This study seeks to improve numerical simulations of the key physics occurring in CO2 enhanced oil recovery (CO2-EOR) processes, with a particular focus on the transition from immiscible to miscible displacements. In the previous work, we have investigated interactions between compositional effects and the underlying heterogeneities of the flow field in near-miscible floods (Wang et al. in Transp Porous Media 129(3):743–759, 2019a). In this current study, we have further analysed the effects of reduction in interfacial tension (IFT) on the flow behaviour, as motivated by the study on the film-flow mechanism previously presented by Sorbie and van Dijke (SPE improved oil recovery symposium, Society of Petroleum Engineers, 2010). We identify two clear mechanisms of oil recovery that may occur in near-miscible CO2 (or other gas) injection processes, which we denote, MCE, as oil stripping or conventional compositional effects, and MIFT as lower IFT oil film-flow effects. The latter MIFT effects are described by an enhanced hydrocarbon relative permeability in the near-miscible three-phase relative permeabilities (3PRP). Various combinations between the MCE and MIFT mechanisms were tested by numerical simulations to evaluate the impact of each mechanism on the flow behaviour, i.e. their separate and joint effects on quantities such as the local oil displacement efficiency, phase flow vectors and the ultimate oil recovery. When acting in combination, the oil stripping and IFT effects can greatly improve the local displacement performance even when viscous fingering flow occurs. Viscous fingering is well known to lead to bypassed oil in the “non-preferential” flow paths between the main fingers. We show that the remaining oil in these non-preferential flow paths (i.e. bypassed oil) can be efficiently recovered by the combined MCE and MIFT mechanisms, but only with the application of water alternating gas (WAG). In contrast to oil stripping effects, the IFT effect is not dependent on continuous contact between oil and CO2. Instead, the remaining oil is mobilized by gas as the IFT is reduced and can be efficiently produced by subsequent water injection. This MIFT mechanism has much less impact in cases with continuous CO2 injection compared to its efficiency in WAG. This is because during continuous injection, gas fingers are dominant in the preferential flow paths, and therefore the local displacement efficiency is very good, but only in these preferential routes. On the other hand, WAG is able to make full use of the IFT effects because of its relatively stable displacing front, which allows the MIFT mechanism to contribute. In this study, the effects of using different three-phase relative permeability methods were investigated and, as expected, different methods yielded different results. However, an important observation is that when IFT effects (MIFT) were included, there was much less difference in the final oil recovery using the different 3PRP models; our analysis shows why this is the case.

中文翻译：

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详细评估成分和界面张力对不混溶和近混溶 CO2 连续置换和 WAG 置换期间流体行为的影响

本研究旨在改进 CO2 提高石油采收率 (CO2-EOR) 过程中发生的关键物理场的数值模拟，特别关注从不混相驱替到混相驱替的转变。在之前的工作中，我们研究了近混相洪水中成分效应与流场潜在异质性之间的相互作用（Wang 等人，Transp Porous Media 129(3):743–759, 2019a）。在当前的这项研究中，我们进一步分析了界面张力 (IFT) 降低对流动行为的影响，这是由 Sorbie 和 van Dijke 先前提出的薄膜流动机制研究推动的（SPE 提高采油率研讨会，Society石油工程师，2010 年）。我们确定了可能发生在近混相 CO2（或其他气体）注入过程中的两种明确的采油机制，我们将 MCE 表示为油剥离或常规成分效应，将 MIFT 表示为较低的 IFT 油膜流动效应。后一种 MIFT 效应被描述为近混相三相相对渗透率 (3PRP) 中碳氢化合物相对渗透率的提高。MCE 和 MIFT 机制之间的各种组合通过数值模拟进行测试，以评估每种机制对流动行为的影响，即它们对局部驱油效率、相流矢量和最终石油采收率等数量的单独和联合影响。联合作用时，即使出现粘滞指流，也可以通过油提和IFT效应大大提高局部驱替性能。众所周知，粘性指法会导致主指状物之间“非优先”流动路径中的油被绕过。我们表明，这些非优先流动路径中的剩余油（即旁路油）可以通过组合 MCE 和 MIFT 机制有效回收，但只能使用水交替气体（WAG）。与油汽提效应相反，IFT 效应不依赖于油和 CO2 之间的持续接触。取而代之的是，随着 IFT 的减少，剩余的油被气体动员起来，并且可以通过随后的注水有效地生产。与其在 WAG 中的效率相比，这种 MIFT 机制在连续注入 CO2 的情况下影响要小得多。这是因为在连续注入过程中，气体指在优先流路中占主导地位，因此局部驱替效率非常好，但仅限于这些优先流路。另一方面，WAG 能够充分利用 IFT 效应，因为它具有相对稳定的位移前沿，这使得 MIFT 机制有所贡献。在这项研究中，研究了使用不同三相相对渗透率方法的影响，正如预期的那样，不同的方法产生了不同的结果。然而，一个重要的观察结果是，当包含 IFT 效应 (MIFT) 时，使用不同 3PRP 模型的最终石油采收率差异要小得多；我们的分析说明了为什么会这样。一个重要的观察结果是，当包含 IFT 效应 (MIFT) 时，使用不同 3PRP 模型的最终石油采收率差异要小得多；我们的分析说明了为什么会这样。一个重要的观察结果是，当包含 IFT 效应 (MIFT) 时，使用不同 3PRP 模型的最终石油采收率差异要小得多；我们的分析说明了为什么会这样。