Abstract Tight/shale reservoirs are extensively containing nanopores, and the confined space in nanopores can greatly alter the phase behavior of reservoir fluids due to the strong capillarity effect. Many researches have been recently conducted to investigate the effect of capillary pressure in nanopores on altering the oleic-vapor two-phase equilibria. Few attempts have been made to describe the effect of capillarity on the aqueous-oleic-vapor three-phase equilibria. This work proposes a new algorithm for performing three-phase pressure-temperature (P-T) flash coupled with capillary effect. This algorithm considers two capillary pressures that exist across the two interfaces dividing the three phases in a nanopore. When describing the three-phase equilibria, two types of reservoir wettability are considered: water-wet formation and oil-wet formation. In each case, distribution of the three phases in a nanopore is determined based on the spreading coefficient which refers to the spreading ability of an oleic phase over the spreading ability of an aqueous phase. Example calculations are conducted to show the robustness of the new algorithm as well as to study the effect of capillarity on the three-phase equilibria. Computation results show that the three-phase P-T envelope for a given hydrocarbons/water mixture in a nanopore can be significantly altered by capillary pressure, but how the envelope moves will depend on the wettability of the nanopore and the spreading coefficient. The general trend is that both the upper branch (i.e., the oleic-aqueous/vapor-oleic-aqueous boundary) and the lower branch (i.e., the liquid-vapor/liquid-liquid-vapor boundary) of the three-phase envelope tend to move downward. Compared to the water-wet case, the oil-wet nanopore will shift the three-phase boundaries in a much larger degree. In addition to the alteration of the three-phase envelope, the presence of capillarity will also lead to the alteration of the phase fractions and phase compositions in the nanopore. Nanofluidic experiments should be conducted in the future to verify the findings obtained from the calculations.

中文翻译：

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毛细管压力对有限孔隙空间三相平衡的影响

摘要 致密/页岩储层中广泛含有纳米孔，纳米孔内的密闭空间由于其强大的毛细管作用可以极大地改变储层流体的相行为。最近进行了许多研究来研究纳米孔中的毛细管压力对改变油酸蒸汽两相平衡的影响。很少有人尝试描述毛细作用对水-油-蒸气三相平衡的影响。这项工作提出了一种新算法，用于执行与毛细管效应耦合的三相压力-温度 (PT) 闪蒸。该算法考虑了存在于划分纳米孔中三相的两个界面上的两个毛细管压力。在描述三相平衡时，考虑了两种类型的储层润湿性：水湿地层和油湿地层。在每种情况下，纳米孔中三相的分布是基于铺展系数确定的，铺展系数是指油相的铺展能力超过水相的铺展能力。进行示例计算以显示新算法的稳健性以及研究毛细作用对三相平衡的影响。计算结果表明，纳米孔中给定碳氢化合物/水混合物的三相 PT 包络线会因毛细管压力而显着改变，但包络线如何移动将取决于纳米孔的润湿性和扩散系数。总的趋势是，上部分支（即油酸-水/蒸气-油酸-水边界）和下部分支（即，三相包络的液-气/液-液-汽边界）倾向于向下移动。与水湿情况相比，油湿纳米孔将在更大程度上移动三相边界。除了三相外壳的改变外，毛细管现象的存在也会导致纳米孔中相分数和相组成的改变。未来应进行纳米流体实验，以验证从计算中获得的结果。