ABSTRACT
In Chapter 2, porosity n was defined as the volume Vp of the pores of a rock or soil sample divided by the total volume Vt of both pores and solid material so that n = Vp/Vt . But porosity at a geometric point cannot be defined since a porous medium is a conglomerate of solid grains and voids. It is then necessary to introduce the concept of representative elementary volume (REV). The REV is sufficiently large to define a space-averaged porosity, but it is small enough that the variation from one REV to the next may be approximated by a continuous function on the scale of the measuring instruments. Thus one could take, for example, an REV of 1 cm3 for a fine sand, but it could be quite larger for a fractured rock. An alternate approach is to consider the porous medium as a realization of a random process. The porosity at a geometrical point is then an ensemble average of an infinite number of realizations (de Marsily, 1986; Charbeneau, 2000). Either of these constructs allows us to use the infinitesimal calculus and thus to apply the concepts of fluid mechanics.
