Globally, CCUS-EOR is mostly implemented in the United States, Canada and other countries. Especially in the United States, a mature CCUS-EOR industrial system has been formed. In the country, CCUS-EOR was initiated in the 1950s. After consistent researches on key techniques in the 1960s-1970s, the industrial test of CCUS- EOR was expanding gradually in the 1970s-1990s, enabling the technology to be increasingly mature. After the 1980s, CCUS-EOR was put into commercialization. Since the 1980s, the industrialization of CCUS-EOR in the United States has continued to expand rapidly, allowing the annual oil production to exceed 100×10⁴ t in the early 1980s, 1000×10⁴ t in the early 1990s, and 1500×10⁴ t in 2012, and remain stable ever since ^[6] (Fig. 1).

According to the Global Status of CCS Report ^[7] of the Global Carbon Capture and Storage Institute (GCCSI), there are 11 large-scale industrial carbon capture projects in operation and 2 suspended projects in the United States for CCUS-EOR. Among these projects in operation, four are natural gas processing projects with a total annual capture capacity of (1275-1285)×10⁴ t, and the remaining seven are other industrial carbon source projects with a total annual capture capacity of (353-579)×10⁴ t (Table 1).

Over nearly 70 years, the CCUS-EOR industrial system in the United States has been mature and continuously expanded. In terms of key technologies, a sophisticated system has been developed covering large-scale CO₂ capture, long-distance pipeline transmission of supercritical CO₂, large-scale CO₂ flooding reservoir engineering design, safety monitoring on large-scale storage and other aspects. Injection, production and surface engineering equipment are simple, efficient and highly automated. The technology of dynamic monitoring and timely optimization and adjustment has been continuously developed. The produced gas recycling technologies can meet the overall efficiency improvement requirements of the project and ensure the realization of the closed zero-emission target of the whole CCUS-EOR process.

CCUS-EOR has been successfully applied in the United States. Here, the CCUS-EOR project in the SACROC block of Kelly-Snyder Oilfield in the Permian Basin is taken as an example ^[8]. In the SACROC block, the reservoir permeability is (1-30)×10^-3 μm² and the original oil in place (OOIP) is 4.1×10⁸ t. The block was put into development in 1949 and reached the peak production of 1020×10⁴ t in 1974, which declined to 40×10⁴ t in 1998. After CO₂ miscible flooding was implemented in 2002, the annual oil production surpassed 150×10⁴ t in 2005. Up to 2020, the production had been stable for 16 years, with a cumulative increase of oil production up to 2456×10⁴ t and a cumulative CO₂ injection of 3.9×10⁸ t. The EOR is expected to be more than 26%. In terms of relevant policies, the United States has issued the 45Q and 43 acts to encourage the industrialization of CCUS-EOR. With the deductible tax of $20-35 for every 1.0 t CO₂ stored for oil displacement, enterprises have been greatly mobilized to participate in CCUS-EOR projects.

The CCUS-EOR industrialization in the United States described above can enlighten China in three aspects. First, long-term technology accumulation in the whole industry chain and a large number of field tests are conducive to the formation of a sophisticated technical standard system. Second, through a large-scale pipeline network, the gas reservoir carbon source and industrial carbon source are integrated for CO₂ capture and closely connected with the reservoir to realize low-cost capture and transportation, which is expected to effectively reduce the wellhead CO₂ prices ($20-25/t ^[9-10]). Third, relevant national incentive policies are issued to continuously support the industrialization of CCUS-EOR, triggering it to expand continuously and begin to move towards CCS.

In China, researches related to CCUS-EOR started early. Oil companies and relevant colleges/universities began to explore CO₂-EOR technology in the 1960s, but the industrialization lags behind due to factors such as gas source, understanding of mechanism and availability of facilities. Since the beginning of the 21^st century, the state and oil companies have successively set up key CCUS-EOR technological research and demonstration projects, which have greatly promoted the breakthrough of key technologies and the success of field tests. So far, CCUS-EOR projects in China have total CO₂ storage of more than 660×10⁴ t, including the 450×10⁴ t of CO₂ storage from the China National Petroleum Corporation (CNPC), which enhanced the oil recovery by 100×10⁴ t cumulatively.

Since 2000, CNPC has accelerated the research, development and application of technologies ^[11-12]. It has successively taken the lead in undertaking a number of national CCUS-EOR technological research and demonstration projects, such as the National Key Basic Research and Development Plan (973 Plan), the National High-tech Research and Development Plan (863 Plan), and the national major science and technology projects^{[13⇓⇓⇓-17]}. In addition, CNPC carried out internal major technology and field test projects ^[18]. Through centralized researches and tests, CNPC found for the first time the important contribution of C₆-C₁₅ components of continental oil to miscible flooding. CNPC established a relatively complete theoretical and technical standard system for CO₂ flooding and storage in continental sandstone reservoirs, and two national CCUS-EOR demonstration projects in the Jilin and Changqing oilfields. By 2021, CNPC had performed 11 key CCUS-EOR tests, with the annual CO₂ injection capacity up to 100×10⁴ t. With the annual CO₂ injection of 56.7×10⁴ t and the annual oil production of 20×10⁴ t in 2021, CNPC has achieved outstanding performance in CO₂ flooding and emission reduction. In total, Jilin Oilfield has 5 CO₂ flooding and storage demonstration zones established with a total CO₂ injection of 212×10⁴ t, annual CO₂ injection capacity up to 40×10⁴ t, and annual oil production of more than 10×10⁴ t; among them, the Hei-79 North test zone with small well spacing for CO₂ miscible flooding is expected to have EOR over 25%. The Daqing Oilfield has a total CO₂ injection of 189×10⁴ t, with annual CO₂ injection capacity of 30×10⁴ t and annual oil production of 10×10⁴ t; particularly, the CO₂ non-miscible flooding in the Shu-101 reservoir with ultra-low permeability is expected to have EOR over 10%. At present, the Daqing, Jilin, Changqing, Xinjiang and other oilfields are in the stage of industrial testing and large-scale application for CO₂ flooding. Currently, CNPC is conducting major technological researches in the whole industry chain of CCUS-EOR. At the same time, Daqing Oilfield-Daqing Petrochemical and Jilin Oilfield-Jilin Petrochemical are constructing key demonstration projects in the Songliao Basin, NW China with an annual CO₂ injection capacity of 300×10⁴ t and an annual oil production of 100×10⁴ t. CNPC strives to achieve the goal of CO₂ injection of 500×10⁴ t and annual oil production of 150×10⁴ t by 2025, and the annual CO₂ injection capacity of 2000×10⁴ t and the annual oil production of over 600×10⁴ t by 2030.

Through technical researches over ten years ^[19⇓-21], the China Petrochemical Corporation (Sinopec) has developed a series of CO₂-EOR technologies for different reservoir types, and performed a number of field tests in the Jiangsu, Shengli, East China and other oilfields, with outstanding performances achieved. Up to now, Sinopec has implemented the CO₂ flooding projects to cover OOIP of 2512×10⁴ t with a cumulative additional oil production of 25.58×10⁴ t. Among these projects, the CO₂ near-miscible flooding pilot test in Block Gao-89-1 of Shengli Oilfield has cumulative CO₂ injection of 31×10⁴ t and cumulative additional oil production of 8.6×10⁴ t up to August, 2021. The EOR of the project is expected to be 17.2%. Recently, it was announced that one CCUS-EOR project with a capacity of one million tons has been constructed jointly by Qilu Petrochemical Company and Shengli Oilfield^[22]. It is expected to have cumulative CO₂ injection of 1068×10⁴ t and additional oil production of 296.5×10⁴ t in the coming 15 years.

In recent years, the Yanchang Oilfield has actively probed into CCUS-EOR ^[23] and made significant progress in integration technology and whole-process low-cost commercialization. It implemented the CCUS demonstration project in Jingbian and Wuqi with an annual processing capacity of 15×10⁴ t, cumulative CO₂ injection of 21.6×10⁴ t, and with expected EOR of more than 8%. In addition, it is expected to have an annual CO₂ injection capacity of 100×10⁴ t during the 14^th Five-year Plan period.

There are few devices with emission of high-concentration CO₂ and few gas reservoirs with high content of CO₂. Generally, high-concentration CO₂ emission facilities are adopted in fertilizer, hydrogenation and coal chemical sectors, with the CO₂ concentration over 90%^[4]. At present, domestic petrochemical enterprises (e.g. Daqing Petrochemical Company, Jilin Petrochemical Company, and Qilu Petrochemical Company) and large coal chemical enterprises (e.g. China Shenhua Coal-to-Liquid and Chemical Co., Ltd.) have sophisticated technologies for capturing high-concentration CO₂ at the cost of less than RMB 200/t. The Jilin Oilfield is mature in physical separation technique for carbon capture in gas reservoirs with high CO₂ content, with the capture cost less than RMB 120/t.

The high costs for the capture of low-concentration carbon seriously restrict the economic and large-scale application. Low- or medium-concentration CO₂ mainly comes from coal power, cement, steel, building materials, refining and chemical sectors. The emissions from these sectors account for more than 90% of the total emissions, and mostly have a concentration of less than 15%. These sectors are characterized by massive emissions and multiple emission points, making large-scale carbon capture challenging and costly (RMB 300-700/t) ^[24-25], which becomes the bottleneck in the application of CO₂-EOR. Carbon capture technologies in the coal-fired power generation sector mainly include capture before, during and after coal combustion. Especially, the capture after coal combustion is relatively well developed; in this aspect, China Huaneng Group Co., Ltd. and CHN Energy take a leading place, with multiple devices of 10×10⁴ t in capturing capacity. Refining and chemical enterprises have carried out research and test on the capture of low- or medium-concentration CO₂; however, the utilization of CO₂ captured is limited for technical and economic reasons. At present, Daqing Petrochemical Company and Jilin Petrochemical Company are doing researches and constructing industrial facilities for the capture of 100×10⁴ t low-concentration CO₂ with a low cost (less than RMB 220/t).

Carbon dioxide is dominantly transported by tank cars, which is characterized by small scale and high cost. This transport mode is adopted in most of the CCUS-EOR projects, suggesting a high cost of RMB 0.8-1.0/(tkm). Jilin Oilfield has constructed a pipeline of 50 km in length for transportation of CO₂ gas with an annual transportation capacity of 50×10⁴ t. Up to now, there is no demonstration project in China for long-distance pipeline transmission of supercritical CO₂.

The efficiency of the CCUS-EOR projects needs to be further improved. At present, the enhanced oil recovery of CO₂ flooding is 10%-25% and the CO₂ storage efficiency is about 60%-70%. Under the current technical conditions, at the end of the oil displacement life cycle, more than 50% of OOIP remains underground. It is of great significance and practical value to further enhance oil recovery and CO₂ storage efficiency, which will inevitably face greater technical and economic challenges.

There is a shortage of experience in safety control and long-term monitoring on large-scale carbon storage projects. Through the 973 Plan, 863 Plan and National Science and Technology Major Project ^{[13⇓⇓⇓-17]}, the mechanisms and controlling factors for CO₂ storage in different geologic structures with different injection techniques are determined. Contribution and characterization methods of mechanisms of CO₂ storage in reservoirs, such as volume replacement, dissolution and retention, and mineralization reaction, are clarified. The evaluation method for CO₂ storage in such geobodies as a gas reservoir, saline aquifer, and CBM reservoir is formulated. The CO₂ leakage monitoring techniques with integration of CO₂ concentration, carbon flux in soil and stable isotope are established. It is found that the leakage points during CO₂ storage are mainly in wellbore, fault and caprocks. Since wellbore may be corroded massively by CO₂, leading to CO₂ leakage, necessary anti-corrosion processes should be taken, such as pipe anti-corrosion or corrosion inhibitors. Insufficient sealing capacities of faults or caprocks may also result in leakage. So far, no CO₂ leakage has been found in field tests. Nonetheless, the safe carbon storage is a long-term process, with the on-site CO₂- EOR project cycle to be 10-20 years. The missing of necessary experience for long-term monitoring of large-scale projects may present challenges for the safety control.

The CO₂ flooding is one of the key techniques available to effectively supplement formation energy and enhance oil recovery. At present, more than 70% of the CO₂ captured by large-scale carbon capture projects in the world is used to enhance oil recovery, especially for low-permeability reservoirs. According to the 2020 estimation of PetroChina Research Institute of Petroleum Exploration and Development (RIPED), the low-permeability reservoirs suitable for CO₂-EOR in CNPC oilfields domestically contain OOIP of 67.3×10⁸ t, with the estimated average EOR of 16.5% and additional recoverable reserves of 11.1×10⁸ t. At the same time, CO₂ of 29.5×10⁸ t can be effectively stored during the oil displacement.

The CO₂ storage potential for oil displacement in China's major oil and gas basins exceeds 140×10⁸ t according to the preliminary evaluation on CO₂ storage potential of major reservoirs under the 973 Plan. The key reservoirs in the Songliao, Ordos, Bohai Bay, Junggar and other basins have significant potential for CO₂ storage, and can serve as key zones for CCUS-EOR. Especially, deep saline aquifers in the major basins demonstrate a theoretically greater CO₂ storage potential - over 6×10¹² t.

CCUS-EOR industrialization has good economic benefits. With reference to the 45Q Act of the United States, given the international oil price of $50/bbl (1 bbl=0.159 m³) and the tax exemption at $35/t for CO₂ flooding in 2026, the potential value of CO₂ EOR and storage in existing reservoirs of China may reach RMB1.0×10¹² and more. Once the CCUS-EOR industrial chain is formed, the annual CO₂ injection capacity of 3000×10⁴ t may induce an annual oil production capacity of 1000×10⁴ t, with a net value of RMB 250×10⁸. At the same time, the annual CO₂ storage capacity is expected to be 2000×10⁴ t with a tax exemption of RMB 48×10⁸. If the international oil price is higher than $50/bbl and the CO₂ wellhead price is lower than RMB 200/t, the CCUS-EOR projects can be expected to be effectively applied on a large scale with the policy support of the state.

CCUS-EOR industrialization can create considerable social benefits. For example, the annual storage of 3000× 10⁴ t CO₂ equals to the carbon emissions of more than ten large refining and chemical enterprises. CCUS-EOR industrialization can buy precious time for the steady adjustment of energy structure. According to the goal of achieving carbon neutrality by 2060, the service life of the existing high carbon emission infrastructures is less than 40 years. CCUS-EOR projects can help solve some CO₂ emission problems in coal power, cement, steel and other sectors, avoid the early shutdown and resulting waste of investment in new infrastructure, and improve the cost-efficiency of carbon neutralization ^[26]. In addition, the large-scale development of CCUS-EOR can provide a large number of job opportunities.

Over decades of years, the development of continental oilfields in China has generally reached the international advanced level. Specifically, many technologies such as fine reservoir description, zonal water flooding and EOR, are internationally leading. Compared with marine reservoirs in other countries, the reservoirs of China are mainly continental deposits with higher heterogeneity and lower reserves. Therefore, large-scale CO₂ storage for oil displacement is more complicated and difficult in China. To build a more efficient CCUS-EOR original technology system for continental reservoirs, it is necessary to develop capture technologies with lower cost and higher efficiency for CO₂ with lower concentration. The focus of the research is finally to develop high-efficiency and low-energy chemical absorbers, and develop new high-efficiency desorption reactors and large-scale facilities, so as to secure large-scale, low-cost and stable capture and gas supply. Transportation of CO₂ will be transformed from tank cars to pipeline transmission of super-critical CO₂. By developing low-cost corrosion-resistant and crack-resistant special steel pipes, long-distance and low-cost safe transportation of CO₂ can be realized. Through the large-scale application of oil flooding technologies, oil recovery can be greatly enhanced. Through innovative research and development of large-scale CCUS-EOR reservoir development program design, the sweep efficiencies can be further promoted. Through the development of supporting systems such as high-efficiency injection production and low-cost corrosion prevention, especially in improving the miscibility of CO₂ and crude oil, CO₂ flooding in formations with large pore volumes, multi-stage profile control and flooding to expand CO₂ sweep efficiencies, zonal CO₂ flooding, high-efficiency coating materials with outstanding CO₂ corrosion-resistant, low-cost recycling of produced gas, and numerical simulation for integration of flooding and storage, it is possible to form a series of technologies for miscible, nearly miscible and immiscible high-efficiency flooding. By upgrading the storage program design, refining the geological and wellbore integrity evaluation, implementing integrated monitoring system of multiple techniques, and improving the safety control technologies, large-scale and ultra-long-term safe storage of CO₂ can be realized.

To realize the industrialization of CCUS-EOR, it is necessary to formulate medium- and long-term development plans as soon as possible, so as to realize the practical connection of the whole industry chain. The core is the implementation and operation of large-scale demonstration application projects. During the 14^th Five-Year Plan period, low-cost and high-efficient low-concentration CO₂ capture projects with the capacity of 100×10⁴ t/a will be constructed to reduce the capture cost below RMB 220/t. At the same time, more carbon-capturing projects will be implemented to form a low-cost and stable CO₂ gas supply with total capacities over 1000×10⁴ t. With consideration to distribution of gas supplies, long-distance CO₂ pipelines will be constructed to form a pipeline network of 100 km with capacity of 100×10⁴ t/a for supercritical CO₂ and with transportation costs below RMB 0.25/(tkm). Additionally, a large-scale high-efficiency oil-flooding demonstration area with an oil production capacity of 100×10⁴ t/a will be constructed to enhance oil recovery by miscible flooding by 30%. By continuously expanding the application, the oil production can be greatly increased. Through safe storage of CO₂ over 1000×10⁴ t/a, the storage efficiency can be enhanced to a level over 70%. By continuously expanding the safe storage, it is possible to make positive contributions to the realization of the "carbon peak and carbon neutrality" goals.

It is necessary to strengthen the cooperation between oil and gas fields and carbon emission sectors such as refining, coal power and coal chemical, thus establishing a CCUS-EOR innovation consortium with complementary advantages. Also, it is necessary to accelerate the development of carbon reduction technologies such as CCUS-EGR, chemical utilization, biological utilization and CCS to promote large-scale applications, and to provide strong technical support for the realization of the "double carbon" goal.

At the national level, the newly launched carbon trading platform plays a positive role in promoting the implementation of CCUS projects. It is suggested that China should further introduce clear and feasible quantitative calculation methods and supporting policies on carbon emission right recognition methods, reasonable emission indicators, emission reduction subsidy standards, comprehensive income of emission reduction (with reference to the 45Q Act of the United States), so as to improve the enthusiasm and initiative of enterprises to implement CCUS on a large scale.