PORE-SCALE STUDY OF RAREFIED GAS FLOW THROUGH FRACTAL AND VORONOI POROUS MEDIA

Investigation of rarefied gas flow through unconventional reservoirs has been a challenging task due to the coexistence of slippage and transition flow regimes. Considering the invalidity of traditional continuum flow theory for describing rarefied gas flow, in this work, the discrete velocity method, which is capable of all flow regimes and accurately predicts the nonequilibrium gas dynamics, is employed to investigate the gas flows of CO2, CH4, N-2, and H-2 through Sierpinski fractals and Voronoi porous media. The effects of porosity, Knudsen number, specific surface area, and pore-throat ratio on gas velocity distribution, streamlines, and apparent permeability are discussed. The results show that the velocity magnitude reduction for CO2 is evidently larger than that for H-2, and the apparent permeability for different gases quantitatively follows k(a,H2) > k(a)(,N2) > k(a)(,)(CH)(4) > k(a)(,)(CO)(2). When the Knudsen number and porosity remain unchanged, the apparent permeability is in inverse proportion to the specific surface area for the sake of the increased friction from the larger specific surface area. Even under the same porosity and specific surface area, the apparent permeability reduces as the pore-throat ratio increases and the gas velocity obviously increases near the pore-pore and pore-throat regions, which can be ascribed to the narrow throat as well as the reduction of available pathways. The present work sheds light on the mechanism of rarefied gas transport in microscale porous media and is of significance for predicting the recovery rate of unconventional gases such as tight gas, shale gas, and coalbed methane.