The phase behavior of fluid is essential for predicting ultimate oil recovery and determining optimal production parameters. The pore size in shale porous media is nanopore, which causes different phase behaviors of fluid in unconventional reservoirs. Nanopores in shale media can be regard as semipermeable membrane to filter heavy components (sieving effect) in shale oil, which leads to the different distributions of fluid components and different phase behaviors. In addition, the phase behavior of fluid in nanopores can be significantly altered by large capillary pressure. In this paper, the phase behavior of fluid in shale reservoirs is investigated by a new two-phase flash algorithm considering sieving effect and capillary pressure. Firstly, membrane efficiency and capillary pressure are introduced to establish a thermodynamic equilibrium model that is solved by Rachford–Rice flash calculation and Newton–Raphson method. The capillary pressures in different pore sizes are calculated by the Young–Laplace equation. Then, the influences of sieving effect and capillary pressure on phase behavior are analyzed. The results indicate that capillary pressure can suppress the bubble point pressure of fluid in nanopores. The distributions of fluid components are different in various parts of shale media. In the unfiltered part, density and viscosity of fluid are higher. Finally, it is found that the membrane efficiency can be improved by CO₂ injection. The minimum miscibility pressure for shale oil–CO₂ system is also studied. The developed model provides a better understanding of the phase behavior of fluid in shale oil reservoirs.

流体的相行为对于预测最终采油率和确定最佳生产参数至关重要。页岩多孔介质中的孔隙尺寸为纳米孔，导致非常规储层中流体的相行为不同。页岩介质中的纳米孔可以看作是半透膜来过滤页岩油中的重组分（筛分效应），从而导致流体组分的不同分布和不同的相行为。此外，大毛细管压力可以显着改变纳米孔中流体的相行为。本文采用一种新的考虑筛分效应和毛细管压力的两相闪蒸算法研究了页岩储层中流体的相行为。首先，引入膜效率和毛细管压力以建立热力学平衡模型，该模型通过 Rachford-Rice 闪蒸计算和 Newton-Raphson 方法求解。不同孔径下的毛细管压力通过杨拉普拉斯方程计算。然后，分析了筛分效应和毛细管压力对相行为的影响。结果表明，毛细管压力可以抑制纳米孔中流体的泡点压力。页岩介质各部位流体组分的分布不同。在未过滤部分，流体的密度和粘度较高。最后发现CO可以提高膜效率 不同孔径下的毛细管压力通过杨拉普拉斯方程计算。然后，分析了筛分效应和毛细管压力对相行为的影响。结果表明，毛细管压力可以抑制纳米孔中流体的泡点压力。页岩介质各部位流体组分的分布不同。在未过滤部分，流体的密度和粘度较高。最后发现CO可以提高膜效率 不同孔径下的毛细管压力由杨拉普拉斯方程计算。然后，分析了筛分效应和毛细管压力对相行为的影响。结果表明，毛细管压力可以抑制纳米孔中流体的泡点压力。页岩介质各部位流体组分的分布不同。在未过滤部分，流体的密度和粘度较高。最后发现CO可以提高膜效率 流体的密度和粘度较高。最后发现CO可以提高膜效率 流体的密度和粘度较高。最后发现CO可以提高膜效率₂注射。还研究了页岩油-CO ₂体系的最小混相压力。开发的模型可以更好地了解页岩油储层中流体的相行为。