Air-Water Interfacial Area and Capillary Pressure: Porous-Medium Texture Effects and an Empirical Function

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Abstract
The relationship between air-water interfacial area and capillary pressure under higher water-content conditions is investigated for four natural porous media. The results show that the magnitude of the air-water interfacial area increases with increasing capillary pressure, consistent with the decrease in water saturation. The maximum observed air-water interfacial areas are dependent upon the magnitude of residual water saturation, which itself is condition-dependent. The more well-sorted porous-medium exhibited a greater rate of change of air-water interfacial area with capillary pressure than the more poorly sorted porous media. The observed relationship between air-water interfacial area and capillary pressure was quantified by coupling an empirical equation describing the air-water interfacial area versus water saturation relationship with the van Genuchten equation relating water saturation and capillary pressure. This equation produced reasonable simulations of the measured data.