Molecular Dynamics Investigation of the Various Atomic Force Contributions to the Interfacial Tension at the Supercritical CO₂–Water Interface

Abstract

Sequestration of carbon dioxide (CO(2)) in deep, geological formations involves the injection of supercritical CO(2) into depleted reservoirs containing fluids such as brine or oil. The interfacial tension (IFT) between supercritical CO(2) and the reservoir fluid is an important contribution to the sequestration efficiency. In turn, the IFT is a complex function of the reservoir fluid phase composition, the molecular structure of each reservoir fluid component, and environmental conditions (i.e., temperature and pressure). Molecular dynamics simulations can be used to probe the dependence of the IFT on these factors, since the IFT can be calculated directly from the simulated atomic forces and velocities at system equilibrium using the mechanical definition of the IFT. Here, we examine the contribution of each type of atomic force to the IFT, including bonded and nonbonded forces, as quantified by the anisotropy of the atomic virial tensor. In particular, we first examine a supercritical CO(2)-pure liquid water interface, at typical reservoir conditions (temperature of 343 K and pressure of 20 MPa), as a reference state against which CO(2)-brine systems can be compared. In this system, we note that the interactions between water molecules and between CO(2) molecules ("self" interactions) contribute positively to the IFT, while the interactions between water and CO(2) molecules ("cross" interactions) contribute negatively to the IFT. We find that the magnitude of the water "self" interactions is the dominant contribution. In terms of specific types of forces, we find that nonbonded electrostatic (QQ), bonded angle-bending, and bonded bond-stretching interactions contribute positively to the IFT, while nonbonded Lennard-Jones (LJ) interactions contribute negatively to the IFT. We also find that the balance between the LJ interactions and the bond-stretching interactions, in particular, plays a significant role in determining the magnitude of the IFT. Using orientational probability distribution functions to study molecular ordering about the interface, we find that the CO(2) molecules prefer to lie parallel to the interface at the Gibbs dividing surface (GDS) and that both the CO(2) and the water molecules are more ordered at the GDS than in the bulk. Finally, we present an initial study of a CO(2)-brine system with CaCl(2) as the model salt at a concentration of 2.7 M. We quantify the effect of the salt on the molecular orientation of water, and show that this effect leads to an increase in the IFT, relative to the CO(2)-water system, which is consistent with experimental measurements.

Molecular dynamics computations of brine–CO2 interfacial tensions and brine–CO2–quartz contact angles and their effects on structural and residual trapping mechanisms in carbon geo-sequestration

“…Our simulated isotherms for subcritical and supercritical states (see Fig. 2) are in good agreement with experimental data [28,54], and also agree well with previous simulations [55] at 300 K, in the low-pressure range. Nonetheless some deviations are noticeable when approaching the vapour-liquid transition.…”

“…Despite this fact the influence of salt on c has not been considered systematically in previous computer simulation studies. In a very recent work the effect of 2.7 M CaCl 2 on c has been investigated, showing that the addition of salt results in an increase of the interfacial tension [54].…”

Molecular Dynamics Characterizations of the Supercritical CO2–Mediated Hexane–Brine Interface

“…27 At the hexane-water interface, differences between the interfacial and bulk liquids were observed, such as the hydrogen-bonding characteristics of water. 28 Special molecular orientational order patterns for hexane molecules were found in the miscible 5 hydrocarbons/fluid interface.…”

High-pressure densities and interfacial tensions of binary systems containing carbon dioxide+n-alkanes: (n-Dodecane, n-tridecane, n-tetradecane)

“…On the other hand, theoretical descriptions of these mixtures have been made by employing Density Functional Theory (DFT) [13,14,22], Density Gradient Theory (DGT) or Square Gradient Theory (SGT) [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. Furthermore, molecular simulations based either on Molecular Dynamics or Monte Carlo schemes have also been employed [19,30,31,39,40,41] to garner information on the interfacial properties of these systems. An analysis of the above mentioned references reveals that there are key points that must be taken into account: First, an unequivocal full description of interfacial properties requires -at least-two independent approaches, as some previous works have demonstrated (see for instance and references therein).…”