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Reduction in interfacial tension of water–oil interface by supercritical CO2 in enhanced oil recovery processes studied with molecular dynamics simulation
Accepted Manuscript Title: Reduction in Interfacial Tension of Water-Oil Interface by Supercritical CO2 in Enhanced Oil Recovery Processes studied with Molecular Dynamics Simulation Author: Bing Liu Junqin Shi Muhan Wang Jun Zhang Baojiang Sun Yue Shen Xiaoli Sun PII: DOI: Reference:
S0896-8446(15)30174-1 http://dx.doi.org/doi:10.1016/j.supflu.2015.11.001 SUPFLU 3497
To appear in:
J. of Supercritical Fluids
Received date: Revised date: Accepted date:
21-7-2015 31-10-2015 1-11-2015
Please cite this article as: B. Liu, J. Shi, M. Wang, J. Zhang, B. Sun, Y. Shen, X. Sun, Reduction in Interfacial Tension of Water-Oil Interface by Supercritical CO2 in Enhanced Oil Recovery Processes studied with Molecular Dynamics Simulation, The Journal of Supercritical Fluids (2015), http://dx.doi.org/10.1016/j.supflu.2015.11.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reduction in Interfacial Tension of Water-Oil Interface by
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Supercritical CO2 in Enhanced Oil Recovery Processes studied
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with Molecular Dynamics Simulation
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Bing Liu a, Junqin Shi a, Muhan Wang a, Jun Zhang a,*, Baojiang Sun b,*, Yue Shen a, Xiaoli
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Sun a
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a
College of Science, China University of Petroleum, 266580, Qingdao, China
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b
School of Petroleum Engineering, China University of Petroleum, 266580, Qingdao, China
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Abstract: The interfacial tension (IFT) has a significant influence on fluid flow in the
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supercritical CO2 (scCO2) enhanced oil recovery process. However, it is still challenging to
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demonstrate the effect of scCO2 on water-oil interface. Therefore, a molecular dynamics
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simulation is performed to investigate the influence of scCO2 on the water-decane interface.
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It is observed that CO2 prefers to accumulate and display surface-active at the water-decane
13
interface. The driving force of CO2 accumulation is the IFT difference between water-decane
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and water-CO2. The IFT of water-CO2-decane decreases linearly with the increase of CO2.
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The interactions of water-CO2 and decane-CO2 are strong, and the difference between them is
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small, providing the mechanism for lowering the IFT of water-decane. The increase in the
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diffusivity for all fluids toward the interface suggests an increase in the mobility at the
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water-decane interface due to the addition of CO2, which is in favor of enhancing oil
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recovery.
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Keywords: water-decane interface; supercritical CO2; interfacial tension; molecular
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dynamics simulation
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1. Introduction Enhanced oil recovery (EOR) by CO2 flooding is a significant technique to help
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common development in terms of energy and environment [1]. It not only increases the crude
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oil recovery from oil reservoirs for energy security but also facilitates carbon dioxide capture
27
and sequestration (CCS) for the reduction in greenhouse gas emission [1-8]. CO2-EOR
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accompanying with geological CO2 storage is largely controlled by the interfacial properties
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among CO2, crude oil and water [9-12]. It is important, therefore, for the interfacial
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phenomenon or behavior pertaining to the crude oil, water and injected CO2 to be understood
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for the improvement of CO2-EOR and CCS process.
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The interfacial tension (IFT) has an effect on capillary pressure and permeability, and
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then the oil flow process through capillary of porous medium is also influenced [1,2].
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Moreover, in the tertiary recovery or EOR methods, when the reservoir pressure or injection
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pressure is above the minimum miscibility pressure (MMP), CO2 and oil become completely
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miscible and thereby their IFT is down to zero [13,14]. Thus, the IFT of the system composed
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of CO2, water and oil should be determined between water and CO2-oil mixture. Experiments
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indicated that the IFT decreases when the mole fraction of CO2 in mixture increases [15-17].
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In addition, the interfacial characteristics of water-oil showed some vital changes due to the
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different solubility of CO2 in oil and water. However, to the best of our knowledge, the effect
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of CO2 on water-oil interface has not been obtained and it is still quite challenging to
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illuminate the interaction mechanism in the ternary CO2, oil and water system from
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experiments.
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Molecular dynamics (MD) simulations have been performed to investigate interfacial 2
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properties for complex binary or ternary systems. Kunieda et al. [18] investigated the oil-fluid
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interface and found that the aromatic molecules in light oil accumulate at the interface. This
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accumulation phenomenon at oil-water interface was ascribed to the weak hydrogen bond
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interaction between water and aromatic hydrocarbon [8]. Mikami et al. [19] studied a similar
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interface and showed that the asphaltene molecules assemble from nanoaggregation to
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thin-film formation at the oil-water interface. Carpenter et al. [20-23] observed the
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differences between the interfacial and bulk liquids, and special molecular orientation for
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hydrocarbon molecules was found at the miscible hydrocarbon-fluid interface. For the
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CO2-water interface system, the MD studies indicated that CO2 molecules exhibit
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hydrophilicity and interact closely with water molecules at the interface [24,25]. However,
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there exist very limited studies for the ternary water-CO2-oil system, especially the
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microscopic behavior of CO2 distributing at interface, the structural and dynamic properties
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of water-oil interface have not been reported.
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In this work, our objective is to shed light on the influence of supercritical CO2 (scCO2)
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on water-oil interface. We hope to illustrate that how and why CO2 accumulate at the
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water-oil interface and reduce IFT. For this purpose, the MD simulation is carried out to
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investigate the interface of water-scCO2-decane system under the conditions of 10 MPa and
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313 K. Here, decane is selected to be a simply representation of crude oil. Through classical
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MD simulation, we study the density profile, structural information for molecules in interface
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systems, IFT, the intermolecular interaction, and transport property. The results are helpful to
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understand the mechanism of scCO2 influencing the water-oil interface, which is of
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paramount importance to future design of methodologies for CO2 EOR. 3
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2. Computational methods
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2.1 Simulation Details MD simulations were carried out using Materials Studio software package [26] with the
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condensed-phase optimized molecular potential for atomistic simulation studies (COMPASS)
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force field describing all atoms in this study. COMPASS, a widely used all-atom force field
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based on ab initio and optimization by the experimental data, has been validated to be capable
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of accurately predicting structural and thermophysical properties for a broad range of organic
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and inorganic substances including water, oil and CO2.[27] This force field includes the
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bonded potential and the non-bonded potential. The bonded potential is composed of bond
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stretching, angular bending, dihedral angle torsion, out-of-plane interactions and cross terms;
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the non-bonded potential consists of the long-range electrostatic interactions and the
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short-range van der Waals (vdW) interactions [28]. The electrostatic interactions are
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calculated by Coulombic equation (1) and vdW interactions are represented by Lennard-Jones
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function (2),
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Eele = C
qi q j
(1)
εR
6 R 9 R EvdW = D0 2 0 − 3 0 R R
(2)
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where C = 332 .0647 ( kcal mol ) Å e 2 is unit conversion factor, ε
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constant, qi and q j are the partial charge of atom i and j , D0 is the depth of potential
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well, R0 is the Lennard-Jones radius, and R is the distance between two atoms.
is relative dielectric
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We performed a series of all-atom MD simulations of systems composed of water,
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decane and CO2 to investigate the effects of scCO2 on the interface between water and decane 4
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at 313 K and 10 MPa. The initial simulation box for the ternary water-CO2-decane system
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consists of three independent phases as shown in Fig. 1. A similar system was reported by
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Makimura [29]. Here, the two water phases involves totally 1800 molecules while the middle
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phase is the decane-CO2 mixture containing 150 decane molecules and different numbers of
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CO2 molecules. The number of CO2 molecules in the mixture is set to be 0, 37, 100, 225, 600
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and 800 corresponding to the mole fraction of CO2 of 0, 0.2, 0.4, 0.6, 0.8 and 1.0,
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respectively. The dimensions of water phase are 40 Å×40 Å×15.8 Å. The dimensions of the
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cross section for decane-CO2 binary phase are also 40 Å×40 Å while it is determined by its
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density in z direction. Periodic boundary conditions were applied in all directions, and the
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primary system thus consisted of two water-CO2-decane interfaces.
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The reasonable decane-CO2 mixtures were determined by their densities obtained by
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carrying out MD simulations in the NPT ensemble for 1.5 ns at 313 K and 10 MPa. The
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simulated densities of the decane-CO2 mixtures are plotted in Fig. S1 (in supplementary
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material). This figure shows the simulated densities are in well agreement with the
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experimental values [30,31], meaning that COMPASS force field is suitable to be applied in
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the water-CO2-decane interfacial system. The effects of CO2 on the water-decane interface
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were obtained from MD simulations of the water-CO2-decane interfacial system in the NPT
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ensemble for 2 ns at 313 K and 10 MPa. Finally, we performed MD simulations of the
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interfacial system in the canonical ensemble (NVT) for 1 ns at 313K. For MD simulations in
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the NPT ensembles, the temperature and the pressure were controlled by the Andersen
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thermostat [32] and Berendsen barostat [33], respectively. For MD simulations in the NVT
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ensembles, the temperature was controlled by Nosé thermostat [34]. For all the simulations, a
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cutoff distance of 13.5 Å was applied to the vdW interactions, the electrostatic interactions
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were treated using the Ewald summation [35], the time step was 1 fs, full trajectory was
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saved and the frames were output every 1 ps for the analysis of the results.
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2.2 Calculation Methods
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The interfacial tension of the water-CO2-decane interface was calculated using the
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formulation of the Gibbs interfacial tension [8]. Two interfaces in the water-CO2-decane
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system are both perpendicular to the z axis and parallel to the xy plane, hence the interfacial
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tension is evaluated from the expression of pressure tensor [36]
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1 Px + Py − Pz Lz 2 2
γ =−
(3)
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where Pa = paa (a = x, y, z) is the diagonal elements of the pressure tensor, and Lz is the
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length of the simulation box in z direction.
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According to the Einstein relation, the diffusion coefficient (D), describing the
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thermodynamic property of the interface, is calculated from the mean square displacement
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(MSD)
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D=
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3. Results and discussions
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3.1 Density profile
2 1 d n lim ∑ Ri ( t ) − Ri ( 0 ) 6 t →∞ dt i
(4)
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Fig. 2 shows the equilibrium configuration and density profile along the direction
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normal to the interface (z-axis) for the water-CO2-decane system with CO2 mole fraction of
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0.6. Herein, the interface is determined by the Gibbs dividing surface (GDS) where the
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excess decane molecules at the water side are approximately equal to the excess water 6
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molecules at the decane side. The interfacial region is established by the “10-90” interfacial
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width (∆z) commonly characterized by the distance between the two surfaces at the 10% and
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90% of the total density along the z axis [8,37,38]. The configuration shows that quite a few
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CO2 molecules penetrate into water phase and a host of CO2 molecules accumulate in the
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interfacial region though CO2 molecules participate in the miscible phase with decane under
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the simulated condition. The density profile shows an obvious peak for CO2 at the interface
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where the density profiles of water and decane intersect. It indicates the formation of CO2
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film in the interfacial region just as the adsorbed layer of CO2 at liquid-gas interface [39-42].
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The CO2 film bridges water and decane phases and thus it could have an important effect on
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the water-decane IFT, resembling the influence of the surfactant absorbed on the water-oil
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interface [43,44].
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Further insight into the accumulation of CO2 molecules at the interface is demonstrated
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by the density profiles in Fig. S2 (in supplementary material) and Fig. 3. The density of CO2
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molecules at the interface is between 3 to 5 times that in the decane phase and raises rapidly
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with the increase of CO2 mole fraction. It shows an enhanced accumulation of CO2 at the
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interface as CO2 mole fraction increases, which is ascribed to that CO2 is more hydrophilic
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than decane driven away from the interface.
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3.2 Structure of water-decane interface
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The accumulation of CO2 molecules will result in an obvious variation of the structure
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of the water-decane interface. Here, the interfacial structure is investigated from several
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aspects as follow. First, we present the snapshots of the surfaces of both water phase and
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decane phase in Fig. 4 where CO2 mole fractions are 0, 0.4 and 0.8, respectively. The 7
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enhanced corrugation or roughness of the surfaces is observed as CO2 mole fraction increases.
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It indicates a higher anisotropic distribution for molecules at the surfaces due to that CO2
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molecules separate from the decane-CO2 mixture and penetrate into water. Therefore, the
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interfacial roughness is related to molecular penetrations (Fig. 2, Fig. 3 and Fig. S2), similar
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to the effect of the amphiphilic surfactant on the water-oil interface [43].
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Then, we determined the interfacial width in Table 1 using the “10-90” interface width.
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The interfacial width obviously increases from 4.02 Å to 11.73 Å as CO2 mole fraction
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increases from 0 to 0.8. It implies the deeper molecular penetrations broadening the
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coexistent region of water and decane. Because of the strong diffusibility of scCO2 regarded
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as a polar molecule with two active and considerably strong bond dipoles [45], CO2 is
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capable of being partially miscible with water. However, decane is immiscible with water for
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its stronger hydrophobic interaction. As CO2 mole fraction increases, more CO2 molecules
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may move towards the interface and form strong interactions with water, such as the
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hydrogen bond, leading to the deeper molecular penetrations featured by interfacial width.
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Finally, based on the interfacial width in Table 1, we depict the configurations of the
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water and decane molecules in the vicinity of interface in Fig. 5 when CO2 mole fractions are
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0, 0.4 and 0.8, respectively. A clear interface between water phase and decane phase is
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observed and the decane molecules in the interfacial region are almost parallel to the interface
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in Fig. 5A. With the increase in CO2 mole fraction, shown in Fig. 5B and C, the interface
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becomes indistinct and the decane molecules in the interfacial region show a tendency to
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rotate from the parallel orientation to the perpendicular orientation. To confirm the variation
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of the orientation, the statistic average of the orientation angle of the decane molecules in the 8
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interfacial region is calculated in Table 1. Here, the orientation angle is defined as the angle
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between the vector joining two carbon atoms at both ends of the decane molecule and the
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interfacial normal vector. As CO2 mole fraction increases, the orientation angle of decane
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molecule decreases. It indicates decane molecules move away from the interface to the bulk
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phase. The rotation of decane molecules from the parallel to the perpendicular to the interface
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provides extra space for the deeper penetrations of water and CO2 molecules into decane
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phase, which further expands and blurs the water-oil interface.
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3.3 Interfacial tension
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Fig. 6 shows the dependence of the IFT of the water-CO2-decane system on CO2 mole
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fraction. The IFT is slightly lower than that of water-CO2-hexane system [29] but larger than
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that of water-CO2-light oil system [46]. The disparity mainly results from the different
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conditions and alkanes we used. The IFT of water-decane is 50.88 mN/m while the IFT of
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water-CO2 is 32.58 mN/m, being a well agreement with the experimental data [6,47]. They
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correspond to CO2 mole fractions of 0 and 1.0 respectively. It shows that COMPASS force
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field is reliable to accurately predict structural and thermophysical properties of the
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water-CO2-decane system.
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Fig. 6 also shows the IFT linearly decreases with the increase in CO2 mole fraction.
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Combining with table 1 showing the increasing interfacial width with CO2 mole fraction, we
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find a negative correlation between the IFT and the interfacial width. It is consistent with the
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conclusion that wide interfaces result in low IFT values [2,48]. In addition, it is noteworthy
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that such a CO2 accumulation is not observed at water-CO2 interface corresponding to the
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CO2 mole fraction of 1.0 (Fig. S2). It shows that CO2 displays “surface-active” at 9
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water-decane interface but “inactive” at water-CO2 interface just as non-amphiphilic aromatic
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at water-oil interface [8,18]. Therefore, we consider that the accumulation of CO2 molecules
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at the interface is driven by the IFT difference between water-CO2 and water-decane. On the
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one hand, when CO2 molecules accumulate in the interfacial region, the configuration
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entropy decreases, similar to the case for the adsorption of surfactants and the accumulation
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of aromatics [8,18,43], and the Gibbs free energy increases within the interface system. On
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the other hand, the IFT of water-CO2 is ~18 mN/m lower than that of water-decane (Fig. 6),
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so the lowered IFT should result from the accumulation of CO2 at the water-CO2-decane
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interface, resulting in the decrease in the potential energy of the total system. As a result, the
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Gibbs free energy reaches a minimum value with the CO2 accumulation at equilibrium.
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Consequently, we conclude that the IFT difference is the “driving force” of accumulation of
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CO2.
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3.4 Intermolecular interaction
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The reduction in the IFT caused by CO2 should be related to intermolecular interactions
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between different components in the interfacial region. To investigate this relation, we
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calculate interaction energies of decane-CO2 and water-CO2 as a function of CO2 mole
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fraction in Fig. 7. As CO2 mole fraction increases, the both interactions dramatically increase
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and they are comparable although the interaction between water and CO2 is slightly stronger
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than that between decane and CO2. The results indicate that CO2 can stably accumulate at the
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water-decane interface, therefore forming the interfacial film due to the balance between the
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interactions acting on CO2 by water and decane respectively. Combining with the reduction
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in the IFT with the increase in CO2 mole fraction (Fig. 6), we infer that the comparable and 10
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strong interactions of water-CO2 and decane-CO2 provide the mechanism for lowering the
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IFT. Therefore, we speculate that if the interaction of CO2-water and that of CO2-oil are
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strong and the difference of these two interactions is small, then CO2 is favorable for the
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decrease in IFT of water-oil.
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In addition, the hydrogen bond (H-bond), found between water and CO2 [49,50], is
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partly responsible for the effect of CO2 on the water-decane interface. In this study, the
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H-bond is calculated within the maximum hydrogen-acceptor distance of 2.50 Å. The H-bond
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is identified by the radial distribution function (RDF) between water and CO2 in the
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interfacial region in Fig. 8 where CO2 mole fraction is 0.6. The first peak of the OW-CC RDF
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occurs at 3.90 Å close to the OW-CC distance of 3.82 Å in the inset of Fig. 8 and in Fig. S3 (in
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supplementary material), and the HW-OC RDF displays a pronounced shoulder in the range of
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1.70 Å to 2.52 Å within the length of H-bond. The shoulder means the saturated H-bond
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forming between water and CO2. It is the saturated H-bond that prevents CO2 molecules from
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gathering around a water molecule, resulting in the tiny variation in the nearest neighboring
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CO2 molecules around the water molecule. It indicates the coordination OC atoms around one
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HW atom decrease in the range of 1.70 Å to 2.52 Å and thereby leads to the shoulder [51,52].
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The results provide an evidence for the formation of H-bond between water and CO2
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molecules in water-CO2-decane system. The number of H-bonds of both water-water and
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water-CO2 in the interfacial region is also shown in Table 2. It shows that the number of
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H-bond between water molecules decreases while that between water and CO2 molecules
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increases with the increase in CO2 mole fraction. The decrease in the number of H-bond
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between water molecules partly weakens the network structure and the interaction between
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water molecules, resulting in the higher degree of the interfacial roughness. Meanwhile, the
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increase in the number of H-bond between water and CO2 molecules enhances the interaction
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of water-CO2, stabilizing CO2 interfacial film and strengthening its bridge effect. As a
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consequence, the H-bond between water and CO2 molecules at the interface has an important
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influence on the reduction in the IFT.
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Besides the importance of H-bond discussed above, water-CO2 complexes may have an
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important influence on the interface of water-decane. It is due to the electron deficiency of
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the C atom of CO2 and bond electron density more polarized toward the O atoms, and CO2 in
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the water-CO2-decane system acts as a Lewis acid favoring the formation of electron
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donor-acceptor (EDA) complexes in the presence of water as a Lewis base [53-56]. The
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vibrational spectra of water-(CO2)n complexes using ab initio method indicated that the
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planar T-shaped structure and linear structure at local energy minimum for CO2-water
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dimmer complex are found mainly stabilized by EDA interaction [56]. The HW-OC distance
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of 2.71 Å in the planar linear structure exceeds the range of 1.70 Å to 2.52 Å in Fig. 8, while
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the OW-CC distance of 2.78 Å in T-shaped structure is close to the range of a small shoulder
256
in OW-CC RDF. This probably proves the existence of T-shaped structure in water-CO2-oil
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system as found by Makimura et al. [29]. Furthermore, a trimer water-(CO2)2 complex is
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observed where the arrangement between water and its second neighbor involves a second
259
weak O···H-O H-bond characterized by a distance of 2.13 Å-2.15 Å [56]. The interactions of
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second weak H-bond and Lewis bases-Lewis acid, tuning the inner structural arrangement of
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the CO2-water dimmer and trimer complexes, maybe have important effect on the change in
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IFT which needs further research.
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3.5 Transport properties for water-CO2-decane interface The diffusion coefficient for a multicomponent system, very difficult to determine
265
experimentally, can be directly determined by MD simulations. To further explore the
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transport properties of the water-decane interface influenced by CO2, the diffusion profile of
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the ternary system along the z-axis is monitored as a function of CO2 mole fraction in Fig. 9
268
and self-diffusion coefficients of water and decane in the interfacial region are calculated in
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Table S1 (in supplementary material). Here, the diffusion profile is evaluated by dividing the
270
system into slabs of thickness of 5 Å parallel to the interface. The results we obtained are in
271
good agreement with the values in similar systems [8,57,58] regardless the small variation of
272
the temperature and pressure. For all fluids considered, we observe an increase in the
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diffusion coefficient toward the interface. It suggests an increase in the mobility at the
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water-decane interface due to the addition of CO2.
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In Fig. 9, the diffusion coefficient of molecules keeps unchanged in bulk water or
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decane, and the diffusion coefficient exhibits a slight increase firstly and then decreases from
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the water side to the decane side. The diffusion coefficient of the system reaches the
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maximum at the position more adjacent to the water side, which is in accordance with the
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density profile in Fig. 2 where the density peak of CO2 is more close to the water density
280
profile. It indicates that the strong mass transfer ability of CO2 plays an important role in
281
influencing the transport property of the interface system. On the one hand, the increased
282
mobility of interfacial molecules caused by the CO2 accumulation leads to wider interface
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and rougher interfacial structure, resulting in the reduction in the IFT. On the other hand, the
284
penetration of CO2 in water results in the increase in viscosity and migration ability of water
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[55] while the dissolution of CO2 in decane results in the expansion and viscosity reduction
286
of decane. These improve the decane-water mobility ratio and thereby enhance the sweep
287
efficiency of water.
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4. Conclusion
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The effect of scCO2 on the water-oil interface has been investigated from MD
290
simulations under the conditions of 10 MPa and 313 K, where decane represents oil
291
molecule. It is interestingly observed that CO2 molecules initially miscible with decane
292
accumulate within the water-decane interface. With the increase in CO2 mole fraction, the
293
enhanced accumulation of CO2 induces the deeper molecular penetrations, which increases
294
the interfacial roughness and width and leads to the decane molecules rotating from the
295
parallel to the perpendicular to the interface. From the viewpoint of the Gibbs free energy, the
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“driving force” of CO2 accumulation is difference of the IFT between water and decane and
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that between water and CO2. The calculated IFT shows that it linearly decreases with the
298
increase in CO2 mole fraction and reveals a negative correlation with the interfacial width.
299
Further investigation shows the enhanced interactions for water-CO2 and decane-CO2 and the
300
smaller difference between them are favorable to lower the IFT. Consequently, we could
301
infer that if the interaction between water and CO2 and that between oil and CO2 are strong
302
enough, and the difference between the two interactions is small enough, then CO2 is
303
favorable to decrease the IFT of water-oil. This inference may not be generalized in more
304
ternary interface systems until more tests to be verified. In addition, the strong mass
305
transportation capability of scCO2 molecules enhances the transport properties of water and
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decane, which is particularly important to improve the oil-water mobility ratio and thereby
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enhance the sweep efficiency of water. The investigation of the effect of scCO2 on
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water-decane interface is helpful to understand many interfacial phenomena in scientific and
309
industrial processes and facilitate the application of CO2 flooding or water-alternating gas
310
flooding for EOR. However, the component of crude oil and the reservoir condition are very
311
complex. Therefore, in the future, we will further investigate the effect of scCO2 on the
312
interfacial properties of water-multicomponent oil system or brine-oil system under different
313
reservoir conditions.
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Acknowledgments
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This work is financially supported by the Key Program of National Natural Science
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Foundation of China (U1262202), National Basic Research Program of China
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(2014CB239204), the Fundamental Research Funds for the Central Universities
318
(14CX05022A),
319
Provincial Natural Science Foundation, China (ZR2014EEM035).
320
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ip t
483
Figures
485
Fig. 1. Initial configuration of water-CO2-decane interface. Color code: red, oxygen; gray,
486
carbon; white: hydrogen.
487
Fig. 2. Configuration and density profile of water-CO2-decane system with CO2 mole fraction
488
of 0.6. CO2 molecules are marked with green for clarity. The label ∆z is the “10-90” interface
489
width, L is the length of system in the direction z. Black, blue, red and deep green
490
lines/symbols represent the density profiles of total, water, decane and CO2, respectively.
491
Fig. 3. Density profile of CO2 in water-CO2-decane system with different CO2 mole fractions.
492
Black, red, blue and deep green lines/symbols represent CO2 mole fractions of 0.2, 0.4, 0.6
493
and 0.8, respectively.
494
Fig. 4. Snapshots of the surfaces of the decane (A1, A2, A3) and water (B1, B2, B3) phases
495
along the direction of z-axis against CO2 mole fractions of 0, 0.4 and 0.8.
496
Fig. 5. Configurations of water and decane molecules in the vicinity of water-decane
497
interface against CO2 mole fractions of (A) 0, (B) 0.4 and (C) 0.8. CO2 molecules are hidden
498
for clarity.
499
Fig. 6. The IFT of water-oil as a function of CO2 mole fraction. Black symbols are the
500
simulated results at 313 K and 10 MPa in this work; red symbols are the simulated results at
501
300 K and 10 MPa in ref 28; c, green symbols are the experimental results in ref 45; the blue
502
line is the fit of decane.
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Page 23 of 35
Fig. 7. Interaction energies of decane-CO2 (black lines/symbols) and water-CO2 (red
504
lines/symbols) as a function of CO2 mole fraction.
505
Fig. 8. RDFs of OW-CC and HW-OC in H2O-CO2 pairs in the interfacial region for the
506
water-CO2-decane system with CO2 mole fraction of 0.6. OW and HW stand for oxygen atom
507
and hydrogen atom in water respectively, CC stands for carbon atoms in CO2. Black
508
lines/symbols represent OW-CC RDF and red lines/symbols represent HW-OC RDF.
509
Fig. 9. Diffusion profiles for all water-CO2-decane interface systems as a function of CO2
510
mole fraction (0.0, 0.2, 0.4, 0.6, and 0.8). For clarity, the interfacial region is colored, and the
511
color deepening from bottom to top layer represent the interfacial region of systems with CO2
512
mole fraction turning from 0.8 to 0. Black, red, green, blue and violet lines/symbols represent
513
CO2 mole fractions of 0.0, 0.2, 0.4, 0.6 and 0.8, respectively.
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ed pt ce Ac Page 25 of 35
c Ac
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ed pt ce Ac Page 27 of 35
ep Ac c Page 28 of 35
ce Ac Page 29 of 35
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1
HIGHLIGHTS: Supercritical CO2 molecules prefer to accumulate at water-oil interface.
3
CO2 accumulation leads to changes in the orientation of oil molecules.
4
Driving force of CO2 accumulation is the water-oil interfacial tension difference.
5
Comparable interactions of water-CO2 and oil-CO2 cause interfacial tension reduction.
ip t
2
6
cr
7
Table 1. Interfacial width and orientation angles of decane molecules in interfacial region in
9
water-CO2-decane system with 150 decane molecules, 1600 water molecules and different
10
CO2 molecules under 313K and 10MPa.
an
us
8
0
0.2
0.4
0.6
0.8
interfacial width /Å
4.02
5.01
6.29
8.70
11.73
orientation angle /±1°
83.25
79.57
69.47
53.00
27.75
d
M
CO2 mole fraction
te
11 12
Table 2. Total number of H-Bonds between water molecules and between water and CO2
14
molecules in interfacial region in water-CO2-decane system with 150 decane molecules, 1600
15
water molecules and different CO2 molecules under 313K and 10MPa.
Ac ce p
13
CO2 mole fraction
0.0
0.2
0.4
0.6
0.8
1.0
H-Bond
water-water
855
782
702
648
552
540
number
water-CO2
0
16
38
72
103
111
16 17 18
Page 35 of 35
S0896-8446(15)30174-1 http://dx.doi.org/doi:10.1016/j.supflu.2015.11.001 SUPFLU 3497
To appear in:
J. of Supercritical Fluids
Received date: Revised date: Accepted date:
21-7-2015 31-10-2015 1-11-2015
Please cite this article as: B. Liu, J. Shi, M. Wang, J. Zhang, B. Sun, Y. Shen, X. Sun, Reduction in Interfacial Tension of Water-Oil Interface by Supercritical CO2 in Enhanced Oil Recovery Processes studied with Molecular Dynamics Simulation, The Journal of Supercritical Fluids (2015), http://dx.doi.org/10.1016/j.supflu.2015.11.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reduction in Interfacial Tension of Water-Oil Interface by
2
Supercritical CO2 in Enhanced Oil Recovery Processes studied
3
with Molecular Dynamics Simulation
ip t
1
Bing Liu a, Junqin Shi a, Muhan Wang a, Jun Zhang a,*, Baojiang Sun b,*, Yue Shen a, Xiaoli
5
Sun a
6
a
College of Science, China University of Petroleum, 266580, Qingdao, China
7
b
School of Petroleum Engineering, China University of Petroleum, 266580, Qingdao, China
8
Abstract: The interfacial tension (IFT) has a significant influence on fluid flow in the
9
supercritical CO2 (scCO2) enhanced oil recovery process. However, it is still challenging to
10
demonstrate the effect of scCO2 on water-oil interface. Therefore, a molecular dynamics
11
simulation is performed to investigate the influence of scCO2 on the water-decane interface.
12
It is observed that CO2 prefers to accumulate and display surface-active at the water-decane
13
interface. The driving force of CO2 accumulation is the IFT difference between water-decane
14
and water-CO2. The IFT of water-CO2-decane decreases linearly with the increase of CO2.
15
The interactions of water-CO2 and decane-CO2 are strong, and the difference between them is
16
small, providing the mechanism for lowering the IFT of water-decane. The increase in the
17
diffusivity for all fluids toward the interface suggests an increase in the mobility at the
18
water-decane interface due to the addition of CO2, which is in favor of enhancing oil
19
recovery.
20
Keywords: water-decane interface; supercritical CO2; interfacial tension; molecular
21
dynamics simulation
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Page 1 of 35
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1. Introduction Enhanced oil recovery (EOR) by CO2 flooding is a significant technique to help
25
common development in terms of energy and environment [1]. It not only increases the crude
26
oil recovery from oil reservoirs for energy security but also facilitates carbon dioxide capture
27
and sequestration (CCS) for the reduction in greenhouse gas emission [1-8]. CO2-EOR
28
accompanying with geological CO2 storage is largely controlled by the interfacial properties
29
among CO2, crude oil and water [9-12]. It is important, therefore, for the interfacial
30
phenomenon or behavior pertaining to the crude oil, water and injected CO2 to be understood
31
for the improvement of CO2-EOR and CCS process.
an
us
cr
ip t
24
The interfacial tension (IFT) has an effect on capillary pressure and permeability, and
33
then the oil flow process through capillary of porous medium is also influenced [1,2].
34
Moreover, in the tertiary recovery or EOR methods, when the reservoir pressure or injection
35
pressure is above the minimum miscibility pressure (MMP), CO2 and oil become completely
36
miscible and thereby their IFT is down to zero [13,14]. Thus, the IFT of the system composed
37
of CO2, water and oil should be determined between water and CO2-oil mixture. Experiments
38
indicated that the IFT decreases when the mole fraction of CO2 in mixture increases [15-17].
39
In addition, the interfacial characteristics of water-oil showed some vital changes due to the
40
different solubility of CO2 in oil and water. However, to the best of our knowledge, the effect
41
of CO2 on water-oil interface has not been obtained and it is still quite challenging to
42
illuminate the interaction mechanism in the ternary CO2, oil and water system from
43
experiments.
44
Ac ce p
te
d
M
32
Molecular dynamics (MD) simulations have been performed to investigate interfacial 2
Page 2 of 35
properties for complex binary or ternary systems. Kunieda et al. [18] investigated the oil-fluid
46
interface and found that the aromatic molecules in light oil accumulate at the interface. This
47
accumulation phenomenon at oil-water interface was ascribed to the weak hydrogen bond
48
interaction between water and aromatic hydrocarbon [8]. Mikami et al. [19] studied a similar
49
interface and showed that the asphaltene molecules assemble from nanoaggregation to
50
thin-film formation at the oil-water interface. Carpenter et al. [20-23] observed the
51
differences between the interfacial and bulk liquids, and special molecular orientation for
52
hydrocarbon molecules was found at the miscible hydrocarbon-fluid interface. For the
53
CO2-water interface system, the MD studies indicated that CO2 molecules exhibit
54
hydrophilicity and interact closely with water molecules at the interface [24,25]. However,
55
there exist very limited studies for the ternary water-CO2-oil system, especially the
56
microscopic behavior of CO2 distributing at interface, the structural and dynamic properties
57
of water-oil interface have not been reported.
cr
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an
M
d
te
In this work, our objective is to shed light on the influence of supercritical CO2 (scCO2)
Ac ce p
58
ip t
45
59
on water-oil interface. We hope to illustrate that how and why CO2 accumulate at the
60
water-oil interface and reduce IFT. For this purpose, the MD simulation is carried out to
61
investigate the interface of water-scCO2-decane system under the conditions of 10 MPa and
62
313 K. Here, decane is selected to be a simply representation of crude oil. Through classical
63
MD simulation, we study the density profile, structural information for molecules in interface
64
systems, IFT, the intermolecular interaction, and transport property. The results are helpful to
65
understand the mechanism of scCO2 influencing the water-oil interface, which is of
66
paramount importance to future design of methodologies for CO2 EOR. 3
Page 3 of 35
67
2. Computational methods
68
2.1 Simulation Details MD simulations were carried out using Materials Studio software package [26] with the
70
condensed-phase optimized molecular potential for atomistic simulation studies (COMPASS)
71
force field describing all atoms in this study. COMPASS, a widely used all-atom force field
72
based on ab initio and optimization by the experimental data, has been validated to be capable
73
of accurately predicting structural and thermophysical properties for a broad range of organic
74
and inorganic substances including water, oil and CO2.[27] This force field includes the
75
bonded potential and the non-bonded potential. The bonded potential is composed of bond
76
stretching, angular bending, dihedral angle torsion, out-of-plane interactions and cross terms;
77
the non-bonded potential consists of the long-range electrostatic interactions and the
78
short-range van der Waals (vdW) interactions [28]. The electrostatic interactions are
79
calculated by Coulombic equation (1) and vdW interactions are represented by Lennard-Jones
80
function (2),
82
cr
us
an
M
d
te
Ac ce p
81
ip t
69
Eele = C
qi q j
(1)
εR
6 R 9 R EvdW = D0 2 0 − 3 0 R R
(2)
83
where C = 332 .0647 ( kcal mol ) Å e 2 is unit conversion factor, ε
84
constant, qi and q j are the partial charge of atom i and j , D0 is the depth of potential
85
well, R0 is the Lennard-Jones radius, and R is the distance between two atoms.
is relative dielectric
86
We performed a series of all-atom MD simulations of systems composed of water,
87
decane and CO2 to investigate the effects of scCO2 on the interface between water and decane 4
Page 4 of 35
at 313 K and 10 MPa. The initial simulation box for the ternary water-CO2-decane system
89
consists of three independent phases as shown in Fig. 1. A similar system was reported by
90
Makimura [29]. Here, the two water phases involves totally 1800 molecules while the middle
91
phase is the decane-CO2 mixture containing 150 decane molecules and different numbers of
92
CO2 molecules. The number of CO2 molecules in the mixture is set to be 0, 37, 100, 225, 600
93
and 800 corresponding to the mole fraction of CO2 of 0, 0.2, 0.4, 0.6, 0.8 and 1.0,
94
respectively. The dimensions of water phase are 40 Å×40 Å×15.8 Å. The dimensions of the
95
cross section for decane-CO2 binary phase are also 40 Å×40 Å while it is determined by its
96
density in z direction. Periodic boundary conditions were applied in all directions, and the
97
primary system thus consisted of two water-CO2-decane interfaces.
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88
The reasonable decane-CO2 mixtures were determined by their densities obtained by
99
carrying out MD simulations in the NPT ensemble for 1.5 ns at 313 K and 10 MPa. The
100
simulated densities of the decane-CO2 mixtures are plotted in Fig. S1 (in supplementary
101
material). This figure shows the simulated densities are in well agreement with the
102
experimental values [30,31], meaning that COMPASS force field is suitable to be applied in
103
the water-CO2-decane interfacial system. The effects of CO2 on the water-decane interface
104
were obtained from MD simulations of the water-CO2-decane interfacial system in the NPT
105
ensemble for 2 ns at 313 K and 10 MPa. Finally, we performed MD simulations of the
106
interfacial system in the canonical ensemble (NVT) for 1 ns at 313K. For MD simulations in
107
the NPT ensembles, the temperature and the pressure were controlled by the Andersen
108
thermostat [32] and Berendsen barostat [33], respectively. For MD simulations in the NVT
109
ensembles, the temperature was controlled by Nosé thermostat [34]. For all the simulations, a
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5
Page 5 of 35
cutoff distance of 13.5 Å was applied to the vdW interactions, the electrostatic interactions
111
were treated using the Ewald summation [35], the time step was 1 fs, full trajectory was
112
saved and the frames were output every 1 ps for the analysis of the results.
113
2.2 Calculation Methods
ip t
110
The interfacial tension of the water-CO2-decane interface was calculated using the
115
formulation of the Gibbs interfacial tension [8]. Two interfaces in the water-CO2-decane
116
system are both perpendicular to the z axis and parallel to the xy plane, hence the interfacial
117
tension is evaluated from the expression of pressure tensor [36]
an
us
cr
114
1 Px + Py − Pz Lz 2 2
γ =−
(3)
M
118
where Pa = paa (a = x, y, z) is the diagonal elements of the pressure tensor, and Lz is the
120
length of the simulation box in z direction.
d
119
According to the Einstein relation, the diffusion coefficient (D), describing the
122
thermodynamic property of the interface, is calculated from the mean square displacement
123
(MSD)
Ac ce p
124
te
121
D=
125
3. Results and discussions
126
3.1 Density profile
2 1 d n lim ∑ Ri ( t ) − Ri ( 0 ) 6 t →∞ dt i
(4)
127
Fig. 2 shows the equilibrium configuration and density profile along the direction
128
normal to the interface (z-axis) for the water-CO2-decane system with CO2 mole fraction of
129
0.6. Herein, the interface is determined by the Gibbs dividing surface (GDS) where the
130
excess decane molecules at the water side are approximately equal to the excess water 6
Page 6 of 35
molecules at the decane side. The interfacial region is established by the “10-90” interfacial
132
width (∆z) commonly characterized by the distance between the two surfaces at the 10% and
133
90% of the total density along the z axis [8,37,38]. The configuration shows that quite a few
134
CO2 molecules penetrate into water phase and a host of CO2 molecules accumulate in the
135
interfacial region though CO2 molecules participate in the miscible phase with decane under
136
the simulated condition. The density profile shows an obvious peak for CO2 at the interface
137
where the density profiles of water and decane intersect. It indicates the formation of CO2
138
film in the interfacial region just as the adsorbed layer of CO2 at liquid-gas interface [39-42].
139
The CO2 film bridges water and decane phases and thus it could have an important effect on
140
the water-decane IFT, resembling the influence of the surfactant absorbed on the water-oil
141
interface [43,44].
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131
Further insight into the accumulation of CO2 molecules at the interface is demonstrated
143
by the density profiles in Fig. S2 (in supplementary material) and Fig. 3. The density of CO2
144
molecules at the interface is between 3 to 5 times that in the decane phase and raises rapidly
145
with the increase of CO2 mole fraction. It shows an enhanced accumulation of CO2 at the
146
interface as CO2 mole fraction increases, which is ascribed to that CO2 is more hydrophilic
147
than decane driven away from the interface.
148
3.2 Structure of water-decane interface
Ac ce p
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142
149
The accumulation of CO2 molecules will result in an obvious variation of the structure
150
of the water-decane interface. Here, the interfacial structure is investigated from several
151
aspects as follow. First, we present the snapshots of the surfaces of both water phase and
152
decane phase in Fig. 4 where CO2 mole fractions are 0, 0.4 and 0.8, respectively. The 7
Page 7 of 35
enhanced corrugation or roughness of the surfaces is observed as CO2 mole fraction increases.
154
It indicates a higher anisotropic distribution for molecules at the surfaces due to that CO2
155
molecules separate from the decane-CO2 mixture and penetrate into water. Therefore, the
156
interfacial roughness is related to molecular penetrations (Fig. 2, Fig. 3 and Fig. S2), similar
157
to the effect of the amphiphilic surfactant on the water-oil interface [43].
cr
ip t
153
Then, we determined the interfacial width in Table 1 using the “10-90” interface width.
159
The interfacial width obviously increases from 4.02 Å to 11.73 Å as CO2 mole fraction
160
increases from 0 to 0.8. It implies the deeper molecular penetrations broadening the
161
coexistent region of water and decane. Because of the strong diffusibility of scCO2 regarded
162
as a polar molecule with two active and considerably strong bond dipoles [45], CO2 is
163
capable of being partially miscible with water. However, decane is immiscible with water for
164
its stronger hydrophobic interaction. As CO2 mole fraction increases, more CO2 molecules
165
may move towards the interface and form strong interactions with water, such as the
166
hydrogen bond, leading to the deeper molecular penetrations featured by interfacial width.
an
M
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Ac ce p
167
us
158
Finally, based on the interfacial width in Table 1, we depict the configurations of the
168
water and decane molecules in the vicinity of interface in Fig. 5 when CO2 mole fractions are
169
0, 0.4 and 0.8, respectively. A clear interface between water phase and decane phase is
170
observed and the decane molecules in the interfacial region are almost parallel to the interface
171
in Fig. 5A. With the increase in CO2 mole fraction, shown in Fig. 5B and C, the interface
172
becomes indistinct and the decane molecules in the interfacial region show a tendency to
173
rotate from the parallel orientation to the perpendicular orientation. To confirm the variation
174
of the orientation, the statistic average of the orientation angle of the decane molecules in the 8
Page 8 of 35
interfacial region is calculated in Table 1. Here, the orientation angle is defined as the angle
176
between the vector joining two carbon atoms at both ends of the decane molecule and the
177
interfacial normal vector. As CO2 mole fraction increases, the orientation angle of decane
178
molecule decreases. It indicates decane molecules move away from the interface to the bulk
179
phase. The rotation of decane molecules from the parallel to the perpendicular to the interface
180
provides extra space for the deeper penetrations of water and CO2 molecules into decane
181
phase, which further expands and blurs the water-oil interface.
182
3.3 Interfacial tension
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175
Fig. 6 shows the dependence of the IFT of the water-CO2-decane system on CO2 mole
184
fraction. The IFT is slightly lower than that of water-CO2-hexane system [29] but larger than
185
that of water-CO2-light oil system [46]. The disparity mainly results from the different
186
conditions and alkanes we used. The IFT of water-decane is 50.88 mN/m while the IFT of
187
water-CO2 is 32.58 mN/m, being a well agreement with the experimental data [6,47]. They
188
correspond to CO2 mole fractions of 0 and 1.0 respectively. It shows that COMPASS force
189
field is reliable to accurately predict structural and thermophysical properties of the
190
water-CO2-decane system.
d
te
Ac ce p
191
M
183
Fig. 6 also shows the IFT linearly decreases with the increase in CO2 mole fraction.
192
Combining with table 1 showing the increasing interfacial width with CO2 mole fraction, we
193
find a negative correlation between the IFT and the interfacial width. It is consistent with the
194
conclusion that wide interfaces result in low IFT values [2,48]. In addition, it is noteworthy
195
that such a CO2 accumulation is not observed at water-CO2 interface corresponding to the
196
CO2 mole fraction of 1.0 (Fig. S2). It shows that CO2 displays “surface-active” at 9
Page 9 of 35
water-decane interface but “inactive” at water-CO2 interface just as non-amphiphilic aromatic
198
at water-oil interface [8,18]. Therefore, we consider that the accumulation of CO2 molecules
199
at the interface is driven by the IFT difference between water-CO2 and water-decane. On the
200
one hand, when CO2 molecules accumulate in the interfacial region, the configuration
201
entropy decreases, similar to the case for the adsorption of surfactants and the accumulation
202
of aromatics [8,18,43], and the Gibbs free energy increases within the interface system. On
203
the other hand, the IFT of water-CO2 is ~18 mN/m lower than that of water-decane (Fig. 6),
204
so the lowered IFT should result from the accumulation of CO2 at the water-CO2-decane
205
interface, resulting in the decrease in the potential energy of the total system. As a result, the
206
Gibbs free energy reaches a minimum value with the CO2 accumulation at equilibrium.
207
Consequently, we conclude that the IFT difference is the “driving force” of accumulation of
208
CO2.
209
3.4 Intermolecular interaction
cr
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an
M
d te
The reduction in the IFT caused by CO2 should be related to intermolecular interactions
Ac ce p
210
ip t
197
211
between different components in the interfacial region. To investigate this relation, we
212
calculate interaction energies of decane-CO2 and water-CO2 as a function of CO2 mole
213
fraction in Fig. 7. As CO2 mole fraction increases, the both interactions dramatically increase
214
and they are comparable although the interaction between water and CO2 is slightly stronger
215
than that between decane and CO2. The results indicate that CO2 can stably accumulate at the
216
water-decane interface, therefore forming the interfacial film due to the balance between the
217
interactions acting on CO2 by water and decane respectively. Combining with the reduction
218
in the IFT with the increase in CO2 mole fraction (Fig. 6), we infer that the comparable and 10
Page 10 of 35
strong interactions of water-CO2 and decane-CO2 provide the mechanism for lowering the
220
IFT. Therefore, we speculate that if the interaction of CO2-water and that of CO2-oil are
221
strong and the difference of these two interactions is small, then CO2 is favorable for the
222
decrease in IFT of water-oil.
ip t
219
In addition, the hydrogen bond (H-bond), found between water and CO2 [49,50], is
224
partly responsible for the effect of CO2 on the water-decane interface. In this study, the
225
H-bond is calculated within the maximum hydrogen-acceptor distance of 2.50 Å. The H-bond
226
is identified by the radial distribution function (RDF) between water and CO2 in the
227
interfacial region in Fig. 8 where CO2 mole fraction is 0.6. The first peak of the OW-CC RDF
228
occurs at 3.90 Å close to the OW-CC distance of 3.82 Å in the inset of Fig. 8 and in Fig. S3 (in
229
supplementary material), and the HW-OC RDF displays a pronounced shoulder in the range of
230
1.70 Å to 2.52 Å within the length of H-bond. The shoulder means the saturated H-bond
231
forming between water and CO2. It is the saturated H-bond that prevents CO2 molecules from
232
gathering around a water molecule, resulting in the tiny variation in the nearest neighboring
233
CO2 molecules around the water molecule. It indicates the coordination OC atoms around one
234
HW atom decrease in the range of 1.70 Å to 2.52 Å and thereby leads to the shoulder [51,52].
235
The results provide an evidence for the formation of H-bond between water and CO2
236
molecules in water-CO2-decane system. The number of H-bonds of both water-water and
237
water-CO2 in the interfacial region is also shown in Table 2. It shows that the number of
238
H-bond between water molecules decreases while that between water and CO2 molecules
239
increases with the increase in CO2 mole fraction. The decrease in the number of H-bond
240
between water molecules partly weakens the network structure and the interaction between
Ac ce p
te
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cr
223
11
Page 11 of 35
water molecules, resulting in the higher degree of the interfacial roughness. Meanwhile, the
242
increase in the number of H-bond between water and CO2 molecules enhances the interaction
243
of water-CO2, stabilizing CO2 interfacial film and strengthening its bridge effect. As a
244
consequence, the H-bond between water and CO2 molecules at the interface has an important
245
influence on the reduction in the IFT.
cr
ip t
241
Besides the importance of H-bond discussed above, water-CO2 complexes may have an
247
important influence on the interface of water-decane. It is due to the electron deficiency of
248
the C atom of CO2 and bond electron density more polarized toward the O atoms, and CO2 in
249
the water-CO2-decane system acts as a Lewis acid favoring the formation of electron
250
donor-acceptor (EDA) complexes in the presence of water as a Lewis base [53-56]. The
251
vibrational spectra of water-(CO2)n complexes using ab initio method indicated that the
252
planar T-shaped structure and linear structure at local energy minimum for CO2-water
253
dimmer complex are found mainly stabilized by EDA interaction [56]. The HW-OC distance
254
of 2.71 Å in the planar linear structure exceeds the range of 1.70 Å to 2.52 Å in Fig. 8, while
255
the OW-CC distance of 2.78 Å in T-shaped structure is close to the range of a small shoulder
256
in OW-CC RDF. This probably proves the existence of T-shaped structure in water-CO2-oil
257
system as found by Makimura et al. [29]. Furthermore, a trimer water-(CO2)2 complex is
258
observed where the arrangement between water and its second neighbor involves a second
259
weak O···H-O H-bond characterized by a distance of 2.13 Å-2.15 Å [56]. The interactions of
260
second weak H-bond and Lewis bases-Lewis acid, tuning the inner structural arrangement of
261
the CO2-water dimmer and trimer complexes, maybe have important effect on the change in
262
IFT which needs further research.
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Page 12 of 35
263
3.5 Transport properties for water-CO2-decane interface The diffusion coefficient for a multicomponent system, very difficult to determine
265
experimentally, can be directly determined by MD simulations. To further explore the
266
transport properties of the water-decane interface influenced by CO2, the diffusion profile of
267
the ternary system along the z-axis is monitored as a function of CO2 mole fraction in Fig. 9
268
and self-diffusion coefficients of water and decane in the interfacial region are calculated in
269
Table S1 (in supplementary material). Here, the diffusion profile is evaluated by dividing the
270
system into slabs of thickness of 5 Å parallel to the interface. The results we obtained are in
271
good agreement with the values in similar systems [8,57,58] regardless the small variation of
272
the temperature and pressure. For all fluids considered, we observe an increase in the
273
diffusion coefficient toward the interface. It suggests an increase in the mobility at the
274
water-decane interface due to the addition of CO2.
te
d
M
an
us
cr
ip t
264
In Fig. 9, the diffusion coefficient of molecules keeps unchanged in bulk water or
276
decane, and the diffusion coefficient exhibits a slight increase firstly and then decreases from
277
the water side to the decane side. The diffusion coefficient of the system reaches the
278
maximum at the position more adjacent to the water side, which is in accordance with the
279
density profile in Fig. 2 where the density peak of CO2 is more close to the water density
280
profile. It indicates that the strong mass transfer ability of CO2 plays an important role in
281
influencing the transport property of the interface system. On the one hand, the increased
282
mobility of interfacial molecules caused by the CO2 accumulation leads to wider interface
283
and rougher interfacial structure, resulting in the reduction in the IFT. On the other hand, the
284
penetration of CO2 in water results in the increase in viscosity and migration ability of water
Ac ce p
275
13
Page 13 of 35
[55] while the dissolution of CO2 in decane results in the expansion and viscosity reduction
286
of decane. These improve the decane-water mobility ratio and thereby enhance the sweep
287
efficiency of water.
288
4. Conclusion
ip t
285
The effect of scCO2 on the water-oil interface has been investigated from MD
290
simulations under the conditions of 10 MPa and 313 K, where decane represents oil
291
molecule. It is interestingly observed that CO2 molecules initially miscible with decane
292
accumulate within the water-decane interface. With the increase in CO2 mole fraction, the
293
enhanced accumulation of CO2 induces the deeper molecular penetrations, which increases
294
the interfacial roughness and width and leads to the decane molecules rotating from the
295
parallel to the perpendicular to the interface. From the viewpoint of the Gibbs free energy, the
296
“driving force” of CO2 accumulation is difference of the IFT between water and decane and
297
that between water and CO2. The calculated IFT shows that it linearly decreases with the
298
increase in CO2 mole fraction and reveals a negative correlation with the interfacial width.
299
Further investigation shows the enhanced interactions for water-CO2 and decane-CO2 and the
300
smaller difference between them are favorable to lower the IFT. Consequently, we could
301
infer that if the interaction between water and CO2 and that between oil and CO2 are strong
302
enough, and the difference between the two interactions is small enough, then CO2 is
303
favorable to decrease the IFT of water-oil. This inference may not be generalized in more
304
ternary interface systems until more tests to be verified. In addition, the strong mass
305
transportation capability of scCO2 molecules enhances the transport properties of water and
306
decane, which is particularly important to improve the oil-water mobility ratio and thereby
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14
Page 14 of 35
enhance the sweep efficiency of water. The investigation of the effect of scCO2 on
308
water-decane interface is helpful to understand many interfacial phenomena in scientific and
309
industrial processes and facilitate the application of CO2 flooding or water-alternating gas
310
flooding for EOR. However, the component of crude oil and the reservoir condition are very
311
complex. Therefore, in the future, we will further investigate the effect of scCO2 on the
312
interfacial properties of water-multicomponent oil system or brine-oil system under different
313
reservoir conditions.
314
Acknowledgments
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ip t
307
This work is financially supported by the Key Program of National Natural Science
316
Foundation of China (U1262202), National Basic Research Program of China
317
(2014CB239204), the Fundamental Research Funds for the Central Universities
318
(14CX05022A),
319
Provincial Natural Science Foundation, China (ZR2014EEM035).
320
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ip t
483
Figures
485
Fig. 1. Initial configuration of water-CO2-decane interface. Color code: red, oxygen; gray,
486
carbon; white: hydrogen.
487
Fig. 2. Configuration and density profile of water-CO2-decane system with CO2 mole fraction
488
of 0.6. CO2 molecules are marked with green for clarity. The label ∆z is the “10-90” interface
489
width, L is the length of system in the direction z. Black, blue, red and deep green
490
lines/symbols represent the density profiles of total, water, decane and CO2, respectively.
491
Fig. 3. Density profile of CO2 in water-CO2-decane system with different CO2 mole fractions.
492
Black, red, blue and deep green lines/symbols represent CO2 mole fractions of 0.2, 0.4, 0.6
493
and 0.8, respectively.
494
Fig. 4. Snapshots of the surfaces of the decane (A1, A2, A3) and water (B1, B2, B3) phases
495
along the direction of z-axis against CO2 mole fractions of 0, 0.4 and 0.8.
496
Fig. 5. Configurations of water and decane molecules in the vicinity of water-decane
497
interface against CO2 mole fractions of (A) 0, (B) 0.4 and (C) 0.8. CO2 molecules are hidden
498
for clarity.
499
Fig. 6. The IFT of water-oil as a function of CO2 mole fraction. Black symbols are the
500
simulated results at 313 K and 10 MPa in this work; red symbols are the simulated results at
501
300 K and 10 MPa in ref 28; c, green symbols are the experimental results in ref 45; the blue
502
line is the fit of decane.
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Page 23 of 35
Fig. 7. Interaction energies of decane-CO2 (black lines/symbols) and water-CO2 (red
504
lines/symbols) as a function of CO2 mole fraction.
505
Fig. 8. RDFs of OW-CC and HW-OC in H2O-CO2 pairs in the interfacial region for the
506
water-CO2-decane system with CO2 mole fraction of 0.6. OW and HW stand for oxygen atom
507
and hydrogen atom in water respectively, CC stands for carbon atoms in CO2. Black
508
lines/symbols represent OW-CC RDF and red lines/symbols represent HW-OC RDF.
509
Fig. 9. Diffusion profiles for all water-CO2-decane interface systems as a function of CO2
510
mole fraction (0.0, 0.2, 0.4, 0.6, and 0.8). For clarity, the interfacial region is colored, and the
511
color deepening from bottom to top layer represent the interfacial region of systems with CO2
512
mole fraction turning from 0.8 to 0. Black, red, green, blue and violet lines/symbols represent
513
CO2 mole fractions of 0.0, 0.2, 0.4, 0.6 and 0.8, respectively.
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1
HIGHLIGHTS: Supercritical CO2 molecules prefer to accumulate at water-oil interface.
3
CO2 accumulation leads to changes in the orientation of oil molecules.
4
Driving force of CO2 accumulation is the water-oil interfacial tension difference.
5
Comparable interactions of water-CO2 and oil-CO2 cause interfacial tension reduction.
ip t
2
6
cr
7
Table 1. Interfacial width and orientation angles of decane molecules in interfacial region in
9
water-CO2-decane system with 150 decane molecules, 1600 water molecules and different
10
CO2 molecules under 313K and 10MPa.
an
us
8
0
0.2
0.4
0.6
0.8
interfacial width /Å
4.02
5.01
6.29
8.70
11.73
orientation angle /±1°
83.25
79.57
69.47
53.00
27.75
d
M
CO2 mole fraction
te
11 12
Table 2. Total number of H-Bonds between water molecules and between water and CO2
14
molecules in interfacial region in water-CO2-decane system with 150 decane molecules, 1600
15
water molecules and different CO2 molecules under 313K and 10MPa.
Ac ce p
13
CO2 mole fraction
0.0
0.2
0.4
0.6
0.8
1.0
H-Bond
water-water
855
782
702
648
552
540
number
water-CO2
0
16
38
72
103
111
16 17 18
Page 35 of 35