Experimental investigation into the permeability of water vapor in shales

Abstract

The transport of water vapor in shale (mudrocks) plays an important role in many natural and engineering issues, such as the global water cycle and shale gas production. However, the permeability of water vapor in shale is not well understood. An experimental approach to investigate the water vapor permeability in shale is presented in this paper. Water vapor sorption was conducted under the dynamic relative humidity condition in which the vapor evaporation was generated from the liquid distilled water at different gas (helium) pressures. Three crushed samples of Carboniferous shale from the eastern Qaidam Basin, China, were obtained and tested. In the experiments, the adsorption process of water vapor into the crushed shale was observed, and the permeability of water vapor through shale was calculated with the data collected during the dynamic water vapor adsorption process. The results show that the permeability decreases with increasing water vapor adsorption. The relationship between permeability and water vapor adsorption amount fits an exponential equation well. The permeability at the equilibrium of water vapor adsorption is 3–4 orders of magnitude lower than that in the initial stage of water vapor adsorption. The decrease in pore size and resulting pore throat blockage cause this permeability reduction. The water film thickness and water saturation in the shale pores at adsorption equilibrium were estimated. The permeability reduction is closely related to the maximum water film thickness. The larger the range of pores blocked by the water film is, the more obvious the permeability reduction.