Position and flux estimation of the unexpected seepage of CO2 purposefully stored in a subseabed geological formation

Abstract To mitigate global warming, the large volume of CO 2 released into the atmosphere must be reduced. CO 2 capture and storage (CCS) are promising means for this. In addition, CCS operators who inject and store CO 2 into subseabed geological formations may be legally required to monitor CO 2 leakage into seawater, even though such events are very unlikely; after detecting unusually high CO 2 concentrations, their social responsibility implies specifying the position and flux of the associated CO 2 seepage in the sea site. In this study, we adopted the adjoint marginal sensitivity method to predict such parameters based on the information recorded by a limited number of CO 2 sensors located on the seafloor. In particular, we newly derived the adjoint marginal sensitivity from the relationship between a source term and the measured concentrations. We successfully conducted two-dimensional tests and three-dimensional simulations with a realistic topography; tidal and ocean currents with CO 2 dispersion were time-forwardly simulated and the seepage information was efficiently evaluated via time-backward numerical tests. However, the limitation of this method also emerged. We accumulated knowledge on its practical utility and proposed some ways to reduce the estimation errors. It is also inferred that this method can be applied for the source detection of various marine pollutants and resources.