Numerical study of the fate of CO2 purposefully injected into the sediment and seeping from seafloor in Ardmucknish Bay

To quantify the impact of leaked CO2 purposefully stored in subsea geological formations on the marine ecosystem, CO2 gas was injected into sandy sediments in a small bay in Scotland in 2012. Alongside the experiment, a numerical study was conducted to predict CO2 fate in the bay. CO2 may take the form both of the gas and dissolved phases when it seeps out from the seafloor. The bubble CO2 rises in the water column forming bubble plumes and dissolves into the seawater during its ascent. Measurements indicated that approximately 8–15% of the injected CO2 escaped the sediments in the gas phase and no empirical evidence was seen for fluxes in the dissolved phase. Therefore, it is thought that 85–92% of the CO2 remained within the sediments. However, the results of our numerical study suggest that 10–40% of the injected CO2 stayed in the sediment. Apart from unexpected errors in the present numerical simulation, a possible explanation for this discrepancy may be the heterogeneous nature of the sediment and observations limited in time and space. It is also recognised that the CO2 concentration away from the injection site is undetectably small and that the readily detectable signal is confined to a small area in the vicinity of the injection point.