Published online 26 May 2006
Published in Vadose Zone J 5:641-648 (2006)
DOI: 10.2136/vzj2005.0063
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
ORIGINAL RESEARCH
Characterizing Pore-Scale Configuration of Organic Immiscible Liquid in Multiphase Systems With Synchrotron X-Ray Microtomography
G. Schnaara and
M. L. Brusseaua,b,*
a Soil, Water and Environmental Science Dep., Univ. of Arizona, 429 Shantz Bldg., Tucson, AZ 85721
b Hydrology and Water Resources Dep., Univ. of Arizona, 429 Shantz Bldg., Tucson, AZ 85721
* Corresponding author (brusseau{at}ag.arizona.edu)
Received 16 May 2005.
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ABSTRACT
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The objective of this study was to examine the pore-scale distribution and morphology of organic immiscible liquid in natural porous media containing three immiscible fluids. High-resolution, three-dimensional images of an organic liquid (tetrachloroethene) in both three-phase (water–air–organic liquid) and two-phase (water–organic liquid) systems were obtained using synchrotron X-ray microtomography. These data were used to quantitatively characterize the morphology of the organic liquid residing within columns packed with one of three natural, sandy porous media. Organic-liquid blobs varied greatly in both size and shape, ranging from small, single spheres (
0.03 mm in diameter) to large, amorphous ganglia with mean lengths of 4 to 5 mm. Singlets comprised the greatest number of blobs, whereas the large ganglia, while much fewer in number, comprised the majority of the organic-liquid surface area and volume. A significant portion of the organic liquid in the three-phase systems was observed to exist as lenses and films in contact with air. These features were not observed in the two-phase water–organic liquid systems. The median of the blob-frequency distributions was smaller and the variance larger for the three-phase systems. In addition, the global specific surface areas of the organic liquid were greater for the three-phase systems. These differences are attributed to the presence of the organic-liquid lenses and films for the three-phase systems.
Abbreviations: GSECARS, GeoSoilEnviroCARS
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INTRODUCTION
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CHARACTERIZING and predicting multiphase flow phenomena in subsurface systems are critical to fields such as contaminant hydrology, petroleum engineering, and soil physics, and a large body of literature has been developed on this topic. One area that has received increased attention lately is that of pore-scale multiphase processes. Recent advances in experimental and computational methods have allowed investigation of fundamental mechanisms controlling the displacement and distribution of immiscible fluids in porous media.
Various imaging methods have been used for many years to examine fluid displacement and distribution in porous media. However, until recently, the visualization and quantification of fluid distributions at the pore scale in three-dimensional porous-medium samples was generally not feasible (e.g., Berkowitz and Hansen, 2001). The development of methods allowing high resolution, pore-scale imaging of immiscible fluids in three-dimensional systems of porous media has been central to recent advances in multiphase flow by providing a means to directly and quantitatively test theoretical and mathematical models. These methods include photoimaging of refractive-index matched systems, magnetic resonance imaging or nuclear magnetic resonance methods, and X-ray microtomography. They have been used, for example, to investigate the distribution and configuration of immiscible fluids in porous media for two-phase systems (Montemagno and Gray, 1995; Johns and Gladden, 1998, 1999, 2000, 2001; Pervizpour et al., 1999; Okamoto et al., 2001; Sederman and Gladden, 2001; Fontenot and Vigil, 2002; Zhang et al., 2002; Wildenschild et al., 2002; Becker et al., 2003; Stohr et al., 2003; Culligan et al., 2004; Al-Raoush and Willson, 2005; Schnaar and Brusseau, 2005; Brusseau et al., 2006). To date, most of these studies have focused on model porous media such as well-sorted silica particles. In addition, three-phase systems comprising water, air, and organic liquid have not been examined.
The objective of this study was to examine the pore-scale distribution and morphology of organic immiscible liquid in natural porous media containing three immiscible fluid phases. High-resolution, three-dimensional images of organic liquid in both three-phase (water–air–organic liquid) and two-phase (water–organic liquid) systems were obtained using synchrotron X-ray microtomography. These data were used to quantitatively characterize the configuration of the organic liquid.
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MATERIALS AND METHODS
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Materials
Tetrachloroethene was used as the model organic liquid for this study. To enhance image contrast, tetrachloroethene was doped with iodobenzene (8% by volume). In addition, the aqueous phase was doped with cesium chloride (60 g L–1), resulting in an ionic strength of 0.36M. Barranco et al. (1997) investigated the influence of ionic strength on wettability and interfacial tension in quartz–water–organic liquid systems. They observed that water–organic liquid interfacial tension increased minimally with increasing ionic strength (<10% for ionic strength values corresponding to those used herein), while the contact angle decreased (i.e., the solid surface became more water wetting). Based on these results, the presence of cesium chloride is not expected to significantly influence the phase-distribution behavior for our system. The densities of the organic liquid and aqueous solution are 1.64 and 1.18 g mL–1, respectively. The selected dopants were chosen to exhibit minimal partitioning to non-target fluids. All chemicals are reagent grade (Sigma-Aldrich Co., St. Louis MO).
Two natural, commercially available silica sands (Accusand, Unimin Corp., New Canaan, CT) and one sandy surface soil (Vinton; sandy, mixed, thermic Typic Torrifluvents) collected locally in Tucson, AZ were used as representative natural porous media. Relevant physical properties of the porous media are given in Table 1. All three media have low (<0.03%) organic C contents. The porous media are considered to be water-wetting.
Establishment and Determination of Organic Liquid Saturation
The porous media were dry-packed into thin-walled, X-ray-transparent columns constructed of aluminum, with stainless-steel end fittings. The columns were 4.4 cm in height, with an outer diameter of 0.635 cm and an inner diameter of either 0.46 or 0.58 cm. Porous frits were placed on both ends of the column to promote uniform flow and retain the porous media. A polypropylene frit (10-µm pores) was placed on the end of the column used for organic-liquid introduction and drainage, and a stainless-steel frit (2-µm pores) was placed on the end of the column used for aqueous-solution imbibition and drainage. The bulk densities and porosities of the packed columns are reported in Tables 1 and 2, respectively; they are similar to those obtained for larger columns packed with the same media.
After packing, the columns were purged with CO2, and de-aired water was pumped upward through the column with a single-piston HPLC pump (Acuflow Series II, Fisher Scientific, Hampton, NH). The equivalent of 2.5 pore-volumes (
1 cm3) of organic liquid was then pumped vertically upward into the column at a Darcy velocity of 4.5 cm h–1 using a syringe pump. An aqueous solution containing tetrachloroethene and iodobenzene at solubility concentrations based on the composition of the organic liquid (assuming Raoult's Law applicability), as well as the cesium chloride salt, was then flushed vertically downward at 20 cm h–1 to displace the organic liquid. The capillary number for this displacement was calculated to be 1 x 10–6, which is similar to values typically used in prior studies to establish a stable, discontinuous distribution of nonwetting phase (e.g., Morrow and Chatzis, 1982; Wardlaw and McKellar, 1985; Johns and Gladden, 1998). Complete water saturation was not attained for some columns, and thus significant fractions of air remained. These columns represent the three-phase systems. The image data were examined to evaluate the comparative distributions of the three fluids, as discussed below. Upon completion of column preparation, the columns were sealed and imaged as described in the following section.
After the imaging procedure was completed, the majority of the columns were extracted with HPLC-grade ethanol (Sigma Aldrich Co.) to independently determine the amount of organic liquid present. Tetrachloroethene and iodobenzene concentrations in ethanol were determined using direct-inject gas chromatography equipped with a flame-ionization detector (GC-14A, Shimadzu, Kyoto, Japan). The quantifiable detection limits of tetrachloroethene and iodobenzene were 6 mg L–1. Saturation values (ratio of organic-liquid volume/pore volume) determined by this method ranged from 14 to 19%. These saturations are in the same range as values obtained for larger columns (5–15 cm long, 2.1-cm inner diameter) packed with the same media (unpublished data).
Synchrotron X-Ray Microtomography
Imaging was conducted at the GeoSoilEnviroCARS (GSECARS) BM-13D beamline at the Advanced Photon Source, Argonne National Laboratory, IL. Methods for collecting three-dimensional images of geologic and environmental samples using synchrotron X-ray microtomography, specific to the instrumentation at GSECARS, have been reviewed elsewhere (e.g., Sutton et al., 2002; Wildenschild et al., 2002). Briefly, the GSECARS bending magnet source provided a monochromatic X-ray beam that was directed to pass through the column, perpendicular to the longitudinal axis. The length of the imaged zone was 5.6 mm. The transmitted X-rays were converted to visible light with a single-crystal synthetic YAG (ytrium-aluminum-garnet) scintillator, and projected onto a mirror inclined 45° to the incoming beam. A snapshot of the image on the mirror was then taken with a high resolution CCD-camera attached to a microscope objective (5x). This image represents a depth-integrated grayscale map of the linear attenuation of the beam passing through the column. If the beam was highly attenuated (absorbed) in a particular location, the grayscale value would be lower (darker). After an image was collected, the column was rotated 0.5°, and the image acquisition process was repeated. A total of 720 two-dimensional images of the sample were collected. Each image was 7.1 by 5.6 mm, with a resultant resolution (pixel size) of 10.9 µm.
The synchrotron beam was tuned to specific incident energies to take advantage of the X-ray absorption K-edge of the doping compounds. Images of the columns were collected sequentially below and above the I K-edge (33.0169 and 33.269 keV) to specifically resolve the I-containing organic liquid, and below and above the Cs K-edge (33.269 and 36.085 keV) to resolve the aqueous phase. Thus, images were collected at three energies, resulting in a total of 2160 scans for a given sample, not including synchrotron beam backfield projections. Eight imaged scanned intervals from six columns were used in the present study: four 45/50-Accusand, two 100/140-Accusand, and two Vinton soil. Of these, three are replicates for the two-phase 45/50-Accusand system used to evaluate reproducibility. The others represent single samples for three-phase 45/50-Accusand and both two-phase and three-phase systems for 100/140-Accusand and Vinton soil. The imaged sections were obtained from the center of the columns.
Image Processing and Analysis
For each set of scans, the 720 two-dimensional images were preprocessed and reconstructed with algorithms developed at GSECARS (Rivers, 2003) to construct a single three-dimensional image array. The reconstruction process reverses the relationship between grayscale and X-ray absorption such that locations of high attenuation appear brighter. After reconstruction, the data can be visualized as "thin sections," two-dimensional slices in either the x–y (perpendicular to the longitudinal [rotational] axis of the column) or x–z (parallel to the rotational axis of the column) direction. The reconstructed thin sections show the attenuation (grayscale) of the X-ray beam in a discrete location (voxel), and thus the internal distribution of the attenuation is obtained. Reconstructed three-dimensional images acquired with incident energy below the I or Cs K-edge were subtracted from the corresponding image above the K-edge to produce images wherein only voxels comprising the fluid of interest displayed a different grayscale value than the background. Use of a subtracted image array simplifies data processing, ensures that voxels comprising the fluid of interest are successfully separated from the surrounding matrix, and eliminates artifacts associated with highly X-ray attenuating components of the porous media, such as metal oxides.
Additional image data processing and extraction of quantitative information were conducted with the software package Blob3D, which was specifically developed for high resolution X-ray microtomography data (Ketcham, 2005). Data were input into Blob3D as 465 continuous x–y thin sections, with a grayscale range of 0 to 255. For the two-phase systems, measurement of the organic liquid and water distributions were conducted with the respective subtracted arrays, as described above. For the three-phase systems, air was resolved using thin-section arrays collected above the Cs K-edge. These images provided sufficient contrast between air and the other fluids.
Median smoothing was used to reduce image noise (Ketcham, 2005). The voxels comprising the fluid of interest were defined to fall within a certain grayscale range. The boundary of this range was set as the midpoint between the average of target-fluid voxels and non-target-fluid voxels in the array. An array of binary images was then created wherein voxels considered to be the fluid of interest were assigned a grayscale of 255 (white) and all others were assigned a grayscale of 0 (black). Contiguous voxels assigned as the fluid of interest were identified and combined to form three-dimensional units (i.e., "blobs"). Quantitative data were then extracted for each individual blob. Volume was calculated as the total volume of all the voxels contained within a blob. Surface area was calculated from the isosurface connecting the grayscale value of 127 in the binarized image. The diameter of each blob was estimated based on a single-sphere assumption. For nonspherical blobs, the equivalent-sphere diameter was compared with the maximum and minimum length scales observed from three-dimensional blob images and was found to be intermediate to them. Thus, the equivalent-sphere diameter appears to be a representative length scale for comparison purposes. A few isolated pockets of air were observed in some of the two-phase columns; in such cases the immiscible-liquid blobs in contact with air were not included in the blob-distribution analysis. The effective resolution with respect to blob size was approximately 10–5 mm3.
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RESULTS AND DISCUSSION
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Fluid Volumes and Distributions
The volumetric fluid contents obtained from processing the image data are reported in Table 2. Organic-liquid content values calculated for the scanned intervals, based on total organic-liquid blob volume, are with one exception in good agreement with values obtained by ethanol extraction of the columns (Table 2). The total porosities calculated from the combined volumes of all fluids imaged in each scanned interval are in most cases relatively similar to the column-scale porosities determined gravimetrically (Table 2). Sources of deviation between the two sets of porosity values include measurement uncertainty associated with both sets of values, and potential spatial variability of porosity.
Inspection of the reconstructed thin sections shows that the fluids, organic liquid, water, air, and the porous-medium grains are well distinguished, as illustrated in Fig. 1
and 2
for a two-phase and three-phase system, respectively. Also shown are false-color overlays developed by combining individual-fluid image sets determined separately for each of the two or three fluids. Organic liquid was observed to be distributed throughout the scanned intervals, both longitudinally and radially, with no apparent preferential accumulation. Organic-liquid blobs in the center and outer sections of the columns appeared to have similar morphologies. Organic-liquid blobs varied greatly in both size and shape, ranging from small spheres (0.03 mm in diameter) to large, amorphous ganglia with mean lengths of 4 to 5 mm. This range of sizes and shapes is consistent with the results of prior studies employing styrene blob casting (e.g., Morrow and Chatzis, 1982; Chatzis et al., 1983; Wardlaw and McKellar, 1985; Conrad et al., 1992; Mayer and Miller, 1992; Powers et al., 1992) and three-dimensional imaging (Okamoto et al., 2001; Fontenot and Vigil, 2002; Zhang et al., 2002; Al-Raoush and Willson, 2005; Schnaar and Brusseau, 2005).
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Fig. 1. Thin sections in the x–y direction (planar view) of a 45/50-Accusand column for a two-phase system (water–organic liquid). (A) Reconstructed raw image collected above the I K-edge. Organic liquid is white, the aqueous phase is dark gray, and porous-medium grains are light gray. (B) False-color images, created by overlapping the binary images obtained separately for each fluid. Organic liquid is red, the aqueous phase is blue, and porous-medium grains (and unresolved fluid) are light gray.
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Fig. 2. Thin sections in the x–y direction (planar view) of a 45/50-Accusand column for a three-phase system (water-air-organic liquid). (a) Reconstructed raw image collected above the I K-edge. Organic liquid is white, the aqueous phase is dark gray, air is black, and porous-medium grains are light gray. (b) False-color images, created by overlapping the binary images obtained separately for each fluid. Organic liquid is red, the aqueous phase is blue, air is black, and porous-medium grains (and unresolved fluid) are light gray.
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Comparison of the thin sections for the three-phase and two-phase systems illustrates visually that organic-liquid blob morphology is more diverse for the three-phase systems. Specifically, a significant portion of the organic liquid exists as lenses and films in contact with air. In contrast, lenses and films are not observed in the two-phase water–organic liquid systems. The differences in blob morphology are illustrated in Fig. 3
by three-dimensional renderings of individual organic-liquid blobs for the two- and three-phase systems. The presence of lenses is evident in the singlet and doublet shown for the three-phase system.
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Fig. 3. Three-dimensional renderings of organic-liquid blobs isolated from the 45/50-Accusand. The top two blobs were isolated from a two-phase system, and the bottom two blobs were collected from a three-phase system. Created with Blob3D (Ketcham, 2005).
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Air was continuous throughout the scanned intervals for all three media for the three-phase systems. Inspection of Fig. 4
clearly shows that air occupied the largest pores for Vinton soil. For this system, 97% of the total air volume in the scanned interval comprised one single, contiguous, interconnected body that spanned the entire length of the scanned interval. The remaining 3% appeared to exist as small isolated singlets. The comparative distributions of air and organic liquid in relation to the pore network were more difficult to evaluate for the two Accusand media given their more uniform pore-size distributions (e.g., Fig. 2a). However, the majority of the air occurred as one contiguous entity in both cases (53% for 45/50-Accusand and 92% for 100/140-Accusand).
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Fig. 4. Thin section in the x–y direction (planar view) of a Vinton soil column for a three-phase system (water–air–organic liquid). Reconstructed raw image collected above the I K-edge. Organic liquid is white, air is black, and the aqueous phase and porous-medium grains are gray.
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Comparison of the false-color overlays (Fig. 1b and 2b), which are developed by combining image sets generated for each fluid individually, with the original thin sections (Fig. 1a and 2a) reveals that some fluid bodies were not completely resolved in the production of the individual-fluid image sets. Specifically, very small fluid bodies or thin liquid films may not have been fully resolved, as influenced by noise inherent to the imaging process and the spatial resolution of the imaging method (e.g., Ketcham and Carlson, 2001). In addition, image smoothing, necessary to reduce image noise, has the effect of removing small features at phase interfaces, such as thin films. The analysis of organic-liquid blob morphology presented herein includes only those blobs that were successfully parsed from the image arrays and resolved. The total unresolved organic liquid is a negligible fraction of total volume in each case. However, a fraction of the unresolved organic liquid is likely to exist as thin films, which would have relatively large surface-area/volume ratios. While this unresolved volume will have minimal impact on calculated total organic-liquid surface areas, it may have a greater impact on calculated specific interfacial areas.
Quantification of Organic-Liquid Blob-Size Distribution and Morphology
Cumulative distributions of organic-liquid blob frequency versus volume are shown in Fig. 5
, along with fitted lognormal distributions. The median and range of blob sizes are similar for the three replicates of the 45/50-Accusand two-phase system. The distribution presented in Fig. 5 comprises a composite of all three data sets for that system. Blob volumes ranging over six orders of magnitude are observed. The frequency distributions are relatively similar for the three media. The lognormal distributions describe the measured data quite well for all three media. The median blob lengths for the two-phase systems are approximately twice as large as those of the three-phase systems (Table 1). The ratios of median blob length to porous-medium grain diameter are approximately 1 and 0.5 for the two-phase and three-phase systems, respectively. The variances of the distributions are greater for the three-phase systems. For example, the coefficient of variation is 3.5 versus 8.4 for the two- and three-phase systems, respectively, for the Vinton soil.
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Fig. 5. Cumulative distribution of the number of organic-liquid blobs of a given volume in two and three-phase systems for (a) Vinton soil, (b) 100/140-Accusand, (c) 45/50-Accusand. Solid lines represent the lognormal distribution fit to each data set.
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Surface areas were calculated for each organic-liquid blob, normalized by the associated blob volume (i.e., surface-area/volume ratio), and plotted as a function of blob volume to examine blob morphology as a function of size (Fig. 6
). The surface areas represent the total surface of each blob, and thus incorporate both water–organic liquid interfaces and air–organic liquid interfaces. The surface-area/volume ratio of a perfect sphere is represented by the solid lines in Fig. 6. The greater the deflection from this line, the larger the deviation from a perfect sphere. For the two-phase systems, the blobs tend to be spherical at small blob volumes (<10–3 mm3), and become increasingly less spherical with increasing size. While the same trend is apparent for the three-phase systems, a significant number of even the smallest blobs (10–5 mm3) appear to deviate significantly from a perfect sphere, resulting in a greater variation in blob morphology for the entire range of volumes. For all three media, the surface-area/volume ratios are observed to be significantly greater in the three-phase systems than the two-phase systems for smaller blob volumes (<10–3 mm3). Conversely, the ratios for the two sets of systems are similar for larger blob volumes. The total numbers of blobs per scanned interval for the two-phase systems are 163, 231, and 327 for 45/50-Accusand, Vinton soil, and 100/140-Accusand, respectively. In contrast, the total numbers of blobs for the three-phase systems are 360, 1167, and 2159, respectively, for the three media.
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Fig. 6. Volume-normalized surface area versus volume for all organic-liquid blobs: (a) Vinton soil, (b) 100/140-Accusand, (c) 45/50-Accusand. Solid lines represent the surface-area/volume ratio for a perfect sphere of equivalent volume.
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The total combined surface area of all organic-liquid blobs residing in a particular scanned interval can be normalized by the total associated blob volume in that interval to produce the global specific surface area, which can be used to compare organic-liquid configurations among systems. For the two-phase systems, the global specific surface areas were 24.9, 30.5, and 25.6 mm–1 for the 45/50-Accusand, 100/140-Accusand, and Vinton soil, respectively. These values are similar to those reported in prior studies of two-phase systems, based on three-dimensional imaging (e.g., Zhang et al., 2002; Al-Raoush and Willson, 2005; Schnaar and Brusseau, 2005) and computational (e.g., Dalla et al., 2002) methods. The values for the three-phase systems, 28.5, 55.9, and 33.1 mm–1 for the 45/50-Accusand, 100/140-Accusand, and Vinton soil, respectively, are larger than those obtained for the two-phase systems.
The fractal geometry of the organic-liquid blobs was assessed, as described by Fontenot and Vigil (2002), to further evaluate blob configuration. The slope of a log-log plot of surface area versus volume represents the ratio of area and volume fractal dimensions (data not shown). This value is 2/3 for perfect Euclidian space-filling objects (e.g., sphere) and approaches unity for increasingly ramified, or branched, geometries. The ratios of fractal dimensions for the smallest blobs (<10–3 mm3) in the two-phase systems are 0.70, 0.69, and 0.68 for 45/50 Accusand, Vinton soil, and 100/140Accusand, respectively, indicating relatively minimal deviation from spherical morphology. For the three phase systems, the ratios of fractal dimensions for the smallest blobs are 0.72, 0.71, and 0.74 for the three media, representing a somewhat greater deviation from spherical morphology. The ratios of fractal dimensions are larger, approaching unity, for larger blob sizes for both the two- and three-phase systems. For the very largest blobs (>10–1 mm3), values range from 0.98 to 1.01 in the two-phase systems and 0.98 to 1.02 in the three-phase systems, signifying blobs of much greater morphological complexity than the smaller blobs. The results of the fractal-dimension analysis are consistent with the results obtained from the surface-area/volume ratio analysis discussed above.
The preceding results indicate several differences exist between the two-phase and three-phase systems with respect to organic-liquid blob morphology. These differences are attributed to the presence of the organic-liquid lenses and films in contact with air that were observed for the three-phase systems. These lenses and films exhibit greater deviation from spherical morphology and have larger specific surface areas compared with the organic-liquid blobs associated with the two-phase systems (e.g., see Fig. 3). The presence of such lenses and films would be expected to significantly impact mass-transfer processes, such as dissolution and evaporation.
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CONCLUSIONS
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The objective of this study was to qualitatively and quantitatively characterize the pore-scale distribution and morphology of organic immiscible liquid in natural porous media comprising three fluid phases. Detailed information regarding organic-liquid blob morphology was successfully obtained using synchrotron X-ray microtomography for three porous media. General characteristics of the blob distributions were similar for all three media. For example, singlets comprised the greatest number of blobs, while the majority of the total surface area and volume of the organic liquid was associated with a relatively few, large ganglia. For the three-phase systems, a significant portion of the organic liquid was observed to exist as lenses and films in contact with air. These features were not observed in the two-phase water-organic liquid systems. The presence of the lenses and films was shown to significantly impact blob-size distribution and morphology. The results presented herein illustrate the utility of synchrotron X-ray microtomography for characterizing fluid distributions at the pore scale in natural porous media. The information obtained with the method should be useful, for example, for evaluating conceptual and mathematical models of pore-scale fluid displacement and retention.
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ACKNOWLEDGMENTS
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This work was supported in part by the NIEHS Superfund Basic Research Program and the USDA National Research Initiative Program. Imaging experiments were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation-Earth Sciences (EAR-0217473), Dep. of Energy-Geosciences (DE-FG01-94ER14466) and the State of Illinois. Use of the APS was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract no. W-31-109-Eng-38. The authors thank Asami Murao (University of Arizona), Dr. Mark Rivers (APS), Dr. Richard Ketcham (Univ. of Texas at Austin), Dr. Molly Costanza-Robinson (Middlebury College), and Dr. Clinton Willson (Louisiana State Univ.) for their kind assistance.
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ABSTRACT
INTRODUCTION
