Structure Of Porous Media Research Articles

Prediction of the capillary pressure of fluid surrounding a cylinder representing an idealized rock structure in porous media

Liquids in oil-bearing porous media assume complex shapes that depend on the reservoir characteristics and the wetting properties of the liquid. The wide variation in the geometry of rock formations makes it difficult to accurately predict the capillary pressure of small volumes of liquid and hence the likelihood of being able to move it. Here, we consider the situation in which a small volume of liquid surrounds an upright cylinder on a flat substrate and predict the shape that the liquid takes and its capillary pressure. We validate our predictions by comparing with Surface Evolver simulations for a range of contact angles and cylinder radii.

Effects of Key Influencing Factors on the Flame Inclination of Low Concentration Methane (LCM) Combustion in Porous Burner

ABSTRACT In present work, a local thermal non-equilibrium numerical model of low concentration methane combustion in porous media is established. The modified thermal effective thermal conductivity of foam ceramics was introduced. The heat and mass dispersion effect as well as the influence of porous media structure on the heat transfer were fully considered. Studies found that the combustion flame front of low concentration methane in porous media always show a S-shaped state under special conditions. The influencing factors of inlet velocity, methane concentration, wall heat loss, and the porous burner size were analyzed. Results show that without initial thermal perturbation and the absence of defects in the porous media, the flame front in porous burner still has the possibility of inclination. As the inclination of the flame front continues to increase, the flame keeps approaching the burner outlet until blow off. Among the considered four influencing factors, the size of porous burner has the greatest influence on the inclination of flame front. As the burner diameter increases from 0.04 m to 0.12 m, the inclination angle of the flame front increases continuously, and the maximum inclination angle is 36°. The flame inclination will cause uneven temperature distribution downstream of the burner and local heat concentration. With the increase of wall heat loss, the high-temperature zone downstream of the burner will evolve into a local high-temperature zone near the flame front. With the reducing of porous burner diameter, the inclinational instability could be suppressed to some extent.

Local entropy generation model for numerical CFD analysis of fluid flows through porous media, under laminar and turbulent regimes

Porous media structures have been proposed as an interesting solution on the design of high-temperature volumetric heat exchangers and sensible thermal energy storage devices. The wide exchange area between the solid matrix and the fluid offers the possibility to reach higher conversion efficiencies, particularly on applications of high-temperature (∼1000°C) gases. Nevertheless, the presence of the solid matrix increases the hydrodynamic resistance on the flow, and consequently, generates irreversibilities. The entropy generation can assess in the same figure of merit the different irreversibilities generation mechanisms. In this context, this work presents a physical and mathematical model to determine the local entropy generation (LEG) rate and recognizes its different generation mechanisms for porous media. The proposed model defines a useful expression to determine the LEG as a post-process variable from the usual CFD scalar and vectorial results (temperature, velocity, TKE, and ), without the necessity of solving an additional entropy transport equation. A numerical experiment was implemented showing inflection points where the porous hydrodynamic resistance forces exceed the heat transfer in the LEG rate. The Forchheimer hydrodynamic resistance effect can domine the LEG in comparison to the volumetric heat transfer for high porous Reynolds regimes (>100) when the porosity is under 0.6.

Prediction of collector contact efficiency for colloid transport in porous media using Pore-Network and Neural-Network models

Conventional colloid filtration theory (CFT) uses the single collector contact efficiency (η) to describe the mass transfer of colloids to a collector surface. However, this approach neglects the full complexity of the pore structure and flow field of real porous media. In this study, the porous medium geometry, flow field, and colloid mass transfer are quantified using a pore-network model (PNM). A database of pore scale η is established by finite-element method to train a Neural-network model (NNM). The reasonable prediction of η indicates the potential of using the developed NNMs as an alternative to correlation equations, which can free the users from repeated numerical simulation. In contrast to the prediction by conventional CFT, the value of η in the PNM occurs as a distribution, which is dependent upon the geometry parameters of the PNM. The mean value of η increases with the standard deviation of pore radius and decreases with the curvature number, but the dependency on coordination number is more complex. Upscaled values of the deposition rate coefficient (kd) corresponding to the distribution of η are calculated by the breakthrough curves by PNMs. The prediction of kd by PNM is then compared with that by CFT. Results show that kd predicted by PNM shows more significant response to velocity change, and less remarkable response to colloid density change than kd predicted by CFT. The comparison between the flow velocity distribution between PNM and CFT shows that the high-velocity region of the flow field in the porous media has been neglected in CFT, which can lead to insufficient consideration of convection. The results of this work imply that it is necessary to consider the influence of the complex pore structure of porous media on the collection of colloids.

Experiments and phase-field simulation of counter-current imbibition in porous media with different pore structure

Spontaneous imbibition is relevant to many processes in nature and industry. In this work, the effect of pore structure of porous media on counter-current imbibition is studied. The one-dimensional counter-current imbibition experiments were conducted in packed porous media with different pore structure. Experimental results showed that counter-current imbibition can only proceed for a short time in homogeneous porous media for all fluids. In heterogeneous porous media, counter-current imbibition can only proceed a short time for water–air experiments as well. However, imbibition can proceed continuously until the imbibition front reaches the end of packed porous media for water–oil experiments and the mass of water infiltration is proportional to the square root of time. The efficiency of water infiltration behind the imbibition front decreases with increase in viscosity of non-wetting phase fluid. In addition, the numerical simulation of counter-current imbibition in porous media with different pore structure were conducted using phase-field method. Simulation results showed that very little oil was produced in homogeneous porous media and more oil was produced in heterogeneous porous media. It is indicated that the difference between capillary driving pressure and capillary back pressure, which are closely related to pore structure and pore size distribution of porous media, has important influence on counter-current imbibition. Furthermore, the snap-off of oil or gas production channels in narrow pore throats may lead to termination of imbibition when the water saturation behind the imbibition front is higher.

The Migration and Deposition Behaviors of Montmorillonite and Kaolinite Particles in a Two-Dimensional Micromodel.

The pick-up, migration, deposition, and clogging behaviors of fine particles are ubiquitous in many engineering applications, including contaminant remediation. Deposition and clogging are detrimental to the efficiency of environmental remediation, and their mechanisms are yet to be elucidated. Two-dimensional microfluidic models were developed to simulate the pore structure of porous media with unified particle sizes in this study. Kaolin and bentonite suspensions were introduced to microfluidic chips to observe their particle deposition and clogging behaviors. Interactions between interparticle forces and particle velocity profiles were investigated via computational fluid dynamics and discrete element method simulations. The results showed that (1) only the velocity vector toward the micropillars and drag forces in the reverse direction were prone to deposition; (2) due to the negligible weight of particles, the Stokes number implied that inertia was not the controlling factor causing deposition; and (3) the salinity of the carrying fluid increased the bentonite deposition because of the shrinkage of the diffused electrical double layer and an increase in aggregation force, whereas it had little effect on kaolin deposition.

Pores integrated fractal (PIF) analysis on transportation in porous media considering spatial distribution of pores and genuine tortuosity

A pores integrated fractal model is established to represent the microscopic structures of porous media in consideration of the spatial distribution of pores and the genuine tortuosity in various transportation processes. Transport equations are obtained based on the PIF model, providing high precision for predicting the effective thermal conductivity, the permeability and diffusion coefficient in porous materials. Spatial distributions of pores in realistic samples are acquired, based on which the influence of the equivalent pore distributions is specifically analyzed in the study. In the premise of the same porosity, broader range of pore size, dominant pores in dozens of micron or bimodal distribution of pores are crucial factors that influence the heat conduction and permeation in porous media. It is noted that the PIF model has significant values in the design of porous structure with improved transportation characteristics and in the accurate prediction of vital parameters in complex porous media.

Proposal for extraction of pore networks with pores of high aspect ratios

We propose a pore network extraction method based on the concept of omnidirectional Euclidean distance (OED) so as to extract the accurate pore network structures of porous media with pores of high aspect ratios. A deterministic method is introduced to identify the pore body and the pore throat regions. The proposed extraction method is validated by comparing the pore network modeling results against the direct numerical simulation results and the experimental data. The proposed pore network extraction method not only preserves the topological and morphological properties of the void spaces in porous media but also is robust and insensitive to the image noise.

A Fractal Model for Thermal Dispersion Coefficients of Porous Media

The thermal dispersion coefficient is an important parameter to characterize heat and mass transfer in porous media, which is related to the physical properties of fluid and the structure of porous media. The pore-throat structure model for fractal porous media was established, and the local head loss and the velocity dispersion effect were studied for the fluid changing from the turbulent state to the laminar state around the pore-throat structure. The thermal dispersion coefficient formula was derived under the influences of the micropore-throat structure and the velocity dispersion effect. The results show that, the thermal dispersion coefficient is directly proportional to the pore-throat ratio, the number of pore-throat structures and the tortuous fractal dimension, and is inversely proportional to the porosity and the area fractal dimension. Furthermore, in the range of 1~150, the pore-throat ratio has a significant influence on the velocity dispersion effect, and the fluid has a local head loss around the pore-throat structure, which leads to an enhancement of the velocity dispersion effect and an increase of the thermal dispersion coefficient.

Review of foam stability in porous media: The effect of coarsening

Foam stability in porous media is crucial to industrial applications such as enhanced oil recovery (EOR) and CO 2 geological sequestration. Though many studies of foam stability in porous media have been carried out, the results from these studies often interpreted the combined effects of drainage, coalescence, and coarsening. This convoluted interpretation does not add to the understanding and contribution of the isolated foam instability mechanisms. Coarsening or Ostwald ripening dominates the evolution of foam structure in porous media due to the scale effect. To bridge the knowledge gap, a comprehensive review of the mass transfer fundamentals and the experimental methods for observing the foam coarsening process in porous media was conducted. Unlike an open system where bubbles can grow freely, the restriction from the geometric confinement in the rock (or rock-like) matrix decreases the foam coarsening rate. The confinement can even reverse the mass transport direction to go from larger bubbles to smaller bubbles (anti-coarsening). Various foam coarsening models based on Fick's diffusion law from the bubble-scale to the bulk-scale are reviewed to better understand the foam coarsening mechanisms. From the reviewed literature, foam coarsening dynamics are dominated by the average bubble size, the liquid film thickness between bubbles, and the gas-liquid interfacial properties. The proper selection of the foaming agents (surfactants and nanoparticles) and the gas phase (N 2 and CO 2 ) is critical to control the foam coarsening kinetics. The review of the experimental methods shows that both optical and Atomic Force Microscopy (AFM) are commonly used in studying the foam coarsening process at the bubble scale. X-ray microtomography (μCT) provides a robust tool to investigate the temporal evolution of trapped gases at the pore scale in real rocks. Microfluidics allow seeing the foam kinetics in 2D and 2.5D feature representations of the porous media. The fabrication of 3D micromodels with controlled wettability remains a challenge. Additionally, the challenge of capturing the dynamics of the foam coarsening process with moving bubbles remains understudied and ill-resolved. • Compared with drainage and coalescence, coarsening is the most significant process impacting foam stability in porous media. • Fick's diffusion law-based coarsening models are introduced from bubble-scale to bulk-scale. • The selection of the foaming agents and the gas phase (N 2 and CO 2 , etc.) is critical to the coarsening kinetics of foams. • A micromodel is a robust visualization tool of the foam structure evolutionin porous media.

3D-PMRNN: Reconstructing three-dimensional porous media from the two-dimensional image with recurrent neural network

The accurate reconstruction of three-dimensional(3D) structure of porous media is crucial for predicting their physical or mechanical properties. Reconstructing the 3D structure from its reference two-dimensional(2D) image is an intractable inverse problem. Although many traditional methods have been proposed to address this problem, their low efficiency and poor performance limit their application. Recently, the deep learning(DL) based methods have attracted widespread attention. However, the existing DL-based methods suffer from some serious defects, including heavy demand for training samples, unstable training and high GPU memory requirement. In this study, a recurrent neural network(RNN) based model, namely 3D-PMRNN, has been proposed to overcome these weaknesses. To the best of our knowledge, it is the first time that RNN model has been applied to solve the 2D-TO-3D reconstruction problem. Benefiting from this innovative architecture, the model just requires one 3D training sample at least and reconstructs the 3D structure layer-by-layer. Furthermore, the proposed model can reconstruct a larger-scale realization(2563 or larger) due to the novel network architecture, less demand for GPU memory and stable training process. The reconstruction efficiency has also been greatly improved compared to the traditional methods. Three experiments are carried out on isotropic and anisotropic porous media to verify the model’s performance on accuracy, diversity and generalization. Experimental results indicate that the synthetic realizations have good agreement with the testing target in terms of visual observation and quantitative comparison.

Natural Hydrocarbon Samples Classification by Topological Analysis Method

Nowadays material science involves powerful 3D imaging techniques such as X-ray computed tomography that generates high-resolution images of different structures. These methods are widely used to reveal information about the internal structure of geological cores; therefore, there is a need to develop modern approaches for quantitative analysis of the obtained images, their comparison, and classification. Topological persistence is a useful technique for characterizing the internal structure of 3D images. We show how persistent data analysis provides a useful tool for the classification of porous media structure from 3D images of hydrocarbon reservoirs obtained using computed tomography. We propose a methodology of 3D structure classification based on geometry-topology analysis via persistent homology.

Pore-scale flow simulation on the permeability in hydrate-bearing sediments

Gas hydrate exploitation involves complex physical and chemical processes. Formation and dissociation of hydrate break the pressure-temperature phase equilibrium and cause change of pore-structure. The permeability of hydrate-bearing sediments plays a significant role for the fluid flow. The permeability change depends on the evolution of pore habit which is affected by hydrate phase behavior. Quantitative analyzing the relationship between pore structure and permeability still keeps a challenging work. To study the microscopic mechanism of hydrate effect on flow characteristic, the pore-scale simulation was carried out considering hydrate morphology and distribution. Five kinds of hydrate occurrence patterns including grain-coating, pore-filling, throat-filling, grain-cementing, and load-bearing in hydrate-bearing sediments are established. The permeability of hydrate-bearing sediments was obtained by solving the Navier-Stokes equation from the direct numerical simulation (DNS). The effect of factors related to the skeleton grain and hydrate distribution on permeability variation was analyzed. The results indicate that the permeability of hydrate-bearing porous media always changes with the variation of hydrate saturation. In most instance, the permeability reduction curve obeys exponential distribution. Permeability decreases as the hydrate saturation increases. The permeability variation is highly sensitive to the grain size, porosity, pore-throat size, hydrate occurrence pattern, and hydrate distribution morphology. From the perspective of the pore-scale, the formation and growth of hydrate cause the reconstruction and complication of pore structure in porous media. The permeability reduction caused by the hydrate formation is pronounced in the porous media with a larger initial pore-throat size. Considering different hydrate occurrence patterns and distribution morphology, the curve shape of the normalized permeability shows different trend. This new workflow has a potential application for analyzing the microscopic flow mechanism of hydrate-bearing sediments.

Computational modeling and conjugate heat transfer study for in situ design of artificial porous media at varied orientations

Computational modeling and design of artificial porous media has gained immense attention in recent years due to its numerous applications, particularly in the biomedical and electrical industries. Conjugate heat transfer study of porous media in an enclosure is performed in several applications to study generated heat dissipation. The present work investigates the characteristics of conjugate heat transfer on various orientations of computationally modeled artificial porous media structure. In the study, the rate of heat transfer is compared for horizontal and vertical orientations of the porous structures which mainly consists of holes and pores. The study also investigates the effect of porosity on heat transfer. Furthermore, the study establishes that for both horizontal and vertical orientations, an increase in porosity results in an increase of the heat transfer rate from the porous media. The work also discusses, the heat transfer occurring due to forced convection in porous structures. Also, we considered a range of velocities from 1 m/s to 4.5 m/s for analyzing the effect of flow rate on the average heat flux.

Analysis of effective thermal conductivity for circular tubes made with porous media based on fractal theory

AbstractAccurately evaluating the relation between heat transfer performance and the complex structure of porous media is still a difficult task. Most previous fractal models of effective thermal conductivity (ETC) are developed to describe the heat‐conducting characteristics of a unit cell or a representative elementary volume in porous media, and few models have paid attentions to the ETC for practical circular tubes made with a porous structure based on fractal theory. This paper proposes a new ETC model for a circular tube made with porous media based on fractals, and the validity of the present model is proved by previous models and testing data in the literature, then the effects of intrinsic thermo‐physical properties of each component and pore structures on the ETC are discussed. The analysis results indicate that a circular tube made with porous media can improve its heat‐insulating performance by about 25% compared with a common parallel circular tube. This can supply an alternative scheme for pipe insulation design in cold/hot fluid supplying systems or air conditioning systems.

Structure resolved simulations of flow around porous coated cylinders based on a simplified pore-scale model

Porous materials play a significant role in flow control and noise reduction of bluff bodies. The commonly used numerical macroscopic model cannot intuitively and accurately simulate the flow field and forces inside the porous material, while the direct numerical simulation using the microscopic model to resolve the complex structure of porous media requires massive computational resources. This study adopts a simplified microscopic porous model to simulate the flow around a porous coated cylinder (PCC) in the subcritical regime (at a Reynolds number of 8.2×104), and is expected to provide improved internal flow characteristics yet at reasonable computational costs. An array of small cylinders are regularly distributed around the large bare cylinder to mimic the pore structure in which the diameters of the small cylinders are varied to represent different porosities. The numerical simulation uses the k-ω Reynolds average turbulence model and the Ffowcs Williams-Hawkings (FW-H) acoustic analogy to calculate the flow and acoustic characteristics. Numerical results of this simplified microscopic model show that such an approximate PCC has significant effects on flow control and noise reduction, that bear strong similarity with other numerical methods and experimental results. Each model with different porosity reveals noise reduction relative to a bare cylinder. Flow field phenomena within the porous coating are revealed that help shed more light on those inside the realistic microstructures of porous media and the mechanisms of flow and noise control by PCCs.

Three-dimensional reconstruction of porous media using super-dimension-based adjacent block-matching algorithm.

As porous media play an essential role in a variety of industrial applications, it is essential to understand their physical properties. Nowadays, the super-dimensional (SD) reconstruction algorithm is used to stochastically reconstruct a three-dimensional (3D) structure of porous media from a given two-dimensional image. This algorithm exhibits superiority in accuracy compared with classical algorithms because it learns information from the real 3D structure. However, owing to the short development time of the SD algorithm, it also has some limitations, such as inexact porosity characterization, long run time, blocking artifacts, and suboptimal accuracy that may be improved. To mitigate these limitations, this study presents the design of a special template that contains two parts of data (i.e., adjacent blocks and a central block); the proposed method matches adjacent blocks during reconstruction and assigns the matched central block to the area to be reconstructed. Furthermore, we design two important mechanisms during reconstruction: one for block matching and the other for porosity control. To verify the effectiveness of the proposed method compared with an existing SD method, both methods were tested on silica particle material and three homogeneous sandstones with different porosities; meanwhile, we compared the proposed method with a multipoint statistics method and a simulated annealing method. The reconstructed results were then compared with the target both visually and quantitatively. The experimental results indicate that the proposed method can overcome the aforementioned limitations and further improve the accuracy of existing methods. This method achieved 4-6 speedup factor compared with the traditional SD method.

A New Method of Central Axis Extracting for Pore Network Modeling in Rock Engineering

Characterizing internal microscopic structures of porous media is of vital importance to simulate fluid and electric current flow. Compared to traditional rock mechanics and geophysical experiments, digital core and pore network modeling is attracting more interests as it can provide more details on rock microstructure with much less time needed. The axis extraction algorithm, which has been widely applied for pore network modeling, mainly consists of a reduction and burning algorithm. However, the commonly used methods in an axis extraction algorithm have the disadvantages of complex judgment conditions and relatively low operating efficiency, thus losing the practicality in application to large-scale pore structure simulation. In this paper, the updated algorithm proposed by Palágyi and Kuba was used to perform digital core and pore network modeling. Firstly, digital core was reconstructed by using the Markov Chain Monte Carlo (MCMC) method based on the binary images of a rock cutting plane taken from heavy oil reservoir sandstone. The digital core accuracy was verified by comparing porosity and autocorrelation function. Then, we extracted the central axis of the digital pore space and characterize structural parameters through geometric transformation technology and maximal sphere method. The obtained geometric parameters were further assigned to the corresponding nodes of pore and throat on the central axis of the constructed model. Moreover, the accuracy of the new developed pore network model was measured by comparing pore/throat parameters, curves of mercury injection, and oil-water relative permeability. The modeling results showed that the new developed method is generally effective for digital core and pore network simulation. Meanwhile, the more homogeneity of the rock, which means the stronger “representative” of binary map the rock cutting plane, the more accurate simulated results can be obtained.

Pore-throat structure characterization of carbon fiber reinforced resin matrix composites: Employing Micro-CT and Avizo technique.

Micro-CT technique poses significant applications in characterizing the microstructure of materials. Based on the CT three-dimensional(3D) reconstruction technology and “Avizo” 3D visualization software, the microscopic pore-throat structure of porous media can be quantitatively characterized. This paper takes the carbon fiber reinforced resin matrix composites as an example to introduce the operation process of “Avizo” in details, which mainly covers the following modules: Volume Edit, Interactive Thresholding, Fill Holes, Mask, Separate Objects and Generate Pore Network Model, then further discuss the difficult problems when the “Avizo” is employed to analyze. The microstructures of carbon fiber reinforced resin matrix composites illustrate that pores in the upper part of sample are dramatically dispersed, and mainly concentrated in the lower part of sample. The porosity of adopted cuboid is 3.6%, accordingly the numbers of pores and throats reach 268 and 7, respectively. The equivalent radius of pores seems mainly distributed in the range of 0.7–0.8μm, accounting for 28.73% of the total pore number. The surface area of pore ranges from 5 to 10μm2, accounting for 14.16% of the total pore number. The pore volume concentrates in the range of 1–20μm3, accounting for 57.46% of the total pore number. In addition, the equivalent radius of throat mainly concentrates in the range of 1–5μm, the overall length of throat is distributed in the range of 37–60μm, and the equivalent area of throat is distributed non-uniformly in the range of 5–75μm2. This work provides a basis for the further investigation of fluid migration mechanism and law in the composite materials by the numerical simulation methodology.

Predicting the permeability of consolidated silty clay via digital soil reconstruction

Numerical modelling of the pore structure of porous media is still one of the most powerful tools for predicting the soil’s permeability and transport properties for geoengineers. However, that approach relies heavily on both the availability and accuracy of microstructural information, which is often challenging to acquire. In this study, statistically equivalent 3D micropore structures of soil samples were reconstructed using a digital soil model, which generated the 3D microstructures using a probabilistic map derived from 2D cross-section images and parameter sets calibrated via a deep learning neural network. The reconstructed images of the 15 micropore structures generated for each of two soil samples with different consolidation pressures (no disturbance and 3200 kPa) indicated that the pores become homogeneous with the consolidation process. The tortuosity factor was found to have a linear correlation with voxel size for simulation. The permeability of each soil sample was computed using the ABAQUS CFD module, and the results of the simulation were comparable to those for the constant head permeability test.