English

Interfacial Mass Transfer Characteristics and Molecular Mechanism of the Gas-Oil Miscibility Process in Gas Flooding

• Hongwei Yu ^1, ,
• Shi Li ^1, ,
• Jinlong Li ^2, ,
• Shaohua Zhu ^3, ,
• Chengzhen Sun ^{3, ,}

• 1.

  State Key Laboratory of Enhanced Oil Recovery, PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China

• 2.

  PetroChina Jilin Oilfield Company, Songyuan 138099, Jilin Province, China

• 3.

  State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China

Corresponding author: Chengzhen Sun, Email: sun-cz@xjtu.edu.cn; Tel: +86-29-82665316

• Received Date: 23 June 2020
  Accepted Date: 29 July 2020
  Revised Date: 28 July 2020
  Available Online: 15 May 2022
• Fund Project:

  the Scientific Research and Technology Development Project of PetroChina 

  the Major Science and Technology Projects of PetroChina 

  the National Natural Science Foundation of China 

Abstract: The interfacial mass transfer characteristics of the gas-oil miscibility process are important in gas flooding technology to improve oil recovery. In this study, the process of gas flooding with actual components of Jilin oilfield is investigated by using molecular dynamics simulation method. We have chosen several alkane molecules based specifically on the actual components of crucial oil as the model oil phase for our study. The pressure of the gas phase is adjusted by changing the number of gas molecules while keeping the oil phase constant in the simulation. After the simulation, we analyze the variations of density in the gas-oil phase and interfacial characteristics to obtain the minimum miscibility pressure (MMP) for different displacement gases. The results show that the density of the gas phase increases while the density of the oil phase decreases with an increase in the displacement gas pressure, resulting in efficient mixing between the gas phase and the oil phase. At higher gas pressures, the thickness of the interface between the gas and oil phases is higher while the interfacial tension is lower. At the same time, we observed that the higher the CO₂ content in the displacement phase, the thicker the oil-gas interface becomes and the better the oil-gas mixing is under the same gas pressure. In this work, the gas-oil miscibility is studied with pure CO₂, pure N₂, and the mixture of these two gases, and it is found that the minimum miscibility pressure for pure CO₂ flooding (22.3 MPa) is much lower than that for pure N₂ flooding (119.0 MPa). When these two gases are mixed in 1 : 1 ratio, the MMP (50.7 MPa) is between the MMPs of the two pure gases. Moreover, the pressure required with CO₂ is lower than that required with N₂ to achieve the same displacement effect. Finally, we explain the mechanisms of the different miscibility processes for different gas pressure and different displacement gases from the perspective of the total energy of the system and the potential of the mean force between the gas and the oil. The total energy of the system increases with the pressure of the gas phase, implying that the number of collisions between the oil and gas molecules increases and the gas-oil miscibility is enhanced. In addition, by analyzing the potential of mean force profiles, it can be concluded that the force of attraction between the oil-phase molecules and CO₂ molecules is greater than that between the oil-phase molecules and N₂ molecules; thus, the CO₂ molecules easily mix with oil, and the effect of displacement is more obvious. These results are of great significance for understanding the interfacial mass transfer characteristics of the gas-oil miscibility process and for guiding the optimization and design of enhanced oil recovery technology by gas flooding.

Key words:
• Gas-flooding
•  /  Oil recovery
•  /  Gas-oil miscibility
•  /  Interfacial mass transfer
•  /  Minimum miscibility pressure
•  /  Molecular dynamics
•  /  Microscopic mechanism

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