Water condensation in shales impacts its hydro-mechanical response. A mechanistic understanding of the pore-water system is made more challenging by significant anisotropy of pore architecture and nano-scale heterogeneity of pore surfaces. We probe the condensation response in two contrasting shales exposed to a vapor of contrast-matching water, as characterized by in situ ultra-small/small-angle neutron scattering (USANS/SANS) techniques under various relative humidities. One shale with a higher content of both kerogen and clay has rougher surfaces and higher anisotropy than the other shale (less clay and no kerogen) over length scales from 2.5 to 250 nm. Scanning electron microscopy with energy-dispersive spectrometry (SEM–EDS) analysis also confirms that the organic-rich shale presents more anisotropic microfabrics and higher heterogeneity compared to the other shale with less clay and no kerogen. USANS/SANS results show that water condensation effectively narrows the pore volume in the way of reducing the aspect ratio of non-equiaxed pores. For the shale with less clays and no kerogen under a relative humidity of 83%, a wetting film uniformly covers the pore-matrix interface over a wide range of length scale (1 nm–1.9 µm) without smoothing the surface roughness. In contrast, for the organic-rich and clay-rich shale with a strong wetting heterogeneity, condensation occurs at strongly curved hydrophilic asperities (1–10 nm) and smoothens the surface roughness. This is consistent with water vapor condensation behavior in a Vosges sandstone by Broseta et al. (Phys. Rev. Lett. 86:5313, 2001). Though well representing the condensation behavior of water vapor in mesopores/macropores (radii > 1 nm), USANS/SANS techniques could underestimate total water adsorption due to potential cation hydration and clay swelling in micropores (radii < 1 nm).

中文翻译：

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页岩中孔隙水汽凝结行为：一项实验研究

页岩中的水凝结会影响其水力机械响应。孔隙结构的显着各向异性和孔隙表面的纳米级异质性使得对孔隙水系统的机械理解更具挑战性。我们探测了暴露于对比匹配水蒸气的两个对比页岩中的凝结响应，如在各种相对湿度下的原位超小/小角度中子散射 (USANS/SANS) 技术所表征的那样。在 2.5 到 250 nm 的长度范围内，干酪根和粘土含量较高的一个页岩比另一个页岩（粘土较少且没有干酪根）具有更粗糙的表面和更高的各向异性。具有能量色散光谱 (SEM-EDS) 分析的扫描电子显微镜也证实，与其他粘土较少且没有干酪根的页岩相比，富含有机质的页岩具有更多的各向异性微结构和更高的非均质性。USANS/SANS 结果表明，水冷凝通过降低非等轴孔的纵横比有效地缩小了孔容。对于粘土较少且没有干酪根的页岩，在 83% 的相对湿度下，润湿膜在很宽的长度尺度范围内（1 nm-1.9 µm）均匀覆盖孔隙-基质界面，而不会平滑表面粗糙度。相比之下，对于具有强润湿非均质性的富含有机质和富含粘土的页岩，在强烈弯曲的亲水性凹凸不平（1-10 nm）处发生冷凝并使表面粗糙度变得平滑。这与 Broseta 等人在 Vosges 砂岩中的水蒸气冷凝行为一致。(Phys. Rev. Lett. 86:5313, 2001)。尽管很好地代表了水蒸气在中孔/大孔（半径 > 1 nm）中的冷凝行为，但由于潜在的阳离子水合和微孔（半径 < 1 nm）中的粘土膨胀，USAS/SANS 技术可能会低估总吸水量。