Tortuosity: a guide through the maze

Abstract: Despite its widespread use in petrophysics, tortuosity remains a poorly understood concept. Tortuosity can have various meanings when used by physicists, engineers or geologists to describe different transport processes taking place in a porous material. Values for geometrical, electrical, diffusional and hydraulic tortuosity are in general different from one another. Electrical tortuosity isdefined in terms of conductivity whereas hydraulic tortuosity is usuaIIy defined geometrically, and diffusional tortuosity is typically computed from temporal changes in concentration. A better approach may be to define tortuosity in terms of the underlying flux of material or electrical current with respect to the forces which drive this flow. Unsteady transport processes, including diffusion, can be described onIy b a population of tortuosities corresponding to the different flow paths taken by particles traversing the medium. In measurements of teady flow (e.g., those normally used to btain resistivity or permeability), information about particle travel times is lost, and so the multiple values of tortuosity are hornogenised. It can be shown that the maximum amount of information about pore structure is embedded in transport processes that combine advective an