Pore-scale characterization of organic immiscible-liquid morphology in natural porous media using synchrotron X-ray microtomography

A steady-state kinetic interface-sensitive tracer (KIS-SST) method to measure capillary associated interfacial area in a simultaneous co-flow condition

We propose a novel method to measure the specific capillary-associated interfacial area (a_wn) between non-wetting and wetting fluids by applying kinetic interface-sensitive (KIS) tracers under steady-state two-phase co-flow conditions. Seven column experiments were conducted with a column filled with glass beads (diameter d₅₀ = 170 μm), serving as the solid grain network of a porous granular material. The experiments were performed for two flow scenarios, i.e., five experiments were performed for drainage conditions (increasing non-wetting saturation) and two experiments for imbibition conditions (increasing wetting saturation). To obtain different saturation levels in the column and, consequently, different capillarity-induced interfacial areas between the fluids, the experiments were performed for different fractional flow ratios (i.e., the ratio between injection rate of the wetting phase and total injection rate). The concentrations of the KIS tracer reaction by-product were recorded at each saturation level and the corresponding interfacial area was calculated. As a result of the fractional flow condition a wide range of wetting phase saturation is created (0.3 < S_w < 0.8). The measured a_wn increases with decreasing wetting phase saturation for the range 0.55 < S_w < 0.8, and then is followed by a drop in wetting phase saturation of 0.3 < S_w < 0.55. A good fit for our calculated a_wn is obtained (RMSE <0.16) using a polynomial model. Additionally, the results of the proposed method are compared to published experimental data and the method's main advantages and limitations are discussed.

Capillary equilibrium of bubbles in porous media

In geologic, biologic, and engineering porous media, bubbles (or droplets, ganglia) emerge in the aftermath of flow, phase change, or chemical reactions, where capillary equilibrium of bubbles significantly impacts the hydraulic, transport, and reactive processes. There has previously been great progress in general understanding of capillarity in porous media, but specific investigation into bubbles is lacking. Here, we propose a conceptual model of a bubble's capillary equilibrium associated with free energy inside a porous medium. We quantify the multistability and hysteretic behaviors of a bubble induced by multiple state variables and study the impacts of pore geometry and wettability. Surprisingly, our model provides a compact explanation of counterintuitive observations that bubble populations within porous media can be thermodynamically stable despite their large specific area by analyzing the relationship between free energy and bubble volume. This work provides a perspective for understanding dispersed fluids in porous media that is relevant to CO₂ sequestration, petroleum recovery, and fuel cells, among other applications.

Magnetic resonance imaging of enhanced mobility of light non aqueous phase liquid (LNAPL) during drying of water wet porous media

Visualization of NAPLs in multiphase systems in porous media is important for determining contaminant transport in the environment. In this study, magnetic resonance imaging (MRI) was used to confirm the recent observations of mobilisation of a light non aqueous phase liquid (LNAPL) trapped in wet sand under natural drying conditions of the wet porous medium. Visualization of LNAPL (motor oil) and water mobility during the drying of wet glass beads (0.5 mm) in a cylindrical glass column (15 mm ID, 45 mm long) was obtained using spin echo-based NMR microimaging performed at 500 MHz, corresponding to a field of ca. 11.75 T. Sagittal and axial images of LNAPL and water in the porous medium were obtained at a spatial resolution of 59 μm/pixel at different time intervals. A rise of 15-20 mm was observed in the presence of evaporation of water as compared to a 2-3 mm rise in the absence of evaporation in a time span of about 1400 min. The spatio-temporal MRI scans of the water and LNAPL in the glass column reveals that LNAPL rise occurs when the water evaporation front reaches the LNAPL layer. This implied that the enhanced LNAPL rise was strongly linked to the process of water evaporation. A linear correlation of the MRI signal intensities of LNAPL and water with reference to different saturation levels of LNAPL and water in the porous media was obtained. This calibration information was used to quantify the saturation levels of the LNAPL and water during the drying process. These findings show the application of non-invasive techniques such as MRI in quantifying and understanding the mechanism of fate and transport of LNAPLs in porous media, towards effective environmental quality assessment.

Pore-scale investigation of the use of reactive nanoparticles for in situ remediation of contaminated groundwater source

Chlorinated solvents are among the most recalcitrant aquifer contaminants, which can cause serious health problems such as kidney and liver damage, and some are considered carcinogenic. They are a global problem due to their wide industrial use since the beginning of the 20th century (e.g., in metal-processing plants). Conventionally, pump and treat technology (ex situ method) has been used to treat such contaminated groundwater resources. Recently, in situ techniques have been applied to lower the remediation costs (e.g., energy/water consumption) while also limiting the disruption. Nanoremediation is a new in situ technology that has shown promising results at laboratory, pilot, and field scales. This study uses 4D (time-resolved 3D) imaging to capture the dynamics of nanoremediation at the pore scale.

Nanoscale zero-valent iron (nZVI) particles have excellent capacity for in situ remediation of groundwater resources contaminated by a range of organic and inorganic contaminants. Chlorinated solvents are by far the most treated compounds. Studies at column, pilot, and field scales have reported successful decrease in contaminant concentration upon injection of nZVI suspensions in the contaminated zones. However, the field application is far from optimized, particularly for treatments at—or close to—the source, in the presence of residual nonaqueous liquid (NAPL). The knowledge gaps surrounding the processes that occur within the pores of the sediments hosting those contaminants at microscale limit our ability to design nanoremediation processes that are optimized at larger scales. This contribution provides a pore-scale picture of the nanoremediation process. Our results reveal how the distribution of the trapped contaminant evolves as a result of contaminant degradation and generation of gaseous products. We have used state-of-the-art four-dimensional (4D) imaging (time-resolved three-dimensional [3D]) experiments to understand the details of this degradation reaction at the micrometer scale. This contribution shows that the gas released (from the reduction reaction) remobilizes the trapped contaminant by overcoming the capillary forces. Our results show that the secondary sources of NAPL contaminations can be effectively treated by nZVI, not only by in situ degradation, but also through pore-scale remobilization (induced by the evolved gas phase). The produced gas reduces the water relative permeability to less than 1% and, therefore, significantly limits the extent of plume migration in the short term.

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 (A_max) 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 A_max 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 A_max 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.

Experimental comparison of agent-enhanced flushing for the recovery of crude oil from saturated porous media

The subsurface remediation of nonaqueous liquid (NAPL) has proven to be challenging even when implementing more aggressive enhanced-flushing techniques. The objective of this study was to evaluate the effectiveness of a combination of alkaline- and surfactant-based enhanced flushing for the removal of crude oil (medium fraction) from saturated porous media. Synchrotron X-ray microtomography (SXM) was used to perform pore-scale examination of NAPL fragmentation and changes in blob morphology, and recovery using three different advective flushing methods: surface-active agent (surfactant) flushing, alkaline flushing, and sequential alkaline-surfactant flushing. This set of experiments was conducted to understand effects on such processes (fragmentation and recovery) as a function of media composition (geochemical/mineralogical) and pH alterations due to calcium-carbonate fraction. Results showed that the sequential flushing technique (alkaline→ surfactant) yielded the highest recovery, 32% after 5 pore volumes (PV) of flushing. The crude oil (NAPL) distribution varied due to differences in porous medium mixture composition and type of fluid (i.e. surfactant vs. alkaline) used for flushing. The results of this study can be used to aid in the understanding of physical and chemical parameters/properties that control mobilization of crude oil in saturated porous media. This can help reduce time and cost during remediation of contaminated sites that contain crude oil or less dense NAPL derivatives consistent with fuel-type petroleum hydrocarbons.

A pore-scale investigation of heavy crude oil trapping and removal during surfactant-enhanced remediation

The presence of nonaqueous phase liquid (NAPL) in the subsurface presents significant challenges for soil and groundwater remediation. In particular, heavy crude oil, coal tar and/or bitumen present unique difficulties for removal and cleanup due to associated high viscosities, low aqueous solubilities, and limited mobility extraction potential. Although surfactant-enhanced aquifer remediation (SEAR) techniques have shown some promise for source removal, overall remediation (mobilization) performance will depend significantly on interfacial effects between the fluid and solid phases. A pore-scale study, implementing synchrotron X-ray microtomography (SXM), was conducted to understand and quantify the trapping and mobilization mechanisms and in-situ emulsification processes of heavy crude oil distributed within increasing complexity (i.e. physical heterogeneity) unconsolidated sands during surfactant flushing events. Pore-scale imaging analyses were conducted to quantify the changes in oil blob morphology before and after surfactant flushing events to assess the primary factors controlling the recovery. Results showed relatively low (10%) net recovery from the homogeneous sand after 5 pore volumes (PVs) of surfactant flushing and may be, in part, due to the more connected ganglia (i.e. single continuous) oil-phase. Such a condition may have limited the surfactant/oil contact resulting in relatively low interfacial activity and correspondingly inefficient oil mobilization and recovery. Negligible net oil recovery was achieved from the mildly-heterogeneous-sand and is likely due to the lower associated permeability of this particular porous medium. Furthermore, the oil-phase distribution within this medium primarily consisted of small disconnected blobs more readily exposed (in contact with) the surfactant solution. For the highly-heterogeneous-sand experiments, an average of 20% heavy-oil recovery resulted after each flushing event (total of ~37% after 5 PVs) and was attributed to more efficient reduction of interfacial tension associated with the increased surfactant-oil contact. The associated higher pH sand/fine‑carbonate system may have aided in maintaining a water-wet porous medium, a condition more conducive to higher oil recovery and displacement efficiency.

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.

Measurement of interfacial area from NMR time dependent diffusion and relaxation measurements

The interfacial area between two immiscible phases in porous media is an important parameter for describing and predicting 2 phase flow. Although present in several models, experimental investigations are sparse due to the lack of appropriate measurement techniques. We propose two NMR techniques for the measurement of oil-water interfacial area: (i) a time dependent NMR diffusion technique applicable in static conditions, similar to those used for the measurement of the solid specific surface of a porous media, and (ii) a fast relaxation technique applicable in dynamic conditions while flowing, based on an interfacial relaxation mechanism induced by the inclusion of paramagnetic salts in the water phase. For dodecane relaxing on doped water, we found an oil interfacial relaxivity of 1.8μm/s, large enough to permit the measurement of specific interfacial surface as small as 1000cm²/cm³. We demonstrate both NMR techniques in drainage followed by imbibition, in a model porous media with a narrow pore size distribution. While flowing, we observe that the interfacial area is larger in imbibition than in drainage, implying a different organization of the oil phase. In a carbonate sample with a wide pore size distribution, we evidence the gradual invasion of the smallest pores as the oil-water pressure difference is increased.

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 (A_ia). Coefficients of variation for A_ia were 3.5% for the glass beads and 11% for the sand. Estimated maximum interfacial areas (A_m) obtained with the GACR method were statistically identical to independent measures of the specific solid surface areas of the media. For example, the A_m 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, A_ias 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.

Comparison of Fluid-Fluid Interfacial Areas Measured with X-ray Microtomography and Interfacial Partitioning Tracer Tests for the same Samples

Two different methods are currently used for measuring interfacial areas between immiscible fluids within 3-D porous media, high-resolution microtomographic imaging and interfacial partitioning tracer tests (IPTT). Both methods were used in this study to measure non-wetting/wetting interfacial areas for a natural sand. The microtomographic imaging was conducted on the same packed columns that were used for the IPTTs. This is in contrast to prior studies comparing the two methods, for which in all cases different samples were used for the two methods. In addition, the columns were imaged before and after the IPTTs to evaluate the potential impacts of the tracer solution on fluid configuration and attendant interfacial area. The interfacial areas measured using IPTT are ~5 times larger than the microtomographic-measured values, which is consistent with previous work. Analysis of the image data revealed no significant impact of the tracer solution on NAPL configuration or interfacial area. Other potential sources of error were evaluated, and all were demonstrated to be insignificant. The disparity in measured interfacial areas between the two methods is attributed to the limitation of the microtomography method to characterize interfacial area associated with microscopic surface roughness due to resolution constraints.

The impact of transitions between two-fluid and three-fluidphases on fluid configuration and fluid-fluid interfacial areain porous media

Multiphase-fluid distribution and flow is inherent in numerous areas of hydrology. Yet, pore-scale characterization of transitions between two and three immiscible-fluids is limited. The objective of this study was to examine the impact of such transitions on the pore-scale configuration of organic liquid in a multi-fluid system comprising natural porous media. Three-dimensional images of an organic liquid (trichloroethene) in two-phase (organic-liquid/water) and three-phase (air/organic-liquid/water) systems were obtained using X-ray microtomography before and after drainage and imbibition. Upon transition from a two-phase to a three-phase system, a significant portion of the organic liquid (intermediate wetting fluid) was observed to exist as lenses and films in contact with air (nonwetting fluid). In these cases, the air was either encased by or contiguous to the organic liquid. The presence of air resulted in an increase in the surface-area-to-volume ratios for the organic-liquid blobs. Upon imbibition, the air was displaced downgradient, and concomitantly, the morphology of the organic-liquid blobs no longer in contact with air reverted to that characteristic of a two-phase distribution (i.e., more spherical blobs and ganglia). This change in morphology resulted in a reduction in the surface-area-to-volume ratio. These results illustrate the impact of transitions between two-phase and three-phase conditions on fluid configuration, and they demonstrate the malleable nature of fluid configuration under dynamic, multiphase-flow conditions. The results have implications for characterizing and modeling pore-scale flow and mass-transfer processes.

Experimental investigation of the influence of grain geometry on residual NAPL using synchrotron microtomography

The objective of this work was to investigate the impact of grain geometry (size and shape) of porous media on the morphology of residual NAPL. Synchrotron microtomography was used to obtain maps of residual NAPL in multiphase systems. High-resolution, three-dimensional images of natural sand systems, comprising a range of grain sizes and shapes were imaged and analyzed. Findings indicate that residual NAPL saturation is influenced by the shapes of grains of the porous medium more than their sizes. In systems composed of grains with similar sphericity and angularity, residual saturations are independent of median grain sizes at the same operating regime (capillary-controlled regime in this work). Residual saturations tend to increase as the system comprised more angular or non-spherical grains where relatively large NAPL blobs are entrapped in such systems. While volumes of individual blobs tend to decrease as grain size decreases, grain geometry has more profound effects on the morphology of the residual NAPL blobs. Within a system composed of grains with similar shape characteristics, total NAPL-water interfacial area increases as grain sizes decrease where a large number of small blobs are trapped. Total meniscus NAPL-water interfacial area exhibits a linear relation with total interfacial area where it tends to increase as grain sizes decrease. However, while meniscus interfacial areas of individual blobs are highly influenced by pore geometry; residual blobs trapped in pores with complex geometry tend to have higher meniscus interfacial areas due to their branched nature which increases contacts with the wetting phase.

A pore scale investigation of crude oil distribution and removal from homogeneous porous media during surfactant-induced remediation

A pore-scale study was conducted to understand interfacial processes contributing to the removal of crude oils from a homogeneous porous medium during surfactant-induced remediation. Synchrotron X-ray microtomography (SXM) was used to obtain high-resolution three-dimensional images of the two-fluid-phase oil/water system, and quantify temporal changes in oil blob distribution, blob morphology, and blob surface area before and after sequential surfactant flooding events. The reduction of interfacial tension in conjunction with the sufficient increase in viscous forces as a result of surfactant flushing was most likely responsible for mobilization and recovery of the two lighter oil fractions. However, corresponding increases in viscous forces as a result of a reduction of interfacial tension were insufficient to initiate and maintain the displacement (recovery) of the heavy crude oil fraction during surfactant flushing. In contrast to the heavy oil system, changes in trapping number for the lighter fraction crude oils were sufficient to initiate mobilization as a result of surfactant flushing. Both light and medium oil fractions showed an increase in the number of blobs and total blob surface area, and a reduction in the total volume after 2 pore volumes (PVs) of surfactant flooding. This increase in surface area was attributed to the change in blob morphology from spherical to more complex non-spherical ganglia shape characteristics. Moreover, the increase in the number of oil blobs from larger to smaller particles after surfactant flushing may have contributed to the greater cumulative oil surface area. Complete recovery of light and medium oil fractions resulted after 5 PVs of surfactant flooding, whereas the displacement efficiency of heavy-oil fraction was severely limited, even after extended periods of flushing. The results of these experiments demonstrate the utility of SXM for quantifying pore-scale interfacial characteristics for specific crude-oil-fraction/porous-medium systems, critical for understanding mobilization/removal relationships in which surfactant-enhanced remediation techniques will be most successful.

Water Distribution Variation in Partially Saturated Granular Materials Using Neutron Imaging

The use of neutron imaging is demonstrated for visualizing and quantifying water distribution in partially saturated granular porous media. Because of the unique difference in the total neutron cross sections of water, sand, and air, a significant contrast for the three phases is observed in a neutron transmission image, and a quantitative analysis provides detailed information on the arrangement and distribution of particles, voids, and water. The experiments in this study are performed at the Neutron Imaging Facility (NIF) at the National Institute of Standard and Technology (NIST). An amorphous silicon flat panel detector was used in this research with a spatial resolution of approximately 250 μm (127 μm/pixel). The effect of particle morphology on water distribution in compacted granular columns is investigated by using round and angular silica sand. Silica sand specimens with different bulk gravimetric water contents (0%, 6%, 9%, and 12%) are studied for evaluating the water phase-distribution spatially for compacted sand specimens in an aluminum cylinder.

Kinetic limitations on tracer partitioning in ganglia dominated source zones

Quantification of the relationship between dense nonaqueous phase liquid (DNAPL) source strength, source longevity and spatial distribution is increasingly recognized as important for effective remedial design. Partitioning tracers are one tool that may permit interrogation of DNAPL architecture. Tracer data are commonly analyzed under the assumption of linear, equilibrium partitioning, although the appropriateness of these assumptions has not been fully explored. Here we focus on elucidating the nonlinear and nonequilibrium partitioning behavior of three selected alcohol tracers - 1-pentanol, 1-hexanol and 2-octanol in a series of batch and column experiments. Liquid-liquid equilibria for systems comprising water, TCE and the selected alcohol illustrate the nonlinear distribution of alcohol between the aqueous and organic phases. Complete quantification of these equilibria facilitates delineation of the limits of applicability of the linear partitioning assumption, and assessment of potential inaccuracies associated with measurement of partition coefficients at a single concentration. Column experiments were conducted under conditions of non-equilibrium to evaluate the kinetics of the reversible absorption of the selected tracers in a sandy medium containing a uniform entrapped saturation of TCE-DNAPL. Experimental tracer breakthrough data were used, in conjunction with mathematical models and batch measurements, to evaluate alternative hypotheses for observed deviations from linear equilibrium partitioning behavior. Analyses suggest that, although all tracers accumulate at the TCE-DNAPL/aqueous interface, surface accumulation does not influence transport at concentrations typically employed for tracer tests. Moreover, results reveal that the kinetics of the reversible absorption process are well described using existing mass transfer correlations originally developed to model aqueous boundary layer resistance for pure-component NAPL dissolution.

COMPARISON OF INTERFACIAL PARTITIONING TRACER TEST AND HIGH-RESOLUTION MICROTOMOGRAPHY MEASUREMENTS OF FLUID-FLUID INTERFACIAL AREAS FOR AN IDEAL POROUS MEDIUM

Fluid-fluid interfacial area for porous-media systems can be measured with the aqueous-phase interfacial partitioning tracer test (IPTT) method or with high-resolution microtomography. The results of prior studies have shown that interfacial areas measured with the IPTT method are larger than values measured with microtomography. The observed disparity has been hypothesized to result from the impact of porous-medium surface roughness on film-associated interfacial area, wherein the influence of surface roughness is characterized to some extent by the IPTT method but not by microtomography due to resolution constraints. This hypothesis was tested by using the two methods to measure interfacial area between an organic immiscible liquid and water for an ideal glass-beads medium that has no measurable surface roughness. The tracer tests yielded a mean interfacial area of 2.8 (± 5 cm^-1), while microtomography produced an interfacial area of 2.7 (± 2 cm^-1). Maximum specific interfacial areas, equivalent to areas normalized by non-wetting fluid volume, were calculated and compared to measures of the specific solid surface area. The normalized interfacial areas were similar to the specific solid surface area calculated using the smooth-sphere assumption, and to the specific solid surface area measured using the N₂/BET method. The results presented herein indicate that both the IPTT and microtomography methods provide robust characterization of fluid-fluid interfacial area, and that they are comparable absent the impact of surface roughness.

A review of non-invasive imaging methods and applications in contaminant hydrogeology research

Contaminant hydrogeological processes occurring in porous media are typically not amenable to direct observation. As a result, indirect measurements (e.g., contaminant breakthrough at a fixed location) are often used to infer processes occurring at different scales, locations, or times. To overcome this limitation, non-invasive imaging methods are increasingly being used in contaminant hydrogeology research. Four of the most common methods, and the subjects of this review, are optical imaging using UV or visible light, dual-energy gamma radiation, X-ray microtomography, and magnetic resonance imaging (MRI). Non-invasive imaging techniques have provided valuable insights into a variety of complex systems and processes, including porous media characterization, multiphase fluid distribution, fluid flow, solute transport and mixing, colloidal transport and deposition, and reactions. In this paper we review the theory underlying these methods, applications of these methods to contaminant hydrogeology research, and methods' advantages and disadvantages. As expected, there is no perfect method or tool for non-invasive imaging. However, optical methods generally present the least expensive and easiest options for imaging fluid distribution, solute and fluid flow, colloid transport, and reactions in artificial two-dimensional (2D) porous media. Gamma radiation methods present the best opportunity for characterization of fluid distributions in 2D at the Darcy scale. X-ray methods present the highest resolution and flexibility for three-dimensional (3D) natural porous media characterization, and 3D characterization of fluid distributions in natural porous media. And MRI presents the best option for 3D characterization of fluid distribution, fluid flow, colloid transport, and reaction in artificial porous media. Obvious deficiencies ripe for method development are the ability to image transient processes such as fluid flow and colloid transport in natural porous media in three dimensions, the ability to image many reactions of environmental interest in artificial and natural porous media, and the ability to image selected processes over a range of scales in artificial and natural porous media.

Characterizing pore-scale dissolution of organic immiscible liquid in a poorly-sorted natural porous medium

Synchrotron X-ray microtomography was used to characterize the pore-scale morphology and distribution of an organic immiscible liquid (trichloroethene) during water flushing to examine dissolution dynamics. The experiments were conducted with a natural porous medium that has a large particle-size distribution. The results were compared to those of a previous experiment conducted with a well-sorted natural sand. The median organic-liquid blob volume was smaller, and smaller blobs composed a larger fraction of the distribution, for the poorly sorted medium. In addition, mass removal was less spatially uniform for the poorly sorted medium. The concentration of trichloroethene in the column effluent was monitored during dissolution to assess mass-flux behavior. A first-order mass transfer equation was used to simulate the measured elution curves. Organic-liquid/water interfacial areas measured with microtomography were used as input, and simulated effluent concentrations were compared to the measured effluent concentrations to determine best-fit values for the mass-transfer coefficient. The value obtained for the poorly sorted medium was approximately 10 times smaller than that obtained for the well-sorted medium. This disparity indicates that hydraulic accessibility of the organic liquid is more constrained for the poorly sorted medium, which would be consistent with a more complex pore-scale flow field for the poorly sorted medium.

Impact of wettability on pore-scale characteristics of residual nonaqueous phase liquids

The objective of this paper was to investigate the impact of wettability of porous media on pore-scale characteristics of residual nonaqueous phase liquids (NAPLs). Synchrotron X-ray microtomography was used to obtain high-resolution three-dimensional images of fractionally wet sand systems with mean grain size of 250 microm. Pore-scale characteristics of NAPL blobs such as volume, lengths, interfacial areas, and sphericity index were computed using three-dimensional image processing algorithms. Four systems comprised of 100, 50, 25, and 0% NAPL-wet mass fractions containing the residual NAPL were imaged and analyzed. Findings indicate that spatial variation in wettability of porous media surfaces has a significant impact on pore-scale characteristics of residual NAPL blobs in saturated porous media systems. As the porous media comprises more water-wet surfaces, residual NAPL blobs increase in size and length due to the entrapment at large pore bodies. NAPL-water interfacial areas tend to increase as the NAPL-wet surface fractions increase in the systems. Overall residual NAPL saturations are less in fractionally wet systems and increase as the systems become more NAPL-wet or water-wet.

Measurement and estimation of organic-liquid/water interfacial areas for several natural porous media

The objective of this study was to quantitatively characterize the impact of porous-medium texture on interfacial area between immiscible organic liquid and water residing within natural porous media. Synchrotron X-ray microtomography was used to obtain high-resolution, three-dimensional images of solid and liquid phases in packed columns. The image data were processed to generate quantitative measurements of organic-liquid/water interfacial area and of organic-liquid blob sizes. Ten porous media, comprising a range of median grain sizes, grain-size distributions, and geochemical properties, were used to evaluate the impact of porous-medium texture on interfacial area. The results show that fluid-normalized specific interfacial area (A) and maximum specific interfacial area (Am) correlate very well to inverse median grain diameter. These functionalities were shown to result from a linear relationship between effective organic-liquid blob diameter and median grain diameter. These results provide the basis for a simple method for estimating specific organic-liquid/water interfacial area as a function of fluid saturation for a given porous medium. The availability of a method for which the only parameter needed is the simple-to-measure median grain diameter should be of great utility for a variety of applications.

SYNCHROTRON X-RAY MICROTOMOGRAPHY AND INTERFACIAL PARTITIONING TRACER TEST MEASUREMENTS OF NAPL-WATER INTERFACIAL AREAS

Interfacial areas between an immiscible organic liquid (NAPL) and water were measured for two natural porous media using two methods, aqueous-phase interfacial partitioning tracer tests and synchrotron X-ray microtomography. The interfacial areas measured with the tracer tests were similar to previously reported values obtained with the method. The values were, however, significantly larger than those obtained from microtomography. Analysis of microtomography data collected before and after introduction of the interfacial tracer solution indicated that the surfactant tracer had minimal impact on fluid-phase configuration and interfacial areas under conditions associated with typical laboratory application. The disparity between the tracer-test and microtomography values is attributed primarily to the inability of the microtomography method to resolve interfacial area associated with microscopic surface heterogeneity. This hypothesis is consistent with results recently reported for a comparison of microtomographic analysis and interfacial tracer tests conducted for an air-water system. The tracer-test method provides a measure of effective, total (capillary and film) interfacial area, whereas microtomography can be used to determine separately both capillary-associated and film-associated interfacial areas. Both methods appear to provide useful information for given applications. A key to their effective use is recognizing the specific nature of the information provided by each, as well as associated limitations.

Three-dimensional visualization and quantification of non-aqueous phase liquid volumes in natural porous media using a medical X-ray Computed Tomography scanner

This study demonstrates the capabilities of a typical medical X-ray Computed Tomography (CT) scanner to non-destructively quantify non-aqueous phase liquid (NAPL) volumes, saturation levels, and three-dimensional spatial distributions in packed soil columns. Columns packed with homogeneous sand, heterogeneous sand, or natural soil, were saturated with water and injected with known quantities of gasoline or tetrachloroethene and scanned. A methodology based on image subtraction was implemented for computing soil porosity and NAPL volumes in each 0.35 mm x 0.35 mm x 1 mm voxel of the columns. Elimination of sample positioning errors and instrument drift artifacts was essential for obtaining reliable estimates of above parameters. The CT data-derived total NAPL volume was in agreement with the measured NAPL volumes injected into the columns. CT data-derived NAPL volume is subject to a 2.6% error for PCE and a 15.5% error for gasoline, at average NAPL saturations as low as 5%, and is mainly due to instrument noise. Non-uniform distributions of NAPL due to preferential flow, and accumulation of NAPL above finer-grained layers could be observed from the data on 3-D distributions of NAPL volume fractions.

Characterization of NAPL source zone architecture and dissolution kinetics in heterogeneous porous media using magnetic resonance imaging

A direct visualization method using magnetic resonance imaging (MRI) was developed to characterize sand grain size distribution, nonaqueous phase liquid (NAPL) source zone architecture, and aqueous flowpaths in a three-dimensional (3-D) flowcell (26.5 cm x 10.5 cm x 10.5 cm) packed with a heterogeneous distribution of five different sand fractions. All images were acquired at a resolution of 0.1875 cm x 0.1875 cm x 0.225 cm. A 1H image of pore water resolved the heterogeneous permeability field; grain size differences as small as 0.1 mm could be distinguished. A time series of 1H images of water doped with the paramagnetic tracer MnCl2 were acquired and used to obtain voxel-scale breakthrough curves. Water preferentially flowed through coarse sands before NAPL release. After NAPL release, the flow bypassed NAPLzones, and bypassing was more evident for high NAPL saturation zones. A time series of 19F images of NAPL were acquired and used to determine voxel-scale NAPL saturation (Sn) during dissolution. Results show that 93% of NAPL mass was in the coarsest sand, most NAPL was trapped as pools and not as residual ganglia, NAPL saturation increased with depth, and the NAPL dissolution front moved vertically from the top to the bottom of the flowcell during the first 170 pore volumes of waterflushed. NAPL component effluent concentrations initially increased due to the development of flow in zones with decreasing NAPL saturation. Flowpath images suggest that this occurs as NAPL transitions from pools (Sn > 0.15) to residual ganglia. The results highlight the importance of flow bypassing and provide the opportunity to develop more accurate NAPL dissolution models.

Measuring air-water interfacial areas with X-ray microtomography and interfacial partitioning tracer tests

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.

Characterizing pore-scale dissolution of organic immiscible liquid in natural porous media using synchrotron X-ray microtomography

The objective of this study was to characterize the pore-scale dissolution of organic immiscible-liquid blobs residing within natural porous media. Synchrotron X-ray microtomography was used to obtain high-resolution, three-dimensional images of the aqueous, organic-liquid, and solid phases residing in columns packed with one of two porous media. Images of the packed columns were obtained after a stable, discontinuous distribution (e.g., residual saturation) of the organic liquid (trichloroethene) had been established, and three subsequent times during column flushing. These data were used to characterize the morphology of the organic-liquid blobs as a function of dissolution, and to quantify changes in total organic-liquid volume, surface area, and water-organic liquid interfacial area. The dissolution dynamics of individual blobs appeared to be influenced by the local pore configuration. In addition to dissolution-induced shrinkage, some blobs were observed to separate into multiple distinct subunits. The median blob size decreased by approximately a factor of 2 at the point where approximately 90% of the initial organic-liquid volume had been removed. The ratio of capillary associated interfacial area to total water-organic liquid interfacial area increased by 50% at the point where approximately 95% of the initial mass had been removed. A nearly linear relationship was observed between both total and capillary associated interfacial area and organic liquid volumetric fraction. Changes in the measured aqueous-phase trichloroethene effluent concentrations were well correlated with changes in the volume, surface area, and number of blobs. The effluent concentration data were adequately described by a first-order mass transfer expression employing a constant value of the mass-transfer coefficient, with values for the water-organic liquid interfacial area obtained independently from the microtomography data.