CO2-Rich Industrial Waste Gas as a Storage-Enhanced Gas: Experimental Study on Changes in Pore Structure and Methane Adsorption in Coal and Shale

A technology that directly injects CO₂-rich industrial waste gas (CO₂-rich IWG) into underground spaces for unconventional natural gas extraction and waste gas storage has received increasing attention. The pore characteristics of coal and shale in a coal-bearing rock series before and after CO₂-rich IWG treatment are closely related to gas recovery and storage. In this study, three coals ranging from low to high rank and one shale sample were collected. The samples were treated with CO₂-rich IWG using a high-precision geochemical reactor. The changes in the pore volume (PV), specific surface area (SSA), and pore size distribution of micropores, mesopores, and macropores were analyzed. The correlations between the Langmuir volume and the PV and SSA of the micropores and mesopores were analyzed. It was confirmed that for micropores, SSA was the dominant factor influencing adsorption capacity. The effectively interconnected pore volume was calculated using macropores to characterize changes in the sample's connectivity. It was found that the PV and SSA of the micropores in the coal samples increased with increasing coal rank. The CO₂-rich IWG treatment increased the PV and SSA of the micropores in all of the samples. In addition, for mesopores and macropores, the treatment reduced the SSA in the coal samples but enhanced it in the shale. The results of this study improve the understanding of the mechanisms of the CO₂-rich IWG treatment method and emphasize its potential in waste gas storage and natural gas extraction.