Time-Lapse Synchrotron Imaging of CO2 Flow in a Reservoir-Caprock Complex System: Implications for Geological CO2 Sequestration

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.