Structures and fractal characteristics of pores in long-flame coal after cyclical supercritical CO2 treatment

Injecting CO2 in deep coal seams for enhanced coalbed methane recovery (ECBM) also benefits the environment through simultaneous CO2. Most studies have concentrated on the changes of pore structures that result from sustained supercritical CO2 (ScCO2) treatment. Conversely, the effects of cyclic ScCO2 treatment have been rarely investigated. In this study, a high-pressure reactor system was used to perform cyclic ScCO2 treatment of longflame coal. Nuclear magnetic resonance techniques and low-pressure nitrogen gas adsorption were employed to determine the fractal dimensions of coal samples and quantitatively characterize their pore structures before and after cyclic treatment. The results demonstrated that the porosity and proportion of macropores increased significantly after treatment. Therefore, new pores were formed, and some small pores might have been converted into macropores. As the treatment duration increased, the daily average porosity rate exhibited an increasing trend that was presumably caused by the effects of coal matrix fatigue. Further, a decreasing tendency was captured in both the pore-surface fractal dimensions of adsorption pores and the pore-volume fractal dimensions of seepage pores, while increasing the cyclic treatment. This phenomenon after treatment indicated that pore roughness and complexity were decreased. A conceptual model was proposed to explain the mechanisms underlying the evolution of coal-pore structures during the treatments, and the effects of the cyclic injection on the CO2-ECBM field were also analyzed. Therefore, our findings have important guiding significance for selecting suitable CO2 injection methods for CO2-ECBM projects.