Carbon capture and storage: where should it go?

Will Dalen Rice and a friendNote: This is a guest blog post from Will Dalen Rice, a graduate student in the Department of Geography and Earth Sciences at UNC Charlotte. He has the misfortune of taking a couple of courses from Anne this semester, and he’ll be contributing a few more blog posts here over the next few months.
Carbon capture and storage gets media attention along with global warming, but few media outlets have attempted to describe what it actually is. The importance of this technology lies in the truth that the largest chunk of our greenhouse emission are a result of creating power. One-third (1/3) of emissions in the world come from power plants producing 10 billion metric tons of CO2. So, the logical step in reducing CO2 levels (putting aside the obvious of “needing” less power) is to intercept them as they leave the power plant, preventing them from going up into the clouds. Once you have a cup full of liquid CO2 though, where do you put it? Hint: Where have we always put things we didn’t want to have around anymore?
As it turns out, the process of natural gas extraction already requires CO2 to be separated and dealt with. A Norwegian oil company has been running an experiment to figure out if we can indeed “bury” this CO2. The more technical term is injection, and it involves putting the carbon inside of an aquifer. Aquifers are geologic sandwiches that are usually of interest since they hold water or gas, which we want to remove. The test aquifer (a sandstone) is located in the North Sea, and has been receiving injected CO2 since 1996. In addition to serving as a viable source for the waste CO2 removed from the natural gas, it also is giving us information about what happens when you put this kind of carbon into the ground and it allows further extraction of gas, as long as you keep “refilling” the aquifer and keeping the sandwich intact (like swapping the meat with, well, carbon).
The only requirements are that the aquifer be very porous and permeable (can you pour water though it like water through sand?) and that the confining units be very thick and impermeable (can you pour water through asphalt?). The test site for this oil company is in deep marine deposits (bottom of the North Sea). On the other side of the spectrum, most efforts in the US have focused on saltwater aquifers located on land. Both types of sites will need to be used to accommodate all the carbon we make.
For now though, the deep geological marine injection seems to be the better option. This is for two reasons. First, at extreme depths and pressures, the CO2 becomes denser than saltwater. This means that any leaked carbon will stay at the bottom of the ocean, as the ocean water “floats” on top of it. The second bonus is that the capping material (“bread”) for deep marine aquifers is unconsolidated clay, which means it cannot form a crack and hold it, giving an easy escape path for CO2.
Deep marine environments offer other advantages as well. Keeping the CO2 in liquid form requires pressure regulation, a more difficult process in land-based aquifers. Proposed land-based aquifers also have chemically complex and toxic saline (salty) solutions that would need to be removed to make room for the CO2. Drilling extra wells to release fluid and pressure in deep marine aquifers just lets out salt water into the ocean, not a problem at all. Lastly, marine land is not disputed, whereas ownership of space at depth on regular land is a more sticky issue. For these reasons, the deep marine CCS systems are likely going to be the first attempt at lowering our CO2 levels in the air.
For more information, you could start here: Schrag, D. 2009. Storage of Carbon Dioxide in Offshore Sediments. Science. 325 (5948): 1658-1659. doi: 10.1126/science.1175750

Categories: climate science, hydrology

Comments (13)

  1. Lab Lemming says:

    Why can’t we just make a trillion tonnes of beer every year?
    Also, how does one stop CO2 and water from dissolving into one another?

  2. Doazic says:

    I once read about how it was discovered that Diamonds were made of Carbon; Lavoisier heated a diamond until it evaporated into Carbon dioxide.
    Why can’t we do the same in reverse?

  3. Ep0pEE says:

    I have often wondered the same thing Doazic. With graphene/nanotubes looking to replace silicon in our electronic gadgets, I would think we wouldn’t want to be burying the stuff.

  4. Jem Cooper says:

    Interesting and informative. As your recommended further information is pay per view, I thought I would ask you.
    Why can’t we just let the CO2 displace the water sideways rather than relieving the pressure upwards? It is not like trying to extract oil or gas where the stuff must percolate through the relatively small cross-sectional area round the well, and cannot be sucked effectively? Your relief wells sound like trouble if someone leaves the tap on in years to come the CO2 could escape up them.

  5. Omega Centauri says:

    On (or more precisely under) land disposal is subject to fearmongering, in much the same manner as prospective nuclear waste sites are routinely made unacceptable. The fact that a concentrated leak could accumulate near the ground and cause a danger of suffocation, however unlikely will be exploited by activists. I suspect getting approval for land based disposal will be problematic. That leaves subsea burial as the politically easier option.

  6. Will Dalen Rice says:

    Hopefully correct responses to above:
    1) Beer still gives off carbon dioxide (burp?). The liquid CO2 and saltwater interact the way fluids of different densities do (i.e. oil and water)
    2) Making diamonds requires intense heat and pressure. Compressing C enough to make a diamond would probably require more energy and create more CO2) than it would eliminate (not to mention the energy required to also seperate out the O2). Thats my guess.
    3) Of course we want to bury it. Its the only way to make sure we cant see it…well, unless we put it behind a wall that nobody can get around.
    4) “Relief wells” would release pressure vertically and horizontally. If the pressure is not relieved in a timely enough matter (i.e. by allowing the CO2 to just flow horizontally through aquifer material) then fracturing and faulting would likely occur (i.e. man-made earthquaking).
    5) yes…agreed. Also, deep land-based, saline aquifers apparently have highly toxic geochemistry…meaning any removal of material to be replaced with CO2 would just present the same problem with a new name. (plus, you gotta keep the liquid CO2 pressurized in the land-based ground aquifer).

  7. Lab Lemming says:

    Oil and water don’t mix because they have low solubilities in each other. CO2 and water are soluble in each other (depending on salinity, of course). So relying on density to keep them separate is like relying on density to keep syrup and ethanol from mixing.
    I strongly encourage everyone to experiment in the ethanol-syrup system at home. Addition of salt and or acidity (preferably lemon) is bonus material.

  8. “CO2 becomes denser than saltwater. This means that any leaked carbon will stay at the bottom of the ocean, as the ocean water “floats” on top of it.”
    This is the same argument used thirty-five years ago when geologists told us chemical disposal through injection wells was the way to go. “The junk is more dense than the water.” They didn’t accound for pressure differentials, fracturing, textural heterogeneity, or chemical concentration gradients. Remember, CO2 in solution is a weathering agent. It will likely alter the mineral composition of the whatever host rock (or sediment) you plan to store it in.

  9. CherryBomb says:

    A couple of things:
    1) Unconsolidated clay would probably not be a very reliable caprock. What is being proposed in that article you referenced is using the water in the aquifer as a “cap”. Being less dense than the liquid CO2, the set-up would be gravitationally stable.
    2) Any liquid CO2 that managed to leak onto the sea floor would not stay liquid for long; the pressures are not high enough until you get several hundred meters down into the rock.

  10. Doazic says:

    What about say, growing trees in farms, cutting down the trees and storing the wood?

  11. Will Dalen Rice says:

    Why are we cutting down the trees again? They do store carbon quite well. Or better yet…peat bogs covered in permafrost. We should find a way to make those.

  12. Dan says:

    I think Doazic was refering to cutting them down to bury them and their stored carbon and growing new ones in their place which in turn will extract more carbon. That’s the theory anyway. There was an interesting article on this in New Scientist some time ago:

  13. Stryke says:

    Sorry to hijack this, but the newest WoGE #189 is just up.