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Q: Why is CO2 stored so deep?

A: CO2 is stored deep underground because it's required by law. The US Safe Drinking Water Act orders that underground injection of any liquid, not just carbon dioxide, is below and isolated from freshwater aquifers.

Rocks are not completely solid; they are porous and liquids fill the microscopic holes. Near the surface, the liquid that fills the holes is freshwater, suitable for drinking. Deeper, it is saltwater, or brine, which is not suitable for drinking. Freshwater occurs above brine because freshwater is less dense than brine. The US Safe Drinking Water Act requires liquids to be injected into the deeper formations that contain non-potable brine.

Reservoirs into which CO2 is injected are also required to have a confining system, made up of non-porous rock through which CO2 cannot migrate. This prevents it from contaminating the freshwater above. [See FAQ: How long will carbon dioxide stored in geological formations remain sequestered?] [See FAQ: How can there be space for so much CO2 underground?]

An added technical benefit is that when we store CO2 greater than 1 km underground it takes up much less space. Like vacuum packing pillows and blankets for more efficient storage, we can fit more CO2 into the same amount of space when we store it deep underground. [See FAQ: What is a supercritical fluid?]