The exploration stage of the global CCUS development was began in 1952 when the CO₂-EOR technology patent was granted to Whorton, Brownscombe and Dyes, three researchers of the Atlantic Refining Company ^[12] (Fig. 1). At this stage, the world had a low industrial CO₂ capture capacity, with relevant technologies under exploration, and oil companies applied CO₂-EOR technology commercially for benefits and enhanced oil recovery.

At this stage, the global industrial CO₂ capture capacity was less than 500×10⁴ t/a, accounting for about 2/10 000 of the global carbon emissions ^[5,15]. Specifically, the capture capacity from natural gas processing took about 2/10 000 of the carbon emissions from the energy sector (Fig. 1) and about 3/10 000 of the carbon emissions from the oil and gas sector ^{[5,16 -17]}. The world's first commercial CO₂-EOR project was implemented in January 1972 by Chevron in the SACROC block of the Kelly-Snyder field in the Permian Basin, the United States ^[18], with CO₂ originally sourced from the Val Verde natural gas processing plant, and currently supported by another contributor - McElmo Dome high-concentration natural CO₂ reservoir, with a CO₂ volume fraction of 98% ^{[18⇓⇓-21]} (Table 1). This is the largest CO₂ miscible flooding project around the world so far ^[18].

The main reason why oil companies launched CO₂ flooding projects in this period was the availability of low-cost carbon sources ^[7,22]. The success of CO₂-EOR in the SACROC block and high oil prices in the late 1970s to early 1980s pushed the construction of three CO₂ pipelines that connected oilfields in the Permian Basin to high-concentration natural CO₂ reservoirs ^[7,22]. High-concentration natural CO₂ reservoirs became the main carbon source for early CO₂ flooding projects.

Following the Norwegian government’s levy of CO₂ emission tax in 1991, the European Union and the United States successively issued policies and acts on climate security and low-carbon economy, recording the onset of a policy-driven stage for global CCUS industry^[11,13] (Fig. 1). At this stage, the global industrial CO₂ capture capacity grew slowly. The industrial carbon sources were mainly natural gas processing plants, and the proportion of other industrial sources increased gradually. Driven and influenced by carbon taxes and related policies, more oil companies began using CO₂ from industrial carbon sources for enhancing oil recovery, and they also started projects to store CO₂ in saline aquifers.

At the end of this stage, the global industrial CO₂ capture capacity was more than 2800×10⁴ t/a, accounting for about 8/10 000 of the global carbon emissions ^[5,15]. Natural gas processing plants accounted for 76% of industrial carbon sources, and their CO₂ capture capacity took less than 7/10 000 of the carbon emissions from the energy sector and about 1/1000 of the carbon emissions from the oil and gas sector ^{[5,16 -17]}. The Century Plant, jointly operated by Occidental Petroleum Corp. and SandRidge Energy, Inc., is a typical CCUS project with natural gas processing plants as the carbon source. It had a CO₂ capture capacity of 500×10⁴ t/a ^[5,20], and its captured CO₂ was used in oilfields in the Permian Basin of the United States for enhancing oil recovery ^[23] (Table 1). At this stage, the global oil production from CO₂-EOR projects increased steadily, reaching more than 1300×10⁴ t in 2014 ^[7].

In addition to CCUS-EOR projects, CO₂ storage in saline aquifers emerged at this stage. In 1996, the Sleipner saline aquifer CO₂ storage project, run by Equinor in the North Sea, was put into operation. It is the world's first large commercial project of CO₂ storage in saline aquifer, with a CO₂ capture capacity of about 100×10⁴ t/a ^[5] and a natural gas processing plant operated by Equinor as the carbon source ^[24⇓-26]. Natural gas produced by Sleipner West gas field operated by the same company contained 9% CO₂. Injecting the CO₂ into the saline aquifer could meet the requirements of commercial gas export, and more importantly, avoid the Norwegian government's carbon tax ^[24⇓-26] (Table 1).

Since the signing of the Paris Agreement at the 21st Conference of Parties to the United Nations Framework Convention on Climate Change (COP21) in 2015, national and international agreements, policies and regulations related to climate change have become more influential, and the CCUS development has entered a stage driven by both economic and social benefits (dual-drive stage) ^[14] (Fig. 1). At this stage, the global industrial CO₂ capture capacity increased rapidly. Although industrial carbon sources were still dominated by natural gas processing plants, the proportion of other industrial carbon sources increased significantly.

At this stage, the global industrial CO₂ capture capacity rose to more than 4000×10⁴ t/a, accounting for more than 1/1000 of global CO₂ emissions ^[5,15]. Natural gas processing plants accounted for 69% of industrial carbon sources ^[5], and their CO₂ capture capacity took more than 8/10 000 of the CO₂ emissions from the energy sector and more than 1/1000 of the CO₂ emissions from the oil and gas sector ^{[5,16 -17]}. Other industrial carbon sources included fertilizer production, synthetic natural gas and methanol production, ethanol production, power generation, hydrogen production, steel production, etc. ^[5]. The Alberta Carbon Trunk Line in Canada is the CCUS-EOR project with the largest CO₂ capture capacity (190×10⁴ t/a) that began operation during this period. Its carbon sources are fertilizer plants and North West Redwater Sturgeon Refinery, and its captured CO₂ is used for enhancing oil recovery in oilfields near Clive, Alberta, Canada ^[37] (Table 1).

Large CCUS projects running around the world have extended their focus from CO₂-EOR to CO₂ storage in saline aquifers. At this stage, the number of CCUS projects with saline aquifer as carbon sink and their proportion of capture increased significantly. In 2022, nine CCUS projects globally had carbon sinks in saline aquifers, with a total CO₂ capture capacity of more than 1000×10⁴ t/a ^[5]. Typically, Gorgon CO₂ injection project in Australia had the largest CO₂ capture capacity, which was about 400×10⁴ t/a ^[5] (Table 1).

The distribution of CCUS projects worldwide is shown in Fig. 2 ^[38]. According to Rystad Energy, there were 65 CCUS projects in operation globally by the end of 2022, with a CO₂ capture capacity of about 4 100×10⁴ t/a. For projects in operation, those in North America took a dominant position by quantity and by CO₂ capture capacity, with proportions of 37% and 61% respectively. Proportions of those in Europe were 14% and 5% respectively, and proportions of those in East Asia were 22% and 8% respectively ^[38]. For projects under construction and planning, those in North America and Europe took an equally leading part by quantity and by CO₂ capture capacity, with proportions of 39% and 46% respectively, and 36% and 34% respectively. Proportions of those in East Asia were both 6% ^[38] (Fig. 2).

By 2017, the cumulative injection of CO₂ from industrial carbon sources globally was approximately 2.2×10⁸ t, with the top three countries being the United States, Canada and Norway ^[39]. The cumulative injection in the United States was about 1.53×10⁸ t, of which about 1.52×10⁸ t injected into reservoirs for enhancing oil recovery and about 0.01×10⁸ t injected into saline aquifers ^[39]. The cumulative injection in Canada was approximately 0.38×10⁸ t, of which about 0.37×10⁸ t injected into reservoirs for enhancing oil recovery and about 0.01×10⁸ t injected into saline aquifers ^[39]. The cumulative injection in Norway was about 0.2×10⁸ t, all of which injected into saline aquifers ^[39].

The carbon sinks of global projects in operation, under construction and planning by the end of 2022 are shown in Fig. 3 ^[38]. According to Rystad Energy, carbon sinks of projects in operation were mainly CO₂-EOR reservoirs. The number of such projects was 28, with a CO₂ capture capacity of 2906×10⁴ t/a, accounting for 71.5% ^[38]. Carbon sinks of projects under construction and planning were mainly other geological bodies for CO₂ storage. The number of such projects was 313, with a CO₂ capture capacity of 3.63×10⁸ t/a, accounting for 77.8% ^[38]. In terms of carbon sources, those of projects in operation were mainly energy sectors such as oil and gas production, accounting for 62%; and those of projects under construction and planning were mainly industrial processes and power generation, accounting for about 71% ^[38].

In order to enhance economies of scale, CCUS projects in North America and Europe have formed large-scale industrial clusters. Carbon sources and sinks in the clusters can share infrastructures. According to the types of carbon sources and sinks, CCUS clusters can be divided into three categories.

The first category is the clusters of oil companies (carbon source) + hydrocarbon reservoirs (carbon sink), where the carbon sources are high-concentration natural CO₂ reservoirs of oil companies and natural gas processing plants, and carbon sinks are hydrocarbon reservoirs. Such clusters were formed in the exploration stage of CCUS development (1952-1990). Two clusters in the Permian Basin and the Rocky Mountains in the United States are the typical ones. In 2020, carbon sources of the cluster in the Permian Basin included 4 high-concentration natural CO₂ reservoirs and 2 industrial carbon sources (the Century natural gas processing facility and the Val Verde natural gas processing plant). CO₂ supplied by natural CO₂ reservoirs accounted for 92.6%, and that supplied by industrial carbon sources accounted for 7.4%. In 2020, the cluster increased crude oil production by 2.2×10⁴ t/d ^[7]. In 2020, carbon sources of the cluster in Rocky Mountains included 1 high-concentration natural CO₂ reservoir and 2 industrial carbon sources (the Shute Creek gas processing facility and the Lost Cabin gas plant). CO₂ supplied by natural CO₂ reservoirs accounted for 22.7%, and that supplied by industrial carbon sources accounted for 77.3%. In 2020, the cluster increased crude oil production by 4341 t/d ^[7].

The second category is the clusters of industrial processes (carbon source) + hydrocarbon reservoirs (carbon sink), where the carbon source is CO₂ production of non-oil companies, and carbon sink is hydrocarbon reservoirs. Such clusters gradually developed in the policy-driven stage of CCUS development (1991-2014). Two clusters in the central United States and Michigan are the typical ones. In 2020, carbon sources of the cluster in the central United States included 1 high-concentration natural CO₂ reservoir and 5 industrial carbon sources (the Coffeyville Resources Nitrogen fertilizer plant, the Enid fertilizer plant, the Agrium fertilizer plant, the Arkalon ethanol facility, and the Conestoga Bonanza ethanol plant). CO₂ supplied by natural CO₂ reservoirs accounted for 20.8%, and that supplied by industrial carbon sources accounted for 79.2%. The cluster increased crude oil production by 1173 t/d ^[7,20]. In 2020, carbon sources of the Michigan cluster included 5 industrial carbon sources mentioned above, and the cluster increased crude oil production by 59 t/d ^[7].

The third category is the clusters of industrial processes (carbon source) + saline aquifers (carbon sink), where the carbon sources are various industrial carbon sources, and carbon sinks are underground saline aquifers. Such clusters gradually developed in the dual-drive stage of CCUS development (since 2015). European Northern Lights cluster and Net Zero Teesside cluster are the typical ones ^[40]. The Northern Lights cluster is a CO₂ storage project invested by Equinor, Shell and TotalEnergies in the North Sea, which transports CO₂ from multiple industrial emission sources in Northern and Western Europe to the North Sea by ship for storage in underground saline aquifers. Emission sources expected to join the cluster in 2024 include the cement plant in Brevik and the power plant in Fortum Oslo Varme ^[40⇓-42]. The Net Zero Teesside cluster in the United Kingdom transports CO₂ from multiple industrial emission sources such as power plants and hydrogen production plants to the North Sea by ship for storage in underground saline aquifers, which is expected to start operations in 2026 ^[43-44].

The current CO₂ capture technologies mainly fall into three series: absorption, membrane separation, and pressure swing adsorption (PSA) ^[6]. The absorption technology is highly mature and has been used as the main method for CO₂ capture for more than 40 years ^[6]. Solvent-based physical absorption is mainly adopted for high- concentration carbon sources, with solvents used including polyethylene glycol dimethyl ether and methanol; while chemical absorption is mainly adopted for low-concentration carbon sources ^{[31,37,45⇓⇓⇓⇓ -50]}. The carbon sources, capture methods, capture capacity and pipeline transportation distances of the eight large CCUS projects in operation in North America are shown in Table 2.

CO₂ transportation technology series mainly involve pipeline, railway, truck and ship ^[6]. The United States has developed a mature supercritical CO₂ pipeline transportation technology, with more than 8000 km of pipelines in operation as of 2019 ^[6]. The maximum source-sink distance for large CCUS projects in North America is 500 km ^{[31,37,45⇓⇓⇓⇓ -50]} (Table 2).

The CCUS-EOR projects are mainly concentrated in the United States and Canada. The United States produced oil of more than 1000×10⁴ t in 1994 and 1365×10⁴ t in 2020 (Fig. 1). It has built large projects for CO₂ flooding, which are equipped with easy, efficient and highly automated injection, production and ground engineering facilities, and the produced gas recycling technology meets the overall efficiency improvement requirements of the projects ^[9]. By 2020, a total of about 3.9×10⁸ t CO₂ was injected into the SACROC block in the Permian Basin of the United States, which had been in stable production for 16 years ^[9,21]. In Canada, the recovery percent before CO₂ injection in the Weyburn field was about 24%, and is expected to increase by more than 15 percentage points from CO₂-EOR ^[51⇓-53].

As to CO₂ geological storage technology, simulation software such as TOUGHREACT and PFLOTRAN have been developed. Storage monitoring methods include surface monitoring, time-lapse seismic monitoring, etc., covering both surface and underground. Typical large saline aquifer storage projects in operation globally and their monitoring technologies are shown in Table 3.

The U.S. government has introduced a number of acts to incentivize the CCUS industrialization. Taking the 45Q tax credit as an example, progressive pricing of CO₂ subsidy was adopted as a stimulus for CCUS investment. The policy was implemented flexibly and diversely, supported with well-established assessment mechanism. In 2022, within the framework of Inflation Reduction Act, the U.S. government amended the 45Q tax credit, adjusting the subsidy price for CO₂ geological storage from $50/t to $85/t, and the subsidy price for CCUS-EOR from $35/t to $60/t ^[59⇓-61]. Moreover, the U.S. Department of Energy invested $2.7×10⁸ in 2020 to support CCUS projects, which also greatly encouraged the initiatives of enterprises ^[8].

In Europe, the support for CCUS projects is reflected in the EU carbon trading market and various funds ^[8]. Horizon 2020, Horizon Europe, and the Innovation Fund with a total amount of €100×10⁸ have provided public funding support for CCUS projects ^[8]. In Australia, a carbon tax mechanism was introduced through the Clean Energy Act in 2011, which came into effect in 2012 ^[62].

The Oil and Gas Climate Initiative (OGCI) is an international organization made up of 12 energy companies. OGCI Climate Investments manages funds worth more than $1 billion, aiming to accelerate global implementation of low-carbon solutions to combat climate change ^[11]. Its member companies are involved in the construction of more than 20 potential CCUS hubs, 7 of which are scheduled to begin construction before 2025 ^[11].

At this stage, China kicked off its exploration of CO₂ flooding. In PetroChina Company Limited (“PetroChina”), Daqing Oilfield started well group test with carbonated water injection in 1965 ^[63], and Jilin Oilfield carried out multi-well group CO₂ huff and puff test in 1994 and CO₂ flooding test in 1999 ^[63]. In 2003, Daqing Oilfield carried out pilot test on CO₂ flooding in ultra-low permeability reservoirs, which realized the effective exploitation of ultra-low permeability reservoirs ^[63]. China Petroleum & Chemical Corporation (“Sinopec”) has begun to carry out CO₂-EOR mechanism experiments since 1967, as well as CO₂ flooding tests and actual production by its Shengli, Huadong, Jiangsu and Caoshe oilfields ^[69].

This stage started with the 2006 Xiangshan Science Conference ^[63], at which a consensus was reached on the following two aspects. First, CO₂ emission reduction and utilization must be closely integrated; and second, the main way of CO₂ utilization is enhanced oil recovery ^[63]. PetroChina has taken the lead in undertaking a number of major CCUS-EOR technological research and demonstration projects, such as the National Key Basic Research Program of China (973 Program), the National High Technology Research and Development Program of China (863 Program), and the National Science and Technology Major Project. It has established a relatively complete series of CCUS-EOR technologies and standards for lacustrine reservoirs, continuously leading the development of CCUS-EOR industry ^[66]. Its Jilin Oilfield, Daqing Oilfield and Xinjiang Oilfield have carried out pilot tests and industrial demonstration application tests ^[63]. The carbon source of Jilin Oilfield comes from the CO₂ captured by the Changling Natural Gas Processing Plant, and that of Daqing Oilfield comes from the CO₂ captured by natural gas processing plants and petrochemical companies ^[10,66,68]. Shaanxi Yanchang Petroleum (Group) Co., Ltd. (“Yanchang Petroleum”) has built pilot test areas in Jingbian and Wuqi and a demonstration project in Ansai for CO₂ flooding and storage ^[66,70]. China Energy Investment Corporation (“CHN ENERGY”), China Huaneng Group Co., Ltd. (“China Huaneng”), China Guodian Corporation, etc. are also making great efforts to develop CO₂ capture technology, and have carried out a series of demonstration projects, e.g. Beitang Thermal Power Plant project in Tianjin of China Guodian Corporation put into operation in 2012 ^[8].

In September 2020, China announced its plan to achieve carbon peak before 2030 and carbon neutrality before 2060 at the 75th session of the United Nations General Assembly. Since then, the CCUS industry in China has entered the stage of industrialization. CO₂ injection by Chinese oil companies has risen rapidly, and a complete CCUS technology system has been basically formed. In 2022, the total amount of CO₂ injected into oil reservoirs in China was 180×10⁴ t, accounting for less than 2/10 000 of the country’s total CO₂ emissions, less than 2/10 000 of the country’s CO₂ emissions from the energy sector, and about 7/10 000 of the country’s CO₂ emissions from the oil and gas sector ^{[15⇓-17,71]}.

According to the Report on China Carbon Dioxide Capture, Utilization and Storage (CCUS)-2023, by the end of 2022, there were nearly 100 CCUS demonstration projects in China that had been put into operation and were under planning and construction, of which the CO₂ capture capacity of those in operation was about 400×10⁴ t/a. China already has the engineering capacity to capture and store CO₂ on a large scale, and is actively preparing to build full-process CCUS clusters ^[8,72]. At the end of 2022, a total of about 780×10⁴ t of CO₂ was injected cumulatively nationwide ^[71]. At present, hydrocarbon reservoirs are the main carbon sinks of CCUS demonstration projects in China. Recently, Shaanxi Coal Chemical Industry Construction (Group) Co., Ltd. has begun to build a saline aquifer CO₂ storage project ^[72-73].

By 2022, the cumulative CO₂ injection amount of PetroChina was 563.0×10⁴ t (of which 111.0×10⁴ t was injected in 2022, resulting in an oil increase of 24.8×10⁴ t), accounting for more than 70% of the country's total. At present, PetroChina has entered the stage of CCUS industrial demonstration and application ^[63,71]. Its Daqing, Jilin, Xinjiang, and Changqing oilfields have built CCUS- EOR full-industry chain demonstration projects, and its Liaohe, Jidong, Dagang, Huabei, and Tuha oilfields and China Southern Petroleum Exploration & Development Corporation have built CCUS-EOR pilot test projects ^[9-10,66]. PetroChina has planned three 10-million-ton CCUS bases in the Ordos Basin, Junggar Basin and Songliao Basin to form clusters ^[9-10,66,74]. In particular, Changqing Oilfield will build three megaton CCUS demonstration zones in three provinces/autonomous regions of Shaanxi, Gansu and Ningxia to drive the development of CCUS industry, and build a ten-million-ton CCUS base in the Ordos Basin during the "14th Five-Year Plan" period. Xinjiang Oilfield has developed a ten-million-ton CCUS vision, planning to achieve the goals of 1000×10⁴ t/a CO₂ flooding in oil reservoirs and 1000×10⁴ t/a saline aquifer storage and build of a ring network for CO₂ pipeline transportation in three five-year periods ^[75-76]. Good results have been achieved in CCUS-EOR field tests in Jilin, Daqing, Changqing, and Xinjiang oilfields. In the CCUS-EOR miscible flooding test of sandstone ultra-low permeability reservoir in Hei-79 North block of Jilin oilfield, the cumulative gas injection was more than 1.2 PV (hydrocarbon pore volume), the recovery was increased by 24.7 percentage points, and the expected increase in recovery was more than 25 percentage points ^[9-10,66].

Sinopec has carried out researches on CO₂-EOR in various types of reservoirs. It carried out field tests in Jiangsu, Shengli, and Huadong oilfields, and achieved remarkable results ^[66]. In 2022, it announced that it had completed the megaton CCUS-EOR project in Qilu Petrochemical-Shengli Oilfield ^[66]. In the same year, it announced that it would jointly launch China's first open ten-million-ton CCUS project in eastern China, together with Shell, China Baowu Steel Group and BASF Group ^[77].

Yanchang Petroleum has been actively exploring CCUS-EOR technology, with positive progress made in tackling key problems of integrated technology and full-process, low-cost commercial engineering demonstration. It built four CO₂ flooding and storage demonstration zones in Jingbian Qiaojiawa, Wuqi Yougou, Wuqi Baibao and Ansai Huaziping, with a gas injection capacity of 15.00×10⁴ t/a. A total of 28.70×10⁴ t gas was injected in these zones, resulting in an increase in oil production by 4.73×10⁴ t and an increase in recovery factor by more than 8 percentage points on the basis of water flooding ^[70].

Shaanxi Coal Chemical Group Yulin Chemical Company has begun to build a 40×10⁴ t/a CO₂ saline aquifer storage pilot project, with the goal of building a 400×10⁴ t/a CO₂ saline aquifer storage demonstration project. At present, Well Yutan-1 of the pilot test project has started drilling ^[73].

For CO₂ capture technology, China has developed amine adsorption method, membrane separation method and pressure swing adsorption method^[66-67]. Jilin Oilfield has applied the amine adsorption method to Changling gas field. When the volume fraction of CO₂ was less than 30%, about 10×10⁸ m³ CO₂ was captured efficiently, helping Changling gas field to produce a total of 137×10⁸ m³ natural gas ^[66-67]. The 15×10⁴ t CCUS demonstration project of Guoneng Jinjie power plant is a post-combustion carbon capture demonstration project of the largest coal-fired power plant in operation in China. Its CO₂ capture rate is more than 90%, CO₂ volume fraction after capture is greater than 99%, and energy consumption of CO₂ regeneration is less than 2.4 GJ/t ^[78].

In the United States, CO₂ is transported mainly through pipelines. In China, however, CO₂ transportation through pipelines has not yet formed its own scale, with tank truck as the main vehicle currently. Some CO₂ in East China oil and gas field and Lishui gas field is transported through ships. Jilin Oilfield has built a 50 km CO₂ transportation pipeline, with a capacity of 50×10⁴ t/a. Sinopec’s Qilu Petrochemical-Gaoqing CO₂ transportation pipeline has recently been fully completed ^[8-9,79].

Lacustrine sedimentary geological bodies in China are significantly different from foreign marine geological bodies in crude oil quality, mineral composition, reservoir heterogeneity and clay content. Therefore, characteristics of lacustrine sedimentary geological bodies should be considered in domestic CCUS-EOR schemes. Chinese oil companies have built a relatively complete theoretical and technical standard system of CO₂ flooding in lacustrine sandstone reservoirs. The results of demonstration projects in Jilin and Daqing oilfields show that CO₂ flooding can increase oil recovery by 10-25 percentage points, and the effect of CO₂ flooding and storage is remarkable ^[66].

PetroChina has established a fine reservoir description process and method suitable for CO₂ flooding, and formed a reservoir engineering optimization design technology based on pressure maintenance and irregular water-alternating-gas (WAG) injection to expand the sweep volume ^[66]. PetroChina has also developed anticorrosive oil well cement, compound corrosion inhibitor system and inhibitor filling process, and established full-process anti-corrosion technology of CO₂ flooding ^[66]. The coiled tubing injection process innovatively developed. It reduced the one-time completion investment by 28%, and the cost during the service life could be reduced by 66%^[66]. By applying its "anticorrosion - gas lift - assisted pumping - casing control" integrated gas-carrying lifting technology, PetroChina has realized the normal production of oil wells with high gas-liquid ratio (>200 m³/m³). By using the gas/liquid separate transportation technology, PetroChina has realized the normal production management of the gathering and transportation system after the increase of gas-liquid ratio and gas channeling, built the first CO₂ cyclic injection station in China, and achieved the airtight recovery of associated gas ^[66,80].

At present, CCUS projects in China have a small scale of storage, most of which focusing on CCUS-EOR. The monitoring methods have been studied mainly from three dimensions, i.e. air, surface and underground, but they have been rarely used in the field, so further analysis on their effect is needed.

Since the 11th Five-Year Plan period, China has introduced a number of policies to support the development of CCUS technology ^[3]. Since 2006, the State Council has issued policy documents related to CO₂ emission control, disposal and utilization technologies, and various ministries/commissions have proposed to formulate a roadmap for the development of CCUS technology in the country ^[3]. In 2011 and 2013, the Ministry of Science and Technology issued the Roadmap for the Development of Carbon Capture, Utilization and Storage Technology in China and the Twelfth Five-Year Plan for the Development of Carbon Capture, Utilization and Storage Technology in China, which evaluated and deployed the development of CCUS technology in the country ^[3].

Since the goal of carbon neutrality was proposed, China has increased its support for the development of CCUS industry. A carbon trading platform has been launched at the national level, playing a positive role in promoting the implementation of CCUS projects. The “1+N” policy system has been introduced to support the "dual carbon" goal. "1" consists of two documents: "Opinions of the Central Committee of the Communist Party of China and the State Council on Full, Accurate and Comprehensive Implementation of the New Development Philosophy and Doing a Good Job in Carbon Peaking and Carbon Neutrality" and the "Action Plan for Peaking Carbon Dioxide Emissions before 2030". "N" refers to the implementation plans for key areas and industries and related support and guarantee schemes ^[81]. As of October 2022, China had issued 70 policies related to the CCUS industry at the national level, including plans, standards, roadmaps, etc. CCUS has been written into China's 14th Five-Year Plan for the first time. Policy documents issued by governments at all levels, such as the “Action Plan for Peaking Carbon Dioxide Emissions before 2030”, have made plans for the scientific research, industrialization and investment of CCUS ^[8,82]. In 2021, China's Ministry of Ecology and Environment issued the Administrative Measures for Carbon Emissions Trading, and established Chinese Certified Emission Reduction (CCER) to quantitatively certify the greenhouse gas emission reduction effects of renewable energy, forestry carbon storage, methane utilization and other projects in China, and register greenhouse gas emission reductions in the national voluntary greenhouse gas emission reduction trading registration system ^[82].

In 2013, under the promotion of the Ministry of Science and Technology, China Technology Strategic Alliance for CO₂ Capture, Utilization and Storage (CTSA-CCUS) was established, consisting of 4 chairman units of China Huaneng, PetroChina, Sinopec and CHN ENERGY and a number of director units ^[83]. The purpose of the alliance is, under the guidance of relevant government policies, through a long-acting, orderly alliance mechanism, and combined with the needs of climate change and industrial development, to integrate and coordinate scientific research forces and industrial resources, to vigorously promote technological innovation and engineering demonstration in China's CCUS field, to rapidly improve the overall technological level, and to give full play to the key role of CCUS technology in ensuring national energy conservation and emission reduction and achieving climate change goals ^[84].

Among CO₂ capture technologies, those for medium and low-concentration CO₂ sources are expensive, restricting their application on a large scale. CO₂ emissions in China are mainly of low concentration, and there are still some challenges in process, technology and equipment for low-cost capture of low-concentration CO₂ in the million-ton range ^[85]. Large CO₂-emitting companies in China are mainly involved in eight sectors, namely coal power, cement, iron and steel, coal chemical, refining, polyethylene, synthetic ammonia and calcium carbide ^[86]. Among them, coal power, cement, and iron and steel sector are low-concentration CO₂ emission sources, while coal chemical sector is a high-concentration CO₂ emission source ^[86]. China's coal-fired power plants, cement plants and steel plants will have (0.071-0.240)×10⁸ t of emission reduction demand through CCUS technology in 2025, accounting for about 80% of the country’s total emission reduction demand ^[8]. In terms of CO₂ transportation, as the source-sink distance of CCUS demonstration projects in China is between 100 and 200 km, high-cost vehicle transportation is the dominant way at present. China lacks experience in large-scale supercritical long-distance pipeline transportation, and it is difficult to commercialize long-distance carbon sources. China's coal power enterprises with annual emissions greater than 2000×10⁴ t are located in the central and eastern coasts of the country, far from sites suitable for storage, and such source- sink dislocation has resulted in high cost pressure ^[8]. In terms of CO₂ flooding, compared with marine sedimentary reservoirs, domestic lacustrine sedimentary reservoirs have high miscible pressure and strong heterogeneity, with flooding efficiency and benefits yet to be increased. In terms of CO₂ storage, efforts should be increased in researches and tests on the screening of storage geological bodies and CO₂ monitoring and regulation to ensure long-term, safe storage.

The development of the CCUS industry requires a lot of new infrastructures, such as pipelines or ships to transport CO₂ from emission sources to geological storage sites or chemical bio-utilization sites. At present, China has no pipelines or ships for large-scale CO₂ transportation, has not fully transformed and utilized existing infrastructures, and lacks a mechanism for cooperation and sharing of infrastructures. The establishment of CCUS clusters can enable related enterprises to share CCUS infrastructures at a lower cost. As there is no profit mechanism for infrastructure construction, it is difficult to build infrastructure on a large scale by relying solely on enterprises. Government leadership and planning, and corresponding financial support are needed.

The development of CCUS industry faces challenges of low initiatives of related enterprises in the entire industry chain and insufficient cooperation between source and sink enterprises, which restricts the large-scale development of the emerging CCUS industrial chain. The construction of CCUS clusters requires the cooperation between source and sink enterprises in different regions and sectors, but there are currently no coordination and incentive policies. The cost of CCUS technology is high, and there are no relevant incentive policies, tax incentives and subsidy policies to encourage relevant enterprises to implement CCUS technology, which affects the coordinated development of the industry.

It is suggested to further evaluate the CO₂ storage potential of basins, establish a database of storage potential units and carbon emission units in each basin, and build a matching relationship between carbon sources and carbon sinks. Additionally, it is necessary to make an overall plan for the research and development of key theories and technologies that restrict the large-scale promotion of CCUS, and build a research and development platform and innovation base for CCUS system.

In terms of capture technology, it is necessary to accelerate the development of low-cost, high efficient CO₂ capture technology for sources of medium and low concentration^[9,66]. In terms of transportation technology, efforts should be made to accelerate the extension of CO₂ transportation technology from vehicle transportation to supercritical CO₂ long-distance pipeline transportation ^[66]. In terms of CO₂ flooding technology, priorities should be given to accelerating the development of CO₂ flooding technology for large-scale and greater EOR, designing innovative large-scale CCUS-EOR reservoir scheme, further expanding the supporting development technologies and systems such as sweep volume, efficient injection and production, low-cost anti-corrosion, and integrated numerical simulation of CO₂ flooding and storage, and establishing a series of efficient miscible, near-miscible and immiscible CO₂ flooding technologies ^[9,66]. More efforts should be made to enhance researches of carbon reduction technologies such as gas reservoir development and utilization (CCUS-EGR), chemical utilization, bio-utilization, direct air capture of CO₂, coordinated development of CO₂ and hydrogen energy, and saline aquifer storage, so as to promote large-scale application ^[66]. For CO₂ storage technology, it is necessary to accelerate long-term, large- scale studies on the integrity of CO₂ storage geological bodies, and achieve large-scale, long-term safe storage by upgrading the design of storage schemes, improving fine geological description and wellbore integrity evaluation, implementing multi-point integrated monitoring, and improving safety prevention and control technology ^[66,85].

On the basis of planning the construction of CCUS clusters, it is necessary to fully deploy CCUS infrastructures, increase investment and construction scale, and promote the transformation and utilization of existing facilities. Governments should fully leverage their leading position in the process of infrastructure construction and provide corresponding financial support. More efforts should be made to establish a mechanism for cooperation and sharing of relevant infrastructures to enable carbon emission enterprises to make full and convenient use of CCUS infrastructures. Additionally, efforts should be increased to strengthen the cooperation between domestic/international oil and gas field enterprises and refining, coal power, coal chemical and other carbon emission enterprises, promote synergistic coupling development with renewable energy, so as to form CCUS industrial innovation consortia and industrial clusters with complementary advantages, and accelerate the development of the whole industry chain on a large scale and with benefits achieved ^[9,85].

It is suggested to improve the CCUS policy support system and the carbon trading market, help the cooperation between source and sink enterprises in different regions and sectors, and promote the coordinated development of relevant enterprises in the whole industry chain of CCUS. Efforts should be made to increase the CCUS industry's absorption of carbon emissions from coal, electric power, steel, cement, chemical and other hard-to-abate sectors, find appropriate business models and project operation models, and explore carbon quota compensation and allocation mechanisms between emitting enterprises and carbon storage enterprises ^[86]. Efforts should be made to fully learn from foreign CCUS incentive policies such as the U.S. 45Q tax credit, establish and improve relevant incentive policy systems, tax incentives and subsidy policies, etc. to encourage relevant enterprises to implement CCUS technology ^[8].

Efforts should be made to strengthen the discipline construction of carbon neutrality through multidisciplinary integration. Specifically, it is necessary to promote the construction of high-level CCUS key laboratories and innovation centers, closely integrate CCUS scientific research and industrialization, and accelerate the research and development, popularization and application of advanced applicable technologies. Additionally, it is suggested to establish interdisciplinary research teams and personnel training teams to reserve talent strength for CCUS industrialization.