Water Sorption and Distribution Characteristics in Clay and Shale: Effect of Surface Force

Characteristics of sorption and distribution of water in nanoporous shale are topics of great interest to evaluate unconventional reservoirs. Also, a study of surface force of water/solid interaction at nanoscale is significant for understanding the storage of initial water and the fate of residual treatment liquid in shale systems. In this work, the thickness and stability of water film were investigated by vapor sorption experiments on clay and shale samples. Meanwhile, an approach based on surface forces (disjoining pressure), which resulted in the instability of adsorbed film transition into condensed bulk liquid, was developed to describe molecule/pore wall interactions. Our experimental results directly demonstrated the occurrence of capillary condensation in hydrophilic clay minerals; however, water would not entirely fill in shale nanopores even under high moisture conditions. This remarkable finding is mainly due to the inaccessibility of water molecules to micropores of hydrophobic organic matter. In addition, the water distribution characteristics are also significantly influenced by pore scale. Under a moist condition with certain relative humidity (e.g., RH = 0.98), the water distributed in hydrophilic inorganic pores with different sizes was mainly classified as (i) capillary water in small pores (e.g., <6-7 nm) and (ii) water film in large pores (e.g., >6-7 nm). In contrast, the surface repulsion prevents water condensing and likely results in a monolayer water film sorption in hydrophobic organic pores (e.g., theta = 100 degrees). Therefore, in an actual shale system with initial moisture content, the inorganic microporosity totally blocked by water might be incapable of gas transport or storage, while the hydrophobic organic pores mainly provide effective space for gas accumulation.