Study of the numerical simulation of tight sandstone gas molecular diffusion based on digital core technology

Diffusion is an important mass transfer mode of tight sandstone gas. Since nano-pores are extensively developed in the interior of tight sandstone, a considerable body of research indicates that the type of diffusion is mainly molecular diffusion based on Fick＇s law. However, accurate modeling and understanding the physics of gas transport phenomena in nanoporous media is still a challenge for researchers and traditional investigation（analytical and experimental methods） have many limitations in studying the generic behavior. In this paper, we used Nano-CT to observe the pore structures of samples of the tight sandstone of western of Sichuan. Combined with advanced image processing technology, threedimensional distributions of the nanometer-sized pores were reconstructed and a tight sandstone digital core model was built, as well the pore structure parameters were analyzed quantitatively. Based on the digital core model, the diffusion process of methane molecules from a higher concentration area to a lower concentration area was simulated by a finite volume method. Finally, the reservoir＇s concentration evolution was visualized and the intrinsic molecular diffusivity tensor which reflects the diffusion capabilities of this rock was calculated. Through comparisons, we found that our calculated result was in good agreement with other empirical results. This study provides a new research method for tight sandstone digital rock physics. It is a foundation for future tight sandstone gas percolation theory and numerical simulation research.

Diffusion behavior of helium in titanium and the effect of grain boundaries revealed by molecular dynamics simulation

The microstructures of titanium（Ti）, an attractive tritium（T） storage material, will affect the evolution process of the retained helium（He）. Understanding the diffusion behavior of He at the atomic scale is crucial for the mechanism of material degradation. The novel diffusion behavior of He has been reported by molecular dynamics（MD） simulation for the bulk hcp-Ti system and the system with grain boundary（GB）. It is observed that the diffusion of He in the bulk hcp-Ti is significantly anisotropic（the diffusion coefficient of the [0001] direction is higher than that of the basal plane）,as represented by the different migration energies. Different from convention, the GB accelerates the diffusion of He in one direction but not in the other. It is observed that a twin boundary（TB） can serve as an effective trapped region for He.The TB accelerates diffusion of He in the direction perpendicular to the twinning direction（TD）, while it decelerates the diffusion in the TD. This finding is attributable to the change of diffusion path caused by the distortion of the local favorable site for He and the change of its number in the TB region.

Dynamics Studies on Molecular Diffusion in Zeolites

A review about the applications of molecular dynamics(MD)simulation in zeolites is presented. MD simulation has been proved to be a useful tool due to its applications in this field for the recent two decades. The fundamental theory of MD is introduced and the hydrocarbon diffusion in zeolites is mainly focused on in this paper.

Molecular dynamics simulation of nanoscale surface diffusion of heterogeneous adatoms clusters

Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface diffusion of heterogeneous adatoms clusters can be vital for the binary island growth on the surface and can be useful for the formation of alloy-based thin film surface through atomic exchange process. The results of the diffusion process show that at 300 K, the diffusion of small adatoms clusters shows hopping, sliding, and shear motion; whereas for large adatoms clusters（hexamer and above）, the diffusion is negligible. At 500 K, small adatoms clusters, i.e., dimer, show almost all possible diffusion mechanisms including the atomic exchange process; however no such exchange is observed for adatoms clusters greater than dimer. At 700 K, the exchange mechanism dominates for all types of clusters, where Zr adatoms show maximum tendency and Ag adatoms show minimum or no tendency toward the exchange process. Separation and recombination of one or more adatoms are also observed at 500 K and 700 K. The Ag adatoms also occupy pop-up positions over the adatoms clusters for short intervals. At 700 K, the vacancies are also generated in the vicinity of the adatoms cluster,vacancy formation, filling, and shifting can be observed from the results.

MORPHOLOGY EVOLUTION IN PTFE AS A FUNCTION OF MELT TIME AND TEMPERATURE——Ⅰ.HIGH MOLECULAR WEIGHT SINGLE-AND MULTI-MOLECULE FOLDED CHAIN SINGLE CRYSTALS AND BAND STRUCTURES

The effect of sintering dispersed dispersion and nano-emulsion particles of high molecular weightpolytetrafluoroethylene(PTFE)on a substrate as a function of“melt”time and temperature is described.Folded chain singlecrystals parallel to the substrate and as ribbons on-edge(with double striations),as well as bands,are produced for longersintering times;particle merger and diffusion of individual molecules,crystallizing as folded chain,single(or few)molecule,single crystals when“trapped”on the substrate by cooling occur for shorter sintering times.It is suggested the observedstructures develop with sintering time,in a mesomorphic melt.The structure of the nascent particles is also discussed.

CFD study of flow-diffusion process in Y-shape micromixer

A CFD simulation was carried out to investigate the mixing process in a Y-shape micromixer with the software Fluent 6.3. The definition of the "diffusion angle" is proposed to describe the molecular diffusion process associated with the flow at low Reynolds number. The linear relationship between the diffusion angle and the Peclet number(Pe) is determined by both theoretical analysis and numerical simulation. Moreover, the simulation results reveal that the diffusion angle is only related to the Peclet number whilst it is irrelevant to the changes of Re(Reynolds number) and Sc(Schmidt number). The range of Peclet number and Reynolds number for experimental measurement are also suggested as Pe≤10000 and Re≤10.

ON DIFFUSION VELOCITY AND THE MASS CONSERVATION EQUATION FOR COMPONENTS

The mass migration velocity(absolute velocitv) of component i in a multicomponent flow is equal to the convection velocity (frame velocity) plus the diffusion velocity (relative velocity). The diffusion velocity as well as the corresponding diffusion coefficient depends on how the convection velocity is adopted.In turbulent flow, the mass migration velocity of component i is( muss-weighted time average velocity). The diffusion velocity consists of turbulent diffusion velocity and molecular diffusion velocity is the simple lime average velocity of component i and a is a certain convection velocity). So, the part of turbulent diffusion velocity is independent of what convection velocity is taken.In the mdss conservation equation for component i, the expression for the diffusion term on its right-hand side will change when the convection velocity on its left-hand side changes. In turbulent flow, there could be no diffusion terms, or a turbulent diffusion term only or both the turbulent and molecular diffusion terms when or any velocity other than these two is taken as the convection velocity. The case, in which there could he molecular diffusion only without turbulent diffusion, occurs in laminar flow. The molecular diffusion term always depends on the adoption of convection velocitv.In two-phase flow, the value of the molecular diffusion term is often near or even exceeds that of the turbulent diffusion term, which is quite different from the case in gas mixture flow.

Diffusion Studies on Polystyrene in 1,4-Dioxane

The diffusion behavior of polystyrene with narrow molecular weight distribution at 20℃ in 1, 4-dioxane was investigated by the photon correlation spectroscopy. The cumulant method was employed for the analysis of the intensity-intensity autocorrelation function measured over a wide range of the scattering vector. The diffusion coefficient D was determined as the function of concentration C in the molecular weight range of 3. 0×104-1. 20×106. In a low concentration range, D was found to be linearly dependent on C, which has been found for some other systems. The dependence of D on molecular weight at infinite dilution can be written as an empirical formula D0 = kDM-γ M, the exponent γ(0. 576 ±0. 01) is in good agreement with the result of the scaling theory.

DYNAMIC LIGHT SCATTERING STUDY ON TRANSLATION DIFFUSION OF 8-ARM STAR POLYSTYRENE IN GOOD AND THETA SOLVENTS

The technique of dynamic light scattering has been used to investigate the translation diffusion behavior of 8-arm star polystyrene (SPS)in a good solvent, tetrahydrofuran (THF) or benzene (BZ) and a theta solvent, cyclohexane (CH), by homodyne photon correlation spectroscopy .The intensityintensity autocorrelation function was analyzed by the method of cumulant. The translation diffusion coefficients have been obtained as a function of temperature and concentration. Under theta condition ,the non-concentration dependence of diffusion coefficient showed the unperturbed Gaussian state o the SPS molecular chain. The different hydrodynamic radii estimated from Stokes- Einstein equation reflected the stretch extent of the arm chain for regular star polymer. The data of diffusion activation energy of SPS in THF, BZ and CH were also obtained respectively.

Comparison of Measurements for Enhanced Diffusion Problem in Chemical Reaction Systems

The problem of molecular diffusion in the soup of chemical reactions attracts mounting interest across fields ranging from chemistry to biophysics to material science.Chemical reactions involve bond breakup and formation,whose time scale is typically on the orders of fs to ps,while molecular diffusion occurs at time scales ofμs to ms.The two processes are often considered orthogonal,given the vastly different scales.The serial results show that the enzyme's diffusion is enhanced in a substrate-dependent manner,which was further extended to small molecule reaction systems,challenging this classical paradigm.However,the results from different groups using different techniques do not quantitatively agree,and a general mechanism is yet to be understood.We summarize experimental studies on diffusion problems and seek to reconcile the interpretation with understanding the limits of measurement tools and the chemical nature of reaction systems.Understanding molecular diffusion in chemical reactions will provide fresh thoughts in designing chemical systems such as molecular machines that harvest work at the nanoscale in a controllable manner.

Zeolites for separation:Fundamental and application

Material based emerging separation techniques are attracting more and more attention as alternatives to the traditional ones such as distillation and extraction,aiming to reduce energy consumption and pollutant emissions.Due to their structure characteristics,zeolites can act as versatile sieves and adsorbents for molecules and have been successfully applied in some very important separation processes.Herein,two major catalogues of zeolite separations,namely membrane separation and adsorptive separation,are discussed and their underlying mechanisms are focused.In the part of membrane separation,the synthesis strategies toward zeolite membranes are introduced and the uniformly-oriented zeolite membranes are emphasized.In the part of the adsorptive separation,the industrial and popular adsorptive separations with the corresponding zeolite adsorbents are summarized.Generally,membrane separation relies on the molecular diffusion behavior within zeolites while adsorptive separation relies on the guest–host interaction in principle.The key challenges and misconceptions in zeolite separations are highlighted throughout the article.

A functional PES membrane for hemodialysis-Preparation,characterization and biocompatibility

In this work, we evaluate the properties of solution casted polysulfone （PSf）/sulfonated polyethersulfone （SPES） blend membranes prepared by non-solvent induced phase inversion technique. The morphologies of these blend membranes, observed using scanning electron microscopy （SEM） and atomic force microscopy （AFM） imaging, indicated a smoother skin layer and an increased number of highly interconnected pores in the sub layer. The efficacy of the prepared membranes was evaluated in terms of porosity, ultrafiltration rate （UFR）, molecular weight cut-off （MWCO） and mean pore size. The hydrophilicity of these membranes was in consonance with contact angle values. It was observed that the selectivity and the UFR of the blend membranes were higher when compared to pristine membranes. Furthermore, these blend membranes demonstrated an increase in biocompatibility - prolonged blood clotting time, suppressed platelet adhesion, reduced protein adsorption and lower complement activation. These membranes were also investigated for uremic solute removal. Diffusive permeability of middle molecular weight cytochrome-c revealed an increase from 8 × 10 ^-4 cm·s ^-1 to 18 × 10^-4 cm· s^- and illustrates the possibility that these sulfonated PES/PSf blend membranes can be used to prepare membrane modules for hemodialysis applications.

Molecular dynamics simulation of diffusion of nanoparticles in mucus

The rapid diffusion of nanoparticles （NPs） through mucus layer is critical for efficient transportation of NPs-loaded drug delivery system. To understand how the physical and surface properties of NPs affect their diffusion in mucus, we have developed a coarse-grained molecular dynamics model to study the diffusion of NPs in modeled mucus layer. Both steric obstruction and hydrodynamic interaction are included in the model capable of capturing the key characteristics of NPs＇ diffusion in mucus. The results show that both particle size and surface properties significantly affect the diffusivities of NPs in mucus. Furthermore, we find rodlike NPs can gain a higher diffusivity than spherical NPs with the same hydrodynamic diameter. In addition, the disturbed environment can enhance the diffusivity of NPs. Our findings can be utilized to design mucus penetrating NPs for targeted drug delivery system.

Resistance of the diffusive boundary layer to salt release from saline sediments to freshwater

The diffusive boundary layer （DBL） is the zone for matter exchange between surface water and aquatic sediments. To elucidate the influence of DBL on salt release from saline sediments to freshwater, two experiments with or without wind blowing were conducted. According to the experiments, a 3.5 cm DBL is formed above the smoothed sediments at a steady wind field and this thickness is greater than other studies. The observed flux of salt through the DBL is 6% larger than the calculated value from Fick＇ s first law. The results indicate that molecular diffusion is the dominant mechanism for salt transport through the DBL. The presence of DBL suppresses the hydrodynamic enhancement for matter exchange between sediments and overlying water. Therefore, salts in the sediments of a polder reservoir may influence the water quality chronically.

Experimental and modeling study on the effect of molecular diffusion during CO_(2) injection

In this work,a new approach based on the effect of molecular diffusion on oil recovery during CO_(2)injection in naturally fractured reservoirs has been proposed to examine the best condition for oil recovery in the matrix block.A homogeneous matrix block from one of Iran’s naturally fractured reservoirs has been investigated.The matrix block is filled with oil and connate water surrounded by fractures filled with injection gas from the top,bottom and right side.In which matrix block oil saturation and behavior of matrix block from inside for specific time are examined simultaneously.Sensitivity analyses on the matrix block at different saturation pressures showed first the positive effect of increasing saturation pressure could be seen when diffusion was involved.Secondly,as the pressure increased,the side of recovery changed from the top to the bottom just absolutely more effective when we had diffusion in the system.Third,gravity drainage at lower saturation pressures was dominant for 2 cases with and without diffusion and at higher saturation pressures capillary force becomes more involved.

Diffusion dynamics and characterization of attogram masses in optically trapped single nanoparticles using laser-induced plasma imaging

In the present work,a wavelength-selected plasma imaging analysis system is presented and used to track photons emitted from single-trapped nanoparticles in air at atmospheric pressure.The isolated nanoentities were atomized and excited into plasma state using single nanosecond laser pulses.The use of appropriate wavelength filters alongside time-optimized acquisition settings enabled the detection of molecular and atomic emissions in the plasma.The photon detection efficiency of the imaging line resulted in a signal>400 times larger than the simultaneously-acquired dispersive spectroscopy data.The increase in sensitivity outlined the evolution of diverse physicochemical processes at the single particle scale which included heat and momentum transfer from the plasma into the particle as wells as chemical reactions.The imaging detection of excited fragments evidenced different diffusion kinetics and time frames for atoms and molecules and their influence upon both the spectroscopic emission readout and fabrication processes using the plasma as a reactor.Moreover,the origin of molecular species,whether naturally-occurring or derived from a chemical reaction in the plasma,could also be studied on the basis of compositional gradients found on the images.Limits of detection for the inspected species ranged from tens to hundreds attograms,thus leading to an exceptional sensing principle for single nanoentities that may impact several areas of science and technology.

Effects of sediment physical properties on the phosphorus release in aquatic environment

Particle size, porosity, and the initial phosphorus concentration in sediments are the main factors affecting phosphorus release flux through the sediment-water interface. Sediments can be physically divided to muddy and sandy matters, and the adsorption-desorption capacity of sediment with phosphorus depends on particle size. According to phosphorus adsorption-desorption experiments, phosphorus sorption capacity of the sediment decreases with the increase of particle dimension. But among the size-similar particles, sediment with a bigger particle size has the larger initial phosphorus release rate. In terms of muddy and sandy sediments, there are inversely proportional relationships between the release rate and the flux. Due to the contact of surface sediment and the overlying water, the release flux from the sediment is either from direct desorption of surface sediment layer or from the diffusion of pore water in the sediment layer, which is mainly determined by sediment particle size and porosity. Generally, static phosphorus release process may include two stages: the first is the initial release. As for coarse particles, phosphorus is desorbed from surface sediment. And for fine particles, phosphorus concentration in water often decreases, mainly from pore water by the molecular diffusion. During the second stage, pore water flows faster in coarse sediment, and phosphorus is easy to desorb from the surface of the particles as diffusion dominates. For the smaller liquid-solid ratio of fine particles and the larger amount of phosphorus adsorption, the release flux from pore water due to diffusion is very small with longer sorption duration.

Revisit of advection-dispersion equation model with velocity-dependent dispersion in capturing tracer dynamics in single empty fractures

An accurate quantification of the contaminant transport through fractured media is critical for dealing with water-quality related scientific and engineering issues, where the dispersion coefficient is an important and elusive parameter for the solute transport modeling. Many previous studies show that the dispersion coefficient(D) in the standard advection-dispersion equation(ADE) model can be approximated by D=avλ(where a is the dispersivity), a formula to be revisited systematically in this study by laboratory experiments and model analysis. First, a series of tracer transport experiments in single empty fractures are conducted in cases of different hydraulic gradients. Second, the tracer breakthrough curves are determined by simulations based on the ADE model, to obtain the dispersion coefficients corresponding to various fracture roughnesses and flow velocities. A varying trend of λ is analyzed under different flow conditions. Results show that although the standard ADE model cannot be used to characterize the late-time tailing of the tracer BTCs, likely due to the solute retention, this simple model can simulate most of the solute mass dynamics moving through fractures and may therefore provide information for estimating the dispersion in parsimonious models appropriate for the non-Fickian transport. The following three conclusions are drawn:(1) the peak of the breakthrough curves comes earlier with increasing the roughness, according to the ADE simulation,(2) the value of λ generally decreases as the relative roughness of the fracture increases,(3) the value of λ is approximately equal to 2.0 when the dispersion is dominated by the molecular diffusion in the smooth fracture.