Effects of the Supercritical ${\mathrm{CO}}_2$ Exposure Duration on Coal Permeability and Microstructural Changes

The injection of ${\mathrm{CO}}_2$ into deep, unmineable coal seams is an effective strategy for reducing greenhouse gas emissions. At depths greater than 800 m, the carbon dioxide transitions to the supercritical state (${\mathrm{Sc}\text{-}\mathrm{CO}}_2$). This study investigated the complex interactions between ${\mathrm{Sc}\text{-}\mathrm{CO}}_2$ and coal, which substantially affected the pore structure, mineralogy, and permeability of the coal. We used low-pressure nitrogen gas adsorption (${\mathrm{LP}\text{-}\mathrm{N}}_2\mathrm{GA}$) to assess the impact of ${\mathrm{Sc}\text{-}\mathrm{CO}}_2$ on the pore characteristics of the coal. X-ray diffraction (XRD) was also used to determine the causal factors. We performed comparative triaxial permeability tests before and after ${\mathrm{Sc}\text{-}\mathrm{CO}}_2$ exposure to evaluate the changes in permeability for various coal types. Our findings suggested that ${\mathrm{Sc}\text{-}\mathrm{CO}}_2$ exposure markedly increased the complexity of the coal pore structure, which in turn affects coal-rock permeability. Specifically, a 10-day exposure period resulted in considerable increases of 43.5% and 50.9% in the pore volume and the specific surface area, respectively, along with a slight increase of 0.01 nm in the average pore diameter. Furthermore, there were notable decreases in the contents of minerals such as kaolinite, calcite, and pyrite, with decreases of 1.5%, 2.8%, and 2.2%, respectively, whereas the quartz content increased by 3%, indicating that significant mineral dissolution influenced the pore structure. A significant positive correlation was observed between the loss of coal mass and the increase in permeability. The effects of ${\mathrm{Sc}\text{-}\mathrm{CO}}_2$ were most pronounced in coals with low permeabilities, particularly during the initial phase of saturation. Subsequent saturation cycles and prolonged exposure resulted in a reduced rate for permeability enhancement, which eventually reached a plateau. This study underscores the critical role of ${\mathrm{Sc}\text{-}\mathrm{CO}}_2$ in long-term geological ${\mathrm{CO}}_2$ storage and improved efficiency for coalbed methane production.