Pore Fluid Concentration Research Articles

Visualization of particle behavior within a porous medium: Mechanisms for particle filtration and retardation during downward transport

A technique for visualizing particle transport in the interior of a porous medium is presented. The technique, which includes the construction of a translucent medium and the use of laser‐induced fluorescence for particle tracking, was used to examine the behavior of a dilute suspension of negatively charged, micron‐sized particles in the interior of uniform glass bead packs during one‐dimensional, downward flow. Particle behavior as a function of pore fluid velocity and bead surface roughness was observed at both the macroscopic and microscopic levels. Experimental results show that particle filtration occurred only at solid‐solid contact points (contact filtration) in smooth bead packs, while particle filtration occurred at the top of bead surfaces (surface filtration) as well as at solid‐solid contact points in rough bead packs. Particle contact filtration was the result of physical straining at solid‐solid contact points, while surface filtration was the result of particles interlocking on surface asperities. In both smooth and rough bead packs the filtration capacity of the medium decreased with the pore fluid velocity. In smooth bead packs the filtration capacity was approximately invariant with transport distance, while in the rough bead packs the filtration capacity showed a decrease with transport distance. This decrease was attributed to the early surface filtration of larger particles as a result of gravitational sedimentation. The accumulation of reversibly attached particles was observed even when particle pore fluid concentrations were stable.

Soft Sediments: Wave-based Characterization

Soft sediments are frequently encountered in geotechnical practice, forming natural deposits or manmade mixtures. These high water content soil-water mixtures involve soils with high specific surface and experience relatively low effective stresses, hence the role of electrical interparticle forces is enhanced and nonstandard soil-like behavior may be observed. Testing such materials presents some unique challenges not faced in soils with a stronger granular skeleton. This experimental study of soft sediments involves kaolinite- electrolyte mixtures. Two test sets are conducted. The first one involves index tests and the second set of tests explores the combined utilization of elastic and electromagnetic waves to assess soft sediments and to monitor their evolution. The kaolinite mixtures tested in both sets of measurements were prepared using fluids of different ionic concentrations to create different soil fabrics. The mixtures change behavior when the ionic concentration of NaCl exceeds ,0.1 mol/ L, indicating internal changes in fabric formation. While the liquid limit is sensitive to fluid-particle interactions, it is less sensitive to fabric effects than viscosity or sedimentation tests. The maximum porosity a soil may attain is determined by the particle slenderness; in the case of kaolinite, a skeleton capable of transmitting a shear perturbation is observed at water content as high as six times the liquid limit. At this incipient skeletal condition, minor differences in shear wave velocity hint to the stiffness of different fabrics. High frequency permittivity is a good indicator of water content and can be effectively used to monitor the evolution of soft sediments. On the other hand, the evolution of the effective conductivity varies with pore fluid concentration: for high ionic concentration fluids, the effective conductivity increases as porosity increases, yet, the opposite is true for low ionic concentration fluids due to the contribution of surface conduction. In a wide range of water contents @0.6 liquid limit ~LL! ,w, 6L Lg, the shear wave velocity is primarily determined by the water content or porosity, and fabric appears to have a limited impact. Test procedures and results can be used to develop parallel field characterization tools and interpretation guidelines.

Loading rate dependence of permeability evolution in porous aeolian sandstones

Mechanical properties of rocks are characterized by their notable dependence on the applied deformation rate. However, little is known about the strain rate dependence of fluid flow properties since most laboratory tests are conducted using a single, high strain rate. We have investigated the effect of loading rate on the permeability of porous sandstones by carrying out triaxial compression tests at four different temperatures and strain rates with continuous monitoring of permeability, acoustic emission (AE), and pore fluid chemistry. All tests are characterized by an initial permeability decrease due to inferred compaction of favorably oriented cracks. The amount of initial permeability reduction increases with decreasing strain rate, thus implying a more efficient initial compaction at slower strain rates. At a later stage of loading, permeability correlates with stress, ion concentration, or AE damage depending on the strain rate used. High strain rate tests are characterized by a positive power law or logarithmic correlation between permeability and AE damage. At slow strain rates, permeabilities decrease exponentially with mean effective stress and axial strain for the Locharbriggs sandstone. The Clashach sandstone exhibits a linear correlation between permeability and exit pore fluid concentrations (Si, Mg, Fe, Al) if a slow strain rate is used. These observations have important implications for the applicability of room temperature, high strain rate laboratory data to the conditions that prevail in the Earth's crust.

A mechanistic model for compaction of granular aggregates moderated by pressure solution

A model is presented for the compaction of granular aggregates that accommodates the serial processes of grain‐contact dissolution, grain‐boundary diffusion, and precipitation at the pore wall. The progress of compaction and the evolution of the mass concentration of the pore fluids may be followed with time, for arbitrary mean stress, fluid pressure, and temperature conditions, for hydraulically open or closed systems, and accommodating arbitrary switching in dominant processes, from dissolution, to diffusion, to precipitation. Hindcast comparisons for compaction of quartz sands [] show excellent agreement for rates of change of porosity, the asymptotic long‐term porosity, and for the development of silica concentrations in the pore fluid with time. Predictions may be extended to hydraulically open systems where flushing by meteoric fluids affects the compaction response. For basins at depths to a few kilometers, at effective stresses of 35 MPa, and temperatures in the range 75°–300°C, rates of porosity reduction and ultimate magnitudes of porosity reduction increase with increased temperature. Ultimate porosities asymptote to the order of 15% (300°C) to 25% (75°C) at the completion of dissolution‐mediated compaction and durations are accelerated from a few centuries to a fraction of a year as the temperature is increased. Where the system is hydraulically open, flushing elevates the final porosity, has little effect on evolving strain in these precipitation‐controlled systems, and depresses pore fluid concentrations. These effects are greatest at lower temperatures.

Eigendecomposition of TDR waveforms: a novel method to determine water content and pore fluid concentration of sandy soils

In this study, an incident pulse signal of several harmonics (i.e., multiples of the fundamental frequency) was used as a source in the time domain reflectometry (TDR) probing technique. Reflected signals were captured by an oscilloscope and their characteristics were determined via eigendecomposition. Autoregressive modeling and singular value decomposition were used to calculate the eigenvalues and the most significant ones were identified based on power spectrum. A multivariate statistical analysis was performed for the two most dominant eigenvalues, which are dependent on water content and salt concentrations, and regression equations were obtained. To determine the water content and salt concentrations in terms of the first and second eigenvalues, a modified Powell hybrid algorithm was used to solve the obtained system of nonlinear equations. Actual and predicted results are in agreement indicating that the developed method is very successful in predicting water content and salt concentrations. Furthermore, on comparing the eigendecomposition method with Fourier spectral analysis, one can observe that the former is superior in predicting water content and salt concentrations.

Development of a Methodology for Evaluating Subsurface Concentrations of Pollutants Using Electrical Polarization Technique

Abstract The study involves the introduction of an electrical polarization technique to in situ analysis of subsurface pollutants by measuring and analyzing the variation of soil-pollutant dielectric properties as a function of concentration of the pollutant in the soil. The developed system was modeled using transmission line theory, and the electrical parameters were optimized using the electrical circuit simulation software (Circuit Maker™) and the experimental results. Also, a methodology was developed to evaluate soil pore fluid concentrations via the use of two important electrical properties for the medium, which are resistance and capacitance.

Osmotic flow through a Cretaceous clay in southern Saskatchewan, Canada

Clays and shales can act as semipermeable membranes, causing osmotic flow and (or) osmotically induced fluid pressures. Despite laboratory proof of the osmotic behaviour of clay-rich geologic material, the influence of this osmotic behaviour on in situ groundwater flow and solute transport is inconclusive. This is due in part to the lack of laboratory experimental work involving undisturbed core samples, and to the lack of field-scale research. The ability of undisturbed clay samples to conduct flow hydraulically and osmotically was investigated using dilute salt solutions. Undisturbed Cretaceous clay samples from southern Saskatchewan were used in a laboratory experimental program. The experiments included constant-head hydraulic conductivity (Kh) tests and osmotic flow tests conducted over a range of pore-fluid concentrations of 0.054–1.12 equiv./L. The clay samples exhibited semipermeable membrane behaviour by conducting flow osmotically. The hydraulic conductivity of the clay increased by a factor of two as the pore-fluid concentration increased from 0.070 to 0.56 equiv./L. Osmotic efficiencies ranged from 0.0028 to 0.42 for concentrations from 0.84 to 0.096 equiv./L. Both the osmotic compressibility and osmotic efficiency decreased with increasing pore-fluid concentration. The behaviour of the clay was consistent with diffuse double-layer theory.Key words: osmotic flow, Cretaceous clay, salt, hydraulic conductivity.

Monitoring sedimentation of a clay slurry

Electrical impedance measurements are monitored during the sedimentation of a clay slurry and provide an in-situ, non-invasive technique to study the sedimentation process. The term normalised resistivity is introduced and data are interpreted in terms of the development of a density profile and the influence of pore fluid concentration on this process. Particle alignment mechanisms are also inferred from the electrical properties of the slurry.

Monitoring sedimentation of a clay slurry

Electrical impedance measurements are monitored during the sedimentation of a clay slurry and provide an in-situ, non-invasive technique to study the sedimentation process. The term normalised resistivity is introduced and data are interpreted in terms of the development of a density profile and the influence of pore fluid concentration on this process. Particle alignment mechanisms are also inferred from the electrical properties of the slurry.

Osmotic flow through a Cretaceous clay in southern Saskatchewan, Canada

Clays and shales can act as semipermeable membranes, causing osmotic flow and (or) osmotically induced fluid pressures. Despite laboratory proof of the osmotic behaviour of clay-rich geologic material, the influence of this osmotic behaviour on in situ groundwater flow and solute transport is inconclusive. This is due in part to the lack of laboratory experimental work involving undisturbed core samples, and to the lack of field-scale research. The ability of undisturbed clay samples to conduct flow hydraulically and osmotically was investigated using dilute salt solutions. Undisturbed Cretaceous clay samples from southern Saskatchewan were used in a laboratory experimental program. The experiments included constant-head hydraulic conductivity (Kh) tests and osmotic flow tests conducted over a range of pore-fluid concentrations of 0.054–1.12 equiv./L. The clay samples exhibited semipermeable membrane behaviour by conducting flow osmotically. The hydraulic conductivity of the clay increased by a factor of two as the pore-fluid concentration increased from 0.070 to 0.56 equiv./L. Osmotic efficiencies ranged from 0.0028 to 0.42 for concentrations from 0.84 to 0.096 equiv./L. Both the osmotic compressibility and osmotic efficiency decreased with increasing pore-fluid concentration. The behaviour of the clay was consistent with diffuse double-layer theory.Key words: osmotic flow, Cretaceous clay, salt, hydraulic conductivity.

Lithium isotopic compositions of pore fluids and sediments in the Costa Rica subduction zone: implications for fluid processes and sediment contribution to the arc volcanoes

Pore fluid and sediment Li concentrations and isotopic ratios provide important insights on the hydrology, sediment contribution to the arc volcanoes and fluid–sediment reactions at the dominantly non-accretionary Costa Rica subduction zone. Ocean Drilling Program Site 1039 in the trench axis provides a reference section of 400 m of the incoming sediments, and Site 1040, situated arcward from the trench, consists of a deformed sedimentary wedge and apron sediments, the décollement, and the partially dewatered underthrust sediment section. At the reference site, pore fluids show important isotopic variations (δ 6Li=−21.7 to −37.8‰), reflecting the interplay of in situ alteration of volcanic material and ion exchange with clay minerals. In the basal section, a reversal of Li concentration and δ 6Li toward seawater values is observed, providing supporting evidence for a lateral seawater flow system in the upper oceanic basement underlying this sediment section. At Site 1040, pore fluid of the lower deformed wedge sediments and within the décollement is enriched in Li and the isotopic compositions are relatively light, suggesting infiltration of a deep-seated fluid. The δ 6Li value of −22‰ of this Li-enriched fluid (261 μM), when compared with the δ 6Li value of the subducted sediment section (−11‰), suggests that the deep source fluid originates from mineral fluid dehydration and transformation reactions at temperatures of 100 to 150°C, consistent with the temperature range of the up-dip seismogenic zone and of transformation of smectite to illite. The distribution of Li and its isotopes in the underthrust section are similar to those at the reference site, indicating near complete subduction of the incoming sediments and that early dewatering of the underthrust sediments occurs predominantly by lateral flow into the ocean. The hemipelagic clay-rich sediment section of the subducting plate carries most of the Li into this subduction zone, and the pelagic diatomaceous and nannofossil calcareous oozes contain little Li. The Li isotopes of both the clay-rich hemipelagic sediments and of the pelagic oozes are, however, similar, with δ 6Li values of −9 to −12‰. The observations that (1) the δ 6Li values of the underthrust sediments are distinctly lower than that of the mantle, and (2) the lavas of the Costa Rican volcanoes are enriched in Li and 7Li, provide an approximation of the contribution of the subducted sediments to the arc volcanoes. A first order mass balance calculation suggests that approximately half of the Li flux delivered by subducted sediments and altered oceanic crust into the Middle American Trench is recycled to the Costa Rican arc and at most a quarter of sedimentary Li is returned into the ocean through thrust faults, primarily the décollement thrust.

Different rates of smectite illitization in mudstones and sandstones from the Niigata Basin, Japan

AbstractThe different rates of smectite illitization have been investigated in interbedded sandstone and mudstone samples from the Neogene to Lower Pleistocene stratigraphy of the Niigata sedimentary basin. For samples of approximately the same age and under the same temperature and pressure conditions, the analytical results of XRD and EDS show that differences in the transformation from R = 1 I-S to R≥3 I-S at a temperature of ~30–40°C and 1000 m in depth are determined by the initial smectite composition and the relative ion concentration of pore-fluid in the sedimentary rock system. The ordered I-S and illite in the mudstone have inherited compositional characteristics from the initial detrital smectite and illite. In the sandstone, the I-S composition was changed by interaction with pore-fluids and by precipitation and crystal growth during diagenesis.

Determination of the reactivity of clay-fluid systems using liquid limit data

The reactivity coefficient, defined as lambda = rhowGsSatheta (rhow is the mass density of water, Gs is the specific gravity, Sa is the specific surface, and theta is the double-layer thickness), has been proposed as a parameter to assess the sensitivity of a clay-fluid system to changes in physical and chemical properties of the pore fluid. In this study, the liquid limit of a clay-fluid mixture, wL, is shown to be related to the reactivity coefficient. Double-layer theory is applied to determine the void ratio at the liquid limit, which is assumed to occur at a stress close to 10 kPa. Liquid limit data were measured for a bentonite clay at different pore-fluid concentrations; other data were collected from the literature. Both measured and collected data followed the proposed simple expression, suggesting that there is a unique relationship between wL and lambda, at least for the three-layer sheet minerals (or for clays with wL > 50). The proposed expression can be used as a simple approach to determine the sensitivity of clay-fluid systems. A parametric study based on the lambda-wL relationship is presented to study the effects of temperature and pore-fluid concentration on the liquid limit.Key words: clay, specific surface, pore fluid, ion concentration, double layer.

Coupled diffusion–fabric-flow phenomena: an effective stress analysis

Concentration diffusion, fluid flow and fabric changes are coupled phenomena in fine soils. Indeed, experimental results previously presented by the authors showed the presence of a pressure front advancing ahead of the diffusing high-concentration front in bentonite and kaolinite specimens. This note presents a simple analysis of diffusion–fabric-flow coupling, based on elementary double-layer repulsion and attraction. Model predictions adequately agree with experimental data. High specific surface, high initial void ratio, and low initial pore-fluide concentration increase the sensitivity of soils to changes in pore-fluid concentration and enhance the potential development of pore pressure fronts. Key words: coupling, diffusion, clay, pore pressure, interparticle forces.

Chloride binding in concrete containing PFA, gbs or silica fume under sea water exposure

The paper reports an experimental investigation on different mix proportions of normal concrete and on concretes incorporating various fractions of cement replacement by pulverized fly ash (PFA), ground blastfurnace slag (gbs) or silica fume. The chloride binding capacity of each matrix was determined after immersion in sea water for up to 270 days, after exposure to laboratory marine cycles (up to 520 cycles) and after exposure to tidal cycles in the North Sea (up to 7.5 years). Extensive data on acid-soluble chloride concentration (total chloride), free chloride concentration in the pore fluid and the evaporable water content of hardened concrete were obtained in order to determine the chloride-binding capacity of the various concrete matrices. Corrosion of rebar electrodes embedded in these matrices was also monitored. The results show that the bound chloride and the chloride concentration decrease with increasing depth into concrete. Higher water-cement ratios or cement contents result in increased bound chloride. The chloride binding near the surface of concrete is not significantly affected by the incorporation of PFA or gbs. At greater depths, the use of PFA or gbs results in a reduction of Cl- bound in the cementitious binder. Replacement of cement with silica fume results in greatly reduced contents of bound chloride. Accelerated penetration of chloride in concrete results in lower binding than under natural exposure conditions. It is also shown that the free Cl- concentration of pore fluid correlates with the corrosion rates of reinforcement embedded in concretes made with various cementitious binders, while the Cl--OH- ratio has no correlation.

Simulation of Microbial Growth Dynamics Coupled to Nutrient and Oxygen Transport in Porous Media

A model for simulating microbial growth‐degradation processes in porous media is developed. It is assumed that the bulk of microorganisms in an aquifer grow in microcolonies attached to matrix surfaces. As developed, the model applies to the growth and decay of aerobic, heterotrophic microorganisms whose growth is limited by lack of a carbon and energy source (substrate), an oxygen source or both simultaneously as described by modified Monod kinetics. Transport of substrate and oxygen in the porous medium is assumed to be governed by advection‐dispersion equations with surface adsorption. A total of five coupled equations result describing substrate and oxygen concentrations in the pore fluid, substrate and oxygen concentrations in the microcolonies and colony density, which is assumed sufficiently small so that aquifer hydraulic conductivity is not diminished. An iterative process involving an Eulerian‐Lagrangian numerical procedure that is highly resistant to numerical dispersion in the presence of small dispersivities is used to solve the overall model, with parameter values selected from the literature or estimated. Results indicate that biodgradation would be expected to have a major effect on contaminant transport when proper conditions for growth exist. For one‐dimensional transport in a column, the most rapid microbial growth always occurred at the influent boundary where oxygen and substrate concentrations were held constant independent of colony density. Anaerobic conditions develop rapidly and aerobic biodegradation ceases if large amounts of substrate are added to the system.

Flux‐Averaged and Volume‐Averaged Concentrations in Continuum Approaches to Solute Transport

Transformations between volume‐averaged pore fluid concentrations and flux‐averaged concentrations are presented which show that both modes of concentration obey convective‐dispersive transport equations of identical mathematical form for nonreactive solutes. The pertinent boundary conditions for the two modes, however, do not transform identically. Solutions of the convection‐dispersion equation for a semi‐infinite system during steady flow subject to a first‐type inlet boundary condition is shown to yield flux concentrations, while solutions subject to a third‐type boundary condition yield volume‐averaged concentrations. These solutions may be applied with reasonable impunity to finite as well as semi‐infinite media if back mixing at the exit is precluded. Implications of the distinction between resident and flux concentrations to laboratory and field studies of solute transport are discussed. It is suggested that perceived limitations of the convection‐dispersion model for media with large variations in pore water velocities may in certain cases be attributable to a failure to distinguish between volume‐averaged and flux‐averaged concentrations.

Controls on pore-fluid concentration of 4He and 222Rn and the calculation of 4He/222Rn ages

Abstract Controls governing the production of 4He and 222Rn in the solid phase as well as parameters and processes contributing to their transport into the pore fluid are discussed. 222Rn activity is used to quantify the uranium sources for 4He and the result is a simplified pore-fluid age equation which is virtually independent of the porosity, the water/rock ratio, the rock density and the uranium concentration, but does require a Th/U estimate. The crucial parameter is the ratio of the release factors for the two species, /GLRn//GLHe, which is discussed in terms of three possible release mechanisms: (1) recoil; (2) recoil followed by diffusion; and (3) weathering release of accumulated 4He. It was found that /GLRn//GLHe can vary over several orders of magnitude, but can be expressed in terms of the effective grain size r, and the diffusive half-length for 222Rn decay, re. 4He measurements are used to “date” gases from known gas fields and the agreement with the assumed source-rock age is good. Application of 4He/ 222Rn measurements to continental freshwater springs indicates that the weathering release of accumulated 4He dominates the input and results in a large overestimate of groundwater age. Measurement in the Lardarello geothermal field indicates that the 4He/222Rn method can indicate relative transport direction. Other possible applications in various geochemical fields are suggested.