Porosity Development Research Articles

Mitigation of spall fracture by evolving porosity

Ductile materials subject to impact loading conditions can undergo spall fracture when an incoming compressive stress wave reflects off interfaces and free surfaces as a tensile stress wave. Experimental observations suggest that in ductile materials, spall fracture is driven by the evolution of porosity. However, the presence of initial porosity in ductile materials also introduces plastic compressibility, which can attenuate the incoming compressive stress wave and, as a result, reduce the amplitude of the reflected tensile stress wave. This, in turn, can mitigate or delay spall fracture. In this work, we report on finite deformation finite element calculations that analyze the response of porous ductile materials subjected to impact loading conditions. Two sets of calculations are carried out, in the first set the material contains initial porosity values ranging from 0% to 5% while in the second set the material also undergoes stress-controlled porosity nucleation. Both sets of calculations are carried out for a wide range of imposed impact velocities. Our results show that porosity in ductile materials can, under certain circumstances, mitigate spall fracture by attenuating stress waves. Results correlating the effects of impact velocity, initial porosity, and porosity nucleation on spall fracture are presented and the underlying mechanisms are discussed.

Fracture behavior of a high pressure die casting AlSi10MnMg alloy with varied porosity levels

The fracture behavior of a high pressure die casting AlSi10MnMg alloy with varied porosity levels is investigated by synchrotron radiation X-ray computed tomography. Attention is focused on origin of crack and the evolution of porosity in the process of plastic deformation. Different from in-situ 3D tensile test, load-off specimen after plastic deformation is applied for this experiment and it is closer to the real scene. Results show that multiple defects are proposed in high pressure die casting AlSi10MnMg alloy including shrinkage, gas pore, externally solidified primary iron-rich phase (ESC-IMCI) and heterogeneous layers. In a lower-porosity sample, the surface defects like skin layer are a main crack source and a radial river pattern exhibits in crack surface. No new pores generated in the process of plastic deformation. In a higher-porosity sample, no obvious crack source and radial crack propagation path can be identified while the large crack surface fluctuation and tortuous morphology are obtained. The collapse of pre-existing small-sized pores exert a vital role in connecting large-scale pores. The micro-crack initiation and propagation originate from the expansion, consolidation and collapse of pores, especially the irregular pores.

On the thermodynamics consistency of Gurson’s model and its computational implications

The aim of this paper is to investigate the thermodynamics consistency of Gurson’s model and notably its relation to the class of standard generalized materials. First, we briefly recall Gurson’s model in its original format and reanalyzed it in the thermodynamic framework of poroplasticity of saturated media. This allows to properly define the coupling between elasticity and plasticity and to demonstrate that Gurson’s model fits into the framework of generalized standard materials model, provided that the internal variables being the plastic strain and the Lagrangian plastic porosity (and not the Eulerian porosity as in the original Gurson model). In particular, by construction, the porosity evolution law is proved to be a full part of the thermodynamic formulation of the model with a generalized normality rule. The implications in terms of numerical implementation of Gurson’s model are then investigated; a new numerical scheme based on the time-implicit discretization of the Lagrangian porosity is notably proposed and discussed with respect to available algorithms.

A novel data-driven dimensional analysis framework for predicting melt pool morphology and porosity evolution in powder bed fusion

A novel data-driven dimensional analysis framework for predicting melt pool morphology and porosity evolution in powder bed fusion

Compressive strength, porosity, and sorptivity of blended Palm Oil Fuel Ash (POFA) concrete containing silica fume in peat water

Blended Palm Oil Fuel Ash (POFA) concrete has the potential for application in organic acid environments such as peat land. However, there is a need to improve resistance of blended POFA concrete using silica fume as a pozzolanic material. There are limited studies of the blended POFA admixed with silica for organic acid exposure. In this study, compressive strength, porosity, and sorptivity of blended POFA concrete admixed with silica such as silica fume has been investigated by exposing the samples in peat water. Portland Composite Cement (PCC) concrete with similar target strength was used as a control mix. The blended POFA concrete contained 0%, 20% and 40% POFA as a cement substitute. Micro silica or silica fume was added with concentrations of 10% in each mixture. Portland Composite Cement (PCC) with similar target strength was used as a control mix. The resistance of the POFA concrete specimens to organic acid attack were measured by compressive strength, porosity and sorptivity development at 7, 28 and 91 days. Compressive strength of control mix declined, while strength of PCC and PCC-POFA concrete containing silica fume increased at 91 days. Porosity and sorptivity of PCC and blended OFA concrete followed the same trend with the compressive strength. Thus, blended POFA concrete containing silica fume has better resistance in peat water than that of the control mix.

Differences in CO2‐Water‐Rock Chemical Reactions among ‘Sweet Spot’ Reservoirs: Implications for Carbon Sequestration

AbstractThe Lucaogou Formation, located in the Jimsar Sag, Junggar Basin, NW China, has great potential for shale oil resources. In the process of CO2‐EOR (CO2 enhance oil recovery), mineral dissolution, precipitation and transformation, leading to the local corrosion or blockage of reservoirs, have a significant influence on recovery. In this study, a combination of high‐temperature and high‐pressure laboratory experiments and coupled temperature/fluid‐chemistry multi‐field numerical simulations are used to investigate CO2‐water‐rock reactions under various reservoir conditions in the upper and lower ‘sweet spots’, to reveal the mechanisms underlying CO2‐induced mineral dissolution, precipitation and transformation. In addition, we quantitatively calculated the evolution of porosity over geological timescales; compared and analyzed the variability of CO2 transformation in the reservoir under a variety of temperature, lithology and solution conditions; and identified the main factors controlling CO2‐water‐rock reactions, the types of mineral transformation occurring during long‐term CO2 sequestration and effective carbon sequestration minerals. The results demonstrate that the main minerals undergoing dissolution under the influence of supercritical CO2 are feldspars, while the main minerals undergoing precipitation include carbonate rock minerals, clay minerals and quartz. Feldspar minerals, especially the initially abundant plagioclase in the formation, directly affects total carbon sequestration, feldspar‐rich clastic rocks therefore having considerable sequestration potential.

Effects of fluorine on dynamic reaction interfaces in hydrothermal feldspar alteration

Fluid-induced mineral reactions exert an important control on the physico-chemical properties of the crust and hydrothermal plumbing systems. Feldspars are the most abundant minerals in the Earth's crust and a major host of the main cations found in geofluids (Na, K, Ca). In previous work, we discovered that K-feldspar-albite-sanidine zonation textures developed dynamically when sanidine ((K,Na)AlSi3O8) was reacted with NaCl or NaF in H216O- and H218O-enriched solutions at 600 °C and 2 kbar in a closed system. Based on a detailed SEM and TEM characterisation, the present study aims to constrain the reaction mechanism and the effect of fluorine on reaction kinetics and overall reaction textures in alkali feldspar replacement reaction interfaces. Replacement of high-sanidine (Sa) by albite (Ab) and/or K-feldspar (Kfs) proceeded via an interface-coupled dissolution-reprecipitation reaction, with the products preserving the crystallographic orientation of the parent Sa in both NaCl- and NaF-bearing solutions. Mineral interfaces with different micro-textures were observed depending on the nature of the interface (Ab-Sa, Kfs-Ab, Kfs-Sa) and whether the reaction occurred in NaCl-only or NaF-bearing solutions. Abundant amorphous domains (25–50 nm thick) occur both within Ab and at Ab-Sa and Kfs-Ab interfaces formed in NaF-bearing solutions. At the Kfs-Sa interface formed in NaCl-only solutions, a complex interfacial zone characterised by a high density of defects may represent sealed fluid pathways. These results reveal that the nature of the reaction interface depends on both mineral properties, in particular microtextures (i.e. polysynthetic twinning in Ab); and solution chemistry; this directly affects porosity development, the chemical environment at the interface, and chemical exchanges between the reaction front and the bulk solution. Fluorine aids the formation of amorphous layers that modify nucleation mechanisms; and facilitates element transport by forming Na-Si-Al-fluoride complexes, resulting in accelerated reaction rates relative to chloride-solutions. These nm- to μm-level processes contribute to efficient coupling between fluid-flow and mass transfer, explaining why sodic and potassic alteration associated with metal and fluid transport in some of the world's largest ore deposits can develop extensively, and subsequently access metals locked within silicate mineral assemblages.

Large-Scale Experimental Simulations of In Situ Coal Gasification in Terms of Process Efficiency and Physicochemical Properties of Process By-Products

This paper presents a series of surface experiments simulating underground coal gasification (UCG). The main goal of the experiments was to investigate the influence of the gasification medium and the coal rank on the gasification process. Four multi-day trials were carried out using a laboratory gasification facility designed for the large-scale experimental simulations of UCG and located in the Experimental Mine “Barbara”, located at Mikołów, Poland. Two Polish bituminous coals were investigated: coal sourced from “Piast-Ziemowit” mine and coal sourced from “Wesoła” mine. Each of the two coals was gasified in two separate experiments using oxygen-enriched air (OEA) and pure oxygen as the respective gasifying agents. Gasification with oxygen resulted in significantly higher gas quality and higher process efficiency than gasification with OEA. Higher concentrations of hydrogen (23.2% and 25.5%) and carbon monoxide (31.8% and 33.4%) were obtained when oxygen was used as a gasifying reagent, while lower concentrations were obtained in the case of gasification with OEA (7.1% and 9.5% of hydrogen; 6.4% and 19.7% of carbon monoxide). Average gas calorific values were 7.96 MJ/Nm3 and 9.14 MJ/Nm3 for the oxygen experiments, compared to 2.25 MJ/Nm3 and 3.44 MJ/Nm3 for the OEA experiments (“Piast-Ziemowit” coal and “Wesoła” coal, respectively). The higher coalification degree of “Wesoła” coal (82.01% of carbon) compared to the “Piast-Ziemowit” coal (68.62% of carbon) definitely improves the gas quality and energy efficiency of the process. The rate of water condensate production was higher for the oxygen gasification process (5.01 kg/h and 3.63 kg/h) compared to the OEA gasification process (4.18 kg/h and 2.63 kg/h, respectively), regardless of the type of gasified coal. Additionally, the textural characteristics (porosity development) of the chars remaining after coal gasification experiments were analyzed. A noticeable development of pores larger than 0.7 nm was only observed for the less coalified “Piast-Ziemowit” coal when analyzed under the more reactive atmosphere of oxygen.

Differential Enrichment Regulation and Genetic Analysis of Tight Sandstone Gas Reservoir

In the deep Paleogene of graben basins in eastern China, not only the tight sandstone oil and gas resources, but also the secondary hydrocarbon generation of source rocks are universal and worth studying. In order to elucidate the laws of oil and gas accumulations in tight sandstone with different phases and explore the diversity of its genetic mechanism, Shahejie Formation in Wendong area was taken as an example in this paper to carry out further research. Firstly, the differential enrichment rules of oil and gas reservoirs are clarified from the aspects of spatial distribution, phase type and fluid properties. Secondly, the reasons for differential accumulation of oil and gas reservoirs were analyzed from multiple perspectives by using a variety of methods, such as source rock thermal evolution simulation, fault activity evaluation and reservoir porosity evolution. A large number of researches have been carried out on hydrocarbon generation and expulsion process of source rocks, paleo-temperature and paleo-pressure changes, the connectivity of skeleton sand bodies, reservoir physical property and densification process, tectonic strength and so on. The results indicate that the deep tight sandstone reservoirs of the Paleogene in the Wendong area have a quasi-continuous distribution characteristic of "planar partition and longitudinal zoning". Secondary kerogen cracking gas, crude oil cracking gas, medium light crude oil, and primary kerogen cracking gas are distributed in a semi-circular manner, from the center of Qianliyuan Sag to Wendong graben and rollover anticline belt at the high structural position. In conclusion, the two periods of hydrocarbon generation and expulsion of source rocks lead to the discontinuity of hydrocarbon accumulation process. Paleo-temperature and paleo-pressure conditions control the phase types of hydrocarbon filling. The lateral connectivity and the difference of physical properties of the skeleton sand bodies limit the distribution range of oil and gas reservoirs. The temporal and spatial availability of faults during the critical accumulation period determines the differential enrichment of oil and gas. The temporal and spatial validity of faults during the critical accumulation period determines the extent of differential enrichment of oil and gas.

Porosity evolution of mafic crystal mush during reactive flow

The emergence of the “mush paradigm” has raised several questions for conventional models of magma storage and extraction: how are melts extracted to form eruptible liquid-rich domains? What mechanism controls melt transport in mush-rich systems? Recently, reactive flow has been proposed as a major contributing factor in the formation of high porosity, melt-rich regions. Yet, owing to the absence of accurate geochemical simulations, the influence of reactive flow on the porosity of natural mush systems remains under-constrained. Here, we use a thermodynamically constrained model of melt-mush reaction to simulate the chemical, mineralogical, and physical consequences of reactive flow in a multi-component mush system. Our results demonstrate that reactive flow within troctolitic to gabbroic mushes can drive large changes in mush porosity. For example, primitive magma recharge causes an increase in the system porosity and could trigger melt channelization or mush destabilization, aiding rapid melt transfer through low-porosity mush reservoirs.

Evolution of soil porosity in loess-palaeosol sequences of the Ebro Valley, NE Iberia

Structure development and porosity evolution are closely related key processes in the formation of soils on loess. In order to better understand changes in soil porosity with time, four loess-palaeosol sequences (LPS) of the Ebro Valley were investigated using micromorphological and multiscalar images of soil thin sections combined with luminescence dating. Scanned and high-resolution mosaic images of thin sections using circular polarized light and backscattered electron scanning images were used to characterize the structural and textural porosity. The results were compared with physical and chemical analyses, and detailed micromorphological descriptions. The main results of our research revealed that 1) the change in loess particle packing, from enaulic to porphyric coarse/fine (c/f) related distribution, is the first process giving cohesion to the loess material and 2) that soil development during stabilization periods is associated with higher bioporosity. Gypsum accumulation is also a relevant process contributing to the increase of packing porosity within channels and chambers in the studied soils. Our results provide new insights into soil formation processes in Mediterranean loess and open new fields of research such as the possibility of using the quantification of bioporosity as an additional palaeoenvironmental marker.

Effect of Powder Processing and Sintering Conditions on the Microstructural, Thermal, and Mechanical Properties of Reticulated Zinc Oxide Ceramic Foams

Ceramic foams are made of zinc oxide using different amounts of Sb2O3 and Bi2O3 as sintering aids. The effect of a ball milling processing of the starting powders and the sintering temperature on the microstructure and the properties of the ZnO foams is investigated. The focus is set on the evolution of the secondary phases formed within the microstructure of ZnO. A determining effect is identified in the amount of an Al2O3 impurity which is introduced by abrasion of the milling vessels during ball milling. Alumina is partially dissolved in a spinel α–Zn7Sb2O12 secondary phase which is stabilized by a reduction of the unit cell volume. Remaining Al2O3 is incorporated into zinc oxide under formation of a defect wurtzite phase. The phase evolution is a complex function of the content of sintering aids, the Al2O3 impurity level and the sintering temperature. The shrinkage during sintering and the porosity evolution are correlated to the phase composition within the ZnO material. The thermal conductivity and the compressive strength of the foams are determined, normalized with respect to their porosity, and correlated to the microstructure and phase composition of the ZnO strut material.

Study of a Glass-Ceramic Seal Porosity

Sealing is a major issue in Solid Oxide Cell (SOC) used for hydrogen production. Currently, a slurry composed of glass powder, organic solvents and adjuvants, is used as a seal. After the deposition of the slurry on the areas to be sealed, a heat treatment allows to obtain a glass-ceramic. This shaping process is very convenient but the obtained material presents porosity. This study aims to understand the evolution of porosity as a function of time and temperature in order to minimize it. A significant increase in porosity is observed when the temperature increases. The porosity evolution is characterized by an increase in pores size and a reduction of pores number, typical of a coalescence phenomenon. At high temperature, the viscosity is low enough to allow pores to rise in the sample. This weakens the upper part of the seal and may cause a loss of sealing.

A solute transport in deformable dual-porosity media using mixture-coupling theory

Rock, soil and many porous-like materials are often fractured or structured media, which can exhibit dual-porosity behaviour. Studies on solute transport in deformable dual-porosity media remain challenging due to the multi-physics coupled effects and the complex interaction between fractures (or macropores) and the porous matrix. Although several studies exist on constitutive modelling of coupled behaviour in deformable dual-porosity media, the previously developed models are not systematic in thermodynamical frameworks. This paper proposes a mixture coupling theory approach based on non-equilibrium thermodynamics to develop a solute transport model with consideration of hydromechanical coupling in dual-porosity media (referred to as the ST-HM model). This paper derives the constitutive equations of fully hydromechanical coupled behaviour in dual-porosity media and considers the pore and fracture porosity evolution influenced by both hydraulic and mechanical fields. Therefore, the governing equations of ST-HM are capable of predicting non-reactive solute transport with a fully hydromechanical coupled effect in dual-porosity media. Then, the model was verified against existing models and validated by relevant experimental results. Further, a numerical example shows that the presented model significantly improves the accuracy of the prediction of porosity, fluid pressure and solute concentration compared with previous models, which ignore the fully hydromechanical coupled effects on solute transport.

Exploring the effects of gravel shapes on vibration compaction behaviours of coarse-grained mixtures via DEM simulations

ABSTRACT Coarse-grained mixtures composed of coarse gravels and fine sands are widely used in subgrade constructions, coarse gravels of which contain rubbles and pebbles with different roundness (R d). To investigate the effect of gravel shapes on the compaction behaviours of subgrades, the real-shaped gravel particles with varying R d (0.2–0.5) were selected to establish coarse-grained mixtures models via the discrete element method (DEM). The contact parameters in DEM models were determined through a series of calibration tests, and the reliability of models was verified by vibration compaction tests with rubble-grained and pebble-grained mixtures. The evolutions of porosity, particle motion, and contact relationship affected by R d were revealed from the macro-and mesoscales, and the influence of R d on the stability of skeleton structures is discussed. As R d decreases, the contact number tends to increase and the distribution of strong contacts is more uniform, the skeleton structure in which can effectively inhibit the motion of fine particles and the sliding between coarse particles. Conversely, the increase in R d will cause the concentration of strong contacts, which further results in the sliding damage of skeleton structures and the improvement of compaction deformation.

Direct monitoring of porosity evolution by dynamic modulus measurements: Three case studies

Porous materials are playing an increasingly relevant role in several fields and industrial sectors with structural and functional applications. Their properties critically depend on the relative density and pore morphology thus it is of great importance monitoring the variations of such structural features. Among the experimental techniques commonly used for investigating the microstructure evolution of porous materials, Mechanical Spectroscopy (MS) provides damping and dynamic modulus of the material during heat treatments. In this work three cases of possible pore structure evolution in different metallic alloys have been examined by MS: (i) growth and coalescence of pores, (ii) closure of pores of nanometric size and (iii) no change of porosity.The results show how dynamic modulus measurements can be successfully employed for a direct monitoring of porosity variations during heating. The technique is quite sensitive and allows to identify the temperature range where pore evolution takes place providing information useful to orientate heat treatments of porous materials.

Numerical investigation of seepage dissolution evolution of the bottom expansion cutoff wall in deep overburden

The bifurcation defect at the bottom of the cutoff wall in the deep overburden leads to a sharp increase in the local seepage gradient of the adjacent overburden at the bottom of the cutoff wall, which affects the evolution process of seepage dissolution of the cutoff wall. This study presents a novel type of bottom expansion cutoff wall to alleviate the problem that the seepage gradient of adjacent strata at the bottom of the cutoff wall far exceeds its allowed value. Considering the effects of the pore, aggregate, and convection on the calcium ion diffusion coefficient of concrete, we propose a coupling analytical model of seepage dissolution for two cutoff wall schemes, including the bifurcation defect schemes and bottom expansion schemes, to investigate the evolution of calcium ion concentration, porosity, and seepage characteristics, and to reveal the influence mechanism of the bottom bifurcation defect and bottom expansion of the cutoff wall on the seepage dissolution of the cutoff wall in deep overburden. The results indicate that subjected to the bottom bifurcation defect, the calcium ion concentration of the cutoff wall decreases faster, and the porosity increases faster with the increase of service life compared to the conventional cutoff wall when subjected to the bottom bifurcation defect. With a bifurcation height of 24 m at the bottom of the wall, the service life of the cutoff wall is reduced by 6.64%. In contrast, the novel type of bottom expansion cutoff wall is used to reveal that the concrete porosity increases slowly with the increase of the expansion radius over the same service life. The seepage gradient of the adjacent overburden at the bottom of the cutoff wall is significantly reduced. The service life of the cutoff wall is extended by 47.33% due to its 5 m bottom expansion radius. This study can provide a theoretical basis for the evolution of long-term service performance of the cutoff wall of an earth-rock dam on deep overburden.

Impact of supercritical CO2 exposure time on the porosity and permeability of dry and wet shale: The influence of chemo-mechanical coupling effects

CO2 storage in shale formation is a potential way to reduce CO2 emissions for carbon neutrality. The variation of permeability in shale triggered by CO2-shale interaction plays an important role in CO2 sequestration. The effect of supercritical CO2 (ScCO2) exposure times (0, 10, 20, 30, 40 days) on the porosity and permeability evolution of dry and wet shale samples was investigated herein. Results show that the porosity and permeability of both dry shale and wet shale increased first and then decreased over exposure time, and there is an inflection point. The porosity and permeability evolution of shale is controlled by the chemical-mechanical coupling effects. The dissolution of mineral induced by ScCO2 exposure increases the porosity and permeability of shale. The mechanical weakening induced by ScCO2 exposure enhances the pores compressibility of shale, makes the porosity and permeability decrease at stressed condition. In the early stage, the chemical effect is dominant, after the time of inflection point, the mechanical weakening effect is dominant. The inflection time of wet shale is earlier than that of dry shale, indicating that the chemical effect dominant stage in wet shale is shorter than that in dry shale. This can be attributed to that the water promoted the chemical reaction and enhanced the weakening of mechanical properties, then shortened the chemical corrosion dominated stage. Our findings suggest that ScCO2-shale interaction can contribute to the decrease in porosity and permeability of shale with the increase of ScCO2 exposure time, and hence adversely affecting the shale gas recovery and the continue injectivity of CO2. Thus, the time dependent chemical-mechanical coupling effects should be considered in evaluating the shale gas production and CO2 storage capacity.

A Compositional Global Implicit Approach for Modeling Coupled Multicomponent Reactive Transport

AbstractReactive transport modeling has become widely used to help improve understanding of hydrogeochemical processes from the pore scale to the watershed scale. In recent years, the scope of reactive transport applications has increased toward a higher level of complexity and process coupling. For example, the production and consumption of water as well as porosity evolution associated with the dissolution and precipitation of hydrated minerals can impact system evolution. Waste rock weathering, carbon sequestration, or the degradation of engineered barriers in radioactive waste repositories all constitute applications in which geochemistry and hydrodynamics can strongly influence each other. For these purposes, the traditional formulation of reactive transport simulators, which decouples groundwater flow and reactive transport processes, is limited. We present a global implicit compositional approach, which integrates the flow processes directly into the reactive transport and geochemical framework. This approach solves the flow field implicitly with the reactive transport equations, simultaneously accounting for water consumption and production due to geochemical reactions. Applications show that the model allows tackling complex reactive transport problems while accounting for intra‐aqueous reactions, redox reactions, and reactions involving mass transfer with the gas and solid phases. The presented simulations also demonstrate that the compositional and traditional approaches yield similar results for complex geochemical systems with relatively low reactivity.

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson