Effects of composition and temperature on water sorption in overmature Wufeng-Longmaxi shales

Water sorption provides a great approach to understand water-shale interaction. In this research, water sorption isotherms of fourteen Wufeng-Longmaxi shale samples collected from the eastern Sichuan Basin of China were generated and then interpreted by classical sorption models. The sorption behavior of water and its relationship with pore structure, organic/inorganic composition, and temperature (278-333 K) were explored using organic geochemistry, mineralogy, and gas physical sorption. Brunauer-Emmett-Teller theory functions well for these isotherms but generates ambiguous model parameters, suggesting that multilayer sorption concept is not suitable for water sorption. Dent model distinguishes between primary and secondary sorption, and the resulting specific surface area (A(Dent)(H2O)) is not comparable to the nitrogen-determined specific surface area (A(BET)(N-2)). However, A(Dent)(H2O)/A(BET)(N-2) linearly decreases with increasing A(BET)(N-2), and water saturation is negatively correlated with total pore volume. These observations collectively support the surface-chemistry-controlled clustering mechanism for water sorption. Clay minerals dominate water sorption at low relative pressure because they provide more hydrophilic sorption sites for water cluster formation and growth. Nevertheless, organic matter becomes increasingly important with increasing relative pressure, primarily because a specific sized organic pore has higher filling pressure than the same-sized clay pore resulting in more pronounced condensation in organic pores at high relative pressure. The water desorption branch shifts to higher relative pressure with increasing temperature and thus, the hysteresis loop shrinks, indicating a mechanism of capillary evaporation similar to wetting fluid nitrogen. The water adsorption branch is insensitive to temperature change, reflecting a temperature-independent clustering mechanism between 278 K and 333 K. The enthalpy of adsorption of collected shale samples is smaller than that of lower-maturity shales but close to that of condensation of bulk water. Based on this finding, we argue that higher-maturity shales have weaker interaction with water, which gives rise to lower temperature sensitivity of water sorption. Therefore, in the future, detailed investigations should revolve around water sorption behavior and its response to temperature in shales with different thermal maturities.

Automated summary

This research investigates water sorption behavior in shale, revealing the relationship between water and pore structure, organic/inorganic composition, and temperature. The study supports a surface-chemistry-controlled clustering mechanism for water sorption in shale.