Comparison of Average and Point Capillary Pressure–Saturation Functions Determined by Steady‐State Centrifugation

Abstract

The capillary pressure–saturation function can be determined from centrifuge drainage experiments. In soil physics, the data resulting from such experiments are usually analyzed by the “averaging method.” In this approach, average relative saturation, 〈S〉, is expressed as a function of average capillary pressure, 〈ψ〉, i.e., 〈S〉(〈ψ〉). In contrast, the capillary pressure–saturation function at a physical point, i.e., S(ψ), has been extracted from similar experiments in petrophysics using the “integral method.” The purpose of this study was to introduce the integral method applied to centrifuge experiments to a soil physics audience and to compare S(ψ) and 〈S〉(〈ψ〉) functions, as parameterized by the Brooks–Corey and van Genuchten equations, for 18 samples drawn from a range of porous media (i.e., Berea sandstone, glass beads, and Hanford sediments). Steady‐state centrifuge experiments were performed on preconsolidated samples with a URC‐628 Ultra‐Rock Core centrifuge. The angular velocity and outflow data sets were then analyzed using both the averaging and integral methods. The results show that the averaging method smoothes out the drainage process, yielding less steep capillary pressure–saturation functions relative to the corresponding point‐based curves. Maximum deviations in saturation between the two methods ranged from 0.08 to 0.28 and generally occurred at low suctions. These discrepancies can lead to inaccurate predictions of other hydraulic properties such as the relative permeability function. Therefore, we strongly recommend use of the integral method instead of the averaging method when determining the capillary pressure–saturation function by steady‐state centrifugation. This method can be successfully implemented using either the van Genuchten or Brooks–Corey functions, although the latter provides a more physically precise description of air entry at a physical point.