Hide carbon in the sea: a new path for CCUS
In 2023, global fossil fuel carbon dioxide emissions will reach 36.8 billion tons (total greenhouse gases 40.9 billion tons), and carbon dioxide concentrations have reached their highest value in 80 years. Among them, my country's carbon dioxide emissions reached 12.6 billion tons, ranking first in the world. Carbon dioxide capture, utilization and storage (CCUS) is regarded as the ultimate guarantee for achieving carbon neutrality in the future. Recently, the reporter learned from Zhou Shouwei, an academician of China Academy of Engineering and an outstanding senior expert of China Offshore Oil Group Co., Ltd., that in addition to displacing oil on land, there may be new paths for carbon dioxide storage deep in the ocean.
"Having been dealing with the ocean all my life, my deepest understanding is that the profound ocean not only provides rich material security for mankind, but also provides ideological enlightenment for people." Zhou Shouwei said,"We must use the power of nature to transform the difficulties of marine natural gas hydrate development into advantages of carbon dioxide hydrate submarine engineering."
Marine CCUS needs to rise
According to the forecast of the International Energy Agency's Energy Technology Outlook Report 2017, to achieve the climate goal of "controlling the increase in global average temperature to within 2 degrees Celsius compared with the pre-industrial period" proposed by the Paris Agreement, cumulative carbon emission reductions will be achieved by 2060. 14% of the amount will come from CCUS. However, as of 2023, there are more than 140 CCUS projects in operation or construction around the world, with a capture capacity of 49 million tons/year, accounting for only 1.33‰ of global carbon emissions throughout the year. From the data point of view, existing carbon neutrality projects are far from achieving the bottom line.
At present, the domestic coal, electricity, petroleum and petrochemical industries and related industries are carrying out CCUS technology research and engineering demonstrations. China Petrochemical has completed the country's first million-ton CCUS full-process project, but the overall scale is still small. In 2023, my country's annual carbon capture scale will reach 6 million tons/year, but this amount only accounts for 4.15‰ of annual carbon emissions. A total of 8 CCS projects are in operation, of which only Enping 15-1CCS project is a saltwater reservoir storage project, and the rest are enhanced oil recovery technology (EOR) projects. It is reported that the Enping project is my country's first offshore carbon dioxide storage project. It was officially put into use in June 2023. More than 300,000 tons of carbon dioxide are injected every year, totaling more than 1.5 million tons; so far, 20 million cubic meters of carbon dioxide have been injected.
"Whether it is 1.33‰ or 4.15‰, it is an order of magnitude away from the final goal of 14%." Zhou Shouwei said that from the perspective of the world, CCUS is nothing more than two major directions: one is carbon dioxide flooding, while simultaneously improving crude oil recovery, and then burying carbon dioxide in geological structures. The other is to sequester carbon dioxide in saltwater beds. Depending on the number of projects under construction, the latter is gradually becoming mainstream.
Stones from other mountains can be used to attack jade
However, stones from other mountains can be used to attack jade. As we all know, the mining of combustible ice (natural gas hydrates) located on the seabed is difficult and has not been commercially developed anywhere in the world. Zhou Shouwei, who has been dealing with the ocean for many years, suddenly thought that the conditions for the formation of carbon dioxide hydrates are similar to those of natural gas hydrates, or even simpler and easier to produce. Why not turn the difficulty of mining combustible ice into the advantages of studying carbon dioxide buried in the ocean?
Marine hydrate storage areas are usually located in water depths of more than 300 meters and 1000 meters below the seabed. Comparing the formation conditions of natural gas hydrates and carbon dioxide hydrates, we can find that marine carbon dioxide sequestration has eight advantages: first, carbon dioxide hydrates are easier to form than natural gas hydrates; second, the low temperature and high pressure marine environment makes the state of natural gas hydrates stable and difficult to develop, but it also constitutes a favorable condition for carbon dioxide sequestration; third, carbon dioxide hydrates, like combustible ice, do not require trap structures and are expected to form solid solids on a large scale and be sealed; Fourth, in the deep-sea environment, the solidified carbon dioxide hydrate will remain solid and will be difficult to vaporize and be easier to seal up; fifth, the shallow layers of the deep sea are mostly muddy silt or non-rock-forming cemented loose mud, sand and water mixed layers, which can provide favorable conditions for carbon dioxide injection and hydrate formation; sixth, storing carbon dioxide in the form of solid hydrates does not need to be evaluated for tightness, and there is no gas leakage problem; Seventh, under appropriate conditions, carbon dioxide hydrate storage, in principle, there is no scale limit on the storage volume to solve current technical problems; eighth, the seabed or deep sea shallow layers provide a wider selection of locations for carbon dioxide storage, breaking through the existing technology must Choose the shackles of closed geological structures or saltwater layers.
"In other words, where natural gas hydrates can be formed, carbon dioxide hydrates can also be formed, and they can be formed more easily." Zhou Shouwei said,"The new path for carbon dioxide sequestration proposed by me through analysis is to learn from the principle of natural formation of marine natural gas hydrates, artificially accelerate the formation rate of carbon dioxide hydrates, and realize the solidification and storage of carbon dioxide hydrates on the deep seabed or in the deep sea."
Turn waste gas into seabed "carbon ore"
"As long as the temperature and pressure are right, carbon dioxide can form hydrates. Like Dongsha in my country and the open sea of New Zealand, natural gas hydrates have been buried on the seabed for millions of years. If we buried carbon dioxide like this, wouldn't it be possible? It can form very stable carbon dioxide hydrates that are permanently sequestered." Zhou Shouwei believes that when it comes to engineering technology, one method is to find a suitable place to directly put carbon dioxide; the other method is to solidify and store carbon dioxide in the underlying layer of natural gas hydrate.
"The direction and goal of our research is to be able to advance, be successful, be sealed, visible, and be ecologically beautiful." He explained that for this reason, the team needs to establish a carbon dioxide hydrate plugging removal technology at the injection port and a multi-layer Fed dynamic control technology; reveal the rapid formation mechanism of carbon dioxide hydrates in the marine environment and form a large-scale carbon dioxide hydrate generation technology; establish deep sea carbon dioxide multiphase stable storage technology, process and equipment; and form a full-time integrated monitoring technology and system for marine carbon dioxide solidification and geological storage. Finally, after carbon dioxide is solidified and sealed, it will be long-term stable and environmentally friendly as an integral part of the marine ecology.
"We used seawater and sediment from the South China Sea to simulate seabed conditions to conduct a series of experiments. These experiments finally supported our idea that marine environmental conditions can quickly form carbon dioxide hydrates, and seabed pressure has little effect on stability. In addition, under the conditions of inducing particles, carbon dioxide hydrate is formed faster, and appropriate addition of reinforcing agents will help solidify carbon dioxide hydrate." Zhou Shouwei concluded,"After preliminary verification, we can determine that the direction of solid storage of marine carbon dioxide is in the right direction, but there are still a large number of problems that need to be continued to explore."
At present, Zhou Shouwei's team is still conducting research on this technology and is trying to establish a multiphase, multi-layered solidification and storage technology system for carbon dioxide hydrate in 3000 meters deep water in the future. "If marine carbon dioxide geological storage technology can be applied, a series of industries such as carbon dioxide carriers, long-distance submarine pipelines, and offshore drilling equipment will usher in spring, and a new CCUS industry chain will also be born." He said prospectively.
