Water-saturated Porous Media Research Articles

Modeling coupled nanoparticle aggregation and transport in porous media: A Lagrangian approach

Changes in nanoparticle size and shape due to particle–particle interactions (i.e., aggregation or agglomeration) may significantly alter particle mobility and retention in porous media. To date, however, few modeling studies have considered the coupling of transport and particle aggregation processes. The majority of particle transport models employ an Eulerian modeling framework and are, consequently, limited in the types of collisions and aggregate sizes that can be considered. In this work, a more general Lagrangian modeling framework is developed and implemented to explore coupled nanoparticle aggregation and transport processes. The model was verified through comparison of model simulations to published results of an experimental and Eulerian modeling study (Raychoudhury et al., 2012) of carboxymethyl cellulose (CMC)-modified nano-sized zero-valent iron particle (nZVI) transport and retention in water-saturated sand columns. A model sensitivity analysis reveals the influence of influent particle concentration (ca. 70 to 700mg/L), primary particle size (10–100nm) and pore water velocity (ca. 1–6m/day) on particle–particle, and, consequently, particle–collector interactions. Model simulations demonstrate that, when environmental conditions promote particle–particle interactions, neglecting aggregation effects can lead to under- or over-estimation of nanoparticle mobility. Results also suggest that the extent to which higher order particle–particle collisions influence aggregation kinetics will increase with the fraction of primary particles. This work demonstrates the potential importance of time-dependent aggregation processes on nanoparticle mobility and provides a numerical model capable of capturing/describing these interactions in water-saturated porous media.

An experiment study on fluid heat and mass transfer properties in porous media using MRI

The objective of this study was to understand fluid heat and mass transfer processes in porous media with different pore structures. High-resolution Magnetic Resonance Imaging was used to measure fluid flow velocity and temperature maps in porous media. Firstly, three orthogonal velocity components (Vx, Vy, and Vz) of single phase flow measurement were evaluated. The flow distribution in porous media is rather heterogeneous, and it is consistent with heterogeneous pore structure, and the velocity in large pore is high. Then we presented initial results from the extension of this work to two-phase flow. The CO2 channeling phenomena were obvious. And the CO2 velocity was calculated from saturation of water. Finally, the linearity relationship between temperature and the MRI parameter was determined for porous media, and we measured the temperature distribution of water saturated porous media. The study provides useful data for heat and mass process during CO2 storage.

PH, geoelectrical and membrane flux parameters for the monitoring of water-saturated silicate and carbonate porous media contaminated by CO2

Characteristics of potable water aquifer contaminated by CO2 are investigated using well-defined laboratory experiments. The porous media domain was prepared with silica sand and limestone in separate experiments. The investigations used combinations of techniques to measure various parameters in the water-saturated porous media domain on which pressure of CO2 was imposed, under various conditions, which correspond to different geological depths. Measured parameters included the pH, geoelectrical parameters, and the diffusion of the CO2 gas through the water-saturated porous media domain using non-porous silicone rubber sheet. Experimental results revealed the existence of three stages in the profile of pH change with time as CO2 dissolved and diffused in the water-saturated porous media domain, which was composed of silica sand. The first stage was characterised by rapid decline in the pH. This is associated with quick dissolution of CO2 and the formation of carbonic acid together with bicarbonate. The second stage showed short rise in pH value, which was attributed to the reverse reaction, i.e., the formation of aqueous and gaseous CO2 and water from the carbonic acid. The third stage was that of the equilibrium in the forward and the reverse reactions, marked by steady state in pH value, which remained unchanged till the end of the experiment. The bulk electrical conductivity (σb) of the water-saturated porous domain increased in the presence of CO2. This is attributed to the formation of ionic species, especially bicarbonate, as CO2 dissolved in the domain. The rise in σb coincided with the first stage of the change in the pH of the system. In addition, the σb was higher in limestone than silica sand, and it increased with pressure of the domain. But, the bulk dielectric constant (εb) showed no change with the dissolution of the CO2 under different conditions. Furthermore, permeation of CO2 through the silicone rubber indicated the diffusion of the CO2 gas through the water-saturated domain. CO2 flux through the membrane was shown to increase with depth or pressure of the domain. A mathematical expression derived in this work shows the dependence of σb on the pH and the initial value of σb. Predictions of the changes in the σb for different porous domains show the reliability of the mathematical expression developed in this work.

Effects of Humic Acid and Solution Chemistry on the Retention and Transport of Cerium Dioxide Nanoparticles in Saturated Porous Media

When released into natural aquatic systems, cerium oxide (CeO2) nanoparticles (NPs) may have toxic effects to the ecosystems and public health; it is thus important to understand their environmental fate and transport. This work studied the effects of humic acid (HA) concentrations (0–10 mg L−1) and solution chemistry (ionic strength (IS) and pH) on the retention and transport of CeO2 NPs in water-saturated porous media under environmental relevant conditions. HA and IS showed remarkable effect on the retention and transport of CeO2 NPs in the porous media. Even at low concentrations (i.e., 5 and 10 mg L−1), HA stabilized CeO2 NPs in the suspensions by introducing both negative surface charge and steric repulsion and thus enhanced their mobility in the porous media. When solution HA concentration increased or ionic strength decreased, mobility of CeO2 NPs in the porous media enhanced dramatically. Solution pH, however, had little influence on the mobility of the CeO2 NPs under the tested experimental conditions, and increasing solution pH only slightly increased the transport of the NPs. Mathematical models were applied to describe the experimental data. Predictions from the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory and advection–dispersion–reaction (ADR) model matched the experimental data well.

Influence of residual polymer on nanoparticle deposition in porous media.

Although surface coatings and free polymers are known to affect the mobility of nanoparticles in water-saturated porous media, the influence of these compounds on nanoparticle deposition behavior has received limited attention. A series of column experiments was conducted to evaluate the transport and retention of quantum dots (QDs) coated with a synthetic polymer, polyacrylic acid-octylamine (PAA-OA). Initial column studies, conducted with three size fractions of Ottawa sand, resulted in unusual solid-phase retention profiles, characterized by low QD deposition near the column inlet and increasing solid-phase concentrations along the column until a plateau or limiting capacity was reached near the column midpoint. Mathematical modeling studies indicated that the observed retention behavior could not be reproduced using one-dimensional simulators based on either clean-bed filtration theory or a modified filtration theory (MFT) model that incorporated a maximum retention capacity. Additional column studies demonstrated that changes in the inlet end plate configuration designed to ensure uniform flow did not alter the observed effluent breakthrough curves (BTCs) or shape of the retention profile. Subsequent QD transport experiments, pretreated by flushing with a pulse of PAA-OA solution, resulted in almost complete QD breakthrough with minimal retention. It is postulated that free polymer was preferentially adsorbed onto the solid surface near the column inlet, thereby preventing QD attachment, whereas in the down-gradient portion of the column, QDs attached to the solid phase without competition from the polymer. These findings reveal the importance of accounting for the influence of coconstituents on nanoparticle deposition and demonstrate the need to simulate both transport and retention data when assessing nanoparticle mobility in porous media.

Transport and Deposition of Alumina Nanoparticles in Water Saturated Porous Media: An Experimental Study

Transport and deposition of nanoparticles (NPs) have drawn great attention in different fields of engineering in recent years because of their potential risk to the environment. In this study, mobility of aluminum oxide NP, one of the most popular metal oxide NPs, was studied in synthetic saturated porous media. The impacts of ionic strength using monovalent (NaCl) and divalent (MgCl2) salt solution, pH, and NPs concentrations on nano-alumina mobility were investigated. The experimental results showed that the transport of nano-alumina was influenced by ionic strength; the highest mobility was observed at the 0.01 M solution and decreased by increasing ionic strength. Moreover, the suspension containing alumina NPs presented the best mobility behavior in the acidic solution (pH = 2) in comparison with neutral and basic solutions. Consequently, transport of NPs with particle size distribution lower than 100 nm through porous media was facilitated at the acidic, low ionic strength condition. In addition, faster elution occurred when the influent concentration was increased from 0.1 wt% to 0.3 wt%. Finally, it is expected that ionic strength, pH of solution, and NPs concentration will be key parameters to control the nano-alumina mobility.

Storage of CO2 in a partially water saturated porous medium at gas hydrate formation conditions

Storage of CO2 in depleted natural gas reservoirs is considered an attractive option to mitigate climate change concerns of CO2 from sources such as thermal power plants. In this regard, a number of depleted gas reservoirs located in Northern Alberta, Canada have been identified as potential sites for CO2 storage at temperature and pressure conditions at which CO2 forms gas hydrate crystals in the presence of water. This offers the possibility of storing more CO2 than what can be normally stored as a gas at the same conditions. The injection of CO2 in a bed of silica sand partially saturated with water (wet sand) at pressure and temperature conditions similar to those in a typical depleted gas reservoir in Northern Alberta was investigated in this work. The CO2 gas was injected into the simulated reservoir under the gas cap mode and the spiral tube mode procedure. Through the pressure drop measurements the CO2 gas consumption was determined and also the amount of water converted to hydrate was calculated. The results indicate that 39–47% of water formed hydrate in the gas cap mode gas injection experiments and 51–55% in the spiral tube mode injection experiments. Hydrate saturation reached 0.11–0.15 after 24h for each gas injection mode. Finally, the capacity of the laboratory reservoir to store CO2 in various forms was determined.

Numerical simulation of two-phase fluid flow

We simulate two-phase fluid flow using a stress–strain relation based on Biot’s theory of poroelasticity for partial saturation combined with the mass conservation equations. To uncouple flow and elastic strain, we use a correction to the stiffness of the medium under conditions of uniaxial strain. The pressure and saturation differential equations are then solved with an explicit time stepping scheme and the Fourier pseudospectral method to compute the spatial derivatives. We assume an initial pressure state and at each time step compute the wetting- and non wetting-fluid pressures at a given saturation. Then, we solve Richards’s equation for the non wetting-fluid saturation and proceed to the next time step with the updated saturations values. The pressure and saturation equations are first solved separately and the results compared to known analytical solutions showing the accuracy of the algorithm. Then, the coupled system is solved. In all the cases, the non-wetting fluid is injected at a given point in space as a boundary condition and capillarity effects are taken into account. The examples consider oil injection in a water-saturated porous medium.

Multi-scale Experimentation and Numerical Modeling for Process Understanding of CO2 Attenuation in the shallow subsurface

Abstract The goal of this study is to quantify the effects of geologic heterogeneity on the dynamic process of CO 2 attenuation in shallow aquifers through the use of multi-scale laboratory experimentation and numerical modelling. First, an experiment was conducted in an intermediate scale two-dimensional cell that was packed with water-saturated porous media in a simple heterogeneous configuration. Constant-head boundary conditions were applied to the system, and then CO 2 -saturated water was injected through it. As the CO 2 -saturated water migrated through the system, gas phase CO 2 evolved within the porous media, as observed by dielectric saturation sensors. The outflow of CO 2 gas from the top of the system was monitored by a gas flow meter, and the outflow of water from the side of the tank was monitored via collection into a container that was placed on a computer-interfaced scale. After the multiphase CO 2 evolution appeared to have reached steady state, clean water was injected through the cell until all of the CO 2 was expelled from the system. In addition to the experiments, numerical models were performed using the Finite Element Heat and Mass transfer (FEHM) code as a first step toward planning larger scale experiments. Results indicate that the presence of geologic heterogeneity, which is ubiquitous in natural environments, can significantly hinder the migration of CO 2 in the shallow subsurface.

CO2/Water Displacement in Porous Medium Under Pressure and Temperature Conditions for Geological Storage

This paper presented CO2 injected into water saturated porous media for immiscible displacement using high-resolution MRI. The porous medium was formed from the packed bed of glass beads 0.2mm. CO2/water displacement processes were implemented by a spin echo sequence with MRI technique. CO2/water displacement process was investigated for different flow rates. Then CO2 and water distribution were distinguished from MRI images. Then residual water saturation were obtained at steady state. The effects of capillary number were investigated. The study provides useful data for transport process in CO2 storage.

A dual-porosity theory for solute transport in biofilm-coated porous media

In this work, we derive a Darcy-scale model for solute transport in porous media colonized by biofilms. Biofilms refer to structured communities of microbial cells that grow attached to an aqueous interface, e.g., the pore walls of a water-saturated porous medium. First, we present the pore-scale description of mass transport within and between the phases (water phase and biofilm phase) including the biologically-mediated reactions. Then, a two-equation Darcy-scale mass balance model (classically referred to as dual-porosity model), suitable both for local equilibrium and non-equilibrium conditions, is obtained from the method of volume averaging. Application of this macroscopic model to complex pore geometry of a two-dimensional porous medium supporting biofilms is presented. Comparisons with direct numerical simulation results confirm the large range of validity of the model. Finally, an illustrative example based on a scenario of reactive transport experimental tests is used to point out conditions for when the use of the proposed model is required.

Particle transport within water-saturated porous media: Effect of pore size on retention kinetics and size selection

The transport and filtration behaviour of fine particles (silt) in columns packed with sand was investigated under saturated conditions by using step-input injections. Three samples of different particle size distributions (coarse medium, fine medium and a mixture of both) were used in order to highlight the influence of the pore size distribution on particle retention and size selection of recovered particles. The main parameters of particle transport and deposition were derived from the adjustment of the experimental breakthrough curves by an analytical model. The higher particle retention occurs in the mixture medium, owing to its large pore size distribution, and the filtration coefficient decreases with increasing flow velocity. Particle size distribution of recovered particles shows a thorough size selection: (i) the first recovered particles are the coarser ones; (ii) the size of the recovered particles increases with increasing flow velocity and enlarger pore distribution of the medium.

Numerical simulation of flow processes in liquefied soils using a soil–water-coupled smoothed particle hydrodynamics method

Liquefaction can result in the damage or collapse of structures during an earthquake and can therefore be a great threat to life and property. Many site investigations of liquefaction disasters are needed to study the large-scale deformation and flow mechanisms of liquefied soils that can be used for performance assessments and infrastructure improvement. To overcome the disadvantages of traditional flow analysis methods for liquefied soils, a soil–water-coupled smoothed particle hydrodynamics (SPH) modeling method was developed to analyze flow in liquefied soils. In the proposed SPH method, water and soil were simulated as different layers, while permeability, porosity, and interaction forces could be combined to model water-saturated porous media. A simple shear test was simulated using the SPH method with an elastic model to verify its application to solid phase materials. Subsequently, the applicability of the proposed SPH modeling method to the simulation of interaction forces between water and soil was verified by a falling-head permeability test. The coupled SPH method produced good simulations for both the simple shear and falling-head permeability tests. Using a fit-for-purpose experimental apparatus, a physical flow model test of liquefied sand has been designed and conducted. To complement the physical test, a numerical simulation has been undertaken based on the soil–water-coupled SPH method. The numerical results correspond well with the physical model test results in observed configurations and velocity vectors. An embankment failure in northern Sweden was selected so that the application of the soil–water-coupled SPH method could be extended to an actual example of liquefaction. The coupled SPH method simulated the embankment failure with the site investigation well. They have also estimated horizontal displacements and velocities, which can be used to greatly improve the seismic safety of structures.

Coupled multiphase flow and elasto-plastic modelling of in-situ gas injection experiments in saturated claystone (Mont Terri Rock Laboratory)

Understanding gas migration in host rock is one of the most important issues to consider in the final disposal of radioactive waste in deep geological formations. In order to investigate gas transport in water-saturated argillaceous rock, constant gas pressure injection tests on the samples from the Mont Terri Rock Laboratory (Switzerland), as well as pulse injection tests with a stepwise increasing of gas pressure, were carried out. During gas migration, desaturation and hydraulic mechanical interaction can be observed. To simulate gas migration in water-saturated porous media, a coupled multiphase flow and mechanical method within the FEM code family OpenGeoSys (OGS) was developed including the van Genuchten function for the relationship between capillary pressure and water saturation, the approach of Mualem for the relative permeability of both gas and water phases, and an elasto-plastic deformation model. Two approaches were applied, taking into consideration the permeability change by pore gas pressure and by deformation, respectively. Both permeability approaches are based on experimental observations and allow for the significant increase in permeability when gas pressure exceeds the minimal stress. The material parameters for the multiphase model are determined using the measured data from laboratory experiments under well-defined test conditions. With the calibrated model parameters, the in-situ experiment was simulated. Good agreement between the numerical results and the experimental data is achieved.

Experimental and Numerical Studies of CO2 Injection Into Water-Saturated Porous Medium: Capillary to Viscous to Fracture Fingering Phenomenon

Injection of anthropogenic carbon dioxide (CO2) into deep saline geological formations is a possible long-term solution of removing CO2 from the atmosphere. The early injection of CO2 into saline aquifer is known as drainage and can cause unstable fingers to extend into the saturated rock pores. In this work, the crossover from capillary invasion to viscous fingering to fracturing is studied. Experimentally, a vertical Hele-Shaw cell filled with micro-beads was set up to study different injection rates of gas invasion into saturated porous media. The visualized invasion patterns showed that the crossover from capillary to viscous to fracture can occur in the same vertical cell but at different heights due to gas bursts occurring at different overburden and pore pressures. Numerically, 2D simulations of the invasion using STOMP (Subsurface Transport Over Multiple Phases) overpredicted the width of fingers obtained experimentally due to the averaging process of the model.

Influence of Capillary Pressure and Injection Rate as well as Heterogeneous and Anisotropic Permeability on CO2 Transport and Displacement Efficiency in Water-Saturated Porous Media

Greenhouse gas emission from industrial production and human living has been widely recognized as one of the primary reasons which cause global climate warming. CO2 sequestration in saline aquifers plays an important role in effectively reducing the greenhouse gas emission. Numerical modeling can be used to quantitatively analyze the effect of some formation properties on the storage processes of CO2 in saline aquifers. Magnetic resonance imaging (MRI) technology may be used to observe the evolution of CO2 plume through time in porous rocks and hence provide an effective means for validate the reliability of the numerical modeling. Based on a combined use of the numerical modeling and the MRI measurement of the vertical injection of the supercritical CO2 into water-saturated rock cores, this study presents a lab-scale investigation on the influence of the heterogeneous and anisotropic permeability, the CO2 injection rate and capillary pressure on the transport of CO2 plume as well as the CO2 utilization efficiency of subsurface space in saline aquifers. Our results indicate: 1) both the heterogeneity and the anisotropy of permeability can enhance the CO2 displacement efficiency. Ignoring these two permeability properties can cause the obvious deviation in the predicted CO2 saturation and transport processes by the model; 2) the enhanced sequestration efficiency by the anisotropy depends on the degree and spatial extension of itself. If the approximately stochastically distributed permeability anisotropy represents natural conditions, specifying the constant ratio of the vertical permeability to the horizontal one in the model to mimic the real permeability can lead to the obvious over-estimation of the CO2 displacement efficiency. Besides, the displacement front of the CO2 plume tends to pass through the region with the higher vertical permeability, thus forming the anomalistic shape of the displacement front; 3) the capillary pressure can influence the CO2 utilization efficiency in the rock core significantly, and tends to inhibit the movement of pore water, which accordingly lowers the CO2 displacement efficiency; 4) the CO2 injection rate can largely affect the CO2 displacement efficiency. The final CO2 saturation of CO2 in the rock core is notably enhanced with the increasing injection rate.

Experimental and Numerical Investigation of CO<sub>2</sub> Injection into Water-Saturated Porous Media: Effect of Capillary Pressure on Displacement Efficiency

CO2 sequestration in deep saline aquifers is regard as the most promising option among all the CO2 storage technologies. Capillary pressure can influence the CO2 storage efficiency in the aquifers. The core-scale experimental and numerical simulation studies are usually used to understand the mechanism and degree of such influence. Based on both magnetic resonance imaging (MRI) technique and numerical simulation method, this study investigates the effect of capillary pressure on the CO2 displacement efficiency in water-saturated porous media especially in quantitative form. Our results indicate: (1) the magnitude of capillary pressure may significantly affect the CO2-water displacement efficiency, and the displacement efficiency declines with increasing capillary pressure; (2) Sensitivity of the numerical model to capillary pressure becomes more unobvious with increasing capillary pressure. Thus, an accurate capillary pressure parameter is particularly required for improving the reliability of the model predictions in the case of the high permeability porous media.

Cotransport of clay colloids and viruses in water saturated porous media

This study examines the cotransport of clay colloids and viruses in laboratory packed columns. Bacteriophages MS2 and ΦX174 were used as model viruses, kaolinite (kGa-1b) and montmorillonite (STx-1b) as model clay colloids, and glass beads as model packing material. The combined and synergistic effects of clay colloids and pore water velocity on virus transport and retention in porous media were examined at three pore water velocities (0.38, 0.74, and 1.21cm/min). The results indicated that the mass recovery of viruses and clay colloids decreased as the pore water velocity decreased; whereas, for the cotransport experiments no clear trend was observed. Temporal moments of the breakthrough concentrations suggested that the presence of clays significantly influenced virus transport and irreversible deposition onto glass beads. Mass recovery values for both viruses, calculated based on total virus concentration in the effluent, were reduced compared to those in the absence of clays. The transport of both suspended and attached onto suspended clay-particles viruses was retarded, compared to the tracer, only at the highest pore water velocity. Moreover both clay colloids were shown to hinder virus transport at the highest pore water velocity. At the lower velocities MS2 transport was hindered and ΦX174 transport was facilitated with the exception of U=0.74cm/min in the presence of KGa-1b. Both MS2 and ΦX174 were attached in greater amounts onto KGa-1b than STx-1b. Also, MS2 exhibited greater affinity than ΦX174 for both clays.

Effects of micro-heterogeneity and hydrodynamic dispersion on the dissolution rate of carbon dioxide in water-saturated porous media

The CO2 dissolution rate is measured in water-saturated porous media by monitoring the transient variation of gas pressure in an isolated CO2 storage tank. Columns of uniform beads and coarse-grained/poorly sorted sand are used as model porous media. For the interpretation of the experimental results, a macroscopic mathematical model combining the advection–dispersion equation with Darcy law and Boussinesq approximation is solved numerically with the aid of Flex-PDE software package The numerical simulations are used to examine the sensitivity of CO2 dissolution rate to variations in hydrodynamic dispersion parameters, and spatial distribution of permeability, both induced by micro-heterogeneities. The experiments reveal that CO2 dissolution accelerates with the micro-heterogeneity strengthening. The observed behavior can be interpreted by macroscopic simulations only if the correlation of hydrodynamic dispersion parameters with the statistics of the pore (or grain) size distribution (quantifying the micro-heterogeneity) is taken into account.

Transport behavior of functionalized multi-wall carbon nanotubes in water-saturated quartz sand as a function of tube length

A series of one-dimensional column experiments was conducted to examine the effects of tube length on the transport and deposition of 4-ethoxybenzoic acid functionalized multi-wall carbon nanotubes (MWCNTs) in water-saturated porous media. Aqueous MWCNTs suspensions were prepared to yield three distributions of tube lengths; 0.02–1.3 μm (short), 0.2–7.5 μm (medium), and 0.2–21.4 μm (long). Results of the column studies showed that MWCNT retention increased with increasing tube length. Nevertheless, more than 76% of the MWCNT mass delivered to the columns was detected in effluent samples under all experimental conditions, indicating that the functionalized MWCNTs were readily transported through 40–50 mesh Ottawa sand. Examination of MWCNT length distributions in the effluent samples revealed that nanotubes with lengths greater than 8 μm were preferentially deposited. In addition, measured retention profiles exhibited the greatest MWCNT deposition near the column inlet, which was most pronounced for the long MWCNTs, and decreased sharply with travel distance. Scanning electron microscope (SEM) images showed that MWCNTs were deposited on sand surfaces over the entire column length, while larger MWCNT bundles were retained at grain intersections and near the column inlet. A mathematical model based on clean bed filtration theory (CBFT) was unable to accurately simulate the measured retention profile data, even after varying the weighting function and incorporating a nonuniform attachment rate coefficient expression. Modification of the mathematical model to account for physical straining greatly improved predictions of MWCNT retention, yielding straining rate coefficients that were four orders-of-magnitude greater than corresponding attachment rate coefficients. Taken in concert, these experimental and modeling results demonstrate the potential importance of, and need to consider, particle straining and tube length distribution when describing MWCNT transport in water-saturated porous media.