Effects of the Supercritical CO2 Exposure Duration on Coal Permeability and Microstructural Changes

Abstract

The injection of CO2 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 (Sc-CO2). This study investigated the complex interactions between Sc-CO2 and coal, which substantially affected the pore structure, mineralogy, and permeability of the coal. We used low-pressure nitrogen gas adsorption (LP-N2GA) to assess the impact of Sc-CO2 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 Sc-CO2 exposure to evaluate the changes in permeability for various coal types. Our findings suggested that Sc-CO2 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 Sc-CO2 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 Sc-CO2 in long-term geological CO2 storage and improved efficiency for coalbed methane production.