Effects of sample size on capillary pressures in porous media

Capillary pressure curves and their hystereses depend not only upon the geometry and wetting properties of the individual pores of a porous sample, but also upon the accessibility of these pores from the surface of the sample. Dependence of accessibility on distance from the surface of the sample implies that capillary pressure curves are sensitive to sample size. This sensitivity is investigated by measuring capillary pressure curves for porous samples of varying thickness. A theoretical investigation of sample thickness effects is made through application of the concepts of percolation theory of chaotic media. In the theory presented here, functions describing accessibility and pore size distribution appear in integral equations which account for all prominent features of drainage and imhibition, including threshold pressure, hystereses, residual saturations and dependence of residual saturation on initial saturation. The character and magnitude of sample thickness effects predicted by percolation theory are generally consistent with experimental results.