Water is ubiquitous within the shale reservoirs and mainly stored in the hydrophilic clay minerals. The water distribution characteristics are important for the gas-in-place and gas production. In our work, water vapor adsorption on montmorillonite (Mt), kaolinite (Kaol) and illite (Il) were performed to investigate the water adsorption behaviors. Then, the samples were conducted with N2 gas-adsorption techniques to investigate the effect of adsorbed water on pore structure characteristics. The results show that (1) the PSD curves under different RH condition has validated the condensation effect and also demonstrated the heterogeneity of water distribution which varies with the pore scale. Under a certain moisture condition (RH=98%), the small pores (approximately less than 5nm) are blocked with the capillary water while large pores are covered with the adsorbed water film. (2) The pre-adsorbed water occupied more pore volume with the increasing of RH, the corresponding quantitative water saturation based on the nanopore size distribution of moisture-equilibration samples can reach to 51.99%, 71.43% and 46.15% at RH of 98% for Mt, Kaol and Il, respectively. The numerical range is enough to represent the value of actual reservoir. (3) The contribution of clay minerals to the methane adsorption capacity is over overestimated in dry condtion. Under certain water saturation, the smaller pore is filled with capillary water while larger pores are covered with water film, the adsorption forces is changing from solid-gas interaction to liquid-gas interaction. This phenomenon also give the reasonable explanation to the critical water content, Up to a this point, the further increasing of water content would not affected the methane adsorption ability. Therefore, the adsorption for clay minerals can be negligible comparing with the hydrophobic organic pores.

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