Impact of surface chemistry and pore structure on water vapor adsorption behavior in gas shale

Abstract

To determine the effects of surface chemistry and the pore structure of shale on water vapor adsorption behavior, water vapor isothermal adsorption experiments were performed with untreated and 15 wt% H2O2 treated shale samples from Longmaxi Formation, Sichuan Basin, China. Additionally, low-pressure N2 ad-/desorption, X-ray diffraction, and X-ray photoelectron spectra measurements were conducted to analyze the pore structure, mineral composition, and functional groups in shale pore surfaces, respectively. The results show that when relative humidity ranged from 0 to 1, water vapor adsorption occurred through primary adsorption sites (monolayer adsorption) and secondary adsorption sites (multi-layer adsorption and capillary condensation). Adsorption by the primary sites was directly related to the number and type of oxygen-containing functional groups and pore size. Furthermore, compared with C–C/C–H and C-O, the functional groups of COO– and C = O/O-C-O provided more adsorption sites. A smaller pore size resulted in more water vapor adsorbed at the primary adsorption sites. The adsorption at secondary sites is not affected by the surface chemical properties; rather, it is directly related to pore volume. Additionally, a larger pore volume results in increased water vapor adsorbed by the secondary adsorption sites. These findings contribute to a better understanding of the primary water distribution in the multiscale pore structure of shale and accurate evaluation of the amount of gas in place.