Abstract CO2 Water-Alternating-Gas injection (CO2-WAG) is still a challenging task to simulate and predict accurately, due to the complex interaction of CO2/oil phase behaviour, 3-phase flow and the heterogeneity of the porous medium. In this paper, we focus specifically on the regime of viscous fingering flow in CO2-WAG in heterogeneous systems because of its importance in elucidating this complex interaction. This work presents a detailed simulation study of both immiscible and near-miscible CO2-WAG and continuous CO2 displacements with unfavourable mobility ratios for 1D and 2D systems. 2D heterogeneous permeability fields were generated as Correlated Random Fields (CRF) with specified degrees of heterogeneity (permeability range, described by the Dykstra-Parsons coefficients, VDP) and structures (defined through the dimensionless correlation range, RL=λ/L). Our central aim is to improve the modelling of CO2 displacement in the transition from immiscible to miscible flows in CO2-WAG processes. To do so, two key physical mechanisms that occur during near-Miscible WAG (nMWAG) processes have been studied in detail, namely compositional effects (denoted as Mechanism 1, MCE) and low-interfacial-tension (IFT) film flow effects (denoted as Mechanism 2, MIFT). The low IFT effects in MIFT manifest themselves in an increased mobility of oil phase due to enhanced film formation and flow processes. This latter mechanism (MIFT) is modelled as an increased oil relative permeability using different well-known models (Bette and Coats) parameterized by the gas/oil IFT (σgo), calculated in the simulation from the compositional PVT model via a built-in correlation (the McLeod-Sugden equation, in this case). A range of various combinations of oil-stripping effects (MCE) and IFT effects (MIFT) has been tested to evaluate the potential impact of each mechanism on the flow behaviour such as the local displacement efficiency and the ultimate oil recovery. Oil bypassed by viscous fingering/local heterogeneity, can be efficiently recovered by WAG in the cases where both MCE and MIFT are taken into account (as opposed to either mechanism being considered alone). We also show that the way these two distinct but related mechanisms (MCE and MIFT) operate in near miscible conditions cannot be observed in (i) a simple 1D system such as a slim tube experiment, or (ii) in a heterogeneous system under continuous CO2 injection. Using tracer analysis in our simulations, we demonstrate that a major recovery mechanism in near-miscible WAG displacement is viscous crossflow between non-preferential (bypassed) flow-paths and preferential flow-paths (i.e. between the viscous fingers). Due to the significance of IFT effects (the MIFT mechanism), we also present comparative results from two of the IFT-dependent relative permeability models (Bette and Coats) showing the impact of each model on the simulation of the near-miscible WAG flow behaviour.

中文翻译：

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CO2 注入的数值研究和粘性横流在近混相 CO2-WAG 中的作用

摘要 由于 CO2/油相行为、三相流和多孔介质的非均质性之间的复杂相互作用，CO2 水交替注气 (CO2-WAG) 仍然是一项具有挑战性的任务，要准确地进行模拟和预测。在本文中，我们特别关注异质系统中 CO2-WAG 中粘性指流的机制，因为它在阐明这种复杂的相互作用方面很重要。这项工作对一维和二维系统的不混溶和接近混溶的 CO2-WAG 以及具有不利迁移率的连续 CO2 置换进行了详细的模拟研究。二维非均质渗透场作为相关随机场 (CRF) 生成，具有指定的非均质度（渗透范围，由 Dykstra-Parsons 系数描述，VDP）和结构（通过无量纲相关范围定义，RL=λ/L）。我们的中心目标是改进 CO2-WAG 过程中从不混溶流向混溶流转变过程中的 CO2 置换建模。为此，详细研究了在近混相 WAG (nMWAG) 过程中发生的两个关键物理机制，即成分效应（表示为 Mechanism 1, MCE）和低界面张力 (IFT) 薄膜流动效应（表示为如机制 2，MIFT）。MIFT 中的低 IFT 效应表现为由于增强的成膜和流动过程导致油相的流动性增加。后一种机制 (MIFT) 被建模为使用由气/油 IFT (σgo) 参数化的不同众所周知的模型（Bette 和 Coats）增加的油相​​对渗透率，在通过内置相关性（在本例中为 McLeod-Sugden 方程）的成分 PVT 模型的模拟中计算得出。已经测试了一系列不同的剥油效应 (MCE) 和 IFT 效应 (MIFT) 组合，以评估每种机制对流动行为的潜在影响，例如局部驱油效率和最终采油率。在同时考虑 MCE 和 MIFT 的情况下（与单独考虑任一机制相反），通过粘性指法/局部异质性绕过的油可以通过 WAG 有效回收。我们还表明，这两种不同但相关的机制（MCE 和 MIFT）在接近混溶条件下运行的方式无法在 (i) 简单的一维系统中观察到，例如细管实验，或 (ii) 在连续条件下的异构系统中二氧化碳注入。在我们的模拟中使用示踪剂分析，我们证明了近混相 WAG 置换的主要恢复机制是非优先（绕过）流路和优先流路（即粘性指之间）之间的粘性交叉流。由于 IFT 效应（MIFT 机制）的重要性，我们还提供了两个依赖于 IFT 的相对渗透率模型（Bette 和 Coats）的比较结果，显示了每个模型对模拟近混相 WAG 流动行为的影响.