The successful development of tight oil reservoirs in the U.S. shows the bright future of unconventional reservoirs. Tight oil reservoirs will be the main target of exploration and development in the future, and CO2 huff-n-puff is one of the most important methods to enhance oil recovery factor of tight oil reservoirs in North America. To improve the performance of CO2 huff-n-puff, injection and production parameters need to be optimized through numerical simulation. The phase behavior and microscopic flow mechanism of CO2 huff-n-puff in porous media need to be further investigated. This paper presents a detailed review of phase behavior and microscopic flow mechanism in tight porous media by CO2 huff-n-puff. Phase behavior in tight porous media is different from that in a PVT cylinder since the capillary pressure in tight porous media reduces the bubble point pressure and increases the miscibility pressure and critical temperature. The condensate pressure in tight porous media and nonequilibrium phase behavior need to be further investigated. The microscopic flow mechanism during CO2 huff-n-puff in tight porous media is complicated, and the impact of molecular diffusion, gas-liquid interaction, and fluid-rock interaction on multiphase flow is significant especially in tight porous media. Nuclear magnetic resonance (NMR) and molecular simulation are efficient methods to describe the microscopic flow in tight oil reservoirs, while the NMR is not cost-effective and molecular simulation needs to be improved to better characterize and model the feature of porous media. The improved molecular simulation is still a feasible method to understand the microscopic flow mechanism of CO2 huff-n-puff in tight oil reservoirs in the near future. The microscopic flow model in micropore network based on digital core is worth to be established, and phase behavior needs to be further incorporated into the microscopic flow model of CO2 huff-n-puff in tight porous media.

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致密油藏CO2 Huff-n-Puff在致密多孔介质中的相行为及微观流动机制研究进展

美国致密油藏的成功开发显示了非常规油藏的美好前景。致密油藏将是未来勘探开发的主要目标，而CO2 huff-n-puff是北美地区提高致密油藏采收率最重要的方法之一。为了提高 CO2 huff-n-puff 的性能，需要通过数值模拟优化注入和生产参数。CO2 huff-n-puff 在多孔介质中的相行为和微观流动机制需要进一步研究。本文详细回顾了 CO2 huff-n-puff 在致密多孔介质中的相行为和微观流动机制。致密多孔介质中的相行为与 PVT 圆柱体中的相行为不同，因为致密多孔介质中的毛细管压力降低了泡点压力并增加了混相压力和临界温度。致密多孔介质中的冷凝水压力和非平衡相行为需要进一步研究。CO2在致密多孔介质中吞吐过程的微观流动机制复杂，分子扩散、气液相互作用和流体-岩石相互作用对多相流的影响尤其在致密多孔介质中显着。核磁共振（NMR）和分子模拟是描述致密油藏微观流动的有效方法，而 NMR 不具有成本效益，需要改进分子模拟以更好地表征和模拟多孔介质的特征。在不久的将来，改进的分子模拟仍然是了解致密油储层 CO2 huff-n-puff 微观流动机制的一种可行方法。基于数字岩心的微孔网络微观流动模型值得建立，相行为需要进一步纳入致密多孔介质中CO2 huff-n-puff的微观流动模型。