Assessing XMT-Measurement Variability of Air-Water Interfacial Areas in Natural Porous Media.

This study investigates the accuracy and reproducibility of air-water interfacial areas measured with high-resolution synchrotron x-ray microtomography (XMT). Columns packed with one of two relatively coarse-grained monodisperse granular media, glass beads or a well-sorted quartz sand, were imaged over several years, encompassing changes in acquisition equipment, improved image quality, and enhancements to image acquisition and processing software. For the glass beads, the specific solid surface area (SSSA-XMT) of 31.6 ±1 cm-1 determined from direct analysis of the segmented solid-phase image data is statistically identical to the independently calculated geometric specific solid surface area (GSSA, 32 ±1 cm-1) and to the measured SSSA (28 ±3 cm-1) obtained with the N2BET method (NBET). The maximum specific air-water interfacial area (Amax) is 27.4 (±2) cm-1, which compares very well to the SSSA-XMT, GSSA, and SSSA-NBET values. For the sand, the SSSA-XMT (111 ±2 cm-1) and GSSA (113 ±1 cm-1) are similar. The mean Amax is 96 ±5 cm-1, which compares well to both the SSSA and the GSSA values. The XMT-SSSA values deviated from the GSSA values by 7-16% for the first four experiments, but were essentially identical for the later experiments. This indicates that enhancements in image acquisition and processing improved data accuracy. The Amax values ranged from 74 cm-1 to 101 cm-1, with a coefficient of variation (COV) of 9%. The maximum capillary interfacial area ranged from 12 cm-1 to 19 cm-1, for a COV of 10%. The COVs for both decreased to 5-6% for the latter five experiments. These results demonstrate that XMT imaging provides accurate and reproducible measurements of total and capillary interfacial areas.

Novel fluid-fluid interface domains in geologic media.

Pore-scale fluid processes in geological media are critical for numerous applications in several fields. Continued improvement of high-resolution image acquisition and processing methods has provided a means to directly characterize pore-scale fluid processes for natural geomedia, and to test the representativeness of theoretical and computational models developed to simulate them. High-resolution synchrotron X-ray microtomography (XMT) combined with advanced 3-D image visualization was used to investigate the impact of larger-scale solid-surface heterogeneity on nonwetting-wetting (air-water) interfacial area for two natural geomedia (a sand and a soil). The studies revealed the presence of air-water interfaces associated with water residing within macroscopic features such as pits and crevices on the surfaces of the solids. The diameters of the features ranged from tens to 100's of µm for the sand, and the aggregate associated air-water interfacial area was estimated to represent ∼12% of the maximum capillary interfacial area. These features and respective fluid interfaces, which are not considered in standard conceptualizations of fluid distribution in geomedia, may have an impact on pore-scale physical and biogeochemical processes.

The Gas-absorption/Chemical-reaction Method for Measuring Air-water Interfacial Area in Natural Porous Media.

The gas-absorption/chemical-reaction (GACR) method used in Chemical Engineering to quantify gas-liquid interfacial area in reactor systems is adapted for the first time to measure the effective air-water interfacial area of natural porous media. Experiments were conducted with the GACR method, and two standard methods (x-ray microtomographic imaging and interfacial partitioning tracer tests) for comparison, using model glass beads and a natural sand. The results of a series of experiments conducted under identical conditions demonstrated that the GACR method exhibited excellent repeatability for maintaining constant water saturation and for measurement of interfacial area (Aia). Coefficients of variation for Aia were 3.5% for the glass beads and 11% for the sand. Estimated maximum interfacial areas (Am) obtained with the GACR method were statistically identical to independent measures of the specific solid surface areas of the media. For example, the Am for the glass beads is 29 (±1) cm-1, compared to 32 (±3), 30 (±2), and 31 (±2) cm-1 determined from geometric calculation, N2/BET measurement, and microtomographic measurement, respectively. This indicates that the method produced accurate measures of interfacial area. Interfacial areas determined with the GACR method were similar to those obtained with the standard methods. For example, Aias of 47 and 44 cm-1 were measured with the GACR and XMT methods, respectively, for the sand at a water saturation of 0.57. The results of the study indicate that the GACR method is a viable alternative for measuring air-water interfacial areas. The method is relatively quick, inexpensive, and requires no specialized instrumentation compared to the standard methods.

Measuring air-water interfacial area for soils using the mass balance surfactant-tracer method.

There are several methods for conducting interfacial partitioning tracer tests to measure air-water interfacial area in porous media. One such approach is the mass balance surfactant tracer method. An advantage of the mass-balance method compared to other tracer-based methods is that a single test can produce multiple interfacial area measurements over a wide range of water saturations. The mass-balance method has been used to date only for glass beads or treated quartz sand. The purpose of this research is to investigate the effectiveness and implementability of the mass-balance method for application to more complex porous media. The results indicate that interfacial areas measured with the mass-balance method are consistent with values obtained with the miscible-displacement method. This includes results for a soil, for which solid-phase adsorption was a significant component of total tracer retention.

Novel methods for measuring air-water interfacial area in unsaturated porous media.

Interfacial partitioning tracer tests (IPTT) are used to measure air-water interfacial area for unsaturated porous media. The standard IPTT method involves conducting tests wherein an aqueous surfactant solution is introduced into a packed column under unsaturated flow conditions. Surfactant-induced drainage has been observed to occur for this method in some cases, which can complicate data analysis and impart uncertainty to the measured values. Two novel alternative approaches for conducting IPTTs are presented herein that are designed in part to prevent surfactant-induced drainage. The two methods are termed the dual-surfactant IPTT (IPTT-DS) and the residual-air IPTT (IPTT-RA). The two methods were used to measure air-water interfacial areas for two natural porous media. System monitoring during the tests revealed no measurable surfactant-induced drainage. The measured interfacial areas compared well to those obtained with the standard IPTT method conducted in such a manner that surfactant-induced drainage was prevented.