Where is carbon dioxide placed? This time there are many new destinations!

Review expert: Gan Qiang, lecturer at Beijing Institute of Technology, PhD in Applied Chemistry

On the boundless sea level in the eastern part of the South my country Sea, the Enping 15-1 oil production platform stands in the middle of the sea. At present, the oil production platform has successfully carried out a carbon dioxide burial project, and the successful implementation of this project marks a new height for my country's carbon dioxide capture and burial technology.

Enping 15-1 Platform Source: China National Offshore Oil Corporation

Carbon dioxide is one of the main greenhouse gases. Excessive carbon dioxide emissions have led to serious problems such as global warming and sea level rise.

According to scientists' predictions, if no action is taken to reduce carbon dioxide emissions, the Earth will face even more severe impacts of climate change in the coming decades, posing huge risks to our lives and the environment.

To address this challenge, carbon dioxide burial technology has been proposed and widely studied.

The core idea of ​​this technology is to store carbon dioxide permanently, preventing it from entering the atmosphere and thus mitigating the greenhouse effect.

Specifically, the "carbon burial" technology will first collect the carbon dioxide produced by the combustion of fossil fuels and industrial production, and bury it in the deep crust using engineering technology, thereby preventing the carbon dioxide from being emitted into the atmosphere.

Source: News

At this point, it seems difficult to link carbon dioxide burial with oil extraction platforms, but the opposite is true. When drilling and developing offshore oil and gas, not only oil is extracted, but some underground gas is also released.

These gases contain a large amount of carbon dioxide. If these associated gases are "ignored" during the mining process, the carbon dioxide hidden in the crust will easily "escape" from the crust and enter the atmosphere.

So, how does CO2 storage technology work?

First, the carbon dioxide needs to be captured and separated, usually through industrial processes or carbon capture facilities.

The CO2 is then transported to the ocean floor and injected into deep geological rock. During burial, the CO2 chemically binds to the rock, forming stable compounds that prevent it from escaping into the ocean or atmosphere.

Finally, a sealing layer is built over the rock layer, ensuring the safety and stability of the buried area.

Carbon dioxide burial principle Source: Polaris Carbon Steward Network

1How to store carbon dioxide?

At present, we mainly bury carbon dioxide in the ocean and underground. The main burial methods are ocean burial, geological storage and mineral burial.

Ocean burial

There are two ways to store carbon dioxide on the seafloor.

Schematic diagram of the main methods of geological storage of carbon dioxide on the seabed

Source: Intergovernmental Panel on Climate Change

Water column burial is to use the water column in the ocean to inject carbon dioxide into the seabed for natural dissolution. This can be achieved by passing carbon dioxide into the seabed through ships or pipelines. Sediment burial is to store carbon dioxide on the seabed, where it gradually turns into a solid and is isolated from the atmosphere. At present, the technical cost of burying carbon dioxide on the seabed is too high. At the same time, we know that carbon dioxide will form acid when it dissolves in water. If the acid content is high, the water will be acidic, lowering the pH value and affecting the marine ecosystem.

Geological storage

The main principle of geological storage is based on the storage mechanism of fossil fuels in nature. Since carbon dioxide needs to maintain a supercritical state when buried in a specific stratum, that is, the temperature needs to be above 31°C and the pressure needs to be greater than 7.38MPa, it is often buried 800m underground. At the same time, carbon dioxide also has special requirements for the strata in which it is stored. At present, these special crustal layers that we have learned about mainly include depleted oil and gas reservoirs, deep saline water layers, and unmineable coal seams.

Mineralization Buried

As we all know, carbonates are very stable. If carbon dioxide can be reacted with metal oxides, it can be buried.

Source: Professional Insights

Mineralization burial is to bury carbon dioxide in mineral layers, and the mineral layers contain a large amount of alkaline oxides (such as magnesium oxide and calcium oxide), which can react with carbon dioxide to solidify carbon dioxide. The compounds formed after carbon dioxide solidifies are relatively stable and cannot diffuse into the atmosphere. Mineralization burial is also called ore carbonization burial. The metal oxides in the reaction are the ores in the stratum. The products of their reaction with carbon dioxide are mainly compounds such as magnesium carbonate and calcium carbonate (i.e. limestone).

2Why hide carbon dioxide?

Carbon dioxide storage technology has important engineering significance.
First, it could significantly reduce greenhouse gas emissions and thus have a positive impact on climate change.

Carbon dioxide is a type of greenhouse gas. The Earth we live on absorbs the sun's heat while also emitting infrared radiation to dissipate heat. Under natural conditions, the gases in the Earth's atmosphere do not absorb the heat energy emitted by infrared radiation, which means that the heat energy emitted by the Earth itself can pass through the atmosphere "intact" and transfer the heat to the universe, thereby cooling the Earth.

Secondly, carbon dioxide burial can make use of existing geological structures such as oil and gas fields or saline formations, allowing them to be reused and promoting the sustainable use of energy and resources.

For oil and gas fields, some oil and gas fields will produce a large amount of carbon dioxide during the exploitation process. By capturing this carbon dioxide and injecting it into the original oil and gas fields, it can not only reduce greenhouse gas emissions, but also effectively utilize oil and gas resources and promote the sustainable use of energy.
Saline layers are usually underground rock layers containing salt water and have no economic value. By injecting carbon dioxide into saline layers, it can not only be permanently sealed to prevent it from entering the atmosphere, but also fully utilize these saline layer resources to achieve sustainable resource utilization.

Source: Xinhua News Agency

In addition, CO2 burial can also reduce the risk of CO2 leakage, thereby protecting marine ecosystems and human health. This is because the dome-like geological structure of the saline layer located 900 meters below the seabed can hold CO2 on the seabed like a "giant bowl". At the same time, the CO2 buried underground will react with water and rocks to mineralize, and the remaining unreacted CO2 will dissolve in the underground water flow. By storing CO2 permanently on the seabed through CO2 burial technology, we can effectively reduce greenhouse gas emissions and combat the risks of global climate change. However, we also need to continue to strengthen research, innovation and regulatory measures to ensure the sustainable development and safe implementation of CO2 burial technology. Only by working together can we meet the global challenge of climate change and pave the way for sustainable development in the future.