Geological Porous Media Research Articles

Application of microfluidic pore models for flow, transport, and reaction in geological porous media: from a single test bed to multifunction real-time analysis tool

Recent advances in microfluidics technology can significantly benefit engineering and science research. Through integration with current optical tools like the optical/confocal microscope, the 3D printing, the soft lithography, and the lithography–etching, microfluidic pore model offers a direct and visual way to study the process of geological related pore-scale transport phenomena. In this paper, recent progress of microfluidic pore model mimicking the geological porous media are discussed, which can apply on many disciplines such as environmental, petroleum engineering and geosciences. The purpose of this review is to highlight the significant features of microfluidic pore model application in geological porous medium pertaining research, as well as to provide engineer/scientist an overview of materials and fabrication methods to manufacture micromodels. Furthermore, the outlook of intensively using micromodel as a multifunction real-time analysis tool is proposed to help the reader to enhance their understanding of pore-scale transport phenomena related to geological porous medium. Also, potential critical issues of micromodels in both research and teaching are discussed. The overall role of this review for microfluidic application is intended to provide benefits to professors, students, and other scientists not only in research but also in teaching fields.

Mathematical modeling of gas transport in porous geological media with contrast of properties and irregular distribution of pores

AbstractIn this work, we perform multiscale modeling of gas transport through the geological media having irregular pore structure and contrast of properties on different spatial scales. We assume that the medium consists of inorganic matrix with organic inclusions imbedded into it. There exist a contrast of properties and spatial scales between the matrix and organic inclusions. The pore sizes vary from micro to nanometers, permeability and diffusivity can differ by several orders of magnitude. We consider filtration and molecular diffusion as mechanisms for free gas transport in both inorganic and organic materials, and surface diffusion as the main mechanism for sorbed gas transport through nanoporous organic inclusions. The irregularities of porous structure we characterize by their deviations from the periodic distribution. We implement multiscale homogenization together with an averaging with respect to random deviations of distribution of pores to derive the macroscopic equation for evaluating the free gas amount in‐place. It turns out that macroscale parameters characterizing gas transport depend on diffusivity, permeability, and porosity of the components of the system, the amount of inclusions and their spatial distribution. We determine the distribution of gas concentration through the production time and investigate its sensitivity to irregularities of pores distribution. We are also interested in the effect of bottom‐hole pressure and study how depletion can be affected by the interchange of gas between kerogen inclusions and inorganic material.

Universal Relationship Between Viscous and Inertial Permeability of Geologic Porous Media

AbstractFluid flow through geologic porous media is represented by Darcy's law and its inertial and nonlinear extension, the Forchheimer equation. These relationships equate the product of the driving potential gradient and phenomenological coefficients representing momentum resistance and dissipation to flux. From decades of research, the coefficient of viscous permeability (kv) in Darcy's law is largely predictable, but this is not the case for the coefficient of inertial permeability (ki) in the Forchheimer equation. Synthesizing results from thousands of laboratory and field flow tests and pore‐scale flow model results, we show that ki can be predicted from kv via the equation ki = 1010kv3/2 across 12 and 20 orders of magnitude in kv and ki, respectively. Since it is related with ki, kv is thus sufficient for predicting flow across viscous to inertial regimes for most geologic porous media.

New Capillary Number Definition for Micromodels: The Impact of Pore Microstructure

AbstractA new capillary number (Nca) definition is proposed for 2‐D etched micromodels. We derive the new definition from a force balance on a nonwetting ganglion trapped by capillarity. It incorporates the impact of pore microstructure on mobilization. The geometrical factors introduced can be estimated directly from image analysis of the pore network etched in the micromodel, without conducting flow experiments. The improved fit of the new Nca to published data supports its validity. The new definition yields a consistent trend in the capillary‐desaturation curve. The conventional Nca definitions proposed for porous rock give a large scatter in the capillary‐desaturation curve for data in micromodels. This is due to the different type of flow in micromodels, as 2‐D networks, relative to 3‐D geological porous media. In particular, permeability is dominated by channel depth in micromodels with shallow depth of etching, and generally, there is no simultaneous multiphase flow under capillary‐dominated conditions. Applying the conventional definitions to results in micromodels may lead to misleading conclusions for fluid transport in geological formations.

Continuous time random walk model with advection and diffusion as two distinct dynamical origins

Modeling the solute transport in geological porous media is of both theoretical interest and practical importance. Of several approaches, the continuous time random walk method is a most successful one that can be used to quantitatively predict the statistical features of the process, which are ubiquitously anomalous in the case of high Péclet numbers and normal in the case of low Péclet numbers. It establishes a quantitative relation between the spatial moment of an ensemble of solute particles and the waiting time distribution in the model. However, despite its success, the classical version of this model is a " static” one in the sense that there is no tuning parameter in the waiting time distribution that can reflect the relative strength of advection and diffusion which are two mechanisms that underlie the transport process, hence it cannot be used to show the transition from anomalous to normal transport as the Péclet numbers decreases. In this work, a new continuous time random walk model is established by taking into account these two different origins of solute particle transport in a geological porous medium. In particular, solute transitions due to advection and diffusion are separately treated by using a mixture probability model for the particle’s waiting time distribution, which contains two terms representing the effects of advection and diffusion, respectively. By varying the weights of these two terms, two limiting cases can be obtained, i.e. the advection-dominated transport and the diffusion-dominated transport. The values of scaling exponent β of the mean square displacement versus time, <inline-formula><tex-math id="M1">\begin{document}${\left( {\Delta {x} } \right)^2} \sim {t^{\rm{\beta }}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M1.png"/></alternatives></inline-formula>, are derived for both cases by using our model, which are consistent with previous results. In the advection dominant case with the Péclet number going to infinity, the scaling exponent β is found to be equal to <inline-formula><tex-math id="M2">\begin{document}$3 - {\rm{\alpha }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M2.png"/></alternatives></inline-formula> where <inline-formula><tex-math id="M3">\begin{document}${\rm{\alpha }} \in \left( {1,2} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M3.png"/></alternatives></inline-formula> is the anomaly exponent in the advection-originated part of the waiting time distribution that <inline-formula><tex-math id="M4">\begin{document}${{\rm{\omega }}_1}\left( {t} \right) \sim {{t}^{ - 1 - {\rm{\alpha }}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M4.png"/></alternatives></inline-formula>. As the Péclet number decreases, the diffusion-originated part of the waiting time distribution begins to have a stronger influence on the transport process and in the limit of the Péclet number going to 0 we observe a gradual transition of β from <inline-formula><tex-math id="M5">\begin{document}$3 - {\rm{\alpha }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20190088_M5.png"/></alternatives></inline-formula> to 1, indicating that the underlying transport process changes from anomalous to normal transport. By incorporating advection and diffusion as two mechanisms giving rise to solute transport in the continuous time random walk model, we successfully capture the qualitative transition of the transport process as the Péclet number is varied, which is, however, elusive from the classical continuous time random walk model. Also established are the corresponding macroscopic transport equations for both anomalous and normal transport, which are consistent with previous findings as well. Our model hence fully describes the transition from normal to anomalous transport in a porous medium as the Péclet number increases in a qualitative and semi-quantitative way.

On mass transport in porosity waves

Porosity waves arise naturally from the equations describing fluid migration in ductile rocks. Here, we show that higher-dimensional porosity waves can transport mass and therefore preserve geochemical signatures, at least partially. Fluid focusing into these high porosity waves leads to recirculation in their center. This recirculating fluid is separated from the background flow field by a circular dividing streamline and transported with the phase velocity of the porosity wave. Unlike models for one-dimensional chromatography in geological porous media, tracer transport in higher-dimensional porosity waves does not produce chromatographic separations between relatively incompatible elements due to the circular flow pattern. This may allow melt that originated from the partial melting of fertile heterogeneities or fluid produced during metamorphism to retain distinct geochemical signatures as they rise buoyantly towards the surface.

On the Theory of Solitons of Fluid Pressure and Solute Density in Geologic Porous Media, with Applications to Shale, Clay and Sandstone

We here analyze a new model of transients of pore pressure p and solute density ρ in geologic porous media. This model is rooted in the nonlinear wave theory, its focus is on advection and effect of large pressure jumps on strain. It takes into account nonlinear and also time-dependent versions of the Hooke law about stress, rate and strain. The model solutions strictly relate p and ρ evolving under the effect of a strong external stress. As a result, the presence of quick and sharp transients in low permeability rocks is unveiled, i.e., the nonlinear “Burgers solitons”. We, therefore, show that the actual transport process in porous rocks for large signals is not only the linear diffusion, but also a solitons presence could control the process. A test of a presence of solitons is applied to Pierre shale, Bearpaw shale, Boom clay and Oznam-Mugu silt and clay. An application about the presence of solitons for nuclear waste disposal and salt water intrusions is also discussed. Finally, in a kind of “theoretical experiment” we show that solitons could also be present in higher permeability rocks (Jordan and St. Peter sandstones), thus supporting the idea of a possible occurrence of osmosis also in sandstones.

Capillary flows across layers and textural interfaces – Pathways and colloid transport considerations in unsaturated layered porous media

Soils and other geologic porous media often display layering and associated textural contrasts that may alter unsaturated flow and transport pathways. Such natural interfaces are usually represented empirically in continuum flow models with limited mechanistic account of the effect of adjusted pathways across layers on colloid transport and on the nature of unsaturated flow. In this study we present the soil foam drainage equation (SFDE) as alternative framework to simulate unsaturated flow in layered porous media. HypothesisIn contrast to standard continuum flow models, the SFDE explicitly accounts for capillary flow pathways and their adjustment across textural interfaces that in turn define flow geometry for colloidal transport not predicted by continuum models. ExperimentsPore scale water distribution in a layered sand sample was measured using X-ray tomography to quantify capillary flow pathways. Literature values of measured saturation and capillary pressure profiles in layered porous columns were used to evaluate solutions based on the SFDE framework and to deduce velocity profiles and their effects for colloid transport. FindingsThe SFDE framework provides new insights into capillary architecture in unsaturated layered media and opens a new way for using pore scale information (from imaging) to provide the necessary SFDE parameters and potentially improve on the standard continuum representation of capillary flows in layered media.

Assessment of a Hybrid Continuous/Discontinuous Galerkin Finite Element Code for Geothermal Reservoir Simulations

FALCON (Fracturing And Liquid CONvection) is a hybrid continuous/discontinuous Galerkin finite element geothermal reservoir simulation code based on the MOOSE (Multiphysics Object-Oriented Simulation Environment) framework being developed and used for multiphysics applications. In the present work, a suite of verification and validation (V&V) test problems for FALCON was defined to meet the design requirements, and solved to the interests of enhanced geothermal system modeling and simulation. The intent for this test problem suite is to provide baseline comparison data that demonstrates the performance of FALCON solution methods. The test problems vary in complexity from a single mechanical or thermal process, to coupled thermo-hydro-mechanical processes in geological porous medium. Numerical results obtained by FALCON agreed well with either the available analytical solutions or experimental data, indicating the verified and validated implementation of these capabilities in FALCON. Whenever possible, some form of solution verification has been attempted to identify sensitivities in the solution methods, and suggest best practices when using the FALCON code.

Pore Structure Characterization of Indiana Limestone and Pink Dolomite from Pore Network Reconstructions

Carbon sequestration in deep underground saline aquifers holds significant promise for reducing atmospheric carbon dioxide emissions (CO2). However, challenges remain in predicting the long term migration of injected CO2. Addressing these challenges requires an understanding of pore-scale transport of CO2 within existing brine-filled geological reservoirs. Studies on the transport of fluids through geological porous media have predominantly focused on oil-bearing formations such as sandstone. However, few studies have considered pore-scale transport within limestone and other carbonate formations, which are found in potential storage sites. In this work, high-resolution micro-Computed Tomography (microCT) was used to obtain pore-scale structural information of two model carbonates: Indiana Limestone and Pink Dolomite. A modified watershed algorithm was applied to extract pore network from the reconstructed microCT volumetric images of rock samples and compile a list of pore-scale characteristics from the extracted networks. These include statistical distributions of pore size and radius, pore-pore separation, throat radius, and network coordination. Finally, invasion percolation algorithms were applied to determine saturation-pressure curves for the rock samples. The statistical distributions were comparable to literature values for the Indiana Limestone. This served as validation for the network extraction approach for Pink Dolomite, which has not been considered previously. Based on the connectivity and the pore-pore separation, formations such as Pink Dolomite may present suitable storage sites for carbon storage. The pore structural distributions and saturation curves obtained in this study can be used to inform core- and reservoir-scale modeling and experimental studies of sequestration feasibility.

Scaling of geochemical reaction rates via advective solute transport.

Transport in porous media is quite complex, and still yields occasional surprises. In geological porous media, the rate at which chemical reactions (e.g., weathering and dissolution) occur is found to diminish by orders of magnitude with increasing time or distance. The temporal rates of laboratory experiments and field observations differ, and extrapolating from laboratory experiments (in months) to field rates (in millions of years) can lead to order-of-magnitude errors. The reactions are transport-limited, but characterizing them using standard solute transport expressions can yield results in agreement with experiment only if spurious assumptions and parameters are introduced. We previously developed a theory of non-reactive solute transport based on applying critical path analysis to the cluster statistics of percolation. The fractal structure of the clusters can be used to generate solute distributions in both time and space. Solute velocities calculated from the temporal evolution of that distribution have the same time dependence as reaction-rate scaling in a wide range of field studies and laboratory experiments, covering some 10 decades in time. The present theory thus both explains a wide range of experiments, and also predicts changes in the scaling behavior in individual systems with increasing time and/or length scales. No other theory captures these variations in scaling by invoking a single physical mechanism. Because the successfully predicted chemical reactions include known results for silicate weathering rates, our theory provides a framework for understanding changes in the global carbon cycle, including its effects on extinctions, climate change, soil production, and denudation rates. It further provides a basis for understanding the fundamental time scales of hydrology and shallow geochemistry, as well as the basis of industrial agriculture.

Challenges and Possibilities in the Multi-Parameter Characterization Techniques for Enhanced Monitoring of CO2 in Geological Carbon Sequestration

Scientific investigations and observations show that carbon dioxide is a major contributor to the increasingly damaging effects of global warming. Thus, geological sequestration of carbon dioxide in saline aquifers is already in operations across the globe. However, geological carbon sequestration in an aquifer should be safeguarded from CO 2 leakage into the atmosphere and/or CO 2 migration into potable water aquifers that are also present in the subsurface. To promote safety of living and non-living species that are near and far from the geological sequestration sites, this work presents techniques for monitoring the greenhouse gas stored in geological porous media, using experiments and numerical simulation techniques. The work employed two-phase flow parameters like capillary pressure (P

Towards a Theory of Solitary Shock Waves of Fluid Pressure and Solute Density in Geologic Porous Media

In this paper we analyze the presence of “Burgers solitons” in geologic porous rocks. These are quick, strong transients of combined fluid pressure and solute density that are generated from an adjacent matrix, characterized by an intense pressure and contaminant density. The effect that produces such transients is a nonlinear advection process that is not often taken into account in hydrologic science. A brief analysis of solitons in Pierre Shale is also shown in the paper.

Self-potential signals from pumping tests in laboratory experiments

Streaming potentials can be generated when geologic porous media are subjected to pumping tests. For a homogeneous medium, theory predicts that input and output points for water circulation generate field responses in the form of electric potentials that are equivalent to those produced by current sources that are externally driven by a power source. We evaluated tank experiments showing that this assumption is valid for common geophysical scenarios and can be used to determine charge density for porous geologic media, a key parameter in interpreting electrokinetic and interfacial properties in hydrogeophysics. We also determined that when water circulation encompasses a heterogeneity, the equivalence with single current poles is lost, and this can be used as a field criterion to detect inhomogeneities near a well. Our experimental results were analyzed with finite-element modeling of water and charge flow, showing that an interfacial distribution of currents must be expected as the cause of distortions in self-potential fields. We developed a procedure that used the background resistivity model to better image the distribution of currents onto media interfaces, pointing out advances still needed and challenges still remaining to improve source imaging.

A pore network model reconstruction method via genetic algorithm

A pore network model could represent the porous medium space to predict rocks' various flow properties. This paper proposes a new method via genetic algorithm (GA) to reconstruct the pore network model with irregular distribution and topologically equivalent based on the real porous medium. Firstly, the properties of pore and throat are based on the Weilbull distribution function. A random pore network model is built by some assumptions and Weilbell parameters. In order to obtain the reliable parameters of pore and throat, the Avizo Fire analysis software is used to analysis the sample's micro-computed tomography (micro-CT) images. The theory of pore network simulation also is introduced simply in this section. Second, two objective functions for GA are given. We build up an objective function to reduce the discrepancy of slices of numerical model with the real micro-CT images, which is controlled by the core porosity. The second objective function makes model much more agreed with the experiment value. Thirdly, according to the characteristics of control parameters, we choose three steps alternating optimization to construct the model via GA. It demonstrates that this method has a quick convergence speed than before experiments. We adopt two kinds of complex geological porous medium to test the validity of our proposed method. One is a rock with high porosity (Sample One). The other is the dense sandstone (Sample Two). After the optimization, two appropriate pore network models are performed excellent of the prediction displacement in primary oil flooding of the target porous medium.

Optimizing contrast agents with respect to reducing beam hardening in nonmedical X-ray computed tomography experiments

Iodine is commonly used as a contrast agent in nonmedical science and engineering, for example, to visualize Darcy flow in porous geological media using X-ray computed tomography (CT). Undesirable beam hardening artifacts occur when a polychromatic X-ray source is used, which makes the quantitative analysis of CT images difficult. To optimize the chemistry of a contrast agent in terms of the beam hardening reduction, we performed computer simulations and generated synthetic CT images of a homogeneous cylindrical sand-pack (diameter, 28 or 56 mm; porosity, 39 vol.% saturated with aqueous suspensions of heavy elements assuming the use of a polychromatic medical CT scanner. The degree of cupping derived from the beam hardening was assessed using the reconstructed CT images to find the chemistry of the suspension that induced the least cupping. The results showed that (i) the degree of cupping depended on the position of the K absorption edge of the heavy element relative to peak of the polychromatic incident X-ray spectrum, (ii) (53)I was not an ideal contrast agent because it causes marked cupping, and (iii) a single element much heavier than (53)I ((64)Gd to (79)Au) reduced the cupping artifact significantly, and a four-heavy-element mixture of elements from (64)Gd to (79)Au reduced the artifact most significantly.

Automated method for determining the flow of surface functionalized nanoparticles through a hydraulically fractured mineral formation using plasmonic silver nanoparticles

Quantifying nanoparticle (NP) transport within porous geological media is imperative in the design of tracers and sensors to monitor the environmental impact of hydraulic fracturing that has seen increasing concern over recent years, in particular the potential pollution and contamination of aquifers. The surface chemistry of a NP defining many of its solubility and transport properties means that there is a wide range of functionality that it is desirable to screen for optimum transport. Most prior transport methods are limited in determining if significant adsorption occurs of a NP over a limited column distance, however, translating this to effects over large distances is difficult. Herein we report an automated method that allows for the simulation of adsorption effects of a dilute nanoparticle solution over large distances under a range of solution parameters. Using plasmonic silver NPs and UV-visible spectroscopic detection allows for low concentrations to be used while offering greater consistency in peak absorbance leading to a higher degree of data reliability and statistics. As an example, breakthrough curves were determined for mercaptosuccinic acid (MSA) and cysteamine (CYS) functionalized Ag NPs passing through Ottawa sand (typical proppant material) immobile phase (C) or bypassing the immobile phase (C0). Automation allows for multiple sequences such that the absorption plateau after each breakthrough and the rate of breakthrough can be compared for multiple runs to provide statistical analysis. The mobility of the NPs as a function of pH is readily determined. The stickiness (α) of the NP to the immobile phase calculated from the C/C0 ratio shows that MSA-Ag NPs show good mobility, with a slight decrease around neutral pH, while CYS-Ag NPs shows an almost sinusoidal variation. The automated process described herein allows for rapid screening of NP functionality, as a function of immobile phase (proppant versus reservoir material), hydraulic fracturing fluid additives (guar, surfactant) and conditions (pH, temperature).

The Development of a Leak Remediation Technology for Potential Non- Wellbore Related Leaks from CO2 Storage Sites.

Abstract In order to reduce global atmospheric greenhouse gas emissions, many pilot-scale and commercial-scale Carbon Capture and Storage (CCS) projects are under development or are operating commercially. There are two main recognized potential leakage mechanisms which may allow CO2 to leak out of the intended storage complex, migrate into overlying aquifers and eventually seep back to the near-surface and atmosphere, with potentially negative impacts upon natural resources and/or the environment. The primary potential leakage pathway is via wellbores, which may provide a direct connection between the storage formation and the surface. Should a well integrity related leak occur, established remediation techniques may be employed to mitigate and/or remediate further leakage. A second potential leakage pathway includes fluid migration through geological faults, fractures and high permeability zones within a caprock. Not only is it more difficult to constrain and characterize these leaks, there is currently no routine method available to intercept and repair solution leakage via such pathways. This experimental study was conducted to assess the injection of chemical solutions capable of physically and chemically interacting with a CO2- containing brine to form a geochemically stable blocking agent capable of preventing further fluid leakage. A number of potential blocking agents were evaluated, and experiments were carried out under quasi formation conditions (i.e. elevated pressure and temperature) using a combination of simulated 2D caprock micromodels and 3D geological porous medium core floods. Experiments succeeded in determining the behaviour of the blocking agent, CO2 saturated brine behaviour, reaction front location, the concentration and amount of blocking agent required and an indicative timescale for remediation. Using experimental parameters as bounding conditions, numerical simulations using PetraSim (TOUGH2 and TOUGHReact) were used to assess the upscaling requirements of the blocking process in preparation for large-scale laboratory tests and a field demonstration.

A New Space-Time CE/SE Numerical Tracking of Contaminant Transport in Fractured Stratified Geologic Profiles

To date, efficient numerical simulation of contaminant transport in geologic porous media is challenged by parametric jumps resulting from stratification and the use of ideal initial/boundary conditions. Thus, to resolve some contaminant hydrology problems, this work presents the development of the Space-Time Conservation Element/Solution Element (CE/SE) scheme for advection-dispersion-reaction a-d-r transport in geologic media. The CE/SE method derives from the native form of Gauss conservation law. Therefore, it is able to effectively handle non-trivial discontinuities that may exist within the problem domain. In freshwater aquifer, stratification and other parametric jumps are examples of such discontinuity. To simulate the Nigerian experience of nitrate pollution of freshwater aquifers; the a-d-r contaminant transport model is herein solved under a time periodic nitrate fertilizer loading condition on farmlands. Results show that this approach is able to recover the well-known field pattern of nitrate profiles under farmlands. Cyclic loading impacts more on the dispersivity of an aquifer. Hence, dispersion coefficient modulates the response of aquifers to loading frequency. However, aquifers with conductivity less than 10-6 m/day are almost insensitive to periodic loads. The CE/SE method is able to sense slight (i.e. order of 10-3) variation in hydrological parameters. Also, CE/SE computes contaminant concentration and its flux simultaneously. Thus, it facilitates a better understanding of some reported phenomena such as contaminant accumulation and localized reverse transport at the interface between fracture and matrix in geologic medium. Clearly, CE/SE is an efficient and admissible tool into the family of numerical methods available for tracking contaminant transport in porous media.

The use of sodium polytungstate as an X-ray contrast agent to reduce the beam hardening artifact in hydrological laboratory experiments

Abstract Iodine is conventionally used as a contrast agent in hydrological laboratory experiments using polychromatic X-ray computed tomography (CT) to monitor two-phase Darcy flow in porous geological media. Undesirable beam hardening artifacts, however, render the quantitative analysis of the obtained CT images difficult. CT imaging of porous sand/bead packs saturated with iodine and tungsten-bearing aqueous solutions, respectively, was performed using a medical CT scanner. We found that sodium polytungstate (Na6H2W12O40) significantly reduced the beam hardening compared with potassium iodide (KI). This result is attributable to the location of the K absorption edge of tungsten, which is nearer to the peak of the polychromatic X-ray source spectrum than that of iodine. As sodium polytungstate is chemically stable and less toxic than other heavy element bearing compounds, we recommend it as a promising contrast agent for hydrological CT experiments.