Proportions Of Clay Minerals Research Articles

Micromechanical properties and fractal homogenization of coal based on nanoindentation

Distinct from hard rock, coal is relatively soft and fragmented. It is not only challenging to prepare test coal samples that meet the requirements of standard mechanical experiments but also impossible to recycle them for repeated testing. There is an urgent need to explore new mechanical testing methods to enhance the study of the mechanical properties of coal. In this study, the micromechanical parameters of the coal matrix solid phase were acquired through targeted nanoindentation technology. The elemental composition, surface morphology, and pore structure characteristics of each indentation point were determined by energy dispersive spectrometer, optical microscope observation, and high-pressure mercury injection experiments. The fractal homogenization equation is deduced based on fractal geometry and the Mori–Tanaka method. The validity of the fractal homogenization approach is verified by integrating the micromechanical parameters and pore structure characteristics of coal, and the impact of the microstructural parameters on the macroscopic mechanical properties of coal is discussed. The results show that the proportion of clay minerals in the solid phase of the coal is the greatest (81.18%), with the main mineral components being kaolinite and illite. The elastic modulus is 1.974 ± 1.036 GPa, the hardness is 0.131 ± 0.108 GPa, and the ratio of upper and lower pore scales conforms to the fractal calibration rate. The macroscopic equivalent elastic modulus rises along with the increase in the fractal dimension. When the fractal dimension is constant, the macroscopic equivalent elastic modulus decreases with the increase in λmin/λmax and increases with the increase in solid phase elastic modulus.

Reservoir Characteristics of Marine–Continental Transitional Taiyuan Formation Shale and Its Influence on Methane Adsorption Capacity: A Case Study in Southern North China Basin

To further study the reservoir characteristics and adsorption capacity of the Taiyuan Formation shale in the South North China Basin (SNCB), the pore structure and adsorption capacity of shale are discussed using various analysis tests, including elemental geochemistry, organic geochemistry, mineral composition, low-temperature nitrogen adsorption (LTNA), and methane adsorption experiments. The results indicate that the Taiyuan Formation shale formed in a poor oxygen and anaerobic sedimentary environment in still water. The average value of total organic carbon (TOC) content is 2.37%. The organic matter type mainly consists of type III kerogen. The vitinite reflectance (Ro) ranges from 3.11% to 3.50%. The clay mineral content varies greatly, averaging at 40.7%, while the quartz content averages at 37.7%. The Taiyuan Formation shale mainly develops interparticle (InterP) pores, followed by organic pores, intraparticle (IntraP) pores, solution pores, and microfractures. BET specific surface area (SSA) is between 9.47 m2/g and 22.14 m2/g, while pore volume (PV) ranges from 0.0098 cm3/g to 0.022 cm3/g, indicating favorable conditions for shale gas storage. According to the results of the CH4 adsorption experiment, Langmuir volume from Taiyuan Formation shales exhibits 1.35~4.30 cm3/g, indicating excellent adsorption capacity. TOC content shows a positive correlation with both Langmuir volume and BET SSA from Taiyuan Formation shales, suggesting that TOC plays a crucial role in controlling microscopic pores and gas adsorption capacity. Organic matter enhances the shale adsorption capacity by providing abundant pore SSA. Due to formation compaction, the pore size of clay minerals decreases, leading to an increase in pore SSA, while kaolinite exhibits weak hydrophilic ability. Consequently, with the increase in clay minerals and kaolinite content, the shale adsorption capacity is enhanced to a certain extent. However, an increase in the carbonate mineral content may result in a decrease in the proportion of clay minerals, therefore reducing the CH4 adsorption capacity of shale.

Study of wellbore instability in shale formation considering the effect of hydration on strength weakening

Shale formations often contain a high proportion of clay minerals, which, upon contact with drilling fluid, undergo hydration expansion. This leads to wellbore instability, a problem that poses significant challenges globally. This study aims to investigate the variation of mechanical properties of shale with respect to hydration time. We employ an empirical model that relates shale strength parameters to the time of drilling through geological formations. Additionally, we consider both shear failure along the wellbore boundary and shear sliding along bedding planes in the analysis. We establish a predictive model for wellbore instability in shale formations. The model quantitatively analyzes the variation of wellbore collapse pressure with drilling time. The research findings indicate that, when the influence of bedding is considered, both the wellbore collapse pressure and the optimal well trajectory undergo significant changes, in addition, for some wellbore trajectories, the collapse pressure can increase by more than 30%. Therefore, it is essential to account for the influence of bedding in wellbore stability analysis in shale formations. As the bedding dip angle changes, both the numerical values and distribution range of wellbore collapse pressure and the optimal well trajectory change noticeably. Changes in bedding dip direction, however, do not affect the numerical values of collapse pressure but do influence the distribution region of the optimal well trajectory. Thus, in wellbore trajectory design within shale formations, it is crucial to determine the orientation of bedding and adjust the well trajectory accordingly to enhance wellbore stability. Furthermore, shale hydration does not impact the optimal well trajectory for a block, but with prolonged hydration, the minimum drilling fluid density required to maintain wellbore stability gradually increases. This suggests that hydration intensifies the weakening effect on bedding plane strength. The research results are helpful to understand the effect of hydration on shale wellbore stability and ensure shale wellbore stability during drilling cycle.

Migration of transition metals and potential for carbon mineralization during acid leaching of processed kimberlite from Venetia diamond mine, South Africa

Carbonation of mafic and ultramafic rocks and mineral wastes provides a permanent way to sequester excess atmospheric CO2. Recent research has shown that this method also offers the potential for enhanced recovery of critical metals from mine tailings. In this study, processed kimberlite from the Venetia diamond mine (South Africa) was used in column acid leaching experiments to assess both its carbonation potential and whether critical metals such as nickel could be recovered during mineral carbonation. Processed kimberlite was treated daily with one pore volume of either deionized water or dilute hydrochloric acid (0.04 M, 0.08 M, 0.12 M and 0.16 M) for 28 days. Iron-rich yellow precipitates consistent with yellow ground formed during the experiments both at the top of the residue columns (corresponding to the inlets of the columns) and within the leachates collected from the bases of columns. The carbonation potential and mobility of transition metals were investigated using a combination of quantitative X-ray diffraction (XRD) using Rietveld refinements, inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM) coupled with EDXS, and synchrotron-based X-ray fluorescence microscopy (XFM). Our results show that the high proportion of clay minerals (e.g., lizardite, smectite, talc, chlorite) in the processed kimberlite act as the primary source for Mg and transition metals such as Ni; however, calcite dissolution is the main source for Ca. The amount of Mg and Ca extracted from processed kimberlite increases with HCl concentration. If acid leaching of processed kimberlite were used at Venetia, the amount of Mg leached from clay minerals would provide an estimated CO2 offset potential ranging from 2.1 to 15.8 % of the mine's total annual emissions. The leached Ca from silicate dissolution could also provide an estimated CO2 offset potential ranging from 2.1 to 8.1 % of the mine's total annual emissions. However, the amount of CO2 released by calcite dissolution during this process is equivalent to 2.1–14.3 % of total annual CO2 emissions at Venetia., thus resulting in a net estimated CO2 offset potential of 2.1–9.6 % if the Ca released from calcite could not be recarbonated. If all of the Ca could be reprecipitated as calcite, the acid leaching techniques employed in this study could offset 4.2–23.9 % of the Venetia mine's CO2 emissions. Ultimately, greater concentrations and/or amounts of acid may be used to access more of the offset potential of Mg phyllosilicates in kimberlite, but the CO2 released by calcite dissolution must also be won back by recarbonation.

Investigation of the Effect of Fracturing Fluids on Shale Pore Structure by Nuclear Magnetic Resonance

Hydraulic fracturing technology significantly enhances the productivity of shale oil and gas reservoirs. Nonetheless, the infiltration of fracturing fluid into shale formations can detrimentally affect the microscopic pore structure, thereby impairing the efficacy of hydraulic stimulation. In this study, nuclear magnetic resonance (NMR) technology was utilized to conduct high-pressure soaking tests on shale specimens treated with EM30+ + guar gum mixed water and CNI nano variable-viscosity slickwater, where various concentrations of a drag reducer were utilized. Additionally, the differences in porosity, permeability, mineral composition, and iron ion concentration before and after the measurements were compared, which were used to analyze the influence on the shale’s microscopic pore structure. It features a reduction in the total pore volume after the interaction with the fracturing fluid, with the pore-throat damage degree, porosity damage degree, and permeability damage degree ranging from 0.63% to 5.62%, 1.51% to 6.84%, and 4.17% to 19.61%, respectively. Notably, EM30+ + guar gum mixed water exhibits heightened adsorption retention, alkaline dissolution, and precipitation compared to CNI nano variable-viscosity slickwater, rendering it more deleterious to shale. Moreover, higher concentrations of drag reducers, such as EM30+ or CNI-B, predominantly result in damage to the shale’s micropores. Shale compositions characterized by lower content of quartz and elevated proportions of clay minerals and iron-bearing minerals showcase augmented mineral dissolution and precipitation, consequently intensifying the shale damage. The hydration expansion of mixed-layer illite/smectite profoundly diminishes the core permeability. Consequently, the mechanisms underpinning the damage inflicted on shale’s microscopic pore structure primarily involve fracturing fluid adsorption and retention, mineral dissolution, and precipitation, such as clay minerals and iron-containing minerals.

Ageing Effect On Shear Strength Of Mixed Kaolinite-Bentonite And Sand Clay Minerals

Ageing Effect On Shear Strength Of Mixed Kaolinite-Bentonite And Sand Clay Minerals

Prokaryotic communities inhabiting a high-radon subterranean ecosystem (Castañar Cave, Spain): Environmental and substrate-driven controls

Castañar Cave (Caceres, Spain) is a unique show cave known for its high natural radiation levels. This study presents a comprehensive analysis of its prokaryotic diversity, specifically focusing on investigating the influence of environmental conditions and substrate characteristics on the prokaryotic community structure in the cave sediments. Additionally, the research aims to evaluate the potential impact of human activities on the cave ecosystem. The identification of distinct bioclimatic zones within the cave was made possible through a combination of environmental and microbial monitoring (ATP assays). The results reveal sediment texture as a significant factor, notably affecting the structure, diversity, and phylogenetic variability of the microbial community, including both Bacteria and Archaea. The proportion of clay minerals in sediments plays a crucial role in regulating moisture levels and nutrient availability. These substrate properties collectively exert a significant selective pressure on the structure of prokaryotic communities within cave sediments. The molecular approach shows that heterotrophic bacteria, including those with chitinolytic enzymes, primarily inhabit the cave. Furthermore, chemoautotrophic nitrifiers such as the archaea Nitrososphaeria and the genus Nitrospira, as well as methanotrophic bacteria from the phyla Methylomirabilota, Pseudomonadota, and Verrucomicrobiota, are also present. Remarkably, despite being a show cave, the cave microbiota displays minimal impacts from human activities and the surface ecosystem. Prokaryotic populations exhibit stability in the innermost areas, while the tourist trail area experiences slightly higher biomass increases due to visitor traffic. This suggests that conservation efforts have successfully limited the entry of external nutrients into the innermost cave areas. Additionally, the results suggest that integrating biomarkers like ATP into environmental monitoring can significantly enhance the methods used to study the negative impacts of tourism on cave ecosystems.

Spatiotemporal Variation in Saline Soil Properties in the Seasonal Frozen Area of Northeast China: A Case Study in Western Jilin Province

Due to the impact of climate change and human activities, the problem of soil salinization is increasingly prominent, posing a threat to the safety of the ecological environment and engineering construction. To understand the development tendency of soil salinization, this paper took the saline soil in Western Jilin province as the research object and carried out a long–term investigation into the basic properties of the soil at several monitoring stations. The results showed that the properties of saline soil in Western Jilin province changed regularly at the spatial and temporal scales. In the longitudinal profile, the water content, soluble salt content, and organic matter content in the soil vary greatly with the seasons at a depth range of 0–50 cm, while their changes below 50 cm are not significant. This is related to the influence depth of the external environment. Meanwhile, the content of sand is relatively stable in the depth direction, mostly between 5 and 15%, while the content of silt and clay fluctuates greatly, and there seems to be a mirror relationship between them. Along the N(W)–S(E) direction, the crystallization proportion of clay minerals gradually increases by about 28% because the relatively humid and hot climate is conducive to mineral crystallization. Over time, in the S(E) study area, the precipitation is relatively abundant, and the shallow soil is desalted due to leaching, resulting in high salt storage in the deep soil. However, in the N(W) study area, salt migrates upwards with water under the dominant effects of evaporation and freeze-thaw, leading to the accumulation of salt in shallow soil and a decrease in salt storage in deep soil. In addition, the saline soil in the study area has strong alkalinity, and the pH increases from 8.2 to 9.8 in the N(W)–S(E) direction. Overall, the soil salinization situation in Western Jilin is not optimistic.

The Effect of Clay Content on the Dilatancy and Frictional Properties of Fault Gouge

AbstractMature fault cores are comprised of extremely fine, low permeability, clay‐bearing gouges. Saturated granular fault materials are known to dilate in response to increases in sliding velocity, resulting in significant pore pressure drops that can suppress instability. Up to now, dilatancy has been measured predominantly in clay‐poor gouges. Clay minerals have low frictional strengths and, in previous experiments, even small proportions of clay minerals were shown to affect the frictional properties of a fault. It is important, therefore, to document in detail the impact of the proportion of clay on the frictional behavior and dilatancy of fault rocks. In this work, a suite of triaxial deformation experiments elucidated the frictional behavior of saturated, synthetic quartz‐clay (kaolinite) fault gouges at effective normal stresses of 60, 25, and 10 MPa. Upon a 10‐fold velocity increase, gouges of all clay‐quartz contents displayed measurable dilatancy with clay‐poor samples yielding comparable changes to previous studies. Peak dilation did not occur in the pure quartz gouges, but rather in gouges containing 10 to 40 wt% clay. The clay content of the simulated gouges was found to control the gouge frictional strength and the stability of slip. A transition occurred at ∼40 wt% clay from strong, unstably sliding quartz‐dominated gouges to weak but stably sliding clay‐dominated gouges. These results indicate that in a low permeability, clay‐rich fault zone, the increases in pore volume could generate pore‐fluid pressure transients, contributing to the arrest of earthquake nucleation or potentially the promotion of sustained slow slip.

Study on the Soil Deterioration Mechanism in the Subsidence Zone of the Wildcat Landslide in the Three Gorges Reservoir Area

The stability of soil mass near the dam bank in the Three Gorges Reservoir is closely related to the periodic variation in the reservoir water level. In order to study the influence of water level variation on soil mass, the soil mass in the water level fluctuation zone of the Wildcat landslide was taken as the research object, and the mechanism of soil mass deterioration in this area was revealed by comparing the strength and mineral structure characteristics of soil mass at different elevations by means of macro- and meso-microscopic analysis. The results show the following: (1) With the increase in sampling elevation, the natural water content of the soil decreases, and the dry density of the soil is a minimum when the elevation is 155 m and at a maximum when the elevation is 175 m. (2) The soil mass in the water dissipation zone of the Wildcat landslide can be divided into three areas: When the elevation is 145–155 m, the fractal dimension increases, the soil fragmentation increases, the cohesion decreases, and the soil deterioration increases. When the elevation is 155–175 m, the fractal dimension decreases, the soil fragmentation decreases, the cohesion increases, and the soil deterioration weakens. When the elevation is greater than 175 m, there is no soil deterioration. (3) X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) were used to test the soil’s mineral composition and pore size at different elevations. It was found that the main reason for the severe deterioration of macro-strength parameters of the soil at the elevation of 155 m was that the proportion of clay minerals and quartz was at the lowest, and the proportion of medium pores and large pores was at the highest. (4) Through the combination of macro and mesoscopic testing and analysis, it was found that the rise and fall of the reservoir water level will lead to the strong chemical action of the skeleton and cemented mineral dissolution in the soil degradation-enhanced area, as well as the physical action of soil particles, resulting in the formation of more medium pores and large pores in the soil and eventually the formation of seepage channels.

Nickel and Chromium Origin in Fluvisols of the Petruševec Well Field, Zagreb Aquifer

Soil plays an important role in the accumulation and transport of potentially toxic elements (PTEs), from surface into aquifer. PTEs can get to the environment naturally, but also from different kinds of contamination sources. In this study, a soil profile located in the vicinity of well field Petruševec, one of the most important well fields related to the public water supply of the City of Zagreb, was analyzed. The main aim of this study was to determine soil properties which can influence retention/mobilization of Ni and Cr in alluvial soil, as well as to define their origin in the investigated soil profile. Results suggest that Cr is geogenic, while Ni is probably of dominantly anthropogenic origin. Observed concentrations, enrichment factors and Igeo values showed no enrichment for Cr, while for Ni, they showed minor to very severe enrichment, i.e., that in some soil horizons, moderate to strong pollution exists. Evaluation of wind directions and location of possible contamination sources that prevail in the study area suggest that Ni can come by aerodeposition from different sources. Results showed that mineral composition can have important influence on retention of analyzed PTEs. Soil horizons, which have very high concentrations of Ni, in general have higher proportion of clay minerals, especially chlorites, as well as Fe oxyhydroxides which can act as an adsorption phase for the investigated PTEs. Results suggest that more detailed research about the investigated PTEs presents a necessity if measures for soil and groundwater protection want to be effectively implemented.

Genesis of clay minerals and its insight for the formation of limestone marl alterations in Middle Permian of the Sichuan Basin

Limestone marl alternations (LMAs) usually contain abundant amounts of clay minerals. A set of Permian LMA source rock gas reservoirs controlled by clay transformations has been found in South China. Apart from the common detrital clay minerals that are rich in LMAs of other regions, Mg-phyllosilicates widely occur in South China. The dilution cycle of detrital clays, sedimentary and diagenetic conditions of authigenic clays, and paleoclimatologic indicators all provide references for understanding the diagenesis and periodic deposition of LMAs. To further understand the formation of the Middle Permian LMAs in South China, in addition to petrological and mineralogical methods, which included field investigations, core and thin section observations, scanning electron microscopy (SEM) and X-ray diffraction (XRD), elemental (major element, trace element and rare earth element) analyses were used to determine the genesis and proportions of clay minerals. Next, a discussion is provided regarding the formation of LMAs in view of the above findings. Based on wavelet matching the GR logs, the original sedimentary and diagenetic differences and rhythmic alterations of LMAs are clarified.The clay minerals in the study area consist mainly of detrital smectite, I/S and illite and authigenic talc and Mg-smectite that transformed from sepiolite, in which Mg originated from high-Mg2+ parent calcites and Si was due to seawater mixing with <∼5% terrigenous and <∼0.05% hydrothermal contributions. Compared with limestone beds, marl beds contain more quartz, detrital or authigenic clay minerals (with authigenic clays being present in higher proportions). Although the dominant detrital clay minerals consist of I/S and illite in both limestones and marls, smectite accounts for a higher proportion in marl layers. By referring to the geochemical analyses, it is certain that the higher smectite contents in the marls indicate a more humid climate. The coordinated variations in quartz or talc contents and paleoclimates match the periods of the wavelets, showing that short eccentricity (Milankovitch cycle) forced intensive weathering and increased rainfall and runoff during the deposition of marl beds, which formed a silica-rich environment and increased the introduction of high Mg2+ calcites, aragonite and detrital clay minerals (smectite) into marl layers. Due to the preferential dissolution of aragonite and stronger pressure dissolution during diagenesis, the greater amount of Mg2+ that was produced promoted the formation of Mg-phyllosilicates.

Geological characterization of natural gas hydrate bearing sediments and their influence on hydrate formation and dissociation

Lithology of host sediments associated with gas hydrate reservoirs have a significant effect on the hydrate formation. The Krishna-Godavari (K-G), Mahanadi and Andaman basin in India have been proven to be rich in natural gas hydrate. In this study, a detailed investigation was carried out for the mineralogical analysis of the sediments collected from K-G, Mahanadi and Andaman basin along with hydrate formation in sediments. The study reveals that, the majority of the minerals in K-G basin samples are clay minerals and quartz. The presence of pyrite crystal (FeS2) in the K-G basin sediment is the indication of anoxic and sulfide environment. Spares fragments of microfossil like diatoms and spicules of fresh-water sponges have been identified from K-G basin sediments. Mahanadi basin is also consisting of clay minerals & quartz, and proportion of clay minerals are higher than K-G basin sediment. Andaman basin is mainly composed of clay and calcite minerals with little amount of quartz and feldspar. The rheological behavior of suspensions of sediments in water was investigated as it has significant effect on the drilling operation through the formation. Considering the similar mineralogical composition, gas hydrate formation and dissociation behavior were investigated in colloidal suspension containing sediments collected from K-G basin. Hydrate formation studies indicated that the presence of sediment solution significantly decreased pressure needed for hydrate formation compared to formation in pure water alone. It was observed that the salinity has delayed the hydrate formation as salt is considered as hydrate inhibitor. The rate of hydrate formation was observed to be high in case of sediment aqueous solution than the pure water. Whereas, in case of sediment solution with salinity shows lower formation rate than both pure water and sediment aqueous solution. The studies are relevant to the origin and stability of natural gas hydrate deposits known to exist in deep permafrost and marine sediments, where the effect of sediment surfaces is largely unknown.

Floc structure and dewatering performance of kaolin treated with cationic polyacrylamide degraded by hydrodynamic cavitation

At present, efficient dewatering of mineral-processing–plant tailings still presents a serious challenge, especially for tailings with a high proportion of clay minerals. In this study, cationic polyacrylamide (CPAM) was degraded using hydrodynamic cavitation and was used for the flocculation and dewatering of kaolin. The properties of CPAM after degradation were characterized using a rheometer. At the same time, focused beam reflectance measurement, particle video microscopy, and low-field nuclear magnetic resonance were used to study the flocculation and filtration characteristics of kaolin treated by degraded CPAM. The results showed that with an increase in the cavitation time, the viscosity of CPAM decreases gradually and a flocculant mixture system with multimolecular weight is formed. In the process of kaolin flocculation, the pretreated CPAM makes the flocs smaller and denser, resulting in the acceleration of kaolin filtration.

Comprehensive microstructural characterization of saline–alkali soils in the Yellow River Delta, China

ABSTRACT Soil salinization in the Yellow River Delta, China, is a matter of concern from agricultural and environmental perspectives. This study characterizes the relationship between the permeability and microstructure of saline–alkali soil. Soil microstructure, including mineral composition, pore distribution, and particle morphology of soils at different depths, was analyzed using X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy. Results indicated that the soil mean saturated hydraulic conductivity and porosity, excluding the surface soil, were only 6.0 × 10−5 cm/s and 33.0%, respectively, and the mean proportion of clay minerals (illite, chlorite and kaolinite) was 6.1%. The pore distribution results showed that small pores (2 ≤ d< 10 μm) and micropores (0.1 ≤ d< 2 μm) constituted the majority of the soil pores. We attributed the deterioration of the soil permeability to low soil porosity, which, in turn, was associated with soil pore size distribution and a closely inlaid ‘brick wall’-type particle arrangement. Quantitative analysis of the scanning electron microscopy images showed that the fractal dimension of pores and roundness of particles increased with depth, whereas the fractal dimension of particles and roundness of pores decreased with depth. However, the azimuth, perimeter, Fred diameter, and orientation of saline–alkali soil did not significantly change with the increase in depth. This study revealed that the microstructure of saline–alkali soil exhibits a good correlation with its permeability.

Mechanical Properties of Mortars Based on Roman Cement with the Addition of Power Plant Fly Ash

Roman cement is a historic hydraulic binder, prepared by firing of limestone with high proportion of clay minerals, usually at temperatures below the sintering point. The firing temperatures of Roman cements range from 800 to 1200°C. At present, this binder is considered to be a promising material with a low carbon footprint and a long lifetime. The properties of Roman cement are influenced by many factors, especially raw materials composition and firing conditions. On the other hand, a disadvantage of mortars based on Roman cement is a slow increase of initial strength. In this paper, the influence of power fly ash admixture on the dynamics of initial strength increase was experimentally investigated. The characteristics of mortars with admixtures were compared with the properties of reference mortar without admixtures. The Roman cement Vicat Prompt TM Natural Cement (manufactured by Vicat SA, French company) was used as a binder. Based on the results of the experiment it can be stated that mortars with the admixture of power plant fly ash show higher dynamics of the increase of initial strengths than the reference mixture.

Selective adsorption of CO2/CH4 mixture on clay-rich shale using molecular simulations

Shale rocks comprise of a lager proportion of clay minerals; shale gas potentially adsorbs on the clay minerals due to its large specific surface area. In this work, selective adsorption behaviors of CO2/CH4 are investigated in clay-mineral nanopores using the grand canonical Monte Carlo (GCMC) simulation method. In addition, the subsequent implications for CH4 recovery and CO2 sequestration are summarized. Results show that CH4 exhibits the strongest adsorption on kaolinite, followed by that on illite and montmorillonite, while adsorption capacity of CO2 is calculated in the order of montmorillonite > illite ≈ kaolinite, suggesting that CO2 injection has the best performance for CH4 recovery when implemented in montmorillonite-rich shale reservoirs. Comparatively, smaller pores are dominated by the adsorbed gas and are also favorable for CO2 sequestration. The adsorption selectivity of CO2 over CH4 on clay minerals is predicted in the order of kaolinite < illite < montmorillonite; moreover, the adsorption selectivity is reduced as pressure decreases for illite and montmorillonite, implying that the injection pressure of CO2 should be low enough to achieve an efficient CH4 recovery and CO2 sequestration. However, the ideal injection pressure of CO2 should be around the pressure where it has the maximum adsorption selectivity for kaolinite-rich shale. This study may achieve a deeper understanding of the adsorption behavior of CO2/CH4 in clay-rich gas-bearing reservoirs; furthermore, it may provide the guidelines for future optimization design of CO2 injection for CH4 recovery and CO2 sequestration in the field applications.

Mud deposition and diagenesis within an Early Palaeozoic clinothem: Power Steps Formation, Newfoundland, Canada

AbstractThe early Ordovician (∼385 Ma) Power Steps Formation, Newfoundland, Canada, exposes a well-preserved mudstone-dominated clinothem that serves as an excellent archive for understanding how mud has been produced, transported and converted into mudstone prior to the evolution of globally widespread, deep soil horizons. Sedimentological analysis of four sandstone and five mudstone facies, along the Ochre Cove clinothem, reveal that mud and sand were delivered by unidirectional currents and experienced episodic reworking by storm waves. Petrographic examination and X-ray diffraction from described mudstone facies reveal significant variability in the distribution of illite versus chlorite between the lower and upper part of the Ochre Cove clinothem. This research highlights that in the present-day clay mineral fraction, illite is often detrital whereas chlorite originated via the alteration of silt-sized, highly unstable, mafic (volcanoclastic?) grains. Throughout all sedimentologic facies, albeit in different proportions, these mafic lithic grains were diagenetically altered via in situ weathering before significant compaction occurred, resulting in the precipitation of significant volumes of pore-bridging, silica- and iron-rich chlorite cement. Compositional, diagenetic and textural attributes across the Ochre Cove mud clinothem vary as a function of starting composition, hydrodynamic sorting and grain density. Given that a significant proportion of clay minerals has been generated via in situ transformation of a mafic, non-stable precursor assemblage, we recommend future studies to incorporate detailed petrographic description along with X-ray diffraction analyses when aiming to employ trends in whole-rock clay mineral data as a proxy in provenance and palaeoclimate studies of very old (pre-Devonian) mudstones and sandstones.

Shear Strength of Compacted Clays as Affected by Mineral Content and Wet‐Dry Cycles

The behaviors of high‐plasticity clays depend largely on the clay mineral content. Recently, it has been observed that sudden slope failures of most clay slopes occur in regions pronounced with repeated rainfall and sunny climate. The reason for this is still unclear. Examining the effect of clay minerals and drastic weather changes on shear strength will be useful in predicting the performance of structures built in such soils and to take precautionary measures to improve the properties before failure. Therefore, a series of quick direct shearing tests were conducted on 11 artificial clay mixtures. The cohesion and frictional strength properties were determined and linked to the proportion of clay minerals and the number of wetting and drying cycles. The results show a significant reduction in shear strength after exposure to wetting and drying. Generally, montmorillonite‐dominated mixtures were less susceptible to the changes in cohesion strength than kaolin‐dominated mixtures, and the reduction in frictional strength was relatively insignificant.

Pore-scale study of the pressure-sensitive effect of sandstone and its influence on multiphase flows

The pressure-sensitive effect on the pore structure of sandstone was investigated using X-ray computed micro-tomography and QEMSCAN quantitative mineral analysis. In a physical simulation study, we extracted the pore network model from digital cores at different confining pressures and evaluated the effect of pressure sensitivity on the multiphase displacement process. In both the pore network model and QEMSCAN scanning, the pore structure was observed to be damaged under a high confining pressure. Due to their different scales, the pores and throats exhibited inhomogeneous changes; further, the throats exhibited a significant variation compared to that exhibited by the pores. Meanwhile, the heterogeneity of the pore structure under the two aforementioned activities was aggravated by the elastic–plastic deformation of the pore structure. The pressure-sensitive effect increased the proportion of mineral particles, such as quartz (the main component of the core skeleton), and reduced the proportion of clay minerals. The clay minerals were originally attached to the pore walls or interspersed in the pores; however, as the pressure increased, the clay minerals accumulated in the pores resulting in blockage of the pores. While simulating the multiphase displacement process, increasing the confining pressure was observed to severely restrict the flowability of oil and water. This study promises to improve the efficiency of reservoir development in terms of oil and gas exploitation.