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

Abstract To quantify the impact of leaked CO 2 purposefully stored in subsea geological formations on the marine ecosystem, CO 2 gas was injected into sandy sediments in a small bay in Scotland in 2012. Alongside the experiment, a numerical study was conducted to predict CO 2 fate in the bay. CO 2 may take the form both of the gas and dissolved phases when it seeps out from the seafloor. The bubble CO 2 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 CO 2 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 CO 2 remained within the sediments. However, the results of our numerical study suggest that 10–40% of the injected CO 2 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 CO 2 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.