Desaturation of porous media. I. Unconsolidated materials

Abstract

A model of the pore space in a random packing of rotund particles is suggested. The assumption that the pores are randomly connected enables the residual number of undrained pores to be calculated, independent of pore size distribution. There are two distinct mechanisms by which liquid can leave a pore; 1. (i) by evaporation 2. (ii) by fluid flow Desaturation by fluid-flow without evaporation is considered in two ways: a) flow through the whole pore space; and b) a modification where flow also occurs through incipient pendular rings. The shape of the capillary pressure curve, normalized to a window size distribution, is given for each of these cases. The relative permeability to both wetting and nonwetting phases is given for monosized windows. The relative permeability for an idealized window size distribution during desorption is derived. An approximation to Archie's Law is obtained. The analysis indicates deficiencies in both mercury porosimetry and desorption as methods for measuring “pore size distributions”. When applied to the random packing of equal spheres, for which an approximate window size distribution is already known, the theory gives a capillary pressure curve which is in reasonable agreement with experiment.