Abstract There is a growing body of evidence that gas situated within the pores of nanoporous materials may not have the same equation of state (pressure, volume, and temperature, PVT) properties as macroscopic free gas. However, there is limited experimental measurement of in-situ fluid properties for gases taken up by nanoporous shales. In this work, we use a gas injection porosimetry approach to measure the gas storage capacity of four different North American shales (Bakken, Marcellus, Haynesville, and Mancos) and in-situ gas density for a few different hydrocarbon and noble gases. We find the porosity measured with helium to be reasonable between 5% and 16.4%. However, when using other gases such as methane, argon, and ethylene, the equivalent porosity estimations are extremely high, with the highest measured value being 309% for ethylene gas in a Marcellus shale sample. Such extreme results raise questions on the validity of the underlying assumptions of the porosimetry equations, in particular, the description of gas density within shale nanopores with macroscopic density. The experimentally measured density of in-situ gas is found to be up to 28 times higher than the theoretically estimated one at the equilibrium PVT conditions. This in-situ densification of gas is independently verified using X-ray CT imaging on one of the samples – the Marcellus. The underlying mechanism for gas densification could be explained by adsorption, in which case the proportion of adsorbed gas is estimated to be between 12% and 96% for the various gas-sample pairs. Surface area measurements show that a monolayer of adsorbed gas can only account for 27% to 42% of the adsorbed gas. This calls into question the commonly assumed Langmuir monolayer model of adsorption, and indicates that gas densification within shale nanopores can be attributed to a multilayer adsorption mechanism and/or other unidentified mechanisms that require further study.

中文翻译：

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纳米多孔页岩中气体致密化和增强储存的实验证据

摘要 越来越多的证据表明，位于纳米多孔材料孔隙内的气体可能与宏观游离气体具有不同的状态方程（压力、体积和温度，PVT）特性。然而，对于被纳米多孔页岩吸收的气体的原位流体特性的实验测量是有限的。在这项工作中，我们使用注气孔隙度测定法来测量四种不同的北美页岩（巴肯、马塞勒斯、海恩斯维尔和曼科斯）的储气能力和几种不同碳氢化合物和稀有气体的原位气体密度。我们发现用氦气测量的孔隙率在 5% 到 16.4% 之间是合理的。然而，当使用其他气体如甲烷、氩气和乙烯时，等效孔隙度估计值非常高，Marcellus 页岩样品中乙烯气体的最高测量值为 309%。这种极端的结果对孔隙度方程的基本假设的有效性提出了质疑，特别是对具有宏观密度的页岩纳米孔内气体密度的描述。发现原位气体的实验测量密度比平衡 PVT 条件下的理论估计密度高 28 倍。这种气体的原位致密化是使用 X 射线 CT 成像对其中一个样品 - Marcellus 进行独立验证的。气体致密化的潜在机制可以通过吸附来解释，在这种情况下，对于各种气体样品对，吸附气体的比例估计在 12% 到 96% 之间。表面积测量表明，单层吸附气体只能占吸附气体的 27% 至 42%。这对通常假设的朗缪尔单层吸附模型提出了质疑，并表明页岩纳米孔内的气体致密化可归因于多层吸附机制和/或其他需要进一步研究的未知机制。