What to do with too much carbon dioxide? Bury it underground!

As an active participant in global climate governance and the initiator and pioneer of building a community with a shared future for mankind, China has put forward the goal of "striving to peak carbon dioxide emissions before 2030 and strive to achieve carbon neutrality before 2060". As one of the most important greenhouse gases in the atmosphere, carbon dioxide has always attracted much attention, and various control methods have emerged one after another, and geological storage of carbon dioxide is one of them.

Where you come from, where you go back to

The greenhouse gases in the atmosphere mainly include carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and ozone. Simply put, the greenhouse effect is that greenhouse gases absorb the long-wave radiation heat released by the earth's surface, causing the atmospheric temperature on the earth's surface to rise. In fact, this most primitive greenhouse effect has long existed, and it is also of great significance to human development. If it did not exist, the temperature difference between seasons and day and night on the earth would be very large, making it unsuitable for human survival. However, after the Industrial Revolution, human activities released a large amount of greenhouse gases, and the greenhouse effect has become increasingly stronger, leading to a series of global climate problems that are now unpredictable by science.

Therefore, a method called carbon dioxide geological sequestration has been proposed for the most important greenhouse gas in the atmosphere, carbon dioxide: since most of the carbon dioxide emitted by humans is released by fossil energy buried underground, why not bury it back underground?

Carbon sealing, there is no single method

Carbon dioxide geological storage refers to the use of engineering technology to capture carbon dioxide emitted by industrial sources, inject it into geological structures such as deep saline water layers on land, depleted oil and gas reservoirs, unexploitable coal seams, and seabed saline water layers at a depth of 800 to 3,500 meters underground, and then seal it in the geological body through a series of rock physical constraints, dissolution and mineralization. Among them, carbon storage in deep saline water layers on land and carbon storage in depleted oil and gas reservoirs are currently the most mature.

Onshore deep saline aquifer carbon storage: taking the lead

Deep saline aquifers on land are considered the best place for long-term storage of carbon dioxide because of their wide distribution and large storage capacity.

Why choose deep saline water layers? Deep saline water is mostly highly mineralized and difficult to exploit. Moreover, a large number of calcium ions and magnesium ions can react with carbon dioxide to form substances such as calcium carbonate and magnesium carbonate, which are the main components of some rocks in nature. In other words, the deep saline water layer turns the carbon dioxide involved in the reaction into solid rock, that is, mineralization occurs. Of course, this process is very slow and may even take millions of years.

Although carbon dioxide can react with calcium ions and magnesium ions, the amount that can participate is limited and cannot meet all needs. Therefore, in deep saline water layers, carbon dioxide will be isolated by the caprock overlying the saline water layer, bound by pores, or dissolved in water and sealed.

Carbon storage in depleted oil and gas reservoirs: saving time and effort

After a certain period of development, the remaining oil and gas in oil and gas fields cannot be extracted due to technical and economic conditions, and are called depleted oil and gas reservoirs. Although they have lost their original value, they have great advantages for geological storage of carbon dioxide: they can make full use of existing oil and gas reservoir exploration and development data, well sites and oil well equipment for storage, saving investment and engineering time.

After carbon dioxide is injected into depleted oil and gas reservoirs, it either dissolves in formation fluids, solidifies into rocks, or is captured by formation structural traps, achieving geological storage.

Carbon storage in unmineable coal seams: the “favored” form of CO2

This method is similar to carbon sequestration in depleted oil and gas reservoirs. In coal-bearing strata, there are generally coal seams that have been abandoned for technical or economic reasons, which is also a potential geological place for storing carbon dioxide.

Coal seams are usually associated with methane. Since the adsorption capacity of carbon dioxide on the surface of coal is twice that of methane, when carbon dioxide is adsorbed by coal or shale rich in organic matter that "prefers" it, it begins to displace methane gases. In this case, as long as the pressure and temperature remain stable, carbon dioxide will remain captured for a long time and eventually exist in the coal seam in an adsorbed or free state.

In addition to the above methods, technologies such as basalt carbon sequestration of carbon dioxide (using the mineralization reaction of basalt to store carbon dioxide) and marine sedimentary basin carbon sequestration (using its saline aquifer to store carbon dioxide) are also developing.

Climate change is related to people's well-being and the future of mankind. Climate change, characterized by global warming, has become one of the most significant environmental and development challenges facing human society. According to surveys, China has great potential for geological storage of carbon dioxide. Scientists are now accelerating their research and striving to make greater contributions to China's realization of carbon neutrality.

Authors: Wang Yao, Land Consolidation Center, Ministry of Natural Resources; Chen Ruishan, Shanghai Jiao Tong University; Yao Shunyu, China Coal Finance Co., Ltd.; Chen Guangfeng, Beijing Municipal Planning and Natural Resources Commission.

Editor-in-charge: Hu Huiwen