Evolution of water vapor adsorption behavior on kerogen: Pyrolysis and molecular investigation

With the advancement of hydraulic fracturing technology, understanding the evolution of shale kerogen and its impact on water vapor adsorption (WVA) behavior is critical for optimizing shale gas production. In this study, high-temperature, high-pressure pyrolysis experiments were conducted on marine kerogen to obtain a series of different maturity kerogen. Furthermore, WVA experiments, gas adsorption, solid-state ¹³C NMR, and X-ray photoelectron spectroscopy (XPS) were performed to analyze the effect of varying kerogen maturities on WVA. Additionally, molecular simulations were employed to elucidate the adsorption mechanisms of water molecules influenced by pore structure and chemical functional groups. The results show that as the maturity of kerogen increases, the content of aromatic structures rises, while the content of aliphatic structures and oxygen-containing functional groups decreases, leading to a reduction in the primary adsorption capacity of water. In particular, at 660 °C, there is a sharp reduction in pore volume and surface area, resulting in a significant decline in capillary-condensed water. The adsorption capacity of kerogen is primarily influenced by its pore structure, followed by the characteristics of functional groups and environmental conditions. Larger pore diameters increase the diffusion coefficient of water molecules, reduce the density of the adsorption layer, and raise the interaction energy. The interaction of oxygen-containing functional groups (R-COOH, R-CHO, R-OH) with water molecules is significantly greater than that of non-polar functional groups (R-C₆H₅, R-CH₂CH₃ & R-CH₃) due to the fact that the oxygen-containing functional groups form a strong hydrogen bond with water molecules, which are more polar and have a higher interaction. The higher the content of polar functional groups, the higher the number of primary adsorption sites, and the more easily water molecules can be adsorbed on the surface of the functional groups through hydrogen bonding and van der Waals forces, thus increasing the initial adsorption capacity. The influence of functional groups on water adsorption capacity extends throughout the adsorption process, with strong interactions present from the early to later stages.