TORTUOSITY OF IMMISCIBLE FLUIDS IN POROUS MEDIA BASED ON PHASE INTERFACIAL AREAS: A NEW DEFINITION AND ITS APPLICATION TO HANFORD’S UNSATURATED MEDIA

Tortuosity is the single most important characteristic of flow through porous media, that determines several flow and transport phenomena. Currently available definitions of tortuosity are empirical, and do not lend themselves to direct and independent measurement. We present a new definition for the tortuosity factor of saturated media (ts) as the ratio S/So (specific surface of real porous medium to that of an idealized capillary bundle). For unsaturated media, tortuosity factor (ta) is defined as the ratio of the specific air-water interfacial area of real and the corresponding idealized porous medium. This tortuosity factor is suitably measured using sorptive tracers (e.g., nitrogen adsorption method) for saturated media and interfacial tracers for unsaturated media. New models based on this approach are presented for the prediction of several fundamental phenomena in unsaturated porous media, such as diffusion, unsaturated water flow and anisotropy, that are influenced by changes in tortuosity with changing water content. Diffusion coefficients and diffusive resistances measured in a number of saturated and unsaturated Hanford media agree well with the predictions of the model. Trends in the prediction of ta as a function water saturation are in agreement with similar recent predictions based on diffusion theory (Moldrup et al., 2001). Unsaturated hydraulic conductivities measured for a number of coarse-textured Hanford sediments agree well with predictions based on a modified Kozeny-Carman relation. Results indicate that the alternative definition of tortuosity is useful to the understanding and prediction of multiphase flow and transport. By defining the tortuosity factor as the phase interfacial area ratio, one overcomes the need to base its definition on the length dimensions of flow through the idealized capillary bundles, which is the most serious deficiency in the tortuosity-based approaches to modeling flow through porous media (Dullien, 1979; Epstein, 1989).