This paper investigates the CO₂/N₂ injection process in tight oil reservoirs considering the confinement effect. To study the microscopic physical mechanisms, the confinement effect is characterized by properties shift and capillarity and introduced into the flash calculation to obtain the phase equilibrium of mixture fluids (tight oil/CO₂/N₂) in tight porous media. The results indicate that the injected nitrogen gas could effectively maintain the reservoir pressure, while it also weakens the effects of the CO₂ injection recovery mechanisms, notably diffusivity and viscosity reduction. In addition, a dual‐pore tight oil reservoir model is set up to investigate the CO₂/N₂ injection with ultra‐low permeability and hydraulic fracturing. The basic CO₂ injection parameters are optimized by the orthogonal method. Based on CO₂ injection process, three injection schemes of CO₂/N₂ injection, which are mixed‐gas injection, CO₂‐alternating‐N₂ (CAN) injection, and N₂‐alternating‐CO₂ (NAC) injection, were investigated and a comparative analysis was made for the pressure distribution, CO₂ mole fraction distribution, and cumulative oil production. Based on this analysis, the CAN injection process proved to be the best injection scheme. A parametric analysis further suggested that the nitrogen gas injection rate was the most important factor. Besides, the effect of gravity drainage, reservoir permeability, nature fractures, and permeability heterogeneity on the oil production of CAN injection process were also investigated in detail. The results show that tight oil reservoir with better vertical connectivity, poor fracture growth, and higher heterogeneity is more favorable for the CO₂/N₂ injection process.

中文翻译：

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致密油藏注CO2 / N2的封闭效应研究

考虑限制作用，本文研究了致密油藏中的CO ₂ / N ₂注入过程。为了研究微观物理机理，以性质变化和毛细管现象为特征来描述封闭效应，并将其引入闪蒸计算中，以获得致密多孔介质中混合流体（致密油/ CO ₂ / N ₂）的相平衡。结果表明，注入的氮气可以有效维持储层压力，同时也减弱了CO ₂注入恢复机制的作用，尤其是扩散率和粘度降低。另外，建立了双孔致密油藏模型研究CO ₂ / N_2次注入具有超低渗透性和水力压裂。基本CO ₂注入参数通过正交方法进行优化。基于CO ₂注入过程，有三种CO ₂ / N ₂注入方案，分别是混合气体注入，CO ₂交替N ₂（CAN）注入和N ₂交替CO ₂（NAC）注入，进行了调查，并对压力分布CO ₂进行了比较分析摩尔分数分布和累计产油量。基于此分析，CAN注入过程被证明是最佳的注入方案。参数分析进一步表明，氮气注入速率是最重要的因素。此外，还详细研究了重力排水，储层渗透率，自然裂缝和渗透率非均质性对CAN注入过程中产油量的影响。结果表明，CO ₂ / N ₂注入过程具有较好的垂向连通性，较弱的裂缝扩展性和较高的非均质性的致密油藏更有利。