Summary When reservoir fluids are confined by nanoscale pores, pronounced changes in fluid properties and phase behavior will occur. This is particularly significant for the natural gas huff ‘n’ puff (HNP) process as a means of enhanced oil recovery (EOR) technology in unconventional reservoirs. There have been considerable scientific contributions toward exploring the EOR mechanisms, yet almost none considered the effects of nanopore confinement and its proportion on the oil recovery dynamics. To bridge this gap, we developed an approach to calculate fluid phase equilibrium in nanopores by modifying the Rachford-Rice equation and Peng-Robinson equation of state (PR-EOS), completed by considering the shifts of fluid critical properties and oil/gas capillary pressure. Afterward, the effect of nanopore radius (rp) on the phase behavior between the injected natural gas and oil was thoroughly investigated. Compositional simulation was performed using a rigorously calibrated model based on typical properties of a tight reservoir to investigate the production response of natural gas HNP, including the effects of nanopore confinement and its proportion. We demonstrated that the critical pressure and temperature of fluid components decreased with the reduction in rp, especially for heavy constitunts. The saturation pressure, density, and viscosity of the oil in the presence of natural gas all declined linearly with 1/rp in the confined space. The suppression of fluid saturation pressure was indicative of an extended single-phase oil flow period during production. The cumulative oil production was approximately 12% higher if the confinement effect was considered in simulation. Moreover, the average reservoir pressure declined rapidly resulting from this effect, mainly caused by the intensified in-situ gas/oil interaction in nanopores. The results of this paper supplement earlier findings and may advance our understanding of nanopore confinement during natural gas HNP, which are useful for field-scale application of this technique.

中文翻译：

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考虑纳米孔约束及其比例影响的非常规油藏天然气吞吐采油动力学：机理研究

总结 当储层流体被纳米级孔隙限制时，流体性质和相行为将发生显着变化。这对于作为非常规油藏提高采收率 (EOR) 技术手段的天然气吞吐 (HNP) 工艺尤为重要。在探索 EOR 机制方面已经做出了相当大的科学贡献，但几乎没有人考虑纳米孔限制的影响及其对石油采收动态的比例。为了弥补这一差距，我们开发了一种通过修改 Rachford-Rice 方程和 Peng-Robinson 状态方程 (PR-EOS) 来计算纳米孔中流体相平衡的方法，通过考虑流体临界性质和油气毛细管的变化来完成压力。之后，深入研究了纳米孔半径 (rp) 对注入的天然气和石油之间的相行为的影响。使用基于致密储层典型特性的严格校准模型进行成分模拟，以研究天然气 HNP 的生产响应，包括纳米孔限制及其比例的影响。我们证明了流体组分的临界压力和温度随着 rp 的降低而降低，特别是对于重组分。在密闭空间中，天然气存在时油的饱和压力、密度和粘度均以 1/rp 线性下降。流体饱和压力的抑制表明生产过程中单相油流动期延长。如果在模拟中考虑限制效应，累计石油产量大约高出 12%。此外，这种效应导致平均储层压力迅速下降，主要是由于纳米孔中的原地气/油相互作用增强。本文的结果补充了早期的发现，并可能促进我们对天然气 HNP 过程中纳米孔限制的理解，这对于该技术的现场规模应用很有用。