Illite Surfaces Research Articles

Elucidating Wettability Alteration on Clay Surface Contacting Mixed Electrolyte Solution: Implications to Low-Salinity Waterflooding

Summary Many mechanisms have been proposed in the literature to explain wettability alteration at low-salinity waterflooding (LSWF). Some of these mechanisms include electrical double layer (EDL) expansion, multi-ion exchange, and cation hydration. However, no consensus has been reached on which one is the key mechanism in low-salinity enhanced oil recovery (EOR). Moreover, the mechanisms by which low-salinity flooding can enhance oil recovery are poorly understood. Parameters such as salinity, electrolyte type, oil components, and presence of clay minerals are often associated with the degree to which the injection of low-salinity water increases oil production. Therefore, an investigation of the geochemistry of the clay/fluid interface is crucial to understand the role of petrophysical properties such as wettability on oil production. We use molecular dynamics (MD) to (i) quantify the impacts of different types of oil components, electrolytes, and its mixture at varying ionic strengths on interfacial properties and (ii) investigate wettability alteration by means of water adsorption quantification. We investigate the brine/clay and oil/brine/clay interfacial interactions involved in the adsorption of ions and water molecules onto the clay surface. Clay is represented by illite, and brine is composed of water molecules and different electrolyte types such as NaCl, CaCl2, and their mixtures at varied concentrations. The oil components investigated in this work include decane (C10H22), decanoic acid (CH₃(CH₂)₈COOH), and sodium decanoate (C10H19NaO2). These components were selected to represent the range of oil molecules typically found in reservoirs, which include nonpolar, polar, and charged polar molecules. Initially, we set up systems composed of different ion composition and salinity to investigate the effect of these parameters on EDL structure and water adsorption. Then, we created systems composed of mixed electrolyte and different oil components to verify the impacts of these molecules on the oil/brine/clay interface. Finally, we increased the NaCl concentration in the system containing sodium decanoate to investigate the role of Na in wettability alteration. MD simulations were performed at 330 K, and particle density profiles of water, hydrocarbon, and ions inside the illite nanopores were computed. We observed that the ion composition and salt concentration of the systems composed of clay minerals and brine do not result in significant changes in the adsorption planes of cations. For the same systems, we also computed the number of water molecules per unit cell in each hydration layer. We observed that the change in the thickness of these hydration layers is also very small. The results showed that nonpolar-charged hydrocarbon molecules have the lowest mobility, suggesting that this component has a more intense interaction with the illite surface compared with other hydrocarbons. Additionally, snapshots of the simulation indicate that calcium preferentially forms bridges with sodium decanoate molecules compared with other organic components. Despite being widely known as an efficient method for achieving EOR, the underpinning mechanism for wettability alteration at LSWF is still not fully understood. The outcomes of this work improve our understanding of the key parameters affecting interactions at the oil/rock/brine interface during LSWF.

Effects of Microheterogeneous Wettability and Pore-Throat Structure on Asphaltene Precipitation in Tight Sandstone: Results from Nuclear Magnetic Resonance.

The pore throat structure and microheterogeneous wettability of tight sandstone reservoirs are complex, which leads to varying asphaltene precipitation locations, contents, and distributions in different pores during CO2 flooding. Clarifying the heterogeneous wettability of different pore throat structures and their effects on asphaltene precipitation and adsorption is crucial for improving CO2 displacement efficiency. A series of experiments were conducted in this study, including X-ray diffraction (XRD), cast thin section (CTS), field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion (HPMI), environmental scanning electron microscopy (E-SEM), nuclear magnetic resonance (NMR), and CO2 flooding experiments, to investigate the pore structure complexity of tight sandstone reservoirs of the Yanchang Formation in the Ordos Basin, China. Furthermore, we investigated the variations in microheterogeneous wettability across diverse pore-throat structures and elucidated the impact of heterogeneous wettability on asphaltene precipitation during CO2 flooding. The findings indicate that the type and configuration of pore throats are crucial factors influencing microheterogeneous wettability. The intergranular pores are dominated by mixed wetting, and most of the dissolution pores exhibit oil wetting. The surface of Illite shows drop-like water under E-SEM, which is mainly oil wetting, whereas the surface of chlorite shows film-like water, which is water wetting. The configuration of chlorite intercrystalline pores and intergranular pores shows water wetting, whereas the configuration of Illite intergranular pores and dissolution pores shows oil wetting. During the CO2 flooding process, asphaltene tends to be adsorbed in the intercrystalline Illite with dissolution pores, reducing the dissolution pore volume and blocking small pores, and the displacement efficiency becomes low. In addition, asphaltene precipitation also occurs in the pore configuration of chlorite intercrystalline and intergranular pores, causing a wetting reversal on hydrophilic mineral surfaces. This reversal increases the pore throat structure complexity but has less of an impact on the flooding efficiency. A high Illite content is more likely to lead to asphaltene precipitation, significantly influencing small pore structure and the oil displacement efficiency.

First Principle Study of the Adsorption of Glyphosate on Illite Clay for Water Depollution

The use of herbicides such as glyphosate in the agricultural sector enables agricultural intensification and thus satisfies the demand for agricultural products on the market. Glyphosate is widely used in agriculture as a herbicide for weeding fields. However, its use has a major drawback that calls into question its many advantages. After use, its toxic residues and metabolites end up in groundwater, which is an important source of drinking water for the population. Adsorption is the most suitable technique for recovering these toxic residues before the water is consumed. Based on Density Functional Theory, implemented in the VASP simulation code, we studied the ability of illite to adsorb glyphosate. The absorbent properties, the abundance of illite throughout the world, particularly in Benin, and the fact that it is free from toxic products were the criteria that guide us in the of illite clay for our work. Our work revealed that the process of adsorption of glyphosate on the illite surface is exothermic. This process does not present any risk of release of toxic by-products on five of the six studied sites. The illite clay can then be used to design water depollution filters to decrease medical as well as financial burden.

The competition of humic acid aggregation and adsorption on clay particles and its role in retarding heavy metal ions

Humic acid (HA) is of great importance in controlling the fate of heavy metals (HMs), however, the pivotal influence of HA aggregation within the HA-clay-HM ternary system on retarding HM mobility remains obscure. This study performed molecular dynamics simulations to delve into the consequences of HA aggregation on the environmental behavior of Cd2+ and Pb2+ (0.1–0.6 M) in the co-existence of illite particles. HA can readily aggregate into clusters, adhering to the illite surface or freely dispersing in the solution. These HA clusters significantly modulate HM mobility, contingent upon their location, arrangement, and interaction with illite. Consequently, HA exhibited a pronounced retardation effect on HM migration, stemming from the competition between HA aggregation and its adsorption on illite. Additionally, the retardation effect of HA aggregation was more obvious for Cd2+ (as compared to Pb2+), owing to its stronger interaction with the functional groups of HA. These findings contribute to the development of potential HA-based strategies for remediation of heavy metal-contaminated sites.

Preparation and Mechanism of Shale Inhibitor TIL-NH2 for Shale Gas Horizontal Wells.

In this study, a new polyionic polymer inhibitor, TIL-NH2, was developed to address the instability of shale gas horizontal wells caused by water-based drilling fluids. The structural characteristics and inhibition effects of TIL-NH2 on mud shale were comprehensively analyzed using infrared spectroscopy, NMR spectroscopy, contact angle measurements, particle size distribution, zeta potential, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy. The results demonstrated that TIL-NH2 significantly enhances the thermal stability of shale, with a decomposition temperature exceeding 300 °C, indicating excellent high-temperature resistance. At a concentration of 0.9%, TIL-NH2 increased the median particle size of shale powder from 5.2871 μm to over 320 μm, effectively inhibiting hydration expansion and dispersion. The zeta potential measurements showed a reduction in the absolute value of illite's zeta potential from -38.2 mV to 22.1 mV at 0.6% concentration, highlighting a significant decrease in surface charge density. Infrared spectroscopy and X-ray diffraction confirmed the formation of a close adsorption layer between TIL-NH2 and the illite surface through electrostatic and hydrogen bonding, which reduced the weakly bound water content to 0.0951% and maintained layer spacing of 1.032 nm and 1.354 nm in dry and wet states, respectively. Thermogravimetric analysis indicated a marked reduction in heat loss, particularly in the strongly bound water content. Scanning electron microscopy revealed that shale powder treated with TIL-NH2 exhibited an irregular bulk shape with strong inter-particle bonding and low hydration degree. These findings suggest that TIL-NH2 effectively inhibits hydration swelling and dispersion of shale through the synergistic effects of cationic imidazole rings and primary amine groups, offering excellent temperature and salt resistance. This provides a technical foundation for the low-cost and efficient extraction of shale gas in horizontal wells.

U(VI) sorption on illite in the Co-existence of carbonates and humic substances

The presence of carbonates or humic substances (HS) will significantly affect the species and chemical behavior of U(VI) in solution, but lacking systematic exploration of the coupling effect of carbonates and HS under near real environmental conditions at present. Herein, the sorption behavior of U(VI) on illite was systematically studied in the co-existence of carbonates and HS including both humic acid (HA) and fulvic acid (FA) by batch technique. The distribution coefficients (Kd) increased as function of time and temperature but decreased with increasing concentrations of initial U(VI), Ca2+, and Mg2+, as well as ion strength. At pH 2.0–10.5, the Kd values first increased rapidly and then decreased visibly, with its maximum value appearing at pH 5.0, owning to the changes in the interaction between illite and the dominant species of U(VI) from electrostatic attraction to electrostatic repulsion. The sorption was a heterogeneous, spontaneous, and endothermic chemical process, which could be well described by pseudo-second-order kinetic and Flory-Huggins isotherm models. When carbonates and HA/FA coexisted, the Kd values always increased first and then decreased as a function of pH, with the only difference for HA and FA being the key pH (pHkey) at which the promoting and inhibiting effects on the sorption of U(VI) onto illite undergo a transition. The carbonates and HS have a synergistic inhibitory effect on the U(VI) sorption onto illite at pH 7.8. FTIR and XPS spectra demonstrated that the hydroxyl groups on the illite surface and in the HS were involved in U(VI) sorption on illite in the presence of carbonates. These results provide valuable data for a deeper understanding of U(VI) migration in geological media.

Flotation behavior and mechanism of microfine apatite and illite in different electrolyte solution systems

The deep sea is rich in mineral resources, and due to the scarcity of freshwater resources, the use of seawater is becoming more widespread. However, the presence of a large number of electrolyte ions in seawater can pose a challenge to mineral flotation and the effects of different electrolytes vary. This study investigated the effects of common chloride (Cl-) electrolyte solutions of Na+, Mg2+ and Ca2+ on the flotation behavior of microfine apatite and illite using sodium oleate (NaOL) as the collector and acidified sodium silicate (ASS) as the depressant. The results showed that ASS can selectively adsorb to the surface of illite as Si(OH)4, hindering the action of NaOL but having little effect on apatite. The electrolyte solution improved the foaming performance of the slurry. It also reduced the critical micelle concentration (CMC) of NaOL and promoted the formation of NaOL micelle, which promoted the adsorption of NaOL on the surface of apatite. However, the reaction of free Mg2+ and Ca2+ in the slurry with oleic anion consumed the effective components of the collector, which was not conducive to mineral flotation. After NaOL interacted with apatite, it had hydrophobic agglomeration effect, which strengthened the flotation recovery of microfine apatite and the separation efficiency from illite.

Hydrocarbon Transportation in Heterogeneous Shale Pores by Molecular Dynamic Simulation.

Shale oil in China is widely distributed and has enormous resource potential. The pores of shale are at the nanoscale, and traditional research methods encounter difficulty in accurately describing the fluid flow mechanism, which has become a bottleneck restricting the industrial development of shale oil in China. To clarify the distribution and migration laws of fluid microstructure in shale nanopores, we constructed a heterogeneous inorganic composite shale model and explored the fluid behavior in different regions of heterogeneous surfaces. The results revealed the adsorption capacity for alkanes in the quartz region was stronger than that in the illite region. When the aperture was small, solid-liquid interactions dominated; as the aperture increased, the bulk fluid achieved a more uniform and higher flow rate. Under conditions of small aperture/low temperature/low pressure gradient, the quartz region maintained a negative slip boundary. Illite was more hydrophilic than quartz; when the water content was low, water molecules formed a "liquid film" on the illite surface, and the oil flux percentages in the illite and quartz regions were 87% and 99%, respectively. At 50% water content, the adsorbed water in the illite region reached saturation, the quartz region remained unsaturated, and the difference in the oil flux percentage of the two regions decreased. At 70% water content, the adsorbed water in the two regions reached a fully saturated state, and a layered structure of "water-two-phase region-water" was formed in the heterogeneous nanopore. This study is of great significance for understanding the occurrence characteristics and flow mechanism of shale oil within inorganic nanopores.

Speciation studies at the Illite - solution interface: Part 2 – Co-sorption of uranyl and phosphate ions

Although it is well known that clays and phosphate ions (hereafter referred to as “P”) can contribute to the long term uptake of uranium (U) in a variety of geochemical systems, work is still needed to document the surface speciation of U(VI) and P sorbed on relevant clay minerals such as illite. Following on from previous work showing strong sorption of phosphate ligands on the surface of a homoionic Na-illite ([1]), we addressed the (competitive/synergistic) mechanisms of (co)sorption of trace levels of uranyl ions (1–10 µM) and phosphate ligands (100 µM) onto this clay, and the identity of the surface species formed, by in situ spectroscopy monitoring of the clay-solution interface along sorption. We also performed complementary batch sorption experiments and electrophoretic mobility (EM) measurements. Macroscopic data indicated a uranyl sorption dependent on pH, aqueous U concentration and clay-to-solution ratio, a signature of P on the U sorption, and a promotion of P sorption with U concentration. Macroscopic and EM data also suggested mostly reversible mechanisms of P and U co-sorption that confer negative charges on the mineral surface and involve several types of uranyl phosphate surface species and/or surface sites present on the clay edges. FTIR interface spectra confirmed that several types of inner sphere uranyl phosphate surface species formed at acidic pH at the illite-solution interface, as a function of U surface coverage and/or reaction time. An inner-sphere uranyl phosphate surface complex, likely with a U-bridging structure, was formed rapidly on high-affinity surface sites existing in limited quantities on the clay edges. Another U-P surface complex (with a possible P-bridging structure) was progressively formed in increasing amounts with U surface coverage and time on low affinity clay edge sites. Finally, a third U-P surface species having an autunite-like structure (probably a U-P polynuclear surface species) was formed on the illite surface at high U concentration (10 µM). The two later species competed with the existence on the illite surface of inner sphere surface complexes and outer-sphere surface complex of phosphate, which were identified in the absence of U. Information on uranyl surface speciation in the presence of phosphate ligands presented here is novel and provides for the first time in situ spectroscopic evidence of synergistic U-P sorption process leading in the formation of several types of uranyl phosphate sorption species on the surface of illite. These results provide a sound basis for better understanding / prediction of U sorption in the environment, including in clayey formations being considered as long term barriers to radionuclide migration in far field of high level radioactive waste repository.

Transient of flow regimes and slip boundary analysis of water and gas in nano clay pores

ABSTRACT Nanoscale two-phase flow is important during shale gas production. In this work, molecular dynamics simulations are employed to investigate fluid flow regimes of water and methane under different pressure gradients inside the nanoscale slit pores. The illite mineral is selected to model the clay pores based on the Small-Angle X-ray Diffraction (SAXD) experiment. The different flow regimes of gas-water two-phase are visualised and the structures are analysed. As the driving pressure gradient increases, it is observed that the two-phase flow in the slit undergoes transitions from intermittent flow to a transition stage between intermittent flow and annular flow, and finally fully converts to annular flow. The slip conditions during the two-phase flow on the hydrophilic illite surface are investigated. We found that the negative slip length stays around 0.25 nm within the range of practical production pressure gradients, which can be used for accurate production prediction. This work provides detailed insights into the nanoscale water and gas two-flow inside the clay nanopores during shale gas exploration.

Efficient removal of cationic dye from wastewater using novel low-cost adsorbent, cellulose-clay composite: Insights from isotherm, kinetic, thermodynamic, and molecular dynamics simulation studies

This study was aimed to evaluate the potential of a natural clay modified with cellulose acetate polymer (CA) as a sustainable and cost-effective material for removing cationic dyes, particularly crystal violet (CV), from aqueous solutions. The cellulose acetate-clay composite (CA-Clay) was created by mixing CA with natural clay and characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscope (SEM) and X-ray fluorescence (XRF) techniques. Batch adsorption studies were conducted to investigate the impact of contact time (10–360 min), initial concentration (50–500 mg/L), temperature (293–318 °K), and pH (2–12) on the removal of CV from aqueous solutions using both CA-Clay and raw clay. Molecular Dynamic simulations (MDS) were employed to explore the adsorptive properties of the surfaces of illite and kaolinite clay fractions and the adsorption behavior of CV on both modified and unmodified clay fractions with (CA). The results showed that the maximum equilibrium adsorption capacity for the CA-Clay and raw-Clay was 67.35 mg/g and 47.62 mg/g, respectively. The kinetic behavior and the isotherms were satisfactorily fitted with the pseudo second-order and Langmuir models, respectively. The thermodynamic parameters showed that the adsorption of CV onto CA-Clay and raw clay was a spontaneous process, and the Gibbs energy increased with increasing temperature. Molecular Dynamic simulations revealed that the higher negative value of the calculated interaction energy in the presence of the (CV-CA-Illite-Water) system was attributed to electrostatic interactions and hydrogen bonds formed between CV molecules and both the illite surface and CA oxygen atoms. Overall, this study suggests that CA-Clay is a promising adsorbent for the removal of cationic dyes from wastewater and provides insights into the adsorption mechanisms and thermodynamic parameters of CV adsorption onto CA-Clay and raw clay.

Molecular simulation on CO2/H2S co-adsorption in organic and inorganic shale nanopores

Geological CO2 sequestration (GCS) in depleted shale reservoirs is recognized as one of promising techniques to mitigate CO2 emissions, while the high cost of CO2 purification greatly hinders its deployment. Co-injection of CO2/H2S mixtures has been proposed as an economical alternative to further promote GCS application. In this work, we used Grand Canonical Monte Carlo (GCMC) and molecular dynamic (MD) simulations to investigate CO2/H2S mixture adsorption in organic and inorganic shale nanopores (at 333.15 K and pressure up to 50 MPa). CO2 and H2S present distinct adsorption behaviors on the different surfaces. H2S adopts unordered structures on kerogen surfaces, but a perpendicular orientation on illite due to H-bonding, which also results in higher adsorption, while CO2 is preferentially distributed parallel along the surfaces. Furthermore, CO2 and H2S accumulate in the concave parts of kerogen surfaces, while they concentrate around the tetrahedral aluminum (AT) atoms and K+ cations on illite surfaces. The adsorption selectivity of H2S over CO2 is always greater than 1, indicating the stronger adsorption propensity of H2S. The preferential adsorption of H2S on illite surface is facilitated by the stronger dipole–dipole interactions. PMF results show that the binding interaction between CO2 or H2S and illite is higher than on kerogen, and H2S always shows a stronger interaction than CO2. Overall, the total CO2 and H2S adsorption is higher in illite nanopores, implying a new promising storage method. This study provides important insights into the competitive adsorption mechanisms of CO2 and H2S in depleted shale reservoirs and sheds light on how sour gas geological sequestration capacity can be further improved.

Investigation of the adsorption of nonpolar adsorbate molecules on the illite surface

Investigation of the adsorption of nonpolar adsorbate molecules on the illite surface

Retention of organic matter on the surface of illite particle under the influence of different cations: A molecular dynamics simulation study

Retention of natural organic matter (NOM) in soils is of great importance in many physicochemical and biochemical processes as well as carbon cycling, however, the effect of cations on such a retention behavior and the underlying mechanism are still elusive. This issue is difficult to be solved using traditional experimental tools, while it is the advantage of molecular dynamics (MD) simulation comes in. Here, we used MD simulation to investigate the retention of TNB (Temple-Northeastern-Birmingham) molecules on illite surface, considering four cation types (K+, Na+, Mg2+, and Ca2+) and three ionic concentrations (0.1, 0.3, and 0.6 mol/L). Different cations greatly affected the adsorption capacity and dynamics of TNB clusters on illite substrate, and the TNB retention ability decreased as Ca2+ > Mg2+ > Na+ > K+. This was achieved by regulating the TNB-illite interaction instead of the TNB-cation interaction. TNB molecules interacted with all four types of cations mainly through carboxyl groups, which were also the main functional group participating in the TNB-illite interaction in the presence of monovalent cations. However, the importance of carboxyl groups in interacting with illite surface decreased with increasing ionic concentration in the presence of divalent cations. There were two kinds of mechanisms for the retention of TNB: hydrogen-bond and cation bridge, but their significance was influenced by cation type and concentration. Two kinds of cation bridges were observed: ternary OS − Cation−COO−TNB complexes for K+, Na+, and Mg2+ while quaternary OS − H2O − Cation−COO−TNB complexes for Ca2+. In addition, suitable amounts of Ca2+ were beneficial to the retention of natural organic matter in soils with high charged minerals, while a too high Ca2+ concentration following an unscientific fertilization may lead to a loss of organic matter in these soils. This study provides a new insight in understanding the stability of organic-mineral association and the soil organic carbon sequestration.

Quantifying Interfacial Interactions Between Minerals and Reservoir/Fracturing Fluids

Chemical interactions between the injected fluids and the minerals during the fracturing process can affect fluid flow and production. However, there is still a need to understand the impact of geochemistry on the interactions at the rock-fluid interface and how these interactions affect the wettability of the rock and fluid flow in organic-rich mudrocks. In this paper, we quantify the mineral-fluid affinity by performing adsorption calculations. Molecular dynamics simulations (MDS) are carried out to (i) quantify the adsorption of fracturing fluids on the surface of minerals, (ii) perform sensitivity analysis on the composition of fracturing fluid and reservoir temperature on adsorption and mobility, and (iii) analyze the spatial distribution of water and chemicals on mineral surfaces. The minerals evaluated include illite and calcite, and the fracturing-fluid components are methanol, citric acid, sodium chloride, and water. We evaluate the effect of each chemical separately. First, systems composed of mineral surfaces in contact with brine are generated. Then, we analyze the mineral in contact with a solution containing brine and methanol and with a solution composed of brine and citric acid. MDS are carried out in the canonical (NVT) ensemble at the temperature of 330 K to evaluate the adsorption of the fracturing fluid. To quantify the impacts of reservoir temperature, we carry out MDS at a temperature of 360 K. Results suggest that methanol does not have a strong effect on water adsorption and the ion spatial distribution on the mineral’s surface. We found that citric acid tends to form aggregates and that some cations present in the solution might participate in these aggregates. When methanol or citric acid is added to the brine solution, the mobility of both sodium and water on the illite surface decreases. The effects of each additive on the affinity between the mineral and fracturing fluids were also investigated. We found that the number of hydrogen bonds between illite and the fluid did not change when additives were added. However, the number of hydrogen bonds between calcite and the fluid was affected when methanol was added to the system. The quantification of adsorption in the molecular scale provides a fundamental understanding of the electrochemical interactions between the rock surface and the fracturing/reservoir fluids at reservoir conditions, which enables the enhanced design of fracturing-fluid composition for different reservoir types. This information can also be used to quantify the impacts of injected and reservoir fluids on the wettability of the rocks.

Ab initio calculation of the adsorption of As, Cd, Cr, and Hg heavy metal atoms onto the illite(001) surface: Implications for soil pollution and reclamation

Elucidating the mechanisms of heavy metal (HM) adsorption on clay minerals is key to solving HM pollution in soil. In this study, the adsorption of four HM atoms (As, Cd, Cr, and Hg) on the illite(001) surface was investigated using density functional theory calculations. Different adsorption configurations were investigated and the electronic properties (i.e., adsorption energy (Ead) and electron transfer) were analyzed. The Ead values of the four HM atoms on the illite(001) surface were found to be As > Cr > Cd > Hg. The Ead values for the most stable adsorption configurations of As, Cr, Cd, and Hg were −1.8554, −0.7982, −0.3358, and −0.2678 eV, respectively. The As atoms show effective chemisorption at all six adsorption sites, while Cd, Cr, and Hg atoms mainly exhibited physisorption. The hollow and top (O) sites were more favorable than the top (K) sites for the adsorption of HM atoms. The Gibbs free energy results show that the illite(001) surface was energetically favorable for the adsorption of As and Cr atoms under the influence of 298 K and 1 atm. After adsorption, there was a redistribution of positions and reconfiguration of the chemical bonding of the surface atoms, with a non-negligible influence around the upper surface atoms. Bader charge analysis shows electrons were transferred from the surface to the HM atoms, and a strong correlation between the valence electron variations and the adsorption energy was observed. HM atoms had a high electronic state overlap with the surface O atoms near the Fermi energy level, indicating that the surface O atoms, though not the topmost atoms around the surface, significantly influence HM adsorption. The above results show illite(001) preferentially adsorbed As among all four investigated HM atoms, indicating that soils containing a high proportion of illite might be more prone to As pollution.

Mineralization mechanism of carbon dioxide with illite interlayer cations using molecular dynamics simulation and experiments

Clay minerals can be identified as a prospective target for long-term CO2 sequestration due to their accessible interlayer cations and periodic sheet structure. Understanding the reactive motion of mineral and fluids has dual advantages of resources and environment. To clarify the storage mechanism, an alternative strategy for CO2 mineralization was investigated through molecular dynamics (MD) simulation and scCO2H2Oillite experiments. The MD simulation predicts the protonation of non-bridging oxygen (NBO) at the illite surface in the first picoseconds, resulting in HCO3- ion formations via the bonding between CO2 molecules and hydroxyl group dissociated from H2O molecules. Surface protonation leads to interlayer K+ cations hopping to the illite/fluids interface since the middle stage, mainly after 1 ns of the reaction. The leached K+ cations bond with the HCO3- ions and later interact with the hydroxyl groups, forming K2CO3 molecules at the interface. In accordance with the experimental results, the K+ cations’ concentration in the filtrates progressively increases throughout the reaction. Results of SEM-EDS, Raman and XPS measurements find that free CO2 clusters in contact with the leached interlayer cations can be converted into carbonate species through the mineralization reaction, precipitating at the surface and thus inducing interlayer swelling. These observations reveal that the clay-related mineralization is estimated to undergo an accumulated process, accessibly enhancing the amount of captured CO2. This is a new report that demonstrates the mechanism of CO2 mineralization in clay minerals, presenting a potential solution for CO2 sequestration enhancement. Insights into K+ cations leaching and mineralization kinetics and their underlying mechanisms during the scCO2H2Oillite reaction is a matter of generalization in clay minerals for CO2 storage.

Effects of the Layer Charge Location and Interlayer Cation on Rectorite Swelling Properties: Comparison between Molecular Dynamics Simulations and Experiments

Understanding the swelling properties of clay minerals is of great significance to environmental science and engineering applications, including environmental remediation, oil and gas wellbore collapse, geological disasters, and commercial additives. The vast majority of experimental and molecular simulation studies have mainly focused on pure clays. However, 70% of clays are mixed-layer clays (MLCs), and the most frequent MLCs is illite-montmorillonite (I-MMT). In this research, grand canonical Monte Carlo and molecular dynamics simulations were used to study the effects of the layer charge location (tetrahedral and octahedral substitution), interlayer cation type (Na+, Cs+, and Ca2+), and water contents on the swelling and hydration properties of rectorite (R1 type I-MMT) and compared with the experimentally measured basal spacing by water vapor adsorption and X-ray diffraction. It turned out that the free swelling ratio was positively correlated with water contents. The order of the three cations’ free expansion rate was Na+, Ca2+, and Cs+. The swelling process of rectorite (REC) was revealed from porosity changing with water contents. The surface of tetrahedral substitution (T-substitution) clay was more likely to bind to interlayer cations to form an inner-sphere surface complex (ISSC), the number of fully hydrated cations of the T-substitution clay was less than that of octahedral substitution (O-substitution), and REC was more swelling in O-substitution, which was supported by the basal spacing, atomic density distribution, and radial distribution function. The cation radius, the hydration energy, and the preference of the inner/outer-sphere surface complex (ISSC/OSSC) all affected REC swelling. Because of the presence of asymmetric charges in the REC layer, interlayer cations tended to distribute on the surface of illite and formed an ISSC when they formed 1-, 2-, and 3-layer (1, 2, and 3 W) hydrate states. When the layer charge density of illite and Mmt was constant, only the layer charge position in Mmt was changed, and the REC swelling behavior was transformed into pure Mmt. The interlayer cations and the layer charge location significantly impacted the REC swelling.

Role of chemical cementation and hydration inhibition on wellbore stability in hydrate bearing sediment: Experimental and molecular dynamics simulation studies

As a sustainable alternative energy source, natural gas hydrate exploitation is limited by the risk of wellbore instability. The hydrate is usually distributed in unconsolidated shallow sediment with high clay content. A modified polyvinyl alcohol was proposed as a novel drilling fluid additive that could increase wellbore stability through chemical cementation and hydration inhibition. Under the action of modified polyvinyl alcohol, the linear expansion rate of the in situ deep-sea soil core was reduced from 18% to 1.9%, which remained the original shape after immersion for 24 h. The adsorption behavior was studied by atomic force microscopy, contact angle measurement and molecular dynamics simulations. The adhesion force was increased from 143.07 to 391.13 kcal/mol on the illite surface compared to the quartz surface by introducing positive functional groups in the modification process. The hydration peak decreased from 4.39 to 3.68 on the illite surface by occupying the interaction site between the rock surface and water molecules. This study provides a new method to ensure the safety of drilling and exploitation of natural gas hydrate and the development of drilling fluid additives for hydrate bearing sediment.

Feasibility study on using CO2-rich industrial waste gas replacement for shale gas exploration based on adsorption characteristics

Clean utilization of CO2-rich industrial waste gas (CO2-rich IWG) is of great significance to realize carbon neutralization. Due to the strong adsorption capacity of clay and organic matter in shale to CO2, it has a good potential for CO2-rich IWG storage and replacement. So, the feasibility of using CO2-rich IWG replacement for shale gas exploration from the perspective of adsorption was studied. A shale composite model was constructed based on the adsorption experiments, and the molecular simulation results were fitted with experiments for the model reliability. Then, the slit models of different minerals were constructed to study the adsorption mechanisms. Finally, the competitive adsorption mechanisms of different components with CH4 were revealed. The results show that the overall adsorption capacity on shale minerals are ranked as HF > CO2 > SO2 > H2S > NO2 > CH4 > CO；Hydrogen bond and electrostatic force are the main adsorption modes of HF, and electrostatic force between negatively charged atoms of gas molecule and mineral surface is the main adsorption modes of CO2, SO2 and H2S. That of NO2, CH4 and CO is the weak van der Waals force. In the competition with CH4, the absolute adsorption capacity of gas decreases, but the relative adsorption capacity increases. The relative adsorption capacities of HF, CO2, H2S and SO2 increase by 35, 26, 15 and 12 times, respectively. During competitive adsorption, negatively charged atoms in HF, CO2, H2S and SO2 are more likely to bind with free cations on the surface of illite and smectite, and generate electrostatic repulsion for C atoms in CH4, leading to the weakening or even disappearance of CH4 adsorption capacity.