Studies on the near-critical carbon dioxide (NC-CO) adsorption behavior of organic-rich shales are important for shallow shale gas exploration and hydrocarbon recovery enhancement of liquid CO fracturing, which further contributes to geological CO sequestration and global carbon neutrality. We performed CO isothermal adsorption and kinetic experiments on six organic-rich shale samples at a temperature of 303.15 K and pressures of up to 5 MPa to investigate the modeling, multifractality, and kinetics of NC-CO adsorption behavior on shales. The equilibrium data were fitted to the Langmuir, Freundlich, BET, modified BET (M−BET), DA, DR, and OK adsorption models; the multifractality of the isotherms was analyzed; and pseudo-first-order, pseudo-second-order, Elovich, and Avrami's fractional-order models were adopted to analyze the adsorption kinetics. Furthermore, an error analysis technique was employed to compare the accuracies of various models for fitting the experimental data. The results showed that various adsorption models presented different fitting effects; however, the M−BET, Freundlich, and DA adsorption models performed better over the entire studied pressure range. The fitting results of the M−BET model indicated that the number of adsorption layers ranged from 2.528 to 4.663 and had a multilayer adsorption mechanism. The multifractality of the adsorption behavior was described by the generalized fractal dimension and multifractal singularity spectra; we demonstrated that some multifractal parameters characterize the NC-CO adsorption behavior on shales well. Additionally, the TOC, clay, and quartz contents show close covariations with the isothermal adsorption and multifractal parameters. Moreover, the adsorption process was TOC-dependent and subject to Avrami's fractional-order model. Finally, we propose conceptual three-stage adsorption and geological storage models to understand the nature of the NC-CO adsorption behavior of shales.

中文翻译：

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富含有机质页岩上近临界二氧化碳的吸附行为：建模、多重分形和动力学

富有机质页岩近临界二氧化碳（NC-CO）吸附行为的研究对于浅层页岩气勘探和液态CO压裂提高油气采收率具有重要意义，从而进一步有助于地质CO封存和全球碳中和。我们在 303.15 K 的温度和高达 5 MPa 的压力下对 6 个富含有机质页岩样品进行了 CO 等温吸附和动力学实验，以研究页岩上 NC-CO 吸附行为的建模、多重分形和动力学。将平衡数据拟合到Langmuir、Freundlich、BET、修正BET（M−BET）、DA、DR和OK吸附模型；分析了等温线的多重分形性；采用准一阶、准二阶、Elovich和Avrami分数阶模型分析吸附动力学。此外，采用误差分析技术来比较各种模型拟合实验数据的准确性。结果表明，不同的吸附模型呈现出不同的拟合效果；然而，M−BET、Freundlich 和 DA 吸附模型在整个研究压力范围内表现更好。 M−BET模型拟合结果表明，吸附层数为2.528~4.663，具有多层吸附机制。用广义分形维数和多重分形奇点谱描述了吸附行为的多重分形性；我们证明了一些多重分形参数可以很好地表征 NC-CO 在页岩上的吸附行为。此外，TOC、粘土和石英含量与等温吸附和多重分形参数表现出密切的协变。此外，吸附过程依赖于 TOC，并服从 Avrami 分数阶模型。最后，我们提出了概念性的三阶段吸附和地质储存模型，以了解页岩 NC-CO 吸附行为的本质。