6.1.1 Local environmental impacts and risks at offshore storage sites

Slow leakages of CO2 from a storage reservoir beneath the ocean would not generally pose an immediate threat to humans. In the open ocean, released CO2 will be partly dissolved in the water column, and any remaining CO2 escaping to the atmosphere will be mixed with air and rapidly diluted (Chadwick et al., 2008). For people on ships and offshore installations, the situation might however be critical in case they are located directly above the site of a catastrophic leakage. The risk that a ship might conceivably sink in a large rising gas bubble has not been assessed.

Leakage from offshore pipelines, wells, and reservoirs could adversely affect a larger area because of the dissolution and acidification of the surrounding seawater. They should be modelled with regard to possible CO2 concentrations in the pelagic and surface zones and biological effects assessed thereafter (Chadwick et al., 2008). Seepage from offshore geological storage sites might pose a hazard to benthic environments and organisms as the CO2 moves from deep geological structures through benthic sediments to the ocean. While leaking CO2 might be hazardous to the benthic environment, the seabed and overlying seawater will also act as a barrier, reducing the escape of seeping CO2 to the atmosphere. These hazards are distinctly different from the environmental effects of the dissolved CO2 on aquatic life in the water column.