The pulse-decay technique is a well-established laboratory method for measuring the permeability of hydraulically tight rocks. Nitrogen or helium is routinely used in measurement of pulse-decay method, but the main components of natural gas reservoirs are hydrocarbons such as methane. In addition, nitrogen or helium has greater access to the fine pore structure of shale than larger molecules such as methane. Utilizing gas such as methane other than N2 or He to measure permeabilities requires corrections for gas rarefaction effect to be incorporated in the analyses. Knudsen number (Kn), the ratio between the gas free molecule distance and the pore diameter, is one of the key parameters to evaluate the rarefaction effect on the transport mechanism in micro/nano channels or porous media structure. When 0.001<Kn<0.1, the velocity slip cannot be ignored in the channel and the permeability will decrease. Due to the Knudsen numbers which indicate the interaction between gas molecules and micro-pores for helium, nitrogen and methane in the same micro-structure of tight rock are significantly different; the permeability measurements are specific for individual working gases. However, our analysis shows that neither of these pulse-decay approaches takes into account the differences in transport mechanisms in shale due to different Kn numbers caused by different working gases. In this paper, we conducted a series of experimental studies to investigate different pulse-decay permeabilities measured using helium, nitrogen and methane for the same specimen under the same reservoir conditions. Specifically, the importance of estimating accurate Kn numbers of different working gases in rock samples directly impact permeabilities of rock samples was demonstrated. In reviewing these approaches, we also would propose the need and possible direction of new models that will be required for making evaluations more accurate and accurate. The new theoretical model was indicated to illustrate the inherent relationship between the microscopic parameter Knudsen number and the macroscopic parameter permeability of a tight rock. The results showed that Knudsen number would strongly affect the pulse-decay permeabilities measured under the same in situ confining-stress state reservoirs because of these existing pulse-decay models of gas flow in nanopores was inapplicable to measure permeabilities since the original assumption of no-slip boundary conditions in the Navier-Stokes equation was incorrect at nanoscale. It would be highly recommended using methane to measure permeability of micro-structure tight rocks (i.e. shale).


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