Research on dissolution-diffusion characteristics of carbon dioxide in fractured saline aquifers

As the dominant channels of CO2 rapid diffusion and escape in the process of CO2 sequestration in saline aquifers, it is urgent to deeply understand the effect of fractures on CO2 transport and sequestration. In this paper, a coupling simulation model for CO2 convective-diffusion transport in fractured saline aquifers was advanced, which was implemented on the platform of COMSOL Multiphysics 6.0. Through the numerical simulation, the characteristics of CO2 convection-diffusion in fractured saline aquifers were analyzed, the mechanisms of the fracture width, inclination angle and combination characteristics on CO2 diffusion were revealed, and the behavior of CO2 diffusion and migration in large-scale fractured saline aquifers with discrete fracture network was further investigated. The results show that fractures play a time-dependent dual effect on the dissolution and diffusion of CO2 in saline aquifers, which increases with the increment of fracture width. In a short time, the fracture can enhance the rise of brine backflow to inhibit the diffusion of CO2, and disturb the uniform development of CO2 fingerings, resulting in the decline of CO2 concentration; while in a long time, it can act as a preferential channel for CO2 diffusion to promote the dissolution and storage of CO2. The process of CO2 dissolution and diffusion can be divided into three stages: CO2 rapid diffusion, relatively-stable diffusion and slow diffusion. The fluctuation of CO2 dissolution in the relatively-stable diffusion stage is closely related to the backflow intensity caused by fractures with varying width. The fracture inclination angle and combination characteristics can affect the CO2 diffusion path and the location and intensity of saline backflow. Under the condition of a single fracture, dissolved CO2 diffuses inward through both ends of the low-angle fracture, and reflux occurs in the middle of the fracture. For high-angle fractures, dissolved CO2 diffuses upward through the lower oblique end of the fracture, and reflux occurs more concentrated at the upper oblique end of the fracture, which is stronger than that of the low-angle fracture. Under the combined fracture condition, the backflow caused by horizontal parallel fractures is similar to that caused by low angle fractures, and the backflow caused by intersecting fractures is mainly concentrated in the smaller area above the intersecting fractures, while the inclined parallel fractures help to alleviate the backflow effect at the upper end of high-angle fractures. For large-scale fractured saline aquifers, highly developed intersecting fractures, as the dominant channel of CO2 diffusion, can enhance the backflow effect in under-developed fracture areas or isolated fracture areas, inhibiting the development of CO2 fine fingerings and reducing the number of fingerings. They also can accelerate the diffusion of CO2 to the depth area of the saline aquifers to enhance the dissolution and storage of CO2. This work will improve the understanding of CO2 transport-sequestration mechanism in fractured saline aquifers, and can also provide a valuable guidance for evaluating the safety of CO2 sequestration in fractured saline aquifers.