OCEAN DISPOSAL OF CO2 AT INTERMEDIATE DEPTHS

One option to reduce atmospheric CO2 levels is to capture and sequester power plant CO2. Commercial CO2 capture technology, though expensive, exists today. However, the ability to dispose of large quantities of CO2 is highly uncertain. The deep ocean is one of only a few possible CO2 disposal options (others are depleted oil and gas wells or deep, confined aquifers) and is a prime candidate because the deep ocean is vast and highly unsaturated in CO2. The term disposal is really a misnomer because the atmosphere and ocean eventually equilibrate on a timescale of 1000 years regardless of where the CO2 is originally discharged. However, peak atmospheric CO2 concentrations expected to occur in the next few centuries could be significantly reduced by ocean disposal. The magnitude of this reduction will depend upon the quantity of CO2 injected in the ocean, as well as the depth and location of injection. From ocean scale mathematical models which compare the impact of releasing fossil-fuel CO2 into the ocean at various depths1-4, we conclude that a sequestration depth of about 1000 m is required to achieve desired residence times on the order of hundreds of years. Below we summarize several modes of CO2 injection which have been identified in the literature. All offer advantages and disadvantages. We conclude that the most feasible initial options will involve a release of liquid CO2 at intermediate depths of 1000 to 1500 m. Either an unconfined release leading to a buoyant droplet plume or a confined release leading to a dense CO2-seawater solution would work. The intermediate discharge depth represents a compromise between the depth of ultimate sequestration, current engineering technology and national jurisdiction limits. We outline steps toward implementation of these options, including needed studies on injector design, transport and costs, site selection and environmental and legal considerations. © 1995.