物理化学学报 >> 2022, Vol. 38 >> Issue (5): 2006061.doi: 10.3866/PKU.WHXB202006061

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气驱油油气混相过程的界面传质特性及其分子机制

俞宏伟1, 李实1, 李金龙2, 朱韶华3, 孙成珍3,*()   

  1. 1 中国石油勘探开发研究院,提高石油采收率国家重点实验室,北京 100083
    2 中国石油吉林油田分公司,吉林 松原 138099
    3 西安交通大学,动力工程多相流国家重点实验室,西安 710049
  • 收稿日期:2020-06-23 录用日期:2020-07-29 发布日期:2020-08-03
  • 通讯作者: 孙成珍 E-mail:sun-cz@xjtu.edu.cn
  • 基金资助:
    中国石油天然气股份有限公司科学研究与技术开发项目(2019B-1111);中国石油天然气股份有限公司重大科技项目(2018E-1805);国家自然科学基金(51876169)

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

Hongwei Yu1, Shi Li1, Jinlong Li2, Shaohua Zhu3, Chengzhen Sun3,*()   

  1. 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
  • Received:2020-06-23 Accepted:2020-07-29 Published:2020-08-03
  • Contact: Chengzhen Sun E-mail:sun-cz@xjtu.edu.cn
  • About author:Chengzhen Sun, Email: sun-cz@xjtu.edu.cn; Tel: +86-29-82665316
  • Supported by:
    the Scientific Research and Technology Development Project of PetroChina(2019B-1111);the Major Science and Technology Projects of PetroChina(2018E-1805);the National Natural Science Foundation of China(51876169)

摘要:

油气混相过程的界面传质特性对气驱提高原油采收率技术非常重要。本文针对吉林某油田的实际油组分,采用分子动力学模拟研究了气驱油过程,分析了不同气体和驱替压力下油气两相的状态变化以及界面特性,获得不同驱替气体的最小混相压力(MMP)。结果表明,随着驱替气体压力的升高,气相的密度逐渐增大,油相膨胀密度降低,气相与油相的混合程度增强,油气两相界面厚度增加,界面张力随之减小。同时发现,驱替相中二氧化碳浓度越高,在同等气体压力下,油气界面更厚,油气混合程度更高。纯CO2驱油得到的MMP远远小于纯N2驱油,当这两种气体摩尔比为1 : 1混合时MMP介于两种纯气体之间,说明要达到同样的驱油效果二氧化碳需要的压力更小。最后,本文从分子微观作用力角度解释了驱替气体不同时影响油气混相程度的机制,通过分子平均作用势曲线发现油相分子对CO2的吸引力要大于N2分子,因此CO2分子更容易与油相混合,驱替效果更明显。

关键词: 气驱油, 石油采收率, 油气混相, 界面传质, 最小混相压力, 分子动力学, 微观机制

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

MSC2000: 

  • O647