Modeling Acid Oil Component Interactions with Carbonate Reservoirs

Article pubs.acs.org/JPCC

Modeling Acid Oil Component Interactions with Carbonate Reservoirs: A First-Principles View on Low Salinity Recovery Mechanisms Verónica M. Sánchez* and Caetano R. Miranda* Nanopetro Research Group, Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, SP 09210-580, Brazil ABSTRACT: Enhanced oil recovery by low salinity water (LSW) injection in carbonate reservoirs constitutes one of the main interests for petroleum industry. A molecular mechanism involving acid oil components and the replacement of surface Ca atom by Mg atom has been proposed. To determine the thermodynamic feasibility of this mechanism, we study the propionic acid adsorption upon calcite (10−14) and Mg-calcite (10−14) surfaces by first-principles calculations based on density functional theory. In vacuum, the propionic acid adsorption energy upon both surfaces is negative, not in favor of the proposed mechanism. In solvent, as a first approach, we include a water monolayer (ML), and we observe a decrease on the adsorption energy due to a change on the surface oxygen atom involved in the acid−surface hydrogen bond, but the acid remains stable upon both surfaces. Additionally, we explore the effect of adding an aqueous media on the adsorption energy. We employ a continuum solvent model without and with explicit water ML. For the first case, we obtain substantial changes on the energy adsorption due to the increasing of the acid−surface hydrogen bond, whereas for the second one, where the effect of explicit water ML is considered, a minor energy variation is observed. In both scenarios the acid becomes unstable upon Mg while remains stable upon Ca, in favor of the proposed mechanism. Furthermore, the effects of salinity, temperature, and pressure are explored in an effective way through the variation of the solvent dielectric constant (50−90). It can be inferred from our results that if Ca−Mg replacement occurs, this mechanism is thermodynamically feasible for the whole dielectric constant range studied.

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system, to improve the EOR process.13 At the molecular level, a change on the chemical interaction between oil components and surface reservoir to a more unstable oil adsorption on reservoir situation would also improve the EOR process. Understanding the improvement either from the wettability or from the oil chemical desorption, the composition of the injected water plays a key role in the oil percentage recovered in the EOR14 process. In particular, it was observed from several experimental and modeling works that injection of low salinity water leads to an improvement on the EOR processes.15,16 Experimental works show that only acid components of oil (especially carboxylic or naphthenic acids) interact with carbonate reservoirs17from which is possible to infer that the interaction between surface and oil components would be controlled by the acid group of the oil component. For low salinity water (LSW) injection, it was also observed by Zhang et al.13 that the role of different ions present on the water may lead to a major recovery percentage. In particular, they studied the interplay between Ca2+, Mg2+, and SO42− ions and their effect on the EOR process. From their observation,

INTRODUCTION The interaction between organic molecules and water upon calcium carbonate surfaces constitutes a key point subject of study in biochemistry1 as well as in geochemistry2 due to its abundance in both scenarios. For the latter case, the chemical behavior of carbonate surfaces in several industry processes such as contaminant migration,3 ion exchange,4 and enhanced oil recovery (EOR) processes5 represents an area of great interest. There is great abundance of carbonate oil reservoirs on Earth.6 Considering the abundance of carbonate reservoirs and the chemical interest in improving the comprehension of its reactivity, we focused our work on the studied of this type of reservoirs through the studied of calcite (CaCO3) surface reactivity. Focusing on the oil industry, one of its main goals is to improve the percentage of oil recovered by the EOR, and several techniques have been employed: gas injection, thermal methods, and water flooding,5 among others. In order to recover the oil present on the reservoir, the injection of water may lead to the displacement of oil present in the porous rock by the mechanical mechanisms. To this end, several experimental and modeling studies have been performed with the aim of reaching the optimal enhance oil recovery (EOR) conditions.7−12 From a microscopic point of view, a change on the wettability of crude oil/brine/rock (OBR) system is desired to be modified to a more water wet © 2014 American Chemical Society

Received: May 28, 2014 Revised: July 28, 2014 Published: July 31, 2014 19180

dx.doi.org/10.1021/jp505259t | J. Phys. Chem. C 2014, 118, 19180−19187

The Journal of Physical Chemistry C

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each condition: (i) vacuum and (ii) dielectric medium. Once the water−surface interaction is known, the first approach to understand the influence of the aqueous solution in the acid adsorption is to consider one explicit water ML in the simulation box. At last, we studied the acid adsorption in the presence of solvent−dielectric continuum medium, by taking into account the influence of explicit water ML and the influence on the variation of the dielectric medium.31−34 The static dielectric constant decreases with the increment on the salinity32,33,35 and temperature31,34,33 and increases with the increment of pressure.34 Therefore, we consider a range of dielectric constant values to account for these variations, considering the oil reservoir conditions.

they proposed the mechanism to explain the desorption of oil components by this water injection. This mechanism could be understood as a two-step mechanism: the first one consists on the sulfate approximation to the calcite surface, which is favored in low salinity water injected, and the second step is the subsequent attraction of the Mg2+ ion by the sulfate group toward the surface, followed by the replacement of a Ca surface atom by this ion. If the Ca is interacting with an acid oil component, then as a consequence of the Mg replacement, the Ca will desorb along with the acid component. This desorption would explain the oil displacement only if the acid oil components are not stable upon the superficial Mg atom remaining in the reservoir surface. Looking at this chemical mechanism, it is possible to study its thermodynamical feasibility by first-principles calculations. In order to model the carbonate reservoir, the calcite (10−14) surface was selected due its major thermodynamical stability.18−21 To study the behavior of acid oil component, we choose the propionic acid as a small molecule with the acid functionality responsible for the interaction with the surface. Some works have been published on the same direction, studying the interaction of acid molecules with calcite surfaces in vacuum and in the presence of water.22,23 In particular, Sakuma et al.22 studied the first step of the proposed mechanism, concluding that the Ca2+ replacement by Mg2+ ion in the presence of SO42− ion is exothermic and that Mg2+ remains on calcite surface site and not inside the crystal. Because of the LSW, a minor NaCl concentration is present in the ionic double layer, favoring the sulfate ion to reach the surface, and the presence of the sulfate anion facilitates the Mg2+−Ca2+ replacement.15 Once this cation replacement occurs, which has been recently shown to be thermodynamically achievable,22 the adsorbed acid on Ca2+ would desorb with it, and no acid adsorption on Mg atom must be observed. Taking this into account, we calculate the propionic acid adsorption energy on Mg and on Ca atom under different conditions. We performed these calculations under periodic conditions in vacuum, in the presence of a water monolayer (ML) and in continuum dielectric medium (solvent). Within continuum or implicit solvent models, the solute is surrounded by a dielectric medium that represent in an average way the static screening of the solution.24,25 Consequently, the polarization induced by the solvent is introduced in an average way, and the computational cost of solvent representation is reduced in comparison to explicit solvent inclusion. However, the solvent structure is disregarded, and solute−solvent specific interaction could be lost unless a first solvation shell (or a part of this) is included explicitly in the calculation. To account for the difference in the acid adsorption energies between surfaces calcite (10−14) and Mg-calcite (10−14), we studied the system in subsequent steps, presented in subsections I−V of the Results and Discussion. In the first one the surface is described as well as the Ca−Mg replacement. The second one shows the difference in the adsorption energy and geometry obtained in vacuum for acid adsorbed upon Ca and Mg atoms. As the mechanism proposed is in the presence of solvent, it is important to describe accurately the interaction between water and surface, with and without the dielectric medium inclusion. There are atomistic calculations that show molecular adsorption of water upon calcite26−29 and experimental works that describe dissociated adsorption type.30 For this reason, we have investigated which type of adsorption would be stable for

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METHODS First-principles calculations were performed using density functional theory (DFT) under periodic boundary conditions, as implemented in the Quantum Espresso package.36 Pseudopotentials and plane wave basis sets were employed. The Perdew−Wang approach (PW91) to the exchangecorrelation energy37,38 and Vanderbilt39 ultrasoft pseudopotentials were selected to compute total energies and forces. The Kohn−Sham orbitals and charge density were expanded in plane waves up to a kinetic energy cutoff of 50 and 580 Ry, respectively. The k-sampling was restricted to the Γ point. Relaxation optimization was carried out with convergence criteria on forces of 0.05 eV/Å. The damping method was used within the Car−Parrinello approach with an electronic mass of 400 au and time steps in the range of 0.35−0.5 au for electrons and 10 times less for ions. We employed the same standard and parameters to perform energy calculations and geometry optimizations in the presence of solvent, for which we used the continuum model.40 Within this scheme, the permittivity is defined as a function of the atomic positions and its correspondence van der Waals radii, varying smoothly from 1.0 inside the surface up to the corresponding dielectric constant into the solvent region (79 in relative units for pure water at 25 °C). In this implementation, the Poisson equation to determine the electrostatic contribution to the total energy is obtained in the real space through a multigrid method.38 As the solvent model accounts for the static screening of the solution by its dielectric constant, it is possible to include the influence of the salinity (S), temperature (T), and pressure (P) in a mean effective media32,34 by considering the influence of this properties on the dielectric constant value. As it has been addressed in the literature, an increase in T and S decreases the dielectric constant value and an increase in P increases the dielectric constant value.33 Therefore, we analyze a range of dielectric constant values of 50−90 to account for the variation in these conditions (S, P, T). The accuracy in the description of this method has been observed for TiO2 surfaces41 where the replacement of explicit water by implicit solvation implies nearly zero energy differences. In Scheme 1, the chemical process study is summarized based on two cases: (a) without and (b) with water ML. For calcite (10−14), the process described before was −0.9 kcal/mol, giving a slightly higher adsorption energy than on titania surface (around 0.2 kcal/mol).39 The latest results emphasize the accuracy of the model, as a consequence of using it for studying other systems. 19181



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Scheme 1. Implicit Solvent Displacement by Explicit Water: (a) Without Water ML and (b) Considering It

RESULTS AND DISCUSSION I. Calcite (10−14) Surface: Electronic Structure and Replacement of One Ca by Mg (Mg-Calcite (10−14)). The calcite (10−14) surface presents three types of oxygen sites: O1, O2, and O3 (Figure 1a). The reactivity of each one

Adsorption energies were evaluated from the total energy obtained with the adsorbate bound to the surface minus the energy calculated with the adsorbate several angstroms away from the interface. The adsorption energy could be defined as eq 1, which could be viewed in Scheme 2a (or in Scheme 2b) for the case of explicit solvent, as were previously reported for titania surfaces.39

Figure 1. (a) Schematic representation of calcite (10−14) surface. (b) PDOS calculation for each surface atom (O1, O2, O3, C, and Ca).

Eadsorption = Esystem − Esurface − Eadsorbate

could be understood through the projected density of states (PDOS) calculation (Figure 1b), where it can be seen that the O1 is the most basic one, being more reactive in this sense. To create the Mg-calcite (10−14) surface, one Ca atom was replaced by a Mg atom; due to the differences in their radius, an optimization calculation was performed. Shorter bond Mg−O distances (around 2.1 Å) were observed with respect to bond Ca−O distances (around 2.3 Å). We perceived the same behavior for supercells 2 × 1, 2 × 2, and 4 × 1. Therefore, the first smallest supercell was selected. As in a previous step of the proposed mechanism, the Mg2+ ion replaces the Ca2+ surface ion. As was mentioned in the Introduction, this process was shown to be thermodynamically favored.22 II. Propionic Acid Adsorption on Pure Calcite (10−14) and Mg-Calcite (10−14). As was already mentioned in the Introduction, the propionic acid molecule is used here to represent acid oil components. As a first step to describe the molecule interaction with calcite surface, we calculate different geometric configurations (the dissociative configuration is not

(1)

Calcite (10−14) surface was represented as three-layer supercell of area (2 × 1), fixing the latest layer as the atomic bulk positions.20 To evaluate the thermodynamical feasibility of this mechanism, one surface Ca atom was replaced by a Mg atom. The surface structure rearrangement due to the Mg atom was analyzed in supercells of (2 × 1), (2 × 2), and (4 × 1). As the three of them lead to similar results, the smallest one (2 × 1) was employed for the subsequent calculations. Cell dimensions of the corresponding (2 × 1) three layer supercell were a = 8.180 Å, b = 10.084 Å, and c = 24.245 Å; the large amount of c dimension was chosen to be able to represent solvent reactions (Scheme 1) accurately.

Scheme 2. Adsorption Energy Process for Acid Adsorption (a) in the Absence of Water Monolayer and (b) in the Presence of Water Monolayera

a

Both processes were performed with and without the solvent inclusion. 19182



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Figure 2. (a−c) Three different propionic adsorption configurations on calcite (10−14), in a vacuum. The first two show hydrogen bondswith O1 surface atomsthat are explicitly marked. (d) and (e) show acid adsorption in the presence of water monolayer for calcite (10−14) and Mg-calcite (10−14), respectively. Hydrogen bondswith O3 surface atomsare displayed.

of O1(sup)−H(acid) increases from 1.57 Å on calcite (10−14) to 1.67 Å on Mg-calcite (10−14) (Figure 4). III. Water Interaction with Calcite (10−14) and Mgcalcite (10−14). In order to mimic the interaction of acid oil components with carbonate reservoirs, the inclusion of aqueous solvent is essential to consider the natural environment of this process. Consequently, the interaction of water molecule with the calcite (10−14) surface was studied first. It has been observed by atomistic simulations that water exhibits a physisorption upon calcite;26,27,42,43 meanwhile, experimentally there are results which show that water dissociates on the calcite surface.30 In this sense, we perform our calculations to be able to represent the water molecule in the most accurate way within the methodology employed. As was already published,28 the molecular interaction obtained for θ = 0.25 (see Figure 3a) was determined as the most stable onethe dissociative configuration is not stable in a vacuumwith an adsorption energy of −15.6 kcal/mol, which slightly differs from Sakuma et al.’s work,22 mainly due to the differences in simulation conditions. Then, we calculate the water adsorption energy as it is shown in Scheme 2b using simulation conditions of Sakuma et al.’s work,22 and a value of −20.8 kcal/mol was obtained, which differs by 2.1 kcal with respect to the one reported in this work. We believe this variance comes from the difference in the adsorption energy scheme calculation: we perform an adsorption calculation for the minimized water monolayer configuration; meanwhile, they obtain the water adsorption energy considering an average of water configurations. The adsorption energies per water molecule for θ = 0.25 and θ = 1 are displayed in Table 1. Calculations performed at θ = 1 correspond to the scheme shown in Scheme 2b, where the propionic acid molecule is replaced by a water molecule. For water ML, a physisorption is also observed and reported in the literature.44 For both surfaces at θ = 0.25, the Hwater−O1sur surface bonds were 1.77 Å in a vacuum. As this type of interaction with the surface is similar, we understand that the main difference between them relies on each Mgsur−Owater and Casur−Owater interaction, the characteristic of each ion being responsible for the difference in adsorption energy.

stable, leading to a molecular adsorption structure). The optimized geometries are shown in Figure 2. The work done by Sakuma et al.22 regarding acetic acid adsorption energy calculations arrived to slightly different energy adsorption values from our work on propionic acid, which we believe is due to the difference in the organic acid used and simulation conditions. For sake of comparison, we calculate the propionic adsorption energy under these conditions, obtaining a value of −29.0 kcal/mol, which differs by 2 kcal/mol with the one reported for acetic acid due to the difference in the acid studied in each case. The adsorption energies obtained in a vacuum (using Scheme 2a) are (a) −25.9, (b) −18.3, and (c) −9.7 kcal/ mol. It can be seen that the configuration shown in Figure 2b is the most stable one and also that the hydrogen bonds present geometries a and b, which leads to a major stabilization of adsorbed propionic molecule. On the Mg-calcite (10−14) surface, the most stable configuration (Figure 2b) was studied (see Table 1 and Figure 4d), resulting in a less adsorption energy (−20.3 kcal/mol). That indicates a minor affinity for Mg than for Ca atom on the calcite (10−14). In accordance with this behavior, the distance Table 1. Adsorption Energies for Water and Propionic Acid Molecules on Surfaces: Calcite (10−14) and Mg-Calcite (10−14), in a Vacuum and in a Solventa vacuum

θ θ θ θ

= = = =

0.25, calcite (10−14) 0.25, Mg-calcite (10−14) 1 (ML)-calcite (10−14) 1 (ML)-Mg-calcite (10−14)

calcite (10−14) Mg-calcite (10−14)

pure water

water adsorption (kcal/mol)/per H2O molecule −15.6 −0.9 −18.3 6.7 −16.8 1.9 −18.4 −0.7 propionic acid adsorption (kcal/mol) −25.9 −5.0 −20.3 14.2

a

For a water molecule, it is also shown the adsorption energy with and without water monolayer (ML). 19183



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Hwater−O1sur bond increment. In order to take into account the explicit solvent interactions, we studied the adsorption energy of water ML in a vacuum and in solvent. For the former calculations, in a vacuum, we can observe that the presence of water ML does not affect significantly the adsorption energy per molecule obtained, meaning that the water−water interactions are not so important in a vacuum. The replacement of one water molecule by the dielectric medium in the presence of water ML gives different results for Mg-calcite (10−14) and calcite (10−14), but it is worth mentioning that both of them give values nearer to zero (which is expected from a replacement from explicit to implicit water). For Mg-calcite surface the replacement of explicit water upon Mg by dielectric medium gives a negative value, indicating that the ML is less stable. Looking at Figure 3d, we could understand this result as a consequence of the longer hydrogen bond (1.85 Å) observed for this case. Meanwhile, for the pure calcite (10−14) surface, a positive value is obtained, and consequently, the ML is more stable. It could be observed an enlargement of hydrogen bond distances (see Figure 3) as a consequence of water interacting with dielectric media. As it was reported in a previous work,39 this enlargement could be understood as a consequence of water molecules rotating and interacting also with the solvent media, leading to a weaker water−surface interaction. IV. Propionic Acid Adsorption on Calcite (10−14) and Mg-Calcite (10−14) in the Presence of Water Monolayer. The first approach to include the aqueous solvent environment on the acid adsorption determination corresponds to what is exposed in Scheme 2b in a vacuum. In this scenario, water molecules belonging to the monolayer interact with O1 surface atoms as it is shown in Figure 3. Consequently, a change in acid adsorption geometry was observed (Figure 2a− c) where the interaction of acid molecule through its hydrogen atom is now with O3 surface atom, instead of O1 atom as it was without water monolayer (Figure 2d,e). The O1 surface atom is less basic than O3 (see Figure 1), and consequently this interaction becomes weaker. We obtain an energy value of

Figure 3. Water monolayer in vacuum on (a) calcite (10−14) and (b) Mg-calcite (10−14). ML in solvent: (c) calcite(10−14) and (d) Mgcalcite (10−14). Hydrogen bonds are described in the figure.

Meanwhile, in the presence of pure water (dielectric inclusion), the Hwater−O1sur surface bond was 1.80 and 1.85 Å for calcite (10−14) and Mg-calcite (10−14) respectively. In the former case, the hydrogen bond between Hwater−O1sur is still present, making the energy difference between implicit solvent (dielectric medium) and explicit interaction (water bonding surface) small. Meanwhile, in the latter case, an increment in the Hwater−O1sur bond distance up to 1.85 Å leads to greater lessening on the hydrogen bond between water and the surface, which could explain a higher positive energy (6.7 kcal/mol) involved in the replacement of dielectric medium by explicit water. Therefore, we understand that the study of water replacement by dielectric medium would be better accomplished in the presence of water monolayer not to obtain unreal

Figure 4. (a) Projection of the dielectric constant on plane yz shows its smooth variation from the matter (ε = 1) to the solvent bulk (ε = 79). Propionic acid adsorption on (b) calcite (10−14) in a vacuum, (c) calcite (10−14) in a solvent, (d) Mg-calcite (10−14) in a vacuum, and (e) Mgcalcite (10−14) in a solvent. Distances of bond lengths are marked. 19184



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−11.0 kcal/mol for calcite (10−14) and a value of −4.7 kcal/ mol for Mg-calcite (10−14). The major acid adsorption instability observed for Mg-calcite (10−14) surface compared to water adsorption (see Table 1) is in line with Sakuma et al.’s22 work, where from the presence of Mg is inferred that the surface exposes a more wet-like behavior. The structures obtained for both surfaces show similar hydrogen bond length with surface and are stable; the adsorption energies for Figure 2d,e are displayed in Table 1 (the adsorption energy values at ε = 79). It can be observed that acid molecule adsorbs on both surfaces, with and without Mg, and therefore the proposed mechanism in the Introduction could not be feasibly from a thermodynamic point of view. Considering the environment in the EOR processes, a more realistic representation is needed to be able to support or dismiss thermodynamically the proposed mechanism.13 Taking it into account, a continuum solvent model representing the solvent by its dielectric constant was employed. V. Propionic Acid Adsorption on Calcite (10−14) and Mg-Calcite (10−14) in the Presence of Solvent: Effect of Dielectric Constant and Explicit Water Monolayer. Solvent inclusion on the acid adsorption calculation was performed in two steps: only dielectric medium included and water ML plus a dielectric medium inclusion. This done was in order to understand: the specific interaction of acid molecule with surface and also the way of including the dielectric medium in a more accurate way. As a first step, only the dielectric constant of solvent environment was included in the propionic acid adsorption. To clarify this model,38 a scheme of dielectric constant variation is included: a smooth variation of dielectric constant from ε = 1 to the dielectric constant of the solvent (ε = 79) is employed, as is shown in Figure 4a. The adsorption energy as is shown in Scheme 2a within the solvent environment were performed for calcite (10−14) and Mg-calcite (10−14) surfaces. The geometry optimization leads (Figure 4c,e) to an enlargement in acid−surface hydrogen bond for both surfaces. As we have observed before, this bond could be understood as the one that determines the magnitude of the acid surface interaction. As a consequence of this enlargement and the presence of the dielectric medium, the propionic acid becomes less stable (see Table 1). The acid adsorption on both surfaces becomes less stable, in particular for the Mg-calcite (10−14) surface. As it can be seen from Figure 4d,e, the hydrogen bond with surface oxygen is 1.85 Å (the same happens with a water molecule), and therefore the adsorption energy is positive. Meanwhile, the hydrogen bond distance is enlarge up to 1.69 Å for the calcite (10−14) surface (Figure 4b,c), remaining a strong interaction with the surface through this bond and a negative adsorption energy, which means a stable acid adsorption. As we have mentioned before39 and it is already known in the literature also for calcite,45,46 in order to consider the explicit interactions due to solvent molecules, a first water ML was added to the calculation. The acid adsorption energy varies with the inclusion of water ML: it becomes less stable. Because of the presence of explicit water ML, the acid interacts to the O3 surface atom, instead of O1; this was discussed for the vacuum results (Figure 2) which represent a more realistic scenario. As a consequence of explicit water ML under the inclusion of dielectric medium, even though the acid surface bond distance is enlarged, it is less affected by the mediumas it is regulated by explicit interactions. In particular, the bond distance O3(sup)−H(acid)

changes from 1.58 to 1.64 Å for calcite (10−14) and from 1.60 to 1.63 Å for Mg-calcite (10−14). A remarkable result is that the propionic acid in the presence of solvent (considering the explicit interactions) is stable upon calcite (10−14), and it is not stable upon Mg-calcite (10−14). This, in terms of the proposed mechanism, it is thermodynamically feasible in pure water. In an attempt to account for reservoir conditions and considering that a high pressure increases the dielectric constant and meanwhile the high temperature 34 and salinity32,35 decrease it, we decided to include a variation of the dielectric constant within a range of 50−90. It could be observed (see Figure 5) that, in general, a major stability on the

Figure 5. Propionic acid adsorption energy (kcal/mol) variation with solvent dielectric medium in the presence of water monolayer. The value of dielectric medium decreases as the salinity32 and temperature32 increase; meanwhile it increases with increasing pressure.32

acid adsorption is observed as the dielectric constant decreases, but the instability of the acid adsorption upon Mg remains along with its stability upon Ca. Consequently, the mechanism proposed13 could be understood as thermodynamically feasible considering variations in S, P, and T. This mechanism was proposed for LSW based on the major probability of Mg2+ and SO42− reaching the surface, but for our results we can infer that if the ions interplay occurs the mechanism proposed is possible for different S, T, and P conditions, as we observed by the dielectric constant variation (50−90).

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CONCLUSIONS The main conclusion of this work is the thermodynamic feasibility of the molecular mechanism proposed,13 considering that the Mg replacement has been already shown to be possible.22 From an atomistic point of view, the inclusion of water ML in the calculation is crucial to describe accurately the acid adsorption on the surface, where it could be seen that the interaction with surface is done with another type of oxygen, which has been observed in both conditions: vacuum and solvent. Both calculations in vacuum, with and without water ML, show that the acid adsorption is stable, which is not in favor of the proposed mechanism and the experimental evidence of a better EOR percentage with an increment in the Mg2+ concentration. If the acid is stable adsorbed to the Mg-calcite (10−14) surface, then an unadsorbed oil acid component could be adsorbed upon Mg atom. This latter situation does not support the proposed mechanism. The analysis of water adsorption type in the presence of solvent is relevant to describe the correct explicit aqueous 19185



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(6) Schlumberger Market Analysis. Carbonate reservoir: Meeting unique challenges to maximize recovery, 2007: available at: http:// www.slb.com/resources/other_resources/brochures/technical_ challenges/carbonates/br_unique_challenge.aspx. (7) Fathi, S. F.; Austad, T.; Strand, S. Water-based enhanced oil recovery (EOR) by “smart water”: Optimal ionic composition for EOR in carbonates. Energy Fuels 2011, 25, 5173−5179. (8) Shariarpanahi, S. F.; Strand, S.; Austad, T. Initial wetting properties on carbonates oil reservoirs: effect of the temperature and presence of sulfate in formation water. Energy Fuels 2011, 25, 3021− 3028. (9) Rigo, V. A.; de Lara, L. S.; Miranda, C. R. Energetics of formation and hydration of functionalized silica nanoparticles: An atomistic computational study. Appl. Surf. Sci. 2014, 292, 742−749. (10) Miranda, C. R.; de Lara, L. S.; Costa Tonetto, B. Stability and mobility of functionalized silica nanoparticles for enhanced oil recovery applications in the Netherlands. SPE International Oilfield Nanotechnology Conference and Exhibition, 2012, SPE-157033-MS. (11) De Lara, L. S.; Michelon, M. F.; Metin, C. O.; Nguyen, P. Q.; Miranda, C. R. Interface tension of silica hydroxylated nanoparticle with brine: A combined experimental and molecular dynamics study. J. Chem. Phys. 2012, 136, 164702. (12) De Lara, L. S.; Michelon, M. F.; Miranda, C. R. Molecular dynamics studies of fluid/oil interfaces for improved oil recovery processes. J. Phys. Chem. B 2012, 116, 14667−14676. (13) Zhang, P.; Tweheyo, M. T.; Austad, T. Wettability alteration and improved oil recovery by spontaneous imbibition of seawater into chalk: Impact of the potential determining ions Ca2+, Mg2+ and SO42−. Colloids Surf., A 2007, 301, 199−208. (14) Zeng, H.; Zou, F.; Hovrvath-Szabo, G.; Andersen, S. Effects of brine compositions on the adsorption of benzoic acid on calcium carbonate. Energy Fuels 2012, 26, 4321−4327. (15) Austad, T.; Shariatpanahi, S. F.; Strand, S.; Black, C. J. J.; Webb, K. J. Conditions for a low-salinity EOR effect in carbonate reservoirs. Energy Fuels 2012, 26, 569−575. (16) Al Shalabi, E. W.; Sepehrnoori, K.; Delshad, M. Mechanisms behind low salinity water injection in carbonate reservoirs. Fuel 2014, 121, 11−19. (17) Thomas, M. M.; Clouse, J. M.; Longo, J. M. Adsorption of organic compounds on carbonate minerals: 1. Model compounds and their influence on mineral wettability. Chem. Geol. 1993, 109, 201− 213. (18) MacInnis, I. N.; Brantley, S. L. The role of dislocations ans surface morphologyin calcite dissolution. Geochim. Cosmochim. Acta 1992, 56, 1113−1126. (19) Heberling, F.; Trainor, T. P.; Lützenkirchen, J.; Eng, P.; Denecke, M. A.; Bosbach, D. Structure and reactivity of the calcitewater interface. J. Colloid Interface Sci. 2011, 354, 843−857. (20) Rigo, V. A.; Metin, C. O.; Nguyen, Q. P.; Miranda, C. R. Hydrocarbon adsorption on carbonate mineral surfaces: A first principles study with van der Waals interactions. J. Chem. Phys. C 2012, 116, 24538−24548. (21) Skinner, A. J.; LaFemina, J. P.; Jansen, H. J. F. Structure and bonding of calcite: A theoretical study. Am. Mineral. 1994, 79, 205− 214. (22) Sakuma, H.; Andersson, M. P.; Bechgaard, K.; Stipp, S. L. S. Surface tension alteration on calcite, induced by ion substitution. J. Phys. Chem. C 2014, 118, 3078−3087. (23) Aschauer, U.; Spagnoli, D.; Bowen, P.; Parker, S. C. Growth modification of seeded calcite using carboxylic acids: Atomistic simulations. J. Colloid Interface Sci. 2010, 346, 226−231. (24) Cramer, C. J.; Truhlar, D. G. Implicit solvation models: equilibria, structure, spectra and dynamics. Chem. Rev. 1999, 99, 2161−2200. (25) Levine, I. N. Quantum Chemistry; Prentice Hall: Englewood Cliffs, NJ, 2000. (26) Lardge, J. S.; Duffy, D. M.; Gillan, M. J. Investigation of the interaction of water with the calcite (10.4) surface using ab inito simulation. J. Phys. Chem. C 2009, 113, 7207−7212.

environment at the solid−liquid interface. Water does not dissociate on the surface, and the molecular adsorption was confirmed to be the more stable one. The inclusion of solvent was fundamental and gives support to the mechanism proposed, turning the acid adsorption on the Mg atom unstable. The inclusion of the water ML is fundamental to an accurate description of the problem, where the explicit interactions between water and adsorbate become relevant. This interaction makes the acid reacts with another type of oxygen surface and also allows us to account for the solvent in a correct way. As the model includes the dielectric constant to represent the interaction of the aqueous phase in an effective media, we considered the variation of S, T, and P through thisfrom where can be observed that for the dielectric constant values range studied, this process is possible. From the results obtained with continuum solvent calculations, we can conclude that in order to analyze the feasibility of chemical processes at surface in contact with fluid, it is required to incorporate it in the calculation. It is not sufficient to include only a ML of the solvent, but it is also needed to consider its effect besides it to account properly for the solvent influence on the chemical reaction.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +55 (11) 33567509 (C.R.M.). *E-mail [email protected] (V.M.S.). Author Contributions

V.M.S. and C.R.M. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by PETROBRAS and the Brazilian agencies FAPESP and CNPq. The calculations have been partially performed at CENAPADSP and UFABC supercomputer facilities. A special thanks to Dr. Damian A. Scherlis (UBA) for the development of the continuum solvent code and the kindness to share it with our group.

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ABBREVIATIONS EOR, enhanced oil recovery; LSW, low salinity water; HSW, high salinity water; S, salinity; P, pressure; T, temperature.

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REFERENCES

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Modeling Acid Oil Component Interactions with Carbonate Reservoirs

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