CT experimental study on the fracture evolution of coal body under supercritical CO2 short duration action

Abstract

The enhanced coalbed methane (ECBM) technology has the potential to enhance coal seam gas extraction efficiency, while also achieving CO2 storage, thereby promoting the dual carbon reduction goals. It holds broad prospects for its application in unconventional natural gas development. The fracture structure and distribution within coal seams determine the effectiveness of supercritical carbon dioxide (SC-CO2) fracturing, which is pivotal in enhancing coal seam gas extraction efficiency. Under prolonged exposure, the adsorption swelling and dissolution-extraction effects generated by SC-CO2 interacting with coal can alter the morphology of coal fractures. However, during the fracturing process, the contact time between SC-CO2 and coal is relatively short, thus the impact of the adsorption swelling and dissolution-extraction effects of supercritical carbon dioxide on coal fractures under short-term conditions remains unclear. Therefore, this study investigated the evolution of coal fractures under a short-term SC-CO2 exposure. Different metamorphic coal samples were studied through CT scans to observe the variations in fractures with accumulated immersion time. A grayscale distribution function of CT two-dimensional scan images was established, and the relationship between immersion time and fracture evolution patterns was constructed. This was combined with the XRD experiments to examine the impact of immersion time on the changes in coal composition. The primary factors driving the evolution of coal fractures under a short-term SC-CO2 exposure were thus clarified. The results indicate that under a short-term SC-CO2 exposure, the adsorption-induced swelling leads to the contraction of coal fractures, while the dissolution-extraction effect causes the fractures to expand. During the coal immersion process, both the adsorption-induced swelling and the dissolution-extraction effect occur simultaneously, with their intensities changing over time. Within different time intervals, they alternately dominate the evolution of coal fractures. Different metamorphic coals affect the intensity and dominant time of adsorption expansion and dissolution extraction. The adsorption and expansion dominated when lignite was immersed for 30 min, and the dissolution and extraction increased and dominated when it was immersed for 90−240 min. The adsorption and expansion were stronger when bituminous coal was immersed for 30−90 min, and the dissolution and extraction were dominant when it was immersed for 90−240 min. Adsorption and swelling dominated when anthracite was immersed for 30−150 min, and dissolution and extraction dominated when it was immersed for 150−240 min.

Keywords

• sc-co2
• short-term action
• fissure development
• adsorption expansion
• dissolved extraction