As an active participant in global climate governance and an initiator and pioneer in building a community with a shared future for mankind, China has put forward the goal and commitment of “strive to peak carbon dioxide emissions by 2030 and strive to achieve carbon neutrality by 2060”. Carbon dioxide (CO2), as one of the most important greenhouse gases in the atmosphere, has always attracted much attention. Various control methods emerge in an endless stream, and geological storage of carbon dioxide is one of them.
where are you from, where are you going
The main greenhouse gases in the atmosphere are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs) and ozone (O3). Simply put, the greenhouse effect is when greenhouse gases absorb long-wave radiation heat released by the earth’s surface, increasing the atmospheric temperature on the earth’s surface. In fact, this most primitive greenhouse effect has existed for a long time, and it is also of great significance to human development. If it does not exist, the temperature difference between seasons and day and night on the earth will be very large, which is not suitable for human survival. However, after the Industrial Revolution, human activities released a large amount of greenhouse gases, and the greenhouse effect has been increasing, resulting in a series of global climate problems that are now unpredictable by science.
As a result, for carbon dioxide, the most important greenhouse gas in the atmosphere, a method called geological storage of carbon dioxide was proposed: since most of the carbon dioxide emitted by humans is released from fossil energy buried in the ground, why not bury it back underground?
Fossil fuel combustion is one of the main sources of anthropogenic carbon dioxide emissions
Carbon sealing, the method is not unique
Geological storage of carbon dioxide refers to the capture of carbon dioxide discharged from industrial sources through engineering and technical means, and then injected into deep onshore salt water layers, depleted oil and gas reservoirs, unexploitable coal seams, and submarine salt water layers within a depth of 800 to 3,500 meters underground. In other geological structures, it is sealed in the geological body through a series of petrophysical constraints, dissolution and mineralization. Among them, carbon sequestration technologies in deep onshore saline waters and carbon sequestration in depleted oil and gas reservoirs are the most mature.
Carbon sequestration in deep saline aquifers on land: Dominance
Onshore deep saline aquifers are regarded as the best sites for long-term carbon dioxide storage due to their wide distribution and large storage capacity.
Why choose deep saline aquifers? The deep salt water is mostly salt water with high salinity, which is difficult to exploit and utilize. Moreover, a large amount of calcium ions (Ca2+), magnesium ions (Mg2+), etc. can react with carbon dioxide to form calcium carbonate (CaCO3), magnesium carbonate (MgCO3) and other substances, which are the main components of some rocks in nature. That is, the deep salt water layer turns the carbon dioxide involved in the reaction into a solid rock, that is, mineralization occurs. Of course, this process is slow, even taking millions of years.
Although carbon dioxide can react with calcium ions and magnesium ions, the amount that can be involved is limited and cannot meet all needs. Therefore, in the deep saline layer, carbon dioxide is also sealed by the capping layer overlying the saline layer, bound by pores, or dissolved in water and stored.
Schematic diagram of the main methods of carbon dioxide geological storage (drawing / Wei Xin)
Carbon sequestration in depleted oil and gas reservoirs: saving time and effort
After a certain period of development of oil and natural gas fields, limited by technical and economic conditions, the remaining oil and gas cannot be recovered, which is called depleted oil and gas reservoirs. Although it has lost its original value, it has great advantages for carbon dioxide geological storage: it 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 is either dissolved in formation fluids, lithogenic and solidified, or captured by stratigraphic structural traps to achieve geological storage.
Carbon sequestration in non-minable coal seams: the ‘favored’ CO2
This approach is similar to carbon sequestration in depleted oil and gas reservoirs. In coal measure strata, there are commonly abandoned coal seams for technical or economic reasons, which is also a potential geological site for carbon dioxide storage.
Coal seams are usually associated with methane. Because carbon dioxide can be adsorbed twice as much as methane on the surface of coal, when carbon dioxide is adsorbed by coal or organic-rich shale that “prefers” it, it begins to displace methane-like gases. In this case, as long as the pressure and temperature remain stable, carbon dioxide will remain trapped for a long time, and eventually exist in the coal seam in an adsorbed state and a free state.
In addition to the above methods, technologies such as basalt carbon sequestration of carbon dioxide (storage by the mineralization reaction of basalt) and carbon sequestration in marine sedimentary basins (storage by its saline aquifers) are also being developed.
Climate change is related to people’s well-being and the future of mankind. Climate change with global warming as the main feature has become one of the most important environmental and development challenges faced by human society. According to the survey, China’s carbon dioxide geological storage potential is huge, and scientists are now accelerating the pace of research, striving to make greater contributions to China’s realization of carbon neutrality.