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Q: How long will carbon dioxide stored in geological formations remain sequestered?

A: Permanently.

In nature, we find carbon dioxide that’s been stored in geological formations for tens of millions of years. Some examples of places where carbon dioxide has accumulated and remained stored underground are the Jackson Dome in Mississippi, the Bravo Dome in Oklahoma, and the McElmo Dome in Colorado.

Carbon dioxide is trapped in these geological formations the same way that oil and natural gas are trapped. In most places deep underground, the rock is saturated with salty water called brine. Like oil and natural gas, carbon dioxide is less dense than the brine. It is buoyant and floats on the brine. So CO2 naturally bubbles upward wherever it can, just like natural gas.

What stops it? Sedimentary basins where CO2 would be stored are layered. Some layers allow fluid to soak into them and move through them; they are called permeable. Between the permeable layers are rock layers that are impermeable to all fluids except water; they do not let CO2, oil or natural gas move through them.  

Buoyant fluids produced deep below the earth’s crust rise upward through rock containing brine until they encounter a layer of rock that is too impermeable for them to pass through. And, like a helium balloon bumping against the ceiling at a kid’s birthday party, buoyant fluids like CO2 remain trapped there.

It might not be the most dignified analogy, but a baby diaper is a similar system. Some of the layers soak up the fluids. Others prevent them from getting out.