Experimental Determination of Surface Area of Residual Phase in Porous Media at Pore- and Continuum-Scale

Since last decade, rapid developments in non-destructive visualization techniques such as X-ray micro-CT provide a tool to directly monitor and study processes at pore scale which previously could only be explored indirectly. Fluid configurations, size distribution of residual clusters, and especially, interface between various phases, which control many transfer processes, can be visualized and quantified with high accuracy. On the other hand, such findings have not been adequately translated into required parameters for modeling, since most experimental attempts have remained case-specified. For example, modeling approaches for mass transfer in multiphase systems still assume single-size spherical bubbles for residual nonwetting phase, while it has been shown that residual clusters are far from being spherical, and have a broad size distribution which usually spans over several orders of magnitude.

In this study, we use micro-CT to investigate the relation between nonwetting phase surface area at residual state and structural properties at four different porous media. First, we performed REV analysis to test the validity of our results for both pore-scale (i.e. sub-REV) and continuum-scale (REV). At pore-scale, surface area of individual clusters shows a power-law relation to their volume, with exponents that vary for each porous medium. At continuum scale, however, the total surface area of nonwetting phase was strongly correlated to surface of pore space, regardless of pore size distribution. Our results improve the accuracy of most of available modeling approaches by avoiding simplifying assumptions without adding complexities to the process model. For example, actual cluster size distribution can replace single bubble size at pore scale, and surface areas can be estimated by power-law relationship, without incorporating complex shape of individual clusters. Similar approach can be used at REV-scale, where surface areas can be coupled with medium properties.