Anisotropic Permeability Research Articles

Anisotropy Study on the Process of Soil Permeability and Consolidation in Reclamation Areas: A Case Study of Chongming East Shoal in Shanghai

Anisotropic permeability is of great significance for assessing the consolidation and drainage mode of soil layers in reclamation areas, as well as for preventing and controlling ground settlement after project construction. This paper analyzes the anisotropic permeability of the inland and nearshore soil layers in Chongming East Shoal, Shanghai, and the formation mechanism of anisotropic permeability through permeability and scanning electron microscope (SEM) tests. The results highlight that compared with dredger fill and sandy silt, the horizontal permeability coefficient of underlying soft clay (USC) is significantly higher than its vertical permeability coefficient, which is more significant in nearshore USC. Interestingly, the upper clay (21.5 m) in the thickest clay layer shows greater anisotropic permeability than the lower clay (41.5 m). Due to the instability of seepage channels, the USC anisotropic permeability increases in a fluctuating manner as the hydraulic gradient increases. Microstructural parameters are used to reveal the mechanism of anisotropic permeability, which shows that a simple soil skeleton and structure, strong particle orientation, decreased particle abundance, increased particle roundness, decreased particle contact area, and increased pore area all contribute to the enhancement of permeability. Moreover, micro-parameters have been proposed to evaluate anisotropic permeability in terms of the effective seepage-pore area. This approach addresses the constraint of water films on the permeability efficiency of USC particles.

Anisotropic flow characteristics in single-fractured tensile and shear fractures in granite subjected to various confining stress

Anisotropic flow characteristics in single-fractured tensile and shear fractures in granite subjected to various confining stresses were investigated by experiments and experimental results-based flow simulations. Anisotropic characteristics in aspects such as the permeability reduction rate, stress dependency of permeability, and flow paths in fracture and fracture aperture distribution were analysed and compared between tensile and shear fractures. The main conclusions are as follows: (1) for tensile fractures, the permeability in the X direction, k X , is slightly lower than the permeability in the Y direction, k Y , and the average permeability anisotropic coefficient, α k ( α k = k X / k Y ), is in the range 0.236–0.779. For shear fractures, k X is obviously lower than k Y , and α k is in the range 0.038–0.163; therefore, the permeability anisotropy in shear fractures is stronger than in tensile fractures. (2) Strong channelling effects exist in both shear and tensile fractures. The stress-dependence coefficient of permeability is 0.731 MPa −1 in the X direction and 0.365 MPa −1 in the Y direction for tensile fractures, and 0.034 and 0.010 MPa −1 , respectively, for shear fractures, indicating that the dependency of permeability on stress also shows anisotropy. (3) An empirical model for α k based on stress and fracture aperture variograms is proposed. Based on α k , permeability along both the X and Y directions can be well predicted, especially under relatively high confining stress.

NUMERICAL SIMULATION METHOD IN LOW PERMEABLE ANISOTROPIC RESERVOIRS IN ORDER TO INCREASE OIL RECOVERY


 
 The object of study is the filtration process in weakly permeable anisotropic formations (SAP). The work is devoted to the evaluation of the process of oil-bearing formation depletion, using as a criterion the distribution of reservoir pressure near the system of production and injection wells, taking into account the anisotropy of the permeability of the oil phase. In the course of the study, a combined numerical modeling method based on the finite element and finite difference methods is presented, which allows determining the value of reservoir pressure at any point in the computational domain. The finite-element-difference method proposed in this paper, which combines the advantages of the finite-element method and the finite-difference method to solve the nonstationary piezoelectric conductivity problem, taking into account the heterogeneous distribution of various filtration parameters within the deformed formation and at its boundaries, allows us to adequately calculate the distribution of reservoir pressure in real-world well operation conditions, which gives a number of advantages over existing methods. It was found that the effect of oil phase permeability in the shear direction dominates the effect of permeability in the axial directions. This is due to the fact that, according to the information obtained, for the effective use of anisotropic low-permeability formations, it is necessary to place production and injection wells in areas with relatively low anisotropy of formation permeability, especially to avoid areas with the presence of shear permeability of the formation. It is important to locate wells so that, on the one hand, there is no blockage of oil from the low permeability side, and, on the other hand, there is no rapid depletion of the formation from the high permeability side and the mutual exchange between the production and injection wells does not stop. When placing a system of production and injection wells in anisotropic formations of an oil field, it is necessary to conduct a systematic analysis of the surrounding anisotropy of the formations in order to place them in such a way that would ensure effective dynamics of filtration processes around these wells and increase oil recovery.
 
 
 
 
 

Understanding subduction infancy to mature subduction in Southwest Japan via the self-consistent formation of a weak slab interface

The weak slab interface controls long-term subduction dynamics. A weak hydrous layer at the slab interface promotes mechanical decoupling between the forearc mantle and the subducting slab and converts a hot forearc mantle to a cold mantle. Often referred to as a cold nose, the cold forearc mantle, plays a key role in the transition from subduction infancy to mature subduction. This study was the first to numerically demonstrate the self-consistent formation of a weak hydrous layer with permeability anisotropy based on the Southwest Japan subduction zone case, where transition-related geological features were present. Our models showed that mechanical decoupling by spontaneous downdip growth of the weak hydrous layer created a cold nose by converting a hot forearc mantle to a cold mantle. The emergence of a cold nose explained the migration of the forearc-to-arc volcanic front, expressed as the formation of mid-Miocene forearc high-magnesium andesite and Quaternary arc adakite. Furthermore, the weak hydrous layer providing a pathway for free-water transport toward the mantle wedge tip elucidates slab/mantle-derived geochemical components in deep groundwater as well as large S-wave delay times and non-volcanic seismic tremors in the forearc.

A New Approach to Predicting Vertical Permeability for Carbonate Rocks in the Southern Mesopotamian Basin

Reservoir performance depends on many factors, and the most important one is permeability anisotropy. In addition, with high heterogeneity, it is essential to find unique relationships to predict permeability. Therefore, this study aims to predict vertical permeability based on horizontal permeability and porosity and to find new equations for carbonate reservoirs. This work relied on the 398 measured points of cores data collected from several wells in carbonate reservoirs. A new correlation for predicting vertical permeability for the whole data (369 samples) as a function of horizontal permeability and porosity has been developed. The results indicate that this new correlation can estimate the vertical permeability with correlation coefficients (RSQ) of 0.853. Then, the used data were divided into four groups depending on the Kv/Kh values: less than 0.1, 1–0.1, 1–10, and more than 10, and a new correlation for permeability prediction for each group has been developed with good RSQ values of 0.751, 0.947, 0.963, and 0.826, respectively. The previous studies lack the correlations to predict vertical permeability in carbonate reservoirs, so this study can be considered as a reference for similar cases.

Hydraulic properties of sediments from the GC955 gas hydrate reservoir in the Gulf of Mexico

The economic feasibility of gas production from hydrate deposits is critical for hydrate to become an energy resource. Permeability in hydrate-bearing sediments dictates gas and water flow rates and needs to be accurately evaluated. Published permeability studies of hydrate-bearing sediments mostly quantify vertical permeability; however, the flow is mainly horizontal during gas production in layered reservoirs. Additionally, ASTM standards require a hydraulic gradient of 10–30 to be used during laboratory permeability measurements, but the gradient is much higher in the field, particularly near a production well. To address these issues, this study focuses on the hydraulic properties of a sandy silt subsample of the hydrate reservoir and a clayey silt subsample of the fine-grained, hydrate-free interbed recovered from a GC955 deep-water Gulf of Mexico gas hydrate reservoir. We characterize the sediment pore space with water retention curves for both hydrate-free and hydrate-bearing samples (hydrate saturation, Sh =80 %). Vertical deformation with increasing stress is also quantified while consolidating the samples to the 4 MPa in situ vertical effective stress. The customized permeameter measures both the horizontal and vertical permeability with increasing stress. Results show that high hydraulic gradients lower permeability in the flow direction, possibly due to increased flow tortuosity and local sediment compaction from the high seepage force. Assuming a single permeability value, even though hydraulic gradients decrease with distance from the well, is not realistic for field estimations. The results highlight that permeability anisotropy, hydrate saturation, stress conditions, and hydraulic gradient all substantially impact reservoir permeability during production.

Assessing the potential of composite confining systems for secure and long-term CO2 retention in geosequestration

A potential geologic target for CO2 storage should ensure secure containment of injected CO2. Traditionally, this objective has been achieved by targeting reservoirs with overlying seals-regionally extensive, low permeability units that have been proven capable of retaining buoyant fluid accumulations over geologic time. However, considering that the amount of CO2 is limited by a decadal injection period, vertical migration of CO2 can be effectively halted by a composite system of discontinuous shale/silt/mudstone barriers in bedded sedimentary rocks. Here, we studied the impact of depositional architectures in a composite confining system on CO2 migration and confinement at reservoir scale. We stochastically generated lithologically heterogeneous reservoir models containing discontinuous barriers consistent with statistical distributions of net-sand-to-gross-shale ratio (NTG) and horizontal correlation lengths derived from well log data and observations of producing hydrocarbon fields in Southern Louisiana. We then performed an extensive suite of reservoir simulations of CO2 injection and post-injection to evaluate the sensitivity of CO2 plume migration and pressure response of the composite system to a series of geologic and fluid parameters including the lateral continuity of barriers, NTG, permeability anisotropy within the sand body, and capillary pressure contrast between the sand and shale facies. The results indicate that lateral continuity of barriers and NTG are the dominant controls on CO2 plume geometry and pressure build-up in the reservoir, while the impact of NTG is particularly pronounced. The significance of intraformational barriers becomes apparent as they facilitate the local capillary trapping of CO2. Those barriers improve the pore space occupancy by promoting a more dispersed shape of the plume and ultimately retard the buoyancy-driven upward migration of the plume post injection.

Subsurface high-grading potential in Coal Seam Gas in eastern Australia

A case study from eastern Australia is used to demonstrate methods for subsurface high-grading and development-concept optimisation in Coal Seam Gas resource plays. We discuss two aspects of subsurface high-grading: (1) improved prediction of net coal using 2D seismic data, and (2) structure-geological analysis and mapping of the fracture network. Net-coal predictions across the study area focused on the upper Juandah Coal Measures interval of the Jurassic Walloon Coal Measures. There is a positive relationship between extracted and normalised upper Juandah Coal Measures seismic-amplitude strength and net coal in the wells. Areas with considerable amplitude dimming commonly coincide with reflector ‘washout’ areas mapped from seismic data and are interpreted as channel incisions into otherwise continuous coal seams. Integration of seismic and well data can improve the mapping of net coal and help optimise the placement of appraisal wells, reducing the number of unexpected or difficult-to-interpret well finds. It may also guide delineation of resource-classification domains in a more robust manner than mapping from well data alone. Fractures in coal seams of the study area are mostly bed-perpendicular joints and cleats. Three sets of fractures/cleats are identified in the coal beds: a dominant set and two more localised sets. Fracture intensity was mapped per set using well data and then combined. Fracture maps are deemed useful as an indication of permeability anisotropy, and hence the expected shape of well drainage areas, as opposed to indicating a range of permeability values. Based on the fracture interpretations, it may be possible to optimise the development concept of specific areas, i.e. varying well spacing and/or drilling wells slanted perpendicular to dominant fracture strike.

Porosity-permeability tensor relationship of closely and randomly packed fibrous biomaterials and biological tissues: Application to the brain white matter

The constitutive model for the porosity-permeability relationship is a powerful tool to estimate and design the transport properties of porous materials, which has attracted significant attention for the advancement of novel materials. However, in comparison with other materials, biomaterials, especially natural and artificial tissues, have more complex microstructures e.g. high anisotropy, high randomness of cell/fibre dimensions/position and very low porosity. Consequently, a reliable microstructure-permeability relationship of fibrous biomaterials has proven elusive. To fill this gap, we start a mathematical derivation from the fundamental brain white matter (WM) formed by nerve fibres. This is augmented by a numerical characterisation and experimental validations to obtain an anisotropic permeability tensor of the brain WM as a function of the tissue porosity. A versatile microstructure generation software (MicroFiM) for fibrous biomaterial with complex microstructure and low porosity was built accordingly and made freely accessible here. Moreover, we propose an anisotropic poro-hyperelastic model enhanced by the newly defined porosity-permeability tensor relationship which precisely captures the tissues macro-scale permeability changes due to the microstructural deformation in an infusion scenario. The constitutive model, theories and protocols established in this study will both provide improved design strategies to tailor the transport properties of fibrous biomaterials and enable the non-invasive characterisation of the transport properties of biological tissues. This will lead to the provision of better patient-specific medical treatments, such as drug delivery. Statement of significanceDue to the microstructural complexity, a reliable microstructure-permeability relationship of fibrous biomaterials has proven elusive, which hinders our way of tuning the fluid transport property of the biomaterials by directly programming their microstructure. The same problem hinders non-invasive characterisations of fluid transport properties in biological tissues, which can significantly improve the efficiency of treatments e.g. drug delivery, directly from the tissues accessible microstructural information, e.g. porosity. Here, we developed a validated mathematical formulation to link the random microstructure to a fibrous material’s macroscale permeability tensor. This will advance our capability to design complex biomaterials and make it possible to non-invasively characterise the permeability of living tissues for precise treatment planning. The newly established theory and protocol can be easily adapted to various types of fibrous biomaterials.

Effects of anisotropic permeability on EMHD nanofluid flow and heat transfer in porous microchannel with wavy rough walls

This theoretical study thoroughly examines the complexities of fluid flow and heat transfer within microfluidic devices by employing anisotropic porous media with wavy walls. The effect of anisotropic permeability and electromagnetohydrodynamics (EMHD) on nanofluid transport is examined in a wavy microchannel under a constant pressure gradient. The nonlinear coupled governing equations for electric double-layer potential, velocity, temperature, and nanoparticle volume fraction are solved numerically using shooting techniques with the Runge–Kutta method. The numerical results are validated with the asymptotic analytical solutions. We explore the impact of parameters like the Hartmann number, Darcy number, and joule heating on nanofluidic flow. The interplay between anisotropic permeability ratio and angle significantly influences flow, temperature profiles, and nanoparticle volume fraction. Reduction in nanoparticle concentration is attributed to constrained entry into the porous medium due to elevated anisotropic permeability. Frictional heating, influenced by the Forchheimer inertial effect, enhances temperature and induces slug flow at low Darcy numbers. The Nusselt number peaks at low Darcy numbers with an anisotropic angle of φ=π/2, experiencing a minimum when φ=0. These findings deepen our understanding of the role of anisotropic permeability in shaping fluid flow and heat transfer in microfluidic systems influenced by EMHD effects.

Physical and Chemical Stability of Nanoparticles in Ferrofluid Before and After Impregnation: Implications for Magnetic Pore Fabric Studies

AbstractMagnetic pore fabrics (MPFs) are a promising approach to explore the 3D pore space of rocks. There exist empirical relationships between the orientation, degree, and shape of MPF with pore space geometry and permeability anisotropy. Nevertheless, the precise nature of these relationships remains elusive. A common assumption in MPF studies is that ferrofluid uniformly fills the pore space, establishing a constant nanoparticle density and magnetic properties in space and time. However, this assumption is challenged by observations of particle sedimentation and time‐dependent magnetic properties, which render the quantitative interpretation of MPFs challenging. This study explores the physical and chemical stability of ferrofluid, mobility of particles after impregnation, and the magnetic properties of impregnated rocks over time. Water‐based ferrofluids are physically more stable than oil‐based ferrofluids, though with higher magnetic variability. In impregnated rock, magnetic nanoparticles display a certain degree of mobility, resulting in changes in MPF degree and shape. When ferrofluid is mixed with epoxy, particles are less mobile, though some changes were observed both during polymerization and aging. The susceptibility of ferrofluid‐epoxy mixtures is lower than for ferrofluid and carrier liquid at the same concentration. Interestingly, the susceptibility of impregnated rock increases over time, regardless of the ferrofluid used for the impregnation. Understanding and controlling these processes will enhance the reliability of MPF interpretations and increase the applicability of the method.

A Study of Geometrical Effects on Permeability Estimation in Three-dimensional Fractures Using the Lattice Boltzmann Method

This study investigates the effect of geometry on permeability estimation in three-dimensional fractures using the Lattice Boltzmann Method. Fractures have irregular and complex shapes, which can significantly impact their permeability. Anisotropy of permeability is important as it indicates the influence of changes in fracture orientation on the flow and transport properties of fluids. To examine the relationship between permeability and fracture geometry, we generated three-dimensional fracture geometries using fractal Brownian motion with a Hurst exponent to describe the roughness on the fracture surface. The fractures in this study have varying mean aperture, ranging from narrow to wide, with surface roughness that can be either uniform or non-uniform. The Lattice Boltzmann Method was performed to calculate permeability and investigate the relationship between permeability and geometric parameters of fractures, such as mean aperture and surface roughness. Our results were in good agreement with previous studies on two-dimensional fractures. We found a clear relationship between permeability and mean aperture. Both in uniform and non-uniform geometry have the same trend as the mean aperture increased, permeability generally increased, while rougher surface roughness permeability generally decreased but in non-uniform geometry the data on the graph appears more random and irregular. We extended our investigation to real fractures by digital rock portal data. The analysis revealed that the geometry of the real fractures closely resembled non-uniform geometry, and the permeability exhibited anisotropic behaviour. In addition, the anisotropy of permeability and computation time were also investigated; and it was found that both can be influenced by the three-dimensional fracture geometry.

Gas drainage and optimal arrangement of boreholes based on permeability anisotropy

Based on the true triaxial dynamic anisotropic permeability model of coal and rock, a multi field coupling mathematical model of structurally anisotropic coal seams under true triaxial stress was established, and a numerical simulation experiment of porous dry gas drainage in anisotropic coal seams was conducted using the COMSOL Multiphysics numerical simulation software. It was found that with an increase in the permeability anisotropy ratio, the greater the ratio of the long and short axes of the elliptical pressure isoline was, the denser was the gas streamline in the dominant seepage direction. As the permeability anisotropy ratio increased, the gas extraction volume and rate decreased. When the permeability was isotropic, the gas drainage volume of the uniformly arranged rectangular boreholes was the largest. However, when the permeability was anisotropic, the gas extraction volume and rate were the largest along the y-direction layout (perpendicular to the dominant seepage direction), followed by the rhombus layout, and the smallest along the x-direction layout. Based on the test results, an optimal arrangement principle of multi holes pumping boreholes along the vertical dominant seepage direction was proposed.

Model of a post-caldera volcano-hosted and vapor-dominated system through a numerical simulation study of the Bedugul geothermal field, Bali Island, Indonesia

To significantly increase the use of geothermal resources, describing geothermal systems in post-calderas is essential because this type of system is infrequent and has not yet been thoroughly researched. The Bedugul geothermal field (BGF), Bali Island, Indonesia, is a representative system with a complex structure composed of a vapor-dominated zone formed above a deep liquid reservoir. This study examines the BGF system and updates a conceptual model, particularly the fluid flow and heat transfer in the reservoir. A dual-porosity model and TOUGH2 EOS1 equation of state for water and steam module are used to simulate natural state conditions, including calderas with a domain area of 148.8 km2 and a thickness of 3.3 km. The BGF model was calibrated using the available geosciences data, temperature core holes and deep explorations wells and then evaluated statistically. A reliable natural state model with decent accuracy was obtained by optimizing the parameter calibration of the physical and structural properties of the reservoir model; the anisotropic and heterogeneous fracture permeability distribution and specific boundary conditions are crucial to the calculation accuracy. The BGF model reveals hot hydrostatic liquid, vapor-dominated, and transition zones and an overpressured liquid domain that is specific to a vapor-dominated geothermal system overlying a liquid reservoir. Occurrence of counterflow is also a noted feature in the vapor-dominated zone. Because of the high pressure in the steam zone, around 47 bar at a near-saturation temperature of 260 °C, the BGF is characterized as a premature field of a vapor-dominated system.

Characterization of a naturally fractured ignimbrite reservoir: Subsurface model validated from worldwide analogue outcrops and production data

Ignimbrites are deposits found in sedimentary basins globally, commonly linked with evolved volcanism. Understanding fractured ignimbrites in sedimentary basins is crucial for various economic applications, including hydrocarbon extraction, CO2 storage, fractured aquifers, and geothermal resources. Fractured ignimbrite reservoirs typically consist of welded deposits with high transmissibility due to the fracture network, but with low matrix permeability. In this study, we demonstrate how a large-scale rhyolite eruption progressed through varying gas-particle ratios and magma discharge rates, which along with the influence of paleotopography, controlled the type and rate of deposition of pyroclastic density currents (PDCs). These factors determined the distribution and thickness of the ignimbrite unit, as well as the post-emplacement processes, such as cooling rate, welding and devitrification, which influenced the occurrence of discrete discontinuities defining the hydraulic system of the Naturally Fractured Reservoir (NFR). Fracture network is closely related to the cooling stage, composed of hybrid, fluid-assisted fractures and extensional fractures associated with fumarolic system and tensional thermo-elastic geomechanics, respectively. The subsurface model was validated through analysis of reservoir dynamic response and comparative studies of worldwide outcrop analogues, focusing on the determining factors of NFRs. In this case, tectonic fractures resulting from the structural evolution of basins may also influence reservoir conditions, albeit to a lesser extent, affecting fracture porosity and permeability of the NFR. However, these tectonic fractures played a significant role in fluid migration and reservoir charging. We conclude that an ignimbrite geobody, with a low porosity and permeability matrix (Øm ∼1 %; Km ∼0.1 mD), constitutes a NFR, where the fracture network provides a low porosity (Øf ∼0.08%) and a high anisotropic permeability of the reservoir (Kf max ∼ 2100 mD and, Kf Av ∼1500 mD). The obtained results have important implications for the study of fluids transmissibility and storage of ignimbrite reservoirs.

Couette-Poiseuille flow in a fluid overlying an anisotropic porous layer

In this study, Couette-Poiseuille flow inside a channel partially filled with a porous medium is treated. Considering the applicability to the practical scenarios, we assume that the porous medium is anisotropic in nature. More precisely, the model under steady-state assumption is described by the coupling of Brinkman-Forchheimer and Stokes equations. We develop the existence and uniqueness results corresponding to this two-dimensional Stokes-Brinkman-Forchheimer coupled system using the Browder-Minty theorem. For this we restrict the permeability to a diagonal matrix. The coupled system involves a non-linear term due to the Brinkman-Forchheimer equation. We propose an algorithm based on shooting and searching techniques to develop a numerical solution for this system, however, for an arbitrary anisotropic permeability matrix. The robustness of the presented numerical solution is established by comparing it with asymptotic solutions corresponding to small and large Darcy numbers. For the case of a large Darcy number, the problem of interest is a regular perturbation while the same is a singular perturbation problem for the case of a small Darcy number, which is dealt with using Prandtl's matching principle. With the velocity distribution determined, we have presented a comparative analysis of the shear stress distribution at the liquid-porous interface and at the bottom plate, which is of interest to analyze the stress distribution on the arterial wall. The asymptotic results are found to be in good agreement with those obtained numerically.

Influence of key geological factors on fluid production behaviors in marine natural gas hydrate reservoirs: A case study

Accurate geological model of hydrate reservoir is the prerequisite of obtaining the credible prediction results. Not only stress sensitivity and anisotropy of permeability but also non-uniformity in hydrate saturation and permeability distribution are key geological factors influencing fluid production behaviors. However, they have been overlooked to some extent in previous numerical simulations. To highlight the significance of these factors, this study develops a robust production model at site AT1 based on geological data and laboratory tests. Then we systematically investigate the effects of the aforementioned factors on production performance. The simulation results exhibit that all the factors have notable impacts on fluid production behaviors, especially the non-uniform hydrate saturation and permeability distributions. Ignoring the permeability stress sensitivity and adopting the homogeneous simplification of hydrate saturation may overestimate gas and water production. Besides, various characterization methods of reservoir permeability can give rise to the significantly different production behaviors for heterogeneous hydrate reservoirs. The stronger the reservoir permeability anisotropy, the better the production performance. In order to achieve the reliable predictions, the geological model built should properly incorporate these factors during numerical modeling. These findings provide valuable guidance for constructing robust models and optimizing production strategies for natural gas hydrate reservoirs.

Numerical Simulations of Carbon-Fibre Impregnation with a Polymer as an Anisotropic Permeability Medium.

The impregnation process of carbon fibres with polymers is challenging to model due to the system's complexity, particularly concerning the following aspects: the complex rheology of the polymeric matrices and the presence of solid, continuous fibres, both with anisotropic properties, and the interaction between solid and fluid, which can change the displacement of fibres into a cyclic dependence. In this work, an interesting approach was considered by setting the fibres as a porous medium whose properties were calculated with microscale/macroscale cycle modelling. In the microscale modelling stage, two main assumptions can be made: (i) a homogeneous distribution with a representative cell or (ii) a stochastic distribution of fibres. The solution to the abovementioned flow and fibre distribution problem can severely differ with only a slight change in a single parameter for a given set of processing parameters. Therefore, the influence of some of them during the fibre impregnation process was evaluated, allowing a shortcut for the polymer through a gap between fibres and the bottom wall of the extrusion die. The range of investigated values regarding the gap enables one to cover good impregnation conditions up to the occurrence of the shortcut and consequent poor impregnation quality. These studies were performed with numerical simulations with circa 126,000 degrees of freedom, considering the discretisation mesh elements and the unknowns (pressure and two velocity components).

Evaluation of the impact of transition from porous to fractured rock media on 3D field-scale DNAPLs contamination

A 3D high-resolution subsurface characteristic (HSC) numerical model to assess migration and distribution of subsurface DNAPLs was developed. Diverse field data, including lithologic, hydrogeologic, petrophysical, and fracture information from both in situ observations and laboratory experiments were utilized for realistic model representation. For the first time, the model integrates hydrogeologic characteristics of both porous (unconsolidated soil (US) and weathered rock (WR)) and fractured rock (FR) media distinctly affecting DNAPLs migration. This allowed for capturing DNAPLs behavior within US, WR, and FR as well as at the boundary between the media, simultaneously. In the 3D HSC model, hypothetical 100-year DNAPLs contamination was simulated, quantitatively analyzing its spatiotemporal distributions by momentum analyses. Twelve sensitivity scenarios examined the impact of WR and FR characteristics on DNAPLs migration, delineating significant roles of WR. DNAPLs primarily resided in WR due to low permeability and limited penetration into FR through sparse inlet fractures. The permeability anisotropy in WR was most influential to determine the DNAPLs fate, surpassing the impacts of FR characteristics, including rock matrix permeability, fracture aperture size, and fracture + rock mean porosity. This study first attempted to apply the field-data-based multiple geological media concept in the DNAPLs prediction model. Consequently, the field-scale effects of WR and media transitions, which have been often overlooked in evaluating DNAPLs contamination, were underscored.

Temporal to spatial instability in a flow system: a comparison

AbstractThe definitions of temporal instability and of spatial instability in a flow system are comparatively surveyed. The simple model of one-dimensional Burgers’ flow is taken as the scenario where such different conceptions of instability are described. The temporal analysis of instability stems from Lyapunov’s theory, while the spatial analysis of instability interchanges time and space in defining the evolution variable. Thus, the growth rate parameter for temporally unstable perturbations of a basic flow state is to be replaced by a spatial growth rate when a coordinate assumes the role of evolution variable. Finally, the idea of spatial instability is applied to a Rayleigh-Bénard system given by a fluid-saturated horizontal porous layer with an anisotropic permeability and impermeable boundaries kept at different uniform temperatures.