Comparing the permeability of dry and moisturized crushed shales determined by the dynamic process data of methane adsorption

Permeability is an important physical characteristic of shales. A lot of researches have been made by scholars to study permeability of shales, but the study of permeability for moisturized shales are insufficient. Water exist in natural shales, therefore, the difference of permeability for dry and moisturized shales and the reasons causing the difference need to be studied carefully. The permeability of dry and wet crushed carboniferous shale samples from the eastern Qaidam Basin, China were calculated by the dynamic process data of methane adsorption equilibrium. To obtain moisturized samples under different water saturation, a portion of the samples was moisturized at relative humidity (RH) values of 65%, 85% and 95%. In addition, the thickness of water in pores of the samples of different RH were calculated. The permeability was reduced by almost one order of magnitude after samples were wetted, and the relationship between permeability and water saturation (S) fit the exponential equation well. By calculating the porosity of dry and moisturized samples, the porosity reduction and connectivity loss accounted for the permeability reduction. Clay minerals and specific surface areas were considered to have a positive effect on permeability reduction. The permeability of moisturized samples were not declined with the water saturation as expectation. The permeability values for each moisturized sample under different RH only fluctuated in a small range. Correspondingly, the biggest water film thickness of moisturized samples under different RH was also almost the same. The difference of the water film thickness between RH = 65% and RH = 95% for each sample is only about 1.8 nm. The small difference in water film thickness under different RH is regarded as the reason for the low permeability difference of moisturized samples.