Impact of Wettability on Pore-Scale Characteristics of Residual

May 26, 2009 - Pore-scale characteristics of NAPL blobs such as volume, lengths, interfacial areas, and sphericity index were computed using ...
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Environ. Sci. Technol. 2009, 43, 4796–4801

Impact of Wettability on Pore-Scale Characteristics of Residual Nonaqueous Phase Liquids RIYADH I. AL-RAOUSH* Department of Civil and Environmental Engineering Southern University and A&M College Baton Rouge, Louisiana 70813

Received September 10, 2008. Revised manuscript received May 2, 2009. Accepted May 4, 2009.

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 threedimensional images of fractionally wet sand systems with mean grain size of 250 µm. 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 waterwet.

Introduction The release of nonaqueous phase liquids (NAPLs), such as gasoline and chlorinated solvents, into the subsurface can lead to contamination of groundwater resources. As NAPLs migrate through porous media, residual portions are entrapped in the complex void space due to capillary forces and serve as a long-term source of pollution to the flowing groundwater. The assessment, design and implementation of an efficient cleanup and remediation strategy, depend on pore-scale distribution, morphology, and mass-transfer characteristics of the residual NAPL. Different methods have been used to characterize the geometry and topology of NAPLs in subsurface systems to gain better understanding of pore-scale processes, which in turn govern macro-scale processes such as dissolution, pressure-saturation relations, and relative permeability. Major techniques used in this regard include direct observation of NAPLs in the pore space (1-5), two-dimensional etched-glass micromodels (6-9), and three-dimensional imaging techniques (10-17). While these investigations provide valuable insights about the characteristics of NAPLs, they assume uniform wettability of the porous media (i.e., either water-wet (hydrophilic) or NAPL-wet (hydrophobic) * Corresponding author phone: (225) 933-7780; fax: (225) 7714320; e-mail: [email protected]. 4796

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systems). In water-wet systems where the solid surfaces are preferentially wetted by water, residual NAPL is entrapped in larger pore bodies as disconnected volumes which range in complexity of shape and geometry from simple singlets which occupy single pore bodies to complex ganglia that extend in length to occupy multiple pore-bodies. In natural subsurface systems, however, it is common that wettability varies spatially where a combination of waterwet and NAPL-wet solid surfaces can be found. Such media are called fractionally wet systems. Variations in wettability of natural subsurface systems occur due to heterogeneity of mineral composition of the porous media, organic matter distribution, surface roughness, and nonuniform sorption of active agents to the surface of the porous media (18-21). Previous work investigated the influence of wettability on pore-scale displacement mechanisms (22-24), flow (25-28), NAPL-water interfacial area (29, 30), NAPL dissolution (31-34), capillary pressure and relative permeability (35-37), residual phase saturation (38), infiltration of fluids into porous media (39), dissolution fingering (40), and virus retention and transport (41). Findings of these studies indicate that the wettability has a significant impact on pore-scale flow and transport processes. The objective of this paper was to investigate the impact of wettability on the characteristics of residual NAPLs in porous media systems. High-resolution three-dimensional images of different fractionally wet sand systems were obtained using synchrotron X-ray microtomography. Quantification of the morphology and entrapment patterns of residual NAPL was obtained using three-dimensional image processing algorithms.

Materials and Methods Materials. Fine sand of uniform grains of mean size (d50 ) 250 µm) was used to represent the porous media. The size of grains ensures that the image has the minimum representative elementary volume (REV), and a uniform size was selected to exclude the impact of heterogeneity of the grains while investigating the impact of wettability on the morphology of residual NAPL. Immiscible fluids used in the experiment were deaired water and Soltrol 220 (Chevron Philips Chemical Company). Soltrol 220, which is composed of a mixture of C13-C17 hydrocarbons, was used to model organic contaminates since it has similar properties to common hydrocarbon contaminants, but less hazardous in such experimental settings. The soltrol was doped with Iodononane (10% by mass) to enhance its contrast. Surface tensions were observed to be similar with and without the dopant (42). After preliminary imaging of the system, we observed that the contrast of the images was sufficient to extract (i.e., segment) the soltrol in the image without the need to dope the aqueous phase. The density and viscosity of the aqueous solution are 0.97 g/cm3 and 1.02 cP, respectively; and for the doped soltrol, 0.78 g/cm3 and 4.79 cP, respectively. The sand was rendered NAPL-wet by its treatment by octadecyltrichlorosilane (OTS) (Acros Organics) according to the procedure presented in refs 43 and 44. The procedure can be summarized as follows: a solution was created by adding 5% by volume of OTS to ethanol. The solution was then added to the sand until it became completely saturated; the mixture was stirred for several hours. The mixture was rinsed off with ethanol that removes most of the OTS, and then evenly placed in a pan in a vented area to air-dry overnight. The remaining mixture was cured in an oven at 100 °C for an hour to allow the remaining ethanol to 10.1021/es802566s CCC: $40.75

 2009 American Chemical Society

Published on Web 05/26/2009

evaporate. The contact angle is ∼0° and 64.9° for the NAPLwet and water-wet sand, respectively. Fractionally wet systems were obtained by combining various mass fractions of untreated and OTS-treated sands. Systems will be designated herein by their percentage of OTS-treated sand (i.e., equivalent NAPL-wet mass fraction). Four different systems containing the residual NAPL were imaged; 100, 75, 50, and 0% OTS-treated mass fraction (i.e., NAPL-wet mass fractions, respectively). Creation of Residual NAPL Saturation. A special cell was fabricated to establish the residual NAPL in the saturated fractionally wet systems and to be mounted in the tomography station for imaging. The cell consisted of a column of polycarbonate connected to aluminum end fittings to allow insertion of tubes to perform drainage and imbibition of fluids. The height of the column was 83.5 mm and the inside diameter was 6.0 mm. Porous polypropylene frits (10 µm) were placed at both aluminum ends of the columns to retain the porous media and to ensure homogeneous injection and displacement of the immiscible fluids. The fractionally wet sand systems were carefully packed under dry conditions to ensure homogeneous distribution of sand grains in the column. Approximately 20 pore-volumes of deaired water were injected upward through the column at a constant rate of 2.5 mL/h using a syringe connected to a Cole-Parmer Series 74900 syringe pump. The doped soltrol was then injected upward at the same constant flow rate used for the deaired water. Water was finally flushed through the column to displace the NAPL and create the residual NAPL saturation. To ensure reproducibility of the packing and the residual NAPL, three replicate experiments of each fractionally wet system were performed including packing, NAPL entrapment, and imaging. Properties of the replicate experiments are given in Supporting Information (SI) Table S1. Microtomography Imaging. The systems were imaged using synchrotron X-ray microtomography at the GeoSoilEnviroCARS 13-BM-C beamline at the Advanced Photon Source, Argonne National Laboratory, IL. Image resolution is 7.47 µm/pixel in all directions. To avoid boundary effects on the spatial distribution and configurations of immiscible fluids, scans were obtained approximately at mid portions of the columns. Image reconstruction was performed using algorithms developed by GSECARS to convert CT attenuation to 3D volumetric data. For each system, three different overlapping scans were obtained at different vertical locations and then stacked for analysis. The size of the image used for residual NAPL characterization of each system was 600 × 600 × 1200 voxels; each volume represented a rectangular portion inside the column to exclude boundary effects of the columns. The NAPL was observed to be evenly distributed throughout the domain, longitudinally and radially, with no apparent preferential accumulation along the walls or centers of the columns (see SI Figure S1). While some air was observed in the columns, the maximum volume of air found in any column was less than 0.19% of the total void space (i.e., voxels of air /voxels of the void space). Characterization of Residual NAPL Using Image Processing Algorithms. Most of the algorithms used for blob characterization in this paper are based on the ones presented in ref 14; coding was modified to improve the efficiency of computations. For the sake of completeness, a brief description of the algorithms is provided herein, with more details found in refs 14, 45, and 46. Microtomography images were segmented to obtain three distinct labels for the phases that exist in the systems (i.e., sand, water, and the residual NAPL). More information on the segmentation and other image processing can be found in the SI. Once the NAPL phase was segmented, a 3D connected component labeling algorithm was then implemented to identify individual NAPL blobs

and ganglia. The algorithm created a new image of the NAPL phase only, where disconnected blobs were labeled by distinct numerical values starting from the value of one to the maximum number of individual blobs in the image. Upon completion of NAPL ganglia labeling, two main operations were performed; removing small isolated NAPL voxels and filling small gaps in the NAPL phase, if they exist. A random elimination procedure was implemented to remove ganglia that have very small volumes which are not realistic (size of 2 × 2 × 2 voxels). Once each disconnected NAPL blob or ganglion was identified, it was treated as a set of voxels from image processing stand point. Attributes such as volume, interfacial area, lengths, sphericity index, orientations, and entrapment pattern in the void space were computed by performing operations on these sets of voxels. It was not possible to distinguish between OTS-treated and untreated sand particles in the 3D images given the resolution and the method of sand coating. Such distinction would be helpful to visually identify the impact of wettability of sand particles on the configuration of the residual NAPL. Such distinction, however, might be possible at very high resolution if the OTS is doped to enhance its contrast. The volume was computed as the total number of voxels that belong to each disconnected blob. The interfacial area was computed using a marching cubes algorithm (45). NAPL interfacial area includes two components, NAPL-water and NAPL-grain interfacial areas, only NAPL-water interfacial area was computed and presented because it is that portion of the total interfacial area that affects mass transfer and hence other processes such as dissolution. Lengths of individual blobs were computed from the principle axes which were obtained from a second-order central moment of inertia matrix. The direction of the major axis points to the largest extent of a given blob and the direction of the minor axis points to its smallest extent. Three lengths (major, minor, and intermediate) were computed for each NAPL blob by multiplying each voxel in the blob by the eigenvector of its inertia matrix to rotate the blob in the image so that its principal axes coincide with the axis of the binary image. In this paper, only distributions of major lengths of the blobs are presented. The second-order central moment of inertia matrix is defined as follows:

[

Ixx -Ixy -Ixz -Iyz In ) -Iyx Iyy -Izx -Izy Izz

]

(1)

where Ixx ) µ020 + µ002, Iyy ) µ200 + µ002, Izz ) µ200 + µ020, Ixy ) Iyx ) µ110, Ixz ) Izx ) µ101, Iyz ) Izy ) µ011 (2) where µijk is the (i + j + k) order-central moment of a given NAPL blob and defined as µi,j,k )

∑ (x - xj) (y - yj) (z - zj) f(x, y, z) i

j

k

(3)

x,y,z

where x, y, and z are indices of row, column, and depth of the blob in the image respectively; xj , yj, and zj are the coordinates of the centroid of a given blob, and f(x,y,z) ) 1 for the blob under consideration. The centroid of a blob is defined as xj )

∑

∑

∑

1 1 1 xf(x, y, z), yj ) yf(x, y, z), zj ) zf(x, y, z) V x,y,z V x,y,z V x,y,z (4)

Where V is the blob volume and computed as VOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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V)

∑ f(x, y, z)

(5)

x,y,z

The sphericity index, ISph, provides a quantification of the similarity of a NAPL blob to a sphere and defined as (41) ISph

2 ) B Nvox

B Nvox

∑ i)1

[

diB diB Lmin Lmax

]

(6)

B is the total number of boundary voxels of the where, Nvox blob; diB is the Euclidean distance between the boundary voxel, i and the centroid of the blob; Lmin, and Lmax are the minor and major lengths of the blob, respectively.

Results and Discussion Properties of the replicate experiments are given in SI Table S1. Each system was characterized and mean values of the properties of the systems are given in the table. Results indicate that there is a good reproducibility for replicas performed with individual fractionally wet media and there is a distinct difference between characteristics of residual NAPL in the different fractionally wet media. Therefore, the analysis of one representative sample from each system is presented and discussed below (sample no. 1 shown in SI Table S1). Figure 1 shows a 3D image of 0% OTS-treated system, only a small portion of the entire image used in analysis is shown in the Figure. The color scheme reflects blob identification where blobs are labeled by distinct numbers (shown as different colors in the image). Both 3D and 2D images of all systems are shown in the SI. Porosities obtained from images are 0.331, 0.327, 0.334, and 0.325 for the 100, 50, 25, and 0% OTS-treated systems, respectively. Residual NAPL saturations are 0.125, 0.067, 0.083, and 0.121 for the 100, 50, 25, and 0% OTS-treated systems, respectively. Residual NAPL saturations are less in fractionally wet systems (i.e., 50 and 25% OTS-treated systems) and increase as the systems become more NAPL-wet or water-wet. The trend of residual NAPL saturations is consistent with the ones obtained from column experiments of fractionally wet Ottawa sand (21, 32, 40). Figure 2 shows frequency distributions and summary statistics of blob volumes normalized by the volume of a sphere that has a diameter equivalent to the mean grain diameter, d50. In all systems, NAPL blobs vary greatly in size and complexity by orders of magnitude. As can be seen in Figure 2, wettability of the porous medium has a significant impact on sizes of residual NAPL blobs and their distributions in the void space. In the 100% OTS-treated system, NAPL has preference for entrapment in small pores, corners, and at contacts of grains through continuous thin films. Since NAPL films were not resolved at the resolution of the images, NAPL in small pores and at the contacts of grains only were detected (see SI Figure S3). Mean blob volumes normalized by the volume of a sphere with mean grain diameter, d50, are 0.97, 1.23, 1.93, and 2.39; and the maximum normalized volumes are 8.81, 13.66, 16.79, and 21.74, for the 100, 50, 25, and 0% OTS-treated systems, respectively. Volume distributions for the 0% OTS and 100% OTS-treated systems are consistent with the values reported independently for water-wet sand (15) and oil-water polyethylene (47), respectively. A trend of decrease in mean blob volume (i.e., individual blobs) is observed as more fractions of the porous media rendered NAPL-wet indicating that spatial distribution of wettability controls locations and entrapment patterns of residual NAPL blobs. This in turn influences processes such as relative permeability and pressure saturation relations. In a complete NAPL-wet system, the effective permeability to NAPL is much lower than in water-wet systems (at any given saturation) 4798

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FIGURE 1. Three-dimensional image of the residual NAPL in the 0% OTS-treated system. because water in the larger pores hinders the flow of NAPL. This becomes more pronounced as the water saturation increases until the residual NAPL saturation is obtained at higher levels than it would be in a water-wet system. On the other hand, the effective permeability to water is high because residual NAPL is located in the small pores and is coating the larger pores with a thin film which does not affect the flow of water. The relative permeabilities are controlled by the distribution of the fluids in the pores of the void space. The relative permeability of a fluid at any saturation is a function of its mobility, which in turn is a function of capillary size and wettability. The wetting phase has a lower mobility if it is located in the smaller pores and is adhering to the surface of the porous media. However, in a water-wet system, where NAPL occupies the large pores and most of them are coated with a thin water film, NAPL flow occurs through the larger pores where the NAPL is located, water does not exist to impede the flow of NAPL, the NAPL effective permeability, relative to water, is very high. On the other hand, the water effective relative permeability is very low, even when the NAPL saturation is reduced to the residual level, because residual NAPL in the large pores remains to effectively block the flow of water. Frequency distributions and summary statistics of blob major lengths normalized by the d50 are shown in Figure 3. Normalized mean blob lengths are 1.10, 1.45, 1.67, and 1.96; and the maximum normalized lengths are 6.27, 11.31, 10.13, and 15.21, for the 100, 50, 25, and 0% OTS-treated systems, respectively. These data indicate that as the porous medium comprises more water-wet surfaces, residual NAPL blobs increase in size and length due to the entrapment preferences discussed previously. Blob sizes observed in the 0% OTStreated system are consistent with values obtained from microtomography studies of Accusand (15) and Berea sandstone (48). Figure 4 shows frequency distributions of NAPL-water interfacial areas normalized by the volume of each blob. Normalized mean blob interfacial areas are 43.75, 35.86, 34.25, and 30.88 cm-1; and the maximum normalized interfacial areas are 112.43, 81.2, 94.78, and 71.90 cm-1, for the 100, 50, 25, and 0% OTS-treated systems, respectively. Normalized interfacial areas reach the highest mean value in the 100%OTS-treated system and the lowest mean value in the 0% OTS-treated system. This trend is consistent with the one observed experimentally for Ottawa sand (31). Interfacial area for the 0% OTS-treated system is consistent with results obtained from computational approaches (49) and microtomography studies of Vinton soil (42) and Berea sandstone (48). The value, however, is higher than values reported for glass bead systems (16). This is mainly due to the difference in texture between soil and glass beads.

FIGURE 2. Frequency distributions of blob volumes normalized by the volume of a sphere that has a diameter equivalent to d50 of grains.

FIGURE 3. Frequency distributions of blob major lengths normalized by the d50 of grains. The interfacial area in the 100% OTS-treated system is consistent with values reported for polyethylene cores obtained from microtomography images (47). The large interfacial area in the NAPL-wet system is due to the existence of large numbers of small blobs in small porebodies and at the contacts of the grains. These blobs are actually connected by continuous thin films, but since the thin films were not resolved at the resolution of the image, the NAPL was characterized as many small blobs (see SI Figure S4). Figure 5 shows frequency distributions of sphericity index for all systems. Mean sphericity index values are 0.216, 0.262, 0.455, and 0.569; and maximum values are 0.857, 1.507, 1.507, and 1.990, for the 100, 50, 25, and 0% OTS-treated systems, respectively. Distributions of sphericity index for all systems are consistent with distributions of volumes and lengths provided in Figures 2 and 3, respectively. To give more

physical meaning of sphericity index, Figure 6 was developed to provide examples of different blobs extracted from the 0% OTS-treated system with different sphericity indices; the corresponding volumes, major and minor lengths are given in Table 1. Figure 6 illustrates that as the sphericity index of a blob diverts from 0, the blob becomes more irregular and branched. These results indicate that spatial variation in wettability of porous media surfaces has a significant impact on porescale characteristics of the residual NAPL in saturated porous media systems. As the porous media comprises more waterwet 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.

TABLE 1. Sphericity Index and Its Relation to Lengths for Each Sample Blob Shown in Figure 6 blob

sphericity index

volume/volume (d50)

major length/d50

minor length/d50

intermediate length/d50

a b c d

0.0160 0.4331 0.8544 0.9024

0.089 4.103 8.406 7.725

0.467 3.093 7.550 6.122

0.462 1.845 3.176 2.345

0.463 2.319 3.324 4.459

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FIGURE 4. Frequency distributions of NAPL-water interfacial areas normalized by the volume of each blob.

FIGURE 5. Frequency distributions of sphericity index for all systems. 109-Eng-38. I would particularly like to thank Maude Johnson, Meagan Pinkney, and Lindsey Thomas (Southern University and A&M College), Dr. Mark Rivers and Dr. Peter Eng (GeoSoilEnviroCARS) for their assistance with the imaging portion of this work.

Supporting Information Available

FIGURE 6. Examples of different blobs with different sphericity indices extracted from the 0% OTS system; the corresponding volumes, major and minor lengths are given in Table 1.

Acknowledgments This research was performed, in part, at Argonne National Laboratory as a research participant in the FaST Program. The program is administered by Argonne’s Division of Educational Programs with funding provided by the U.S. Department of Energy and the National Science Foundation (Contract No. HRD-0310426). The experiment was performed at the GeoSoilEnviroCARS beamline (13-BM-C) at the Advanced Photon Source. GeoSoilEnviroCARS is supported by the National Science Foundation Earth Sciences (EAR0217473), Department of Energy Geosciences (DE-FG0194ER14466), 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-314800

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Table summarizes properties of replicas of each system, additional details of image processing and 2D and 3D images of the different systems.This material is available free of charge via the Internet at http://pubs.acs.org.

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