Why giant CO2 bags on the bottom of the ocean should't explode

Via Inhabitat:

Plastic bags are the center of a huge green debate, but here’s one use we’d never imagined: Underwater ocean carbon sequestering!? With all the talk about carbon sequestration, which involves storing liquefied CO2 deep beneath the earth, Dr. David Keith from the Department of Chemical and Petroleum Engineering at the University of Calgary has devised a scheme to store it underwater in the ocean floor, contained in giant sausage-like plastic bags. Could this possibly be a good idea?

Just to give this perspective, visualize this: in order to hold around 160 million tons of CO2 (around 2.2 days worth of current global emissions), it would take a cylindrical bag measuring 100 meters in radius and several kilometers long. The obviously immense weight of the bag, as well as the pressure from the ocean, would contain the CO2 and prevent the bag from floating up. If you’re wondering where a gas-filled bag of this size might be located, it would sit 3 kilometers below the ocean’s surface. Just like in subterranean carbon sequestration, the CO2 would be pumped and moved via pipes to the bag.

Maybe it’s just us, but giant plastic bags filled with carbon dioxide, submerged in the oceans depths sounds like a recipe for disaster. What if one of those things leaks or explodes? And with that kind of deep-ocean pressure on a bag so large.. how would this really work?

I wish to address the "bag explosion" issue. Let's assume a few things:

1. The depth the bag is placed at is roughly 3800 m (the average ocean depth).
2. The temperature of the surrounding water is roughly 4 C (the average water temperature at that depth).

This means that the pressure on the bag (and the CO2 in the bag) is roughly 368bar, and the temperature of the CO2 will be roughly 277Kelvin. If you look at a phase diagram of CO2, you'll note that when CO2 is in this range of pressure and temperature, it is actually a liquid. (True, CO2 sublimates at ranges near standard temperature and pressure, but the ocean floor is anything but standard.)

It is likely, therefore, that if the bag bursts, there will not be a catastrophically large explosion. Instead, there will be a "pool" of liquid CO2 (CO2 is heavier than H2O) at the bottom of the ocean.

"So this isn't too bad," you say, right? Well, you have to remember that CO2 is also highly dissolvable in water. The pool of CO2 will slowly dissolve into the surrounding ocean waters - up to its saturation point, but here's the caveat. CO2 changes into carbonic acid when mixed with water:

CO2 + H20 <=> H2CO3

Near the atmosphere, this isn't really an issue, since there is gas exchange with the atmosphere, and the partial pressure of H2CO3 remains relatively low. However, at the ocean floor, this might be very different. (I'm not a physicist, nor a chemist, so I'll leave that train of thought there for now.)

The additional "downside" of all this is that you now have a pool of heavy liquid CO2 sitting on the ocean floor. That CO2, like any liquid, will slowly flow downhill, to the lowest point it can fill. This will mean that it could easily wash over organisms living on or near the sea floor (sponges, nudibranchs, etc.), suffocating them.

Therefore, long story short, the scary thought of an exploding bag of CO2 is not likely to occur (since the CO2 will change into a liquid at that temperature and pressure). However, a burst giant bag of CO2 will mean a liquid pool of suffocation streaming across the sea floor until it reaches a nadir.