Low-carbon oil exploitation: Carbon dioxide flooding technology

Introduction

It is well known that there are many technical methods for improving oil recovery, including gas miscible flooding, thermal oil recovery, chemical flooding, and microbial oil recovery. Among these, CO₂ flooding is one of the key technologies for improving oil recovery that achieves the goal of reducing carbon emissions (Jia et al., 2019). Given the increasing awareness regarding human environmental protection, the development of a low-carbon economy is an irresistible global trend. The concepts of “carbon peak” and “carbon neutrality” have gradually gained popularity. Carbon neutrality has also spawned the carbon capture, utilization, and storage (CCUS) industry (Zou et al., 2021). CO₂ flooding is one of the main carbon utilization technologies that makes full use of carbon resources to enhance oil recovery while promoting the development of a low-carbon economy. This paper reviews and summarizes the developments regarding CO₂ flooding in domestic and foreign scenarios, discusses the mechanism and process of CO₂ flooding, analyzes the technological challenges, and provides some corresponding solutions.

Research status

CO₂ flooding has been used globally to enhance oil recovery for more than 30 years and is becoming more popular. Therefore, an increasing number of researchers are engaged in research on CO₂ flooding (Chen et al., 2015). There are many reported CCUS-EOR projects from domestic and foreign investigators, as shown in Table 1. Research on CO₂ flooding began in the 1950s (Whorton et al., 1952), and some European countries had earlier used CO₂ flooding to enhance oil recovery. The former Soviet Union first began studying CO₂ injection enhanced oil recovery (EOR) technology in 1952. In 1968, CO₂ flooding tests were carried out in the Tuimajin Oilfield, which achieved a 15% increase in the final crude oil recovery (Qin et al., 2015). CO₂ flooding developed the fastest in the United States, which has the largest number of CO₂ flooding projects accounting for more than 90% of the global total. CO₂ flooding affords good economic benefits, with an annual oil production of about 1,500 × 104 t for five consecutive years and oil recovery increase of 7–15% (Hitesh et al., 2008; British Petroleum Company, 2017; British Petroleum Company, 2018). In the 21st century, greenhouse gas emission reductions have further promoted rapid development of CO₂ flooding technology (Shen and Liao, 2009; Koottungal, 2014).

TABLE 1

TABLE 1. International CCUS-EOR projects [modified according to Xiang et al. (2022)].

Since the 1960s, China has started paying attention to CO₂ flooding technology and has successively carried out indoor research on CO₂ flooding. In 1963, the first study on CO₂ flooding was carried out in the Daqing Oilfield in China. From 1991 to 1993, immiscible displacement tests of CO₂ by alternating gas and water injection were carried out in the eastern Sanan Oilfield. In 1995, Jilin Oilfield carried out the CO₂ single-well huff and puff test and obtained 1420 t of increased crude oil through multiple tests. In 1998, Shengli Oilfield carried out the CO₂ single-well huff and puff test to obtain increased crude oil outputs by more than 200 t. Subsequently, China has continued to strengthen research on CO₂ flooding technology. At present, research on CO₂ flooding for EOR in the Jilin Oilfield is relatively mature (Chen et al., 2012).

Carbon dioxide flooding mechanisms, methods, and processes

CO₂ flooding has been increasingly used in recent times. Understanding and mastering the oil displacement mechanism can therefore promote technological progress.

Carbon dioxide flooding mechanisms

There are nine mechanisms of CO₂ flooding:

1) Viscosity reduction: Carbon dioxide and crude oil have good mutual solubilities. Temperature and pressure also affect the solubility of CO₂ in crude oil, and extremely high temperatures are not conducive to viscosity reduction (Li et al., 2008).

2) Improve oil–water mobility ratio: CO₂ increases the viscosity of water and decreases the viscosity of crude oil. These two viscosities tend towards each other, thus improving the mobility ratio of crude oil to water and expanding the swept volume (Luo et al., 2012).

3) Expansion effect: The dissolution of CO₂ in crude oil greatly expands the volume of the oil. The greater the degree of expansion, the stronger is the ability to flow and easier it is to replace (Gu et al., 2007).

4) Extraction and gasification of light hydrocarbons in crude oil: CO₂ can be used to extract and gasify light components in crude oil, thereby reducing the relative density of crude oil and enhancing recovery (Roper et al., 1992).

5) Miscible effect: When the pressure reaches the miscible pressure, CO₂ mixes with the light hydrocarbons in crude oil to form an oil zone, which greatly improves oil recovery during the oil displacement process (Grigg and Siagian, 1998).

6) Molecular diffusion: The water phase hinders diffusion of carbon dioxide molecules into the oil phase, completely inhibiting the release of the light hydrocarbons from the oil phase to carbon dioxide. Therefore, sufficient time is required for the carbon dioxide molecules to fully diffuse into the oil phase.

7) Reduce interfacial tension: In the process of CO₂ flooding, mixing CO₂ and the light components in crude oil (C2–C6) effectively reduces the oil–water two-phase interfacial tension, reduces the resistance of CO₂ in the displacement process, and enhances oil recovery (Desch et al., 1984).

8) Dissolved gas flooding: CO₂ injected into the oil layer increases its pressure, but in the displacement process, the pressure reduction causes CO₂ to escape from the crude oil and occupy the pore spaces, thereby allowing dissolved gas flooding that is conducive to flooding (Gu et al., 2007).

9) Acidizing plugging improves permeability: CO₂ dissolved in water generates carbonic acid and renders the aqueous solution acidic. To a certain extent, massive injection of CO₂ can flush out blockages, dredge oil circulation, and restore the production capacity of a single well (Rao et al., 2004).

Carbon dioxide flooding methods

CO₂ flooding can be divided into two types: CO₂ immiscible and CO₂ miscible flooding. Expansion viscosity reduction is the main mode of CO₂ immiscible flooding. For reservoirs with low pressures, high temperatures, and heavier components, the surface tension between CO₂ and crude oil is large. CO₂ can be used to extract some light components of crude oil, thereby expanding the crude oil, reducing its viscosity, and improving recovery. The essence of CO₂ huff and puff is immiscible flooding, and the mechanism involves expanding the volume of crude oil continuously so that the tension and viscosity of the crude oil interface can be reduced. This method allows injection of carbon dioxide into the bottom of the production well. After shutting down, the carbon dioxide penetrates the reservoir, which is ultimately conducive to continued production from the well. When the formation pressure is greater than the CO₂ miscible pressure and less than the formation fracture pressure, CO₂ miscible flooding occurs. The injection methods may be continuous or simple, among others. Studies have shown that miscible flooding enables greater recovery than immiscible flooding.

Carbon dioxide flooding processes

The flooding processes include primary, secondary, and tertiary flooding. Primary flooding is the method of extracting crude oil using only natural energy. Secondary flooding is the method of increasing the reservoir pressure by injecting gas or water. EOR technology involves the use of high pressures to inject supercritical/dense-phase CO₂ into the reservoir, which allows the CO₂ to drive the flow of crude oil to the production well, thereby increasing oil recovery. The CO₂-EOR process flow chart is as shown in Figure 1. After the CO₂ is compressed, it is injected into the injection well. Then, the production well produces the oil–gas mixture, which further helps separation of the crude oil.

FIGURE 1

FIGURE 1. CO₂ enhanced oil recovery (EOR) process flowchart.

Prospects

In recent years, with increasing awareness regarding environmental protection, CO₂-EOR technology has developed gradually. This not only eases the pressure on environmental pollution but also enhances oil recovery. However, there are still some problems and challenges: 1) it is necessary to strengthen measures to tackle the problem of low CO₂ oil recovery; 2) the economic issues with regard to use of CO₂, such as the fact that CO₂ is corrosive to the pipeline during gathering and transportation, accelerate the costs from pipeline damage; 3) gas source of CO₂ flooding: owing to the increased distance between the oilfield and the city, the costs of gathering and transportation are high, which limit the development of CO₂ flooding technologies.

This paper therefore suggests two areas of improvement: 1) China should continue to strengthen technical research on improving oil recovery; 2) China needs to develop breakthrough carbon dioxide capture technologies and provide inexpensive natural gas sources.

Author contributions

All authors conceived and designed the study. Write the first draft, XX and QD; Writing review and editing, PS and LT; Format modification, LY, JB, and WL.

Funding

This work was supported by the Open Foundation of Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University (Ministry of Education and Hubei Province, No. UOG 2022-03), Open Fund of Hubei Key Laboratory of Drilling and Production Engineering for Oil and Gas (Yangtze University, No. YQZC202206), and Hubei Provincial Natural Science Foundation Youth Project of 2022 (No. ZRMS2022000846).

Conflict of interest

PS was employed by the company Engineering Technology Research Institute of PetroChina Southwest Oil and Gas Field Company, LT was employed by the company Chuanxi Drilling Company, CNPC Chuanqing Drilling Engineering Co. Ltd.; QD and LY were employed by the company Carbon Hydrogen Epoch Technology Co. Ltd. WL was employed by the company CNOOC Ltd.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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