High Porosity Regions Research Articles

Fracture Propagation in Heterogeneous Porous Media: Pore-Scale Implications of Mineral Dissolution

Open-mode fractures in sediments and rock occur both in natural processes and in engineering practice. In contrast to long planar fractures predicted by solutions in homogeneous media, laboratory and field observations often evidence complex and non-planar open-mode fracture networks in geological media. One cause of fracture complexity is the heterogeneity of material mechanical properties. This study focuses on the effects of mineral dissolution on pore structure modification in heterogeneous porous media and ensuing changes on fracture propagation characteristics. We performed semicircular bending experiments on limestone samples pre- and post-acidizing, mapped 3-D pore structure with X-ray microtomography, and used a finite element formulation based on the phase-field approach and linear elastic fracture mechanics to model initiation and propagation of open-mode fractures. Semicircular bending experiments on acidized carbonate rock samples show that non-planar fractures follow high porosity regions and large pores, and that fracture toughness correlates well with local porosity. Numerical modeling shows that a direct relationship between fracture toughness and porosity permits replicating fracture stress intensity at initiation and non-planar fracture propagation patterns observed in experiments. The results of this study indicate that mineral dissolution can affect fracture propagation patterns and that a coupling of mechanical and reactive processes in porous media formulations can be achieved through porosity and toughness. The understanding of fracture propagation patterns and geometry manipulation is critical for safe and effective use of reactive fluids in the subsurface, such as in hydraulic fracturing and carbon geological sequestration applications.

Monte Carlo phonon transport simulations in hierarchically disordered silicon nanostructures

Hierarchical material nanostructuring is considered to be a very promising direction for high performance thermoelectric materials. In this work we investigate thermal transport in hierarchically nanostructured silicon. We consider the combined presence of nanocrystallinity and nanopores, arranged under both ordered and randomized positions and sizes, by solving the Boltzmann transport equation using the Monte Carlo method. We show that nanocrystalline boundaries degrade the thermal conductivity more drastically when the average grain size becomes smaller than the average phonon mean free path. The introduction of pores degrades the thermal conductivity even further. Its effect, however, is significantly more severe when the pore sizes and positions are randomized, as randomization results in regions of higher porosity along the phonon transport direction, which introduce significant thermal resistance. We show that randomization acts as a large increase in the overall effective porosity. Using our simulations, we show that existing compact nanocrystalline and nanoporous theoretical models describe thermal conductivity accurately under uniform nanostructured conditions, but overestimate it in randomized geometries. We propose extensions to these models that accurately predict the thermal conductivity of randomized nanoporous materials based solely on a few geometrical features. Finally, we show that the new compact models introduced can be used within Matthiessens rule to combine scattering from different geometrical features within approximately 10 per cent accuracy.

Nonlinear convection regimes in superposed fluid and porous layers under vertical vibrations: Negative porosity gradients

We study numerically a nonlinear problem on the excitation of average convection in a fluid layer heated from below and partially filled with a porous medium in the gravity field. The porous medium is non-uniform in the vertical direction, and its permeability and porosity grow with depth at a negative porosity gradient. The layer oscillates vertically with high frequency and small amplitude. It is shown that in the absence of vibrations harmonic velocity oscillations can occur in the layer. In this case, the heat flux performs small-amplitude oscillations, to it is assumed to be quasi-stationary. With increasing supercriticality, the velocity oscillations become irregular and are accompanied by heat flux fluctuations with high enough amplitude. Changes in the heat flux are caused by the instability of long-wave convective rolls that penetrate pores. Additional vortices having a shorter wavelength and interacting with long-wave rolls are observed within a high porosity region near the hot lower boundary of the layer. This region constitutes the major part of the total temperature difference at the solid boundaries of the layer. Vibrations of a high enough intensity suppress the supercritical velocity and heat flux oscillations, thus preventing additional vortex formation.

Investigating the interaction between internal structural changes and water sorption of MDF and OSB using X-ray computed tomography

Both medium density fiberboard (MDF) and oriented strand board (OSB) are increasingly used in construction, yet when exposed, water sorption can cause internal structural changes, as such decreasing mechanical strength and increasing decay risk. It is, therefore, essential to understand the interaction between structural changes and water sorption of MDF and OSB. Detailed water sorption behavior and related structural changes in two types of MDF and two types of OSB were periodically monitored using a gantry-based X-ray CT scanner. Water sorption causes local swelling in MDF, resulting in reduced swelling or even shrinkage in neighboring regions. Then again, local shrinkage can increase water resistance and possibly decrease water content locally. Obviously, local swelling results in structural changes. Such structural changes predominantly seem to appear in the transition area between high and low porosity regions. Structural changes, such as glue bonding detachment and cracking, could facilitate water intake in both MDF and OSB. For the studied T-MDF and OSB-A, the relative position of wood fibers/strands and the volume of the voids hardly changed during water sorption, which probably contributes to the decrease in water sorption along newly formed cracks and voids induced by structural changes.

Fluid Migration in a Subducting Viscoelastic Slab

AbstractMetamorphic dehydration reactions in a subducting slab release fluids that trigger arc volcanism and are thought to be responsible for intermediate‐depth seismicity. The fluid flow from the source is controlled by buoyancy and compaction pressure which is modified by viscous and elastic effects. In this paper, we investigate how fluid migrates in viscoelastic slab by using 2‐D and 3‐D numerical models based on a theory of two‐phase flow. When bulk viscosity is sufficiently low, viscosity plays a dominant role and fluid goes up almost vertically soon after its release producing porosity waves. When a higher bulk viscosity is assumed, a large amount of fluid is trapped in a high porosity region produced by the fluid source and migrates along the source except for a case where the ratio of permeability (K) to fluid viscosity (μ) is relatively low. We also find that porosity increases in the deeper part of the fluid source in cases with intermediate and low values of . In 3‐D, fluid focusing occurs where the slab bends away from the trench causing a local increase in porosity and compaction pressure. These findings may help us explain several types of observations in subduction zones including slow earthquakes at the plate interface, low seismic wave velocities in the oceanic crust, double seismic zones in the slab, and shallow subduction angle at the bend of the slab.

Headward growth and branching in subterranean channels.

We investigate the erosive growth of channels in a thin subsurface sedimentary layer driven by hydrodynamic drag toward understanding subterranean networks and their relation to river networks charged by ground water. Building on a model based on experimental observations of fluid-driven evolution of bed porosity, we focus on the characteristics of the channel growth and their bifurcations in a horizontal rectangular domain subject to various fluid source and sink distributions. We find that the erosion front between low- and high-porosity regions becomes unstable, giving rise to branched channel networks, depending on the spatial fluctuations of the fluid flow near the front and the degree to which the flow is above the erodibility threshold of the medium. Focusing on the growth of a network starting from a single channel, and by identifying the channel heads and their branch points, we find that the number of branches increases sublinearly and is affected by the source distribution. The mean angles between branches are found to be systematically lower than river networks in humid climates and depend on the domain geometry.

An Experimental Study on the Influence of CO2 Containing N2 on CO2 Sequestration by X-ray CT Scanning

The CO2 stream to be transported and sealed in CCS project usually contains various impurities, and purifying the CO2 stream costs utmost in the CO2 capture and storage process. Co-sequestration of impurities with supercritical CO2 will decline the capture requirement. Few studies have discovered the impact on CO2 storage capacity at porous scale when remaining non-condensable gas in the CO2 stream such as N2 which occupied a large proportion in flue gas stream. We performed comparison experiments between different N2 proportion, capillary number, temperatures and pressures to discover preliminary research on the performance of CO2 containing N2 by X-ray CT scanning. The 3-D saturation distributions implies the higher temperatures and pressures slightly increase the saturation and such gas flow will show priority at small scale heterogeneity for the same capillary number. Meanwhile when the N2 proportion increase, the saturation will slightly promoted at high porosity region, and vice versa. However at the logCa=-7.5, the high N2 percentage in CO2 stream at high temperature and pressure reveal strong assets among the whole porosity region. Under such conditions the saturation of gas mixture achieves relative high values and enhances the CO2 storage capacity.

Evolution of Porosity and Channelization of an Erosive Medium Driven by Fluid Flow

We demonstrate that a homogeneous porous medium composed of sedimentary particles develops channels due to curvature driven growth of fluid flow coupled with an increase in porosity. While the flux is increased linearly, the evolution of porosity is observed to be intermittent with erosion occurring at the boundaries between low and high porosity regions. Calculating the spatial distribution of the flow within the medium and the fluid stress given by the product of the fluid flux and the volume fraction of the particles, we find that the system organizes itself to be locally near the threshold needed to erode the weakest particles. A statistical model simulating the coupling of the erosion, transport, and deposition of the particles to the local fluid flow and porosity is found to capture the overall development of the observed channels.

Critical porosity of melt segregation during crustal melting: Constraints from zonation of peritectic garnets in a dacite volcano

The presence of leucogranitic dikes in orogenic belts suggests that partial melting may be an important process in the lower crust of active orogenies. Low seismic velocity and low electrical resistivity zones have been observed in the lower crust of active mountain belts and have been argued to reflect the presence of partial melt in the deep crust, but volcanoes are rare or absent above many of these inferred melt zones. Understanding whether these low velocity zones are melt-bearing, and if so, why they do not commonly erupt, is essential for understanding the thermal and rheologic structure of the crust and its dynamic evolution. Central to this problem is an understanding of how much melt can be stored before it can escape from the crust via compaction and eventually erupt. Experimental and theoretical studies predict trapped melt fractions anywhere from <5% to >30%. Here, we examine Mn growth-zoning in peritectic garnets in a Miocene dacite volcano from the ongoing Betic–Rif orogeny in southern Spain to estimate the melt fraction at the time of large-scale melt extraction that subsequently led to eruption. We show that the melt fraction at segregation, corresponding approximately to the critical melt porosity, was ∼30%, implying significant amounts of melt can be stored in the lower crust without draining or erupting. However, seismic velocities in the lower crust beneath active orogenic belts (southern Spain and Tibet) as well as beneath active magmatic zones (e.g., Yellowstone hotspot) correspond to average melt porosities of <10%, suggesting that melt porosities approaching critical values are short-lived or that high melt porosity regions are localized into heterogeneously distributed sills or dikes, which individually cannot be resolved by seismic studies.

Free radical polymerisation of methacrylates with thermal initiator in clay rock

Clay rock porosity analysis with C-14-PMMA autoradiography was the focus of this work. Methodology for impregnating clay rock with C-14 labelled methacrylate, and fixing it via free radical polymerisation initiated with thermal initiator was developed towards this end. The effect of drilling was of particular interest because of their effect on the porosity and therefore they were studied in detail. Two different methacrylates were tested in this work. C-14 labelling enables measuring porosity through the autoradiography of samples impregnated with methyl methacrylate (MMA). The supporting analysis was done with thermogravimetric analysis (TGA), Scanning Electron Microscopy supplemented by energy dispersive X-ray spectrometry (SEM/EDAX) and X-ray diffraction (XRD).Procedures for impregnating clay rock and fixing C-14-labelled methacrylates in the sample were developed and optimised successfully. The two clay rock discs studied had mean porosities of (7.9±0.8) % and (7.6±0.8) % as measured by autoradiography. Autoradiography also shows that a disturbed zone can be seen as a region of high porosity in the drilled clay rock cores. In this case, the porosity of the disturbed zone is more or less double of the bulk porosity. TGA measurements gave bulk porosity values that varied between 4.1–15.0%. The TGA values were uncertain due difficulties associated with the sample matrix degrading at the same time as the methacrylate inside the sample. They could still be used to estimate the general range of the porosity. Pure MMA gave bulk porosity of (12.9±7.5) % while mixture composed of 75% Hydroxyethyl methacrylate (HEMA) and 25% MMA gave a bulk porosity of (4.1±6.0) %. SEM/EDAX results showed that calcite grains were more fractured than quartz grains in the disturbed zones. Quartz grains were mostly unaffected. This supports the conclusion that the porosity increase in the disturbed zone is intergranular and result of the break-up of calcite grains.

Understanding Water Transport in Polymer Electrolyte Fuel Cells Using Coupled Continuum and Pore‐Network Models

AbstractWater management remains a critical issue for polymer electrolyte fuel cell performance and durability, especially at lower temperatures and with ultrathin electrodes. To understand and explain experimental observations better, water transport in gas diffusion layers (GDLs) with macroscopically heterogeneous morphologies was simulated using a novel coupling of continuum and pore‐network models. X‐ray computed tomography was used to extract GDL material parameters for use in the pore‐network model. The simulations were conducted to explain experimental observations associated with stacking of anode GDLs, where stacking of the anode GDLs increased the limiting current density. Through imaging, it is shown that the stacked anode GDL exhibited an interfacial region of high porosity. The coupled model shows that this morphology allowed more efficient water movement through the anode and higher temperatures at the cathode compared to the single GDL case. As a result, the cathode exhibited less flooding and hence better low temperature performance with the stacked anode GDL.

Morphological models of trabecular bone suitable for high-porosity regions and vertebrae

The complex microarchitecture of trabecular bone makes it difficult to perform computational analyses on the real structure. Researchers have often resorted to using morphological idealizations employing simplified geometries. One such idealized structure, based on the gyroid, was found to mimic trabecular bone well. However, structures generated using the basic gyroid equation manifested discontinuities at high porosities. Another disadvantage of the gyroid is that it cannot model vertebral trabecular bone, which generally resembles cubic cells. To address these two shortcomings, we describe: (i) a modified structure based on the skeletal gyroid, which remains connected even at extremely high porosities, (ii) a cubic grid-like structure, based on the primitive minimal surface, for studying vertebrae.

Journal of Bone and Mineral Research: Volume 30, Number 12, December 2015

Abstract Cover: Maps of apparent total water (aTW) and apparent bound water (aBW) 1H concentrations, in mol/L, in bone specimens from four representative donors. Age and gender of the donors are given within each quadrant, and the endosteal surface of each specimen faces left. Arrows indicate a region of high porosity, which has elevated total water and reduced matrix densities, and correspondingly increased aTW and decreased aBW. Non-zero background signal surrounding the bone specimen arises from extremely short-T2 signal contributed by the plastic body of the RF probe (see Seifert et al. beginning on page 2229).

Joint Inversion of Seismic and Magnetotelluric Data in the Parkfield Region of California Using the Normalized Cross-Gradient Constraint

We present jointly inverted models of P-wave velocity (Vp) and electrical resistivity for a two-dimensional profile centered on the San Andreas Fault Observatory at Depth (SAFOD). Significant structural similarity between main features of the separately inverted Vp and resistivity models is exploited by carrying out a joint inversion of the two datasets using the normalized cross-gradient constraint. This constraint favors structurally similar Vp and resistivity images that adequately fit the seismic and magnetotelluric (MT) datasets. The new inversion code, tomoDDMT, merges the seismic inversion code tomoDD and the forward modeling and sensitivity kernel subroutines of the MT inversion code OCCAM2DMT. TomoDDMT is tested on a synthetic dataset and demonstrates the code’s ability to more accurately resolve features of the input synthetic structure relative to the separately inverted resistivity and velocity models. Using tomoDDMT, we are able to resolve a number of key issues raised during drilling at SAFOD. We are able to infer the distribution of several geologic units including the Salinian granitoids, the Great Valley sequence, and the Franciscan Formation. The distribution and transport of fluids at both shallow and great depths is also examined. Low values of velocity/resistivity attributed to a feature known as the Eastern Conductor (EC) can be explained in two ways: the EC is a brine-filled, high porosity region, or this region is composed largely of clay-rich shales of the Franciscan. The Eastern Wall, which lies immediately adjacent to the EC, is unlikely to be a fluid pathway into the San Andreas Fault’s seismogenic zone due to its observed higher resistivity and velocity values.

Microstructure evolution during 300 °C storage of sintered Ag nanoparticles on Ag and Au substrates

A silver nanoparticle based die attach material was used in a pressure free process to bond 2.5mm square Ag plated Si die to Ag and Au plated substrates. The assemblies were stored at 300°C for up to 500h and the morphology of the sintered Ag and the shear strength were monitored as a function of time. On Ag substrate it was found that die shear strength increased and that the Ag grains grew in size and porosity decreased over time. There was also a clear difference in morphology between sintered Ag at the die edge and centre. On Au substrate, it was observed that the initially high die shear strength decreased with storage time and that voids migrated away from the Ag/Au interface and into the Ag joint. This has led to the formation of a void free layer at the interface followed by a high porosity region, which weakened the joint. The microstructure reveals a high density of grain and twin boundaries which facilitate the Ag and Au atomic diffusion responsible. The grain structure of the plated Au led to diffusion of Au into the Ag via high-angle tilt grain boundaries, and grain boundary migration further dispersed the Au into the Ag layer.

Three-phase plane composites of minimal elastic stress energy: High-porosity structures

The paper establishes exact lower bound on the effective elastic energy of two-dimensional, three-material composite subjected to the homogeneous, anisotropic stress. It is assumed that the materials are mixed with given volume fractions and that one of the phases is degenerated to void, i.e., the effective composite is porous. Explicit formula for the energy bound is obtained using the translation method enhanced with additional inequality expressing certain property of stresses. Sufficient optimality conditions of the energy bound are used to set the requirements which have to be met by the stress fields in each phase of optimal effective material regardless of the complexity of its microstructural geometry. We show that these requirements are fulfilled in a special class of microgeometries, so-called laminates of a rank. Their optimality is elaborated in detail for structures with significant amount of void, also referred to as high-porosity structures. It is shown that geometrical parameters of optimal multi-rank, high-porosity laminates are different in various ranges of volume fractions and anisotropy level of external stress. Non-laminate, three-phase microstructures introduced by other authors and their optimality in high-porosity regions is also discussed by means of the sufficient conditions technique. Conjectures regarding low-porosity regions are presented, but full treatment of this issue is postponed to a separate publication. The corresponding “G-closure problem” of a three-phase isotropic composite is also addressed and exact bounds on effective isotropic properties are explicitly determined in these regions where the stress energy bound is optimal.

Fault zone identification in the eastern part of the Persian Gulf based on combined seismic attributes

Faults, as main pathways for fluids, play a critical role in creating regions of high porosity and permeability, in cutting cap rock and in the migration of hydrocarbons into the reservoir. Therefore, accurate identification of fault zones is very important in maximizing production from petroleum traps. Image processing and modern visualization techniques are provided for better mapping of objects of interest. In this study, the application of fault mapping in the identification of fault zones within the Mishan and Aghajari formations above the Guri base unconformity surface in the eastern part of Persian Gulf is investigated. Seismic single- and multi-trace attribute analyses are employed separately to determine faults in a vertical section, but different kinds of geological objects cannot be identified using individual attributes only. A mapping model is utilized to improve the identification of the faults, giving more accurate results. This method is based on combinations of all individual relevant attributes using a neural network system to create combined attributes, which gives an optimal view of the object of interest. Firstly, a set of relevant attributes were separately calculated on the vertical section. Then, at interpreted positions, some example training locations were manually selected in each fault and non-fault class by an interpreter. A neural network was trained on combinations of the attributes extracted at the example training locations to generate an optimized fault cube. Finally, the results of the fault and nonfault probability cube were estimated, which the neural network applied to the entire data set. The fault probability cube was obtained with higher mapping accuracy and greater contrast, and with fewer disturbances in comparison with individual attributes. The computed results of this study can support better understanding of the data, providing fault zone mapping with reliable results.

Lift-off of Porous Germanium Layers

We demonstrate reproducible formation of mesoporous germanium layers suitable for solar energy applications by electrochemical etching in highly concentrated electrolytes. For long anodization times or thick layer formation a porosity gradient is observed leading eventually to high porosity regions and cavity formation at the bottom of the porous layer. A 30 min annealing step at a temperature of 575°C in hydrogen atmosphere allows for reorganization and subsequent lift-off. The mean surface roughness increases from 0.31 nm for unprocessed Ge up to 7.85 nm for reorganized Ge as measured by atomic force microscopy. μ-Raman confocal spectroscopy analysis confirms that etching and annealing do not affect the crystalline structure.

Instabilities in the dissolution of a porous matrix

[1] A reactive fluid dissolving the surrounding rock matrix can trigger an instability in the dissolution front, leading to spontaneous formation of pronounced channels or wormholes. Theoretical investigations of this instability have typically focused on a steadily propagating dissolution front that separates regions of high and low porosity. In this paper we show that this is not the only possible dissolutional instability in porous rocks; there is another instability that operates instantaneously on any initial porosity field, including an entirely uniform one. The relative importance of the two mechanisms depends on the ratio of the porosity increase to the initial porosity. We show that the “inlet” instability is likely to be important in limestone formations where the initial porosity is small and there is the possibility of a large increase in permeability. In quartz-rich sandstones, where the proportion of easily soluble material (e.g., carbonate cements) is small, the instability in the steady-state equations is dominant.

핀-휜 구조물을 삽입한 채널의 열전달 특성과 압력강하에 관한 수치해석

핀-휜 구조물이 삽입된 채널은 열전달이 향상되는 것으로 알려져 있다. 복잡한 핀-휜 구조물은 다공성 매체로 모사할 수 있을 만큼 매우 불규칙적인 유동을 보여준다. 본 연구는 수치해석을 통하여 핀-휜 구조물이 삽입된 채널의 열전달과 압력강하의 특성을 분석하였다. 유효유동공간비가 높고 직경이 큰 경우 실험데이터를 잘 예측 하였지만, 유효유동공간비가 낮은 경우 비교적 오차가 큰 편이었다. 유효유동 공간비가 낮을수록 유동교란이 강해져서 열전달이 증가하지만 압력강하도 증가하여 팬의 동력이 증가하기 때문에 최적점을 찾아서 설계하여야 한다. 본 연구에서는 전면유속이 5m/s 일 때, D25 모델에서는 유효유동공간비가 0.5인 지점에서, D05 모델에서는 0.9에서 압력강하가 급격하게 늘어나기 때문에 이보다 더 작은 유효유동공간비에서의 작동은 바람직하지 않다.