Clay Grains Research Articles

Towards understanding the microstructural and structural changes in natural hierarchical materials for energy recovery: In-operando multi-scale X-ray scattering characterization of Na- and Ca-montmorillonite on heating to 1150 °C

Understanding the changes in the microstructures and structures of clays with varying intercalated metal ions at elevated temperatures is of importance for many applications ranging from the recovery of shale gas from unconventional formations to developing effective nuclear waste containment technologies, and engineering materials such as ceramics for fuel cell applications. In this study, synchrotron-based in-operando multi-scale X-ray scattering analyses are used to determine dynamic microstructural and crystal structural changes in Na- and Ca-montmorillonite on heating from 30°C to 1150°C. Larger cations such as Ca2+ confer more defined morphological regimes compared to Na+ ions in compacted clays, as evident from the ultra-small-angle X-ray scattering results. The hierarchical morphology of clays is characterized to distinguish between nano-scale interlayer swelling porosity, meso-scale porosity, and intergranular pore spaces between powdered clay grains. On heating from ambient temperature to 200°C, the removal of interlayer water reduced the basal distances to 9.6Å. On further heating to 800°C, gradual dehydroxylation of the clay sheets is evident from the structural changes. The effects of sintering at temperatures greater than 800°C are evident from significant reductions in the intrinsic porosities of the clay sheets, and the formation of newer phases such as mullite. By connecting the in-operando microstructural and structural changes across spatial scales ranging from micrometers to Angstroms, the possibility of engineering high temperature processes for achieving morphologies and chemical compositions of interest is presented.

Influence of deposition in dam reservoir on the deep marine hemipelagic environment off Niigata, central Japan

It has been well known that dam construction on rivers can cause a decrease in sediment supply and serious coastal erosion, while influence of dams on the sedimentary environment beyond the coastal area has not been well known. The mass accumulation rate (MAR) in the hemipelagic environment (500–700 m water depth) off Niigata, central Japan since the early half of the twentieth century was obtained from core samples, based on excess 210Pb activity. Decreases in MAR during the mid-twentieth century were recognized in several core samples located northern off the Agano and Shinano river mouths. Using the CIC model, combining correction for the influence of surface bioturbation based on the ratio of MARs, decreases in MAR were estimated as occurring during 1959–1964 and 1949–1955. These dates correspond with construction of the Okutadami dam (1956–1961), possessing the largest capacity and reservoir sediment volume of 3.9 × 107 m3 in the Agano and Shinano river systems. The other core located NW off the Agano river mouth showed constant MAR through the mid-twentieth century. A synchronicity of huge dam construction and decrease in MAR in deep marine environment, the similarity between silt–clay-sized grain size of dam reservoir sediment and that seen in the hemipelagic environment, and horizontal distribution of decreasing MAR in cores according to north–northeastward sediment transport on shelf, makes it reasonable to interpret that the decrease in MAR in hemipelagic environment was caused by entrapment of silt–clay grains in the huge dam reservoir.

Are polygonal faults the keystone for better understanding the timing of fluid migration in sedimentary basins?

The initial sediment lithification starts with complex interactions involving minerals, surface water, decomposing organic matter and living organisms. This is the eogenesis domain (0 to 2 km below the seafloor) in which the sediments are subject to physical, chemical and mechanical transformations defining the early fabric of rocks. This interval is intensively prospected for its energy/mining resources (hydrocarbons, metal deposits, geothermal energy). In most basins worldwide it is composed of very fine-grained sediments and it is supposed to play the role of a seal for fluids migration. However, it is affected by polygonal faulting due to a volume loss during burial by contraction of clay sediments with a high smectite content. This process is of high interest for fractured reservoirs and/or cover integrity but it is not well constrained giving an uncertainty as this interval can either promote the migration of deeper fluids and the mineralized fluids intensifies diagenesis in the fracture planes, rendering this interval all the more impermeable. The next challenge will be to define where, when and how does this polygonal fault interval occur and this can only be done by understanding the behavior of clay grains and fluids during early burial.

Compaction of diagenetically altered mudstones – Part 1: Mechanical and chemical contributions

At low temperatures, siliciclastic mudstones compact mechanically. Above 70 °C, where the smectite-to-illite transformation dominates clay diagenesis, they also compact chemically, provided that excess pore water can escape. There are two prevailing conceptual models for the compaction of siliciclastic mudstones at higher temperatures, above ∼100 °C. One holds that precipitation of minerals during diagenesis cements the mudstones such that mechanical compaction no longer occurs even if the mudstones can drain freely. According to the other model, diagenetically altered mudstones continue to compact mechanically in response to increasing effective stress. We found that wireline-log and pressure data from Cretaceous mudstones at Haltenbanken are consistent with ongoing mechanical compaction accompanying chemical compaction up to at least 130 °C. We suggest that mechanical compaction continues because grain contacts in siliciclastic mudstones following smectite-to-illite transformation are still mostly between clay grains.

Interfacial polarization of disseminated conductive minerals in absence of redox-active species — Part 1: Mechanistic model and validation

Electrically conductive mineral inclusions are commonly present in organic-rich mudrock and source-rock formations such as veins, laminations, rods, grains, flakes, and beds. Laboratory and subsurface electromagnetic (EM) measurements performed on geomaterials containing electrically conductive inclusions generally exhibit frequency dispersion due to interfacial polarization phenomena at host-inclusion interfaces. In the absence of redox-active species, surfaces of electrically conductive mineral inclusions are impermeable to the transport of charge carriers, inhibit the exchange of charges and behave as perfectly polarized (PP) interfaces under the influence of an externally applied EM field. Interfacial polarization phenomena involving charge separation, migration, accumulation/depletion, and relaxation around PP interfaces is referred to as PP interfacial polarization; it influences the magnitude and direction of the electric field and charge carrier migration in the geomaterial. We have developed a mechanistic model to quantify the complex-valued electrical conductivity response of geomaterials containing electrically conductive mineral inclusions, such as pyrite and magnetite, uniformly distributed in a fluid-filled, porous matrix made of nonconductive grains possessing surface conductance, such as silica and clay grains. The model first uses a linear approximation of the Poisson-Nernst-Planck equations of dilute solution theory to determine the induced dipole moment of a single isolated conductive inclusion and that of a single isolated nonconductive grain surrounded by an electrolyte. A consistent effective-medium formulation was then implemented to determine the effective complex-valued electrical conductivity of the geomaterial. Model predictions were in good agreement with laboratory measurements of multifrequency complex-valued electrical conductivity, relaxation time, and chargeability of mixtures containing electrically conductive inclusions.

Analysis of Dimension Stone Waste Addition to the Clayey Mass Used in Roof Tile Production

Addition of dimension stone waste to clayey mass is an alternative to make the dimension stone sector more environmentally sustainable and to reduce the consumption of clayey raw material. Our aim is to study the technical feasibility of dimension stone waste addition to the clayey mass used in roof tile production. Samples of waste and clay were tested for physical, chemical, mineralogical and microstructural characterization. Samples containing dimension stone waste contents, varying from 10% to 90% (wt.), were produced and burned in industrial furnace of a structural ceramic factory at a 900°C maximum temperature for 24 hours. The technological characteristics evaluated were water absorption, apparent porosity, apparent density, loss on ignition, linear shrinkage and flexural strength. The results show that the samples with up to 60% waste content can be used in the production of ceramic bodies, since the waste promoted the physical packing of the grains, which increased the densification of ceramic bodies. With that, the waste-improved ceramic characteristics were analyzed. From 60% content onwards, the waste was not be able to fill the remaining interstices between the clay grains to densify the ceramic body because both raw materials have similar particle size distribution. Furthermore, the excess silica, in the form of quartz contained in the waste, presents an inert behavior in the factory firing temperature used in this research, causing fracture points and decreasing the flexural strength.

The mineralogy and fabric of ‘Brickearths’ in Kent, UK and their relationship to engineering behaviour

Mineralogical and petrographical investigation of two loessic brickearth profiles from Ospringe and Pegwell Bay in north Kent, UK have differentiated two types of brickearth fabric that can be correlated with different engineering behaviour. Both sequences comprise metastable (collapsing) calcareous brickearth, overlain by non-collapsing ‘non-calcareous’ brickearth. This study has demonstrated that the two types of brickearth are discretely different sedimentary units, with different primary sedimentary characteristics and an erosional junction between the two units. A palaeosol is developed on the calcareous brickearth, and is associated with the formation of rhizolithic calcrete indicating an arid or semi-arid environment. No evidence has been found for decalcification being responsible for the fabric of the upper ‘non-calcareous’ brickearth. Optically-stimulated dates lend further support for the calcareous and ‘non-calcareous’ brickearth horizons being of different age or origins. The calcareous brickearth is metastable in that it undergoes rapid collapse settlement when wetted under applied stresses. It is characterised by an open-packed arrangement of clay-coated, silt-sized quartz particles and pelletised aggregate grains (peds) of compacted silt and clay, supported by an inter-ped matrix of loosely packed, silt/fine-grained sand, in which the grains are held in place by a skeletal framework of illuviated clay. The illuviated clay forms bridges and pillars separating and binding the dispersed component silt/sand grains. There is little direct grain-to-grain contact and the resultant fabric has a very high voids ratio. Any applied load is largely supported by these delicate clay bridge and pillar microfabrics. Collapse of this brickearth fabric can be explained by a sequence of processes involving: (1) dispersion and disruption of the grain-bridging clay on saturation, leading to initial rapid collapse of the loose-packed inter-ped silt/sand; (2) rearrangement and closer stacking of the compact aggregate silt/clay peds; (3) with increasing stress further consolidation may result from deformation and break up of the peds as they collapse into the inter-ped regions. Smectite is a significant component of the clay assemblage and will swell on wetting, further encouraging disruption and breaking of the clay bonds. In contrast, the ‘non-calcareous’ brickearth already possesses a close-packed and interlocking arrangement of silt/sand grains with only limited scope for further consolidation under load. Minor authigenic calcite and dolomite may also form meniscus cements between silt grains. These have either acted as “scaffolds” on which illuviated clay has subsequently been deposited or have encrusted earlier-formed grain-bridging clay. In either case, the carbonate cements may help to reinforce the clay bridge fabrics. However, these carbonate features are a relatively minor feature and not an essential component of the collapsible brickearth fabric. Cryoturbation and micromorphological features indicate that the calcareous brickearth fabric has probably been developed through periglacial freeze–thaw processes. Freezing could have produced the compact silt/clay aggregates and an open porous soil framework containing significant inter-ped void space. Silt and clay were remobilised and translocated deeper into the soil profile by water percolating through the active layer of the sediment profile during thawing cycles, to form the loosed-packed inter-ped silt matrix and grain-bridging meniscus clay fabrics. In contrast, the upper ‘non-calcareous’ brickearth may represent a head or solifluction deposit. Mass movement during solifluction will have destroyed any delicate grain-bridging clay microfabrics that may have been present in this material.

The influence of freeze-thaw cycles on the granulometric composition of Moscow morainic clay

ABSTRACT The freeze-thaw cycling process considerably changes the composition, structure, and properties of soils. Since the grain size is the most important factor in determining soil characteristics, our current research primarily aims to investigate dynamic changes of the soil fraction when exposed to freeze-t haw conditions. We observed two series of Moscow morainic clayey specimens (gQ IIm ): (I) the original series, and (II) the remolded series. We subjected each series of soil specimens to different frequencies of freeze-thaw cycles (3, 6, 20, and 40 cy cles), and we used granulometric tests to analyze both series before and after exposure to freeze-thaw conditions. As a resu lt of our experiments, the granulometric compositions tended to be distributed evenly after 40 freeze-thaw processes ( i.e. , content of fraction for 0.1–0.05 mm was increased after 40 freeze-thaw cycles) because the division of coarse grains and the aggregation of fine grains were synchronized during the freeze-thaw process. The soil grains in both series changed bi-directionally. In the original series, changes of the sand grains were conjugated with the clay grains, and in the remolded series, changes of the sand grains were conjugated with the silt grains, because potential energy difference caused the division and aggregation processes to relate to the coun-teraction process. The even distribution of soil grain size indicated the state of equilibrium or balance. The granulometric compositions were altered the most during the sixth freeze-thaw cycle, because the coefficient of the intensity variation of the grain fineness (

Clast-cortex aggregates in experimental and natural calcite-bearing fault zones

We investigated the formation mechanisms of rounded clast-cortex aggregates, a composite grain found in the slipping zones of faults hosted in calcite- and clay-rich rocks. The natural aggregates contain a central clast commonly made of host-rock fragments or reworked cataclasite from the slipping zone. The central clasts are surrounded by an outer cortex of calcite or clay grains a few μm or less in size. In laboratory experiments on calcite gouges using two rotary-shear apparatus we investigated the dependence of clast-cortex aggregate formation on the applied slip rate, normal stress, total displacement and ambient humidity. Clast-cortex aggregates formed at all investigated slip rates (100 μm/s to 1 m/s) but only at relatively low normal stresses (≤5 MPa). The aggregates were better developed with increasing displacement (up to 5 m) and did not form in experiments with water-dampened gouges. In the experiments, aggregates formed in low-strain regions within the gouge layers, adjacent to the highest-strain slip zones. We propose that clast-cortex aggregates in calcite-bearing slip zones form in the shallow portions of faults during shearing in relatively dry conditions, but our experiments suggest that they cannot be used as indicators of seismic slip. Formation involves clast rotation due to granular flow accompanied by accretion of fine matrix material possibly facilitated by electrostatic forces.

Geochemical and grain-size distribution of radioactive and stable cesium in Fukushima soils: implications for their long-term behavior

Availability and mobility of radioactive cesium, 137Cs, in soils are crucial for recovery from the accident in the Fukushima Dai-ichi Nuclear Power Plants. In this study we investigated the geochemical and grain-size distribution of 137Cs in 11 soil samples collected in the eastern area of Fukushima Prefecture after the accident. Sequential extractions were performed to evaluate the distribution of 137Cs having different geochemical interactions with soil components. The result was further compared with that of the stable cesium, 133Cs, which had occurred in the soils before the accident. The distribution of 137Cs in different grain-size fractions was also determined. Radioactive cesium was predominantly found in the extract obtained by strong-acid dissolution and the extraction residue and was more concentrated in silt and clay grains. X-ray diffraction analyses revealed that micaceous minerals as well as kaolin minerals were predominantly dissolved by the strong-acid treatment. Correlation between the fraction of 137Cs and the content of micaceous minerals in different grain-size fractions of soil minerals suggests that micaceous minerals are responsible for the fixation of 137Cs in the soils. The isotopic ratio of 137Cs and 133Cs in the extract by strong-acid dissolution was more than three times smaller than those in the extracts by water, ion exchange, and reductive dissolution. This indicates that the distribution of 137Cs was not in the steady state in 2 y after the accident due to relatively slow fixation by the soil clay minerals.

Interfacial reactions between humic-like substances and lateritic clay: Application to the preparation of “geomimetic” materials

The aim of this study was to understand the mechanisms responsible for the strengthening of “geomimetic” materials, especially the chemical bonding between clay and humic substances. The mineral matter is lateritic clay which mainly consists in kaolinite, goethite, hematite and quartz. The other starting products are fulvic acid (FA) and lime. The preparation of these geomimetic materials is inspired from the natural stabilization of soils by humic substances occurring over thousands of years. The present process involves acidic and alkaline reactions followed by a curing period of 18days at 60°C under a water saturated atmosphere. The acceleration of the strengthening process usually observed in soils makes this an original process for treatment of soils. The consolidation of the “geomimetic” materials could result from two major phenomena: (i) chemical bonding at the interface between the clay particles and iron compounds and the functional groups of the fulvic acid, (ii) a partial dissolution of the clay grains followed by the precipitation of the cementitious phases, namely calcium silicate hydrates, calcium aluminate hydrates and mixed calcium silicum and aluminum hydrates. Indeed, the decrease of the BET specific area of the lateritic clay after 24h of reaction with FA added to the structural reorganization observed between 900 and 1000°C in the “geomimetic” material, and to the results of adsorption measurements, confirm the formation of organo-ferric complexes [1]. The presence of iron oxides in clay, in the form of goethite, appears to be another parameter in favor of a ligand exchange process and the creation of binding bridges between FA and the mineral matter. Indeed all faces of goethite are likely to be involved in complexation reactions whereas in lateritic clay only lateral faces could be involved [2]. The results of the adsorption experiments realized at a local scale will improve our understandings about the process of adsorption of FA on lateritic clays and its involvement in the strengthening process of materials.

Atomic-scale modelling of elastic and failure properties of clays

The elastic and failure properties of a typical clay, illite, are investigated using molecular simulation. We employ a reactive (ReaxFF) and a non-reactive (ClayFF) force field to assess the elastic properties of the clay. As far as failure is concerned, ReaxFF was used throughout the study; however, some calculations were also performed with ClayFF. A crack parallel to the clay layers is found to have low fracture resistance when submitted to a tensile loading perpendicular to the crack. The mechanism of both yield and fracture failures is decohesion in the interlayer space. In contrast, under shear loading, the nanoscale failure mechanism is a stick-slip between clay layers. No fracture propagation is observed as the clay layers slide on top of each other. The low fracture resistance in mode I and the stick-slip failure in mode II are both the consequence of the lack of chemical bonds between clay layers where the cohesion is provided by non-covalent interactions. This work, which provides a description of the failure of clays at the microscopic scale, is the first step towards describing the failure of clays at a larger scale where the polycrystalline distribution of clay grains must be taken into account.

Numerical simulation of desiccation cracking in a thin clay layer using 3D discrete element modeling

Desiccation cracking of clay soil is of critical importance in many applications, such as industrial waste containment, hydraulic barriers, road embankments, and agricultural operations. The factors that influence cracking are known qualitatively, but it is not clear how to predict the initiation and propagation of cracks. This study presents a discrete element approach to modeling desiccation cracking in thin clay layers, considering material property changes. First, an aggregate shrinkage model based on the aggregate structure of clay was proposed, and the drying shrinkage of clay soil was modeled by imposing drying shrinkage kinetics for each aggregate at the micro-scale. Second, the clay soil was represented by an assembly of aggregates linked by bonds, and desiccation cracking of the clay layer was modeled using a three-dimensional discrete element code (PFC3D), with the aid of the embedded programming language FISH. When the clay layer is sufficiently thin, the water content gradient along the section can be neglected; thus, the shrinkage kinetics are the same for all of the grains of clay. In the model based on the discrete element method (DEM), the bond strength and contact stiffness changed during drying. Their changes were determined by matching the simulation results with the experimental data. Third, the DEM approach was validated by reproducing experimental desiccation tests performed on a thin clay layer in a disk shape. The geometric parameters of surface cracks were quantified using image analysis techniques and were compared with experimental observations. Fourth, some factors of influence, such as the sample thickness, the properties of the soil–base interface, micro-mechanical parameters, and shrinkage parameters, were investigated using the DEM model. The results obtained from the DEM analyses were compared with the results of prior research in this field of study. The approach used in this study is very promising for simulating desiccation cracking in thin clay soil because the model captures the initiation and propagation mechanism of desiccation cracks. Although this study was carried out on surface cracking in a thin clay layer, the extension of this methodology is of potential benefit not only for predicting three-dimensional desiccation cracking in real clay liners but also for modeling cracking in other materials with properties that vary with water content or temperature, such as concrete and rock.

Attenuation Measurement Method to Control Clay Slurries: Application of Dams Water

We propose to study the behavior of diluated suspensions of clay and non colloidal particles by a combination of experiments. We plan to study these diluate particulate systems by changing the masse fraction of clay. We have developed a technique based on the ultrasound, by measuring ultrasonic attenuation waves in clay slurries. This technique makes it possible to control in real time the presence of the clay grains whose dimension is about 10 micron, using an interface conceived under Labview program. The slurries with different weight of clay were controlled. The detection of weak amount of clay’s grains and colloid were provided by the transducer 30Mhz. High sensitivity is obtained by analyzing the attenuation measurements founded from multiple paths through the slurry. The experimental results obtained show that the attenuation of waves due to particles varies linearly with mass fraction and the frequency of transducers. Therefore, such measures of the attenuation at a given frequency give the concentration of clay in the suspension. These frequency charts will be used for the quality control of the dam’s water. General Terms 0.N.E.P: Office National de l’eau Potable, (National office of drinking water). FFT: Fast Fourier Transform.

Consistent joint elastic‐electrical differential effective‐medium modelling of compacting reservoir sandstones

AbstractImproved reservoir characterization and monitoring can be achieved by combining seismic and controlled‐source electromagnetic techniques. This requires developing coherent rock physics descriptions. In this paper we demonstrate consistent joint elastic‐electrical modelling according to the differential effective‐medium theory. We test our modelling against data from a compaction experiment on a set of 11 sandstone core samples from the same quarry location. The presented approach is analogous to calibrating a rock physics model to a particular reservoir based on data from possible well logs and core samples. For simplicity we choose to use multivariable non‐linear regression in the inversion. It shows that this technique is able to identify solutions that are physically sound. However, a more rigorous inversion method might be considered in future implementations. To identify the critical parameters we test the elastic‐electrical sensitivity of the various unknown variables involved. The most sensitive parameters identified are then perturbed during the modelling. The mineralogy consists mainly of quartz, which we assume to be spherical and kaolinite. We use the resistivity to calibrate the aspect ratio of the clay grains and estimate the porosity reduction due to compaction. These values are in turn used in inverse modelling of the bulk and shear moduli. The solid minerals make up the inclusion material in the differential effective‐ medium modelling for both the elastic and electrical properties. Hence, this formulation constitutes a consistent joint elastic‐electrical modelling scheme. We achieve good fits between the model results and the laboratory measurements for most of the samples. The reason for the less good fit with some of the samples might be due to measurement errors in the laboratory. This is supported by the observed abnormal stiffness compaction trends associated with those samples.

How clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones: Observations and experiments

Observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones. Samples of deeply buried, porous sandstones with different types of clay coats were split in two, coats removed from one of each pair of splits, and grain surfaces inspected with scanning electron microscopy. Quartz grains in a fluvial-deltaic sandstone buried to 115C had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats, though well-developed overgrowths occurred on nearby, naturally uncoated grains. However, in similar sandstones buried to 164C, quartz-grain surfaces exposed by chlorite-coat removal were covered with small (5 m), mainly anhedral, syntaxial quartz overgrowths. Similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215C. We conclude that clay coats may retard quartz nucleation at moderate temperatures, but at high temperatures, many coats permit quartz nucleation and preserve porosity by limiting cement growth. To investigate cement growth-limitation mechanisms, samples with coats removed were subjected to quartz-cementing conditions in a hydrothermal reactor. During experiments, the naturally occurring small overgrowths on clay-cleaned grains coalesced and grew, suggesting that clay particles in coats inhibit cement growth by forming barriers to early-overgrowth coalescence. Although the fraction of grain-surface coverage is the primary control on cement inhibition by coats, cement growth–interference textures vary with coat type, providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness. In deeply buried sandstones, quartz cement can fill significant microporosity within diagenetic chlorite coats, potentially affecting mechanical and petrophysical rock properties.

Examination of effective parameters for the production of expanded clay aggregate

It is important to know which parameters are effective for the manufacture of expanded clay aggregate and to determine how effective these parameters are on expansion. In this study, expansion tests were conducted by using samples from three separate fields to determine how different parameters influence manufacturing. Clay grain size, pellet size, temperature in the furnace and firing time were selected in the expansion tests as parameters. Their effect on production was evidenced separately. It was found that decreasing the clay size has a positive effect on expansion while a decrease in pellet size has a negative effect on it. Also, temperature in the furnace and the time period during which the clay remains in the furnace are critical values for the resultant aggregate quality. The conditions for optimum manufacturing were explained in detail.

High Resolution Gas Adsorption Study on Montmorillonite Clays from Côte d’Ivoire: Assessment of Surface Energetic Properties

The surface heterogeneity of two samples of montmorillonite clays from Katiola (C?te d’Ivoire), referenced K1 and K2, has been explored by low-pressure of argon and nitrogen adsorption at 77 K. The adsorption isotherms of the studied samples have been modelled by the Derivative Isotherms Summation method (DIS method) in a satisfactory manner, with the help of local isotherms to argon and nitrogen. The use of the DIS method allowed us to quantify the basal and lateral surfaces, then to determine the total surface of studied materials. Beyond the quantitative aspect, the DIS method permitted us to determine the lamellarity index of the clay grains and to describe the two studied samples of a family of high energy site (–14 kT to nitrogen), that shows a strong affinity for the polar adsorbates.

Pore types in the Barnett and Woodford gas shales: Contribution to understanding gas storage and migration pathways in fine-grained rocks

The identification of organoporosity (microscale and nanoscale pores within organic matter in shales), its importance to storage and perhaps transfer of gas molecules through shales, and methods for gathering three-dimensional images of the pores, such as by argon-ion milling and/or field emission scanning electron microscopy, have all been well documented and discussed for unconventional gas shales. However, other types of pores exist within shales that can be important to storage and migration of gas (and oil), and other technologies are available for their identification and imaging. The different pore types found in the Barnett and Woodford shales are described and classified in this article. Scanning electron microscopy revealed the presence of porous floccules in both the Barnett and Woodford shales, which appear similar to laboratory-produced floccules and to those in other ancient shales. Published experimental and observational studies indicate that floccules are hydraulically equivalent to coarser grains and are transported by traction processes. Current-induced microsedimentary structures and textures within the Barnett and Woodford shales, as well as the preserved floccules, suggest such processes were active during transport and deposition. Pore spaces between the floccules are open and can provide storage space as well as permeability pathways for gas molecules through the shales. Pores are also found within organic matter that occurs both as discrete particles and as adsorbed coatings around clay grains in the shale. They are referred to here as “organopores.” Porous fecal pellets are also common in the Barnett Shale. Preserved fossil fragments such as organic-walled spores and inorganic sponge spicules have hollow central chambers, which may remain partially or completely open even after burial. Intraparticle pores occur between grains of various minerals (e.g., pyrite framboids). Microchannels within shale matrix, which may be the bounding surfaces of scours or microsedimentary structures, may also provide permeability pathways for hydrocarbon migration. Mircrofractures are also common, and their initiation might be related to mineral crystal structure. When present in sufficient quantity, these pore types offer potential gas (and oil) molecule storage spaces and permeability pathways through the shales.

Permeability anisotropy and fabric development: A mechanistic explanation

We develop a model to describe development of permeability anisotropy and fabric in clay‐rich sediments due to clay grain reorientation during consolidation and shearing. The model considers porosity, grain aspect ratio, and average angle of grains with respect to the horizontal plane. To validate the model, we determined permeability anisotropy ratios (ratio of horizontal permeability to vertical permeability) of porous media composed of flat cylindrical particles by lattice‐Boltzmann simulations. Over representative ranges of grain aspect ratio (diameter/thickness = 1–20) and porosity (44%–82%) the simulation results match the predicted values well. We show that permeability anisotropy ratios up to ∼20 can be attained within highly sheared (shear strain >20), low‐porosity (<40%) materials with aspect ratios >20, and that the maximum anisotropy ratio attainable by grain rotation is limited by grain aspect ratio. We further show that the anisotropy ratio of mixtures of low aspect ratio and high aspect ratio particles, like silty clays, are low (<2). This occurs because the low aspect ratio particles reduce the difference in tortuosity in transverse directions. Our results demonstrate why larger permeability anisotropy ratios are possible only through diagenesis, layering, or development of aligned microcracks.