Nanoporous Media Research Articles

Assessment of Various Approaches in the Prediction of Methane Absolute Adsorption in Kerogen Nanoporous Media

It is crucial to accurately characterize methane absolute adsorption in kerogen nanoporous media for gas-in-place evaluation and well productivity prediction. Assuming that methane forms a single-l...

Adsorption characteristics of copper ion on nanoporous silica

Adsorption by nanoporous media is critically involved in many fundamental geological and geochemical processes including chemical weathering, element migration and enrichment, environmental pollution, etc. Yet, the adsorption behavior of metal ions on nanoporous materials has not been systematically investigated. In this study, MCM-41 material with a monodisperse pore size (4.4 nm) and a large BET specific surface area (839 m2/g) was hydrothermally prepared and used as a model silica adsorbent to study the adsorption characteristics of Cu2+ as a representative metal ion. The Cu2+ adsorption capacity was found to increase with increasing suspension pH in the range from 3 to 5 and to decrease in the presence of NaNO3. At 25 °C, pH = 5, and a solid-to-liquid ratio of 5 g/L, the adsorption capacity was determined to be 0.29 mg/g, which can be converted to a dimensionless partition coefficient of 45, indicating a strong enriching effect of nanoporous silica. The adsorption isotherm and kinetic data were fitted to several commonly used thermodynamic, kinetic, and diffusion models. The adsorption mechanism was also studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and synchrotron-based X-ray absorption spectroscopy. The results suggest that Cu2+ ion adsorption is an entropy-driven endothermal process, possibly involving both outer-sphere and inner-sphere complexes.

Spraying synthesis and ion permeation in polyvinyl chloride/graphene oxide membranes

Abstract An inexpensive kind of paper-like (film) membranes was synthesized by a new simple spraying-coating method to reject ions from water. Polyvinyl Chloride/Graphene Oxide membranes resulted to be flexible films, their morphology, stability and permeation changed with GO concentration (0–40 GO wt%). Each GOM presented one smooth and one rough surface. Contact angle measurements confirmed a maximum perceptible difference of approximately 30° between both surfaces of the same GOM. Hydrophilicity and permeability of GOM were enhanced with the amount of GO as was revealed by Fourier-transform infrared and X-ray photoelectron spectroscopies. Permeation rejection was between 82 and 97%, in the same order or better than membranes synthetized with other methods. It was found that both the diffusion coefficient and tortuosity values were 2 × 10−7 cm2 s−1 and 14, respectively, which are in the range of those reported for ion transport within a nanoporous medium with high tortuosity. These paper-like films could be cost – effective candidates for ion rejection due to its ease of fabrication at low cost and long-term stability under laboratory conditions.

Pore‐scale Study of Ion Transport Mechanisms in Inhomogeneously Charged Nanoporous Rocks: Impacts of Interface Properties on Macroscopic Transport

AbstractThe electrokinetic transport mechanisms of multispecies ions through 3‐D nanoporous rocks with chemical reaction at the solid‐aqueous solution interfaces are investigated. We systematically study the multiphysics transport phenomena by considering either inhomogeneous (local surface charge based on local pH and ion concentrations) or prescribed homogeneous surface charge at solid‐aqueous solution interface while the pores are screened via electric double layers. We develop a lattice Boltzmann numerical framework to solve the set of governing equations (Poisson‐Nernst‐Planck plus Navier–Stokes). Our modeling results reveal that the averaged local electric potential of the nanoporous rock is significantly underestimated (about 83%) when a homogeneous surface charge is prescribed based on the bulk solution properties. It is shown that increasing the porosity of the nanoporous media considerably increases the absolute values and inhomogeneity of the surface charge, which means that while the electric double layers screened the pores, increasing the porosity enhances the ion selectivity of the porous medium. When the scenario with inhomogeneous charge is taken into account, the predicted electroosmotic permeability and tortuosity are higher in comparison with the prescribed homogeneous case. Moreover, we have studied the electrostatic tortuosity, coupling coefficient, and the effective excess charge density of the nanoporous rocks. The results demonstrate that ignoring the inhomogeneity of surface charges may cause erroneous prediction of the ion transport through porous rocks with chemically active surfaces.

Modeling elastic properties of Vycor glass saturated with liquid and solid adsorbates

A combination of ultrasonic experiments with gas adsorption is a promising tool for improved characterization of nanoporous materials. The use of ultrasound requires understanding of the effects of adsorbates on the elastic properties of nanoporous medium. This issue is not trivial, because nanostructured materials, as well as nanoconfined matter, may exhibit physical properties that differ substantially from the properties of “normal” bulk materials. In this paper, we investigate the change of elastic properties of Vycor glass filled with adsorbed liquid and solid argon within the context of elasticity and compare the modeling results with the ultrasonic measurements. The modeling requires the knowledge of solid moduli of Vycor glass and the pore geometry, which cannot be measured directly. Instead, we estimate these parameters from the dry moduli using so-called Differential Effective Medium (DEM) theory, in which the pores are assumed to be of spheroidal shape characterized by a single aspect ratio. Predictions of the Gassmann equation give an excellent fit to the measured elastic moduli of Vycor glass completely filled with liquid argon at temperature 80 K. Estimates of the DEM show a reasonable agreement with ultrasonic measurements on the elastic moduli of Vycor glass fully saturated with solid argon at 74 K in shear modulus but a significant overestimate in bulk modulus. This might be due to the effects of the confinement on the moduli of argon in nanopores. Although the validation and generalization of this conclusion requires further laboratory experiments for a number of well characterized solid–fluid systems, our finding shed light on the understanding of elastic properties of nanoporous materials mixed with adsorbates in various phases. These results provide steps toward development of methods for ultrasonic characterization of confined fluid and solid phases.

Water Vapor and Carbon Monoxide Broadening and Line Shifts Inside Aerogel Nanopores

Line halfwidths and shifts are calculated for water vapor and carbon monoxide confined in nanoporous media. Physically adsorbed H2O and CO molecules are considered as scattering centers. The influence of changes in the rotational structure of levels in physically adsorbed molecules is numerically analyzed. The comparison with the existing experimental data is performed.

Oil Recovery from Nanoporous Media via Water Condensation

AbstractOil–nanoporous materials interplay is ubiquitous in oil recovery from unconventional reservoirs, environmental clean‐up, and membrane technology, so there is a genuine need for novel approaches that can monitor, manipulate, and also extract the oil present in nanoporous media. Here demonstrated is a water condensation–assisted method of extracting oil from nanopores. Under cooling‐induced condensation at room conditions, distinctive droplet modes act as both decoupling and collecting elements to transport oil from the pore space toward the nanomaterial surface after water evaporation. Even the movement of the oil through the nanoporous network can be visualized by taking advantage of the interference response of visible light in a nanoporous thin‐film platform. The method and results presented in this work open different routes to exploit oil–water–nanopore interactions and enable novel perspectives for oil recovery and environmental clean‐up.

Pressures Inside a Nano-Porous Medium. The Case of a Single Phase Fluid

We define the pressure of a porous medium in terms of the grand potential, and compute its value in a nano-confined or nano-porous medium, meaning a medium where thermodynamic equations need be adjusted for smallness. On the nano-scale, the pressure depends in a crucial way on the size and shape of the pores. According to Hill, two pressures are needed to characterize this situation; the integral pressure and the differential pressure. Using Hill's formalism for a nano-porous medium, we derive an expression for the difference between the integral and the differential pressures in a spherical phase $\alpha$ of radius $R$, $\hat{p}^\alpha-p^\alpha = {\gamma}/{R}$. We recover the law of Young-Laplace for the differential pressure difference across the same curved surface. We discuss the definition of a representative volume element for the nano-porous medium and show that the smallest REV is half a unit cell in the direction of the pore in the fcc lattice. We also show, for the first time, how the pressure profile through a nano-porous medium can be defined and computed away from equilibrium.

Pore-scale simulation of shale oil flow based on pore network model

The pore-grain characteristics of shale oil reservoir rocks are complex: nano-size pore throats, multi-scale of pore size and topology, and rich in organic matter. Generally, shale oil flow in nanoporous media is significantly affected by microscopic fluid slippage and adsorption on the pore surfaces, which give rise to variable slip length, viscosity ratio of adsorbed phase to bulk phase, and thickness of adsorption layer in organic & inorganic pore throats. In this study, a new pore network model is developed based on a modified shale oil flow equation to consider those combinational effects on shale oil permeability under different organic matter contents, and then is applied to provide an order analysis of those effects on the permeability of a representative shale model. Different flow patterns characterizing fluid-solid interaction are also discussed. Analysis results show that shale oil permeability is strongly influenced by the slip length. The effect of adsorption on permeability is negligible for cases considered. Shale oil flow is mainly controlled by inorganic pores when volumetric TOC is low. The influence of TOC on permeability depends on slippage and adsorption conditions in organic and inorganic matters, referred in this paper to as flow patterns. As the organic matter content increases, the connectivity of organic pores and throats improves, which further affects network permeability.

Spontaneous Selective Preconcentration Leveraged by Ion Exchange and Imbibition through Nanoporous Medium

Manipulating mechanism of particle’s motion has been extensively studied for the sample preparation in microfluidic applications including diagnostics, food industries, biological analyses and environmental monitoring. However, most of conventional methods need additional external forces such as electric field or pressure and complicated channel designs, which demand highly complex fabrication processes and operation strategies. In addition, these methods have inherent limitations of dilution or mixing during separation or preconcentration step, respectively, so that a number of studies have reported an efficient selective preconcentration process, i.e. conducting the separation and preconcentration simultaneously. In this work, a power-free spontaneous selective preconcentration method was suggested based on leveraging convective flow over diffusiophoresis near the water-absorbing nanoporous ion exchange medium, which was verified both by simulation and experiment. Especially, the velocity of the convective flow by an imbibition deviated from the original tendency of t−1/2 due to non-uniformly patterned nanoporous medium that has multiple cross-sectional areas. As a result, the direction of particle’s motion was controlled at one’s discretion, which led to the spontaneous selective preconcentration of particles having different diffusiophoretic constant. Also, design rule for maximizing the efficiency was recommended. Thus, this selective preconcentration method would play as a key mechanism for power-free lab on a chip applications.

Tackling the challenges in the estimation of methane absolute adsorption in kerogen nanoporous media from molecular and analytical approaches

Accurate characterization of methane absolute adsorption in shale nanoporous media is of great importance to the gas-in-place (GIP) estimation and well productivity. Because experimental measurement can only provide the excess adsorption, the absolute adsorption is generally converted from the excess adsorption based on the single-layer adsorption model. However, it is well known that shale has a widespread pore size distribution (PSD), ranging from sub 2-nm to hundreds of nanometers. In micropores (<2 nm), methane may have layering structures, which deviates from the commonly used adsorption model. Thus, it is necessary to take into account the varying methane adsorption behavior in micropores and mesopores and consider the PSD effect to obtain the absolute adsorption from the experimentally measured excess adsorption. In this work, we propose a number of artificially generated PSDs and study methane adsorption in each nanopore by using grand canonical Monte Carlo (GCMC) simulations. By coupling GCMC simulations and varying PSDs, we effectively model methane adsorption in nanoporous media.Based on the varying density profiles in different nanopores obtained from GCMC, we propose the corresponding methane adsorption model in each nanopore, which is applied in Ono-Kondo (OK) lattice model. By fitting the excess adsorption in nanoporous media and explicitly considering the PSD, OK model can readily obtain the absolute adsorption. In order to validate our model, 1000 sets of randomly generated PSDs are used. We find that our proposed OK model has an excellent agreement with GCMC simulation, while the commonly used method to convert the excess adsorption to the absolute adsorption without considering the PSD shows noticeable deviations. Moreover, the optimized constant adsorbed phase densities are very different from the commonly used values as 424 kg/m3 and 373 kg/m3. Our work proposes a simple, efficient and accurate empirical model to obtain the absolute adsorption in nanoporous media. This work should provide important insights into accurate characterization methane absolute adsorption and the gas-in-place estimation in shale.

The transport behaviors of oil in nanopores and nanoporous media of shale

Shale, known as the “tight” rock with abundant nanopores, exhibits extremely low permeability on the order of micro/nanodarcy. The classic Darcy law, being widely and successfully used in conventional porous media, becomes insufficient for the shale. In this work, on the basis of molecular dynamics (MD) simulations data available in the literature, a model for oil transport through a single nanopore is established by considering the boundary slip and the varying viscosity of the confined oil. Then, the established model for the single nanopore is integrated into the generalized lattice Boltzmann method (GLBM) to mimic the oil transport in the shale with nanopore networks, successfully scaled up to the nanoporous media. The results show that, to accurately predict the oil transport properties in inorganic and organic nanopores, the viscosity correction for the confined oil transport in the nanopores is necessary. The oil transport capability in organic nanopores is greatly enhanced compared with that predicted by the no-slip Poiseuille equation, significantly enhancing the flow capability in the scale of nanoporous media, while the small slip length in the inorganic matter (IOM) has neglected effect. The large liquid slip in organic matter (OM) also results in the apparent oil permeability of shale increases with the total organic content (TOC), although the fact that pore sizes within OM are universally smaller than that in IOM. This work provides a better understanding of the oil transport behaviors in a single nanopore and the nanoporous shale, and paves a feasible way to the exact numerical simulation for oil transport in nanoporous shale.

Requirements to Determine the Average Pore Size of Nanoporous Media Using Ultrasound.

Liquids in nanoporous media are exposed to an adsorption-induced pressure, a consequence of the interaction with the pore surface. The smaller the pore diameter, dP, the higher the pressure at saturation and thus the bulk modulus of the confined liquid. Therefore, it has been proposed to use ultrasonic measurements on saturated nanoporous media for the determination of the average pore size. Here, we discuss the requirements for such an analysis. Although predictions for the size-dependent pore pressure and the liquid’s modulus, Kiso(dP), are based on isothermal simulations, an experimentalist studying the propagation of ultrasonic waves determines adiabatic moduli, Kad(dP). We show that the quantity relating adiabatic and isothermal moduli, the heat capacity ratio γ = cp/cv = Kad/Kiso, exhibits a strong pressure dependence for many bulk liquids. In nanopores, this translates into a size-dependent γ(dP), provided the confinement does not alter the heat capacity ratio. Disregarding this effect in the analysis of ultrasonic data would yield an underestimate of the isothermal modulus and thus an overestimate of the average pore size. For a correct analysis, an experimentalist thus needs to know the size dependence of three quantities: the isothermal modulus, adsorption-induced pressure, and heat capacity ratio.

Experimental study of pore size distribution effect on phase transitions of hydrocarbons in nanoporous media

The phase behavior of a fluid in a nanoporous system is altered from that in the bulk due to confinement effects. There have been many experimental studies on this important effect, yet the vast majority are limited to porous media with a narrow or singular pore size distribution. However, natural porous media exhibit a broad pore size distribution. For example, in shale rocks the pore size ranges from several nanometers to hundreds of nanometers. It is not clear if a broad pore size distribution has a significant effect on the phase transition of nanoconfined fluids, and, if effects are present, how and why the phase behavior is altered. The effects of a distributed pore size on the bubble point of confined fluids as measured by differential scanning calorimetry (DSC) are not known. Here, we present an experimental study on the effect of pore size distribution on the liquid-vapor phase transition of n-hexane, n-octane and n-decane confined in nonporous media. We report a method to discretize the pore size distribution of natural shale rocks so that it can be mimicked using mixtures of synthetic nanoporous media. Using DSC, we show that the presence of a broad pore size distribution significantly alters the vapor-liquid phase transition of confined hydrocarbons. In fully saturated (i.e. filled) nanoporous media, the largest pore size dictates the onset of liquid-vapor phase transition (.i.e. bubblepoint). However, when the porous media is partially saturated, smaller pore sizes influence the bubblepoint. These findings suggest that there is a dependency of the confined fluid phase behavior on the fluid saturation in nanoporous media with a broad pore size distribution, which contributes to a broader understanding of phase transitions in natural porous media.

An integrated approach for gas-water relative permeability determination in nanoscale porous media

This paper provides an integrated approach to simulate gas-water relative permeability in nanoscale porous media with interfacial effects using fractal theory. The calculation results with present model considering the interfacial effects match the laboratory results very well. The characteristics of relative permeability were analyzed under different values of Hydrogen bond (∏h), Double layer repulsive force (∏e), Long-range van der Waals force (∏w) and Short-range structure repulsive force (∏s). The result shows that the interfacial effects can promote the fluid flow in the nanoporous media and the relationship is as follows: ∏h>∏e>∏w>∏s. ∏h and ∏e improve spreading obviously, which are accounted for more than 70%.Then the ∏w affects spreading accounts for 1/5 nearly. In addition, the analysis of fractal dimension on relative permeability is carried out to indicate that the gas-water relative permeability decreases with the increase of the fractal dimension. While the capillary force has impact on relative permeability slightly. The new insight and theoretical basis could be obtained for CO2 geological storage and shale gas extraction from nanoscale porous media.

Scaling law for slip flow of gases in nanoporous media from nanofluidics, rocks, and pore-scale simulations

In unconventional reservoirs, as the effective pore size becomes close to the mean free path of gas molecules, gas transport in porous media begins to deviate from Darcy’s law. The objective of this study is to explore the similarities of gas flows in nanochannels and core samples as well as those simulated by direct simulation BGK (DSBGK), a particle-based method that solves the Bhatnagar-Gross-Krook (BGK) equation.Due to difficulties in fabrication and experimentation, previous study on gas flow experiments in nanochannels is very limited. In this work, steady-state gas flow was measured in reactive-ion etched nanochannels with a controlled channel size on a sillicon wafer. A core-based permeability measurement apparatus was used to perform steady-state gas flow measurements on carbonate and shale samples. Klinkenberg permeability was obtained under varying pore pressures but constant temperature and effective stress. Methane was used in nanofluidic and rock experiments, making them directly comparable. Results from both experiments were then compared to gas flow simulations by DSBGK method carried out on several independently constructed geometry models. DSBGK uses hundreds of millions of simulated molecules to approximate gas flow inside the pore space. The intermolecular collisions are handled by directly integrating the BGK equation along each molecules trajectory, rather than through a sampling scheme like that in the direct simulation Monte-Carlo (DSMC) method. Consequently, the stochastic noise is significantly reduced, and simulation of nano-scale gas flows in complex geometries becomes computationally affordable.The slippage factors obtained from these independent studies varied across three orders of magnitude, yet they all appear to collapse on a single scaling relation where the slippage factor in the slip flow regime is inversely proportional to the square root of intrinsic permeability over porosity. Our correlation could also fit the data in the literature, which were often obtained using nitrogen, after correcting for temperature and gas properties. This study contributes to rock characterization, well testing analysis as well as the understanding of rarefied gas transport in porous media.

A redox fuel cell capable of converting biofuels to electricity at high rate

Membraneless nanofluidic fuel cells are devices that utilize fluid flow through nanoporous media which serve as three-dimensional electrodes. In the case of hybrid fuel cells (HFC) an enzymatic and an abiotic catalyst are incorporated on the electrodes. Here we compared two different HFC. In the first one (HFC-1), glucose oxidase- and Pt-based electrodes were used as bioanode and cathode respectively. This cell reached an open circuit voltage (OCV) of 0.55 V and a maximum power density of 5.7 mWcm−2. In the second one (HFC-2), AuAg- and laccase-based electrodes were used as anode and biocathode respectively. This cell exhibited an OCV of 0.91 V and a maximum power density of 17 mWcm−2. Finally, enzymatic electrodes were used to develop a high performance biofuel cell (3.2 mWcm−2) that exhibited high stability over 4 days. These preliminary results indicate that the incorporation of enzymes into the 3D carbon structures is an efficient alternative for miniaturized nanofluidic power sources.

A Transient Productivity Model of Fractured Wells in Shale Reservoirs Based on the Succession Pseudo-Steady State Method

After volume fracturing, shale reservoirs can be divided into nonlinear seepage areas controlled by micro- or nanoporous media and Darcy seepage areas controlled by complex fracture networks. In this paper, firstly, on the basis of calculating complex fracture network permeability in a stimulated zone, the steady-state productivity model is established by comprehensively considering the multi-scale flowing states, shale gas desorption and diffusion after shale fracturing coupling flows in matrix and stimulated region. Then, according to the principle of material balance, a transient productivity calculation model is established with the succession pseudo-steady state (SPSS) method, which considers the unstable propagation of pressure waves, and the factors affecting the transient productivity of fractured wells in shale gas areas are analyzed. The numerical model simulation results verify the reliability of the transient productivity model. The results show that: (1) the productivity prediction model based on the SPSS method provides a theoretical basis for the transient productivity calculation of shale fractured horizontal well, and it has the characteristics of simple solution process, fast computation speed and good agreement with numerical simulation results; (2) the pressure wave propagates from the bottom of the well to the outer boundary of the volume fracturing zone, and then propagates from the outer boundary of the fracturing zone to the reservoir boundary; (3) with the increase of fracturing zone radius, the initial average aperture of fractures, maximum fracture length, the productivity of shale gas increases, and the increase rate gradually decreases. When the fracturing zone radius is 150 m, the daily output is approximately twice as much as that of 75 m. If the initial average aperture of fractures is 50 μm, the daily output is about half of that when the initial average aperture is 100 μm. When the maximum fracture length increases from 50 m to 100 m, the daily output only increases about by 25%. (4) When the Langmuir volume is relatively large, the daily outputs of different Langmuir volumes are almost identical, and the effect of Langmuir volume on the desorption output can almost be ignored.

Phenomenon of non-outflow of a non-wetting liquid dispersed in nanoporous medium. The influence of modification and size of granules.

In this paper was studied the non-outflow phenomenon of a non-wetting liquid dispersed in nanoporous medium and the influence of porous medium surface modification and size of porous medium granules. The study of this phenomenon is important for development of mechanical energy dampers and for extraction of medicinal substances under certain conditions devices. The presented results for four commercial hydrophobized porous media Fluka 100 C18 (60756), Fluka 100 C18 (60758), Fluka 100 C8 (60755) and Fluka 100 C8 (60759) with the different thickness of the modifying layer and the size of the granules are show that the less of granules size and more narrow pore size distribution lead to decreasing of non-outflow phenomenon. The study of non-outflow phenomenon of non-wetting liquid from the pores of a nanoporous medium was carried out by time relaxation method and dispersed non-wetting liquid distribution recovery method at different temperatures.

Study of local configurations in the systems “disordered nanoporouse medium – non-wetting liquid”

The analysis of redistribution functions of non-wetting liquid (water) dispersed in disordered nanoporous media of Libersorb 23, obtained in work [22]. Local configurations which describe the time relaxation of a non-wetting liquid dispersed in a disordered nanoporous medium are studied. It is shown that various possible local configurations can be used to describe the temporal relaxation of a liquid dispersed in a nanoporous medium. A qualitative picture is presented of the possible mutual arrangement of the pores in the porous medium Libersorb 23. The presented approach allows qualitatively to restore the spatial distribution of pores of a porous medium.