CO2 injection into oil reservoirs is widely accepted as an effective enhanced oil recovery and CO2 storage technique. While oil recovery and CO2 storage potential of this technique have been studied extensively at the core-scale, complex multiphase flow and fluid–fluid interactions at the pore scale during near-miscible CO2 injection have not, and this area needs more study. To address this, a unique high-pressure microfluidic system was implemented which allows for the optical visualisation of the flow using optical microscopy. The results show that during tertiary near-miscible CO2 injection, when CO2 phase contacts the oil, the oil spreads as a layer between the CO2 phase and water preventing CO2 phase from contacting the water phase. This is attributed to the positive value of the spreading coefficient. Furthermore, due to the presence of pore-scale heterogeneity in the chip, an early breakthrough of CO2 was observed causing a large amount of oil to be bypassed. However, after CO2 breakthrough, CO2 gradually started to diffuse and flow inside the bypassed oil zones in the transverse directions which is a characteristic of capillary crossflow. The driving force for this capillary crossflow was the interfacial tension gradient formed by the diffusion of CO2 into the oil phase and the extraction of light to medium hydrocarbon components from the oil into the CO2 phase. The same mechanism led to the recovery of the bypassed oil trapped in dead-end pores. This unique mechanism produced the majority of the bypassed oil after CO2 breakthrough and significantly increased the oil recovery. In our three-phase flow water-wet system, CO2 flow displaced the water through a multiple displacement mechanism which is unique to three-phase flow. CO2 displaced the oil in oil-filled pores thorough bulk flow, and the spreading oil layers were gradually produced by layer flow.

中文翻译：

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芯片上的油藏：近混相 CO2 EOR 和存储过程中多相流的孔隙尺度研究

CO2 注入油藏作为一种有效的提高石油采收率和 CO2 封存技术被广泛接受。虽然该技术的石油采收率和 CO2 封存潜力已在岩心尺度上进行了广泛的研究，但在近混相 CO2 注入过程中，复杂的多相流和孔隙尺度的流体-流体相互作用尚未得到广泛研究，该领域还需要更多的研究。为了解决这个问题，实施了一种独特的高压微流体系统，该系统允许使用光学显微镜对流动进行光学可视化。结果表明，在三次近混相 CO2 注入过程中，当 CO2 相接触油时，油在 CO2 相和水之间形成一层，阻止 CO2 相与水相接触。这归因于传播系数的正值。此外，由于芯片中存在孔隙尺度的异质性，观察到 CO2 的早期突破导致大量石油被绕过。但是，CO2 突破后，CO2 逐渐开始在绕过的油层内横向扩散和流动，这是毛细管错流的特征。这种毛细管横流的驱动力是由 CO2 扩散到油相中以及从油中提取轻到中等碳氢化合物成分到 CO2 相中形成的界面张力梯度。同样的机制导致被困在死胡同中的旁通油被回收。这种独特的机制在 CO2 突破后产生了大部分旁通油，并显着提高了石油采收率。在我们的三相流水湿系统中，CO2 流通过三相流独有的多重置换机制置换水。CO2通过整体流动置换含油孔隙中的石油，层流作用逐渐产生扩散的油层。