Knudsen and Molecular Diffusion Coefficients for Gas Transport in Unconsolidated Porous Media

Abstract

This study was an assessment of the Knudsen and molecular diffusion cocfficienrs in relation to gas-phase permeability and air-filled porosity, respectively, for gas transport in unconsolidated porous media. Single gas How experiments at various gas pore pressures and equimolar binary experiments were conducted to determine apparent, effective gas permeabilities as well as the Knudsen and molecular diffusion coefficients. Experimental results integrated with data from previous studies showed that for dry porous media, the Knudsen diffusion coefficient value for kaolin with total porosity ranging from 0.675 to 0.85 is on the order of lO""* m', while it is at least one order of magnitude greater for sea sand and one order less for silica flour. Additionally, the existing correlations of the Knudsen diffusion coefficient can give lower and upper limits to the order of this coefficient value for porous media with effective gas permeabilities in the order of 10"''' to 10"'^ m^. The gas-phase molecular diffusion coefficient for an unsaturated silica flour sample with total porosity of 0.386 and water saturation levels >50% was measured to be one order of magnitude less than those for the other samples with total porosities ranging from 0.402 to 0.417 at similar air-filled porosities. The former also had lower gas diffusivity under rewetting than under desaturation conditions. The diffusivity factor values obtained from all ofthe unsaturated silica flour samples were far less than the predictions from other commonly used difîusivity relationships.Abbreviations: DGM, dusty gas model: DSF, dry-packed silica flour: DSS, dry-packed sea sand: WSF, wet-packed silica flour: WSS, wet-packed sea sand.

Assessing Vadose Zone Biodegradation by a Multicomponent Gas Transport Model

“…hese magnitudes are very small in comparison with the permeability of the sandy layer (1.0 ´ 10 −11 m 2 ). his implies high resistance to gas difusion in lowpermeability zones (i.e., the cemented limestone layer), as pointed out by Davis et al (2005) and Fen et al (2011).…”

“…Molins et al (2010) further demonstrated that the DGM approach (coupled with aqueous-phase transport and biogeochemical reactions) can be used to assess the concentration distributions of reactive and nonreactive gas components in an unsaturated soil above a smear zone of a crude oil spill site. If the DGM approach is used, however, another parameter, the Knudsen difusion coeicient, may also afect gas transport in soils with low permeability or at a high water content (Fen et al, 2009(Fen et al, , 2011. Being related to soil air permeability, the Knudsen difusion coeicient may also afect model ittings to soil gas concentration depth proiles where degradation occurs.…”

CO2 Diffusion and Dispersion in Porous Media: Review of Advances in Experimental Measurements and Mathematical Models

“…Knudsen and molecular diffusion are two primary diffusion mechanisms that can take place during fluid flow in porous media. In general, during gas injection, the molecular diffusion occurs as a result of the collision between the molecules of the gas, while Knudsen diffusion occurs when the gas molecules collide with the rock wall and are reflected back by the rock particles. , Hence, the Knudsen diffusion is dominant when the pore diameter is less than or nearly equal to the free path of the molecules. Consequently, the Knudsen diffusion is more likely to occur in tight reservoirs, with permeability less than 10 –14 m 2 .…”