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Q: How can there be space for so much CO2 underground?

A. Trillions of tiny holes make a big a rock-hard sponge.

Underground is not a cave or a balloon. It is made up of layers of different types of rocks. The kind of rock that CO2 will be stored in is porous. This means the rock is made up of small grains stuck together, and between those grains are tiny spaces.

Think about a jar packed full of marbles. There is space between the marbles. If you had a liquid, you could pour the liquid into the jar and it would fill the space between the marbles.

This is similar to how underground reservoirs for CO2 work. The marbles are like the grains of the rock. The CO2 that is stored underground is in a compressed phase that flows like a liquid. The CO2 fills the pores by displacing the salty water called brine between the grains of the rock.

Now imagine that you didn’t just have one jar of marbles. Imagine you had trillions of jars of marbles extending over many square miles of land that is many 100's of feet thick. You could store a lot of liquid.

Dr. Tip Meckel
 of the Gulf Coast Carbon Center approved this FAQ.



The factors that influence the capacity of particular rocks and reservoirs are summarized in a presentation by Dr. Susan Hovorka of the Gulf Coast Carbon Center. For slides of that talk, check out the link at the left. 



The Gulf Coast Carbon Center's Offshore Miocene Repository project is developing an assesment of the capacity of the geologic reservoirs deep under the Gulf of Mexico. You can read more about it on their website.



For an atlas of all potential storage reservoirs in the the United States, click the link to the right. Note that for each reservoir you can map the thickness, as well as other physical properties.