Note: 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
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