页岩油-CO_(2)、CH_(4)、N_(2)混合物界面性质的分子动力学模拟研究

Molecular dynamics simulations of interfacial properties of shale oil-CO_(2),CH_(4),N_(2)mixtures

油气界面张力是气驱提高采收率过程中分析油气两相混合程度、影响油气最小混相压力和注气开发效果的重要参数。针对页岩油的主要成分正辛烷(C8),通过分子动力学模拟方法研究了纯CO_(2)和混合气体(CO_(2)、CH_(4)、N_(2))与C8的界面张力及界面微观特征变化规律,并考虑了温度、压力和气体组分等外界因素对界面特征的影响。结果表明,随着气相压力的增大,气体与C8混合程度增大,界面厚度、粗糙度、相对吸附量等微观特征增强,导致油气界面张力逐渐减小。界面张力随温度在不同压力区间表现出相反的变化趋势:在低压时温度越高,界面张力越小;在高压时温度越高,界面张力反而越大。相较于纯CO_(2)、CH_(4)和N_(2)的加入会增大混合气体与油相的界面张力,其中N_(2)对油气界面张力的影响更大;同时CH_(4)和N_(2)会使得油气界面厚度、相对吸附量等微观特征减弱。在三元CO_(2)+CH_(4)+N_(2)/C8体系中,CO_(2)的相对吸附量最大,CH_(4)次之,N_(2)最小,证明了3种气体与油的相互作用由强到弱为CO_(2)>CH_(4)>N_(2)。此外,所有体系相对吸附量均大于0,吸附量越大,界面张力随压力的增大下降得越快,这与吉布斯吸附理论相一致。

Oil-gas interfacial tension is an important parameter for analyzing the mixing degree of oil and gas phases and influencing the minimum mixing pressure of oil and gas and the development effect of gas injection during gas flooding for enhanced oil recovery.Aiming at n-octane(C8),which is the main component of shale oil,the changing rules of interfacial tension and interfacial microscopic characteristics of pure CO_(2)and mixed gases(CO_(2),CH_(4),N_(2))with n-octane were investigated by molecular dynamics simulation.Additionally,the effects of external factors such as temperature,pressure,and gas components on interfacial characteristics were also taken into account.The results show that with the increase in gas-phase pressure,the mixing degree of gas and noctane increases,and the microscopic features such as interfacial thickness,roughness,and relative adsorption are enhanced,resulting in a gradual decrease in the interfacial tension between the oil and gas.The interfacial tension shows opposite trends with temperature in different pressure intervals.The interfacial tension declines as the temperature rises in low pressure intervals.While the interfacial tension grows as the temperature rises in high pressure intervals.Compared with pure CO_(2),the addition of CH_(4)and N_(2)increases the interfacial tension between the mixed gases and the oil,of which N_(2)has a greater effect on the oil-gas interfacial tension.At the same time,CH_(4)and N_(2)weaken the microscopic features such as the oil-gas interfacial thickness and relative adsorption.The relative adsorption of CO_(2)is the largest,followed by CH_(4)and the smallest N_(2)in the ternary CO_(2)+CH_(4)+N_(2)/C8 system,which proves that the interactions between the three gases and the oil are from the strong to the weak,i.e.,CO_(2)>CH_(4)>N_(2).In addition,the relative adsorption of all the systems is greater than zero.As the adsorption is larger,the interfacial tension decreases faster with the increasing pressure,which is in agreement with Gibbs’adsorption theory.