Showing papers on "Permeability (earth sciences) published in 1996"

The relationship of the unsaturated soil shear strength to the soil-water characteristic curve

Abstract: The measurement of soil parameters, such as the permeability and shear strength functions, used to describe unsaturate soil behaviour can be expensive, difficult, and often impractical to obtain. T...

How Permeability Depends on Stress and Pore Pressure in Coalbeds: A New Model

Abstract: In naturally fractured formations, such as coal, permeability is sensitive to changes in stress or pore pressure (i.e., effective stress). This paper presents a new theoretical model for calculating pore volume compressibility and permeability in coals as a function of effective stress and matrix shrinkage, using a single equation. The equation is appropriate for uniaxial strain conditions, as expected in a reservoir. The model predicts how permeability changes as pressure is decreased (i.e., drawdown). Pore volume compressibility is derived in this theory from fundamental reservoir parameters. It is not constant, as often assumed. Pore volume compressibility is high in coals because porosity is so small. A rebound in permeability can occur at lower drawdown pressures for the highest modulus and matrix shrinkage values. We have also history matched rates from a {open_quotes}boomer{close_quotes} well in the fairway of the San Juan basin using various stress-dependent permeability functions. The best fit stress-permeability function is then compared with the new theory.

Evaluation of transport and storage properties in the soil and groundwater zone from induced polarization measurements1

Abstract: Spectral induced polarization as well as complex electrical measurements are used to estimate, on a non-invasive basis, hydraulic permeability in aquifers. Basic laboratory measurements on a variety of shaly sands, silts and clays showed that the main feature of their conductivity spectra in the frequency range from 10-3 to 103 Hertz is a nearly constant phase angle. Thus, a constant-phase-angle model of electrical conductivity is applied to interpret quantitatively surface and borehole spectral induced polarization measurements. The model allows for the calculation of two independent electrical parameters from only one frequency scan and a simple separation of electrical volume and interface effects. The proposed interpretation algorithm yields the true formation factor, the cation exchange capacity and the surface-area-to-porosity ratio, which corresponds to the inverse hydraulic radius. Using a Kozeny–Carman-like equation, the estimation of hydraulic permeability is possible.

Hydro-mechanical aspects of the sand production problem

Abstract: This paper examines the hydro-mechanical aspect of the sand production problem and sets the basic frame of the corresponding mathematical modelling. Accordingly, piping and surface erosion effects are studied on the basis of mass balance and particle transport considerations as well as Darcy's law. The results show that surface erosion is accompanied by high changes of porosity and permeability close to the free surface. Quantities which can be measured in experiment, like the amount of produced solids or fluid discharge, can be used in an inverse way to determine the constitutive parameters of the problem.

Macroscopic Models for Predicting Changes in Saturated Porous Media Properties Caused by Microbial Growth

Abstract: Analytical equations are developed to model changes in porosity, specific surface area, and permeability caused by biomass accumulation in porous media. The proposed equations do not assume any specific pattern for microbial growth but instead are based on macroscopic estimates of average biomass concentrations. For porous media with a pore-size distribution index value (λ) equal to 3, the macroscopic model predictions of porosity, specific surface area, and permeability changes are in exact agreement with biofilm-model predictions. At other values of λ between 2 and 5, simulated porosity profiles are identical and relative specific surface area and permeability profiles show minor deviations. In comparison to biofilm-based models, the macroscopic models are relatively simple to implement and are computationally more efficient. Simulations of biologically reactive flow in a one-dimensional column show that the macroscopic and biofilm approach based transport codes predict almost identical porosity and permeability profiles. The macroscopic models are simple and useful tools for estimating changes in various porous media properties during bioremediation of contaminated aquifers.

Concurrent methods for permeability measurement in resin transfer molding

Abstract: The numerical simulation of the resin transfer molding process (RTM) requires knowledge of the physical properties of the fibrous material. In particular, the resistance to the resin flow is measured by the permeability of the preform in the mathematical model of Darcy's law. A concurrent method for low-cost permeability estimation is proposed. The method uses a rectangular mold for the numerical determination of the principal permeabilities. The experimental data include a built-in correlation with Darcy's law and allow an estimation of both experimental and numerical errors. Since the experimental procedure can introduce a significant uncertainty on the estimated permeability, practical considerations are pointed out and some relevant parameters such as the minimum injected length and maximum injection pressure are introduced to increase the reliability of permeability measurements.

Hydraulic conductivity of homogenized tailings from hard rock mines

Abstract: Tailings produced by milling for one extraction from hard rock mines, which range in size from clay to fine sand, have relatively low hydraulic conductivity, k. The value of k must be known with a ...

Analysis of transverse flow in aligned fibrous porous media

Abstract: The permeability of preform materials used in liquid moulding processes such as resin transfer moulding and structural reaction injection moulding is a complex function of weave pattern, packing characteristics, tow structure and intra-tow properties. The development of tools for predicting permeability as a function of these parameters is of great practical importance because such capability would speed process design and optimization, and provide a step towards establishing processing-performance relations. In this study, transverse flow in aligned fibrous porous media has been investigated. A semi-analytical model based on lubrication analysis is derived to predict the effect of tow shape and intra-tow permeability on the overall bed permeability for flow through rectangular arrays of porous elliptical cylinders. The Brinkman equation is used to model flow inside the porous structures, and the Stokes equation to model flow in the open media between the structures. The model predictions are verified by comparing with rigorous finite element calculations. The model shows that the influence of the intra-tow permeability on the overall bed permeability increases with inter-tow packing, and increasing degree of tow ellipticity. The influence is particularly critical for ‘stacked’ geometries for which previous models predict a zero permeability. A method for predicting intra-tow permeability is also proposed and investigated by comparing the model predictions for overall bed permeability with some experimental data for flow in a model porous medium. The comparison shows that use of the method enables the experimentally measured permeability value to be properly bounded.

Permeability model for a woven fabric

Abstract: The resin transfer molding (RTM) method is used to manufacture composite parts. The reinforcing fibers are placed in a mold cavity and the resin is injected to fill up the empty spaces. After the resin cures, the mold is opened and the part ejected. To predict necessary pressures and filling times and the proper locations for the inlet ports for resin injection and vents for air ejection it is necessary to model the resin infiltration process. A key to this modeling is permeability which characterizes the resistance of fibers to the flow of infiltrating resin. A simplified model for in-plane permeability of fabric reinforcement (preform) is developed here. This model uses lubrication theory for modeling the flow through open pores and Darcy's law for the transverse flow through the reinforcement. Scaling analysis is provided to justify the simplification and to estimate the range of validity for resulting expressions. Extension of the model to cover multi-layered preforms is derived. Boundary conditions and the data necessary to specify the problem geometry are discussed. A numerical experiment is conducted to estimate the influence of the transverse permeability of the preform on the solution. A calculation is provided for the permeability of a plain weave fabric.

Effect of desiccation cracking on the hydraulic conductivity of a compacted clay liner

Abstract: Despite our best efforts to reduce the waste stream, there will always remain some residues which cannot be further treated and must be disposed in landfills. One critical aspect of landfill construction is the integrity of the landfill liner. Current landfill liner technology includes a composite liner which consists of a FML component and a compacted soil component. The primary characteristic for selecting a soil for use in composite liner construction is that the soil have a saturated hydraulic conductivity of 1 × 10−7 cm s−1 or less. In the present study the effects of desiccation cracks on the hydraulic conductivity of the compacted soil were measured. Two soils of diverse mineralogy and typical of soils used for clay liner construction were selected for use. Each was tested in its native state plus after the addition of 30% sand. Laboratory measurements were made of the volumetric shrinkage of each soil. In addition, the hydraulic conductivity was determined using 10 cm diameter fixed wall permeameters. Additional conductivity measurements were made using 60 cm diameter fixed wall double ring permeameters which had been exposed to 0, 1, and 2 periods of desiccation prior to hydraulic conductivity determinations. The data show that laboratory measurements using 10- cm diameter fixed wall permeameters underestimate the hydraulic conductivity of the same soils when packed in large diameter permeameters. It was also found that exposure to two cycles of desiccation resulted in large increases in hydraulic conductivity. The time required to reach a steady outflow volume decreased as the amount of desiccation increased. The hydraulic conductivities of soils which had been allowed to dry were greater than those which were not allowed to dry prior to measurement. The relationship between volumetric shrinkage and the increase in hydraulic conductivity after desiccation indicates that soils which exhibit less than 11% shrinkage in the laboratory, exhibit increases in K of less than a factor of 2 upon desiccation. Clay soils with greater than 11% shrinkage can potentially be amended with sand to decrease the volumetric shrinkage and their response to desiccation.

Inferring shallow groundwater flow in saprolite and fractured rock using environmental tracers

Abstract: The Ridge and Valley Province of eastern Tennessee is characterized by (1) substantial topographic relief, (2) folded and highly fractured rocks of various lithologies that have low primary permeability and porosity, and (3) a shallow residuum of medium permeability and high total porosity. Conceptual models of shallow groundwater flow and solute transport in this system have been developed but are difficult to evaluate using physical characterization or short-term tracer methods due to extreme spatial variability in hydraulic properties. In this paper we describe how chlorofluorocarbon 12, 3H, and 3He were used to infer groundwater flow and solute transport in saprolite and fractured rock near Oak Ridge, Tennessee. In the shallow residuum, fracture spacings are <0.05 m, suggesting that concentrations of these tracers in fractures and in the matrix have time to diffusionally equilibrate. The relatively smooth nature of tracer concentrations with depth in the residuum is consistent with this model and quantitatively suggests recharge fluxes of 0.2 to 0.4 m yr−1. In contrast, groundwater flow within the unweathered rock appears to be controlled by fractures with spacings of the order of 2 to 5 m, and diffusional equilibration of fractures and matrix has not occurred. For this reason, vertical fluid fluxes in the unweathered rock cannot be estimated from the tracer data.

A model for sedimentary compaction of a viscous medium and its application to inner-core growth

Abstract: SUMMARY A detailed study of the physics of a 1-D sedimentary compaction of a viscous medium was carried out both numerically and analytically for columnar and self-gravitating spherical cases, in view of applying it to the inner-core growth process of the Earth. The effects of sedimentation rate and surface porosity upon the porosity profile were investigated. It was found that the porosity profile differs depending on whether or not the sedimentation rate is larger than the Darcy velocity (velocity of the solid matrix when the fluid flows by buoyancy alone). When the sedimentation rate is larger than the Darcy velocity, a thick, constant-porosity layer develops at the surface, and below it, the porosity decreases gradually towards the bottom. When the sedimentation rate is smaller than the Darcy velocity, the porosity profile is characterized by a mushy layer at the top, where the fluid is expelled by the deformation of the solid, underlain by a thick layer of constant porosity, termed the residual porosity. Such a porosity profile can be understood as the propagation of a half-sided solitary wave. The study was extended further for the self-gravitating spherical case. Formation of an unstable porosity structure and the appearance of solitary waves were discovered for the case of monotonically decreasing sedimentation rate. Given the size of the sphere formed by sedimentary compaction, according to the magnitude of the ratio of sedimentation rate to Darcy velocity, three types of porosity structure, which differ in force balance and the typical length scale required for porosity decrease, were discovered. One such structure is where a low-porosity layer forms at the top, accompanied by solitary waves beneath it, indicating that a crust-like region can develop at the surface of the inner core.

Cracking and permeability of concrete under tension

Abstract: Cracking of concrete, whatever its origin (mechanical, physico-chemical, thermal,…) is a key factor for the materials durability. Knowledge of the transfer properties of sound and of cracked concrete is essential for predicting its durability since the deteriorating mechanisms (freezing, corrosion, leaching) depend on the flow of aggressive (liquid or gaseous) agents through the porous or cracked body. A series of experiments is intended to estimate the increase in permeability resulting from mechanically-induced cracking. A concrete specimen is subjected to uniaxial tension, and the water permeability is measured in a direction perpendicular to the axis of loading. The tests are designed and monitored in order to collect useful data for modelling concrete structures: the material is in tension (and not in compression as it is in more classical studies of damage-permeability coupling) and the permeability is measured through open cracks covering a large width range (0.1 μm–0.1mm). Thus, the transfer properties are being evaluated in the most unfavorable context. Herein we detail the principle of the tests and the design of the specimen, using numerical simulations, as well as present and comment the first experimental results.

Fluid flow in fault zones: Influence of hydraulic anisotropy and heterogeneity on the fluid flow and heat transfer regime

Abstract: Fluid circulation, heat transfer, and the development of thermal springs are examined for vertical fault zones with anisotropic permeability and internal heterogeneity. Interactions between thermally driven convective circulation in the fault zone and topographically driven groundwater flow through the surrounding country rock are mapped in permeability space (permeability of the country rock versus fault zone permeability) and compared to earlier results for homogeneous, isotropic fault zones. Simulations with a fault zone 4–10 times more permeable in the strike than in the dip direction show that the field of steady convection expands in permeability space, promoting stable convection at both higher and lower flux rates. Higher groundwater discharge temperatures (by 12–18°C) are predicted relative to an isotropic fault because this anisotropy favors the formation of a smaller number of convection cells, creating a flow pattern that is more efficient in capturing heat from the country rock and transmitting it to a reduced number of discharge sites. Simulations with a fault zone 4–10 times less permeable in the strike than the dip direction indicate the regional flow from the country rock overrides buoyancy-driven convective circulation in the fault zone at lower values for the country-rock permeability. Heterogeneity internal to the fault creates complex patterns of flow and variations in the geothermal gradient that reflect the connections of higher-permeability regions interior to and along the surface trace of the fault. Channeling of the flow leads to minor differences in the maximum discharge temperature relative to the homogeneous case but creates significant enhancement in the local heat flux owing to the higher groundwater discharge rates.

Hydrodynamic permeability of hydrogels stabilized within porous membranes

Abstract: The purpose of this work was to demonstrate that cross-linked polymer gels can be stabilized against mechanical and osmotic forces by confining them in a microporous support. The hydrodynamic (Darcy) permeability was measured for neutral and charged polyacrylamide (PA) gels synthesized in semirigid membranes having a hydraulic mean pore diameter of 0.5 μm and a porosity of 67%. The permeability was determined by measuring the flow rate of aqueous solutions as a function of pressure drop across the membranes. The membrane-supported gels were stable and yielded a constant permeability when the pressure drop was increased to 300 bar/cm. No swelling/deswelling was observed with the charged gels (0.3−0.4 equiv/L) when the ionic strength was varied between 0.01 and 1.0 M, and the permeability was essentially independent of ionic strength. The permeability of the neutral gel varied as φ-3.3 where φ is the polymer volume fraction, whereas literature data for bulk PA gels shows the dependence to be φ-1.4. The perm...

A generalized model for the transverse fluid permeability in unidirectional fibrous media

Abstract: In liquid molding processes such as resin transfer molding (RTM), fluid is injected into a mold filled with fiber reinforcement. The microstructure of the reinforcement strongly influences the resistance it offers to fluid flow. This resistance is characterized by the permeability that determines the ratio between the superficial velocity and the pressure drop in the porous medium. Currently values of the permeability have to be determined experimentally. Therefore, each type of reinforcement has to be characterized before a computer simulation can be used to predict the overall mold filling pattern. A model for predicting the permeability as a function of structure would help reduce the number of experiments needed to determine the input parameters for mold filling simulations. Also, by understanding the physics of the flow through such materials, one may tailor the microstructure such that it has both the desired reinforcing capability and the necessary permeability to fill efficiently. In response to this need, we have developed a predictive semi-analytical solution for flow across arrays of aligned cylinders with elliptical cross sections modeling the fiber mats. The shape of the tow, its porosity, and the packing configuration are found to influence the transverse permeability of such an array significantly. Predicted results of the permeability from this model compare very well with numerical results obtained from finite element calculations over a range of volume fractions, cross-sectional shapes, and tow permeabilities.

An attempt to quantify fibre bed permeability utilizing the phase average Navier-Stokes equation

Abstract: The mathematical development relating the micro-scale surface integral term of the phase average Navier-Stokes equation and the Darcy permeability is reviewed, as well as a proposed closed-form solution for the orthotropic permeability of unidirectional fibre beds. A simplistic extension of the solution is proposed to account for pinch-off effects during crossflow through the fibre bed. A solution for the in-plane permeability of plain weave fabrics is then constructed, following a similar methodology to that employed for unidirectional fibre beds. Encouraging agreement with experimental permeability measurements is listed throughout the paper in an attempt to demonstrate that sufficient potential exists to pursue this approach further.

Single fluid flow in porous media

Abstract: A comprehensive study on single fluid flow in porous media is carried out. The volume averaging technique is applied to derive the governing flow equations. Additional terms appear in the averaged governed equations related to porosity e, tortuosity τ, shear factor F and hydraulic dispersivity D h. These four parameters are uniquely contained in the volume averaged Navier-Stokes equation and not all of them are independent. The tortuosity can be related to porosity through the Brudgemann equation, for example, for unconsolidated porous media. The shear factor models are reviewed and some new results are obtained concerning high porosity cases and for turbulent flows. It is known that there are four regions of flow in porous media: pre-Darcy's flow, Darcy's flow, Forchheimer flow and turbulent flow. The transitions between these regions arc smooth. The first region, the pre-Darcy's flow region represents the surface-interactive flows and hence is strongly dependent on the porous media and the flowing fluid...

Geotechnical Properties of Paper Mill Sludges for Use in Landfill Covers

Abstract: This study investigates the geotechnical properties of seven paper mill sludges. Paper mill sludges have a high water content and a high degree of compressibility and behave like a highly organic soil. Consolidation tests reveal a large reduction in void ratio and high strain values that are expected due to the high compressibility. Triaxial shear-strength tests conducted on remolded and undisturbed samples showed variations in the strength parameters resulting from the differences in sludge composition (i.e., water content and organic content). Laboratory permeability tests conducted on in-situ specimens either met the regulatory requirement for the permeability of a landfill cover or were very close. With time, consolidation and dewatering of the paper sludge improved the permeability of cover. Freezing and thawing cycles increased the sludge permeability about one to two orders of magnitude. Maximum permeability changes occurred within 10 freeze and thaw cycles.

Effects of stress on the two‐dimensional permeability tensor of natural fracture networks

Abstract: The effects of stress on the 2-D permeability tensor of natural fracture networks were studied using a numerical method (Universal Distinct Element Code). On the basis of three natural fracture networks sampled around Dounreay, Scotland, numerical modelling was carried out to examine the fluid flow in relation to the variations in burial depth, differential stress and loading direction. It was demonstrated that the permeability of all the networks decreased with depth due to the closure of aperture. The permeability approached the minimum value at some depth below which little further variation occurred. Also, differential stress had a significant effect on both the magnitude and direction of permeability. The permeability generally decreased with increasing major horizontal stress for a fixed minor horizontal stress, but the various networks considered showed different behaviours. A factor, termed the average deviation angle of maximum permeability (Am), was defined to describe quantitatively the deviation degree of the direction of the major permeability component from the applied major stress direction. For networks whose behaviour is controlled by set(s) of systematic fractures, Am is significantly greater than zero, whereas those comprised of non-systematic fractures have Am close to zero. In general, fractured rock masses, especially those with one or more sets of systematic fractures, cannot be treated as equivalent porous media. Specification of the geometry of the network is a necessary, but not sufficient, condition for models of fluid flow. Knowledge of the in situ stress, and the deformation it induces, is necessary to predict the behaviour of the rock mass.

Measurements of spectral induced polarization for environmental purposes

Abstract: Hydraulic permeability is one of the most important parameters for the evaluation of sediments relevant to environmental and hydrogeological problems. Up to now, permeability could be determined only by time-consuming and expensive methods like pumping tests or sampling and laboratory investigations. The results are confined to few locations, and they depend on the scale of the investigation method. Measurements on rock samples in a laboratory can differ significantly from well test results. Geophysical measurements are performed on different scales from high resolution measurements in boreholes up to large-scale soundings. Variations in permeability are mainly caused by varying grain size and by changes in porosity. A decrease of average grain diameter results in an increasing internal surface area. Petrophysical investigations have shown a reliable correlation between the imaginary part of electrical conductivity and the porespace-related internal surface. The formation resistivity factor, which is related to porosity, can be determined by geoelectrical measurements if the electrical conductivity of the pore fluid is known. The internal surface area and the formation factor are the only two parameters used by a Kozeny-Carman-like equation to evaluate the permeability or hydraulic conductivity for the investigated representative volume. Complex electrical conductivity is determined by measurements of induced polarization in the frequency domain. Frequencies below 10 Hz are used to avoid electromagnetic coupling. The permeability values determined by electrical measurements in boreholes can well be compared with those derived from the grain size distribution of samples. The same algorithm can be applied to evaluate the hydraulic conductivity of subsurface layers by complex resistivity soundings. The high sensitivity of the imaginary conductivity component to changes at the internal surface may be used as an indicator for contaminations.

Water and solute transport along developing maize roots

Abstract: Hydraulic and osmotic properties were measured along developing maize (Zea mays L.) roots at distances between 15 and 465 mm from the root tip to quantify the effects of changes in root structure on the radial and longitudinal movement of water and solutes (ions). Root development generated regions of different hydraulic and osmotic properties. Close to the root tip, passive solute permeability (root permeability coefficient, Psr) was high and selectivity (root reflection coefficient, σsr) low, indicative of an imperfect semipermeable root structure. Within the apical 100–150 mm, Psr decreased by an order of magnitude and σsr increased significantly. Root hydraulic conductivity (Lpr) depended on the nature of the force (hydrostatic and osmotic). Osmotic Lpr was smaller by an order of magnitude than hydrostatic Lpr and decreased with increasing distance from the root tip. Throughout the root, responses in turgor of cortical cells and late metaxylem to step changes in xylem pressure applied to the base of excised roots were measured at high spatial resolution. The resulting profiles of radial and longitudinal propagation of pressure showed that the endodermis had become the major hydraulic barrier in older parts of the root, i.e. at distances from the apex a 150 mm. Other than at the endodermis, no significant radial hydraulic resistance could be detected. The results permit a detailed analysis of the root's composite structure which is important for its function in collecting and translocating water and nutrients.

Permeability and stress in crystalline rocks

Abstract: Groundwater from crystalline rocks is a significant resource in many areas of the world. It is also an important medium for contaminant transport from, for example, deep nuclear waste repositories. Stress distributions in fractured rocks are important in controlling groundwater flow in several ways: (i) palaeostress fields are responsible for the evolution of fracture systems which transmit groundwater; (ii) current in situ stress fields will influence the shape and aperture of fractures; (iii) humans can influence the natural stress field in a rock mass to enhance fracture flows. The significance of stresses for groundwater flow can be investigated by field techniques (hydraulic fracturing), laboratory techniques (stress cells) or by numerical modelling.

Porous composite sheet and process for the production thereof

Abstract: A porous composite sheet comprising a layer wherein both a component (A) which is easily heat-meltable and a component (B) which is more thermally stable than the component (A) coexist in a fused state. The layer of the component (A) may be constituted of a hydrophilic material, while the layer of the component (B) a hydrophobic one. Further, both of the layers may contain a common component made of an easily heat-meltable material. This sheet advantageously exhibits suitable permeability to air or moisture and excellent drape characteristics, and therefore is suitable for the raw material of sanitary or medical supplies.