Structure and reactivity of the calcite–water interface

CO2 Geostorage and Enhanced Oil Recovery: Challenges in Controlling Foam/Emulsion Generation and Propagation

CO2 injection in subterranean reservoirs for storage, oil recovery, or both is challenging because of its very high mobility. Using a CO2 foam or emulsion is a way to remedy this problem by increasing CO2's apparent viscosity. However, the generation of the foam and its propagation in porous media present several issues that have to be overcome for this process to be economically realistic in practice. For example, it may take time, i.e., a number of pore volumes to be injected, before the foam is created. It is the objective of this Article to investigate these issues thoroughly and to identify the mechanisms underlying them by looking at the effects of various parameters. It is found that surfactant adsorption on the surface of the rock is an important factor involved in the delay of foam formation, but this may not explain all of the results. The nature and morphology of the porous medium may be, in some cases, the dominant factors for foam generation and propagation. From an understanding of the origin of the encountered problem, relevant mitigation strategies are envisioned and evaluated. It is found, for example, that when appropriately formulated and injected with the proper process, foam or emulsion generation is strongly accelerated, which very significantly shortens the delay for achieving CO2 storage.

Interfacial processes underlying the temperature-dependence of friction and wear of calcite single crystals

HYPOTHESIS

This study posits that thermal effects play a substantial role in influencing interfacial processes on calcite, and consequently impacting its mechanochemical properties.

EXPERIMENTS

This work interrogates the temperature-dependence of friction and wear at nanoscale contacts with calcite single crystals at low air humidity (≤ 3-10 % RH) by AFM.

FINDINGS

Three logarithmic regimes for the velocity-dependence of friction are identified. BelowTc ∼ 70 °C, where friction increases with T, there is a transition from velocity-weakening (W1) to velocity-strengthening friction (S1). AboveTc ∼ 70 °C, where friction decreases with T, a second velocity-strengthening friction regime (S0) precedes velocity-weakening friction (W1). The low humidity is sufficient to induce atomic scale changes of the calcite cleavage plane due to dissolution-reprecipitation, and more so at higher temperature and 10 % RH. Meanwhile, the surface softens above Tc -likely owing to lattice dilation, hydration and amorphization. These interfacial changes influence the wear mechanism, which transitions from pit formation to plowing with increase in temperature. Furthermore, the softening of the surface justifies the appearance of the second velocity-strengthening friction regime (S0). These findings advance our understanding of the influence of temperature on the interfacial and mechanochemical processes involving calcite, with implications in natural processes and industrial manufacturing.

Surface charge change in carbonates during low-salinity imbibition

Optimizing the injection water salinity could present a cost-effective strategy for improving oil recovery. Although the literature generally acknowledges that low-salinity improves oil recovery in laboratory-scale experiments, the physical mechanisms behind it are controversial. While most experimental low-salinity studies focus on brine composition, this study investigated the influence of carbonate rock material on surface charge change, wettability alteration, and spontaneous imbibition behavior. Zeta potential measurements showed that each tested carbonate rock material exhibits characteristic surface charge responses when exposed to Formation-water, Seawater, and Diluted-seawater. Moreover, the surface charge change sensitivity to calcium, magnesium, and sulfate ions varied for the tested carbonate materials. Spontaneous imbibition tests led to high oil recovery and, thus, wettability alteration towards water-wet conditions if the carbonate-imbibing brine system's surface charge decreased compared to the initial zeta potential of the carbonate Formation-water system. In the numerical part of the presented study, we find that it is essential to account for the location of the shear plane and thus distinguish between the numerically computed surface charge and experimentally determined zeta potential. The resulting model numerically reproduced the experimentally measured calcium, magnesium, and sulfate ion impacts on zeta potential. The spontaneous imbibition tests were history-matched by linking surface charge change to capillary pressure alteration. As the numerical simulation of the laboratory-scale spontaneous imbibition tests is governed by molecular diffusion (with a time scale of weeks), we conclude that molecular diffusion-driven field scale wettability alteration requires several hundred years.

Salinity and pH effects on water-oil-calcite interfaces by using molecular dynamics

Smart water injection is a technology that allows changing the wettability of the oil rock by injecting water at different salinities, in a cheap and environmentally friendly way compared to other traditional methods. In this study, the individual effect of some typical salts on the wettability of the (104) surface of calcite toward non-polar and polar crude oil models was explored by molecular dynamics as a function of the salinity and pH. The results obtained show that the electrical double layer plays a principal role in the detachment of crude oil models. The divalent ion salts, i.e., CaCl2, CaSO4, MgCl2, and MgSO4, do not form the electrical double layer on calcite, but salts of NaCl and Na2SO4 form it. Moreover, the surface affinity of calcite to the non-polar crude oil is not affected by the salinity. However, the affinity of the calcite surface toward polar crude is affected by salinity and pH conditions. This research provides new insights into the action mechanisms that could help optimize its uses in enhanced oil recovery.

Upcycling of sugar refining mud solid waste as a novel adsorbent for removing methylene blue and Congo red from wastewater

The feasibility of utilizing the mud solid waste (MSW) produced during the carbonation process of sugar refining as a cost-effective and environmentally friendly alternative for the water removal of methylene blue (MB) and Congo red (CR), being highly utilized organic dyes representing cationic and anionic species, respectively is presented. Prior to its use, the MSW was dried at 110 °C for 24 h and sieved through a 100-mesh screen. The chief constituent of the MSW utilized was CaCO3, with a point of zero charge (PZC) found at pH 8.4 and 7.96 m2 g-1 total surface area. XRD and FTIR data indicate the presence of interactions between the dyes and the MSW surface, indicating effective adsorption. Different variables, such as initial dye concentration, MSW weight, solution pH, contact time, and temperature, were all examined to determine the optimal dye removal conditions. A central composite design (CCD) approach based on response surface methodology (RSM) modeling was utilized to identify statistically significant parameters for MB and CR adsorption capacities onto the MSW adsorbent. The removal equilibrium was typically reached in 120 minutes, with the greatest removal efficiency of CR taking place at pH 2 and 328 K, while the highest MB removal efficiency was obtained at pH 12 and 296 K. Kinetic studies suggest the adsorption of both dyes on the MSW follow pseudo-second-order rates, as evident through the high correlations obtained. Linearized and non-linearized Langmuir models showed strong correlations indicating maximum adsorption capacities of 86.6 and 72.3 mg g-1 for MB and CR, respectively. High regeneration and reusability potential of the MSW was demonstrated especially for the adsorption of CR, where the removal efficiency was nearly constant throughout five adsorption cycles, ranging from 93 to 91%, while the reduction in the removal for MB was much more significantly impacted, diminishing from 95 to 79% after the five cycles.

The dynamic nature of natural and fatty acid modified calcite surfaces

Calcium carbonate, particularly in the form of calcite, is an abundant mineral widely used in both human-made products and biological systems. The calcite surface possesses a high surface energy, making it susceptible to the adsorption of organic contaminants. Moreover, the surface is also reactive towards a range of chemicals, including water. Consequently, studying and maintaining a clean and stable calcite surface is only possible under ultrahigh vacuum conditions and for limited amounts of time. When exposed to air or solution, the calcite surface undergoes rapid transformations, demanding a comprehensive understanding of the properties of calcite surfaces in different environments. Similarly, attention must also be directed towards the kinetics of changes, whether induced by fluctuating environments or at constant condition. All these aspects are encompassed in the expression "dynamic nature", and are of crucial importance in the context of the diverse applications of calcite. In many instances, the calcite surface is modified by adsorption of fatty acids to impart a desired nonpolar character. Although the binding between carboxylic acid groups and calcite surfaces is strong, the fatty acid layer used for surface modification undergoes significant alterations when exposed to water vapour and liquid water droplets. Therefore, it is also crucial to understand the dynamic nature of the adsorbed layer. This review article provides a comprehensive overview of the current understanding of both the dynamics of the calcite surface as well as when modified by fatty acid surface treatments.

A microfluidic chip for geoelectrical monitoring of critical zone processes

We miniaturize geoelectrical acquisition using advanced microfabrication technologies to investigate coupled processes in the critical zone. We focus on the development of the complex electrical conductivity acquisition with the spectral induced polarization (SIP) method on a microfluidic chip equipped with electrodes. SIP is an innovative detection method that has the potential to monitor biogeochemical processes. However, due to the lack of microscale visualization of the processes, the interpretation of the SIP response remains under debate. This approach at the micrometer scale allows working in well-controlled conditions, with real-time monitoring by high-speed and high-resolution microscopy. It enables direct observation of microscopic reactive transport processes in the critical zone. We monitor the dissolution of pure calcite, a common geochemical reaction studied as an analog of the water-mineral interactions. We highlight the strong correlation between SIP response and dissolution through image processing. These results demonstrate that the proposed technological advancement will provide a further understanding of the critical zone processes through SIP observation.

Mineral phase transition characteristics and its effects on the stabilization of heavy metals in industrial hazardous wastes incineration (IHWI) fly ash via microwave-assisted hydrothermal treatment

Toxic heavy metals in industrial hazardous waste incineration (IHWI) fly ash can be effectively stabilized by using microwave-assisted hydrothermal technology. However, few works have focused on the relationship between mineralogical conversion and stability of heavy metals of fly ash during hydrothermal process. This study investigated the effect of mineral phase transition process on the stabilization and migration behavior of heavy metals in IHWI fly ash using coal fly ash as silicon‑aluminum additive. Mineral composition analysis reveals that after microwave-assisted hydrothermal treatment (MAHT) of IHWI fly ash, zeolite-like minerals (e.g., tobermorite, katoite and sodalite), secondary aluminosilicate minerals (e.g., prehnite and anorthite) and other newly-formed minerals (e.g., wollastonite, pectolite and larnite) were found. The leaching concentrations of heavy metals (Cr, Ni, Cu, Zn, Cd and Pb) in IHWI fly ash decrease sharply after MAHT with the most obvious decreases in Cu, Pb and Zn. Spearman correlation analysis show significantly negative correlation between the content of zeolite-like minerals and the leaching concentrations of most heavy metals (e.g., Ni, Cu, Zn, Cd and Pb). These results suggest that the immobilization effects of heavy metals in IHWI fly ash can be effectively enhanced by promoting the formation of zeolite-like minerals during the MAHT. This study is expected to further promote the development of IHWI fly ash harmless treatment technology.

Distance-dependent dielectric constant at the calcite/electrolyte interface: Implication for surface complexation modeling

HYPOTHESIS

The electrical double layer formed at the mineral/electrolyte interface is often modeled using mean-field approaches based on a continuum description of the solvent whose dielectric constant is assumed to decrease monotonically with decreasing distance to the surface. In contrast, molecular simulations show that the solvent polarizability oscillates near the surface similar to the water density profile - as shown previously, for example, by Bonthuis et al. (D.J. Bonthuis, S. Gekle, R.R. Netz, Dielectric Profile of Interfacial Water and its Effect on Double-Layer Capacitance, Phys Rev Lett 107(16) (2011) 166102). We showed that molecular and mesoscale pictures agree by spatially averaging the dielectric constant obtained from molecular dynamics simulations over the distances relevant to the mean-field representation. In addition, the values of capacitances used to describe the electrical double layer in Surface Complexation Models (SCMs) of the mineral/electrolyte interface can be estimated using molecularly informed spatially averaged dielectric constants and positions of hydration layers.

EXPERIMENTS

First, we used molecular dynamics simulations to model the calcite 101¯4/electrolyte interface. Next, by using atomistic trajectories, we calculated the distance-dependent static dielectric constant and water density in the direction normal to the. Finally, we applied spatial compartmentalization consistent with the model of parallel-plate capacitors connected in series to estimate SCM capacitances.

FINDINGS

Computationally expensive simulations are required to determine the dielectric constant profile of interfacial water near the mineral surface. On the other hand, water density profiles are readily assessable from much shorter simulation trajectories. Our simulations confirmed that dielectric and water density oscillations at the interface are correlated. Here, we parametrized linear regression models to estimate the dielectric constant directly from the local water density. This is a significant computational shortcut compared to slowly converging calculations relying on total dipole moment fluctuations. The amplitude of the interfacial dielectric constant oscillation can exceed the dielectric constant of the bulk water, suggesting an ice-like frozen state, but only if there are no electrolyte ions. The interfacial accumulation of electrolyte ions causes a decrease in the dielectric constant due to the reduction of water density and re-orientation of water dipoles in ion hydration shells. Finally, we show how to use the computed dielectric properties to estimate SCM's capacitances.

Colloidal aspects of calcium carbonate scaling in water-in-oil emulsions: A fundamental study using droplet-based microfluidics

HYPOTHESIS

As a mainstream process in the extraction and recovery of crude oil, water is injected into reservoirs in the so-called waterflooding process to facilitate the oil displacement through the wellbore, typically generating water-in-oil (W/O) emulsions. Based on economic considerations, sea water is used in the flooding process; however, the ionic incompatibility between the injected water and the formation water inside the reservoir may precipitate sparingly-soluble inorganic salts (scale). We hypothesize that calcium carbonate (CaCO₃) scale dynamically interacts with cationic surfactants in W/O emulsions, resulting in (i) scale growth retardation and (ii) emulsion destabilization.

EXPERIMENTS

We developed stable W/O emulsions via combining droplet-based microfluidics with multifactorial optimizations to investigate the influence of emulsion properties, such as surfactant type and concentrations, temperature, and pH, as well as calcium ions on the CaCO₃ scaling kinetics and emulsion stability. The CaCO₃ scale was characterized based on particle size and charge, lattice structure, interactions with the surfactant, and time-dependent effects on emulsion stability.

FINDINGS

The interfacial interactions between the cationic surfactant (cetyltrimethylammonium bromide, CTAB) and CaCO₃ retarded scale growth rate, decreased crystal size, and destabilized emulsion within hours as a result of surfactant depletion at the water-oil interface. The surfactant did not affect the crystal structure of scale, which was formed as the most thermodynamically stable crystalline polymorph, calcite, at the ambient condition. This fundamental study may open new opportunities for engineering stable W/O emulsions, e.g., for enhanced oil recovery (EOR), and developing scale-resistant multiphase flows.

How water desorbs from calcite

The desorption of water from calcite (10.4) reveals that the (2 × 1) surface reconstruction not only persist at room temperature but crucially influences the surface properties.

Can calcite play a role in the adsorption of glyphosate? A comparative study with a new challenge

Calcite as a sorbent can interact with both inorganic and organic substances through their functional groups. To measure its adsorption ability, another sorbent, saponite was selected because it can sorb glyphosate, an organic compound with a polar molecule and widely used as a herbicide. In this study, the two sorbents calcite and saponite were saturated by calcium chloride, and characterized by SEM-EDX, X-ray diffraction, and Zeta Potential Analyzer to investigate their capacity to sorb glyphosate. After saturation, the saponite became homoionic Ca-saponite with minor changes in morphology and specific surface area. But, the morphology of calcite transformed from rhombohedron to scalenohedron, with an increase of 75-folds in its specific surface, and the zeta potential became positive in alkaline pH, which contradicts the results of all previous research. The modified sorbents (Ca-calcite and Ca-saponite) were added to two soil samples to investigate each sorbent's effect on glyphosate sorption. Adsorption isotherm and percentage of glyphosate desorbed revealed the difference in binding and adsorption sites. The Langmuir and Temkin models fitted isotherm data in low concentrations better and suggested chemosorption for the uptake of glyphosate. FTIR analyses of samples with and without glyphosate were compared and results suggested that the bulk of adsorption happened in siloxane groups and on calcium carbonates surfaces.

Solid-Water Interface Interaction of Selenium with Fe(II)-Bearing Minerals and Aqueous Fe(II) and S(-II) Ions in the Near-Field of the Radioactive Waste Disposal System

Selenium can be highly toxic in excess for both animals and humans. However, since its mobile forms can be easily adsorbed with ferric minerals, its mobility in the natural oxic environment is generally not an issue. Still, the removal and immobilization of the long-lived radioactive isotope 79Se from the contaminated anoxic waters is currently a significant concern. 79Se can be accessible in the case of radionuclides' leaching from radioactive waste disposals, where anoxic conditions prevail and where ferrous ions and Fe(II)-bearing minerals predominate after corrosion processes (e.g., magnetite). Therefore, reductive and adsorptive immobilizations by Fe(II)-bearing minerals are the primary mechanisms for removing redox-sensitive selenium. Even though the information on the sorptive interactions of selenium and Fe(II)-bearing minerals seems to be well documented, this review focuses specifically on the state of the available information on the effects of the redox properties of Fe(II)-bearing solid phases (e.g., ferrous oxides, hydroxides, sulfides, and carbonates) on selenium speciation via redox transformation and co-occurring coprecipitation.

Solution NMR investigation of phytic acid adsorption mechanisms at the calcite-water interface

As one of the most abundant organic phosphorus (P) species in soils, phytic acid could serve as a mineralizable P reservoir in soils and sediments. It has been widely acknowledged that the adsorption of phytic acid to soil minerals retards P mineralization in soils. However, the adsorption mechanisms of phytic acid in the minerals are not clearly understood. Using solution ³¹P NMR and ¹H-³¹P 2D NMR, the adsorption mechanism of phytic acid was investigated at the calcite-water interface at pH 6 and 8. Maximum phytic acid adsorption reached 3.07 mmol/g, 2.60 mmol/g, 2.39 mmol/g at pH 6, 8, and 9.5, respectively. The presence of outer-sphere surface complex was evident by a lack of significant change in zeta-potential of phytic acid reacted calcite. Solution NMR analysis showed a fast exchange process between adsorbed and unreacted phytic acid at the mineral surface on an NMR time scale, also indicating the outer-sphere complexation mechanism at both pH values. Interestingly, a more active role of P5 and P4,6 in binding with calcite surface was observed at pH 6. Adsorbed phytic acid on the calcite surface should be labile and is not limiting P mineralization in the terrestrial environment.

Iron Sulfide Scale Inhibition in Carbonate Reservoirs

Hydrocarbon production operations include water injection, varying stimulation approaches, and enhanced oil recovery techniques. These treatments often affect reservoir formation, production, and injection facilities. Such sorts of well operations cause the formation of organic and inorganic scales in the near-wellbore region and various production and injection structures. Downhole squeeze treatment is commonly used as a control measure to prevent scale precipitation. A scale inhibitor solution is introduced into a formation by applying a squeeze treatment. The method allows scale inhibitors to adsorb on the internal rock surface to avoid settling down the scale precipitates. Thus, the study of adsorption of different types of inhibitors to prevent scale formation on the reservoir rock through the execution of downhole squeeze treatment is becoming necessary. This study incorporated different experimental techniques, including dynamic adsorption experiments of chelating agents employing a coreflooding setup, inductively coupled plasma-optical emission spectrometry (ICP-OES) to inhibit the formation of iron-containing scales in limestone rocks, and ζ-potential measurements targeting determination of iron precipitation in varying pH environments on calcite minerals. The influence of the inhibitor soaking time and salt existence in the system on chelating agent adsorption was also evaluated in the coreflooding experiments. The findings based on the coreflooding tests reveal that the concentration of chelating agents plays a significant role in their adsorption on carbonate rocks. The treatments with 20 wt % ethylenediaminetetraacetic acid (EDTA) and 20 wt % diethylenetriaminepentaacetic acid produced the highest adsorption capacity in limestone rock samples by inhibiting 84 and 85% of iron(III) ions, respectively. Moreover, the presence of the salts (CaCl₂ and MgCl₂) considerably decreased the adsorption of 10 wt % EDTA to 56% (CaCl₂) and 52% (MgCl₂) and caused nearly 20% more permeability reduction, while more inhibitor soaking time resulted in comparably higher adsorption and lesser permeability diminution. The results of ζ-potential measurements showed that the pH environment controls iron(II) and (III) precipitation, and iron(III) starts to deposit from a low pH region, whereas iron(II) precipitates in increased pH environments in calcite minerals.

Difficulties and Recent Achievements in Flotation Separation of Fluorite from Calcite—An Overview

As an important strategic non-metallic mineral resource, fluorite has been widely used in various industrial fields, such as metallurgy, optics and semiconductor manufacturing, as well as fluorine-related chemical engineering. Since the major gangue minerals of fluorite ore are silicate and carbonate ones, flotation is the main beneficiation method for the concentration. Compared with the relatively easy operation for silicate-type fluorite ore, fluorite concentration from calcite has always been the most difficult challenge in the field of mineral processing. In this review, analyses of the fundamental reasons for the difficulties of flotation separation of fluorite from calcite are performed, from the similar surface properties of both calcium minerals to the deterioration by the interference of dissolved ions in the pulp during grinding and flotation. Recent achievements in the flotation separation of fluorite from calcite as the main contents are comprehensively summarized, covering all aspects of flotation reagents of collectors, depressants and modifiers. Finally, successful examples of industrial practices forfluorite and calcite flotation separation are introduced. This overview provides a detailed and comprehensive reference source for the current research status of fluorite and calcite flotation separation, and some suggestions for future research are provided.

Ordered and Disordered Carboxylic Acid Monolayers on Calcite (104) and Muscovite (001) Surfaces

The adsorption of carboxylic acid molecules at the calcite (104) and the muscovite (001) surface was investigated using surface X-ray diffraction. All four investigated carboxylic acid molecules, hexanoic acid, octanoic acid, lauric acid, and stearic acid, were found to adsorb at the calcite surface. Whereas the shortest two carboxylic acid molecules, hexanoic acid and octanoic acid, showed limited ordering and a flexible, disordered chain, the two longest carboxylic acid molecules form fully ordered monolayers, i.e., these form highly structured self-assembled monolayers. The latter molecules are oriented almost fully upright, with a tilt of up to 10°. The oxygen atoms of the organic molecules are found at similar positions to those of water molecules at the calcite-water interface. This suggests that in both cases, the oxygen atoms compensate for the broken bonds at the calcite surface. Under the same experimental conditions, stearic acid does not adsorb to K⁺ and Ca²⁺-functionalized muscovite mica because the neutral molecules do not engage in the ionic bonds typical for the mica interface. These differences in adsorption behavior are characteristic for the differences of the oil-solid interactions in carbonate and sandstone reservoirs.

Role of Sporopollenin Shell Interfacial Properties in Protein Adsorption

Sporopollenin shells isolated from natural pollen grains have received attention in translational and applied research in diverse fields of drug delivery, vaccine delivery, and wastewater remediation. However, little is known about the sporopollenin shell's potential as an adsorbent. Herein, we have isolated sporopollenin shells from four structurally diverse pollen species, black walnut, marsh elder, mugwort, and silver birch, to study protein adsorption onto sporopollenin shells. We investigated three major interfacial properties, surface area, surface functional groups, and surface charge, to elucidate the mechanism of protein adsorption onto sporopollenin shells. We showed that sporopollenin shells have a moderate specific surface area (<12 m²/g). Phosphoric acid and potassium hydroxide treatments that were used to isolate sporopollenin shells from natural pollen grains also result in the functionalization of sporopollenin shell surfaces with ionizable groups of carboxylic acid and carboxylate salt. As a result, sporopollenin shells exhibit a negative ζ potential in the range of -75 to -82 mV at pH 10 when dispersed in water. The ζ potentials of sporopollenin shells remain negative in the pH range of 2.5-11, with the absolute value of ζ potential showing a decrease with the decrease in pH. The negative surface charge promotes the adsorption of protein onto the sporopollenin shell via electrostatic interaction. Despite having a moderate surface area, sporopollenin shells adsorb a significant amount of lysozyme (145-340 μg lysozyme per mg of sporopollenin shells). Lysozyme adsorption onto sporopollenin shells alters the surface, and the surface charge becomes positive at acidic pH. Overall, this study demonstrates the potential of sporopollenin shells to adsorb proteins, highlights the critical role of sporopollenin shell's interfacial properties in protein adsorption, and identifies the mechanism of protein adsorption on sporopollenin shells.

Electrokinetic behavior of artificial and natural calcites: A review of experimental measurements and surface complexation models

The surface charge of calcite in aqueous environments is essential to many industrial and environmental applications. Electrokinetic measurements are usually used to assess the calcite charging behavior and characterize its electrical double layer (EDL). Numerous surface complexation models (SCMs) have been proposed to interpret the effect of different surface interactions on the zeta potential. Because of their versatility, SCMs have also become important tools in reactive transport modeling. The research on enhanced oil recovery within the last decade has led to an increased number of publications reporting both zeta potential measurements and SCMs for calcite. Nonetheless, the measurements are often inconsistent and the reasons for choosing one model over another are unclear. In this work, we review the models proposed for calcite and address their main differences. We first collect a large number of published zeta potential measurements and then we fit a Diffuse Layer, Basic Stern, and Charge-Distribution Multi-Site Complexation models to a selected reliable dataset. For each model, we maintain a similar number of adjustable parameters. After optimizing the parameters of the models, we systematically compare their prediction capabilities against data obtained in monovalent and divalent electrolyte systems containing calcium, magnesium, sulfate, or carbonate. We show that, often, the discrepancies between the models and the experimental data can be explained by different levels of disequilibrium. Nonetheless, assumptions used in the development of the models may significantly reduce their extrapolability to variable chemical conditions. The poor agreement between the models tuned to electrokinetic data with surface charge measurements and dynamic retention from single-phase flowthrough tests show that zeta potential may not be the best type of data to characterize ion binding at the calcite surface. Including the effect of mineral impurities and temperature on the calcite surface speciation and electrokinetic behavior prevail as main challenges for reactive transport modeling.

Ionic Interactions at the Crude Oil-Brine-Rock Interfaces Using Different Surface Complexation Models and DLVO Theory: Application to Carbonate Wettability

The impact of ionic association with the carbonate surface and its influence toward carbonate wettability remains unclear and is an important topic of interest in the current literature. In this work, a triple layer model (TLM) approach was used to capture the electrokinetic interactions at both calcite-brine and oil-brine interfaces. The developed TLM was assembled against measured ζ-potential values from the literature, successfully capturing the trends and closely matching the ζ-potential magnitudes. The developed TLM was compared to a diffused layer model (DLM) presented in previous works, with the DLM showing a better match to the ζ-potential values for seawater brine solutions. The ζ-potential values predicted from both surface complexation models (SCMs) were used to calculate the total interaction energy (or potential) based on the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. It was observed that low Mg2+ and high SO4 2- concentrations in modified composition brine (MCB) made the calcite-brine interface more negative. However, at the oil-brine interface, low Mg2+ made the oil-brine interface more negative but high SO4 2- concentrations slightly shifted the oil-brine ζ-potential toward negative. At the crude oil-brine-rock (COBR) interfaces, low Mg2+ and high SO4 2- concentrations in the MCB were observed to generate a greater repulsive interaction energy, which could trigger carbonate wettability alteration toward water wetness. The absolute sum of the ζ-potential at both interfaces was observed to be correlated to the total interaction potential at a 0.25 nm separating distance. Thus, an increase in the absolute sum of the ζ-potentials would generate a greater repulsive interaction potential and trigger wettability alteration. Therefore, these SCMs can be applied to design modified composition brine capable of triggering a repulsive interaction energy to alter carbonate wettability toward water wetness.

Predicting the electrokinetic properties on an outcrop and reservoir composite carbonate surfaces in modified salinity brines using extended surface complexation models

Surface complexation models (SCM), based mainly on the diffuse double layer (DDL) theory, have been used to predict zeta potential at the crude oil-brine-rock (COBR) interface with limited success. However, DDL is inherently limited in accurately predicting zeta potential by the assumptions that all the brine ions interact with the rock surface at the same plane and by the double layer collapse at higher brine ionic strength (>1M). In this work, a TLM-based SCM captured zeta potential trends at the calcite-brine interface with ionic strength up to 3 M. An extended DDL and TLM-based SCMs were used to predict the electrokinetic properties of a composite carbonate rock showing a different mineralogical composition. The extended TLM-based SCM captured the zeta potential prediction trends and magnitude, highlighting the contribution of the inorganic minerals and organic impurities on the composite carbonate surface. In contrast, the extended DDL-based SCM captured the zeta potential trends but failed to capture the magnitude of the measured zeta potential. Interestingly, the TLM-based SCM predicted a positive SP for the rock-brine interface, which could explain the oil-wet nature of composite carbonate rocks due to electrostatic adsorption of negatively charged carboxylic acids. Conversely, the DDL-based SCM predicted a negative SP, leading to an inaccurate interpretation of the electrokinetic properties at the rock-brine interface. Thus, the use of extended TLM-based SCM was required to accurately predict the zeta potential and account for the adsorption of carboxylic acids on the reservoir composite carbonate surface.

Surfactant-induced wettability reversal on oil-wet calcite surfaces: Experimentation and molecular dynamics simulations with scaled-charges

HYPOTHESIS

Surfactant flooding is the leading approach for reversing the wettability of oil-wet carbonate reservoirs, which is critical for the recovery of the remaining oil. Combination of molecular dynamics (MD) simulations with experiments on simplified model systems can uncover the molecular mechanisms of wettability reversal and identify key molecular properties for systematic design of new, effective chemical formulations for the enhanced oil recovery.

EXPERIMENTS/SIMULATIONS

Wettability reversal by a series of surfactant solutions was studied experimentally using contact angle measurements on aged calcite chips, and a novel MD simulation methodology with scaled-charges that provides superior description of the ionic interactions in aqueous solutions.

FINDINGS

The MD simulation results were in excellent agreement with the experiments. Cationic surfactants were the most effective in reversing the calcite wettability, resulting in complete detachment of the oil from the surface. Some nonionic surfactants also altered the wettability, but to a lesser degree, while the amphoteric and anionic surfactants had no effect. From the tested cationic surfactants, the double-tailed one was the least effective, but the experiments were inconclusive due to its poor solubility. Contributions of specific interactions to the wettability reversal process and implications for the design and optimization of surfactants for the enhanced oil recovery are discussed.

Predictive surface complexation model of the calcite-aqueous solution interface: The impact of high concentration and complex composition of brines

Electrochemical interactions at calcite-water interface are characterized by the zeta potential and play an important role in many subsurface applications. In this work we report a new physically meaningful surface complexation model that is proven to be efficient in predicting calcite-water zeta potentials for a wide range of experimental conditions. Our model uses a two-stage optimization for matching experimental observations. First, equilibrium constants are optimized, and the Stern layer capacitance is optimized in the second stage. The model is applied to a variety of experimental sets that correspond to intact natural limestones saturated with equilibrated solutions of low-to-high salinity, and crushed Iceland Spar sample saturated with NaCl at non-equilibrium conditions. The proposed linear correlation of the Stern layer capacitance with the ionic strength is the main novel contribution to our surface complexation model without which high salinity experiments cannot be modelled. Our model is fully predictive given accurately known conditions. Therefore, the reported parameters and modelling protocol are of significant importance for improving our understanding of the complex calcite-water interfacial interactions. The findings provide a robust tool to predict electrochemical properties of calcite-water interfaces, which are essential for many subsurface applications including hydrology, geothermal resources, CO₂ sequestration and hydrocarbon recovery.

Ca2+ Ions Decrease Adhesion between Two (104) Calcite Surfaces as Probed by Atomic Force Microscopy

Solution composition-sensitive disjoining pressure acting between the mineral surfaces in fluid-filled granular rocks and materials controls their cohesion, facilitates the transport of dissolved species, and may sustain volume-expanding reactions leading to fracturing or pore sealing. Although calcite is one of the most abundant minerals in the Earth's crust, there is still no complete understanding of how the most common inorganic ions affect the disjoining pressure (and thus the attractive or repulsive forces) operating between calcite surfaces. In this atomic force microscopy study, we measured adhesion acting between two cleaved (104) calcite surfaces in solutions containing low and high concentrations of Ca²⁺ ions. We detected only low adhesion between calcite surfaces, which was weakly modulated by the varying Ca²⁺ concentration. Our results show that the more hydrated calcium ions decrease the adhesion between calcite surfaces with respect to monovalent Na⁺ at a given ionic strength, and thus Ca²⁺ can sustain relatively thick water films between contacting calcite grains even at high overburden pressures. These findings suggest a possible loss of cohesion and continued progress of reaction-induced fracturing for weakly charged minerals in the presence of strongly hydrated ionic species.

Structure and Surface Complexation at the Calcite(104)-Water Interface

Calcite is the most stable polymorph of calcium carbonate (CaCO₃) under ambient conditions and is ubiquitous in natural systems. It plays a major role in controlling pH in environmental settings. Electrostatic phenomena at the calcite-water interface and the surface reactivity of calcite in general have important environmental implications. They may strongly impact nutrient and contaminant mobility in soils and other subsurface environments, they control oil recovery from limestone reservoirs, and they may impact the safety of nuclear waste disposal sites. Besides the environmental relevance, the topic is significant for industrial applications and cultural heritage preservation. In this study, the structure of the calcite(104)-water interface is investigated on the basis of a new extensive set of crystal truncation rod data. The results agree with recently reported structures and resolve previous ambiguities with respect to the coordination sphere of surface Ca ions. These structural features are introduced into an electrostatic three-plane surface complexation model, describing ion adsorption and charging at the calcite-water interface. Inner surface potential data for calcite, as measured with a calcite single-crystal electrode, are used as constraints for the model in addition to zeta potential data. Ion adsorption parameters are compared with molecular dynamics simulations. All model parameters, including protonation constants, ion-binding parameters, and Helmholtz capacitances, are within physically and chemically plausible ranges. A PhreeqC version of the model is presented, which we hope will foster application of the model in environmental studies.

Water Orientation at the Calcite-Water Interface

Mineral-water interfaces play an important role in many natural as well as technological fields. Fundamental properties of these interfaces are governed by the presence of the interfacial water and its specific structure at the surface. Calcite is particularly interesting as a dominant rock-forming mineral in the earth's crust. Here, we combine atomic force microscopy, sum-frequency generation spectroscopy, and molecular dynamics simulations to determine the position and orientation of the water molecules in the hydration layers of the calcite surface with high resolution. While atomic force microscopy provides detailed information about the position of the water molecules at the interface, sum-frequency generation spectroscopy can deduce the orientation of the water molecules. Comparison of the calcite-water interface to the interfaces of magnesite-water, magnesite-ethanol, and calcite-ethanol reveals a comprehensive picture with opposite water orientations in the first and second layer of the interface, which is corroborated by the molecular dynamics simulations.

Charge regulated solid-liquid interfaces interacting on the nanoscale: Benchmarking of a generalized speciation code (SINFONIA)

Surface chemistry of mineral phases in aqueous environments generates the electrostatic forces involved in particle-particle interactions. However, few models directly take into account the influence of surface speciation and changes in solution speciation when the diffuse layer potential profiles of approaching particles overlap and affect each other. These electrostatic interactions can be quantified, ideally, through charge regulation, considering solution and surface speciation changes upon particle approach by coupling state-of-the-art surface complexation models for the two particle surfaces with a Poisson-Boltzmann type distribution of electrostatic potential and ions in the inter-particle space. These models greatly improve the accuracy of inter-particle force calculations at small inter-particle separations compared to constant charge and constant potential approaches. This work aims at advancing charge regulation calculations by including full chemical speciation and advanced surface complexation models (Basic Stern-, three-, or four plane models and charge distribution concepts), for cases of similar and dissimilar surfaces involving the numerical solution of the Poisson-Boltzmann equation for arbitrary electrolytes. The concept was implemented as a Python-based code and in COMSOL. The flexibility and precision of both, concept and implementations are demonstrated in several benchmark calculations testing the new codes against published results or simulations using established speciation codes, including aqueous speciation, surface complexation and various interaction force examples. Due to the flexibility in terms of aqueous chemistry and surface complexation models for various geometries, a large variety of potential applications can be tackled with the developed codes including industrial, biological, and environmental systems, from colloidal suspensions to gas bubbles, emulsions, slurries like cement paste, as well as new possibilities to assess the chemistry in nano-confined systems.

Atomistic insight into salinity dependent preferential binding of polar aromatics to calcite/brine interface: implications to low salinity waterflooding

This paper resolve the salinity-dependent interactions of polar components of crude oil at calcite-brine interface in atomic resolution. Molecular dynamics simulations carried out on the present study showed that ordered water monolayers develop immediate to a calcite substrate in contact with a saline solution. Carboxylic compounds, herein represented by benzoic acid (BA), penetrate into those hydration layers and directly linking to the calcite surface. Through a mechanism termed screening effect, development of hydrogen bonding between –COOH functional groups of BA and carbonate groups is inhibited by formation of a positively-charged Na⁺ layer over CaCO₃ surface. Contrary to the common perception, a sodium-depleted solution potentially intensifies surface adsorption of polar hydrocarbons onto carbonate substrates; thus, shifting wetting characteristic to hydrophobic condition. In the context of enhanced oil recovery, an ion-engineered waterflooding would be more effective than injecting a solely diluted saltwater.

First Steps towards Understanding the Non-Linear Impact of Mg on Calcite Solubility: A Molecular Dynamics Study

Magnesium (Mg2+) is one of the most common impurities in calcite and is known to have a non-linear impact on the solubility of magnesian calcites. Using molecular dynamics (MD), we observed that Mg2+ impacts overall surface energies, local free energy profiles, interfacial water density, structure and dynamics and, at higher concentrations, it also causes crystal surface deformation. Low Mg concentrations did not alter the overall crystal structure, but stabilised Ca2+ locally and tended to increase the etch pit nucleation energy. As a result, Ca-extraction energies over a wide range of 39 kJ/mol were observed. Calcite surfaces with an island were less stable compared to flat surfaces, and the incorporation of Mg2+ destabilised the island surface further, increasing the surface energy and the calcium extraction energies. In general, Ca2+ is less stable in islands of high Mg2+ concentrations. The local variation in free energies depends on the amount and distance to nearest Mg in addition to local disruption of interfacial water and the flexibility of surface carbonate ions to rotate. The result is a complex interplay of these characteristics that cause variability in local dissolution energies. Taken together, these results illustrate molecular scale processes behind the non-linear impact of Mg2+ concentration on the solubility of magnesium-bearing calcites.

Effects of Surfactant Charge and Molecular Structure on Wettability Alteration of Calcite: Insights from Molecular Dynamics Simulations

Wettability alteration of oil-wet calcite by surfactants was studied by means of molecular dynamics (MD) simulations. The simulations use the recently developed model for positively charged calcite surface, whose oil-wet state originates from binding of negatively charged carboxylate molecules contained in the oil, consistently with the bulk of the available experimental data. The ability to alter the surface wettability, which can be directly quantified by the release of the surface-bound carboxylates, is tested for nine different surfactants of all charge types-cationic, anionic, nonionic, and zwitterionic-and compared to that of brine. It was found that only the cationic surfactants are able to detach the organic carboxylates more efficiently than brine, while the neutral and anionic surfactants do not seem to have any measurable effect on the wettability. The outperformance of the cationic surfactants is generally consistent with the majority of previously published experimental observations. The data also point toward a consistently better performance of single-tailed cationic surfactants over the two-tailed structure. Molecular mechanism of the wettability alteration by different types of surfactants is discussed, along with the implications of the results for the design of new surfactant formulations for the enhanced oil recovery.

The Impact of Micelle Formation on Surfactant Adsorption-Desorption

The monomer-micelle equilibrium is shown to be responsible for an asymmetry between surfactant adsorption and desorption rates. When a solution containing micelles is brought into contact with a solid surface, the micelles dissociate to supply monomers that adsorb to the surface. When the same surface is subsequently exposed to a surfactant-free solution, desorption occurs slowly because of the higher affinity of the monomers to remain to the surface than to form micelles. As a result, the number of monomers that desorb is limited by the critical micelle concentration (CMC) of the surfactant. This effect is particularly pronounced for surfactants with low CMC values and in systems with high surface-to-volume ratios, such as porous media. A generic model is developed and applied to simulate the Ca2+-mediated adsorption and desorption of surfactants in limestone cores.

Ion-specific interactions at calcite-brine interfaces: a nano-scale study of the surface charge development and preferential binding of polar hydrocarbons

This research provides an atomic-level insight into the synergic contribution of mono- and divalent ions to interfacial characteristics of calcite surfaces exposed to electrolyte solution containing organic compounds. The emphasis was placed on the ionic interactions responsible for charge developing mechanisms of calcite surfaces and also the capacity for adsorption of polar hydrocarbons, represented by benzoic acid (BA), at different brine compositions. For this purpose, molecular dynamics (MD) simulations were employed to explore the interplay of the main constituent ions of natural brines (Na+, Cl-, Mg2+, and SO42-) and BA at the interface of CaCO3. It was observed that surface accumulation of Na+ cations produces a positively charged layer immediate to the basal plane of calcite, validating the typical positive surface charge of carbonates reported by laboratory experiences. Meanwhile, a negatively charged layer appears beyond the sodium layer as a result of direct and solvent-mediated pairing of anions with Na+ cations lodged on the calcite substrate. In this process, sulfate adsorption severely diminishes surface charge to even a negative value in the case of a SO42--rich solution, providing an interpretation for the measurements reported in the literature. Our results revealed the inhibition of direct binding of BA molecules onto the calcite surface through complexation with protruding oxygen atoms of basal carbonates by the residing Na+ cations. Further, we noticed the sulfate-mediated pairing of BA molecules to the Na+ layer, which in effect intensifies surface adsorption of BA. However, BA-SO42- interaction is considerably reduced by magnesium cations shielding sulfate sites in the Mg2+-augmented brine. The findings presented in this study are of fundamental importance to advance our microscopic understanding of interfacial interactions in brine/oil/carbonate systems; with broad scientific and applied implications in the context of mobilizing organic contaminants trapped in aquifer sediments and enhancement of hydrophilicity of subsurface oil-bearing carbonate reservoirs by injecting ion-modified brine solutions.

The Impact of Spatially Correlated Heterogeneity and Adsorption on Modified Salinity Water in Carbonates

Modified salinity water (MSW) core flooding tests conducted in carbonates often exhibit a delay in the additional oil recovery. It has been suggested that the ionic adsorption process controls this delay. In this study, we examine the adverse effect of the adsorption process on the performance of MSW flooding in various models categorized as layered and heterogeneous reservoirs and a North Sea field sector model. To evaluate the impact of porous media's heterogeneity on the delay caused by the adsorption, we introduce the net present volumetric value based on which the cost of delay is calculated. This evaluation is achieved by comparing the calculated cost of delay for heterogeneous systems and that of their equivalent homogeneous porous media. It is found that, as the level of reservoir heterogeneity increases, the adverse effect of ionic adsorption on the improved oil production decreases. Further, computational results suggest that the connectivity index, which is defined as the effective permeability between injection and production wells divided by the average permeability, is a better alternative to the vorticity index to describe the impact of the delay of additional oil recovery in heterogeneous reservoirs subjected to MSW flooding.

Review of low salinity waterflooding in carbonate rocks: mechanisms, investigation techniques, and future directions

This review analyses the fundamental thermodynamic theory of the crude oil-brine-rock (COBR) interface and the underlying rock-brine and oil-brine interactions. The available data are then reviewed to outline potential mechanisms responsible for increased oil recovery from low salinity waterflooding (LSWF). We propose an approach to studying LSWF and identify the key missing links that are needed to explain observations at multiple length scales. The synergistic effect of LSWF on other chemical enhanced oil recovery methods such as surfactant, alkaline, nanoparticle and polymer flooding are also outlined. We specifically highlight key uncertainties that must be overcome to fully implement the technique in the field.

Adsorption of enhanced oil recovery polymer, schizophyllan, over carbonate minerals

Schizophyllan is a natural polysaccharide that has shown great potential as enhanced oil recovery (EOR) polymer for high-temperature, high-salinity reservoirs. Nevertheless, the adsorption behavior of schizophyllan over carbonate minerals remains ambiguous element towards its EOR applications. Here, we investigate the adsorption of schizophyllan on different carbonate minerals. The effect of mineral type, salinity, and background ions on adsorption is analyzed. Our results indicate the adsorption capacity is higher on calcite and dolomite compared to silica and kaolin and the adsorption capacity decreases with salinity. Moreover, the adsorption kinetics follows pseudo-second order mechanism regardless of the mineral type. Adsorption over calcite is diminished in presence of water structure making ions and enhanced in presence of structure breaking ion and in presence of urea. Gel permeation chromatography results reveal the preferential adsorption of longer chains. The adsorption over carbonate minerals proceed via complex formation between polymer molecule and mineral surface.

A positively charged calcite surface model for molecular dynamics studies of wettability alteration

A new model for a positively charged calcite surface was developed to allow realistic molecular dynamics studies of wettability alteration on carbonate rocks. The surface charge was introduced in a manner consistent with the underlying calcite geochemistry and with the conclusions of recent quantum mechanical studies. The simulations using the new surface model demonstrate that the experimentally observed wettability behavior of calcite is represented correctly. In particular, the model surface became oil-wet due to the adsorption of the carboxylate species. Furthermore, the oil-wet conditions were reversed more effectively by a cationic surfactant than by an anionic one, in agreement with the majority of experimental observations. Finally, with simulated smart water, the well-documented wettability alteration abilities of Ca²⁺ and SO₄^2- could be explained by the formation of ion-pairs and competitive adsorption onto the surface, respectively. The simulation results with the new surface model conceptually agree with the electric double layer expansion being the predominant mechanism for the low salinity effect in oil recovery enhancement. The proposed calcite surface model will benefit future simulation studies on the wettability characteristics of carbonate rocks, and facilitate the design and optimizations of chemical agents and formulations to enhance the oil recovery from carbonate reservoirs.

Towards an Understanding of Crystallization by Attachment

Crystallization via particle attachment was used in a unified model for both classical and non-classical crystallization pathways, which have been widely observed in biomimetic mineralization and geological fields. However, much remains unknown about the detailed processes and driving mechanisms for the attachment. Here, we take calcite crystal as a model mineral to investigate the detailed attachment process using in situ Atomic Force Microscopy (AFM) force measurements and molecular dynamics simulations. The results show that hydration layers hinder the attachment; however, in supersaturated solutions, ionic bridges are formed between crystal gaps as a result of capillary condensation, which might enhance the aggregation of calcite crystals. These findings provide a more detailed understanding of the crystal attachment, which is of vital importance for a better understanding of mineral formation under biological and geological environments with a wide range of chemical and physical conditions.