Summary Tracking carbon dioxide (CO2) flow and its consequent effects in subsurface rocks has received considerable attention in geological carbon sequestration. However, existing research on CO2 flow in reservoir-caprock systems with well-developed pores and microfractures is limited, and our understanding of its mechanisms remains incomplete. Here, we present the first observation of gaseous and supercritical CO2 (scCO2) behavior within a reservoir-caprock couplet using high-resolution synchrotron X-ray imaging. We found that high-speed gaseous CO2 flow reshaped part of the fracture framework but did not induce additional secondary fractures. High-speed scCO2 caused significant fracturing, connecting natural fractures in shale with newly developed secondary branches and creating larger pore spaces in sandstone. Consequently, the simulated permeability increased by approximately 2.6-fold in shale and 8.6-fold in sandstone. The concentrated strains around the main fracture in shale and the web-like strain patterns along granular mineral boundaries in sandstone highlight the distinct modes of scCO2 action during its passage through the reservoir-caprock system. This work provides new insights into the complex reservoir-caprock system and offers practical guidelines for fluid injection and production activities.
Wang et al. (Wed,) studied this question.