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Binary segmentation of volumetric images of porous media is a crucial step towards gaining a deeper understanding of the factors governing biogeochemical processes at minute scales. Contemporary work primarily revolves around primitive techniques based on global or local adaptive thresholding that have known common drawbacks in image segmentation. Moreover, the absence of a unified benchmark prohibits quantitative evaluation, which further undermines the impact of existing methodologies. In this study, we tackle the issue on both fronts. First, by drawing parallels with natural image segmentation, we propose a novel, and automatic segmentation technique, 3D Quantum Cuts (QCuts-3D) grounded on a state-of-the-art spectral clustering technique. Secondly, we curate and present a publicly available dataset of 68 multiphase volumetric images of porous media with diverse solid geometries, along with voxel-wise ground truth annotations for each constituting phase. We provide comparative evaluations between QCuts-3D and the current state-of-the-art over this dataset across a variety of evaluation metrics. The proposed systematic approach achieves a 26% increase in AUROC (Area Under Receiver Operating Characteristics) while achieving a substantial reduction of the computational complexity over state-of-the-art competitors. Moreover, statistical analysis reveals that the proposed method exhibits significant robustness against the compositional variations of porous media. publishedVersion

The migration of fine particles in porous media has been studied for different applications, including gas production from hydrate-bearing sediments. The clogging behavior of fine particles is affected by fine particle-pore throat size ratio, fine particle concentration, ionic concentration of fluids, and single/multiphase fluid flow. While previous studies presented valuable results, the data are not enough to cover a broad range of particle types and sizes and pore throat size in natural hydrate-bearing sediments. This paper presents a novel micromodel to investigate the effects of fine particle-pore throat size ratio, fine concentration, ionic concentration of fluid, and single/multiphase fluid flow on clogging or bridging in porous media. The results show that (1) the concentration of fine particles required to form clogging and/or bridging in pores decreased with the decrease in fine particle-pore throat size ratio, (2) the effects of ionic concentration of fluid on clogging behaviors depend on the types of fine particles, and (3) fine particles prefer to accumulate along the deionized water- (DW-) CO2 interface and migrate together, which in turn easily causes clogging in pores. As a result, multiphase fluid flow during gas production from hydrate-bearing sediments could easily develop clogging in pore throats, where the relative permeability of DW-CO2 in porous media decreases. Accordingly, the relatively permeability of porous media should be evaluated by considering the clogging behavior of fines. 2019 Jongwon Jung et al. This research was made possible by an NPRP grant # NPRP8- 594-2-244 from the Qatar National Research Fund (a member of Qatar Foundation). Also, this research was supported by a grant (2018-MOIS31-009) from the Fundamental Technology Development Program for Extreme Disaster Response funded by the Korean Ministry of the Interior and Safety (MOIS) and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A3B03 031369). This research was supported by the Ministry of Trade, Industry, and Energy (MOTIE) through the Project Gas Hydrate Exploration and Production Study (19-1143) under the management of the Gas Hydrate Research and Development Organization (GHDO) of Korea and the Korea Institute of Geoscience and Mineral Resources (KIGAM). Scopus

Gas production studies from natural gas hydrate reservoirs have been the subject of extensive research in recent years. Although CH4-CO2 replacement production method from gas hydrates has many advantages, the number of the studies related to this production method are less than depressurization production method studies, especially in microscale porous media. Hence, this paper presents a comprehensive literature review on CH4-CO2 replacement to better understand the associated processes and mechanisms in microscale porous media with emphasis on micromodel experiments, 3D imaging, other visualization testing method and pore network modelling. Moreover, the advantages and disadvantages of currently available CH4-CO2 replacement studies were investigated. The critical issues of the replacement method were underlined and new suggestions have been offered for future investigations.

Recently, researchers have been attracted towards the gas production from hydrate bearing sediments considering its abundance in marine continental margins and persisting demand for alternate energy. Dissociation of hydrate into gas and water is the preliminary technique for gas production in hydrate bearing sediments. Expanded fluid volume and gas pressure upon dissociation detach the fines from the grain surface and result in pore throat entrapment. Migration of fines associated with gas flow greatly influence the alteration of permeability of the sediment by clogging pore throats in the flow path. A pore-scale visualization study was implemented to provide a clear insight into the actual mechanisms associated with mobilization and clogging of fines during two-phase flow through a microfluidic chip. Carboxylate modified polystyrene latex particles deposited in the porous media were migrated during drainage with CO2 gas. The detachment of fine particles from the grain surfaces was observed and were retained on the new interface; gas-water interface. The images and videos captured during the experiment were helpful in observing additional pore scale mechanisms responsible for permeability impairment in the porous media. Interface pinning, deformation and resistance to coalescence were found to be other mechanisms in addition to pore clogging.

Release of deposited colloids in the soil porous media during two-phase flow poses potential health hazard due to the facilitated transport of contaminants towards groundwater reservoirs. Considerable uncertainties exist concerning the impact of ionic strength on pore-scale mechanisms of colloid mobilization during transient flow. This study aims to investigate the effect of ionic strength on colloid retention and mobilization using a glass micromodel. The behavior of Carboxylate modified Polystyrene latex particles of 5 ?m diameter in saline solution (i.e., 100 mM & 1 mM of NaCl at pH 10) was visualized with an optical microscope during saturated and twophase flow. We found that colloid aggregation and attachment on Solid-Water Interfaces (SWI) was increased with increase in ionic strength. CO2 injection into the saturated micromodel mobilized the previously attached colloids on SWI, retained at the Gas- Water Interfaces (GWI) due to capillary forces and thus were transported through the micromodel. Imbibition mobilize colloids from GWI and are transported or reattached on SWI depending on the ionic strength of pore water. The greater adhesive forces of colloids at higher ionic strength was resulted in thin film attachment during drainage and reattachment of colloids mobilized from GWI on SWI during imbibition. The acquired images showed the application of a micromodel for the visualization of colloid retention and re-mobilization through the porous media. This publication was made possible by partial funding from NPRP grant # NPRP8- 594-2-244 from the Qatar National Research Fund (a member of Qatar Foundation). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of funding agencies.

Tortuosity is a parameter that plays a significant role in the characterization of complex porous media systems and it has a significant impact on many engineering and environmental processes and applications. Flow in porous media, diffusion of gases in complex pore structures and membrane flux in water desalination are examples of the application of this important micro-scale parameter. In this paper, an algorithm was developed and implemented as a MATLAB code to compute tortuosity from three-dimensional images. The code reads a segmented image and finds all possible tortuous paths required to compute tortuosity. The code is user-friendly, easy to use and computationally efficient, as it requires a relatively short time to identify all possible connected paths between two boundaries of large images. The main idea of the developed algorithm is that it conducts a guided search for connected paths in the void space of the image utilizing the medial surface of the void space. Once all connected paths are identified in a specific direction, the average of all connected paths in that direction is used to compute tortuosity. Three-dimensional images of sand systems acquired using X-ray computed tomography were used to validate the algorithm. Tortuosity values were computed from three-dimensional images of nine different natural sand systems using the developed algorithm and compared with predicted values by models available in the literature. Findings indicate that the code can successfully compute tortuosity for any unconsolidated porous system irrespective of the shape (i.e., geometry) of particles. 1 2017 Elsevier B.V. Scopus

Fine particles within porous media may migrate with the flowing fluid and cause bridging or clogging in the pore space. Bridging and clogging reduce the flow permeability of porous media, which has a significant influence on petroleum engineering applications such as water and oil extraction, sand production, and gas production from hydrate-bearing sediments. Although the migration of fine particles and its impact on bridging and clogging have been investigated for a single-phase flow, it has not been understood clearly for a multi-phase flow. This work reports an approach using microfluidic pore models to study the migration of fine particles and the bridging/clogging behavior in a structure mimicking porous media. Results from the microfluidic model show that: (1) fine particles accumulated along the water and gas (CO2) interface; (2) fine particle concentrations in pores locally increased due to the accumulated particles at the interface, and (3) consequently bridging and clogging occurred in the pore throat. Findings from this work provide a starting point in understanding complex phenomena including a reduced flow permeability of porous media on a fluid containing fine particles for a variety of petroleum engineering applications. 2017, Springer-Verlag GmbH Germany. Acknowledgement This research was made possible by NPRP Grant # NPRP8-594-2-244 from Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors. Scopus

Recently, researchers are attracted towards the gas production from hydrate bearing sediments considering its abundance in marine continental margins and persisting demand for alternate energy. Dissociation of hydrate into gas and water is the preliminary technique for gas production in hydrate bearing sediments. Expanded fluid volume and gas pressure upon dissociation detach the fines from the grain surface and result in pore throat entrapment. Migration of fines associated with gas flow greatly influence the alteration of permeability of the sediment by clogging pore throats in the flow path. A pore-scale visualization study was implemented to provide a clear insight into the actual mechanisms associated with mobilization and clogging of fines during two-phase flow through a microfluidic chip. Carboxylate modified polystyrene latex particles deposited in the porous media were migrated during drainage with CO2 gas. The detachment of fine particles from the grain surfaces was observed and were retained on the new interface; gas-water interface. The images and videos captured during the experiment was helpful in observing additional pore scale mechanisms responsible for permeability impairment in the porous media. Interface pinning, deformation and resistance to coalescence was found to be other mechanisms in addition to pore clogging. EAGE 2019. This research was partially funded by National Priority Research Program (NPRP) grant # NPRP8-594-2-244 from Qatar National Research Fund (a member of Qatar Foundation). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of funding agencies. Scopus

Availability of natural hydrates and ongoing rise in demand for energy, motivated researchers to consider hydrates as a potential energy source. Prior to gas production operations from hydrate-bearing sediments, hydrate dissociation is required to release gas into sediments. To reliably predict natural hydrate reservoir gas production potential, a better understanding of hydrate dissociation kinetics is needed. Hydrate dissociation models assume the relationship between hydrate surface area and (hydrate volume)2/3 to be linear due to hydrate sphericity assumptions. This paper investigates the validity of the spherical hydrate assumption using in-situ three-dimensional (3D) imaging of Xenon (Xe) hydrate dissociation in porous media with dynamic 3D synchrotron microcomputed tomography (SMT). Xe hydrate was formed inside a high-pressure, low-temperature cell and then dissociated by depressurization. During dissociation, full 3D SMT scans were acquired continuously and reconstructed into 3D volume images. A combination of cementing, pore-filling, and surface coating pore-habits were observed in the specimen. It was shown that hydrate surface area can be estimated using a linear relationship with (hydrate volume)2/3 during hydrate dissociation in porous media based on direct observations and measurements from 3D SMT images. EAGE 2019. Funding was partially provided by National Priority Research Program (NPRP) grant # NPRP8-594-2-244 from Qatar National Research Fund (a member of Qatar Foundation) and the Institute for a Secure and Sustainable Environment (ISSE), University of Tennessee-Knoxville, USA. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of funding agencies. This paper used resources of the Advanced Photon Source (APS),a U.S.DepartmentofEnergy(DOE) Office of Science User Facilitypoerated for the DOE Office of Science by Argonne National Laboratory (ANL) under Contract No. DE-AC02-06CH11357. The SMT images presented in this paper were collected using the x-ray Operations and Research Beamline Station 13-BMD at Argonne Photon Source (APS), ANL. We thank Dr. Mark Rivers of APS for help in performing the SMT scans. We also acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation, Earth Sciences (EAR-1128799), and the DOE, Geosciences (DE-FG02-94ER14466). Scopus

A better understanding of the kinetics of hydrate dissociation is essential to reliably predict gas production potential from natural hydrate reservoirs. Most hydrate dissociation models assume hydrates to be a constant number of equal-sized spheres dissociating at a constant rate. This paper uses dynamic 3D synchrotron micro-computed tomography (SMT) imaging to study hydrate surface area evolution during Xenon hydrate dissociation. Hydrates are formed inside a high-pressure low-temperature cell filled with partially saturated ASTM 20-30 Ottawa sand. Hydrate dissociation is initiated through depressurization in the first experiment and through thermal stimulation in the second experiment. During dissociation, continuous full 3D SMT images were acquired where each scan took 45 s to complete. A combination of cementing, pore-filling, and surface coating pore habits were observed for the depressurization experiment and pore-filling for the thermal stimulation experiment. Surface coating hydrates dissociate faster than hydrates with pore-filling pore habit due to the higher specific area which allows for more surface for hydrates to dissociate it. Direct measurements of hydrate volume and hydrate surface area suggest that even with a combination of hydrate pore habits formed within the 3D porous media, estimation of hydrate surface area as a linear relation with (hydrate volume)2/3 is best for hydrate saturation less than a threshold value depending on the dissociation method and driving force. (hydrate volume)2 and (hydrate volume)3 were found to better estimate hydrate interfacial area in comparison to (hydrate volume)2/3 for the depressurization experiment and thermal stimulation experiment, respectively. 2019 The Authors This publication was made possible by partial funding from NPRP grant # NPRP8-594-2-244 from the Qatar national research fund (a member of Qatar Foundation) and the Institute for a Secure and Sustainable Environment (ISSE) , University of Tennessee-Knoxville, USA . Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of funding agencies. The authors would like to thank Mr. Wadi Imseeh for his help during scanning and Mr. Jamal Hannun for his help in interfacial area analysis. This paper used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (ANL) under Contract No. DE-AC02-06CH11357. The PSMT images presented in this paper were collected using the x-ray Operations and Research Beamline Station 13-BMD at Argonne Photon Source (APS), ANL. We thank Dr. Mark Rivers of APS for help in performing the PSMT scans. We also acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation, Earth Sciences (EAR-1128799), and the DOE, Geosciences (DE-FG02-94ER14466). The authors would also like to thank the anonymous reviewers who contributed with comments and suggestions to improve this paper. Scopus

The availability of natural gas hydrates and the continuing increase in energy demand, motivated researchers to consider gas hydrates as a future source of energy. Fundamental understanding of hydrate dissociation kinetics is essential to improve techniques of gas production from natural hydrates reservoirs. During hydrate dissociation, bonds between water (host molecules) and gas (guest molecules) break and free gas is released. This paper investigates the evolution of hydrate surface area, pore habit, and tortuosity using in-situ imaging of Xenon (Xe) hydrate formation and dissociation in porous media with dynamic three-dimensional synchrotron microcomputed tomography (SMT). Xe hydrate was formed inside a high- pressure, low-temperature cell and then dissociated by thermal stimulation. During formation and dissociation, full 3D SMT scans were acquired continuously and reconstructed into 3D volume images. Each scan took only 45 seconds to complete, and a total of 60 scans were acquired. Hydrate volume and surface area evolution were directly measured from the SMT scans. At low hydrate saturation, the predominant pore habit was surface coating, while the predominant pore habit at high hydrate saturation was pore filling. A second-degree polynomial can be used to predict variation of tortuosity with hydrate saturation with an R2 value of 0.997. The Authors, published by EDP Sciences, 2020. This publication was made possible by partial funding from NPRP grant # NPRP8-594-2-244 from the Qatar national research fund (a member of Qatar Foundation) and the Institute for a Secure and Sustainable Environment (ISSE), University of Tennessee-Knoxville, USA. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of funding agencies. The authors would like to thank Mr. Wadi Imseeh for his help during scanning and Mr. Jamal Hannun for his help in interfacial area analysis. This paper used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (ANL) under Contract No. DE-AC02-06CH11357. The PSMT images presented in this paper were collected using the x-ray Operations and Research Beamline Station 13-BMD at Argonne Photon Source (APS), ANL. We thank Dr. Mark Rivers of APS for help in performing the PSMT scans. We also acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation, Earth Sciences (EAR-1128799), and the DOE, Geosciences (DEFG02-94ER14466). Scopus

The structure of natural hydrate-bearing sediments that exist offshore or onshore is a combination of coarse-grained sediments and fine-grained particles. During gas production from hydrate-bearing sediments, fine particles may migrate with the flowing fluids within pore space and cause clogging of the pore space of the porous media. Therefore, fine particles play a significant role during methane production from hydrate-bearing sediments as it impact the overall sediment formation performance and production efficiency. The migration of fine particles and its impact on clogging have been investigated in a single-phase flow, but it has not been clearly understood in a multi-phase flow. This research focuses on the study of fines migration and clogging behavior during single and multi-phase flow which can be implicated in gas production from hydrates bearing sediments. Microfluidic pore models that mimic porous media with different pore throat sizes were fabricated and utilized to study fines migration and clogging behavior in porous media. Artificial particles and natural fine particles were selected to represent fine particles. The impact of flow rate, pore-fluid types, particle concentration, and pore-throat to fine particle size ratio was investigated. Fine particles used in this research include polystyrene latex particles, silica, and kaolinite. Pore-fluids used in this study include deionized (DI) water, and sodium chloride (NaCl) brine (2M concentration). The particle concentrations covered from 0.1% to 10%. And the pore-throat widths were fabricated from 40 μm to 100 μm. Single-phase flow experiments were conducted to show that the concentration of fine particles required to form clogging in pores increased as flow rate decreased. The results obtained using polystyrene latex particles provide the insight at a relatively higher flow rate (50 μl/min) than literature studies that fine particles with 2% concentration can migrate in the pore throat without bridge or clogging at the various pore throat and fine particle size ratios (o/d = 2.6∼36.4). Furthermore, silica presents higher critical clogging concentration (0.5% in brine) compared with kaolinite (0.2% in brine) when the pore-throat width equal to 60 μm due to the larger pore throat and fine particle size ratio. On the other hand, the findings show that clogging easily occurred at a lower pore-throat to fine particle size ratio even with a few number of fine particles. In addition, pore-fluid type directly influences the tendency of fine's to form clusters which in turn impacts the clogging behavior. For instance, silica fines clogging easier occurs in brine solution compare within deionized water due to larger cluster size in brine, while kaolinite shows an opposite result which means the kaolinite has higher clogging possibility in deionized water compared within brine solution. On the other hand, findings of multi-phase flow experiments show that fine particles accumulate along the liquid-gas interface and migrate together, which in turn cause bridging or clogging to occur easily in pores. These observations imply that a multi-phase flow during gas production could easily form clogging in pores, in which the flow permeability of porous media decreases even though clogging has not occurred in the same conditions with a single-phase flow. Thus, the permeability of porous media in engineering applications should be estimated by considering relatively easy clogging in pores in a multiphase flow compared to a single-phase flow. Findings of this research show the vital impact of pore-fluids and fluid-fluid interphase on fine particles migration and clogging in porous media. It provides a better understanding of the fines migration and clogging mechanisms. In addition, the results indicate the need to understand the types of fines and fluids in reservoir before evaluating if there will be a clogging potential during gas production from hydrates bearing sediments.

The concept of representative elementary volume (REV) is critical to understand and predict the behavior of effective properties of complex heterogeneous materials in a multiscale manner. Porosity is usually used to define the REV of a given material, if porosity is stationary over consecutive volumetric increments, the sample is considered as REV regardless of the behavior of other microstructure properties. To investigate this common assumption, the change of key microstructure parameters such as local void ratio, sphericity, angularity, and coordination number of particles was computed over different volumes of REV for porosity. X-ray computed tomography was used to obtain three-dimensional volumes of natural sand packed at four different particle fragments. Efficient and robust algorithms were used to compute the microstructure properties from three-dimensional images. Findings revealed that the REVmin for porosity may not be adequate to be considered as a REV for parameters such as particle size dist...

The concept of representative elementary volume (REV) is critical to understand and predict the behaviour of effective parameters of complex heterogeneous media (e.g., soils) in a multiscale manner. Porosity is commonly used to define the REV of a given sample. In this paper we investigated whether the use of a REV for porosity can be used as a REV for other parameters such as particle size distribution, local void ratio and coordination number. X-ray computed tomography was used to obtain 3D images (i.e., volumes) of natural sand systems with different particle size distributions. 3D volumes of four different systems were obtained and a REV analysis was performed for these parameters utilizing robust 3D algorithms.Findings revealed that the REVmin for porosity may not be adequate to be considered as a REV for parameters such as particle size distribution, local void ratio and coordination number. The REVmin for these parameters was observed to be larger than the REVmin for porosity. Heterogeneity of the systems was found to be an important factor to determine the REV for the parameters analyzed in this paper. The REV analysis revealed that as the uniformity coefficient increased, a larger volume was required to obtain the REVmin for the distribution of particle sizes and coordination number whereas a smaller volume was required to obtain the REVmin for local void ratio. Therefore, determination of the REV for parameters described in this paper or any microscale parameter of concern should not be derived based on REV for porosity and should be determined based on their distributions over different volumes. © 2010 Elsevier B.V.

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.

An optimization based model that simulates random packings of polydisperse particles to mimic porous media systems is presented in this paper. The model simulates systems with properties that map back to physically measured quantities. The model is composed of three main procedures: detachment of an existing sphere from a target sphere by moving its center to a new position, movement of an existing sphere to touch the target sphere, and addition of a new sphere to touch the target sphere. Each procedure is performed to satisfy a constrained nonlinear optimization formulation. Input parameters include radius and coordination numbers density functions. A monodisperse and polydisperse packings were simulated by incorporating physically based properties of glass bead packings obtained from 3D computed tomography images. In each simulated system, the packing density obtained was very close to the packing density of the physical system. The packings were tested for isotropy, homogeneity and randomness and then compared to experimental microtomography images. Findings indicate that the packings generated were isotropic, homogenous, random, and in a good agreement with the experimental data.

We present in this paper a methodology to calculate the distribution of local void ratio in porous media systems from high-resolution three-dimensional images. We introduce an algorithm to calculate the distribution of local void ratio from 3D images based on distance and watershed transforms. The watershed transform is used to segment touched or overlapped particles in an efficient way and the distance transform is used to calculate the boundaries of local void regions. The algorithm is validated using computer simulated 3D images of regular packing, irregular (non-spherical particles) packing, and random uniform spherical packing. Results show that the algorithm is robust, accurate and can be used to calculate local void ratio distribution of 3D systems regardless of irregularity in shapes, sizes, or arrangement of particles. X-ray microtomography images of different glass bead systems are used to calculate distributions of local void ratio. Parameters of distributions are function of porosity and particle-size distribution. The maximum local void ratio in each system is less than 3.0 and the minimum is greater than 0.2.

Pore-scale processes govern fundamental behavior in multiphase porous media systems. A high-resolution, three-dimensional image of the interior of a multiphase porous media system was obtained using synchrotron X-ray tomography. The system was imaged at a resolution of 12.46 mum following entrapment of the nonwetting phase at residual saturation. First, the physically representative network structure of the porous media system is extracted from the void space. This provides a direct mapping of the pore bodies and throats and enables pore-level calculations of coordination numbers, aspect ratios, and pore body and throat correlations. Next, algorithms developed to calculate properties of the entrapped nonwetting phase, such as volume, sphericity, interfacial area, and orientation, are applied to the residual nonwetting phase blobs. Finally, correlations between the pore network structure and nonwetting phase characteristics are examined. As expected, it was found that the nonwetting phase was trapped primarily in the largest pore spaces, the pore bodies with the highest aspect ratios, and the pore bodies with the highest coordination numbers. This work shows that, while there may be limitations related to the ability to capture REV-sized domains for some of the multiphase flow properties and phenomena, high-resolution X-ray tomography is able to provide the high quality datasets needed to observe and quantify the pore-scale phenomena and processes that govern multiphase flow in unconsolidated porous media systems.

This paper presents application of a series of algorithms used to extract pore network structure from high-resolution three-dimensional synchrotron microtomography images of unconsolidated porous media systems. These algorithms are based on the three-dimensional skeletonization that simplifies the pore space to networks in the form of nodes connected to paths. Dilation algorithms were developed to generate inscribed spheres on the nodes and paths of the medial axis to represent pore-bodies and pore-throats of the network, respectively. The end result is a physically representative pore network structure, i.e. three-dimensional spatial distribution (i.e. x-, y-, and z-coordinates) of pore-bodies and pore-throats, pore-body size distribution, pore-throat size distribution, and the connectivity. Systems analyzed in this study include different glass bead systems and natural marine sand. The media ranged in size from 0.123 to 1.0 mm, while the image volumes ranged between 7.7 and 108.9 mm3. In addition to extracting the pore network structure, the porosity, specific surface area, and representative elementary volume analysis on the porosity were calculated. Spatial correlation between pore-body sizes in the network was investigated using semivariograms and integral scale concepts. The impact of resolution on the calculated property was also investigated. In this work, we show that microtomography is an effective tool to non-destructively extract the structure of many systems. The quality of the datasets depends on photon energy, photon flux, size of the sample, type of the sample, and size of the sample ‘features’. Results show that the developed method of extracting pore network structure is applicable to ideal and natural porous media systems. The impact of resolution on the quantification of the network structure properties varies in its significance based on feature size of the system and the properties being calculated. Therefore, a thorough resolution sensitivity analysis should be carried out to determine the degree of error associated with a system imaged at a given resolution.

While network models of porous materials have traditionally been constructed using regular or disordered lattices, recent developments allow the direct modeling of more realistic structures such as sphere packings, microtomographic images, or computer-simulated materials. One of the obstacles in these newer approaches is the generation of network structures that are physically representative of the real systems. In this paper, we present and compare two different algorithms to extract pore network parameters from three-dimensional images of unconsolidated porous media systems. The first approach, which utilizes a pixelized image of the pore space, is an extension to unconsolidated systems of a medial-axis based approach (MA). The second approach uses a modified Delaunay tessellation (MDT) of the grain locations. The two algorithms are validated using theoretical packings with known properties and then the networks generated from random packing are compared. For the regular packings, both methods are able to provide the correct pore network structure, including the number, size, and location of inscribed pore bodies, the number, size, and location of inscribed pore throats, and the connectivity. Despite the good agreement for the regular packings, there were differences in both the spatial mapping and statistical distributions in network properties for the random packings. The discrepancies are attributed to the pixelization at low resolution, non-uniqueness of the inscribed pore-body locations, and differences in merging processes used in the algorithms, and serve to highlight the difficulty in creating a unique network from a complex, continuum pore space.

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. U.S. Department of Energy, the National Science Foundation (Contract No. HRD-0503362) and Louisiana Board of Regents (Contract No. LINK-45). Scopus