Figure 6.9 Shallower than 2500 m, liquid CO. is less dense than sea water, and thus tends to float upward. Deeper than 3000 m, liquid CO. is denser than sea water, and thus tends to sink downwards. Between these two depths, the behaviour can vary with location (depending mostly on temperature) and CO. can be neutrally buoyant (neither rises nor falls). Conditions shown for the northwest Atlantic Ocean.

Figure 6.8 CO. sea water phase diagram. CO. is stable in the liquid phase when temperature and pressure (increasing with ocean depth) fall in the region below the blue curve; a gas phase is stable under conditions above the blue dashed line. In contact with sea water and at temperature and pressure in the shaded region, CO. reacts with sea water to from a solid ice-like hydrate CO.6H.O. CO. will dissolve in sea water that is not saturated with CO.. The red line shows how temperature varies with depth at a site off the coast of California; liquid and hydrated CO. can exist below about 400 m (Brewer et al., .004).

Figure 6.10 Liquid CO. released at 3600 metres initially forms a liquid CO. pool on the sea floor in a small deep ocean experiment (upper picture). In time, released liquid CO. reacts with sea water to form a solid CO. hydrate in a similar pool (lower picture).

3000 m, liquid CO2 is denser than the surrounding sea water and sinks. CO2 nozzles could be engineered to produce large droplets that would sink to the sea floor or small droplets that would dissolve in the sea water before contacting the sea floor. Natural ocean mixing and droplet motion are expected to prevent concentrations of dissolved CO2 from approaching saturation, except near liquid CO2 that has been intentionally placed in topographic depressions on the sea floor.

Solid. Solid CO2 is denser than sea water and thus would tend to sink. Solid CO2 surfaces would dissolve in sea water at a speed of about 0.2 cm hr-1 (inferred from Aya et al., 1997). Thus small quantities of solid CO2 would dissolve completely before reaching the sea floor; large masses could potentially reach the sea floor before complete dissolution.

Hydrate. CO2 hydrate is a form of CO2 in which a cage of water molecules surrounds each molecule of CO2. It can form in average ocean waters below about 400 m depth. A fully formed crystalline CO2 hydrate is denser than sea water and will sink (Aya et al., 2003). The surface of this mass would dissolve at a speed similar to that of solid CO2, about 0.2 cm hr-1 (0.47 to 0.60 ^m s1; Rehder et al, 2004; Teng et al, 1999), and thus droplets could be produced that either dissolve completely in the sea water or sink to the sea floor. Pure CO2 hydrate is a hard crystalline solid and will not flow through a pipe; however a paste-like composite of hydrate and sea water may be extruded (Tsouris et al., 2004), and this will have a dissolution rate intermediate between those of CO2 droplets and a pure CO2 hydrate. Behaviour of injected CO2 in the near field:

CO2-rich plumes As it leaves the near field, CO2 enriched water will reside at a depth determined by its density. The oceans are generally stably stratified with density increasing with depth. Parcels of water tend to move upward or downward until they reach water of the same density, then there are no buoyancy forces to induce further motion.

The dynamics of CO2-rich plumes determine both the depth at which the CO2 leaves the near-field environment and the amount of initial dilution (and consequently the amount of pH change). When CO2 is released in any form into seawater, the CO2 can move upward or downward depending on whether the CO2 is less or more dense than the surrounding seawater. Drag forces transfer momentum from the CO2 droplets to the surrounding water column producing motion in the adjacent water, initially in the direction of droplet motion. Simultaneously, the CO2 dissolves into the surrounding water, making the surrounding water denser and more likely to sink. As the CO2-enriched water moves, it mixes with surrounding water that is less enriched in CO2, leading to additional dilution and diminishing the density contrast between the CO2-enriched water and the surrounding water.

CO2 releases could be engineered to produce CO2 plumes with different characteristics (Chen et al., 2003; Sato and Sato, 2002; Alendal and Drange, 2001; Crounse et al, 2001; Drange et al., 2001; Figure 6.11). Modelling studies indicate that pH

Droplet concentration (kgCOj m"3)

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