Pore Structure Of Porous Media Research Articles

Pore scale numerical simulation of flow and heat transfer non-equilibrium in randomly packed bed

The complex pore structure of porous media in packed beds leads to randomness and non-uniformity of flow and heat transfer. In this paper, the pore-scale method is used to numerically study the flow and heat transfer in 3D randomly packed bed. The average value and root-mean-square (RMS) of variables are used to evaluate the non-equilibrium characteristics of temperature and velocity. The results demonstrate that the packed bed consistently exhibits non-equilibrium phenomena due to the influence of porous media structure. The peak of thermal non-equilibrium occurs at the location with the largest temperature difference between gas and solid phases, while the peak of flow non-equilibrium occurs after reaching maximum temperature. With increasing inlet velocity, there is an expansion in both local velocity and temperature disturbance range, leading to a wider non-equilibrium region. However, the trend of non-equilibrium between flow and heat transfer is different.

Experimental and Numerical Simulations of Pore Structures and Seepage Characteristics of Deep Sandstones

Previously conducted studies have established that deep underground rock masses have complex pore structures and face complex geological conditions. Therefore, the seepage problem of such rock masses seriously affects engineering safety. To better explore the seepage law of deep rock masses and ensure engineering safety, indoor experimental methods such as casting thin sections, scanning electron microscopy, and mercury intrusion testing were utilized in this study. The microscopic pore shape, size, distribution, and other structural characteristics of sandstone in coal bearing strata were analyzed. The tortuosity calculation formula was obtained by the theoretical derivation method. And a numerical model was established for seepage numerical simulation research through microscopic digital image methods. The seepage law of surrounding rocks in the Tangkou Coal Mine roadway under different conditions is discussed. The research results indicate that the complexity of the pore structure in porous media leads to an uneven distribution of flow velocity and pressure within the medium. Meanwhile, with the change of physical properties, the fluid flow characteristics also undergo significant changes. The research results can effectively guide micropore water blocking, reduce the impact of groundwater on the environment, ensure the environment and safety of the project, and provide guidance for other geological projects.

Fractal analysis of rock pore structure: A comparative study of the Matlab-based image method and the mercury intrusion method

As a naturally porous medium, rocks have a very strong heterogeneity in their internal pore structure, which has an important impact on the mechanical and chemical properties of rocks, etc. Therefore, it is very important to quantitatively characterise the heterogeneity of the pore structure of rocks. Fractal dimension has long been recognised as an effective means of characterising the heterogeneity of the pore structure of porous media and is widely used in oil and gas exploitation, construction materials, mining, and water engineering. There are different methods to calculate the fractal dimension. To verify the consistency between different methods, this paper compares the results of fractal analysis using the mercury intrusion method and MATLAB image fractal analysis using three rock samples with large differences in porosity and calculates the fractal dimension in three different ways on the basis of the mercury intrusion method. The results demonstrate that the fractal dimension of the mercury intrusion method and the box counting dimension of the image analysis obtained by the three methods, although slightly different in numerical value, are consistent in their numerical relationship, i.e., they all conform to the rule that the stronger the non-homogeneity of the pore throat, the larger the fractal dimension. The results of this paper show that fractal dimension is indeed an effective means of characterising rock homogeneity.

Effect of gas hydrate formation and dissociation on porous media structure with clay particles

Natural gas hydrates are a potential future energy modality with the advantages of clean burning and large resource reserves and have attracted worldwide attention. Natural gas hydrates formation and dissociation impact the skeletal structure and mechanical properties of porous media sediments. In addition, there is a synergistic effect between the migration of fine clay particles in porous media and hydrate behavior. In this study, methane hydrate was examined in-situ using a low-field nuclear magnetic resonance system in the presence of two suspensions of clay particles with different stability. The results showed that clay particles impacted methane hydrate formation and dissociation and the pore structure of porous media. Hydrate nucleated preferentially in large pores, causing them to split into smaller pores; the presence of clay particles, especially illite, improved the water conversion rate. The results indicated that water content in large pores increased after hydrate dissociation but was discontinuous among different pores. When illite was present, the distribution was more continuous; when montmorillonite was present, the water distribution of the large pores was similar to that of the original state. This work increases our understanding of the kinetics, water migration, and pore structure alteration of methane hydrate formation and dissociation in sediments with clay particles and provides support for the safe and efficient development of natural gas hydrates.

Saltwater intrusion induced micro-scale mineral precipitation and evolution in porous media

Saltwater intrusion is often occurring in coastal regions, and can substantially alter the chemical properties of groundwater. Thus, the micro-processes of mineral precipitation and the subsequent pore structure of porous media will evolve. However, these micro-processes have not yet been thoroughly studied. In this work, a series of column experiments were conducted to assess the evolution of ion exchange and simulate the mixing process of saltwater-freshwater that occurred in coastal aquifers after saltwater intrusion. The spatiotemporal distribution, morphological characteristics, and elemental composition of the precipitated minerals were characterized using nano-computed tomography, scanning electron microscopy, and energy dispersive spectroscopy mapping. The results revealed that mineral precipitation occurred throughout the columns. The number of smaller-diameter minerals with larger specific surface areas increased the most. The precipitated minerals contained mainly calcium and magnesium carbonates, and the morphology of the precipitated minerals varied, including block-, needle-, cube-, and rod-like precipitates. The mineral precipitates first filled the cracks in the quartz sand and then precipitated on the surface of quartz sand. Under the effects of ion exchange and mixing, quartz sand grains in the saltwater-freshwater mixing zone tended to become oversaturated and form precipitates. Moreover, the mineral precipitates preferentially nucleated rather than increasing in volume during the precipitation.

Characterization method of core pore structure based on truncated Gaussian and its application in shale cores

The core pore structure is mainly described by fractal theory. Conventional single fractal models cannot express the multi-peak distribution characteristics of coal rock, shale, carbonate rock, and other cores. A multifractal model has numerous dimensions and discontinuous functions, making the model parameter selection difficult. Based on the capillary bundle theory and the truncated Gaussian distribution model, a model for characterizing the multi-peak distribution structure of cores is developed in this study. The model's accuracy is verified using the tested NMR experimental data of shale bimodal distribution and the cited coal trimodal distribution. This study further analyzes the influence of tortuosity and tortuosity fractal dimensions on this model. The results show that tortuosity and tortuosity fractal dimensions are conducive to improving the accuracy of the multi-peak pore structure model. The structure model, which can characterize the multi-scale and multi-peak distribution characteristics of shale pore diameter, provides a new concept for describing the pore structure of porous media.

Prediction of colloid sticking efficiency at pore-scale and macroscale using a pore network model

The sticking efficiency (α) is a vital parameter to predict the transport and deposition of colloids in porous media. The value of α depends on various factors such as the interaction energy between colloids and the solid-water interface (SWI), kinetic energy fluctuations of diffusing colloids, and the hydrodynamics of the flow field in the pore structure. However, α is usually assumed to be spatially constant and fitted from experimental data. In this study, a theoretical method is proposed to predict the distribution of the sticking efficiency at pore scale (αt) using a pore network model (PNM) and the macroscale sticking efficiency of the whole porous media (αM) by means of upscaling. A PNM is established to simulate the pore structure of porous media and to obtain the flow field, and energy and torque balances are calculated throughout the spatial pore structure to determine the distribution of αt under different physiochemical conditions. The value of αM is determined through breakthrough curves provided by the PNM under favorable and unfavorable conditions. Results show the distribution of αt is sensitive to various factors including colloid characteristics, pore surface features, geochemical environment and hydrodynamic conditions. Colloid characteristics like colloid surface zeta potential, geochemical environment such as solution ionic strength, and pore surface characteristics including nanoscale roughness and especially charge heterogeneity have a controlling influence on αt for nanoparticles (<100 nm) due to their impact on interaction energies. Hydrodynamic conditions played an increasingly important and eventually dominant role in determining αt for larger colloids by changing the sticking of weakly associated colloids (e.g., at secondary minima). When hydrodynamic torques are weak, the influence of colloid size on αt can be non-monotonic due to the combined influence of interaction energy and hydrodynamic torques. As a result, higher values of αM occurred for smaller colloid sizes, lower flow velocities, larger pore sizes, and in the presence of microscopic roughness. The method proposed by this study can be used to predict the sticking efficiency under different solution and solid phase chemistries, nanoscale heterogeneities, microscopic roughness, flow velocities, and pore structure conditions.

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.

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.

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.

Characterization of the Pore Structure of Porous Media Using non‐Newtonian Fluids

AbstractWe demonstrate a simple, cheap method for pore size characterization of porous media that generates a distribution of pore radii for improved flow and transport modeling. The new method for pore structure characterization utilizes recent theoretical developments in non‐Newtonian fluids. Numerical evaluations and validations with synthetic porous media showed potential for obtaining a distribution of effective pore radii and their contribution to total flow only by complementing water with non‐Newtonian fluids in saturated infiltration experiments. To demonstrate this ability on real sands, a series of one‐dimensional column experiments was conducted with varying porous medium packings, including Accusands and a polydisperse sand/glass bead mixture. For each packing, distilled water and varying concentrations of guar and xanthan gum were injected over a range of flow rates and pressure gradients. The model‐generated pore radii were compared with pore radius distributions measured by X‐ray microcomputed tomography (μCT), with results demonstrating good agreement between the model and μCT data. Simulations of saturated water flow and drainage curves using model‐generated pore radii compared favorably to experimental data, with errors typically between 2% and 10% for single‐phase flow and approaching the error of the μCT measured radius distributions for the drainage curves.

Monte Carlo method for determining radon diffusion coefficients in porous media

Radon diffusivity is a key parameter for estimating radon emanation from porous media. We have developed a Monte Carlo method for determining radon diffusion coefficients in porous media. In the proposed method, the probability model for pore diameter derived from fractal theory is used to describe the microscopic pore structure of porous media. The Monte Carlo technique is utilized to search for the pore size that satisfies the probability model for porous media. The convergence rate of the proposed is fast (∼3 min). Radon diffusion coefficients measured by the closed chamber method are used to validate the accuracy of the proposed method.

Capillary Pressure Curve Determination Based on a 2‐D Cross‐Section Analysis Via Fractal Geometry: A Bridge Between 2‐D and 3‐D Pore Structure of Porous Media

AbstractPore structure is a crucial attribute in characterizing fluid flow through porous media. However, direct experimental measurements or numerical reconstructions are commonly expensive and not environmentally friendly, with great uncertainty caused by the complex nature of porous media. In this study, we demonstrate that one special bridge function, which is a function of the apparent length and tortuosity fractal dimension, can characterize the relationship of pore structures between two dimensions (2‐D) and three dimensions (3‐D), and it can serve as a conversion bridge of the radius to determine the capillary pressure curve (CPC). We compare estimations by the proposed method with experimental results obtained by mercury intrusion porosimetry in six typical natural sandstones with varying porosities and permeabilities. The result shows that cross sections of the global pore structure, such as thin section, electronic probe, and microcomputed tomography slices, give a reliable estimation of the CPC using the bridge function in porous media with a medium porosity. However, in unconventional porous media with a relatively low porosity (~10%) or extra high porosity (~30%), due to the empirical nature of the equation widely used to calculate the tortuosity fractal dimension, the necessary modification is necessary to obtain the CPC when applying the bridge function in such porous media. This insight can significantly simplify the procedure for obtaining the petrophysical properties of a porous medium, which may shed light on the inherent differences and correlations between the 2‐D and 3‐D pore structures of porous media.

Anomalous Solute Transport in Cemented Porous Media: Pore‐scale Simulations

Core Ideas The porous media with higher cementation degree have wider velocity spreads. The solute transport becomes more anomalous with increasing cementation degree. It is the changing characteristics of the flow field that lead to more anomalous solute transport. The pore structure of porous media varies significantly with the change in the degree of cementation among solid grains, and recent studies have shown that pore structure greatly influences anomalous solute transport. Nevertheless, limited effort has been devoted to explore the change in the degree of anomalous solute transport with increasing cementation. In this study, we performed pore‐scale numerical simulations to investigate the anomalous dispersion in computer generated two‐dimensional porous media with different degrees of cementation. The simulated pore‐scale fluid fields show that porous media have wider spreads of velocities when the cementation degree increases. When the percentage of cemented solid grains increases from 0.0 to 57.8%, the stagnant regions' area extends more than 11 times, and preferential flow region increases by a factor of more than 6. These changing characteristics of the flow field as the cementation degree increases make solute transport more anomalous. Therefore, we should be careful when applying an advection–dispersion equation (ADE) to predict solute transport in highly cemented porous media.

A fractal model of effective stress of porous media and the analysis of influence factors

The basic concept of effective stress describes the characteristics of fluid and solid interaction in porous media. In this paper, based on the theory of fractal geometry, a fractal model was built to analyze the relationship between the microstructure and the effective stress of porous media. From the microscopic point of view, the influence of effective stress on pore structure of porous media was demonstrated. Theoretical analysis and experimental results show that: (i) the fractal model of effective stress can be used to describe the relationship between effective stress and the microstructure of porous media; (ii) a linear increase in the effective stress leads to exponential increases in fractal dimension, porosity and pore number of the porous media, and causes a decreasing trend in the average pore radius.

Size Distribution and Fractal Characteristics of Coal Pores through Nuclear Magnetic Resonance Cryoporometry

Characterization of the coal pore structure plays a critical role in the adsorption and flow of coalbed methane (CBM) during CBM exploitation. The accuracy of conventional techniques is relatively low, especially for micropores. Nuclear magnetic resonance cryoporometry (NMRC), as a new technique that is used to detect the pore structure of porous media, has been applied to many fields. However, it is rarely used for CBM reservoirs. In this study, the pore size distribution (PSD) and fractal characteristics of semianthracites and anthracites are investigated through NMRC, routine nuclear magnetic resonance (NMR), and low-temperature nitrogen adsorption methods. The results show that the PSD obtained from NMRC is divided into three types, which are mainly affected by the metamorphic degree of the selected coals (coal rank). Type I PSD from NMRC shares a high consistency with that yielded by NMR. The comparison between PSD from NMRC and NMR shows that the NMR method yields a higher pore volume for adsorption...

Non‐Newtonian Fluids in Action: Revisiting Hydraulic Conductivity and Pore Size Distribution of Porous Media

Core Ideas Experimenting with non‐Newtonian fluids allows for improved media characterization. This unfolds unique signatures of flow–pore interaction. Those signatures can be used in the hydraulic characterization of the medium. Model potential for unsaturated media characterization with only saturated flow experiments. The model allows for obtaining multiple hydraulic conductivities for porous media. Classic porous media flow experiments focus on water as the fluid of choice, thus limiting estimates of saturated hydraulic conductivity to a single averaged value that is rarely informative of the complexity of the pore structure. A theoretical framework is presented for a new method for experimentally estimating a functional pore structure of porous media using a combination of Newtonian and non‐Newtonian fluids. The proposed method builds an analog geometry of parallel capillary tubes (with tortuosity) that has the same functional behavior of real porous media in terms of saturated flow and porosity. The equivalent pore structure is formed by transforming results from N saturated infiltration experiments, comprised of water and N − 1 non‐Newtonian solutions, into a system of equations that yields N representative pore radii (Ri) (size distribution of tubes) and their corresponding number (derived from percent contribution to water flow, wi). This feature further allows estimating the soil water retention curve using only saturated experiments. We further show the ability of the proposed theoretical framework to infer additional information about flow in the porous medium as compared to what can be inferred from N (or even more) Newtonian fluids. This example shows the potential of this method and the ability of the proposed framework to extract additional information that an equal or larger number of experiments with only Newtonian fluids cannot extract.

Numerical simulation of displacement characteristics of CO2 injected in pore-scale porous media

Pore structure of porous media, including pore size and topology, is rather complex. In immiscible two-phase displacement process, the capillary force affected by pore size dominates the two-phase flow in the porous media, affecting displacement results. Direct observation of the flow patterns in the porous media is difficult, and therefore knowledge about the two-phase displacement flow is insufficient. In this paper, a two-dimensional (2D) pore structure was extracted from a sandstone sample, and the flow process that CO2 displaces resident brine in the extracted pore structure was simulated using the Navier–Stokes equation combined with the conservative level set method. The simulation results reveal that the pore throat is a crucial factor for determining CO2 displacement process in the porous media. The two-phase meniscuses in each pore throat were in a self-adjusting process. In the displacement process, CO2 preferentially broke through the maximum pore throat. Before breaking through the maximum pore throat, the pressure of CO2 continually increased, and the curvature and position of two-phase interfaces in the other pore throats adjusted accordingly. Once the maximum pore throat was broken through by the CO2, the capillary force in the other pore throats released accordingly; subsequently, the interfaces withdrew under the effect of capillary fore, preparing for breaking through the next pore throat. Therefore, the two-phase displacement in CO2 injection is accompanied by the breaking through and adjusting of the two-phase interfaces.