Experimental study for visualizing CO-dissolved water plume migration under hydraulic gradient conditions and implication for field monitoring data.

Investigating the possible direction of a CO-dissolved water plume migration near the potential CO leakage area is a significant task because it helps estimate the spatial and temporal monitoring scale to detect the signal of released CO from the storage. Accordingly, the Korea CO Storage Environmental Management (K-COSEM) research center tried to develop an intensive monitoring system and applied it to the artificial CO release test in the actual field. Monitoring data from the field tests depicted the horizontal movement of the CO-dissolved water plume along the direction of the groundwater flow. However, it remains unclear how the CO-dissolved water plume migrates vertically and how gas accumulation occurs near the capillary zone. The present study simulated the CO release test with a visual expression method utilizing a Hele-Shaw cell with hydraulic gradient conditions (i = 0, 0.1, and 0.01) and tried to estimate the significant influences on a diffusive-advective transport of the dissolved gas plume with the shallow aquifer condition. The visualization experiment results were intuitively verified to determine whether the theoretical principles of action related to plume flow applied in this context. The results suggest that a CO-dissolved water plume is distributed by hydraulic gradients and density-driven CO convective flow. The plume shape, center, and area were analyzed using an image analyzer program; the results demonstrated that the plume characteristic evolved depending on the significant effects on the plume. When the plume was mainly affected by the hydraulic gradient, it rapidly moved from the injection point to the last boundary; in contrast, when it was influenced primarily by density-driven CO convective flow, it flowed diagonally downward in the shape of varied branches. The numerical model calculated the migration of the CO-dissolved water plume affected by both factors. The laboratory experiment and numerical simulation results suggest that the migration of a CO-dissolved water plume may be affected by the hydraulic gradient and density-driven CO convective transport. As such, these factors should be considered when designing and analyzing CO monitoring signals to detect CO leaks from shallow aquifer systems.