Numerical analysis on the direction of groundwater flux effects on the early post-injection migration of a CO2 plume

Abstract

Several parameters have been analysed in the field of carbon dioxide (CO2) subsurface storage efficiency and security, including the aquifer dip and background water flow, for proper selection of CO2 storage aquifer sites. However, less importance has been assigned to the impact of the direction of groundwater flow on the migration of the injected CO2 plume, and hence the aquifer storage performance. This study investigates, using a numerical modelling method, the effect of the direction and magnitude of groundwater flux on the CO2 plume displacement and trapping in dipping aquifers during the early stages of the post-injection migration. The groundwater flow is established using water injection and production wells at the extremities of the model, while the direction of flow is altered from downdip to updip by swapping the locations of these wells. The study estimates and compares the extent of the total injected plume and the mobile portion of the plume, in addition to the instantaneous velocity of the leading tip of the plume, when groundwater flows downdip and when it flows updip. Our results indicate that when the groundwater flows downdip, counter to the overall direction of the CO2 plume migration, the plume is more stretched in both the updip and downdip directions than when groundwater flows updip. The size of mobile CO2 plume enlarges to a greater extent with downdip groundwater flow, causing the advancing tip to accelerate during its updip migration due to increasing buoyancy, with the effect being more pronounced at lower flow velocities. This increases the risk of CO2 leakage during the early stages of the plume migration when groundwater flows downdip. On the other hand, updip groundwater flow reduces the risk of leakage by reducing the velocity and extent of the plume, and increasing the impact of trapping mechanisms. Thus, the direction of groundwater flow can control the migration velocity of the CO2 plume in dipping storage aquifers, not due primarily to any induced viscous force, but due to the impact of trapping on buoyancy. Therefore, choosing the appropriate injection location for CO2 relative to the direction of flux is important for storage efficiency.