Ionic Effects on Supercritical CO2-Brine Interfacial Tensions: Molecular Dynamics Simulations and a Universal Correlation with Ionic Strength, Temperature, and Pressure

Estimation of CO2-Brine Interfacial Tension Based on an Advanced Intelligent Algorithm Model: Application for Carbon Saline Aquifer Sequestration

The emission reduction of the main greenhouse gas, CO2, can be achieved via carbon capture, utilization, and storage (CCUS) technology. Geological carbon storage (GCS) projects, especially CO2 storage in deep saline aquifers, are the most promising methods for meeting the net zero emission goal. The safety and efficiency of CO2 saline aquifer storage are primarily controlled by structural and capillary trapping, which are significantly influenced by the interactions between fluid and solid phases in terms of the interfacial tension (IFT) between the injected CO2 and brine at the reservoir site. In this study, a model based on the random forest (RF) model and the Bayesian optimization (BO) algorithm was developed to estimate the IFT between the pure and impure gas-brine binary systems for application to CO2 saline aquifer sequestration. Then three heuristic algorithms were applied to validate the accuracy and efficiency of the established model. The results of this study indicate that among the four mixed models, the Bayesian optimized random forest model fits the experimental data with the smallest root-mean-square error (RMSE = 1.7705) and mean absolute percentage error (MAPE = 2.0687%) and a high coefficient of determination (R2 = 0.9729). Then the IFT values predicted via this model were used as an input parameter to estimate the CO2 sequestration capacity of saline aquifers at different depths in the Tarim Basin of Xinjiang, China. The burial depth had a limited influence on the CO2 storage capacity.

Ab Initio Characterization of the CO2-Water Interface Using Unsupervised Machine Learning for Dimensionality Reduction

Precise characterization of the supercritical CO2-water interface under high pressure and temperature conditions is crucial for the geological storage of carbon dioxide (CO2) in deep saline aquifers. Molecular dynamics (MD) simulations offer a valuable approach to gaining insight into the CO2-water interface at a microscopic level. However, no attempt has been made to characterize the CO2-water interface with the accuracy afforded by ab initio calculations. In this study, we performed ab initio MD (AIMD) simulations to investigate the structural and dynamical properties of the CO2-water interface, comparing the results with those obtained from classical force-field MD (FF-MD) simulations. Molecular orientation at the interface was well reproduced in both AIMD and FF-MD simulations. Characteristic structural fluctuations of water at the interface were unveiled by applying multidimensional scaling and time-dependent principal component analysis to the AIMD trajectories; however, they were not prominent in the FF-MD simulations. Furthermore, our study demonstrated a marked difference in the residence time of molecules in the interface region between AIMD and FF-MD simulations, indicating that time-dependent properties of the CO2-water interface strongly depend on the description of the intermolecular forces.

Atomic-Scale Insights into the Phase Behavior of Carbon Dioxide and Water from 313 to 573 K and 8 to 30 MPa

We performed molecular dynamics (MD) simulations of CO2 + H2O systems by employing widely used force fields (EPM2, TraPPE, and PPL models for CO2; SPC/E and TIP4P/2005 models for H2O). The phase behavior observed in our MD simulations is consistent with the coexistence lines obtained from previous experiments and SAFT-based theoretical models for the equations of state. Our structural analysis reveals a pronounced correlation between phase transitions and the structural orderliness. Specifically, the coordination number of Ow (oxygen in H2O) around other Ow significantly correlates with phase changes. In contrast, coordination numbers pertaining to the CO2 molecules show less sensitivity to the thermodynamic state of the system. Furthermore, our data indicate that a predominant number of H2O molecules exist as monomers without forming hydrogen bonds, particularly in a CO2-rich mixture, signaling a breakdown in the hydrogen bond network's orderliness, as evidenced by a marked decrease in tetrahedrality. These insights are crucial for a deeper atomic-level understanding of phase behaviors, contributing to the well-grounded design of CO2 injection under high-pressure and high-temperature conditions, where an atomic-scale perspective of the phase behavior is still lacking.

Interfacial Tensions, Solubilities, and Transport Properties of the H2/H2O/NaCl System: A Molecular Simulation Study

Data for several key thermodynamic and transport properties needed for technologies using hydrogen (H2), such as underground H2 storage and H2O electrolysis are scarce or completely missing. Force field-based Molecular Dynamics (MD) and Continuous Fractional Component Monte Carlo (CFCMC) simulations are carried out in this work to cover this gap. Extensive new data sets are provided for (a) interfacial tensions of H2 gas in contact with aqueous NaCl solutions for temperatures of (298 to 523) K, pressures of (1 to 600) bar, and molalities of (0 to 6) mol NaCl/kg H2O, (b) self-diffusivities of infinitely diluted H2 in aqueous NaCl solutions for temperatures of (298 to 723) K, pressures of (1 to 1000) bar, and molalities of (0 to 6) mol NaCl/kg H2O, and (c) solubilities of H2 in aqueous NaCl solutions for temperatures of (298 to 363) K, pressures of (1 to 1000) bar, and molalities of (0 to 6) mol NaCl/kg H2O. The force fields used are the TIP4P/2005 for H2O, the Madrid-2019 and the Madrid-Transport for NaCl, and the Vrabec and Marx for H2. Excellent agreement between the simulation results and available experimental data is found with average deviations lower than 10%.

Interfacial Tension-Temperature-Pressure-Salinity Relationship for the Hydrogen-Brine System under Reservoir Conditions: Integration of Molecular Dynamics and Machine Learning

Hydrogen (H2) underground storage has attracted considerable attention as a potentially efficient strategy for the large-scale storage of H2. Nevertheless, successful execution and long-term storage and withdrawal of H2 necessitate a thorough understanding of the physical and chemical properties of H2 in contact with the resident fluids. As capillary forces control H2 migration and trapping in a subsurface environment, quantifying the interfacial tension (IFT) between H2 and the resident fluids in the subsurface is important. In this study, molecular dynamics (MD) simulation was employed to develop a data set for the IFT of H2-brine systems under a wide range of thermodynamic conditions (298-373 K temperatures and 1-30 MPa pressures) and NaCl salinities (0-5.02 mol·kg-1). For the first time to our knowledge, a comprehensive assessment was carried out to introduce the most accurate force field combination for H2-brine systems in predicting interfacial properties with an absolute relative deviation (ARD) of less than 3% compared with the experimental data. In addition, the effect of the cation type was investigated for brines containing NaCl, KCl, CaCl2, and MgCl2. Our results show that H2-brine IFT decreases with increasing temperature under any pressure condition, while higher NaCl salinity increases the IFT. A slight decrease in IFT occurs when the pressure increases. Under the impact of cation type, Ca2+ can increase IFT values more than others, i.e., up to 12% with respect to KCl. In the last step, the predicted IFT data set was used to provide a reliable correlation using machine learning (ML). Three white-box ML approaches of the group method of data handling (GMDH), gene expression programming (GEP), and genetic programming (GP) were applied. GP demonstrates the most accurate correlation with a coefficient of determination (R2) and absolute average relative deviation (AARD) of 0.9783 and 0.9767%, respectively.

Interfacial properties of the hexane + carbon dioxide + water system in the presence of hydrophilic silica

Molecular dynamics simulations were conducted to study the interfacial behavior of the CO₂ + H₂O and hexane + CO₂ + H₂O systems in the presence of hydrophilic silica at geological conditions. Simulation results for the CO₂ + H₂O and hexane + CO₂ + H₂O systems are in reasonable agreement with the theoretical predictions based on the density functional theory. In general, the interfacial tension (IFT) of the CO₂ + H₂O system exponentially (linearly) decreased with increasing pressure (temperature). The IFTs of the hexane + CO₂ + H₂O (two-phase) system decreased with the increasing mole fraction of CO₂ in the hexane/CO₂-rich phase x_CO2 . Here, the negative surface excesses of hexane lead to a general increase in the IFTs with increasing pressure. The effect of pressure on these IFTs decreased with increasing x_CO2 due to the positive surface excesses of carbon dioxide. The simulated water contact angles of the CO₂ + H₂O + silica system fall in the range from 43.8° to 76.0°, which is in reasonable agreement with the experimental results. These contact angles increased with pressure and decreased with temperature. Here, the adhesion tensions are influenced by the variations in fluid-fluid IFT and contact angle. The simulated water contact angles of the hexane + H₂O + silica system fall in the range from 58.0° to 77.0° and are not much affected by the addition of CO₂. These contact angles increased with pressure, and the pressure effect was less pronounced at lower temperatures. Here, the adhesion tensions are mostly influenced by variations in the fluid-fluid IFTs. In all studied cases, CO₂ molecules could penetrate into the interfacial region between the water droplet and the silica surface.

Interfacial behavior of the decane + brine + surfactant system in the presence of carbon dioxide, methane, and their mixture

Molecular dynamics simulations are carried out to get insights into the interfacial behavior of the decane + brine + surfactant + CH₄ + CO₂ system at reservoir conditions. Our results show that the addition of CH₄, CO₂, and sodium dodecyl sulfate (SDS) surfactant at the interface reduces the IFTs of the decane + water and decane + brine (NaCl) systems. Here the influence of methane was found to be less pronounced than that of carbon dioxide. As expected, the addition of salt increases the IFTs of the decane + water + surfactant and decane + water + surfactant + CH₄/CO₂ systems. The IFTs of these surfactant-containing systems decrease with temperature and the influence of pressure is found to be less pronounced. The atomic density profiles show that the sulfate head groups of the SDS molecules penetrate the water-rich phase and their alkyl tails are stretched into the decane-rich phase. The sodium counterions of the surfactant molecules are located very close to their head groups. Furthermore, the density profiles of water and salt ions are hardly affected by the presence of the SDS molecules. However, the interfacial thickness between water and decane/CH₄/CO₂ molecules increases with increasing surfactant concentration. An important result is that the enrichment of CH₄ and/or CO₂ in the interfacial region decreases with increasing surfactant concentration. These results may be useful in the context of the water-alternating-gas approach that has been utilized during CO₂-enhanced oil recovery operations.

Molecular Dynamics Simulations Study on the Shear Viscosity, Density, and Equilibrium Interfacial Tensions of CO2 + Brines and Brines + CO2 + n-Decane Systems

The shear viscosity, density, and interfacial tensions (IFT) of two systems, namely, brine and brine/n-decane, blended with carbon dioxide (CO₂) were investigated via molecular dynamics simulations over broad ranges of temperature, pressure, CO₂ mole fraction, and brine concentration. The operating conditions for the molecular simulations to be studied are similar to the CO₂ geological storage processes. The effects of temperature, pressure, and concentrations on the viscosity and IFT have been investigated and analyzed. All four influencing parameters affect the shear viscosity and IFT. The pressures and temperatures up to 1000 bar and 573 K, respectively, were used for predicting the viscosity and IFT by considering intermolecular interactions, while salinities up to 32 000 ppm and CO₂ mole fractions between 0 and 0.5 were used in the simulations. Comparisons were made between simulated values and the predicted results of an empirical correlation, both against experimental data. Both monovalent and divalent ions and their mixtures were used in the simulations, and the results showed that monovalent ions impose stronger interactions in the solution than divalents. The results have revealed that the supercritical CO₂'s capability to reduce the IFT of the brine/n-decane interface is remarkable, which makes it a promising agent for underground geological injection for enhanced oil recovery. Also, viscosity and density ratio analysis have confirmed the viability of CO₂ storage in deep saline aquifers, where harsh geothermal conditions of high salinities limit the extent of the experiments. The molecular simulation results are in good qualitative agreement with the experimental data available in the literature for the viscosity, density, and IFT.

Interaction of low salinity surfactant nanofluids with carbonate surfaces and molecular level dynamics at fluid-fluid interface at ScCO2 loading

HYPOTHESIS

The advanced low salinity aqueous formulations are yet to be validated as an injection fluid for enhanced oil recovery (EOR) from the carbonate reservoirs and CO₂ geosequestration. Interaction of various ionic species present in the novel low salinity surfactant nanofluids with scCO₂/CO₂ saturated aqueous phase interface and at the interface of CO₂ saturated aqueous phase/mixed wet (with CO₂ and Decane) limestone surface at the conditions of low salinity at reservoir conditions are to yet to be understood.

EXPERIMENTS

This study, carried out for the first time in low salinity at scCO₂ loading conditions at 20 MPa pressure and 343 K temperature, comprises of wettability study of the limestone surface by aqueous phase contact angle measurements using ZrO₂ nanoparticles (in the concentration range of 100-2000 mg/L) and 0.82 mM Hexadecyltrimethylammonium bromide (CTAB) surfactant. Molecular dynamics simulations results were used to understand the underlying mechanism of wettability alteration and interfacial tension (IFT) change.

FINDINGS

This study reveals that a low dosage (100 mg/L) of ZrO₂ nanoparticles forming ZrO₂-CTAB nano-complexes helps in wettability alteration of the rock surface to more water-wetting state; certain ionic species augment this effect when used in appropriate concentration. Also, these nano-complexes helps in scCO₂/CO₂ saturated aqueous phase IFT reduction. This study can be used to design advanced low salinity injection fluids for water alternating gas injection for EOR and CO₂ geosequestration projects.

Wetting Properties of the CO2-Water-Calcite System via Molecular Simulations: Shape and Size Effects

Assessment of the risks and environmental impacts of carbon geosequestration requires knowledge about the wetting behavior of mineral surfaces in the presence of CO₂ and the pore fluids. In this context, the interfacial tension (IFT) between CO₂ and the aqueous fluid and the contact angle, θ, with the pore mineral surfaces are the two key parameters that control the capillary pressure in the pores of the candidate host rock. Knowledge of these two parameters and their dependence on the local conditions of pressure, temperature, and salinity is essential for the correct prediction of structural and residual trapping. We have performed classical molecular dynamics simulations to predict the CO₂-water IFT and the CO₂-water-calcite contact angle. The IFT results are consistent with previous simulations, where simple point charge water models have been shown to underestimate the water surface tension, thus affecting the simulated IFT values. When combined with the EPM2 CO₂ model, the SPC/Fw water model indeed underestimates the IFT in the low-pressure region at all temperatures studied. On the other hand, at high pressure and low temperature, the IFT is overestimated by ∼5 mN/m. Literature data regarding the CO₂/water/calcite contact angle on calcite are contradictory. Using our new set of force field parameters, we performed NVT simulations at 323 K and 20 MPa to calculate the contact angle of a water droplet on the calcite {10.4} surface in a CO₂ atmosphere. We performed simulations for both spherical and cylindrical droplet configurations for different initial radii to study the size dependence of the water contact angle on calcite in the presence of CO₂. Our results suggest that the contact angle of a cylindrical droplet, is independent of droplet size, for droplets with a radius of 50 Å or more. On the contrary, spherical droplets make a contact angle that is strongly influenced by their size. At the largest size explored in this study, both spherical and cylindrical droplets converge to the same contact angle, 38°, indicating that calcite is strongly wetted by water.

Effect of Ion Valency on the Properties of the Carbon Dioxide-Methane-Brine System

Molecular dynamics simulations and theoretical analysis were carried out to study the bulk and interfacial properties of carbon dioxide-methane-water and carbon dioxide-methane-brine systems under geological conditions. The density gradient theory with the bulk phase properties estimated using the cubic-plus-association (CPA) equation of state (EoS) can well describe the increase in the interfacial tension (IFT) of the CO₂-water system in the presence of methane. The theoretical estimates of species mole fractions in the carbon dioxide-methane-water system are in good quantitative agreement with the experimental results. Furthermore, simulations of carbon dioxide-methane-brine system show that the IFT of the CaCl₂ case is generally higher than that of the NaCl case. This is probably due to the stronger hydration of Ca²⁺ ions and their stronger repulsion from the interface compared to Na⁺. While the overall shape of the ionic profiles is not much affected by the ion type, the water profiles show a local enrichment at the interface in the system with CaCl₂. In contrast to the case of NaCl, the slopes of the plots of IFT vs CaCl₂ concentration are dependent on temperature. Species mole fractions in the carbon dioxide-methane-brine system predicted by combining the CPA EoS with the Debye-Hückel electrostatic term are in good agreement with simulation results.

Molecular Gibbs Surface Excess and CO2-Hydrate Density Determine the Strong Temperature- and Pressure-Dependent Supercritical CO2-Brine Interfacial Tension

In CO₂ geological storage, the interfacial tension (IFT) between supercritical CO₂ and brine is critical for the storage capacitance design to prevent CO₂ leakage. IFT relies not only on the interfacial molecule properties but also on the environmental conditions at different storage sites. In this paper, supercritical CO₂-NaCl solution systems are modeled at 343-373 K and 6-35 MPa under the salinity of 1.89 mol/L using molecular dynamics simulations. After computing and comparing the molecular density profile across the interface, the atomic radial distribution function, the molecular orientation distribution, the molecular Gibbs surface excess (derived from the molecular density profile), and the CO₂-hydrate number density under the above environmental conditions, we confirm that only the molecular Gibbs surface excess of CO₂ molecules and the CO₂-hydrate number density correlate strongly with the temperature- and pressure-dependent IFTs. We also compute the populations of two distinct CO₂-hydrate structures (T-type and H-type) and attribute the observed dependence of IFTs to the dominance of the more stable, surfactant-like T-type CO₂-hydrates at the interface. On the basis of these new molecular mechanisms behind IFT variations, this study could guide the rational design of suitable injecting environmental pressure and temperature conditions. We believe that the above two molecular-level metrics (Gibbs surface excess and hydrate number density) are of great fundamental importance for understanding the supercritical CO₂-water interface and engineering applications in geological CO₂ storage.