Estimating interfacial areas for multi-fluid soil systems

Knowledge of the fluid-fluid and fluid-solid interfacial areas is important to better understand and quantify many flow and transport processes in porous media. This paper presents estimates for interfacial areas of porous media containing two or three fluids from measured capillary pressure (Pc)-saturation (S) relations. The thermodynamic treatment of two-fluid Pc−S relations presented by Morrow (1970) served as the basis for the predictions. In media containing two fluids (air-oil, air-water, oil-water), the solid-nonwetting interfacial area (AsN*) equaled zero when the solid was completely wetted by the wetting fluid. The area under the Pc−S curve was directly proportional to the nonwetting-wetting interfacial area (ANW*). If the solid surface was not completely wetted by one fluid, ANW* and AsN* were estimated by weighed partitioning of the area under the Pc−S curve. For porous media with fractional wettability, the procedure was applied separately to water- and oil-wet regions. The values of ANW* and AsN* were highest and lowest, respectively, in systems that were strongly wetted. In three-fluid media the wetting and spreading behavior of the liquids greatly affected the estimated interfacial areas. For a water-wet medium with a continuous intermediate oil phase, the interfacial areas were predicted from Pc−S data in a similar manner as for two-fluid media. The oil-water and oil-solid interfacial areas were estimated from the oil-water Pc−S curve, while the air-oil interfacial area was obtained from the air-oil Pc−S curve. For a fractional wettability or oil-wet medium there may be as many as six interfaces. These interfacial areas were estimated from three-fluid Pc−S relations based on previously developed methods for predicting three-fluid Pc−S relations from two-fluid data.