Water-saturated Conditions Research Articles

Variations in water saturation states and their impact on eruption size and frequency at the Aso supervolcano, Japan

The exsolution of a magmatic volatile phase in the plumbing systems of volcanoes plays a key role in controlling growth dynamics of subvolcanic reservoirs and eruptive styles. By using common petrological proxies found in volcanic deposits, such as melt inclusions and apatite crystals, the presence of such an exsolved magmatic volatile phase can be traced, specifically the exsolution of water from silicate melt. To monitor variations in the water saturation state of magmas from the Aso volcanic complex (Kyushu, Japan) prior to and during the catastrophic Aso-4 caldera-forming event (at ∼86 ka BP), we investigate a set of pre-Aso-4 and Aso-4 deposits, combining volatile budgets (F, Cl, OH, S) of melt inclusions, matrix glasses, and apatite crystals, with sulfur isotope signatures in apatite. F-Cl-OH partitioning in apatite along with melt inclusion data from pre-Aso-4 units indicate water-undersaturated conditions during magma evolution until around 10 ka prior to the Aso-4 event. In contrast, eruptions occurring within the last ∼10 ka prior to the Aso-4 event indicate the presence of a water-rich exsolved magmatic volatile phase in the eruptible portions of the subvolcanic reservoir. Likewise, sulfur systematics in apatite and melt inclusions from the Aso-4 event suggest the exsolution of a water-rich magmatic volatile phase at some point prior to the caldera-forming event. Recharge of mafic magmas shortly before the Aso-4 eruption induced chemical hybridization in the resident upper crustal mush, bringing the reservoir back to less evolved compositions and water-undersaturated conditions. This hybridization event is recorded by volatile contents of both apatite and matrix glasses from late-erupted, crystal-rich products of the Aso-4 event, all yielding water-undersaturated signatures. During this hybridization event, chemical dilution and partial redissolution of the exsolved volatile phase reduced the magma compressibility significantly, so that additional magma influx from depth might have allowed a sharply increasing overpressurization in the subvolcanic reservoir and served as a potential trigger for the cataclysmic Aso-4 eruption. Drawing from our observations made on Aso, we propose that recharging of large silicic upper-crustal reservoirs with increased volumes of drier and more mafic melts can influence their water saturation states and associated physical properties. Such changes could contribute to the triggering of large-scale caldera-forming events.

Effects of Water Saturation and Loading Condition on Rock Tensile Strength: Insights from Acoustic Emission Analysis

Effects of Water Saturation and Loading Condition on Rock Tensile Strength: Insights from Acoustic Emission Analysis

Physical and mathematical modeling of frost resistance for cement concretes

The key factor of engineering structures made of concrete and reinforced concrete is insufficient frost resistance of concretes. It is important to identify the causes and prevent concrete damage caused by frost. The research provides the basic points of the developed physical and mathematical theory of cement concretes frost resistance. Under consideration are the processes occurring during cyclic freezing and thawing of concrete in a water-saturated state. The results of the performed theoretical and experimental studies are presented. The criterion of concrete frost resistance, which estimates the pore structure of concrete, was derived on the basis of the obtained results. The suggested criterion has a close correlative relation with the frost resistance of concrete. Using this interrelation, the method was proposed for accelerated determination of actual frost resistance of concrete, as well as the method of selection concretes' compositions for a specified design frost resistance of concretes. The methods of accelerated determination of concrete frost resistance, as well as ways to increase it, are described. The developed physical and mathematical model was used to carry out computational modeling for freezing of an extended concrete structure. This made it possible to determine the changes in humidity, temperature, and pressure in concrete during cyclic freezing and thawing at different distances from the surface of its freezing and also draw graphs of changes in these parameters. The recommendations on assignment of concrete design grades by frost resistance for various elements of concrete and reinforced concrete structures are given.

Formation processes of paper-thin raft and coated bubble: Calcium carbonate deposition at gas–water interface

Travertines are hot spring carbonate deposits that exhibit characteristic fabrics, including a paper-thin raft and a coated bubble formed by the calcification of water and bubble surfaces, respectively. A previous study interpreted that compared with the water surface, the bubble surface displays more active CO2 degassing and resultant active CaCO3 precipitation. However, considering the CO2 partial pressure and the volumes of atmosphere and bubbles, it is possible that more active CO2 degassing occurs on the water surface. In addition, the surfaces of water and bubbles at the travertine-depositing sites are negatively charged, but it is still unclear whether the CaCO3 nucleation on these surfaces occurs via amorphous precursors, like the CaCO3 nucleation on negatively charged organic matter. This study provides a solution to these uncertainties by examining the aragonitic travertines formed in the Nagayu area of Japan. Through field observations, two types of paper-thin rafts were recognized: one with a smooth surface and formed in approximately 1 h, and the other with a rough surface and formed in approximately 3 h. In addition, the bubbles generated on the microbial mat during the daytime were covered with white minerals within an hour, and with ivory-colored minerals over 8 h after sunset, forming firm coated bubbles. Microelectrode measurements revealed that the active CO2 degassing on the water surface significantly increased the CaCO3 saturation state to cause active CaCO3 precipitation. In contrast, less active CO2 degassing on the bubble surface did not increase the saturation state, and moderate CaCO3 precipitation occurred due to the moderately high saturation state of the hot spring water. Various microscopic observations revealed that the smooth-surfaced paper-thin raft comprises a micritic layer of approximately 10–20 μm, which represents tightly arranged submicron-sized granular aragonite. At the lower surface of this layer, hemispherical aragonite partially grew toward the hot spring water. In addition, the rough-surfaced paper-thin raft and coated bubble comprise bundled and acicular aragonites arranged along the surfaces of water and bubble, from which hemispherical aragonite grew toward the hot spring water. Neither the paper-thin raft nor the coated bubble contains primary amorphous CaCO3. These results suggest that the smooth-surfaced paper-thin raft is formed by active CaCO3 nucleation on the water surface and subsequent crystal growth, and that the water surface is unfavorable for preserving the amorphous precursors potentially involved in the nucleation. The rough-surfaced paper-thin raft and the coated bubble could have been formed by the attachment of externally formed acicular aragonite to the water/bubble surface and subsequent crystal growth. The revealed formation processes of the paper-thin raft and coated bubble provide valuable information for interpreting their formation mechanism in other travertine deposits, including those in the geological past.

Interactions between C-steel and blended cement in concrete under radwaste repository conditions at 80 °C

Deep geological repository is widely considered as the preferred solution for the final disposal of high-level radioactive waste. Investigation representative of the Hungarian disposal concept was conducted using mock-up diffusion cells to study the chemical changes of S235JR carbon steel canister and CEM II/B concrete of the Public Limited Company for Radioactive Waste Management under anerobic and water saturated conditions at 80 °C. Micro-Raman, Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy, fluid and potentiometric analysis were performed over a period of 12 months. The analysis was supported by thermodynamic and reactive transport modeling using the HYTEC code. The findings revealed that a uniform corrosion process occurred, leading to rapid passivation of the C-steel with magnetite as the primary corrosion product. Modeling demonstrated that the increase in temperature to 80 °C and the chemical evolution of the concrete did not significantly affect the corrosion passivation process. Although the formation of Fe-siliceous hydrogarnets is thermodynamically possible at 80 °C, it did not jeopardize magnetite passivation. The results show that the passivation of the containers occurred under the test conditions and this is a promising result for further investigations.

Retention and transport of PFOA and its fluorinated substitute, GenX, through water-saturated soil columns

Perfluoro-2-propoxypropanoic acid (GenX) has emerged as a substitute for perfluorooctanoic acid (PFOA) especially since PFOA was listed among the persistent organic pollutants (POPs) by the Stockholm Convention in 2019. However, limited knowledge exists regarding the behavior and mobility of GenX in natural soils hindering the prediction of its environmental fate. This study investigated the mobility and retention of GenX and PFOA in soils under batch and water-saturated flow-through conditions. Batch experiments revealed that GenX has a lower binding affinity to soil than longer-chained PFOA, potentially threatening groundwater resources. Unlike metal-oxides/minerals (ferrihydrite, gibbsite and manganese dioxide), biochar (BC) and activated carbon (AC) amendments significantly enhanced the sorption of both GenX and PFOA in soil. Sorption data on minerals and carbonaceous materials implied that for shorter-chained GenX, the predominant mode of sorption was through electrostatic (ionic) interactions, while for longer-chained PFOA, hydrophobic interactions became progressively more important with increasing chain length. The dynamic flow experiments demonstrated that these soil amendments enhanced the retention of both compounds, thereby decreasing their mobility. Simultaneous injection of both compounds into columns pre-loaded with either PFOA or GenX increased their retardation. GenX sorption was more affected by pre-sorbed PFOA compared to the minimal impact of pre-loaded GenX on PFOA sorption. A newly developed reactive transport model, which incorporates a two-site sorption model and accounts for kinetic-limited processes, accurately predicted the sorption and transport of both compounds in single and binary contamination systems. These findings have important implications for predicting and assessing the fate and mobility of per- and polyfluoroalkyl substances (PFAS) in soils and groundwaters.

Experimental characterization of dynamic strength and failure behavior of saturated reef limestone concrete under biaxial stress constraint

On the basis of not destroying the island's ecological environment, concrete mixed with reef limestone aggregates in situ is beneficial to solve the problem of shortage of terrestrial rock resources in the island area. Considering the influence of water-saturated state, lateral stress constraint and strain rate, the dynamic mechanical properties and failure characteristics of reef limestone concrete under biaxial stress constraint are studied by using true triaxial Hopkinson bar test technology. The dynamic failure behavior of reef limestone concrete is analyzed from macro-meso level by means of SEM test method, and the stress intensity factor theory of crack tip is cited to reveal the enhancement effect of dynamic strength of water-saturated reef limestone concrete under the coupling effect of biaxial stress constraint and impact load. The results show that the dynamic compressive strength of reef limestone concrete increases with the strain rate in a power function, and the growth rate decreases gradually. Under the same impact load, the compressive strength of reef limestone concrete in water-saturated state increases more significantly than that in dry state. The lateral stress constraint has a significant enhancement effect on the dynamic strength of reef limestone concrete samples, and changes the Poisson's ratio of reef limestone concrete samples. In addition, based on the equivalent crack model theory, it is verified that under the combined action of biaxial stress constraint and high strain rate, the dynamic stress intensity factor of the crack tip of the reef limestone concrete sample in water-saturated state is larger than that in dry state.

Quantifying hydrolytic degradation of poly(ethylene terephthalate glycol) under storage conditions and for fused filament fabrication mechanical properties

Poly(ethylene terephthalate glycol) (PETG) is a promising polyester for additive manufacturing (AM) applications. However, an important question is its thermal and hydrolytic stability under storage conditions. The current work investigates which measuring technique is most suited to qualitatively answer this question, completing the field for polyester stability analysis by focusing on more amorphous structures. In combination with regression analysis under lab accelerated conditions to practically estimate characteristic degradation time scales and currently lacking (overall) Arrhenius parameters, a distinction is made between techniques dealing with molecular and larger scale properties, distinguishing between Fourier Transfer Infrared (FTIR) spectroscopy, size exclusion chromatography (SEC), rheological measurements, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). It is shown that hydrolytic scission is dominant over thermal scission, and mainly affects the lower mass region of the molar mass distribution. However, the degradation sensitivity can only be assessed using the SEC number average molar mass, and a more pronounced qualitative sensitivity is only obtained applying rheological analysis, with an extrapolated very low half-life time under water-saturated conditions of ca. 3 days in the Newtonian region and of 2 days in the shear thinning region. It is further highlighted that hydrolytic degradation influences the tensile properties of parts fabricated with fused filament fabrication, which is an important AM technique.

Analyzing the Energy and Damage Constitutive of Cemented Backfill with Different Water Content under Dynamic Load.

The dynamic characteristics of the filling body are the key parameters for designing the filling ratio and evaluating the stability of an underground stope. The different environment (water-bearing state) of the filling body in the underground stope exerts a complex impact on the mechanical behavior of the filling body. Therefore, six groups of cemented filling body specimens with different states were formed and subjected to dynamic uniaxial impact tests. The effects of water content on the mechanical properties, fractal dimension, and deformation damage characteristics of the cemented backfill under dynamic load were analyzed in depth, and a dynamic damage constitutive model that considers water damage and the compaction stage was established. The results indicate the following: (1) Due to the change of the specimen from the dry state to the water saturation state, the dynamic compressive strength of the cemented filling body decreases from 5.03 Mpa to 1.79 Mpa; however, the ductility of the specimen generally increases, and the filling body specimens with different water contents mainly exhibit tensile failure. (2) There is a significant nonlinear relationship between the water content and the fractal dimension Db of the cemented backfill specimen, and the growth rate of the fractal dimension Db tends to slow down with the increase in the water content. (3) From the energy evolution perspective, the water content of the specimen exerts a significant effect on the elastic deformation and failure stage of the stress-strain curve, and the slope of the dissipated energy-strain curve decreases with the increase in water content. (4) Based on the Weibull distribution and damage theory, a statistical damage constitutive model of cemented backfill was established, and it was compared with the experimental curve to verify the rationality of the model. Therefore, the relationship between stress and damage and the strain curves is discussed, and it is inferred that the damage evolution curve of cemented backfill is a typical S-shaped curve that exhibits a stable development-rapid increase-tending to be gentle. This study can provide a theoretical reference for further understanding the dynamic behavior and stability of backfill under different water conditions.

Assessment of the theoretical models of effective thermal conductivity based on digital rock physics for clean sandstones

Knowledge of the effective thermal conductivity (ETC) of porous media provides valuable insights into the heat flow and profoundly affects the feasibility analysis of thermal enhanced oil recovery methods and geothermal field development. Theoretical assessment of ETC based on structural properties of the porous medium and various simplification assumptions by prediction models have shown acceptable results towards ETC quantification; however, their applicability must be investigated further due to the availability of various types of such models. In this study, we first employed a digital rock approach to simulate the heat conductivity of micro-CT images of 10 clean sandstone samples using Avizo software in different directions for evacuated, air-saturated, and water-saturated conditions. The filtered and segmented images were applied to perform voxel-based ETC simulations under steady state condition. Then, the simulation results were compared to the computed ETCs for each sample by 11 theoretical models. Two well-known thermal conductivity bounds, i.e., Hashin and Shtrikman bounds and Wiener bounds were used to confirm the validity of models and simulation outputs. It was shown that the Haung model generates the best outputs for vacuum (MAPE = 0.08 and RMSE = 0.37) and air-saturated (MAPE = 0.07 and RMSE = 0.32) states. While the water-saturated condition was competently predicted by the Ribaud model (MAPE = 0.05 and RMSE = 0.31). The overall analysis indicated that the Haung model is the best model for the prediction of ETC in all three pore-filling states for clean sandstones.

Autogenous self-healing of polypropylene fiber reinforced ettringite-based composite

This study intends to advance the poor knowledge on the ability of an ettringite-based binder to obtain autogenous self-healing and its relevant mechanism. We developed a high-performance polypropylene fiber-reinforced cementitious composite using a slag-based ettringite binder and investigated its autogenous healing property under water-saturated conditions. Cracks were created via four-point bending loads on prismatic specimens at different crack opening displacements (i.e., 0.5, 1, and 2 mm in total crack opening). After curing times, the mechanical retention and recovery of the damaged composite were compared with the sound sample. We found that the material effectively heals micro cracks (i.e., under 100 μm) after 1 month of aging, while larger cracks could not completely close during the total curing times (up to 3 months). The healing products were mainly CaCO3 and ettringite observed through both experiments and thermodynamic modelling. These crystals mainly formed from the carbonation of ettringite and other AFm phases in aqueous conditions. The strength retention slightly reduced (about 10%) after healing, however, the composite still exhibited deflection-hardening behavior. Findings show the high potential of ettringite-based binder to be a matrix for self-healing cementitious composites. Further improvements, nevertheless, are needed to increase the healing rate and strength recovery of the composite.

Water-Weakening and Time-Dependent Deformation of Organic-Rich Chalks

The Ghareb Formation is a shallowly buried porous chalk in southern Israel that is being considered as a host rock for a geologic nuclear waste repository. Setup and operation of a repository will induce significant mechanical, hydrological and chemical perturbations in the Ghareb. Developing a secure repository requires careful characterization of the rock behavior to different loads. To characterize hydromechanical behavior of the Ghareb, several short- and long-term deformation experiments were conducted. Hydrostatic loading tests were conducted both dry and water-saturated, using different setups to measure elastic properties, time-dependent behavior, and permeability. A set of triaxial tests were conducted to measure the elastic properties and rock strength under differential loading at dry and water-saturated conditions. The hydrostatic tests showed the Ghareb began to deform inelastically around 12–15 MPa, a relatively low effective pressure. Long-term permeability measurements demonstrated that permeability declined with increasing effective pressure and was permanently reduced by ~ 1 order of magnitude after unloading pressure. Triaxial tests showed that water saturation significantly degrades the rock properties of the Ghareb, indicating water-weakening is a significant risk during repository operation. Time-dependent deformation is observed during hold periods of both the hydrostatic and triaxial tests, with deformation being primarily visco-plastic. The rate of deformation and permeability loss is strongly controlled by the effective pressure as well. Additionally, during holds of both hydrostatic and triaxial tests, it is observed that when water-saturated, radial strain surpassed axial strain when above effective pressures of 13–20 MPa. Thus, deformation anisotropy may occur in situ during operations even if the stress conditions are hydrostatic when above this pressure range.

Comparative study on the electric resistance of mortars made of low carbon binders

Against the background of the high CO2 emissions generated in the production of Portland cement (OPC), research is being carried out worldwide into alternative binders that can be produced with lower energy consumption. These have a lower clinker-cement factor or contain no Portland cement at all. To enable these binders to be used on a large scale, also in reinforced concrete structures, their performance in terms of durability must be ensured. The specific electric resistance is a parameter for characterizing different binders. At a young age, the hydration progress and the formation of the pore structure can be observed based on the specific electric resistance. In connection with the electrical conductivity of the pore solution, statements about the pore structure and the resistance to the penetration of corrosion-promoting substances are possible. Finally, the electric resistance is an influencing factor for estimating possible corrosion rates after depassivation of the steel reinforcement. In this study, a wide range of mortars produced with different alternative binders were characterized in terms of their resistivity as a function of age and water content. From the group of clinker substitutes (SCMs), calcined clays and modified steel slag were investigated. The other binders included in the investigation program are produced entirely without Portland cement: In addition to a calcium sulfoaluminate (CS̅A) cement and the C-S-H binder Celitement®, alkali-activated materials (AAMs) in the form of geopolymers and alkali-activated slags were also investigated. The results show a wide range of specific electric resistances in a naturally water-saturated state of the mortar specimens between 5.5Ωm for an alkali-activated metakaolin and 971Ωm for the Celitement®. Clinker replacements with calcined clays metakaolin, or metaillite result in increased specific electric resistances. In addition, the different binders show a very different influence on the dependence of the resistivity on the water content.

Numerical Simulation of Electrical Properties of Carbonate Reservoirs Using Digital Rocks

Rock electrical experiments are essential means of researching the conductive properties of rocks and are fundamental to interpreting resistivity logging. Carbonate rocks have more complex pore structures than sandstone, which results in more complex conductive properties. However, conducting experiments on representative rock samples from carbonate reservoirs is difficult, making it challenging to study the micro factors affecting electrical properties. Therefore, researching the conductive properties of carbonate rocks is difficult. To address this, in this paper, three-dimensional (3D) digital rock models with different porosities are generated, and conductive simulations are carried out on these models using the finite element method (FEM). Firstly, a micro-computed tomography (μ-CT) 3D image of a carbonate rock is obtained. Secondly, mathematical morphology-based methods are used on the μ-CT image to generate cores with varying porosities and fluid distributions. Then, the electrical properties are simulated using the FEM method, and the results are analyzed. The results reveal that the formation factor of the reservoir is mainly influenced by the shape and structure of the pores. The Archie equation is more suitable for carbonate reservoirs with water saturation levels greater than 60%. The wettability of the rock can alter the distribution of fluid in the reservoir space under different water saturation conditions. In pure water-wet rocks, the water phase mainly occupies small pores, while the oil phase occupies larger pores. As a result, compared to pure oil-wet rocks, water-wet rocks have more conductive channels and better conductivity. Therefore, it is important to determine the wettability of the rock when calculating water saturation using the Archie equation. The saturation index value of water-wet carbonate rock is about 2, while that of oil-wet rock is around 3–4. This research lays a foundation for studying the electrical conductivity of carbonate reservoirs using digital rocks.

Modelling biogeochemical reactions triggered by graphene's addition in a fertilized calcareous sandy soil

Graphene production has dramatically increased in the last years and new ways to recycle this engineered material need to be investigated. To this purpose, a reactive model network was developed using PHREEQC-3 code to quantify the relevant biogeochemical reactions induced by graphene scraps' incorporation in a calcareous sandy soil. The numerical model was calibrated versus a complete dataset of column experiments in water saturated conditions using two different fertilizers, a synthetic NPK fertilizer and fertigation water produced in a wastewater treatment plant. Column experiments consisted of 50 cm columns filled with a mixture of graphene scraps (0.015 % dry weight) and soil in the first 10 cm, while the remaining 40 cm had only soil. The model performance was tested using classical statistical indices (R2, Modelling Efficiency, and Index of Agreement), resulting to be satisfactory. Besides, a simple sensitivity analysis via the perturbation of relevant parameters showed a low degree of uncertainty. The main outcome of this study was the quantification of the increased denitrification rate triggered by graphene incorporation into the soil. Moreover, graphene incorporation substantially increased soil CEC and DOC sorption capacity, demonstrating a good adsorption capacity for ammonium and organic compounds, thus decreasing nutrients leaching that represents a major concern related to agricultural practice. Indeed, Graphene incorporation increased by 40 % the CEC in the first 10 cm of the CSG_NPK column (2.50e−02 mol/L) respect to the CS_NPK column (1.75e−02 mol/L) and increased it by 150 % in the first 10 cm of the CSG_FW column (2.50e−02 mol/L) in comparison with the CS_FW column 1.00e−02 (mol/L). pH fluctuations were most likely due to the precipitation of Ca5(PO4)3OH, indeed the consumption of H+ ions could have triggered the pH lowering during the experiment. These results could be relevant for future graphene applications as a soil improver or as suitable material to enhance soil bioremediation in order to include graphene in a circular economy loop.

Dynamic properties and fragmentation fractal characteristics of water-saturated reef limestone concrete under impact loading

Reef limestone was used as a coarse–fine aggregate to prepare concrete with a strength class of C30. The mechanism of influences of water-saturated state on dynamic compressive strength, energy dissipation, and macroscopic and microscopic failure modes of reef limestone concrete was studied, and the relationships between fractal dimension and loading rate, fragmentation characteristics, and energy dissipation were discussed. The results indicated that the dynamic compressive strength of reef limestone concrete presents power function growth with increasing loading rate, the free water reduces the degree of cementation among particles and weakens the quasi-static state and dynamic mechanical properties of reef limestone concrete. Meanwhile, under the same loading rate, water-saturated reef limestone concrete demonstrates a stronger energy absorption effect and a greater energy dissipation density, and the energy dissipation density is linearly correlated with the dynamic compressive strength. In addition, the irregular shape and porous structure of reef limestone aggregate particles effectively enhances the friction strengths between the aggregates and cement slurry, causing most of the dynamic cracks in reef limestone concrete to occur in the cement-sand slurry and aggregates themselves. Finally, the results also show that the fractal dimension of the fragments of reef limestone concrete increases linearly with increases in the loading rate and fragmentation energy dissipation density. At approximately the same loading rate, the fractal dimension of dry specimens was larger, and when the fractal dimensions in the two states were similar, the energy consumed per unit mass water-saturated specimen exceeds that of dry specimens.

Experimental Validation and Calibration of the Galvin Model with Artificial Tight Sandstones with Controlled Fractures

The study of fractures in the subsurface is very important in unconventional reservoirs since they are the main conduits for hydrocarbon flow. For this reason, a variety of equivalent medium theories have been proposed for the estimation of fracture and fluid properties within reservoir rocks. Recently, the Galvin model has been put forward to model the frequency-dependent elastic moduli in fractured porous rocks and has been widely used to research seismic wave propagation in fractured rocks. We experimentally investigated the feasibility of applying the Galvin model in fractured tight stones. For this proposal, three artificial fractured tight sandstone samples with the same background porosity (11.7% ± 1.2%) but different fracture densities of 0.00, 0.0312, and 0.0624 were manufactured. The fracture thickness was 0.06 mm and the fracture diameter was 3 mm in all the fractured samples. Ultrasonic P- and S-wave velocities were measured at 0.5 MHz in a laboratory setting in dry and water-saturated conditions in directions at 0°, 45°, and 90° to the fracture normal. The results were compared with theoretical predictions of the Galvin model. The comparison showed that model predictions significantly underestimated P- and S- wave velocities as well as P-wave anisotropy in water-saturated conditions, but overestimated P-wave anisotropy in dry conditions. By analyzing the differences between the measured results and theoretical predictions, we modified the Galvin model by adding the squirt flow mechanism to it and used the Thomsen model to obtain the elastic moduli in high- and low-frequency limits. The modified model predictions showed good fits with the measured results. To the best of our knowledge, this is the first study to validate and calibrate the frequency-dependent equivalent medium theories in tight fractured rocks experimentally.

Determination of Ce3+, Co2+, Mn2+ and Fe2+ diffusion coefficients in Nafion® membrane

Concentration gradient diffusion coefficients for Ce3+, Co2+, Mn2+ and Fe2+ cations are determined in Nafion®211 membranes over a wide range of environmental conditions. Measurements are made using finite-width cation-rich bands introduced to NR211 via a hot-pressing procedure. A robust method of accurate diffusion coefficient determination of the finite-width deposits is developed using a Fick's second law of diffusion solution for one-dimensional systems. The transition metal dications are found to have nearly identical NR211 diffusion coefficients under water saturated conditions over the 22–80 °C range. The powerful chemical mitigant, Ce3+, is about one-half as diffusive as the dications under identical conditions. Derived diffusion coefficients for Ce3+ and Co2+ are found to be independent of initial cation concentration over the range of 12.5–70 mol% exchange level. The diffusion behavior of Ce3+ in 80 °C liquid water and saturated water vapor is identical. The diffusion coefficients of Ce3+ are shown to have a strong dependence on membrane hydration level varying by a factor of more than 30 over the range of 95 to 40% RH. The implications of these new diffusion findings are applied to a discussion of potential guidelines for the development of highly durable fuel cell systems, which employ mobile metal cations as lifetime-extending redox stabilizers.

Composite Dielectric Model for Cement Concrete Considering Water Saturation

Water has a significant impact on the dielectric constant of materials. As a multiphase composite material, the effect of water content must be taken into account when modeling the composite dielectric of cement concrete. Cement concrete traditionally has been thought of as a three-phase mixed material consisting of mortar, aggregate (limestone), and pores (air). Water was used as a constituent material of cement concrete in this paper, and its content was described in terms of water saturation. By abstracting the components of cement concrete into independent medium units and arranging them according to certain rules, the four-phase mixing model of cement concrete can be established. To investigate the effect of water, the dielectric constant of cement concrete was measured using a network analyzer at various water saturation levels. The experimental results show that water significantly can enhance the dielectric constant of cement concrete, and this effect is related to porosity: the higher the water saturation, the faster the dielectric constant increases. According to the established concrete four-phase mixed model, combined with the parallel plate capacitor theory, a composite dielectric model considering water saturation was developed which can improve the prediction accuracy under different water saturation conditions and provide a foundation for the application of ground-penetrating radar (GPR) in the quality inspection of cement concrete pavement.

Simulation of nuclear magnetic resonance response based on 3D CT images of sandstone core

In recent years, nuclear magnetic resonance (NMR) logging has become increasingly prevalent in the characterization of rock properties such as porosity, permeability, saturation, and pore size distribution. However, interpreting such properties accurately from NMR logging data is challenging for some reservoirs. In particular, the impact of salinity, viscosity, and saturation on NMR measurements is not always clear, which can lead to inaccuracies in the resulting data interpretation. Properly accounting for these factors is essential in order to obtain accurate and reliable measurements for effective characterization of subsurface formations. This study utilized a random walk technique to simulate the NMR response of homogeneous sandstones using 3D CT images and conducted a sensitivity analysis under various salinity, crude oil viscosity, and water saturation conditions. The results indicate that the T2 relaxation time slightly shifts toward the short relaxation direction as the salinity of the formation water increases. In addition, longer echo intervals result in a more significant forward shift in the T2 value than shorter intervals. Whereas, for crude oil, the T2 relaxation time becomes shorter as its viscosity increases. Furthermore, the effect of echo interval on the forward T2 shift is less pronounced for crude oil than it is for formation water. Under water wet conditions, the T2 spectrum of crude oil exhibits a peak at the volume relaxation position. As the water saturation decreases, the left two peaks in the T2 spectrum shift toward shorter relaxation times. Under oil wet conditions, the T2 spectrum exhibits a complex three-peak structure. The method provides a physical basis for interpreting NMR macroscopic responses, and the simulated NMR responses can help identify fluids in reservoirs.