(302c) Theoretical and Experimental Study of Gas Permeability and Klinkenberg Effects

To predict accurately the transport properties of coals and shales, it is important to study a non-adsorptive gas such as helium to investigate the slippage phenomenon independently of adsorption effects. Non-equilibrium molecular dynamics (NEMD) simulations are carried out with an external driving force imposed on the system to investigate the transport of helium and predict Klinkenberg parameters. The structure is modeled using a three-dimensional pore network, generated atomistically using the Voronoi tessellation method, to represent the carbon-based porous structure of natural systems. Simulations are performed to determine the effect of the pore structure and pressure gradient on gas permeability. In addition, experimental analysis is conducted using the classical Brace pressure solution to measure helium permeability and Klinkenberg parameters of a shale core plug at similar conditions. Simulation results are compared with experiments to understand both the merit and limitations of the simulation approach. 

The results indicate the gas permeability, obtained by the pulse decay experiment, is about a magnitude greater than that calculated by the molecular dynamics simulations. The estimated experimental permeability, however, is extracted from the early-time pressure profile that corresponds to macropores as indicated by the estimated apparent pore diameter and referred to as Darcy permeability. Crushing the intact core and sieving the sample to control the grain size could bridge the gap in scale between two methods in a way that the permeability measurements are comparable to the simulations. More complex molecular models of graphite structures containing fractures, local charge and defect sites within the pores, in addition to the inclusion of chemical functional groups to generate a more realistic model of natural systems of interest such as coal and shales could also bridge the gap.

The results have potentially important implications on gas transport in carbon-based materials and geologic formations and may provide an understanding of the limitations of the use of continuum fluid to model transport properties for confined fluids.