Atomistic Molecular Dynamics Simulations of Crude Oil/Brine Displacement in Calcite Mesopores

Effect of ethylene oxide groups on calcite wettability reversal by nonionic surfactants: An experimental and molecular dynamics simulation investigation

HYPOTHESIS

Ethoxylated nonionic surfactants are promising candidates for enhanced oil recovery (EOR) from oil-wet carbonate reservoirs due to their ability to reverse the mineral wettability. The wettability-reversal efficiency increases with the number of the ethoxy (EO) groups in the surfactant molecule.

METHODOLOGY

Contact angle measurements, scanning electron microscopy (SEM) and molecular dynamics (MD) simulations were combined to investigate the wettability reversal of an oil-wet calcite by three ethoxylated nonionic surfactants with 1, 4 and 8 EO groups, respectively, to directly probe the role of the EO groups and to uncover the molecular mechanism responsible for the wettability reversal.

FINDINGS

Both experiments and simulations consistently show a clear correlation between the number of EO groups and the wettability reversal efficiency of the surfactants, whereby the higher number of EO groups results in greater degree of wettability reversal. This is due to 1) the more hydrophilic surfactant headgroup weakening the carboxylate interactions with the surface by expanding the surface-adjacent water layer, and 2) the physically larger surfactant molecule attracting the carboxylates more strongly, thus aiding in their removal from the surface.

New insights into interaction between oil and solid during hydrothermal treatment of oily sludge

Hydrothermal treatment (HT) can effectively dehydrate and reduce oily sludge (OS) volume, but the resulting hydrothermal oily sludge (HOS) presents greater challenges for washing than the initial oily sludge (IOS). This study examines the effects of HT on OS by analyzing changes in water, oil, and solid. Results indicate that HT considerably decreases the water content in OS while increasing resin and asphaltenes contents. In addition, condensation, side-chain scission, and oxidation reactions occur during the HT process, resulting in coking, agglomeration, and an increase in oxygen-containing groups. This increase, further confirmed by X-ray photoelectron spectroscopy (XPS), enhances the interaction between oil and solids. Calcite, the most prevalent solid-phase component, may form a calcium bridge with the oxygen-containing groups. Moreover, HT reduces the solid particle size, thereby increasing the oil-solid contact area. Interestingly, the process of deasphalting diminishes the interaction between oil and solids, facilitating sludge washing. After washing, the residual oil content in HOS is reduced to less than 0.34%. This study elucidates why HOS is challenging to separate from oil and solids and introduces a novel method that combines dodecylbenzene sulfonic acid (DBSA)-assisted heptane deasphalting with conventional washing techniques. This method shows promise for applications in OS affected by weathering processes.

Adsorption Behaviors of Different Components of Shale Oil in Quartz Slits Studied by Molecular Simulation

Understanding the adsorption state and molecular behavior of the diverse components of shale oil in shale slits is of critical importance for exploring novel enhanced shale oil recovery techniques, but it is hard to be achieved by experimental measurements. In this paper, molecular dynamics (MD) simulations are performed to quantitatively describe the microbehavior of shale oil mixtures containing different kinds of hydrocarbon components, including asphaltene, in quartz slits. The spatial distributions of all the presenting components are given, the interaction energy between the components and quartz is analyzed, and the diffusion coefficients of all the components are calculated. It was found that asphaltene molecules play a vitally important role in restricting the detachment and diffusion movement of all hydrocarbon components, which is actually a key problem limiting the recovery efficiency of shale oil. The effects of temperature, slit aperture, and the appearance of CO₂ on the adsorption behavior of the different shale oil components are examined; the results suggest that the light and medium components are the fractions with the most potential in thermal exploitation, while injection of CO₂ is beneficial for the extraction of all the components, especially the medium components. This work gives insights into the effect of asphaltene on shale oil recovery in quartz slits and might provide guidance on the utilization of thermal and CO₂-enhanced enhanced oil recovery (EOR) techniques in shale oil production.

Resonance-Induced Reduction of Interfacial Tension of Water-Methane and Improvement of Methane Solubility in Water

Molecular dynamics simulations were performed to study the effect of the periodic oscillating electric field on the interface between water and methane. We propose a new strategy that utilizes oscillating electric fields to reduce the interfacial tension (IFT) between water and methane and increase the solubility of methane in water simultaneously. These are attributed to the hydrogen bond resonance induced by an electric field with a frequency close to the natural frequency of the hydrogen bond. The resonance breaks the hydrogen bond network among water molecules to the maximum, which destroys the hydration shell and reduces the cohesive action of water, thus resulting in the decrease of IFT and the increase of methane solubility. As the frequency of the electric field is close to the optimum resonant frequency of hydrogen bonds, IFT decreases from 56.43 to 5.66 mN/m; water and methane are miscible because the solubility parameter of water reduces from 47.63 to 2.85 MPa^1/2, which is close to that of methane (3.43 MPa^1/2). Our results provide a new idea for reducing the water-gas IFT and improving the solubility of insoluble gas in water and theoretical guidance in the fields of natural gas exploitation, hydrate generation, and nanobubble nucleation.

Molecular dynamics simulation on water/oil interface with model asphaltene subjected to electric field

HYPOTHESIS

The droplet-medium interfaces of petroleum emulsions are often stabilized by the indigenous surface-active compounds (e.g., asphaltenes), causing undesired issues. While demulsification by electric field is a promising technique, fundamental study on the droplet-medium interface influenced by electric field is limited. Molecular dynamics (MD) simulations are expected to provide microscopic insights into the nano-scaled water/oil interface.

METHODS

MD simulations are conducted to study the adsorption of model asphaltene molecules (represented by N-(1-hexylheptyl)-N'-(5-carboxylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide (C5Pe)) on a water-toluene interface under various strengths of electric field. The adsorption amount and structural feature of C5Pe molecules at water-toluene interface are investigated, and the effects of electric field and salt are discussed.

FINDINGS

C5Pe molecules tend to adsorb on the water-oil interface. As the electric field strength increases, the adsorption amount first slightly increases (or remains constant) and then decreases. The electric field disrupts the compact π-π stacking between C5Pe molecules and increases their mobility, causing a dispersed distribution of the molecules with a wide range of orientations relative to the interface. Within the studied range, the addition of salt ions appears to stabilize the interface at high electric field. These results provide useful insights into the mechanism and feasibility of demulsification under electric field.

Breakthrough pressure of oil displacement by water through the ultra-narrow kerogen pore throat from the Young-Laplace equation and molecular dynamic simulations

As one common unconventional reservoir, shale plays a pivotal role to compensate the depletion of conventional oil resources. There are numerous nanoscale pores and ultra-narrow pore throats (sub 2 nm) in shale media. To displace oil through ultra-narrow pore throats by water, one needs to overcome excessively-high capillary pressure. Understanding the water-oil two-phase displacement process through pore throats is critical to numerical simulation on tight/shale oil exploitation and ultimate oil recovery estimation. In this work, we use molecular dynamics simulations to investigate oil (represented by n-octane) displacement by water through a ~2 nm kerogen (represented by Type II-C kerogen) pore throat. Besides, the applicability of the Young-Laplace equation to the ultra-narrow kerogen pore throat has been assessed. We find that although the Type II-C kerogen is generally oil-wet, water has an excellent displacement efficiency without the oil film on the substrate, thanks to the hydrogen bonding formed between water and heteroatoms (such as O, N, and S) on the kerogen surface. Unlike previous studies, the capillary pressure obtained from the widely used Young-Laplace equation shows a good agreement with the breakthrough pressure obtained from MD simulations for the ∼2 nm kerogen pore throat. Our work indicates that explicitly considering intermolecular interactions as well as atomistic and molecular level characteristics is imperative to study the two-phase displacement process through ultra-narrow pore throats.

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.

Phase Behavior and Composition Distribution of Multiphase Hydrocarbon Binary Mixtures in Heterogeneous Nanopores: A Molecular Dynamics Simulation Study

In this study, molecular dynamics (MD) simulation is used to investigate the phase behavior and composition distribution of an ethane/heptane binary mixture in heterogeneous oil-wet graphite nanopores with pore size distribution. The pore network system consists of two different setups of connected bulk and a 5-nm pore in the middle; and the bulk connected to 5-nm and 2-nm pores. Our results show that nanopore confinement influences the phase equilibrium of the multicomponent hydrocarbon mixtures and this effect is stronger for smaller pores. We recognized multiple adsorbed layers of hydrocarbon molecules near the pore surface. However, for smaller pores, adsorption is dominant so that, for the 2-nm pore, most of the hydrocarbon molecules are in the adsorbed phase. The MD simulation results revealed that the overall composition of the hydrocarbon mixture is a function of pore size. This has major implications for macro-scale unconventional reservoir simulation, as it suggests that heterogenous shale nanopores would host fluids with different compositions depending on the pore size. The results of this paper suggest that modifications should be made to the calculation of overall composition of reservoir fluids in shale nanopores, as using only one overall composition for the entire heterogenous reservoir can result in significant error in recovery estimations.

Molecular Origin of Wettability Alteration of Subsurface Porous Media upon Gas Pressure Variations

Upon extraction/injection of a large quantity of gas from/into a subsurface system in shale gas production or carbon sequestration, the gas pressure varies remarkably, which may significantly change the wettability of porous media involved. Mechanistic understanding of such changes is critical for designing and optimizing a related subsurface engineering process. Using molecular dynamics simulations, we have calculated the contact angle of a water droplet on various solid surfaces (kerogen, pyrophyllite, calcite, gibbsite, and montmorillonite) as a function of CO₂ or CH₄ gas pressure up to 200 atm at a temperature of 300 K. The calculation reveals a complex behavior of surface wettability alteration by gas pressure variation depending on surface chemistry and structure, and molecular interactions of fluid molecules with surfaces. As the CO₂ gas pressure increases, a partially hydrophilic kerogen surface becomes highly hydrophobic, while a calcite surface becomes more hydrophilic. Considering kerogen and calcite being the major components of a shale formation, we postulate that the wettability alteration of a solid surface induced by a gas pressure change may play an important role in fluid flows in shale gas production and geological carbon sequestration.

Ion Valency and Concentration Effect on the Structural and Thermodynamic Properties of Brine-Decane Interfaces with Anionic Surfactant (SDS)

Salt ion valency and concentration vary in actual oil reservoirs, which play an important role in the functionalities of surfactant formula during chemical flooding processes to enhance oil recovery. Herein, we report a molecular dynamics (MD) study to investigate the ion valency and concentration effect on the structural and thermodynamic properties of brine-decane interfaces with anionic surfactant (SDS), under typical reservoir conditions (353 K and 200 bar). We use two different cations (Na⁺ and Ca²⁺) and a wide range of ion concentrations (up to 3.96 M) to simulate reservoir conditions. We find that ion valency has a significant effect on the molecular configurations, which further influences the thermodynamic properties. Ca²⁺ ions can have a strong adsorption at the interface due to the strong electrostatic interactions between Ca²⁺ ions and SDS, which also results in the Cl^- ion enrichment at the interface. Furthermore, Ca²⁺ ions can form pentagon-like SDS-Ca²⁺ complexes through SDS-Ca²⁺-SDS cation bridging, which renders a nonuniform distribution of SDS at the interface. On the other hand, the cation bridging density monotonically increases as ion concentration increases for the systems without Ca²⁺ ions, while first increases, then decreases for the systems with Ca²⁺ ions. This is because the accumulation of Cl^- ions at the interface at high salt concentrations can melt SDS-Ca²⁺ complexes. This work should provide new insights into the structural and thermodynamic properties of brine-oil interfaces with an anionic surfactant, which can facilitate the optimization of chemical flooding processes.

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.

A Deep Look into the Dynamics of Saltwater Imbibition in a Calcite Nanochannel: Temperature Impacts Capillarity Regimes

This research concerns fundamentals of spontaneous transport of saltwater (1 mol·dm^-3 NaCl solution) in nanopores of calcium carbonates. A fully atomistic model was adopted to scrutinize the temperature dependence of flow regimes during solution transport under CaCO₃ nanoconfinement. The early time of capillary filling is inertia-dominated, and the solution penetrates with a near-planar meniscus at constant velocity. Following a transition period, the meniscus angle falls to a stabilized value, characterizing the capillary-viscous advancement in the calcite channel. At this stage, brine displacement follows a parabolic relationship consistent with the classical Lucas-Washburn (LW) theory. Approaching the slit outlet, the meniscus contact lines spread widely on the solid substrate and brine leaves the channel at a constant rate, in oppose to the LW law. The brine imbibition rate considerably increases at higher temperatures as a result of lower viscosity and greater tendency to form wetting layers on slit walls. We also pointed out a longer primary inertial regime and delayed onset of the viscous-capillary regime at higher temperatures. Throughout the whole span of capillary displacement, transport of sodium and chloride ions is tied to dynamics and diffusion of the water phase, even at the mineral interface. The results presented in this study are of broad implications in diverse science and technological applications.

Transport Properties of Waxy Crude Oil: A Molecular Dynamics Simulation Study

To study the effects of paraffin on viscosity of waxy crude oil and transport properties of small molecules, light and waxy crude oil models were investigated at atmospheric pressure and 293-323 K temperature range using atomistic molecular dynamics simulations. The optimized parameters for liquid simulations all-atom (OPLS-AA) and atomistic polarizable potential for liquids, electrolytes, and polymers (APPLE&P) force fields were employed. The self-diffusion coefficients, viscosity, and paraffin configurations were compared for two oil models and between the two employed force fields. However, the behavior of paraffin molecules predicted by two force fields was quite different. Simulations using the OPLS-AA force field showed crystallization of longer paraffin molecules below 323 K, while simulations with the APPLE&P force field demonstrated a homogeneous mixture down to 293 K. To provide additional validation of the employed force fields, the density, diffusion coefficient, and crystallization of pure alkanes were compared with experimental data. The density and diffusion coefficients of n-C₆ and n-C₁₄ simulated with the APPLE&P force field were found to be in much closer agreement with the experimental data. The OPLS-AA force field was found to overestimate the crystallization temperature of pure alkanes. Therefore, simulations with the APPLE&P provide more realistic description of the waxy oil structure and transport properties. In this temperature range, the paraffin molecules are homogeneously distributed in the mixture, and viscosity of the system increased by a factor of two compared to light oil. Crystallization of paraffins requires lower temperatures or/and the presence of other components such as nanoparticles or asphaltene molecules to facilitate nucleation.

Mechanism of wettability alteration of the calcite {101[combining macron]4} surface

To understand the mechanism of wettability alteration of calcite, a typical mineral in oil reservoirs, the interactions of deionized water and brine (with different compositions) with the calcite {101[combining macron]4} surface are investigated using a combination of molecular dynamics and first-principles simulations. We show that two distinct water adsorption layers are formed through hydrogen bonding and electrostatic interactions with the calcite {101[combining macron]4} surface as well as hydrogen bonding between the water molecules. These highly ordered water layers resist penetration of large stable Mg²⁺ and Ca²⁺ hydrates. As Na⁺ and Cl^- hydrates are less stable, Na⁺ and Cl^- ions may penetrate the ordered water layers to interact with the calcite {101[combining macron]4} surface. In contact with this surface, Na⁺ interacts significantly with water molecules, which increases the water-calcite interaction (wettability of calcite), in contrast to Cl^-. We propose that formation of Na⁺ hydrates plays an important role in the wettability alteration of the calcite {101[combining macron]4} surface.

Brine-Oil Interfacial Tension Modeling: Assessment of Machine Learning Techniques Combined with Molecular Dynamics

The physical chemistry mechanisms behind the oil-brine interface phenomena are not yet fully clarified. The knowledge of the relation between brine composition and concentration for a given oil may lead to the ionic tuning of the injected solution on geochemical and enhanced oil recovery processes. Thus, it is worth examining the parameters influencing the interfacial properties. In this context, we have combined machine learning (ML) techniques with classical molecular dynamics simulations (MD) to predict oil/brine interfacial tensions (IFT) effectively and compared this process to a linear regression (LR) method. To diversify our data set, we have introduced a new atomistic crude oil model (medium) with 36 different types of hydrocarbon molecules. The MD simulations were performed for mono- and multicomponent (toluene, heptane, Heptol, light, and medium) oil systems interfaced with sulfate and chloride brines with varying cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) and salinity concentration. Thus, a consistent IFT data set was built for the ML training and LR fitting at room temperature and pressure conditions, over the feature space considering oil density, oil composition, salinity, and ionic concentrations. On the basis of gradient boosted (GB) algorithms, we have observed that the dominant quantities affecting the IFT are related to the oil attributes and the salinity concentration, and no specific ion dominates the IFT changes. When the obtained LR model was validated against MD and experimental data from the literature, the error varied up to 2% and 9%, respectively, showing a robust and consistent transferability. The combination of MD simulations and ML techniques may provide a fast and cost-effective IFT determination over multiple and complex fluid-fluid and fluid-solid interfaces.

The Impact of Salinity on the Interfacial Structuring of an Aromatic Acid at the Calcite/Brine Interface: An Atomistic View on Low Salinity Effect

This study aims to elucidate the impact of salinity on the interactions governing the adsorption of polar aromatic oil compounds onto calcite. To this end, molecular dynamics simulations were employed to assess adsorption of a model polar organic molecule (deprotonated benzoic acid, benzoate) on the calcite surface in NaCl brines of different concentration levels, namely, deionized water (DW), low-salinity water (LS, 5000 ppm), and sea water (SW; 45,000 ppm). Calcite was found to be completely covered by several well-ordered water layers. The top hydration layer is very compact and prevents direct adsorption of benzoates onto the substrate. Instead, Na⁺ ions form a distinct positively charged layer by adhering on the calcite substrate through inner-sphere complexion mode. Cl^- ions mostly lodge on top of the adsorbed sodium cations, forming a negatively charged layer. The distribution of ions at the calcite/brine interface thus exhibits the features of an electrical double layer, composed of a Stern-like positive layer followed by a negative one. The positive charged layer attracts benzoates toward the surface. As such, the sodium ions attached onto the calcite can act as adsorption sites to connect benzoates to the surface. By increasing the salinity, more Na⁺ ions adsorb onto the calcite surface, and the density of benzoate molecules at the interface is enhanced as a result of more Na⁺ bridging ions. The monotonic salinity-dependent adsorption of benzoate molecules on calcite offers a mechanism driving additional oil recovery upon injection of diluted brine into subsurface carbonate reservoirs.

Distribution and Mobility of Crude Oil-Brine in Clay Mesopores: Insights from Molecular Dynamics Simulations

The value of crude oil accommodated in shale has been recognized and has attracted increasing attention from the academic and industrial society. The occurrence and mobility of crude oil in clay pores, therefore, become essential issues for evaluation and recovery of shale oil. The distribution, structure, and transport of the oil-brine mixture confined in a slit-shaped montmorillonite mesopore with different water amounts have been investigated using equilibrium molecular dynamics and nonequilibrium molecular dynamics (NEMD) simulations. A mimic model of crude oil, a mixture of 19 organic molecules, was employed, and thus the behavior of different organic molecules could be characterized in detail. A temperature of 410 K and a pressure of 300 atm corresponding to a buried depth of 3 km were employed. The simulations indicate that the water amount determines the distribution of crude oil. Water and metal ions prefer to cover on hydrophilic montmorillonite surfaces, while nonpolar hydrocarbons tend to be far away from clay surfaces. As the water amount is too low to completely cover the clay surfaces, some polar organic molecules will come into contact with the uncovered clay surface. Abundant organic acid molecules adsorb onto montmorillonite surfaces mainly through participating in the inner-sphere complexes of Na⁺ ions closely located at montmorillonite surfaces (i.e., Na⁺ cation bridge) and forming hydrogen bonds with water molecules in the vicinity. Carbazole molecules tend to aggregate together due to π-π stacking, while thioether molecules mix within alkane molecules and exhibit no characteristic distributions. The mobility of all oil components decreases with the decrease of the water amount, and the mobility of polar components (i.e., organic acid and carbazole) is relatively lower than that of nonpolar hydrocarbons. NEMD simulations clearly indicate that the transport velocity of crude oil markedly increases with the water amount under a specific pressure gradient. The brine covering on clay surfaces significantly weakens oil-clay interfacial interactions. Polar components, especially organic acid, exhibit relatively low transport velocity compared with nonpolar hydrocarbons. These findings highlight the understanding of physical-chemical behaviors of shale oil and provide atomistic information for technology development for enhancing oil recovery.

Distinguishing between bulk and edge hydroxyl vibrational properties of 2 : 1 phyllosilicates via deuteration

Observation of vibrational properties of phyllosilicate edges via a combined molecular modeling and experimental approach was performed. Deuterium exchange was utilized to isolate edge vibrational modes from their internal counterparts. The appearance of a specific peak within the broader D2O band indicates the presence of deuteration on the edge surface, and this peak is confirmed with the simulated spectra. These results are the first to unambiguously identify spectroscopic features of phyllosilicate edge sites.

Mechanistic Study of Wettability Changes on Calcite by Molecules Containing a Polar Hydroxyl Functional Group and Nonpolar Benzene Rings

Oil extraction efficiency strongly depends on the wettability status (oil- vs water-wet) of reservoir rocks during oil recovery. Aromatic compounds with polar functional groups in crude oil have a significant influence on binding hydrophobic molecules to mineral surfaces. Most of these compounds are in the asphaltene fraction of crude oil. This study focuses on the hydroxyl functional group, an identified functional group in asphaltenes, to understand how the interactions between hydroxyl groups in asphaltenes and mineral surfaces begin. Phenol and 1-naphthol are used as asphaltene surrogates to model the simplest version of asphaltenes. Adsorption of oil molecules on the calcite {101̅4} surface is described using static quantum-mechanical density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations. DFT calculations indicate that adsorption of phenol and 1-naphthol occurs preferentially between their hydroxyl group and calcite step edges. 1-Naphthol adsorbs more strongly than phenol, with different adsorption geometries due to the larger hydrophobic part of 1-naphthol. MD simulations show that phenol can behave as an agent to separate oil from the water phase and to bind the oil phase to the calcite surface in the water/oil mixture. In the presence of phenol, partial separation of water/oil with an incomplete lining of phenol at the water/oil boundary is observed after 0.2 ns. After 1 ns, perfect separation of water/oil with a complete lining of phenol at the water/oil boundary is observed, and the calcite surface becomes oil-wet. Phenol molecules enclose decane molecules at the water-decane boundary preventing water from repelling decane molecules from the calcite surface and facilitate further accumulation of hydrocarbons near the surface, rendering the surface oil-wet. This study indicates phenol and 1-naphthol to be good proxies for polar components in oil, and they can be used in designing further experiments to test pH, salinity, and temperature dependence to improve oil recovery.

Simulations of Interfacial Tension of Liquid-Liquid Ternary Mixtures Using Optimized Parametrization for Coarse-Grained Models

In this work, liquid-liquid systems are studied by means of coarse-grained Monte Carlo simulations (CG-MC) and Dissipative Particle Dynamics (DPD). A methodology is proposed to reproduce liquid-liquid equilibrium (LLE) and to provide variation of interfacial tension (IFT), as a function of the solute concentration. A key step is the parametrization method based on the use of the Flory-Huggins parameter between DPD beads to calculate solute/solvent interactions. Parameters are determined using a set of experimental compositional data of LLE, following four different approaches. These approaches are evaluated, and the results obtained are compared to analyze advantages/disadvantages of each one. These methodologies have been compared through their application on six systems: water/benzene/1,4-dioxane,water/chloroform/acetone, water/benzene/acetic acid, water/benzene/2-propanol, water/hexane/acetone, and water/hexane/2-propanol. CG-MC simulations in the Gibbs (NVT) ensemble have been used to check the validity of parametrization approaches for LLE reproduction. Then, CG-MC simulations in the osmotic (μ_soluteN_solventP _zzT) ensemble were carried out considering the two liquid phases with an explicit interface. This step allows one to work at the same bulk concentrations as the experimental data by imposing the precise bulk phase compositions and predicting the interface composition. Finally, DPD simulations were used to predict IFT values for different solute concentrations. Our results on variation of IFT with solute concentration in bulk phases are in good agreement with experimental data, but some deviations can be observed for systems containing hexane molecules.

Molecular mechanism of viscoelastic polymer enhanced oil recovery in nanopores

Polymer flooding is a promising chemical enhanced oil recovery (EOR) method, which realizes more efficient extraction in porous formations characterized with nanoscale porosity and complicated interfaces. Understanding the molecular mechanism of viscoelastic polymer EOR in nanopores is of great significance for the advancement of oil exploitation. Using molecular dynamics simulations, we investigated the detailed process of a viscoelastic polymer displacing oil at the atomic scale. We found that the interactions between polymer chains and oil provide an additional pulling effect on extracting the residual oil trapped in dead-end nanopores, which plays a key role in increasing the oil displacement efficiency. Our results also demonstrate that the oil displacement ability of polymer can be reinforced with the increasing chain length and viscoelasticity. In particular, a polymer with longer chain length exhibits stronger elastic property, which enhances the foregoing pulling effect. These findings can help to enrich our understanding on the molecular mechanism of polymer enhanced oil recovery and provide guidance for oil extraction engineering.

Atomistic insights into the nanofluid transport through an ultra-confined capillary

Nanofluid or nanoparticle (NP) transport in confined channels is of great importance for many biological and industrial processes. In this study, molecular dynamics simulation has been employed to investigate the spontaneous two-phase displacement process in an ultra-confined capillary controlled by the surface wettability of NPs. The results clearly show that the presence of NPs modulates the fluid-fluid meniscus and hinders the displacement process compared with the NP-free case. From the perspective of motion behavior, hydrophilic NPs disperse in the water phase or adsorb on the capillary, while hydrophobic and mixed-wet NPs are mainly distributed in the fluid phase. The NPs dispersed into fluids tend to increase the viscosity of the fluids, while the adsorbed NPs contribute to the wettability alteration of the solid capillary. Via capillary number calculations, it is uncovered that the viscosity increase of fluids is responsible for the hindered spontaneous displacement process by hydrophobic and mixed NPs. The wettability alteration of the capillary induced by adsorbed NPs dominates the enhanced displacement in the case of hydrophilic NPs. Our findings provide guidance for modifying the rate of capillary filling and reveal the microscopic mechanism transporting NPs into porous media, which is significant to the design of NPs for target applications.

CO2 activating hydrocarbon transport across nanopore throat: insights from molecular dynamics simulation

In tight oil reservoirs, nanopore throat acting as the narrowest section of fluidic channel determines the oil transport performance; injecting CO₂ is found to significantly promote the oil flow.

Investigations on oil detachment from rough surfaces in an aqueous solution

Herein, detachment of oil molecules from perfect and defective aluminum oxide surfaces in an aqueous solution was investigated using atomistic molecular dynamics simulations.