Bury carbon dioxide in the ground

Half green, half destroyed earth.

Since the 20th century, global warming has become a common challenge faced by all mankind, which has had a profound impact on the earth's natural ecosystem.

China is also a sensitive and significant area of the impact of global climate change, mainly as follows:

The average annual surface temperature has increased obviously, and the annual average precipitation has also increased slightly; the overall fluctuation of sea level has affected marine ecological security; the retreat of glaciers and the melting of permafrost have affected the ecological security of the Qinghai-Tibet Plateau; changing the original water balance; increasing the risk of water security; resulting in ecosystem changes and biodiversity; the adverse impact on agricultural production and food security is more prominent. Extreme weather events occur frequently, aggravating the risk of natural disasters.

Virtual picture: global warming, sea level rise

The United Nations Panel on Climate change (IPCC) pointed out that human activities are most likely to be the main cause of global warming since the middle of the 20th century, with a probability of more than 95%. Carbon dioxide (CO2) is the main greenhouse gas. In the past 200 years, human beings have emitted trillions of tons of CO2 into the atmosphere, which is equivalent to creating a greenhouse for the earth and making the earth "feverish". Therefore, it is necessary to vigorously reduce CO2 emissions to curb climate warming.

As a major country responsible for dealing with climate change actively, China proposes that CO2 emissions should reach the peak by 2030 and strive to achieve the goal of carbon neutrality by 2060.

By improving the utilization efficiency of fossil fuels such as coal, oil, and natural gas, and vigorously promoting clean energy such as wind power and photovoltaic, so that our annual CO2 emissions no longer increase, reaching a peak, it is called "carbon peak"; the CO2 that cannot be reduced is fully absorbed through artificial carbon sequestration technologies such as afforestation, carbon capture and storage (CCS), that is, "carbon neutralization".

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What is carbon capture and sequestration technology

Carbon capture and storage technology (CCS) refers to the process of long-term isolation from the atmosphere by engineering and technical means to capture the CO2 emitted from industrial sources, purify and compress it, transport it by highways or pipelines, and inject it into salt water layers and oil and gas reservoirs at a depth of 800m. Below 800m, CO2 enters a supercritical state, which is about 1/400 of the volume of standard atmospheric pressure, thus allowing the same space to store more CO2.

The density of carbon dioxide (CO2) will be greatly increased at the storage depth.

Carbon capture and sequestration includes three important technical links, namely capture, transportation and sequestration.

The capture of CO2 is realized by adding adsorption devices to the flue gas emitted by coal-fired power plants, chemical industry, iron and steel, cement and other industries. The mode of transportation of CO2 includes roads, pipelines, ships, etc., usually liquid CO2 transport. The sealing and storage of CO2 means that after transporting CO2 to a suitable location, liquid CO2 is injected into the deep underground salt water layer, oil and gas reservoir and so on.

Schematic diagram of carbon capture and sequestration (according to the Australian greenhouse Gas Technology Cooperation Research Centre)

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Main technical types of carbon sequestration

As far as carbon sequestration technology is concerned, there are mainly two ways to achieve large-scale application worldwide, including oil displacement in carbon dioxide (CO2) injection reservoirs and salt water storage.

CO2 flooding

CO2 flooding refers to the injection of CO2 into the reservoir to supplement reservoir energy, and the interaction between CO2 and crude oil can increase the oil recovery rate by more than 7%, so as to achieve the purpose of increasing crude oil production and sealing CO2.

In the process of CO2 flooding, about 50% to 67% of CO2 will be extracted together with crude oil. However, in order to reduce the technical cost, the extracted CO2 is often separated from the crude oil and injected back into the formation to realize the reuse of CO2. Part of the injected CO2 can also be dissolved in formation fluid, mineralized and solidified, or captured by structural traps to achieve permanent storage in the reservoir.

When the reservoir has no economic value, the CO2 flooding technology is transformed into the geological storage of CO2 in the depleted reservoir, and the injected CO2 will be completely sealed in the formation.

Storage of carbon dioxide (CO2) in saltwater layer

In the deep underground space of the sedimentary basin, there are a wide range of salt water layers, which are almost static and can not be extracted and utilized, so they can be used to seal CO2. After injection into the salt water layer, the CO2 will be blocked by the argillaceous caprock (figure a) and pore bound (figure b), and further dissolved in salt water (figure c) or fixed by reaction with water, rocks and minerals (figure d).

By the end of 2020, there are more than 12 proposed and under construction salt water storage projects around the world, and they are gradually developing from small-scale demonstration to large-scale integration projects. Countries such as Norway, the United States and Australia are carrying out full-process carbon capture and sequestration commercial projects with annual injections of up to one million tons.

The geological storage potential of CO2 in deep salt water layer in China is huge, accounting for more than 90% of the total potential of salt water layer and oil and gas reservoir, and it is the main storage space to support the target of carbon neutralization in the future. Thirteen large and medium-sized basins such as Tarim, Ordos, Songliao and other large and medium-sized basins in China, as well as 16 large and medium-sized sedimentary basins such as the East China Sea Shelf, Bohai Sea and Pearl River Estuary, have great CO2 storage potential and relatively good caprock conditions.

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Possible environmental risks of carbon sequestration technology

It is undeniable that a large amount of CO2 injected into salt water layers or reservoirs may produce CO2 leakage. The leakage of CO2 into the shallow aquifer will cause the pollution of potable groundwater; when CO2 enters the shallow soil, it will change the soil physical and chemical properties, affect the organisms in the soil, and then enter the vadose zone and atmospheric environment, which will affect groundwater, soil, ecosystem, atmospheric environment and human health.

Secondly, after CO2 is injected into the salt water layer or reservoir, the formation pressure may be greatly increased, which may lead to the risk of ground deformation and earthquake in extreme cases.

However, the risk of CO2 leakage can be avoided by scientific geological location of storage site, improving construction quality and strengthening monitoring.

CO2 geological sealing of salt water layer is essentially equivalent to finding or creating an underground "artificial gas reservoir", which is the reverse technology of oil and gas development. Analogically, oil and gas can be preserved underground for millions of years. As long as a reasonable storage site is selected, CO2 can be sealed in the underground salt water layer for a long time until it is dissolved and finally fixed in carbonate rocks. Secondly, in the process of CO2 geological storage or after stopping injection, scientific construction technology and monitoring technology of deployment system can be used to effectively prevent or monitor whether CO2 leakage occurs.

Monitoring system of Shenhua CCS demonstration Project

The long-term effective monitoring results of the ex