Abstract：

CO₂ flooding is a promising enhanced oil recovery (EOR) method. Since the 1960s, many scholars have conducted studies of CO₂ flooding, including laboratory experiments and oil field pilot tests. The effects of CO₂ injection on crude oil are complex, mainly including dissolution, expansion, viscosity reduction, diffusion and mass transfer. The effects of different mechanisms on the level of miscibility are also different due to the different compositions of crude oil. In addition, the mixing of impurity gases, water shielding, and pore scale effects aggravate the difference of various mechanisms. This paper reviews the current research into the CO₂   flooding method. Besides, combined with the actual demand of oil and gas exploration and development, this paper summarizes the status and existing problems of CO₂ flooding during its promotion and application. At present, the research and field application of CO₂ flooding often pays too much attention to the minimum miscibility pressure, which makes the realization of achieving a miscible supplement the determinant of implementing CO₂ flooding in oil fields. Many reservoirs with favorable conditions are rejected simply because they cannot meet the minimum miscibility pressure. Therefore, focusing on the near-miscible CO₂ flooding technique, which is easier to realize in practical applications, three key factors limiting the effective development are proposed. The application and development of CO₂ near-miscible flooding in the future are discussed. Both experiments and field tests have fully verified the advantages of this technique in improving oilfield development. Lower injection pressure and operation cost obviously eased the limitation of miscible flooding in the characteristics of reservoir, quality of oil and gas sources of China. However, a definition of near-miscible flooding is still not agreed, the determination of the near-miscible pressure interval is still a blank, and the typical phase behavior and mechanisms of displacement are not yet clear. In the process of practical application, the effect of the mixing of contaminant gases, water shielding, and the effect of pore size cannot be ignored. Problems such as the determination of the lower limit of CO₂ content in the production gas, the characterization of water shielding with the forms of by-pass, cross flow and Marangoni flow under three phase conditions including oil, gas and water, the effect of wettability, pore distribution and pore size on miscibility degree, and the synergistic effect of these factors, all need to be included in further research. Thus, the key scientific problems during the promotion and application of CO₂ near-miscible flooding and the subsequent engineering problems may be able to be tackled effectively.  

Key words：near-miscible pressure interval; production gas re-injection; water shielding; Pore size effect

Received： 2019-08-30

Corresponding Authors： chenhao@cup.edu.cn

Cite this article：CHEN Hao, LIU Xiliang, JIA Ninghong, ZHANG Ke, YANG Ran, YANG Shenglai. Prospects and key scientific issues of CO2 near-miscible flooding. Petroleum Science Bulletin, 2020, 03: 392-401

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