At room temperature and pressure, CO₂ is a colorless and odorless gas with inactive chemical properties. When the pressure is standard atmospheric pressure and the temperature is 0 °C, CO₂ has a thermal conductivity of 0.073 J/(m·K) and a density of 1.98 kg/m³, which is about 1.53 times that of air. In view of the dissolution, competitive adsorption, viscosity reduction and conduction properties of CO₂ under the formation conditions, various utilizations can be realized in the geological field: (1) CO₂ can extract light crude oil in the formation and reduce the oil-water interfacial tension, so it can be used for enhanced oil recovery (CO₂-EOR) (Figs. 3 and 4). (2) The leaching solution composed of light crude oil and CO₂ can dissolve uranium salt, so it can be used for uranium mining. (3) Thick oil reservoirs and condensate gas reservoirs face mobility problems in the development process, while CO₂ has the characteristics of low viscosity and high mobility, which can enhance the exploitation of thick oil and condensate gas reservoirs. (4) As a conduction medium, CO₂ has better mobility than water, which is conducive to the protection of water-sensitive reservoirs, so it can be used for the development of dry heat rock. (5) CO₂ is more strongly adsorbed in coal and shale reservoirs than CH₄, so it can improve the development effect of coalbed methane and shale gas by means of competitive adsorption.

Chemical utilization is to take the means of chemical transformation to generate target products through reaction of CO₂ with reducing agents under the action of efficient catalysts, so as to realize the conversion of greenhouse gases into high value-added chemical products and thereby achieve revenue generation and efficiency improvement in the manufacturing industry. From the perspective of chemical reaction, CO₂ can be reduced to a variety of carbon-containing organic chemicals. However, different from the scientific issue of “CO₂-based chemical synthesis”, the economic value of “CO₂ resource utilization” needs to be taken into account comprehensively. For bulk chemical production, CH₄ or C₂H₆O can be produced by means of CO₂ hydrogenation, whose industrial applications are already available. Electrocatalysis and visible light catalysis are effective strategies to achieve the conversion of CO₂ to CH₄, and the key point of such technologies is a highly active and selective catalyst. For example, Rao et al. ^[24] reported an iron-based molecular-level photocatalyst that can efficiently catalyze the reduction of CO₂ to CH₄ under room temperature and atmospheric pressure driven by visible light with a total selectivity of 82% and a quantum yield of 0.18%. Lin et al.^[25] performed tandem catalysis over non-Cu-based catalysts for the electrochemical reduction of CO₂ to methane. This combination effectively electrochemically reduced CO₂ to CH₄ via cobalt phthalocyanine (CoPc) and zinc-nitrogen-carbon (Zn-N-C) tandem catalysts, and the CH₄ generation rate was more than 100 times higher than that of cobalt phthalocyanine or zinc-nitrogen-carbon alone, providing a new strategy for the electrocatalytic reduction of CO₂ to hydrocarbons. In the production of fine chemicals, the direction of environment-friendly new energy and new material technology support should be considered comprehensively, and dimethyl carbonate, the main raw material of battery fluid and environmental protection gasoline additive, and vinyl carbonate, polycarbonate and polyurethane carbonate, the main raw material of biodegradable plastics should be taken in priority as the main research objects.

Bio-utilization is based on cutting-edge science and technology, converting CO₂ into biofertilizer, biofuel, artificial starch and other products through artificial photosynthesis or biomimetic catalytic technology, so that CO₂ can re-enter the ecological cycle system in the way of “returning to the living body” while achieving carbon sequestration. The efficiency of natural photosynthesis is extremely low, and the maximum efficiency of plant photosynthesis is 4.6% to 6.0% ^[26]. In the face of high CO₂ emissions, the research trend is to simulate the mechanism of CO₂ photocatalytic redution (PCR) in plant photosynthesis, and explore and break through artificial photosynthesis technology and CO₂ photocatalytic reduction to prepare high value-added products ^{[7,27⇓ -29]}. Ayuba ^[27] developed an effective catalyst for the reduction of CO₂ to acetic acid and ethylene, namely the S.ovata bacteria. A stand-alone solar system for efficient production of acetic acid was established under simulated light irradiation for over 100 h, and its energy conversion efficiency is equivalent to that of plant photosynthesis, which further demonstrates the feasibility of artificial photosynthesis. A genetic approach was used to reconstitute thermophilic bacteria (alias Pyrococcus furiosus), which can directly convert atmospheric CO₂ to 3-hydroxyl propionic acid, so it can be used to produce acrylic resins and other chemicals. The entire conversion process releases the same amount of CO₂ as is needed to make it, achieving carbon balance while producing a cleaner fuel that can replace natural gas, coal and oil. In summary, research on bio-utilization opens up a new pathway for “solar-to-chemical” (Eqs. (1)-(5)), which is of practical significance ^[30-31].

At present, microalgae carbon sequestration technology focuses on the conversion of CO₂ fixed by microalgae into liquid fuels and chemicals, biofertilizers, food and feed additives. A new pathway for the bio-utilization of CO₂ resources is microbial electrosynthesis (MES), which is the process in which microorganisms use electrical energy as a reducing force to reduce CO₂, glucose or other organic substrates into high value-added chemicals. In the actual situation that non-stationary electricity cannot yet be effectively integrated into the power grid system, the research and promotion of CO₂ reduction by microbial electro-synthesis technology can not only achieve CO₂ sequestration, but also store natural energy (wind and light energy) in the form of high value-added chemicals through electrical energy conversion, while realizing the efficient coupled metabolism of microbial cell energy and substance ^[32]. The biofuel butanediol was successfully synthesized using CO₂, light energy and solar power as feedstock with the help of bacterially modified rhodopseudomonas palustris ^[33]. When electrolytic hydrogen bubbling reaction device was used, the coulombic efficiency was 14 times higher than that of crude oil and the rate of acetic acid production was more than 10 times higher than that of biofilm MES reactor under the same bacterial population and electrode current density conditions, which effectively solved the problem of lower product synthesis rate of microbial electrosynthesis ^[34-35].

CO₂ sequestration in deep saline formations and depleted reservoirs includes physical storage and chemical storage. Physical storage includes geological structure storage and residual gas storage, and chemical storage includes dissolution storage and mineralization storage^{[39⇓⇓-42]}. CO₂ geological storage is closely related to the CO₂ fluid-solid interaction after entering the formation. After entering the rocks, CO₂ is in a supercritical state and miscible with formation water and oil and gas to form a multiphase fluid, and exchanges substances with minerals, either dissolving existing minerals or forming new minerals, thus changing the pore structure of the reservoir (Fig. 5), which affects the CO₂ sequestration efficiency ^[42-43].

Ketzer et al. ^[44] studied CO₂ water-rock interactions in saline formations in southern Brazil and confirmed that CO₂ could react with rocks to produce calcium carbonate for effective carbon sequestration under subsurface conditions. Mohamed et al. ^[45] studied sulfate precipitation during CO₂ sequestration and suggested that temperature was the main parameter affecting sulfate precipitation and injection rate has no significant effect through comparative studies of temperature and injection rate. Even if the sulfuric acid concentration is low, calcium sulfate precipitation occurs under high salinity conditions; Liu et al. ^[46] took into account the regional fluid flow while studying CO₂ sequestration in the Mt. Simon sandstone formation in the Midwestern U.S., and found a large amount of feldspar dissolution and clay mineral precipitation. Yu et al. ^[47] studied the water-rock interaction in the displacement process of formation water saturated with CO₂ in the southern part of the Songliao Basin, and pointed out the variability of different mineral evolutionary characteristics: Calcite dissolved to the greatest extent, followed by smectite, iron dolomite was the weakest, and authigenic sodium feldspar and microcrystalline quartz did not undergo significant dissolution. Elkhoury et al. ^[48] studied the dissolution and deformation of minerals in fractured carbonate reservoirs. Dávila et al. ^[49] studied the problems related to CO₂ sequestration in high NaCl and sulfate-rich formation water in Hontomín of Spain, and systematically analyzed the change of Ca²⁺, S²⁻, Fe²⁺ and Si⁴⁺ before and after the reaction, pointing out that the dissolution of calcite, the precipitation of gypsum and the dissolution of small amounts of silicate are the main mineralogical changes. Taking the tight sandstone of the 7^th Member of Triassic Yanchang Formation in the Ordos Basin as an example, the author systematically analyzed the mineral and physical property changes during CO₂ sequestration. It is indicated that that the dissolution intensity of potassium feldspar, sodium feldspar and calcite is the greatest, and the dissolution, migration and precipitation of clay minerals such as chlorite and kaolinite have important effects on the storage performance. As for the carbon fixation minerals, in addition to calcite, dolomite and montmorillonite, the author also found carbon fixation minerals such as rhodochrosite and kaolinite. From the in-situ comparison of field emission scanning electron microscopy before and after the CO₂ water-rock interaction, it can be seen that the particle size and morphology of rhodochrosite minerals shows a significant increase after the reaction (Fig. 5a-5c), and the area of kaolinite distribution also shows a significant increase (Fig. 5d-5f). Numerical simulation results of medium- and long-term supercritical CO₂ injection into sandstone strata further validate the contribution of kaolinite and rhodochrosite to carbon sequestration (Fig. 6). With the increase of sequestration time from 200 to 1000 years, the distribution area of kaolinite and rhodochrosite in the precipitation zone gradually increases; at the sequestration time of 1000 years, the volume change coefficient (the difference between the instantaneous volume and the initial volume divided by the initial volume) of siderite in the precipitation zone is 0.004 50 (Fig. 6c), the volume change coefficient of kaolinite is 0.002 86 (Fig. 6f), and the distribution range can be as far as up to 600 m from the injection well.

With the growth of storage time, the CO₂ storage pattern under subsurface conditions will change. Taking the Fuyu oil reservoir in the Qian'an area of the southern Songliao Basin as an example, the evolution of CO₂ sequestration mode in 10 000 years was systematically simulated with the adoption of ToughReact numerical simulation software. The numerical simulation results show that from 10 to 10 000 years, the CO₂ sequestration mode gradually changes from mainly tectonic residual storage (83%) to both tectonic residual storage and mineral storage, with 38% and 37% respectively, and 25% of dissolved storage in formation water (Fig. 7). The evolution of the sequestration mode is the result of the integrated geological and engineering evolution in the time frame. Overall, CO₂ sequestration is a complex project, so it is necessary to fully investigate the geological structure, rock composition, pore structure, formation water distribution, and temperature and pressure field characteristics of the sequestration site before implementation, and carefully evaluate the factors such as economic benefits and operational costs. The factors affecting CO₂ storage include seal, reservoir dip, reservoir heterogeneity, porosity, permeability, formation temperature and pressure, formation water salinity and mineralogy. The factors that affect the amount of CO₂ injection are mainly fracture pressure, reservoir thickness, maximum injection rate, geothermal gradient, burial depth, permeability, pressure gradient and porosity ^{[41,50 -51]}. With the development of technology, waste treatment based on CO₂ mineralization has become an important option for carbon sequestration, among which CO₂ blended with concrete becomes a very promising way for large-scale CO₂ sequestration. High-performance recycled aggregates are produced from CO₂ and waste concrete, and the carbon hydroxide and hydrated calcium silicate gels on the surface of the aggregates interact with CO₂ to form calcium carbonate and silica gel, which improve the performance of the aggregates ^[52].

The current methods for evaluating the CO₂ sequestration potential of saline aquifer include volumetric method, capacity coefficient method, and rapid intuitive dynamic method ^[53⇓-55]. According to the assessment results of the U.S. Geological Survey (USGS), the cumulative storage capacity of 36 basins in the U.S. is approximately 3×10¹² t CO₂. The Global Carbon Capture and Storage Institute (GCCSI) suggests that the U.S. has a sequestration potential of (2-21)×10¹² t CO₂ ^[56]. The North American Carbon Sequestration Atlas (NACSA) shows that the sequestration potential of oil and gas bearing basins in the US and Canada are 1200×10⁸ t CO₂ and 160×10⁸ t CO₂ respectively, and the sequestration potential of saline aquifers are (1.610-20.155)×10¹² t CO₂ and (0.028-0.296)×10¹² t CO₂ respectively. The sequestration potential of saline aquifers in Mexico is over 0.1×10¹² t CO₂ ^[54]. The European Union's Geo Capacity project assessment results show that the sequestration potential of European hydrocarbon-bearing basins is 300×10⁸ t CO₂ and the sequestration potential of deep saline aquifers is 3250×10⁸ t CO₂ ^[7]. The geological sequestration potential in Japan is about 1400×10⁸ t CO₂, which is mainly distributed in the large sedimentary basins around the Japanese islands, including the Tokyo Bay Basin, the Osaka Bay Basin, northern Kyushu region and the Ise Bay Basin ^[7]. The sequestration potential of the deep saline aquifers in Korea is about 9.4×10⁸ t CO₂, including 9.0×10⁸ t CO₂ in the Bukpyeong Basin and 0.4×10⁸ t CO₂ in the Pohang Basin; the oil and gas-bearing basins in Korea are mainly oil reservoirs, including sequestration potential of 30×10⁸ t CO₂ in the Oryong Basin, 235×10⁸ t CO₂ in the Jeju Basin, and 3×10⁸ t CO₂ in the Gunsan Basin ^[7]. The total sequestration potential in Indonesia, Thailand, Philippines and Vietnam is about 540×10⁸ t CO₂ ^[56-57].

The storage capacity of the deep saline aquifer in China is about 2.42×10¹² t CO₂, mainly concentrated in the Songliao Basin, Bohai the Bay Basin, the Sichuan Basin, the Ordos Basin and the Junggar Basin ^[58]. Among them, the Songliao Basin (6950×10⁸ t CO₂), Tarim Basin (5530×10⁸ t CO₂) and Bohai Bay Basin (4910×10⁸ t CO₂) are the three largest onshore saline aquifer storage areas, accounting for half of the total storage volume. Besides, the deep saline aquifers in the Subei Basin (4360×10⁸ t CO₂) and Ordos Basin (3360×10⁸ t CO₂) also have a large CO₂ sequestration potential ^[59].

Due to the problems of high energy consumption and cost of single CO₂ storage projects, and uncertainty of long-term storage safety and reliability, the research on the integration of CO₂-EOR and storage has become an important direction for CO₂ storage in oil and gas industry ^[37,60]. At present, there are more than 140 CO₂-EOR projects all over the world, 121 of which are in the U.S. The large-scale industrial application of CO₂-EOR technology is mainly in the U.S., with annual oil production maintained at about 0.15×10⁸ t, improved recovery rate of 7% to 22%, and oil production cost per barrel of 18–28, and it has become the greatest enhanced recovery technology in the U.S. In recent years, the U.S. has been actively developing a new generation of CO₂-EOR technologies with the goal of increasing recovery factor by 25%, such as nanoparticle steady-state CO₂ foam based swept volume enlarging technology, silicate polymer gel research to increase CO₂-EOR flow control, CO₂-EOR and storage planning software research, CO₂-EOR flow control and geomechanical simulator research, and small molecule-conjugated CO₂ thickener for improving flow control. In terms of the number of CCS projects and the amount of storage carried out worldwide, CO₂ capture, enhanced oil recovery and storage (CCS-EOR) is the main approach and direction (Table 1).

China has explored CO₂-EOR technology in the Daqing Oilfield since 1960s, and has carried out scientific and technological researches such as national “973”, “863” and national major projects, and has built demonstration areas of CO₂-EOR and storage in the Jilin and Changqing Oilfield. By 2020, China has conducted 21 CCUS tests with a total storage capacity of about 130×10⁴ t CO₂ ^[7]. The CCUS tests include projects in Jilin, Daqing, Changqing and Xinjiang experimental areas, among which the on-site CCUS in Jilin Oilfield has been continuously monitored for 14 years, verifying the storage safety of the oil reservoir (Table 1). In terms of the storage capacity of depleted oil reservoir, about 51×10⁸ t CO₂ can be stored by CO₂ enhanced oil recovery technology (CO₂-EOR) in the Songliao Basin, Bohai Bay Basin, Ordos Basin and Junggar Basin. In terms of the storage capacity of depleted gas reservoirs, about 153×10⁸ t CO₂ can be stored using depleted gas reservoirs in the Ordos Basin, Sichuan Basin, Bohai Bay Basin and Tarim Basin, and about 90×10⁸ t CO₂ can be stored using CO₂ enhanced gas recovery technology (CO₂-EGR).

Terrestrial carbon sink is the capacity of the land to absorb and store carbon from the atmosphere, and terrestrial carbon pools lie in the terrestrial lithosphere, biosphere and pedosphere, etc. ^[61]. The lithosphere is the largest carbon pool on the earth, it is estimated that the total carbon stock of the whole lithosphere is about 9×10¹⁶ t CO₂, including organic carbon stock about 2×10¹⁶ t CO₂, of which the carbon stock in fossil fuels is about (5-10)×10¹² t CO₂ ^[62-63], while the earth has a total of about 10×10¹⁶ t CO₂ ^[64]. The carbon stock of the biosphere is about 0.686×10¹² t CO₂, including 662×10⁸ t CO₂ in forests and 240×10⁸ t CO² in grasslands ^[65]. The total carbon stock of pedosphere is (1.4-1.5)×10¹² t CO₂ ^{[66⇓⇓-69]}.

Overall, carbon emissions-sinks of source-sink system of the global carbon cycle system during 1850-2018 are largely balanced (Fig. 8), indicating that global ecosystems are playing an active role as carbon sinks. The cumulative terrestrial carbon sinks from 1900 to 2005 are (35.1-213.1)×10⁸ t CO₂ in the United States_, (57.8-129.3) ×10⁸ t CO₂ in Russia and (35.3-125.8) ×10⁸ t CO₂ in Canada. Overall, terrestrial carbon emissions in the United States, Europe, Canada, and Russia from 1900 to 1949 are offset (or nearly offset) by terrestrial carbon sequestration from 1950 to 1989, respectively, leaving terrestrial ecosystems in a net state of sequestration after 1990. By analyzing the organic carbon stock of Chinese cultivated soils in the past 20 years ^[70], the average annual carbon sink of Chinese cultivated soils was estimated to be (0.41-0.71)×10⁸ t CO₂. The National Greenhouse Gas Inventory of China shows that the greenhouse gas emissions grew very fast from 1994 to 2014, and the emissions in 2014 were 3.3 times those in 2004. Meanwhile, the carbon sink of land use, land-use change and forestry (LULUCF) also grew faster, from 4.07×10⁸ t CO₂ in 1994 to 11.25×10⁸ t CO₂ in 2014, an increase of 1.76 times. The carbon sink of LULUCF accounted for 10.94% of carbon emissions in 2014. In terms of carbon budget, China's terrestrial ecosystem is playing a role of carbon sink.

Ocean carbon sink is the total amount of CO₂ from the atmosphere that the ocean absorbs and fixes in various ways. 93% (38.4×10¹² t) of the earth's CO₂ is stored in the ocean. Therefore, the use of ocean “carbon sinks” and the development of ocean low-carbon technologies are of great significance to achieve China's “dual carbon” goal. In 2018, Shenzhen Dapeng New Area took the lead in conducting a study on ocean carbon sink accounting covering the sea area under its jurisdiction, and compiled China's first "Ocean Carbon Sink Accounting Guide", signaling that China will start to develop ocean carbon sinks vigorously ^[7].

As an important element of the carbon industry system, the maturity of carbon market and carbon finance is of great significance to promote the rapid development of carbon industry. The tracking and accounting of carbon footprint and the assessment of carbon asset value have become the necessary inherent demand of carbon trading market and the objective need of ecological civilization construction. Carbon footprint is a measurement indicator of greenhouse gas emissions, and carbon asset is a new type of important asset in the low carbon economy.

Carbon trading is the trading of greenhouse gas emission rights (emission reductions) established using market mechanisms to promote global carbon emission reduction. Specifically, one party with a purchase contract pays the other party to obtain a set amount of greenhouse gas emission rights. Carbon trading market regulation refers to the supervision and management of the initial allocation, exercise and trading of carbon emission rights through legal, economic and administrative means ^[7]. China's carbon emission trading market was officially launched on July 16, 2021, marking the curtain of carbon emission trading of national nature in China is opened. China uses market mechanisms to regulate greenhouse gas emissions and actively promotes green and low-carbon development, effectively contributing to the achievement of the “dual carbon” goal.

The subjects of carbon trading market include emission control enterprises, financial institutions, individual investors, trading institutions, registration authorities, third-party institutions, technical support institutions and governments. Carbon market trading products include carbon emission rights (carbon emission allowances) and emission reduction credits, etc. Carbon emission rights are issued by the government based on the total carbon emission target and represent the right to use the carbon emission space; emission reduction credit refers to the spontaneous emission reduction behavior of enterprises/residents, whose emission amount is calculated according to the standard methodology and recognized by the competent authorities ^[7]. The overall workflow of carbon trading is divided into six major steps: enterprise registration, allowance issuance, allowance trading, emission verification, enterprise compliance and cancellation. Fig. 9 shows the landmark events in the development of the global carbon trading market. The statistical report of the International Carbon Action Partnership (ICAP) shows that 38 national jurisdictions and 24 states, cities or regions are operating carbon trading markets globally ^[7]. Jurisdictions account for 54% of global GDP; the 24 carbon trading systems in operation cover 16% of global GHG emissions ^[79].

The carbon industry system is a globally recognized sunrise industry and a product of the high development of human society. It not only involves the traditional energy industry, construction industry and financial industry, but also has an important impact on environmental protection and the green development of the earth. Like other environmental protection actions, the carbon industry system with carbon neutrality as its core is a realistic action for human beings to protect our common home. Carbon industry is a decarbonized energy revolution, ecological technology revolution and green industrial revolution for all human beings, which will bring new changes to human society and economy.

(1) From the perspective of the energy revolution, the carbon industry will accelerate the transformation of the world energy system to low-carbon and carbon-free. The world energy consumption structure will change from the “four equal shared” pattern of coal, oil, natural gas and new energy to the “one major, three minors” pattern with new energy as the main source. In China, for example, in 2021, the proportion of carbon-based energy consumption was 80% with coal, oil and natural gas as the main sources, and the proportion of zero-carbon energy consumption was 15.9% with new energy as the main sources; it is expected that by 2060, the proportion of zero-carbon energy consumption will increase to 80% with the revolutionary conversion of zero-carbon energy and carbon-based energy. On this basis, China's carbon emissions will be reduced from the current level of about 105×10⁸ t to about 20×10⁸ t, achieving a double reduction of total carbon emission reduction of 85×10⁸ t and carbon emission reduction ratio of 80%. Therefore, under the carbon industry system, new energy has risen from a new resource to a new strategy and a new mission, playing an important role in promoting the construction of an energy power and achieving global carbon neutrality.

(2) From the perspective of ecological scientific and technological revolution, the scientific and technological revolution is the driving force for the development and progress of human society, and the impelling force for industrial and energy revolutions, and scientific innovation and technological progress is the key to achieving the goal of carbon neutrality. The world is in the period of the forth industrial revolution and the sixth scientific and technological revolution, which is an information-controlled scientific and technological revolution with atomic energy technology, intelligent technology, space technology and bioengineering technology as the main symbols, involving many technical fields such as new energy, new materials, information, space, ocean and biology. The integration of the carbon neutral goal with the new round of science and technology revolution will guide the scientific and technological revolution to the direction of ecological development. The new scientific and technological revolution and the industrial revolution will drive the third world energy transformation. In particular, under the goal of carbon neutrality, the low-carbon and ecological characteristics of the new scientific and technological revolution will become more prominent. The new energy utilization technology represented by renewable energy will become the leading force, and green, ecological and sustainable development will be the important theme of the new science and technology revolution.

(3) From the perspective of green industrial revolution, the advanced information technology represented by big data and artificial intelligence has set off the fourth industrial revolution. The carbon industrial revolution will change the traditional high-carbon and coarse development model, and accelerate the construction of a low-carbon, environmentally friendly and efficient green development model. The scientific and technological revolution triggered by carbon industry will lead to significant changes in social and economic development and guarantee the successful achievement of carbon neutrality goal. In the process of human industrialization, it will give birth to new industries such as carbon industry with CCUS as the core and hydrogen industry with green hydrogen as the core.

The development of carbon industry system and the construction of carbon neutrality society is a feat for human beings to save the earth and human civilization, which requires every person, every enterprise and every country to agree and develop the idea of carbon neutrality society, establish carbon neutrality social order and make up for previous damages to the earth. The carbon industry is the basic condition to ensure fresh air, pleasant temperature, vigorous vitality, and clean environment on the earth. When the whole human society is included in the carbon neutrality system, human beings will regain and enjoy the green and livable earth continuously.