Jiang Wen, Lv Weifeng, Jia Ninghong, Cheng Kai, Wan Yidi, Wang Kai
College of Engineering Science, University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
Institute of Porous Flow and Fluid Mechanics, University of Chinese Academy of Sciences, Langfang 065007, China.
Langmuir. 2024 Jul 23;40(29):14922-14940. doi: 10.1021/acs.langmuir.4c00989. Epub 2024 Jul 11.
Hybrid N-CO huff-n-puff (HnP) has been experimentally demonstrated to be a promising approach for improving oil recovery from tight/ultratight shale oil reservoirs. Despite this, the detailed soaking process and interaction mechanisms remain unclear. Adopting molecular dynamic simulations, the soaking behavior of hybrid N-CO HnP was investigated at the molecular and atomic levels. Initially, the soaking process of fluid pressure equilibrium after injection pressure decays in a single matrix nanopore connected to a shale oil reservoir is studied. The study revealed that counter-current and cocurrent displacement processes exist during the CO and hybrid N-CO soaking, but cocurrent displacement occurs much later than counter-current displacement. Although the total displacement efficiency of the hybrid N-CO soaking system is lower than that of the CO soaking system, the cocurrent displacement initiates earlier in the hybrid N-CO soaking system than in the CO soaking system. Moreover, the N soaking process is characterized by only counter-current displacement. Next, the soaking process of fluid pressure nonequilibrium before the injection pressure decays is investigated. It was discovered that counter-current and cocurrent displacement processes initiate simultaneously during the CO, N, and hybrid N-CO soaking process, but cocurrent displacement exerts a dominant influence. During the CO soaking process, many hydrocarbon molecules in the nanopore are dissolved in CO while simultaneously exhibiting a substantial retention effect in the nanopore. After pure N injection, there is a tendency to form a favorable path of N through the oil phase. The injection of hybrid CO-N facilitates the most significant cocurrent displacement effect and the reduction in residual oil retained in the nanopore during the soaking process, thus resulting in the best oil recovery. However, the increase rate in total displacement efficiencies of the different soaking systems over time (especially the hybrid N-CO soaking system) was significantly larger before than after injection pressure decays. Additionally, the displacement effect induced by oil volume swelling is significantly restricted before the injection pressure decays compared to the soaking process after the injection pressure decays. This study explains the role of CO-induced oil swelling and N-induced elastic energy played by hybrid N and CO at different stages of the hybrid N-CO soaking process before and after pressure decays and provides theoretical insights for hybrid gas HnP-enhanced recovery. These pore-scale results highlight the importance of injection pressure and medium composition during the soaking process in unconventional oil reservoirs.
混合N-CO吞吐（HnP）已通过实验证明是一种提高致密/超致密页岩油藏采收率的有前景的方法。尽管如此，详细的浸泡过程和相互作用机制仍不清楚。采用分子动力学模拟，在分子和原子水平上研究了混合N-CO HnP的浸泡行为。首先，研究了连接页岩油藏的单个基质纳米孔中注入压力衰减后流体压力平衡的浸泡过程。研究表明，在CO和混合N-CO浸泡过程中存在逆流和顺流驱替过程，但顺流驱替比逆流驱替发生得晚得多。虽然混合N-CO浸泡系统的总驱替效率低于CO浸泡系统，但混合N-CO浸泡系统中的顺流驱替比CO浸泡系统中更早开始。此外，N浸泡过程仅以逆流驱替为特征。接下来，研究了注入压力衰减前流体压力不平衡的浸泡过程。发现CO、N和混合N-CO浸泡过程中逆流和顺流驱替过程同时开始，但顺流驱替起主导作用。在CO浸泡过程中，纳米孔中的许多烃分子溶解在CO中，同时在纳米孔中表现出显著的滞留效应。注入纯N后，有形成N通过油相的有利通道的趋势。混合CO-N的注入促进了最显著的顺流驱替效应，并在浸泡过程中减少了纳米孔中残留油的保留，从而导致最佳的采收率。然而，不同浸泡系统的总驱替效率随时间的增加率（特别是混合N-CO浸泡系统）在注入压力衰减前明显大于注入压力衰减后。此外，与注入压力衰减后的浸泡过程相比，注入压力衰减前油体积膨胀引起的驱替效应受到显著限制。本研究解释了在压力衰减前后混合N-CO浸泡过程的不同阶段，混合N和CO所引起的CO诱导油膨胀和N诱导弹性能的作用，并为混合气体HnP强化采收提供了理论见解。这些孔隙尺度的结果突出了非常规油藏浸泡过程中注入压力和介质组成的重要性。
