Four-region process modeling of offshore CO2 well blowouts

Interest in offshore geologic carbon sequestration is motivating risk assessment of subsea pipeline and well leakage in the Gulf of Mexico (GoMCarb project). In this study, we address whether rare offshore CO₂ blowouts result in CO₂ emissions at the sea surface. Four distinct regions or domains control CO₂ transport: (1) the CO₂-containing reservoir deep below the seafloor; (2) the well and pipeline system; (3) the water column; and (4) the atmosphere above the sea surface. We model these four regions by linking together three simulation models. The reservoir containing the CO₂ is tightly coupled to the well and pipeline and we use T2Well for modeling this coupled system. T2Well models two-phase flow in the porous medium and in the well-pipeline system based on Darcy's law and the drift-flux model, respectively. For CO₂ transport in the water column, we use TAMOC which models jet and buoyant plume flow in ambient seawater by discrete and Lagrangian particle and integral model approaches. For the atmospheric dispersion above the sea surface, we use the NRAP MSLR which is a simple nomograph approach derived from empirical data. Each upstream model feeds output to the next downstream model (one-way coupling). Preliminary T2Well simulations of a very rare large-scale CO₂ blowout scenario involving a 2-inch diameter hole in a pipeline feeding a CO₂ injection well results in 35 kg/s leakage rate at the seafloor. Using established methods, we predict that CO₂ emitted into the water column at this rate forms bubbles with an average size of ~0.5 mm which facilitates strong dissolution of CO₂ into the seawater. TAMOC results suggest that for a blowout in 50 m of water, less than 1% of the leaked CO₂ will make it to the sea surface, while in 10 m of water 94% of the CO₂ will arrive at the surface and further leak into the atmosphere. Using a 1% CO₂ concentration in air above the sea surface as the criterion for safety with a 1 m/s wind at an elevation of 10 m, the NRAP MSLR forecasts a downwind safety distance of 0.5 km for the 10 m case, and 90 m for the 50 m case. In general, our preliminary modeling finds that large CO₂ well blowouts, which will be very rare events, are unlikely to manifest as acute CO₂ emissions at the sea surface in deep water, while large blowouts in shallow water (<10 m) will result in strong emissions at the sea surface.