Bead Packs Research Articles

The coupled dynamics of foam generation and pipe flow

Foam generation in porous media and foam flow through pipes have been studied extensively to date as uncoupled processes. The coupled dynamics of foam generation and pipe flow, however, are comparatively understudied, despite their importance to industrial applications. We consider an experimental system that generates foam by flowing air and surfactant solution through a bead pack, after which the foam flows through a pipe to a downstream pressure chamber. We systematically vary the foam flow rate, bead diameter, bead pack length, pipe length, and chamber pressure. We measure the corresponding impact on flow pressures, densities, velocities, composition, and textures throughout the system, including direct measurements of bubble diameter distributions. We show that with the proper non-dimensionalization, the generator and pipe flows collapse to surprisingly simple friction-factor relationships. We also show that the mean foam bubble diameter exiting the bead pack decreases with increasing flow rate, up to a minimum value set by the pore length-scale K, where K is the bead pack permeability. We also explore the impact of foam texture and composition on the effective foam viscosities within the pipe. Finally, we show that foam properties evolve within the pipe, such that the pipe and bead pack play equally important roles in determining outlet foam properties.

Quantification of Uncertainty and Best Practice in Computing Interfacial Curvature from Complex Pore Space Images.

Recent advances in high-resolution three-dimensional X-ray CT imaging have made it possible to visualize fluid configurations during multiphase displacement at the pore-scale. However, there is an inherited difficulty in image-based curvature measurements: the use of voxelized image data may introduce significant error, which has not—to date—been quantified. To find the best method to compute curvature from micro-CT images and quantify the likely error, we performed drainage and imbibition direct numerical simulations for an oil/water system on a bead pack and a Bentheimer sandstone. From the simulations, local fluid configurations and fluid pressures were obtained. We then investigated methods to compute curvature on the oil/water interface. The interface was defined in two ways; in one case the simulated interface with a sub-resolution smoothness was used, while the other was a smoothed interface extracted from synthetic segmented data based on the simulated phase distribution. The curvature computed on these surfaces was compared with that obtained from the simulated capillary pressure, which does not depend on the explicit consideration of the shape of the interface. As distinguished from previous studies which compared an average or peak curvature with the value derived from the measured macroscopic capillary pressure, our approach can also be used to study the pore-by-pore variation. This paper suggests the best method to compute curvature on images with a quantification of likely errors: local capillary pressures for each pore can be estimated to within 30% if the average radius of curvature is more than 6 times the image resolution, while the average capillary pressure can also be estimated to within 11% if the average radius of curvature is more than 10 times the image resolution.

Ordered macro/micro-porous metal-organic framework of type ZIF-8 in a steel fiber as a sorbent for solid-phase microextraction of BTEX.

An ordered array of macropores on microporous metal-organic framework crystals was developed. This array facilitates analyte diffusion in microextraction applications. A prototypical zeolitic imidazolate framework (ZIF-8) was synthesized in the interstitial voids of a polystyrene bead packing of sub-μm polystyrene beads. After removal of polystyrene by dimethylformamide, a single-crystal ordered macroporous ZIF-8 material (SOM-ZIF-8) was obtained. The resulting μm-sized SOM-ZIF-8 crystals are based on a fully-microporous structure containing a macroporous network. The SOM-ZIF-8 crystals were placed in a stainless-steel fiber and applied as a sorbent for the extraction of benzene, toluene, ethylbenzene, and xylenes (BTEX) by headspace solid-phase microextraction (HS-SPME). The fiber was applied to the HS-SPME of BTEX from wastewater samples followed by GC with flame ionization detection. A Plackett-Burman design and Box-Behnken design were carried out to evaluate the variables affecting the method. Figures of merit include (a) limits of detection of 1.0-12ng·L-1, (b) linear ranges of 0.004-50μg·L-1, (c) relative standard deviations of 4.6-6.7%, and (d) excellent fiber-to-fiber reproducibility (5.6-6.7% for n = 3). Spiking recoveries between 92 and 106% were obtained for BTEX analysis in wastewater samples. The introduction of an ordered macroporous network on microporous ZIF-8 crystals enhanced analyte uptake. This increases the extraction performance by a factor of 2.5-3.1 when compared to analogous ZIF-8 crystals that lack templated macropores. Graphical abstract BTEX extraction is enhanced by templating an ordered macroporous network in microporous crystals as exemplified with the single-crystal ordered macropore zeolitic imidazolate framework-8 (SOM-ZIF-8). Graphical Abstract contains poor quality of image inside the artwork. Please do not re-use the file that we have rejected or attempt to increase its resolution and re-save. It is originally poor, therefore, increasing the resolution will not solve the quality problem. We suggest that you provide us the original format. We prefer replacement figures containing vector/editable objects rather than embedded images. Preferred file formats are eps, ai, tiff and pdf.The file in original format has been attached.

Ultrasonic tracking of a sinking ball in a vibrated dense granular suspension

Observing and understanding the motion of an intruder through opaque dense suspensions such as quicksand remains a practical and conceptual challenge. Here we use an ultrasonic probe to monitor the sinking dynamics of a steel ball in a dense glass bead packing (3D) saturated by water. We show that the frictional model developed for dry granular media can be used to describe the ball motion induced by horizontal vibration. From this rheology, we infer the static friction coefficient and effective viscosity that decrease when increasing the vibration intensity. Our main finding is that the vibration-induced reduction of the yield stress and increase of the sinking depth are presumably due to micro-slips induced at the grain contacts but without visible plastic deformation due to macroscopic rearrangements, in contrast to dry granular packings. To explain these results, we propose a mechanism of acoustic lubrication that reduces the inter-particle friction and leads to a decrease of the yield stress. This scenario is different from the mechanism of liquefaction usually invoked in loosely packed quicksands where the vibration-induced compaction increases the pore pressure and decreases the confining pressure on the solid skeleton, thus reducing the granular resistance to external load.

Efficient hybrid algorithm for the dynamic creation of wormlike chains in solutions, brushes, melts and glasses

We present an updated version of a program that had been published earlier in this journal. The program executes an algorithm for the creation and relaxation of large, dense or diluted homogeneous many particle systems made of wormlike, finite extendable, semiflexible multibead chains and – optionally – solvent particles, which repulse each other. The key feature is its efficiency, its output has been proven to serve as an excellent basis for any subsequent off-lattice computer simulation. The application allows to choose (i) the bead number density or packing fraction, temperature, chain length, system size, concentration, (ii) the interaction potentials, hence the local features such as bond length and bending rigidity of the chains, and (iii) the degree of pre-relaxation, parametrized and expressed through a minimum intermolecular distance. The monodisperse polymers are represented by chains of monomer coordinates in 3D space. During the dynamical two-step process of sample creation the initially (Monte Carlo step 1) predicted global characteristics of the molecular conformations remain as unaffected as possible (during molecular dynamics step 2) and the potential energy and the entropy production are relaxing towards their minima. The potentials, the distribution of bond lengths, the integration time step and temperature are smoothly controlled during the creation/relaxation process until they finally approach their prescribed or physical values. The quality of the algorithm is by its nature independent of concentration, system size or degree of polymerization; the CPU speed is quite independent of the latter quantity and linear in the system size. Chains tethered to a surface (dry polymer brushes) can be generated as well. New version program summaryProgram Title: GenPolProgram Files doi:http://dx.doi.org/10.17632/5kgz4bmt2h.1Licensing provisions: MITProgramming language: Fortran, PerlJournal reference of previous version: Comput. Phys. Commun. 118 (1999) 278.Does the new version supersede the previous version?: The basic methods remained unchanged. The new version runs on modern compilers, and the caller script had been adapted to enter parameters more conveniently.Reasons for the new version: Mainly compatibility issues.Summary of revisions: original k-shell script replaced by Perl script. Outdated Fortran commands replaced by state-of-the-art commands. Resulting configuration is Z1-formatted so that it can directly be used by CPC program Z1 (ADVG).Nature of problem: The problem is to place and relax flexible, semiflexible or stiff, tethered or free model polymer chains within a finite volume with periodic boundaries such that (i) the configurational statistics is obtained from a microscopic potential which determines the local and – affected by concentration and excluded volume – global conformational features of the system in a ‘physical’ way. The resulting configuration obeys a minimum distance criterion.Solution method: In a first step, according to the chosen system parameters, a mixture of phantom and excluded volume chains plus solvent particles is placed into the (finite and final) simulation box by a Monte Carlo algorithm. A subsequent molecular dynamics algorithm solves Newtons equations of motion during the relaxation phase, while the strength of the repulsive and attractive forces, the temperature and integration time step control interact with each other by a global optimization procedure which minimizes the CPU request for the goals (i) the minimum distance between particles is reaching (finally above) its lower limit and (ii) the changes in both local and global conformational properties – as determined by the Monte Carlo procedure – are kept at a very low level. The algorithm interrupts the relaxation process when a break off condition (actually the minimum distance criterion) is fulfilled.Additional comments including restrictions and unusual features: None, except that the machine must provide the needed main memory, proportional to the number of particles. Installation instructions and script syntax are described in the README.txt file accompanying the new version.

Modelling of multispecies reactive transport on pore-space images

We present a new model, named poreReact, to simulate multispecies reactive transport on pore space images. We solve the Navier–Stokes equations and the advection-diffusion equation for concentration on an unstructured grid using the finite volume method implemented in OpenFOAM. We couple it with the chemical model Reaktoro, which we use to calculate the chemical equilibrium of homogeneous reactions in each grid cell, considered as a completely mixed batch reactor. We validate the model against analytical solutions and experimental data, and investigate, for a range of Péclet numbers, the interplay between transport and reaction for multispecies reactive transport in a 3D bead pack where two streams of reactants at different pH are injected in parallel. We analyse the distribution of species and the rates of formation and consumption in the pore space and find that, despite the relative homogeneity of the bead pack and symmetry in injection conditions, the concentration fields of the products can be asymmetric because of the interplay between transport and chemical equilibrium. For different Péclet numbers, we calculate relative yields (the ratio between the observed change in concentration and the change that would be obtained if the reactants were completely mixed). We observe that lower Péclet numbers give rise to higher relative yields because of increased transverse mixing by diffusion. However, higher absolute yields are obtained at higher injection velocities because of the larger amount of matter available for reaction in a given time. Reaction is more favoured in the faster-flowing regions of the pore space. However, this effect is more marked for species for which advection is the dominant mechanism of transport to reactive sites, as opposed to diffusion-mediated reactions where the full velocity distribution is sampled before reaction occurs.

An Experimental Approach to Formulate Lignin-Based Surfactant for Enhanced Oil Recovery

The higher cost of chemical surfactants has been one of the main reasons for their limited used in enhanced oil recovery (EOR) process. Hence, the reason for developing lignin-based surfactant is to lower the cost of chemicals as it does not tie to the price of crude oil as compared to petroleum-based surfactants. Besides, lignin is biodegradable and easily extracted from plant waste. The objectives of this study are to determine the formulations of the lignin-based surfactant for EOR applications and to determine the oil recovery performance of the formulated surfactants through surfactant flooding. The lignin-based surfactants were formulated by mixing the lignin with the amine (polyacrylamide or hexamethylenetetramine) and the surfactant sodium dodecylbenzenesulfonate in a 20,000 ppm NaCl brine. Interfacial tension (IFT) of the formulated lignin-based surfactant is measured at ambient temperature using the spinning drop method. The displacement experiments were conducted at room temperature in glass beads pack holders filled with glass beads, saturated with paraffin and brine. The results of the study showed that the best formulation of lignin-based surfactant is using hexamethylenetetramine as the amine, lignin, and sodium dodecylbenzenesulfonate at 2% total active concentration. The oil recovery and interfacial tension using the lignin amine system is comparable with the commercial petroleum sulfonate system.

Slow-dynamical nonlinearity in a glass bead pack probed with diffuse ultrasound and coda-wave interferometry

Low amplitude diffuse coda ultrasound probe signals received through unconsolidated glass bead packs under static vertical loads are used to measure the bead pack's slow dynamics, in which the speed of low amplitude probe waves varies slowly with time long after loading. Coda-wave interferometry reveals tiny stretches in the waveform, with precision of parts in 105. These stretches are ascribed to decreased contact stiffnesses or decreased sound speeds. The coda-wave stretch is observed to be sensitive to low frequency conditionings at strains of order 10-5. Such conditionings include taps on the sides, low frequency harmonic vibrations and impulses on top, and step changes in vertical load. Typical behavior after the conditioning is an initial loss of stiffness followed by a slow healing that proceeds with the logarithm of time. The initial loss of stiffness and its slow dynamic healing are investigated for sensitivity to sundry parameters.

Diffuse ultrasonic transport in an unconsolidated glass bead pack

We study the transport of diffuse ultrasound with frequencies of hundreds of kHz through random aggregates of d = 3.0 and 1.0 mm diameter spherical glass beads in air under static loads of 100 to 300 kPa. Highdensity polystyrene foam on top and bottom transmits the static loads while maintaining ultrasonic isolation. A floating polystyrene foam wall helps establish a uniform hydrostatic load through the 10 to 70 mm depths. Findings include a band gap extending—for the 3 mm beads—from a lower edge at about 200 kHz (that scales weakly with load and inversely with bead diameter.) Amongst the 3mm beads, we observe an upper edge to the band gap at about 900 kHz corresponding to an optical branch passband associated with the lowest internal resonance of an isolated bead. Higher optical branches are observed also. The lower edge at 200 kHz corresponds well with estimates of the upper band edge for the rotational-wave vibrations of a hexagonal close packed array of beads in Hertzian contact. The observed first arrival times correspond well with Hertzian predictions for low frequency effective longitudinal wavespeeds. Within the low frequency pass band we see diffuse transport, with diffusivities comparable to simple theoretical expectations.

Controlled imbibition in a porous medium from a soft wet material (poultice).

We provide a first approach of the mechanisms of liquid imbibition in a porous medium from a wet paste in contact with this substrate. Through Magnetic Resonance Imaging (MRI) we first show that, in contrast with intuition, the liquid can invade the substrate even if it has a larger pore size than the paste, which induces a lower capillary pressure in the substrate. This phenomenon happens because the paste can easily shrink. We then show that the imbibition stops when the capillary pressure in the substrate balances the stress needed to further contract the paste. The dynamics of the process then mainly results from the competition of these two effects plus the pressure gradient associated with the liquid flow through the paste. This in particular shows that the liquid penetration in a porous medium, from a poultice in contact with this medium, may be controlled by adjusting the poultice characteristics.

Saturation-dependent gas transport in sand packs: Experiments and theoretical applications

Understanding gas transport in porous media and its mechanism has broad applications in various research areas, such as carbon sequestration in deep saline aquifers and gas explorations in reservoir rocks. Gas transport is mainly controlled by pore space geometrical and morphological characteristics. In this study, we apply a physically-based model developed using concepts from percolation theory (PT) and the effective-medium approximation (EMA) to better understand diffusion and permeability of gas in packings of angular and rounded sand grains as well as glass beads. Two average sizes of grain i.e., 0.3 and 0.5 mm were used to pack sands in a column of 6 cm height and 4.9 cm diameter so that the total porosity of all packs was near 0.4. Water content, gas-filled porosity (also known as gas content), gas diffusion, and gas permeability were measured at different capillary pressures. The X-ray computed tomography method and the 3DMA-Rock software package were applied to determine the average pore coordination number z. Results showed that both saturation-dependent diffusion and permeability of gas showed almost linear behavior at higher gas-filled porosities, while deviated substantially from linear scaling at lower gas saturations. Comparing the theory with the diffusion and permeability experiments showed that the determined value of z ranged between 2.8 and 5.3, not greatly different from X-ray computed tomography results. The obtained results clearly indicate that the effect of the pore-throat size distribution on gas diffusion and permeability was minimal in these sand and glass bead packs.

Multidimensional Observations of Dissolution-Driven Convection in Simple Porous Media Using X-ray CT Scanning

We present an experimental study of dissolution-driven convection in a three-dimensional porous medium formed from a dense random packing of glass beads. Measurements are conducted using the model fluid system MEG/water in the regime of Rayleigh numbers, Ra=2000{-}5000. X-ray computed tomography is applied to image the spatial and temporal evolution of the solute plume non-invasively. The tomograms are used to compute macroscopic quantities including the rate of dissolution and horizontally averaged concentration profiles, and enable the visualisation of the flow patterns that arise upon mixing at a spatial resolution of about (2times 2times 2),hbox {mm}^3. The latter highlights that under this Ra regime convection becomes truly three-dimensional with the emergence of characteristic patterns that closely resemble the dynamical flow structures produced by high-resolution numerical simulations reported in the literature. We observe that the mixing process evolves systematically through three stages, starting from pure diffusion, followed by convection-dominated and shutdown. A modified diffusion equation is applied to model the convective process with an onset time of convection that compares favourably with the literature data and an effective diffusion coefficient that is almost two orders of magnitude larger than the molecular diffusivity of the solute. The comparison of the experimental observations of convective mixing against their numerical counterparts of the purely diffusive scenario enables the estimation of a non-dimensional convective mass flux in terms of the Sherwood number, Sh=0.025Ra. We observe that the latter scales linearly with Ra, in agreement with both experimental and numerical studies on thermal convection over the same Ra regime.

Electrochemical Peroxide Generation for in Situ Disinfection

Novel technologies with limited earth support are required to enable energy-efficient maintenance of closed air, water, and waste systems in microgravity spacecraft habitats such as International space station (ISS). One area of need for the interstellar travel community is cleaning/sanitizing supplies to meet personal hygiene requirements, which is currently accomplished through the use of pre-packaged, disposable, wetted wipes, or ion exchange cartridges. These items represent an appreciable carry-along mass and disposal/replacement burden requiring earth based support. Therefore, if a system could be developed to use onboard utilities to create cleaning and disinfecting solutions it would reduce the astronaut’s dependence on earth based supplies. We are addressing this challenge by demonstrating a technology that will use utilities on-board the spacecraft habitat to create sanitizing solutions and eliminate the need for regular delivery of disinfecting wipes. This concept is founded on the electrochemical reduction of oxygen to hydrogen peroxide using readily available on-board supplies of O2 and water. Peroxide is well-established disinfectant with non-toxic decomposition products (viz., O2 and H2O), that is safe enough for human contact to be sold commercially as a 3% w/w solution. Thus, it is an ideal disinfecting solution for closed space environments. The electrochemical reduction of oxygen to hydrogen peroxide occurs by two electron transfer reactions which proceeds as [O2 + 2H+ + 2e- → H2O2 or O2 + 2H2O + 2e- → H2O2 + 2OH-][1]. We designed and constructed an electrochemical reactor for generating peroxide that is compatible with the available low conductivity water stream based on a patented concept of one of our commercialization partners (De Nora Technology, Inc. U.S. patent number 6,254,762). The peroxide generation unit (PGU) system consists of oxygen chamber, catholyte chamber (with catalyzed GDE cathode[2]), whose volume can be modified as desired by changing the catholyte plate (width/length) and size/density of the NAFION bead packing material, and anolyte chamber with commercially-available mixed-metal oxide anode pressed against a cation exchange membrane, as shown in the conceptual schematic (Figure - left) and the modified PGU (Figure - right). Initial trials confirmed that the PGU can generate peroxide utilizing high conductivity sodium sulfate as catholyte. Further, the PGU was used to demonstrate the potential of generating peroxide in very low conductivity electrolytes such as distilled water (DI) and reverse osmosis water (RO) by enabling transfer conductivity through the use of NAFION ionomer “beads”. These results are promising and an early indication that the PGU will be able to generate peroxide with high performance efficiency. Faraday is continuing the technology development efforts by: (1) demonstrating the potential of the device design by testing under zero gravity; (2) optimizing the gas diffusion electrode (GDE) catalytic structure, and wettability, (3) demonstrating the potential to use onboard International Space Station (ISS) water utilities, (4) developing an electrochemical peroxide detector, and (5) designing and building an α-scale reactor to produce 1 L/day of 2 w/w% H2O2. Some of the results aimed at evaluating the system in zero-gravity environments will be presented at the conference. In addition to being an integral component of long term life support on NASA manned space missions; this technology has the potential to be an alternative method for synthesis of commercial hydrogen peroxide for on-demand onsite disinfection of process waste streams and/or field remediation systems. Acknowledgements: The financial support of NASA Contract No. NNX16CA43P and NNX17CJ12C is acknowledged. We would also like to thank Dr. Jeffrey J. Sweterlitsch from NASA Johnson Space Center (Houston, TX), who provided insight and expertise that greatly assisted the research

Displacement and Dissolution Characteristics of CO $$_{2}$$ 2 /Brine System in Unconsolidated Porous Media

This study investigated the dynamic displacement and dissolution of \(\hbox {CO}_{2}\) in porous media at 313 K and 6/8 MPa. Gaseous (\(\hbox {gCO}_{2}\)) at 6 MPa and supercritical \(\hbox {CO}_{2 }(\hbox {scCO}_{2}) \) at 8 MPa were injected downward into a glass bead pack at different flow rates, following upwards brine injection. The processes occurring during \(\hbox {CO}_{2}\) drainage and brine imbibition were visualized using magnetic resonance imaging. The drainage flow fronts were strongly influenced by the flow rates, resulting in different gas distributions. However, brine imbibition proceeded as a vertical compacted front due to the strong effect of gravity. Additionally, the effects of flow rate on distribution and saturation were analyzed. Then, the front movement of \(\hbox {CO}_{2}\) dissolution was visualized along different paths after imbibition. The determined \(\hbox {CO}_{2}\) concentrations implied that little \(\hbox {scCO}_{2}\) dissolved in brine after imbibition. The dissolution rate was from \(10^{-8}\) to \(10^{-9}\, \hbox {kg}\, \hbox {m}^{-3} \, \hbox {s}^{-1}\) and from \(10^{-6}\) to \(10^{-8}\, \hbox {kg}\, \hbox {m}^{-3} \, \hbox {s}^{-1}\) for \(\hbox {gCO}_{2}\) at 6 MPa and \(\hbox {scCO}_{2 }\) at 8 MPa, respectively. The total time for the \(\hbox {scCO}_{2}\) dissolution was short, indicating fast mass transfer between the \(\hbox {CO}_{2}\) and brine. Injection of \(\hbox {CO}_{2}\) under supercritical conditions resulted in a quick establishment of a steady state with high storage safety.

Evaporation with the formation of chains of liquid bridges

The objective of the present work is to study the drying of a quasi-two-dimensional model porous medium, hereafter called the micromodel, initially filled with a pure liquid. The micromodel consists of cylinders measuring $50~\unicode[STIX]{x03BC}\text{m}$ in both height and diameter, radially arranged as a set of neighbouring spirals and sandwiched between two horizontal flat plates. As drying proceeds, air invades the pore space and elongated liquid films trapped by capillary forces form along the spirals. These films consist of ‘chains’ of liquid bridges connecting neighbouring cylinders. They provide hydraulic connectivity between the central bulk liquid cluster and the external rim of the cylinder pattern, where evaporation takes place during a first constant-evaporation-rate drying stage. The first goal of the present paper is to describe experimentally the phase distribution during drying, notably the evolution of liquid films, which controls the evaporation kinetics (e.g. the depinning of the films from the external rim signals the end of the constant-evaporation-rate period). Then, a viscocapillary model for the drying process is presented. It is based on numerical simulations of a liquid film capillary shape and viscous flow within a film. The model shows a reasonably good agreement with the experimental data. Thus, the present study is a step towards direct modelling of the effect of films on the drying of more complex porous media (e.g. packing of beads) and should be of interest for multiphase flow applications in porous media, involving transport within liquid films.

High-efficiency removal of NOx using dielectric barrier discharge nonthermal plasma with water as an outer electrode

With the rapid increase in the number of cars and the development of industry, nitrogen oxide (NOx) emissions have become a serious and pressing problem. This work reports on the development of a water-cooled dielectric barrier discharge reactor for gaseous NOx removal at low temperature. The characteristics of the reactor are evaluated with and without packing of the reaction tube with 2 mm diameter dielectric beads composed of glass, ZnO, MnO2, ZrO2, or Fe2O3. It is found that the use of a water-cooled tube reduces the temperature, which stabilizes the reaction, and provides a much greater NO conversion efficiency (28.8%) than that obtained using quartz tube (14.1%) at a frequency of 8 kHz with an input voltage of 6.8 kV. Furthermore, under equivalent conditions, packing the reactor tube with glass beads greatly increases the NO conversion efficiency to 95.85%. This is because the dielectric beads alter the distribution of the electric field due to the influence of polarization at the glass bead surfaces, which ultimately enhances the plasma discharge intensity. The presence of the dielectric beads increases the gas residence time within the reactor. Experimental verification and a theoretical basis are provided for the industrial application of the proposed plasma NO removal process employing dielectric bead packing.

Reconstruction of three-dimensional porous media using generative adversarial neural networks.

To evaluate the variability of multiphase flow properties of porous media at the pore scale, it is necessary to acquire a number of representative samples of the void-solid structure. While modern x-ray computer tomography has made it possible to extract three-dimensional images of the pore space, assessment of the variability in the inherent material properties is often experimentally not feasible. We present a method to reconstruct the solid-void structure of porous media by applying a generative neural network that allows an implicit description of the probability distribution represented by three-dimensional image data sets. We show, by using an adversarial learning approach for neural networks, that this method of unsupervised learning is able to generate representative samples of porous media that honor their statistics. We successfully compare measures of pore morphology, such as the Euler characteristic, two-point statistics, and directional single-phase permeability of synthetic realizations with the calculated properties of a bead pack, Berea sandstone, and Ketton limestone. Results show that generative adversarial networks can be used to reconstruct high-resolution three-dimensional images of porous media at different scales that are representative of the morphology of the images used to train the neural network. The fully convolutional nature of the trained neural network allows the generation of large samples while maintaining computational efficiency. Compared to classical stochastic methods of image reconstruction, the implicit representation of the learned data distribution can be stored and reused to generate multiple realizations of the pore structure very rapidly.

Incoherent Waves in Fluid-Saturated Sintered Granular Systems: Scattering Phenomena

The incoherent transport of ultrasound waves in water-saturated sintered glass bead packings is experimentally investigated. The spectral energy density of scattered high-frequency waves is explained by a diffusion wave equation. Immersion broadband transducers with central frequencies of 1 MHz are positioned at a distance of 73 mm to the porous sample. The diffusion coefficient and quality factor are predicted from a diffusion approximation of the time-dependent intensity curve to the ensemble-averaged measurement data. From the diffusion coefficient, we deduce a mean-free path for scattering events at l*=0.87±0.03 mm close to the range of particle diameters of the samples (1.0<dp<1.2 mm). Results are in good agreement with observations from Jia (2004, “Codalike Multiple Scattering of Elastic Waves in Dense Granular Media,” Phys. Rev. Lett., 93(15), p. 154303) observed for nonsintered and consolidated bead packings (0.6<dp<0.8 mm). The low-quality factor Q=190±10 indicates a high amount of intrinsic damping of the scattered waves although water was used as saturating and coupling fluid.

Pore‐scale investigation of effects of heterogeneity on CO2 geological storage using stratified sand packs

AbstractThe geological sequestration of CO2 is considered to have great potential. However, the heterogeneity that characterizes most geological reservoirs significantly affects the migration, trapping, and leakage of CO2. The present study involved a pore‐scale investigation of the effects of heterogeneity in pore size and wettability using stratified sand packs. A stratified glass bead (BZ06+BZ02) pack and a quartz+dolomite pack were introduced in flow experiments at gaseous CO2 (gCO2) and supercritical CO2 (scCO2) conditions. Different injection rates, namely, 0.1, 0.3, and 0.6 mL/min, were applied to the drainage while a constant rate of 0.1 mL/min was maintained for the imbibition. High resolution CT imaging and subsequent processing of the images were used to analyze the effects of heterogeneity in pore size and wettability. The results indicate that heterogeneity in pore size and wettability alters the intrinsic percolation abilities of the individual layers of the stratified structure and weaken the linear correlation between CO2 volume and slice‐averaged porosity. It was also observed that water films sometimes exist in the layers with smaller pore spaces after drainage and it can trigger snap off events. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Multidimensional Imaging of Density Driven Convection in a Porous Medium

Carbon dioxide (CO2) sequestration is a climate change mitigation technique which relies on residual and solubility trapping in injection locations with saline aquifers. The dissolution of CO2 into resident brines results in density-driven convection which further enhances the geological trapping potential. We report on the use of an analogue fluid pair to investigate density-driven convection in 3D in an unconsolidated bead pack. X-ray computed tomography (CT) is used to image density-driven convection in the opaque porous medium non-invasively. Two studies have been conducted that differ by the Rayleigh number (Ra) of the system, which in this study is changed by altering the maximum density difference of the fluid pair. We observe the same general mixing pattern in both studies. Initially, many high density fingers move downward through the bead pack and as time progresses these coalesce and form larger dominate flow paths. However, we also observe that a higher Rayleigh number leads to the denser plume moving faster towards the bottom of the system. Due to the finite size of the system, this in turn leads to early convective shut-down.