Industrial CT Experimental Study on Coal Fractures Induced by Short-Duration Exposure to Supercritical Carbon Dioxide.

Enhanced coalbed methane (ECBM) technology offers the potential to increase coalbed gas extraction efficiency while concurrently enabling CO2 sequestration, thereby facilitating the realization of dual carbon goals. The structure and distribution of coal fractures determine the effectiveness of supercritical carbon dioxide (SC-CO2) fracturing, making them crucial for enhancing coalbed gas extraction efficiency. Under prolonged exposure, SC-CO2 induces adsorption expansion and dissolution-extraction effects on coal, altering the morphology of coal fractures. However, the influence of these two effects on coal fractures remains unclear during the short duration of fracturing. Hence, research was conducted on the evolution of coal fractures under short-term SC-CO2 exposure. CT scanning was employed to study the variation of fractures in different metamorphic coals with cumulative immersion time. A relationship between immersion time and fracture evolution was established by constructing a grayscale distribution function of CT two-dimensional scanning images, quantitatively characterizing the changes in coal fractures. XRD experiments were combined to investigate the influence of immersion time on the variation of coal substance composition, elucidating the main reasons behind the evolution of coal fractures under short-term SC-CO2 exposure. The results indicate that under short-term SC-CO2 exposure, adsorption expansion leads to coal fracture contraction, while dissolution-extraction induces fracture expansion. During coal immersion, these two effects occur simultaneously, with their intensities changing over time, alternately dominating coal fractures during different time intervals. The degree of metamorphism affects the strength and dominant time of these two effects.