A numerical analysis of background flow velocity effects on long-term post-injection migration of CO₂ plumes in tilted storage aquifers

Even though groundwater flow exists in many saline aquifers, very few studies have investigated its significance on the injected CO₂ migration and trapping processes. Here, a numerical simulation approach is used to study the late post-injection migration and trapping of CO₂ injected into a tilted aquifer. The analysis highlights that although the migration of the CO₂ and its dissolution in brine is induced by buoyancy, the existence of background flow can further affect the plume velocity, convective dissolution, the dissolved CO₂ flux and its distribution in the storage complex. Our analysis shows that the background flow removes the residual CO₂, by dissolution, before the convective dissolution of the mobile part becomes dominant. The plume decelerates during its vertical migration by a factor of 6.5; then, its height increases with time to more than 15% as background flow velocity increases, hence reducing its rate of deceleration. However, when the plume reaches its maximum height, it migrates with a constant velocity. Greater background flow velocity not only allows the plume to migrate further, but it may hinder CO₂ dissolution. This is because it can transport the dissolved CO₂ underneath the plume for a long time, thus slowing down the interaction at the CO₂-brine interface. The weak and strong background flows can impact the tendency of the dissolved CO₂ to persist underneath the caprock. Our results indicate the existence of a critical background flow velocity which can control the distribution of the dissolved CO₂ at the bottom of the aquifer, further away from the caprock.