气驱油油气混相过程的界面传质特性及其分子机制

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。

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

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.