4h. Storing Carbon Dioxide
Storing carbon dioxide presents unique challenges. There are two main types of carbon dioxide storage: geologic and terrestrial.
Geologic Storage
Geologic storage permanently stores CO2 underground below impermeable layers of rocks, preventing the gas from escaping into the atmosphere.
Terrestrial Storage
Terrestrial storage techniques maximize the amount of carbon dioxide stored in plants and soils. Examples of terrestrial storage practices include no-till farming, reforestation and land/wetland management.
Geologic Storage
In geologic storage, CO2 is stored deep underground as a supercritical fluid. In this state, the CO2 takes up less space and can flow between tiny spaces in the rocks. The CO2 is injected into deep rock layers through tubing encased in 3 layers of steel and 2 layers of durable cement. The rock layers used for CO2 storage must meet the following criteria:
Pore Spaces
Sedimentary rock layers are used for storage because they have small spaces, or pores, which fill with CO2 like a sponge soaking up water. Sandstone or limestone are typically good sedimentary rock layers for storage.
Impermeable Top Layers
The rock layers above the sedimentary rock layers must be impermeable so the CO2 cannot migrate towards the surface or into underground water aquifers. Claystone and shale make good top layers.
Stable
The rock layers cannot have any faults (large cracks) that may shift or open up, allowing CO2 to escape. The rocks also can’t contain any chemicals that will react with CO2.
Deep
Rock layers need to be deep enough so that the temperature and pressure is high enough to keep CO2 a supercritical fluid. The layers chosen for storage are typically located more than 3,000 feet below the surface.