Gas transport mode criteria in ultra-tight porous media

Conventional Darcy law cannot accurately describe the combined gas transport mechanisms in porous media with free gas only (such as tight gas reservoirs) and porous media with both free gas and adsorbed gas (such as shale gas reservoirs). The gas transport mechanisms in tight porous media including viscous flow, Knudsen diffusion, surface diffusion and molecular diffusion are investigated. Both the equivalent hydraulic radius and transport properties including intrinsic permeability, tortuosity and porosity, are used to build the coupled transport models describing the combined mechanisms of gas transport in ultra-tight porous media with free gas only and porous media with both free gas and adsorbed gas respectively. The effect of the pore-volume occupied by the adsorbed layer and surface diffusion through the adsorbed layer is considered in the transport model of porous media with adsorbed gas. The influences of equivalent hydraulic radius and transport properties on the gas transport mode in ultra-tight porous media are analyzed and the gas transport mode criteria in ultra-tight porous media are obtained. The result shows that viscous flow is the dominant mechanism, Knudsen diffusion can be ignored and Darcy law is applicable in porous media with equivalent hydraulic radius more than 100 nm. Surface diffusion can be ignored in the porous media with equivalent hydraulic radius less than 10 nm, while the effect of volume reduction by adsorption of porous media is less than 100 nm. The critical value of the transport function of intrinsic permeability, tortuosity and porosity also is given in this paper. (C) 2014 Elsevier Ltd. All rights reserved.