How is carbon stored?

Answer 1

• Structural Storage

  CO2 is more buoyant than water meaning that when injected into a reservoir it will rise up through the pore spaces in the reservoir rock until it reaches the impermeable cap rock. Good cap rocks such as shales and mudstones are impenetrable to the CO2 and will prevent it from leaking back to the atmosphere. Many natural gas and oil fields around the world have stored both CO2 and natural gas for millions of years in this way.

• Residual Storage

  The porous rocks in oil and gas reservoirs behave like a tight, rigid sponge. A sponge traps air by 'residual trapping' and for this reason to soak a sponge in water it must be squeezed several times to replace the trapped air with water. In a similar fashion when liquid CO2 is injected into a rock formation much of it is trapped within the pore spaces of the rock and this is known as residual trapping.

• Dissolution Storage

  In the same way that sugar dissolves in tea, CO2 dissolves readily in water. This water containing CO2 is denser than the CO2 free water surrounding it and therefore it will sink to the bottom of the reservoir, trapping the CO2.

• Mineral Storage

  When CO2 is dissolved in water it forms a weak acid (carbonic acid) which can dissolve and react with the minerals in the reservoir rock. If the conditions in the reservoir are favourable, new minerals can be formed which coat the inside of microscopic pores in the reservoir rock, and lock away the CO2. However, under most conditions this process is extremely slow and therefore this is the least useful storage mechanism.

Answer 2

Carbon is stored in various forms on Earth, including in the atmosphere as carbon dioxide, in plants and trees as organic matter, in soil as organic carbon, and in fossil fuels such as coal, oil, and natural gas. The ocean also acts as a carbon sink, absorbing carbon dioxide from the atmosphere.