gas law

In order to reveal the effect of changes in coal microstructure on gas desorption, this study selected coal samples from the Eighth Mine in Pingdingshan, Henan Province as the research object, and used two surfactants, 1-Butyl-3-methylimidazolium tetrafluoroborate and Sodium dodecyl sulfate, to soak the coal samples. The coal samples were investigated by low temperature liquid nitrogen adsorption, infrared spectroscopy, X-ray diffraction, the measurement experiment of adsorption constant a, contact angle measurement, and gas displacement and desorption experiment. The findings indicate that surfactants can dramatically alter the pore properties, molecular groups, and microcrystalline structure of coal. The specific surface area of the coal sample micropores decreases and the average pore diameter increases, so that the pore and fracture network is more smooth. The chemical functional groups of the coal samples change, the average chain length and the fat ratio decrease, and the oxygen enrichment degree increases. The aromatic ring layer spacing and aromaticity of the coal sample increase, while the aromatic layer stacking height and the number of aromatic stacking layers decrease, indicating that the structure arrangement of aromatic layer is more disordered and loose, the degree of condensation of aromatic ring is reduced, the arrangement of coal molecules is disrupted, and the range of ordered structure is narrowed. These changes in pore characteristics, molecular groups and microcrystalline structure greatly improve the wettability of coal and reduce the difficulty of solution intruding into coal samples, which helps to strengthen gas displacement and enhance the efficiency of gas extraction. At the same time, the solution in the pores of coal body increases, so after pressure relief, the coal sample gas desorption and discharge are blocked, the gas desorption rate decreases, and the water lock effect is enhanced. The results of this study provide a theoretical guidance for the exploitation of coal seam gas resources. By adding surfactants and other methods, the microstructure of coal is artificially changed, and then the gas desorption characteristics are directionally changed to improve the gas extraction efficiency. During coal seam mining, the sudden outflow of a large amount of gas can be avoided, so as to achieve an efficient and safe extraction of coal seam methane. © 2023 China Coal Society. All rights reserved.