Measuring Air−Water Interfacial Areas with X-ray Microtomography and Interfacial Partitioning Tracer Tests

Abstract

Air-water interfacial areas as a function of water saturation were measured for a sandy, natural porous medium using two methods, aqueous-phase interfacial partitioning tracer tests and synchrotron X-ray microtomography. In addition, interfacial areas measured in a prior study with the gas-phase interfacial partitioning tracer-test method for the same porous medium were included for comparison. For all three methods, total air-water interfacial areas increased with decreasing water saturation. The interfacial areas measured with the tracer-test methods were generally larger than those obtained from microtomography, and the disparity increased as water saturation decreased. The interfacial areas measured by microtomography extrapolated to a value (147 cm(-1)) very similar to the specific solid surface area (151 cm(-1)) calculated using the smooth-sphere assumption, indicating that the method does not characterize the area associated with microscopic surface heterogeneity (surface roughness, microporosity). This is consistent with the method resolution of approximately 12 microm. In contrast, the interfacial areas measured with the gas-phase tracer tests approached the N2/BET measured specific solid surface area (56000 cm(-1)), indicating that this method does characterize the interfacial area associated with microscopic surface heterogeneity. The largest interfacial area measured with the aqueous-phase tracer tests was 224 cm(-1), while the extrapolated maximum interfacial area was approximately 1100 cm(-1). Both of these values are larger than the smooth-sphere specific solid surface area but much smaller than the N2/BET specific solid surface area, which suggests that the method measures a limited portion of the interfacial area associated with microscopic surface heterogeneity. All three methods provide measures of total (capillary + film) interfacial area, a primary difference being that the film-associated area is a smooth-surface equivalent for the microtomography method. An advantage of the microtomography method is the ability to determine explicitly both total and capillary-associated interfacial areas, which is problematic for the tracer-test methods.