The impact of background water flow on the early migration of a CO2 plume in a tilted aquifer during the post-injection period Mawda Awag*, Eric Mackay, Saeed Ghanbari *Institute of GeoEnergy Engineering, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom Abstract: The study presents a numerical modelling analysis on CO2 plume migration in a dipping storage aquifer with background flux, which incorporates residual and dissolution trapping of CO2. The purpose of this analysis is to investigate the effect of the background flow velocity on the CO2 plume migration during the early postinjection period. Different velocities of groundwater flow from low to high were considered in the aquifer model. The distribution, migration distance and velocity of the injected CO2 plume as well as the remaining mobile CO2 plume extent are estimated to determine how fast and far the plume propagates with time. Comparison of the results indicate that increasing the background flux velocity causes the plume to migrate longer distances up-dip, while it reduces the height distribution of the plume with time. This reduces the volume of mobile CO2 in the storage aquifer at larger velocities of background flux, hence decreasing the leakage risk of CO2 to the surface. In addition, the CO2 plume decelerates immediately after cessation of injection as its bottom rises vertically and the buoyancy force reduces as the thickness of the plume reduces. However, the plume then accelerates during the initial period of its subsequent lateral migration, as the plume becomes extended, and the buoyancy forces increases somewhat. The degree of lateral extension increases with increasing background water flow velocity, with the leading tip of the plume migrating faster than the trailing edge, until residual and dissolution trapping sufficiently reduce the volume of free phase CO2 that its migration is arrested.