CO₂ flooding and huff-n-puff are novel injection strategies for underground gas recovery and are expected to realize carbon storage simultaneously, which drive sustainable energy production and advance net-zero emission targets. However, determining which method outperforms the other is not well discussed from a fundamental perspective in the current literature. In this study, we employ coarse-grained molecular dynamics to investigate microscale CO₂-CH₄ displacement in nanoconfined porous media, addressing key limitations such as the absence of porous media heterogeneity and the oversimplification of nanopore structure to a single nanochannel. After calibrating gas–gas and gas–solid interactions and validations, different displacement mechanisms are studied in a slit microchannel by comparing CO₂ flooding and huff-n-puff under three schemes. Furthermore, ten nanoporous structures are constructed to mimic the organic matrix in shale, with varying porosities (0.45–0.6) and pore sizes (35.4–106.2 nm). Results show that gas flooding can lead to ∼ 1.5 times more CO₂ being stored in the nanoporous media but might not significantly improve CH₄ recovery in low-porosity shale matrix compared to huff-n-puff (0.51–0.53 vs. 0.52–0.54). Moreover, neither CO₂ sequestration nor CH₄ recovery is sensitive to pore size changes with the coefficient of variation less than 10 %. The recovery rate of CH₄ is found to be only correlated to the injected pore volume of CO₂ during flooding where ∼ 50 % of CH₄ is recovered after the first injection volume. Our microscale findings provide essential insights to optimize CO₂-EGR and carbon storage at the field scale, contributing to the development of underground clean energy technologies.

中文翻译：

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用于增强甲烷回收和碳储存的 CO2 注入技术的微尺度建模


CO₂ 驱油和 huff-n-puff 是地下天然气回收的新型注入策略，有望同时实现碳储存，从而推动可持续能源生产并推进净零排放目标。然而，在目前的文献中，从基本角度确定哪种方法优于另一种方法并没有得到很好的讨论。在这项研究中，我们采用粗粒分子动力学来研究纳米受限多孔介质中的微尺度 CO_{2-CH 4} 置换，解决了关键限制，例如缺乏多孔介质异质性和将纳米孔结构过度简化为单个纳米通道。在校准气-气和气-固相互作用和验证后，通过比较三种方案下的 CO₂ 驱油和 huff-n-puff 来研究狭缝微通道中不同的驱替机制。此外，构建了 10 个纳米多孔结构来模拟页岩中的有机基质，具有不同的孔隙率 （0.45-0.6） 和孔径 （35.4-106.2 nm）。结果表明，气驱可导致纳米多孔介质中储存的_CO2 增加约 1.5 倍，但与 huff-n-puff 相比，可能无法显著提高低孔隙度页岩基质中 CH₄ 的回收率（0.51-0.53 对 0.52-0.54）。此外，CO₂ 封存和 CH₄ 回收均对孔径变化不敏感，变异系数小于 10%。CH₄ 的回收率仅与驱油过程中注入的 CO₂ 孔体积相关，其中 ∼ 50% 的 CH₄ 在第一次注入体积后被回收。 我们的微观发现为在现场规模优化 CO_2-EGR 和碳储存提供了重要的见解，为地下清洁能源技术的发展做出了贡献。