Impact of fractures on convective-mixing characteristics of carbon dioxide in saline aquifers

Fractures serve as the main channels for rapid CO₂ diffusion and escape during sequestration in saline aquifers. While existing studies have focused on how fracture characteristics and chemical reactions affect CO₂ convective mixing, the specific mechanisms at each stage of CO₂ dissolution and sequestration are still not well understood. It is crucial to gain a deeper understanding of the overall impact of fractures on CO₂ transport and sequestration. This paper presents a new coupling simulation model for CO₂ convective–diffusion transport in fractured saline aquifers, developed using COMSOL Multiphysics 6.0. The numerical simulation analyzes how CO₂ convection–diffusion in fractured saline aquifers, focusing on the effects of fracture width, inclination angle, and combination characteristics on CO₂ convection and diffusion. It also examines CO₂ behavior in large-scale fractured saline aquifers with a discrete fracture network. The results show that fractures located in the middle of saline aquifers have a dual, time-dependent effect on CO₂ dissolution and diffusion, which becomes more pronounced as fracture width increases. Initially, these fractures enhance brine backflow, inhibiting CO₂ migration and disrupting the uniform development of CO₂ fingerings, leading to a decrease in CO₂ concentration. Over time, however, they become preferential channels for CO₂ migration, promoting its dissolution and storage capacity. In saline aquifers with fractures, the amount of dissolved CO₂ can increase significantly by 11.03% with a fracture width of 0.1 mm, and this increase continues as the fracture width increases. The CO₂ convective-mixing process is divided into three stages: rapid migration, relatively stable migration, and slow migration. The inclination angle and combination characteristics affect the CO₂ diffusion path and the location and intensity of saline backflow. In a single low-angle fracture, dissolved CO₂ migrates inward from both ends, with reflux occurring in the middle. By contrast, high-angle fractures caused dissolved CO₂ to migrate upward through the lower oblique end with more concentrated and stronger reflux at the upper oblique end compared to low-angle fractures. In aquifers with combined fractures, horizontal parallel fractures create backflow patterns similar to low-angle fractures. Intersecting fractures help reduce backflow in the smaller area above them, while inclined parallel fractures help reduce backflow at the upper end of high-angle fractures. In large-scale fractured saline aquifers, well-developed intersecting fractures serve as primary channels for CO₂ migration. They enhance the backflow effect in less developed or isolated fracture areas, inhibiting the formation of CO₂ fine fingerings and reducing their number. These fractures also accelerate CO₂ migration to deeper areas, improving its dissolution and storage. This work improves the understanding of CO₂ transport and sequestration mechanisms in fractured saline aquifers and offers valuable guidance for evaluating the safety of CO₂ sequestration in these environments.