Effects of Oil Leaching on the Physicochemical Structure of Coal

To investigate the variation in the gas adsorption performance of oil-impregnated coal under crude oil intrusion conditions, infrared spectroscopy experiments, combined with the author’s research, analyzed the main active functional groups of raw coal and oil-impregnated coal and calculated the pore fractal dimensions of both. This study aims to elucidate the mechanism by which crude oil intrusion affects the gas adsorption performance. The total hydroxyl functional group content of oil-impregnated coal exhibited an average increase of 22.87% compared to the original coal, while the percentage decreased by 15.38%. The total content and percentage of aliphatic hydrocarbons and oxygenated functional groups increased significantly, whereas the total content and percentage of aromatics showed a minimal change. The aromaticity of oil-impregnated coal decreased, the content of aliphatic hydrocarbons increased, the length of its aliphatic chains increased, the number of branched chains decreased, the degree of aromatic cyclization was high, and the content of C═O increased. The dimensional shape D₁ of the percolation pore fraction of the oil-impregnated coal, based on N₂ adsorption experiments, exhibited values between 2.7149 and 2.7474, representing a decrease of 1.508% compared to the original coal. The adsorption pore D₂ exhibited values between 2.6694 and 2.7998, with an average reduction of 2.38% compared with the original coal. The fractional dimensional shape of oil-invaded coal from the piezo mercury experiments exhibited a range from 2.9598 to 3.1867. The pore structure of oil-impregnated coal is simpler than that of raw coal. It is hypothesized that the limiting adsorption of gas is reduced. The pore complexity is lower than that of the original coal. This leads to a weakening of the gas adsorption capacity of coal, and the gas adsorption performance of oil-impregnated coal is diminished compared with that of the original coal. This provides a reasonable basis for the prevention of gas drainage in coal–oil coexistence mines.