Heterogeneous Porous Systems Research Articles

A novel upscaling procedure for characterising heterogeneous shale porosity from nanometer-to millimetre-scale in 3D

Microstructures and pore systems in shales are key to understanding the role of shale in many energy applications. This study proposes a novel multi-stage upscaling procedure to comprehensively investigate the heterogeneous and complex microstructures and pore systems in a laminated and microfractured shale, utilising 3D multi-scale imaging data. Five imaging techniques were used for characterisation from sub-nanoscale to macroscale (core-scale), spanning four orders of magnitude. Image data collected using X-ray tomography, Focused Ion Beam, and Electron Tomography techniques range in voxel size from 0.6 nm to 13 μm.Prior to upscaling, a novel two-step analysis was performed to ensure sub-samples were representative. Following this, a three-step procedure, based on homogenising descriptors and computed volume coefficients, was used to upscale the quantified microstructure and pore system. At the highest resolution (nanoscale), four distinct pore types were identified. At the sub-micron scale equations were derived for three pore-associated phases. At the microscale, the volume coefficients were recalculated to upscale the pore system to the millimetre- scale. The accuracy of the upscaling methodology was verified, predicting the total porosity within 7.2% discrepancy. The results provide a unique perspective to understand heterogeneous rock types, breaking though prior scale limitations in the pore system.

Prediction of reservoir quality in carbonates via porosity spectrum from image logs

The Majiagou Formation is an important gas-bearing stratigraphic unit in the Ordos basin of west China. The highly complex lithology and heterogeneous pore systems make it difficult to predict the reservoir quality via petrophysical and image logs. This study investigates the lithology, pore systems and well log responses of the Majiagou Formation Member 5 (Ma 5) carbonate reservoirs using a comprehensive analysis of cores, thin sections, routine core analysis, conventional and porosity spectrum derived from image logs. The results show that the lithologies of the Ma 5 are dominantly of mud-sized to silt-sized crystalline dolomite, and there are some dolomites containing gypsums mudstones and limestone. The pore systems include vugs, intercrystalline pores, dissolution pores and microfractures. The vuggy dolomites and the fractured dolomites are the best quality reservoir rocks, and the mud-sized to silt-sized crystalline dolomites containing intercrystalline pores are good quality reservoirs with high porosity and permeability.Open fractured zones are recognized as dark sinusoidal waves, and the vuggy dolomites can be recognized by the dark spots on the image logs. However, the fractured and vuggy dolomites are only occasionally detected, and most of the dolomite reservoirs in Ma 5 contain only intercrystalline pores, which are difficult to be recognized by image logs. In addition, good quality reservoirs have very similar log responses (low natural gamma-ray (GR), low-moderate bulk density (DEN), and moderate-high resistivity) with the poor reservoir quality intervals. By applying the Archie's formula to the flushed zone, a total of 150 porosity curves can be obtained from the XRMI image logs. Then the porosity spectrum is derived from the histogram of porosity distribution for a certain interpretation interval. The best quality reservoir rocks (fractured or vuggy dolomites) have very broad porosity spectrums with long tails due to the presence of vugs and fractures. The abundance of intercrystalline pores containing in the mud-sized to silt-sized crystalline dolomites also result in the broad porosity spectrums without any tails. In contrast, the very narrow porosity spectrums correspond to the poor reservoir quality intervals. The interpretation results are verified by core observations and thin section analysis. By using the porosity spectrum analysis derived from the image logs, the reservoir quality can be predicted. The results can help improve the understanding and prediction of the reservoir quality in carbonates using well logs.

Effect of Ionomer Equivalent Weight and Side Chain Length on Catalyst Layer Transport Resistances

Current state of the art Proton Exchange Membrane fuel cell cathode catalyst layer can be described as a complex, heterogeneous porous system with multiple components across various length scales at which both the electrochemical reaction and mass transport of reactants/products occur. The functional components in the catalyst layer consists of the catalyst nanoparticles (few nm in diameter) for electrochemical reaction, carbon support (few tens of nm to µm) for electron conduction, ionomer chains (few nm to µm) for proton transport and pore structure (few nm to few hundred nm) for gas transport. [1] There is a strong impetus to decrease the catalyst loading to minimize Pt material cost and ease the supply-demand scenario. While extensive research into the development of highly active Pt-alloy nanoparticle catalyst systems has led to tremendous improvements in the low current density kinetic performance region, the current state of the art cathode electrodes suffer from a poor high current density (HCD) performance at low Pt loadings which leads to an increase in stack area and cost. This brings into focus the need to fundamentally understand the functional role and microstructure-property relationships of the various components in the cathode catalyst layer towards mitigating the HCD mass transport losses at low precious metal loadings. [2] Recent studies have pointed to the presence of a local transport resistance in the cathode catalyst layer that affects high current density performance via i) decrease in O2 permeability through the ionomer thin film covering the Pt/C agglomerates, and/or ii) O2 transport within the Pt/C agglomerates. [2, 3] In this context, the chemical composition of the PFSA ionomer and its aggregate structure plays a key role in not only enabling the bulk proton transport in the catalyst layer but also in directly affecting the local O2 transport resistance through the ionomer thin film and at the catalyst-ionomer interface. Current state-of-the-art perfluorosulfonic acid (PFSA) ionomers that are composed of both hydrophobic fluoroethylene backbone and sulfonic acid end groups are understood to form a non-homogeneous thin film with varying thickness of few nanometers around the catalyst particles. The phase-segregated structure of these ionomers plays a key role enabling a high proton conductivity and O2 permeability through the film. There are two characteristic factors of PFSA ionomers that control ionomer phase segregation. These are i) side chain length/chemistry and ii) equivalent weight (EW). These two factors are related by the equation EW = 100m + MWside-chain, where m is the fluorocarbon backbone length (number of TFE units) and MWside-chain is the molecular weight of the side chain. For a given EW, ionomers with shorter side chain length (m) yield a higher backbone fraction. Similarly, for a given side-chain length, increasing EW indicates a higher backbone fraction. Hence, both the side-chain length/chemistry and the EW could be tuned to affect the backbone length of the ionomer and hence the phase separation behavior. [4] In this study, we have systematically examined the ex situ and in situ characteristics of a few select PFSA ionomers of varying side-chain length and EW. Ex situ analysis of the ionomer include x-ray scattering, size exclusion chromatography, ionic conductivity, and water uptake measurements of the thin films. In situ measurements include H2/air polarization curves under differential cell conditions and electrochemical diagnostics to quantify proton and local-O­2 transport resistances. Figure 1 shows a snapshot of H2/air polarization curves and local O2 transport resistances measured using PFSA ionomers of different chain lengths (short – SSC, medium – MSC and long – LSC) and EW. The two lower EW ionomers were found to significantly decrease the transport resistance and increase the high current density performance. The measured differential cell data will be used in a 1-D model to quantify the performance loss terms. The effects of ionomer EW and side-chain length on the catalyst layer transport resistance and the corresponding impact on the high current density performance will be discussed. References S. Holdcroft, Fuel Cell Catalyst Layers: A Polymer Science Perspective, Chem. Mater. 2014, 26 (1), pp 381-393A. Kongkanand, M. F. Mathias, The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells, J. Phys. Chem. Lett., 2016, 7 (7), pp 1127–1137S. Jomori et al., An Experimental Study of the Effects of Operational History on Activity Changes in a PEMFC, J. Electrochem. Soc. 2013, 160 (9), pp F1067-F1073A. Kusoglu, A. Weber, New Insights into Perfluorinated Sulfonic-Acid Ionomers, Chem. Rev., 2017, 117 (3), 987-1104 Figure 1

An efficient distribution method for nonlinear two-phase flow in highly heterogeneous multidimensional stochastic porous media

In the context of stochastic two-phase flow in porous media, we introduce a novel and efficient method to estimate the probability distribution of the wetting saturation field under uncertain rock properties in highly heterogeneous porous systems, where streamline patterns are dominated by permeability heterogeneity, and for slow displacement processes (viscosity ratio close to unity). Our method, referred to as the frozen streamline distribution method (FROST), is based on a physical understanding of the stochastic problem. Indeed, we identify key random fields that guide the wetting saturation variability, namely fluid particle times of flight and injection times. By comparing saturation statistics against full-physics Monte Carlo simulations, we illustrate how this simple, yet accurate FROST method performs under the preliminary approximation of frozen streamlines. Further, we inspect the performance of an accelerated FROST variant that relies on a simplification about injection time statistics. Finally, we introduce how quantiles of saturation can be efficiently computed within the FROST framework, hence leading to robust uncertainty assessment.

Development of Ordered Mesoporous Carbon Supported Fuel Cell Cathode Catalysts for Improved O2 Transport

Current state of the art PEMFC cathode catalyst layer can be described as a complex, heterogeneous porous system with multiple components across various length scales at which both the electrochemical reaction and mass transport of reactants/products occur. The functional components in the catalyst layer comprises of the catalyst nanoparticles (few nm in diameter) for electrochemical reaction, carbon support (few tens of nm to µm) for electron conduction, ionomer chains (few nm to µm) for proton transport, and pore structure (few nm to few hundred nm) for gas transport. Mass transport of reactants and products are supported by the intrinsic pore structure in the carbon support and the secondary pore structure induced by the ionomer chains in the catalyst ink formulation stage. [1] There is a strong impetus to decrease the catalyst loading to minimize Pt material cost and also, ease the supply-demand scenario. While extensive research into the development of Pt-alloy nanoparticle catalysts has led to improvements in the low current density kinetic region, the current state of the art cathode electrodes suffer from a poor high current density (HCD) performance at low Pt loadings which leads to an increase in stack area and cost. This brings into focus the need to fundamentally understand the functional role and microstructure-property relationships of the various components in the cathode catalyst layer towards mitigating the HCD mass transport losses at low PGM loadings. [2] Recent studies have pointed out to the presence of a local transport resistance in the cathode catalyst layer that affects HCD performance via i) decrease in O2 permeability through the ionomer thin film covering the Pt/C agglomerates, and/or ii) O2 transport within the Pt/C agglomerates. The latter resistance arises due to the necessity of O2to be transported within the tortuous pore structure in the Pt/C agglomerates to reach the active site. [2, 3] Perhaps, the most complicating aspect of the catalyst layer microstructure arises from i) the complex pore structure of the carbon support upon which the Pt-alloy nanoparticles are deposited, and ii) the extensive aggregation of both the porous carbon support & the colloidal ionomer particles. A well optimized cathode catalyst layer could be envisioned as one where the ionomer is distributed around the Pt/C agglomerates with an optimal coverage in such a fashion that there is no direct Pt-ionomer interface formation (to minimize sulfate poisoning), but with Pt nanoparticles present at vantage locations within the agglomerates that are more open, less tortuous and easily accessible by O2. Such a structure would essentially maximize the mass activity in the kinetic regime and minimize the local-O2transport resistance in the mass transport regime. Current state-of-the-art carbon supports for cathode catalyst applications involve moderate to high surface area materials that feature either a disordered-porous (e.g. Ketjen Black) or non-porous (e.g. Vulcan) structure. While the porous carbon supports host the Pt nanoparticles within its micro-meso pores thereby preventing it from coming into direct contact with the sulfate functional groups in the ionomer, the disorder in these pores leads to high tortuous pathways for O2transport that affects the HCD performance. Contrarily, the non-porous carbon supports host Pt nanoparticles on the surface in direct contact with the ionomer leading to a poor low mass activity. In this study, we demonstrate the use of ordered mesoporous carbon (OMC) supports for cathode catalyst applications to mitigate the HCD performance losses. As shown in Figure 1, OMC support features a unique pore structure with the majority of surface area in the mesoporous region along with the presence well-ordered, open, continuous pores for reactant transport. [4] High mass activity is achieved since some or all of the Pt nanoparticles are deposited inside the pores so that there is no direct contact with the ionomer but at the same time the pores have an open, continuous and ordered structure that enables better O2 transport at high current densities. The merits and challenges that arise from the use of OMC supports for cathode catalyst applications will be discussed. References 1) S. Holdcroft, Fuel Cell Catalyst Layers: A Polymer Science Perspective, Chem. Mater. 2014, 26 (1), pp 381-393 2) A. Kongkanand, M. F. Mathias, The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells, J. Phys. Chem. Lett., 2016, 7 (7), pp 1127–1137 3) S. Jomori et al., An Experimental Study of the Effects of Operational History on Activity Changes in a PEMFC, J. Electrochem. Soc. 2013, 160 (9), pp F1067-F1073 4) R. Ryoo et al., Ordered Mesoporous Carbon, Adv. Mater. 2001, 13 (9), pp 677-681 Figure 1

Permeability evolution across carbonate hosted normal fault zones

Carbonate lithologies tend to have highly heterogeneous and tortuous pore systems that are created and/or modified by diagenetic and tectonic processes following deposition. The correlation between porosity and permeability in carbonate lithologies is often poor as a result of their heterogeneous and complex pore systems. To effectively predict permeability, it is necessary to understand the processes that modify pore systems and quantify the impact of these modifications on permeability. Using outcrop exposures of normal fault zones hosted in carbonate lithologies on the Maltese Islands, this study documents the evolution of textures in contrasting carbonate lithofacies (wackestones, packstones and pack/grainstones) across two normal fault zones of varying displacement (c. 10 and 100 m). The pore system modifications associated with these textural changes are quantified using image analysis and point count methods, while porosity and permeability are measured across the studied fault zones using core plug porosimetry and permeametry techniques.The fault related processes that occur within the fault zones are controlled by the primary lithofacies and to a lesser extent the fault displacement. Aggrading neomorphism is observed within the damage zones in the grain supported lithofacies and is postdated by fracturing. In the micrite supported lithofacies in the same damage zones, aggrading neomorphism is absent, but fracturing is prevalent. In the fault core, brecciation occurs in both lithofacies within the 10 and 100 m displacement fault zones, while cataclasis is only active in the grain supported lithofacies in the higher displacement fault zone. The mineralogical and textural compositions of the primary lithofacies dictate the processes that occur in the fault zones. These processes variably modify the pore systems and hence control the temporal evolution of permeability in the fault zones. Such observations can help understand reservoir quality distribution around fault zones in the subsurface reservoirs.

Carbonate pore system evaluation using the velocity–porosity–pressure relationship, digital image analysis, and differential effective medium theory

Abstract Carbonate reservoirs exhibit heterogeneous pore systems and a wide variety of grain types, which affect the rock's elastic properties and the reservoir parameter relationships. To study the Albian carbonates in the Campos Basin, a methodology is proposed to predict the amount of microporosity and the representative aspect ratio of these inclusions. The method assumes three pore-space scales in two representative inclusion scenarios: 1) a macro–mesopore median aspect ratio from the thin-section digital image analysis (DIA) and 2) a microporosity aspect ratio predicted based on the measured P-wave velocities. Through a laboratory analysis of 10 grainstone core samples of the Albian age, the P- and S-wave velocities (Vp and Vs) are evaluated at effective pressures of 0–10 MPa. The analytical theories in the proposed methodology are functions of the aspect ratios from the differential effective medium (DEM) theory, the macro–mesopore system recognized from the DIA, the amount of microporosity determined by the difference between the porosities estimated from laboratorial helium-gas and the thin-section petrographic images, and the P-wave velocities under dry effective pressure conditions. The DIA procedure is applied to estimate the local and global parameters, and the textural implications concerning ultrasonic velocities and image resolution. The macro–mesopore inclusions contribute to stiffer rocks and higher velocities, whereas the microporosity inclusions contribute to softer rocks and lower velocities. We observe a high potential for this methodology, which uses the microporosity aspect ratio inverted from Vp to predict Vs with a good agreement. The results acceptably characterize the Albian grainstones. The representative macro–mesopore aspect ratio is 0.5, and the inverted microporosity aspect ratio ranges from 0.01 to 0.07. The effective pressure induced an effect of slight porosity reduction during the triaxial tests, mainly in the microporosity inclusions, slightly changing the amount and the aspect ratio of the microporosity.

Effect of disorder on condensation in the lattice gas model on a random graph.

The lattice gas model of condensation in a heterogeneous pore system, represented by a random graph of cells, is studied using an exact analytical solution. A binary mixture of pore cells with different coordination numbers is shown to exhibit two phase transitions as a function of chemical potential in a certain temperature range. Heterogeneity in interaction strengths is demonstrated to reduce the critical temperature and, for large-enough degreeS of disorder, divides the cells into ones which are either on average occupied or unoccupied. Despite treating the pore space loops in a simplified manner, the random-graph model provides a good description of condensation in porous structures containing loops. This is illustrated by considering capillary condensation in a structural model of mesoporous silica SBA-15.

Carbon black directed synthesis of ultrahigh mesoporous carbon aerogels

A simple modification of the conventional sol–gel polymerization of resorcinol–formaldehyde mixtures allowed a facile preparation of ultrahigh mesoporous carbon gels. In the conventional synthesis the growth of the cluster polymer particles leading to the development of the porosity is controlled by the R/C ratio. In the presence of a carbon conductive additive, the polymerization of the reactants proceeded through the formation of less-branched polymer clusters resulting in carbon gels with large pore volumes within the micro/mesoporous range. The obtained materials displayed unusual heterogeneous pore systems characterized by large mesopores interconnected by necks of variable sizes, along with an enhanced electrical conductivity provided by the carbon black additive. The gels showed stable electrochemical response in neutral aqueous electrolyte, being reversibly charged/discharged at large potential windows, without significant losses in the current density, chemical modifications or structural collapse. The enhanced life cycle of these electrodes makes them good candidates for their use in electrochemical applications where a fast response and high cycleability is required.

Effect of surface functionalities on gas adsorption in microporous carbons: a grand canonical Monte Carlo study

In an attempt to offer a more realistic picture of adsorption in highly heterogeneous porous systems, such as oxygen functionalized porous carbons, we consider a series of carbon surfaces baring different amounts of oxygen functionalities (hydroxyl and epoxy). These surfaces are used to construct “oxidized” slit pores of varying width and functionality. With the aid of such inhomogeneous structures we study the interaction of Ar (87 K) inside “functionalized” pores and report grand canonical Monte Carlo adsorption simulations results. Based on our simulation data, we discuss the role of chemical heterogeneity on adsorbed/gas phase equilibrium properties such as density, heat of adsorption, and molecular packing within the pores. Comparisons are made with the case of the oxygen–free (completely homogeneous) slit pore models and conclusions on the suitability of Ar based pore size distributions for functionalized porous carbons are drawn.

Molecular simulation of methane adsorption in micro- and mesoporous carbons with applications to coal and gas shale systems

Methane adsorption in porous carbon systems such as coal and the organic matrix of gas shales is an important factor in determining the feasibility of CO2 injection for enhanced natural gas recovery and possible sequestration of CO2. Methane and CO2 adsorb competitively on carbon surfaces and an understanding of each gas individually is important for determining a model to predict the feasibility of this approach for permanent CO2 storage. Coal and gas shales have a very heterogeneous pore system, ranging from the micro, meso, and macro-scales, with the pore size strongly affecting the adsorption behavior. In micropores, the force fields of opposing pore walls are close enough that they will overlap and significantly influence the adsorption behavior, which affects adsorbate packing and density. To determine the size at which these effects become non-negligible and to determine the magnitude of this impact, grand canonical Monte Carlo simulations have been carried out to estimate the adsorption isotherms of methane across a range of pore sizes and at various temperature and pressure conditions characteristic of subsurface conditions. These isotherms have been calculated on graphitic surfaces as an initial model of coal and kerogen of gas shales. The general trend within pore sizes is that larger pores exhibit lower excess density compared to smaller pores. However, at pressure above 1MPa, the adsorption capacities of 0.6-nm pores drop below those of the wider pores, ultimately decreasing below that of the 1.2-nm pore at 18MPa. The density of adsorbed methane changes non-monotonically with increasing pore width, and drops to a minimum in 1.2-nm pores at 12MPa. The isotherms have been compared with experimental data to gauge their accuracy, and the behavior of the adsorbed layer has been examined in detail. At pressures less than 2.5MPa, the molecular simulation estimates underpredict the excess adsorption, while at pressures greater than 2.5MPa up to 20MPa, the simulation estimates overpredict the excess adsorption. This discrepancy is likely due to the limitation of the experimental-based model that was used to generate the pore size distribution and the surface functionalities of the porous media that were ignored in the molecular simulation investigations, but likely play an important role in determining accurate capacities under confinement at the nanoscale.

AN EXPLANATION OF LISIANTHUS AND LETTUCE BEHAVIOR: MICROMORPHOLOGICAL FEATURES AND PHYSICAL PROPERTIES OF DIFFERENT GROWING MEDIA

The micromorphological features and physical properties of several materials for growing media were compared with response variables from two greenhouse experiments, so as to determine the limiting and/or promoting factors of plant development. The first greenhouse experiment used lisianthus (Eustoma grandiflorum) and included organic (coconut fiber, Cf; and peat moss, Pm) and inorganic (pumice, P; and Perlite, Pe) materials, with varying particle size (1-2 and 2-3.36 mm), using the same organic:inorganic ratio (60:40 v/v), and using bulk material as blank tests. The second experiment used lettuce (Lactuca sativa), with one particle size, but with varying ratios (100 Cf, 75:25, 50:50, 25:75, 100 P v/v) of Cf:P mixtures. Thin sections were prepared from the mixtures of both experiments, to describe sorted degree, distribution patterns and porosity (macro and micro). Furthermore, the water release curve was determined (air capacity, AC; easily available water, EAW) for each treatment in both experiments. In lisianthus, it was observed that using a single particle size Cf:P (1-2 or 2-3.36 mm) renders higher yields, because of the loose packing particles, well sorted material and macro- and micropores, resulting in sufficient EAW (36%) and AC (33%). In contrast, the worst effects were in bulk peat moss, because of the poor sorted, close packing particles, and high micropores content (95%) resulting in a low AC (12%) and EAW (49%), overall. Regarding lettuce, the best ratio resulted in Cf:P 75:25 v/v compared to other ratios, because particles presented a banded distribution pattern generating an heterogeneous porous system (vughs, fissures, and simple and compound packing voids). We conclude micromorphology helps to a better understanding of physical properties of mixtures, and to identify which features for growing materials may improve or restrict lisianthus and lettuce development.

X-ray radiography and tomography for monitoring the penetration depth of consolidants in Opuka – the building stone of Prague monuments

Abstract Stone consolidation is one of the major restoration treatments used for historical monuments preservation. A natural stone is a complicated heterogeneous porous system making the process of consolidation dependent on many variables. In practical restoration aims, for a given stone type, the selection of a suitable consolidant and consolidation conditions therefore remains a complex issue. The impregnation depth is a key factor for the assessment of the treatment efficiency. The combination of state-of-the-art hybrid pixel semiconductor detectors with newly available micro-focus X-ray sources makes possible to apply X-ray radiography, an ideal non-destructive tool, for penetration depth monitoring. In this study, high-resolution X-ray radiography is used for monitoring the penetration depth of organosilicon consolidants in the Opuka stone. The penetration depth has been evaluated in relation to the time of consolidation, stone porosity and consolidation mixtures properties. The exact influence of the X-ray contrast agent on the consolidation depth has been investigated as well. The information obtained provides supplementary knowledge on the suitability of investigated products for the treatment of this type of stone. In our study, the capabilities of X-ray radiography have been demonstrated on X-ray radiography simple projections, high-resolution computed tomography (CT) as well as on the dynamic processes monitoring. The results thus can serve also as an instrumental and methodological example applicable for consolidation monitoring of other stone types.

Oriented calcite concretions in Upper Miocene carbonate rocks of Menorca, Spain : evidence for fluid flow through a heterogeneous porous system

Elongate calcite concretions in Upper Miocene dolomitic, shallow-marine grainstones and packstones of Menorca document fluid flow through heterogeneous systems. These post-dolomitization concretions are thought to have grown with elongation axes parallel to groundwater flow direction, and to reflect the hydraulic gradient and the anisotropy of the hydraulic conductivity. Differences in shape, size and orientation of concretions, as well as the spatial and crosscutting relationships reflect two phases of calcite-cementing fluids. This is in contrast to most examples in the literature that distinguish just one phase of precipitating fluids. The first phase of cementing fluids flowed horizontally, most likely in the phreatic zone, across hydraulic high-conductivity layers that resulted from sediment-packing heterogeneities and preferential dissolution during dolomitization. These first-phase fluids were most likely injected into the host rock through fracture zones and probably originated in deeper settings. The second phase of cementing fluids was downward directed and possibly of meteoric origin. First-generation concretions, acting as permeability barriers, partly controlled the fluid pathways, precipitation patterns, and concretion type and loci.

RELACIÓN POROSIDAD-RETENCIÓN DE HUMEDAD EN MEZCLAS DE SUSTRATOS Y SU EFECTO SOBRE VARIABLES RESPUESTA EN PLÁNTULAS DE LECHUGA

The relationship among porosity, size and particle distribution (1-2 mm and 2-3.36 mm) with water retention for different mixtures of materials: coir (Fc), stone (P) or volcanic stone (T), combined in variables proportions (0:100, 25:75, 50:50, 75:25 and 100:0 v/v) was studied. The effect of the combination Fc:P with a particle size (1-2 mm) on response variables in lettuce seedlings var. Summertime was analyzed. The water retention curve was determined. The types of pores and the particle distribution pattern of the mixtures was analyzed though thin sections. Porosity was quantified using an image analyzer. Results showed that Fc:T and Fc:P mixtures (75:25 v/v), showed the highest volumetric (98 %) and gravimetric (230 %) water retention capacity, as well as an anisotropic or heterogeneous pore system (macro and micro), and a band distribution, that allowed percolation and optimal water retention for the development of lettuce seedlings, represented in the best response variables and quality of root ball. In contrast, the rest of the mixture with an isotropic or homogeneous pore system, either simple or compound packing pores, and a basic random distribution of particles, generating more percolation or more water retention, that limited the growth of seedlings. Micromorphology can be useful for understanding the physical properties of the subs¬trates by directly determining the type, size and continuity of pores, characteristics that affect water-air availability.

Improved capillary pressure model considering dual-pore geometry system in cabonate reservoirs

In cabonate rocks, it is difficult to precisely represent the relationship between capillary pressure and water saturation due to the high degree of heterogeneity in pore geometry. Cabonate rocks have various types of pore systems such as fractures and vugs, and this heterogeneity should be considered for more realistic prediction of oil production in carbonate reservoirs. However, the previously developed capillary pressure models like Leverett J-function and Thomeer method cannot accurately describe the cabonate rocks having great heterogeneity in its pore system. These models consider the pore as a single pore system. Therefore, we present a capillary pressure model that can handle the pore system as macropore and micropore, separately. We compare the result of this model against experimental data measured for special core analysis of six different cores of cabonate rocks. As a result, it was found that this model favorably matches with experimental results in the entire range of water saturation. The applicability of this model for the greater heterogeneous pore system was also investigated. From the comparison with experimental results for three core samples with different degrees of heterogeneity, one can realize that the error is much greater in Thomeer method and Leverett J-function than this model especially in heterogeneous system. When the developed model is applied to the case without capillary pressure data, the model firstly quantifies the matching parameters, i.e. pore geometrical factor, displacement pressure and residual saturation for each macropore and micropore. And then, the applicability of this model was tested with these matching parameters by comparing to experimental data. The result showed excellent matches in cases of both macropore and micropore systems with correlation coefficient value of 0.97. Therefore, it can be concluded that the capillary pressure model proposed in this study could apply appropriately to the greater heterogeneous pore system like cabonate rocks.

Experimental and modeling investigation of multicomponent reactive transport in porous media

We present an experimental and modeling study of solute transport in porous media in the presence of mixing-induced precipitation of a solid phase. Conservative and reactive transport experiments were performed in a quasi-two-dimensional laboratory flow cell, filled with homogeneous and heterogeneous porous media. Conservative experiments were performed by injecting solutions containing sodium chloride and calcium chloride into the domain. In reactive transport experiments, inlet solutions of calcium chloride and sodium carbonate were injected in parallel, resulting in calcium carbonate precipitation where the solutions mix. Experimental results were used as a benchmark to examine the performance of a reactive transport numerical model. Good agreement between model predictions and experimental results was obtained for the conservative transport experiments. The reactive transport experiments featured the formation of a calcium carbonate mineral phase within the mixing zone between the two solutions, which controlled the spatial evolution of calcium carbonate in the domain. Numerical simulations performed on high resolution grids for both the homogeneous and heterogeneous porous systems underestimated clogging of the system. Although qualitative agreement between model results and experimental observations was obtained, accurate model predictions of the spatial evolution of calcium concentrations at sample points within the flow cell could not be achieved.

Two-domain description of solute transport in heterogeneous porous media: Comparison between theoretical predictions and numerical experiments

This paper deals with two-equation models describing solute transport in highly heterogeneous porous systems and more particularly dual permeability structures composed of high- and low-permeability regions. A macroscopic two-equation model has been previously proposed in the literature based on the volume averaging technique [Ahmadi A, Quintard M, Whitaker S. Transport in chemically and mechanically heterogeneous porous media V: two-equation model for solute transport with adsorption, Adv Water Resour 1998;22:59–86; Cherblanc F, Ahmadi A, Quintard M. Two-medium description of dispersion in heterogeneous porous media: calculation of macroscopic properties. Water Resour Res 2003;39(6):1154–73]. Through this theoretical upscaling method, both convection and dispersion mechanisms are taken into account in both regions, allowing one to deal with a large range of heterogeneous systems. In this paper, the numerical tools associated with this model are developed in order to test the theory by comparing macroscopic concentration fields to those obtained by Darcy-scale numerical experiments. The heterogeneous structures considered are made up of low-permeability nodules embedded in a continuous high-permeability region. Several permeability ratios are used, leading to very different macroscopic behaviours. Taking advantage of the Darcy-scale simulations, the role of convection and dispersion in the mass exchange between the two regions is investigated. Large-scale averaged concentration fields and elution curves are extracted from the Darcy-scale numerical experiments and compared to the theoretical predictions given by the two-equation model. Very good agreement is found between experimental and theoretical results. A permeability ratio around 100 presents a behaviour characteristic of “mobile–mobile” systems emphasizing the relevance of this two-equation description. Eventually, the theory is used to set-up a criterion for the existence of local equilibrium conditions. The potential importance of local-scale dispersion in reducing large-scale dispersion is highlighted. The results also confirm that a non-equilibrium description may be necessary in such systems, even if local-equilibrium behaviour could be observed.

Methods for colloid transport visualization in pore networks

Prediction of colloid transport in the subsurface is relevant to researchers in a variety of fields such as contaminant transport, wastewater treatment, and bioremediation. Investigations have traditionally relied on column studies whereby mechanistic inferences must be drawn on the basis of colloid behavior at the outlet. Over the past decade, development of noninvasive visualization techniques based on visible light, magnetic resonance, and X rays have provided insight into a number of colloid transport mechanisms by enabling direct observation of individual colloids at the pore scale and colloid concentrations at longer length scales. As research focus shifts from transport of ideal colloids in ideal media such as glass beads to natural colloids in natural porous media, these noninvasive techniques will become increasingly useful for studying the collection of mechanisms at work in heterogeneous pore systems. It is useful at this juncture to review recent progress in colloid transport visualization as a starting point for further development of visualization tools to support investigation of colloids in natural systems. We briefly discuss characteristics of visualization systems currently used to study colloid transport in porous media and review representative microscale and mesoscale visualization studies conducted over the past decade, with additional attention given to two optical visualization systems being developed by the authors.

Closed‐Form Expression for the Multi‐Modal Unsaturated Conductivity Function

In this note we derive a closed‐form expression representing the hydraulic conductivity function for soils with a multi‐modal pore size distribution. By combining the multi‐modal representation of the retention function of Durner with the conductivity representation model of Mualem and following van Genuchten, we derive a simple analytical expression for the conductivity of soils with heterogeneous pore systems. Examples for the representation of bi‐modal and tri‐modal retention and corresponding conductivity curves demonstrate the applicability of the simple analytical expressions. It is concluded that the usefulness of the multi‐modal representation of retention functions is increased by providing an analytical expression to directly calculate unsaturated hydraulic conductivities.