Crack-like Pores Research Articles

Evaluation of the stress sensitivity in tight reservoirs

Abstract The expressions of three types of stress sensitivity coefficients ( S, α, β ) are obtained on the basis of analysis of the empirical models and theoretical models on the relationships between permeability and effective stress, and the stress sensitivity evaluation standard is proposed considering experiment data and rock micro-structural features. Then the empirical models and theoretical models were used to fit experiment data of low-permeability and tight sandstones and different types of granites, which promotes the understanding of the empirical models and the physical meanings of the stress sensitivity coefficients. According to the study of the three types of stress sensitivity coefficients and the relationship of effective stress and permeability, it is found that the stress sensitive coefficient S is suitable for evaluating the stress sensitivity (strong stress sensitivity: S >0.40; low stress sensitivity: S S ≤0.4). Meanwhile, it is also found that strong-stress-sensitivity cores are more suitable to be characterized by the logarithmic model, while the exponential model (or binomial model) occurred more frequently in low-stress-sensitivity cores, and medium-stress-sensitivity cores are more likely to be described with the power model. Finally, the evaluation standard is discussed based on the micro-structure in the low-permeability and tight sandstones. The results show that the crack-like pores in the strong-stress-sensitivity cores are obvious, but low-stress-sensitivity cores have the characteristics of porous rocks; moreover, the types of cemented clay minerals, the size of rock grain and lithology have impact on stress sensitivity.

Suppressing Lateral Conduction Loss of Thin-film Cathode by Inserting a Denser Bridging Layer

To reduce the lateral conduction loss of thin-film-processed cathodes, the microstructure of the thin-film cathode is engineered to contain a denser bridging layer in the middle. By doing so, the characteristic crack-like pores that separate the cathode domains in thin-film-processed cathodes and hamper lateral conduction are better connected and, as a result, the sheet resistance of the cathode is effectively reduced by a factor of 5. This induces suppression of the lateral conduction loss and expansion of the effective current collecting area; the cell performance is improved by more than 30%.

ChemInform Abstract: Generic Principles of Crack‐healing Ceramics

AbstractReview: 138 refs.

Ceramic injection moulding of alumina–10 vol.-%YAG composites

Alumina composite containing 10 vol.-%YAG was processed by injection moulding. The injection moulding feedstock was prepared by milling an aqueous dispersion of submicrometre size alumina and YAG powder synthesised by aqueous sol–gel method using aluminium tri-isopropoxide and yttrium oxide as starting powders. Binder and additives were further added to plastify the feedstock and facilitate the shaping process. Small and complex near net shape components can be efficiently mass produced by ceramic injection moulding reaching high surface quality and high dimensional tolerances. The influence of sintering in air and hydrogen atmosphere on the mechanical properties and density was investigated. It was shown that sintering in hydrogen atmosphere enhances densification, accelerates grain growth and develops elongated crack-like pores along the grain boundaries. The hardness and strength of alumina–YAG composites were influenced by changes in the sintering atmosphere.

Generic principles of crack-healing ceramics

Ceramic materials able to heal manufacture or damage induced microstructure defects might trigger a change in paradigm for design and application of load bearing ceramics. This work reviews thermodynamic and kinetic aspects governing the regeneration of solid contact able to transfer stress between disrupted crack surfaces in ceramics. Major crack healing processes include perturbation of crack-like pores followed by sintering of isolated pores, as well as reaction with an environmental atmosphere and filling of the crack space with an oxidation product. Since thermally activated solid state reactions require elevated temperatures which may exceed 1000 °C, processes able to trigger crack healing at lower temperatures are of particular interest for transferring into engineering applications. Generic principles of microstructure modifications able to facilitate crack repair at lower temperatures will be considered: (i) acceleration of material transport by grain boundary decoration and grain size reduction, and (ii) reduction of thermal activation barrier by repair filler activation. Examples demonstrating crack healing capability include oxidation reaction of low energy bonded intercalation metal from nano-laminate MAX phases and catalyzed surface nitridation of polymer derived ceramics containing repair fillers.

Time-lapse acoustic, transport, and NMR measurements to characterize microstructural changes of carbonate rocks during injection of CO2-rich water

We investigated how changes induced in the microstructure of carbonate rocks by the injection of [Formula: see text]-rich water affect pore-network properties. In particular, we investigated from multiple perspectives the microstructural changes and types of porosity that alter the observable geophysical properties. We thereby refined our understanding of induced modification of the pore network. Our experimental protocol included a suite of time-lapse acoustic, transport, and nuclear magnetic resonance (NMR) measurements, along with scanning electron microscopy (SEM) and CT-scan images; these gave us complementary sensitivity to changes in different properties of the pore space. Induced porosity variations were smaller than in previous reported results because of chemomechanical compaction resulting from dissolution under pressure. No porosity enhancements larger than 0.8 pu were observed. Results indicated that dissolution occured primarily in the grain-coating cement and the microporosity of the micritic phase. Both caused the formation of cracklike pores around larger grains leading to a more compliant frame, causing both velocity reductions and an increased sensitivity of velocity to pressure. Chalky micritic facies exhibited velocity reductions of [Formula: see text], whereas micritic limestones, less prone to compaction and grain sliding, experienced smaller velocity reductions ([Formula: see text]). Because porosity enhancement was minimal, we hypothesized that the reductions were due to injection-induced reduction of grain-contact stiffness. Dissolution-induced compaction played an integral role also in the permeability response during injection. Compaction in pressure-sensitive chalky facies strongly counteracted the effects of dissolution, leading to negligible permeability and NMR response changes. In contrast, stiff micritic limestones with little dissolution-induced compaction exhibited larger permeability increases ([Formula: see text]). This work demonstrated the advantages of utilizing a suite of concurrent and independent measurements to build a more comprehensive interpretation of microstructure changes induced by injecting fluids that are in chemical disequilibrium with the host formation.

Elastic moduli of solids containing spheroidal pores

We use asymptotic approximations for the elastic compliances ( P, Q) of a spheroidal pore as input in the differential effective medium scheme to derive approximate analytical expressions for the effective moduli of an isotropic solid containing randomly oriented spheroids. The approximations are valid for crack-like pores having aspect ratios α as high as 0.3, needle-like pores having aspect ratios as low as 3, and nearly spherical pores (0.7 < α < 1.3). Analytical solutions for the differential scheme have previously only been available for the limiting cases of infinitely thin-cracks ( α = 0) and spherical pores ( α = 1). The relatively simple approximations found between the limiting cases can account for more realistic pore shapes, and are valid for a wide range of porosities. The behaviour of the effective Poisson’s ratio in the high concentration limit shows that ν is bounded between the Poisson’s ratio of the solid and a fixed point ν c that only depends on the aspect ratio of the pores. The asymptotic expressions for P and Q can also successfully be used as input in any other effective medium theory, such as the Mori–Tanaka or Kuster–Toksoz schemes. The relatively simple expressions found for the various effective medium schemes, as well as the bounds found for the effective Poisson’s ratio, will be useful to simplify the process of inversion of elastic velocities in porous solids.

Compressibility and shear compliance of spheroidal pores: Exact derivation via the Eshelby tensor, and asymptotic expressions in limiting cases

We explicitly calculate the elastic compliance of a spheroidal pore in an isotropic solid, starting from Eshelby’s tensor. The exact expressions found for the pore compressibility, P, and the shear compliance, Q, are valid for any value of the aspect ratio α, from zero (cracks) to infinity (needles). This derivation clarifies previous work on this problem, in which different methods were used in different ranges of α, or typographical errors were present. The exact expressions obtained for P and Q are quite complex and unwieldy. Simple expressions for both P and Q have previously been available for the limiting cases of infinitely thin-cracks ( α = 0), infinitely long-needles ( α = ∞), and spherical pores ( α = 1). We have now calculated additional terms in the asymptotic expansions, yielding relatively simple approximations for P and Q that are valid for crack-like pores having aspect ratios as high as 0.3, needle-like pores having aspect ratios as low as 3, and nearly spherical pores. Their relatively simple forms will be useful for incorporation into various schemes to estimate the effective elastic moduli.

A simplified model of effective elasticity for anisotropic shales

A simple method to deal with cracklike pores in anisotropic matrix rock such as shales enhances analytical models and their applications. Actually, clayrocks (shales, in particular) are the dominant clastic component in sedimentary basins, representing about two-thirds of all sedimentary rocks. Shales are usually assumed to be transversely isotropic (TI) media. They are known to be highly anisotropic because of (1) intrinsic elastic anisotropy of the solid phase (matrix) forming the rock (more or less ordered clay layers) and (2) anisotropy induced by the presence of cracklike pores. We focus on this second component of anisotropy. Current analytical models deal with it, but they are complex and are restricted in the case of matrix TI symmetry to cracks lying in the symmetry plane. We simplify such models within a reasonably good approximation and develop an analysis scheme in which cracklike pore effects are calculated in an equivalent isotropic matrix. This simplifies the theoretical approach and potentially broadens its application to any crack and/or pore orientation, e.g., damaged shale with horizontal and vertical (perpendicular to the bedding plane) cracks. A high-pressure confinement test provides experimental data for checking the proposed tool against a reference model in the case of cracklike pores lying in the bedding plane. The results (in terms of Thomsen parameters) are consistent with results from large-scale field data.

Micromechanics Contribution to Coupled Transport and Mechanical Properties of Fractured Geomaterials

This article is devoted to the modelling of interdependent mechanical and hydraulic behaviours of geomaterials in presence of a single through-wall fracture by means of micromechanics arguments. Experimental results of fractured concrete samples show non-linear evolutions for both mechanical and hydraulic behaviours with respect to confinement intensity. These non-linear responses are interpreted by the progressive closure of crack-like pores defining the pore volume of the fracture interfacial domain. Disregarding tortuosity effects, we adopt a 2D representation for these cracks. The key role of the fracture initial porosity is also emphasized. It allows to discuss the shape of the distribution of the local apertures distribution function classically used, intercepted here in terms of the distribution of initial crack–aspect ratio within the fracture domain. Application on fractured concrete samples shows the capability of the theoretical model to accurately reproduce the experimental results.

Pore migration under high temperature and stress gradients

An analytical model is proposed for the evolution of ellipsoidal pores by surface diffusion under the influence of large temperature gradients and the associated thermoelastic stress field. It is found that both of these influences affect the migration velocity of a pore as well as its shape. The shape of a pore is determined by competition between the thermoelastic stress field, the interfacial energies of the pore and grain boundaries, and the kinetics of the mass transport process. The dramatic case of void migration in a uranium dioxide nuclear fuel rod is considered, in which very large temperature gradients of the order of 4 × 10 5 K/m are predicted. It is found that crack-like pores are expected to form on the radial grain boundaries prevalent in the fuel rod microstructure, and that these crack-like pores lead to the eventual structural failure of the component. This agrees with experimental observations. The temperature gradient is the dominant driving force for the migration of near-spheroidal and prolate pores, whereas the stress field is the dominant driving force for oblate crack-like pores.

Anisotropy of elastic wave velocities in deformed shales: Part 2 — Modeling results

This study was devoted to the interpretation of the evolution of elastic wave velocities in anisotropic shales that are subjected to deformation experiments in the laboratory. A micromechanical model was used to describe the macroscopic effective elastic properties and anisotropy of the rock in terms of its microscopic features, such as intrinsic anisotropy and crack/pore geometry. The experimental data (reported in Part 1) were compared quantitatively with the micromechanical model predictions to gain some insight into the microstructural behavior of the rock during deformation. The inversion of the experimental data using the micromechanical model was carried out by means of a numerical minimization of the least-squares distance between data and model in terms of effective compliances. Under isotropic mechanical loading, the overall behavior of the dry shale is consistent with the closure of crack-like pores, which are aligned in theplane of symmetry of the transversely isotropic background matrix. Those cracks represent a low fraction of the total porosity, but they have a strong effect on elastic wave velocities. The data are consistent with an initial (horizontal) crack density of 0.07. Crack closure also is evidenced at early stages of axial loading applied perpendicular to the shale bedding plane, whereas crack density increases significantly as axial stress is increased. Interpretation of the wet experiment is less straightforward, although some preliminary conclusions could be drawn. Under isotropic stress, crack closure also is evidenced, whereas crack density remains constant at the early stages of deviatoric loading. When axial peak stress is approached, crack density increases drastically, which likely indicates onset and development of vertical cracking. Wet experiments probably are more complex because water is likely to be expelled from crack-like pores toward equant pores in response to the mechanical loading.

Effects of fluids and dual-pore systems on pressure-dependent velocities and attenuations in carbonates

The effects of fluid substitution on P- and S-wave velocities in carbonates of complex texture are still not understood fully. The often-used Gassmann equation gives ambiguous results when compared with ultrasonic velocity data. We present theoretical modeling of velocity and attenuation measurements obtained at a frequency of [Formula: see text] for six carbonate samples composed of calcite and saturated with air, brine, and kerosene. Although porosities (2%–14%) and permeabilities [Formula: see text] are relatively low, velocity variations are large. Differences between the highest and lowest P- and S-wave velocities are about 18% and 27% for brine-saturated samples at 60 and [Formula: see text] effective pressure, respectively. S-wave velocities are measured for two orthogonal polarizations; for four of six samples, anisotropy is revealed. TheGassmann model underpredicts fluid-substitution effects by [Formula: see text] for three samples and by as much as 5% for the rest of the six samples. Moreover, when dried, they also show decreasing attenuation with increasing confining pressure. To model this behavior, we examine a pore model made of two pore systems: one constitutes the main and drainable porosity, and the other is made of undrained cracklike pores that can be associated with grain-to-grain contacts. In addition, these dried rock samples are modeled to contain a fluid-filled-pore system of grain-to-grain contacts, potentially causing local fluid flow and attenuation. For the theoretical model, we use an inclusion model based on the [Formula: see text]-matrix approach, which also considers effects of pore texture and geometry, and pore fluid, global- and local-fluid flow. By using a dual-pore system, we establish a realistic physical model consistently describing the measured data.

Permeability Evolution During Non-linear Viscous Creep of Calcite Rocks

Permeability, storage capacity and volumetric strain were measured in situ during deformation of hot-pressed calcite aggregates containing 10, 20, and 30 wt% quartz. Both isostatic and conventional triaxial loading conditions were used. The tests were performed at confining pressure of 300 MPa, pore pressures between 50 to 290 MPa, temperatures from 673 to 873 K and strain rates of 3 × 10−5 s−1. Argon gas was used as the pore fluid. The initial porosities of the starting samples varied from 5% to 9%, with higher porosity correlated to higher quartz content. Microstructural observations after the experiment indicate two kinds of pores are present: 1) Angular, crack-like pores along boundaries between quartz grains or between quartz and calcite grains and 2) equant and tubular voids within the calcite matrix. Under isostatic loading conditions, the compaction rate covaries with porosity and increases with increasing effective pressure. Most of the permeability reduction induced during compaction is irreversible and probably owes to plastic processes. As has been found in previous studies on hot-pressed calcite aggregates, permeability, k, is nonlinearly related to porosity, o. Over small changes in porosity, the two parameters are approximately related as \( k{\mathbf{ }}\alpha {\mathbf{ }}\phi ^n \) . The exponent n strongly increases as porosity decreases to a finite value (from about 4 to 6% depending on quartz content), suggesting a porosity percolation threshold. When subjected to triaxial deformation, the calcite-quartz aggregates exhibit shearenhanced compaction, but permeability does not decrease as rapidly as it does under isostatic conditions. During triaxial compaction the exponent n only varies between 2 and 3. Non-isostatic deformation seems to reduce the percolation threshold, and, in fact, enhances the permeability relative to that at the same porosity during isostatic compaction. Our data provide constraints on the governing parameters of the compaction theory which describes fluid flow through a viscous matrix, and may have important implications for expulsion of sedimentary fluids, for fluid flow during deformation and metamorphism, and melt extraction from partially molten rocks.

Structure of strength-limiting flaws in alumina ceramics made by the powder granule compaction process

Alumina ceramics were made by the powder granule compaction process and the structure of strength-limiting flaws was examined. Optically projected images obtained by transmission optical microscopy for a thin alumina specimen revealed that there were many round-shape large flaws of several tens of microns. The cross-sectional views of the large flaws observed after polishing the thinned specimen clearly showed that their structure was a crack-like pore rather than a spherical one. The fractography indicated that the fracture of the alumina ceramics originated at the crack-like pore which was a part of the corresponding large flaws located near the tensile surface of the specimen. The internal structure of the compact before firing was examined by the liquid immersion method. It was clearly shown that the large flaws resulted from non-uniform powder packing associated with granule deformation during compaction.

Foaming behaviour of Al–Si–Cu–Mg alloys

The powder metallurgical route was utilised to obtain the Al–5Si–4Cu–4Mg (alloy 544) and Al–3Si–2Cu–2Mg (alloy 322) foams. Various steps such as centrifugal atomisation, mixing alloy powder and foaming agent (1 wt-%TiH2), cold compaction of mixture, hot extrusion and foaming in a preheated furnace were performed. Foaming behaviour of the alloys was investigated by digital microscopy, image analysis, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) mapping in this study. It was found that alloy 544 takes a shorter period of time to initiate pore nucleation than alloy 322. Alloy 544 had a higher pore growth rate than alloy 322 at the same pre-set furnace temperature. In both alloys, crack-like pore nucleation occurred between aluminium alloy powders elongated in a direction parallel to the extrusion direction. Both alloys showed the same foaming sequence of crack-like pore nucleation, spherical pore growth, coalescence of neighbouring pores and collapse of pores adjacent to the free surface of specimen. The time required to start pore nucleation decreased with the increase of foaming temperature. The cell walls of both alloys consisted of α-Al phase and eutectic phase.

Effects of aluminium powder content and cold rolling on foaming behaviour of xAlp/Al5Si4Cu4Mg/0.8TiH2 composites

Aluminium foams were produced by applying powder metallurgy technology. The process began by making aluminium powder and mixing it with alloy powder (Al5Si4Cu4Mg) and foaming agent (TiH2). The mix was compacted to the form of a billet by cold pressing and then it was hot extruded to a dense foamable strip, which was cold rolled to give 40% thickness reduction. The resulting precursor composites of both the extruded strip and the extruded plus rolled strip were then freely foamed without a mould at a constant temperature of 700°C for different foaming times. The effects of aluminium powder content and cold rolling on the foaming characteristics of the foamable composite strip were studied. It is noted that aluminium powder fibre in the extruded composite strip acts as a barrier to pore initiation and evolution due to the higher melting point of pure aluminium fibre than that of the alloy matrix. Cold rolling promotes foaming of the composite strip due to the TiH2 cracking and debonding between TiH2 particles and metal matrix. The morphological and microstructural evolution of composite foams was also investigated. The foaming mechanism can be described by the following sequence: cracklike pore nucleation between elongated powder fibres; ellipsoidal, spherical, and polygonal pore growth; and the collapse of pores as a result of coalescence.

Gassmann fluid substitutions: A tutorial

Fluid substitution is an important part of seismic attribute work, because it provides the interpreter with a tool for modeling and quantifying the various fluid scenarios which might give rise to an observed amplitude variation with offset (AVO) or 4D response. The most commonly used technique for doing this involves the application of Gassmann's equations.Modeling the changes from one fluid type to another requires that the effects of the starting fluid first be removed prior to modeling the new fluid. In practice, the rock is drained of its initial pore fluid, and the moduli (bulk and shear) and bulk density of the porous frame are calculated. Once the porous frame properties are properly determined, the rock is saturated with the new pore fluid, and the new effective bulk modulus and density are calculated.A direct result of Gassmann's equations is that the shear modulus for an isotropic material is independent of pore fluid, and therefore remains constant during the fluid substitution process. In the case of disconnected or cracklike pores, however, this assumption may be violated. Once the values for the new effective bulk modulus and bulk density are calculated, it is possible to calculate the compressional and shear velocities for the new fluid conditions.There are other approaches to fluid substitution (empirical and heuristic) which avoid the porous frame calculations but, as described in this tutorial, often do not yield reliable results. This tutorial provides the reader with a recipe for performing fluid substitutions, as well as insight into why and when the approach may fail.

Resintering Behavior in Oxidativdy Sintered UO2 and UO2-5wt%CeO2 Pellet

The cause of large density drop, 2 ~ 3%TD., of oxidatively sintered UO2 and UO2-5wt%CeO2 pellets during the resintering in H2 atmosphere has been studied for powders admixed with zinc stearate lubricant for pressing. The density of UO2-5wt%CeO2 pellet admixed with zinc stearate and sintered in CO2-H2 atmosphere was decreased by about 3%TD. and a number of crack-like pores were observed in the pellet after the resintering, while that of the pellets admixed with stearic acid or Acrawax and sintered in the same condition was little changed. The pellets admixed with zinc stearate and sintered in H2-CO2-H2 or H2 atmospheres showed the density increase Mow 1%T.D. during the resintering. Oxidatively sintered UO2-5wt%CeO2 pellets mixed with Zn or ZnO also displayed the density drop of about 2.7%T.D. and crack-like pores after the resintering. Therefore, the major cause of large swelling of the pellets admixed with zinc stearate and sintered in CO2-H2 atmosphere after the resintering is found to be Zn residues.

Sintering of Nanocrystalline ZrO&lt;SUB&gt;2&lt;/SUB&gt;(3Y) by Hot-Pressing

Processing of synthesising nanocrystalline ZrO 2 (3Y) by hot-pressing was investigated. Large crack-like pores have been found in the sintered samples. The reason of this phenomenon was that the agglomerate in the powder had not been destroyed efficiently. This agglomerate sintered quickly and turned into aggregate when hot-pressing was applied. Large pores formed as a result of the interaction between the aggregate and the matrix. Since the applied pressure was limited because of the low stress tolerance of the graphite die, no plastic deformation could happen and those large pores could not be squashed. As a result, the densities of the samples sintered by hot-pressing were lower than by pressureless sintering at the similar sintering temperature. To overcome this shortage of hot-pressing, sinter forging was applied. Nano Y-TZP materials with grain size of about 85 nm and relative density of 99% could be obtained after sinter-forging at 1100°C for only 30 min.