Brine Interfaces within Thin

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Interactions between rock/brine and oil/brine interfaces within thin brine film wetting carbonates: a molecular dynamics simulation study Mohammad Mehdi Koleini, Mohammad Hasan Badizad, Zahra Kargozarfard, and Shahab Ayatollahi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00496 • Publication Date (Web): 28 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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Interactions between rock/brine and oil/brine interfaces within thin brine film wetting carbonates: a molecular dynamics simulation study

Mohammad Mehdi Koleini*1, Mohammad Hasan Badizad2, Zahra Kargozarfard3, Shahab Ayatollahi1

Corresponding Author: M.M. Koleini; Email: [email protected] & [email protected] Phone & Fax: +98 21 6616 6461 1 Sharif Upstream Petroleum Research Institute (SUPRI), Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran. 2 Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran; Email: [email protected] & [email protected] 3 Department of Chemical Engineering, Tarbiat Modares University, Tehran, Iran. *

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Abstract The thin brine film that wets rock surfaces governs the wettability of underground reservoirs. The ionic composition and salinity of this nano-size brine film influences the wetting preference of rock’s pore space occupied by hydrocarbons. Despite numerous investigations in the last decades, a unanimous fundamental understanding which concerns the contribution of ions in original wetting state of the reservoir is lacking and hence the mechanisms responsible for the wettability reversal of the mineral are still in doubt. This wettability reversal is the main consequence of ion-tuned waterflooding. Although the method is widely accepted in practice, there is not a universal consensus on the underlying mechanisms involved. Molecular dynamics simulation is an excellent choice to remove such ambiguities. This method can capture an atomic level picture of the phenomena that affect reservoir wettability upon injecting low salinity water. For the purpose, we performed simulations of brine films confined between calcite substrate and a layer of an oil model. The films were at two different salinities to represent initial state of high salinity connate water and low salinity brine. We found the development of ionic aggregates, mainly comprised of Na⁺ and Cl‾, within the high salinity thin brine film. These aggregates act as pinning entities to localize polar oil components within oil/brine interface and connect the hydrocarbon phase to the calcite surface. This results in the adhesion of oil components to the rock surface though a high salinity thin brine film. Simulation results suggest that the aggregates do not form after changing the brine content to low salinity. From these observations we concluded that diluting the brine content of the reservoir leads to disintegration of such ionic aggregates. As a 2 ACS Paragon Plus Environment

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consequence, electrical double layers (EDL) form at both rock/brine and brine/oil interfaces, which is supposed to be reflected by additional oil recovery at macroscopic scale. Furthermore, we pointed out that EDL at oil interface is established by negatively charged oleic polar species and cations around those compounds. Likewise, the EDL in proximity to calcite is composed of a positive Stern layer of Na⁺ cations and a negative diffuse layer of Cl‾ anions beyond that.

Keywords: Thin brine film; Low salinity effect; Pinning points; Anchoring effect; Double layer expansion; Molecular dynamics simulation

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1. INTRODUCTION Low salinity waterflooding (LSW) is an efficient affordable technique in improving oil recovery. This method which originated in the mid-1990s, is also known as low salinity effect (LSE) in the literature.1,2 According to several increasing studies since then, wettability alteration of oil reservoirs is thought as the major factor of LSE.2 Despite those studies, Yousef et al. are amongst the first researchers who observed improved oil recovery from LSE in carbonates at the beginning of the present decade.3,4 They reported that LSW might alter carbonate wettability towards more water-wet conditions and the optimal LSE is expected for weakly water-wet rocks. Wettability of oil reservoirs is affected by a residual thin brine film that wets the pores of rock saturated with oil.5 This persisting thin brine film influences the initial wetting state of the mineral and hence the efficiency of LSW.6 The thin brine film separating oil and rock, is initially composed of high salinity (HS) brine. Injection of low salinity (LS) brine into carbonate reservoirs is expected to disturb the established thermodynamic equilibrium within the thin brine film.7 However, the major mechanisms that alter mineral wettability due to such disturbance is still a subject of debate.8–11 The complex nature of oil/brine/rock interactions might be the main reason behind the uncertainties around the mechanisms of LSW. This complexity has led into the acknowledgement of a combination of several mechanisms governing LSE rather than a single one. Amongst those mechanisms, double layer expansion (DLE) which focuses on the stability of thin brine films is of particular importance.12,13 In DLE mechanism, the electrical charges at oil/brine and brine/rock interfaces adjust the thickness of brine film. Injection of LS brine increases the electrostatic 4 ACS Paragon Plus Environment

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repulsions between double layers developed at both interfaces. This results in a thicker and more stable brine film that creates a wetting state more favorable for oil recovery.5,13 DLE mechanism is based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.14,15 However, chemical mechanisms has been also proposed that rely on non-DLVO interactions.16 Both theories have been employed in the area of oil recovery in order to explain the stability of the water film wetting surface of pore walls and also wettability alteration in response to modifying salinity.17 For example, multi-component ionic exchange (MIE)18 and cation bridging11 mechanisms affect non-DLVO interactions between oil/brine and brine/rock interfaces in LSE. In both mechanisms, divalent cations act a linkage role between polar oil components (e.g., charged acidic groups; COO‾) and rock surface. The common feature of DLVO and non-DLVO interactions is their definite influence on the stability of wetting thin brine film. Hence, DLE and chemical mechanisms are known as important factors in brine film stability. Modifying the composition of thin brine film by injecting LS brine changes the thickness of water film depending on the net charge of ions distributed within oil/brine and brine/rock interfaces. Jackson et al.19 reported improvement in oil recovery due to the advent of same electrical charges at both interfaces upon LSW. On the other hand, LSW impacts interactions between components of oil phase and rock surface through chemical bonding. At high salinities, polar oil components (specifically carboxylates) that populate oil/brine interface, bind to the surface via divalent cations despite the thin persisting monolayer of water. The injection of LS brine is believed to break up the cation bridging and release oil components.20 Nevertheless, the thin brine film covering mineral surface acts as a barrier between rock and oil. The wettability of the mineral somehow depends on the 5 ACS Paragon Plus Environment

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stability of this film and oil/brine/rock interactions are expected to govern its stability. However, these interactions are intrinsically complex and a fundamental understanding of the underlying mechanisms to interpret experimental observations is lacking. Molecular dynamics (MD) simulations have been recommended to fulfill the necessities of thorough understanding of these phenomena.5 Recently several studies have been published which applied MD simulations to gain insights into interfacial phenomena.21,22 The issue of confined fluids within nanopores has been of great interest among researchers. Papavasileiou et al.23 examined the adsorption behavior inside kaolinite nanopores of various aqueous solutions and hydrocarbons. They showed that polar hydrocarbons don’t strongly affect the distribution of water and ions over surfaces. Anvari and Choi24 observed the formation of water bridge connecting the two basal surfaces of kaolinite at all ion concentrations. They reported the breakage of water bridge at high salt concentrations and as a result a phaseseparation occurred. Therefore, a three-layer structure of water/hydrocarbon/water was formed within the nanopore because of the screening effect of the adsorbed counterions. In general, the salt ions adsorbed onto clay surfaces are expected to promote to the mineral hydrophilicity.25 Likewise, the formation of water bridges was also observed across silica nanopores which span pore volume, thus hindering volatile hydrocarbon gases transport within the pore.26,27 Therefore, the hydration structure within narrow pores was found to lead to significant selectivity to permeation of gases.28 Wu and Firoozabadi29 simulated the selective adsorption in the nanopores and verified the impact of transport process on the composition of gas mixture within the pore. The study of solid/liquid interfaces is a similar topic to confinement 6 ACS Paragon Plus Environment

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which also gained numerous contributions in the literature. MD simulations validated the development of charged Stern layer at silica/water and mica/water interfaces.30,31 Hartkamp et al.32 studied a negatively charged silica/electrolyte interface by means of MD simulation and detected strong cation adsorption at the surface. The surface charge over-screening led to an effective positive surface next to the Stern layer, followed by a negatively charged diffuse layer. Adapta et al.33 showed that mono- and divalent cations diversely affect the structure and adsorption behavior of water on mica surface. This distinction is a consequence of different hydration shell size and hydration energy of the ions. More specifically, calcite has been the subject of solid/liquid interface studies. Kirch et al.34 investigated the structure of water and ions at calcite/aqueous solution interface. They detected strong ordering of water molecules in contact with the solid surface. As a consequence, the Na+ and Cl⁻ electrolyte ions occupied the available sites near the solid/brine interface, resulting in a screening effect.35 BrekkeSvaland and Bresme36 also provided a molecular insight into the solvation structure of calcite. Likewise, they reported well-ordered three hydration layers at calcite/water interface up to 1 nm above surface. Ricci et al.37 emphasized that these highly ordered water layers at the calcite surface prevent the hydrated ions from directly interacting with calcite due to the energy penalty incurred by the necessary restructuring of the ions’ solvation shells. Further studies have investigated calcite/fluid interface by concerning surface wettability. Kim et al.38 applied simulations to indicate that while polar oil components locate at water/oil interface, the nonpolar ones are repelled from the water phase. They reported that the polar components are responsible for creating the link between the nonpolar fraction of the oil and polar calcite surface through an adsorbed layer of 7 ACS Paragon Plus Environment

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water. Chang et al.39 performed simulations to classify calcite among water-wet minerals. Based on the inherent water-wetness of calcite, the tuning of ions in LSW is expected to impact the mineral wettability. Santos et al.40 described the formation of an electrical double layer (EDL) within calcite/aqueous electrolyte interface at atomistic level. Zhao et al.41 verified that the EDL is composed of oppositely-charged layers of Na+ and Cl⁻ ions and attributed it to DLE mechanism. Prabhakar and Melnik42,43 showed that the first charged layer close to calcite is composed of sodium cations. They proposed that the formation of Na-carboxylate complexes over calcite surface makes the mineral more oil-wet. Their ab initio MD simulations suggested that addition of Mg2+ and SO42- ions alters the relative wettability of calcite to less oil-wet state. Despite those studies, the influence of LSW on thin brine films in oil/brine/carbonate system, as a three-phase interaction phenomenon, has not been directly the subject of MD simulations so far. Hence, the proposed mechanisms in experimental studies are still seeking quantitative validations at molecular level. In this study we performed MD simulations to gain molecular understanding of phenomena within thin brine film confined between calcite slab and model oil. To the best of our knowledge, the studies concerning the simulation of nano-sized interphases are mostly devoted to identical confining mediums, e.g. a fluid confined within a slit44 or another fluid45, or the contact region of fluid/solid explored by a single interface.46 Dealing with heterogeneous confining phases is the core novel aspect of the present investigation compared to other ones. The focus of the work is on ionic characteristics of the film in HS 8 ACS Paragon Plus Environment

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and LS conditions. The former condition is aimed to resemble the initial wetting state of the pore while the latter is similar to that after LSW. The distribution of ions specially at oil/brine and calcite/brine interfaces can provide quantitative information around oil/brine/rock interactions and dominant mechanisms dealing with the stability of thin brine film.

2. METHOD We performed MD simulations to study the interactions within oil/brine/rock system with a focus on ionic composition of the brine. For the purpose, we considered a 2 nm water film confined between calcite slab and hydrocarbon layer to imitate carbonate reservoir nanopores with the aid of MD simulations (Figure 1). The chosen brine thickness is in the typical range of experimental data, 1 to 10 nm, reported for the reservoir rocks.5 The stability of brine films with an upper limit of 10 nm thickness has been validated from free energy calculations by Sharma.47,48 The film thickness is deemed to be balanced by DLVO interactions between oil/brine and brine/rock interfaces as well as nonDLVO interactions taking place within the thin film.14 Disjoining pressure is a good parameter that can simultaneously reflect the impact of both DLVO and nonDLVO interactions on the stability of thin films. Hirasaki49 proved that the magnitude of forces responsible for film stabilization strongly depends on the thickness of the enclosed water layer. As a result, he verified that disjoining pressure of thin films with thicknesses of ca. above 5 nm decays to zero at moderate concentrations (1.0 M and above). His results emphasized on the optimum influence of hydration forces within very thin water films. Therefore, the interfaces are not practically expected to interact above such thicknesses. On the 9 ACS Paragon Plus Environment

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other hand, the most stabilizing forces identified by negative disjoining pressures appear within films with thicknesses below 3 nm. Hence, the choice of a 2 nm thick brine film in our work seems to be optimum to study the interactions among interfaces and within brine. The choice of carbonate in this study is due to the novelty of LSW in carbonate reservoirs as compared to other minerals such as sandstones. In this study, carbonate is represented by calcite which is the most stable form of the mineral and the main component of carbonate reservoirs.50,51 The naturally stable calcite (1014) cleavage plane is put in contact with the brine.52,53 The alternative distribution of positive calcium ions and negative carbonate groups of calcite slab is the reason of the charge neutrality of this cleavage plane of the mineral.54 The stability of the (1014) cleavage plane of calcite partly stems from this charge neutrality. The calcite slab is composed of 120 unit cells (5 × 8 × 3) building a 4.05 nm × 3.99 nm × 1.75 nm supercell. The hydrocarbon layer that represents the oil phase, is composed of 1:3 molar mixture of decanoate and decane molecules. It is a common procedure in several MD studies to represent the oil phase with a mixture of saturate aliphatic and carboxylic acid hydrocarbons.55–57 This mixture is used to characterize an oil sample with polar components. The formation brine in carbonate reservoirs is naturally buffered to a slightly basic pH (ca. 8)58,59 and LSW also causes pH increase.20 At such basic pH, the carboxylic groups of the oil are hydrolyzed and become negatively charged within oil/brine interface.60 The pH-dependent dissociation of acidic functional groups has been verified by the net negative charge of crude oil/water interface at basic pH.61 Therefore, we used carboxylates rather than carboxylic acids to abide the necessities of basic pH media within oil/brine interface. In reality, a brine film entirely covers the pores’ wall of a 10 ACS Paragon Plus Environment

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calcite rock, as depicted in Fig. 1. However, for modelling and computational purposes, we shall take a nano-sized part of that media to focus on the ions’ contribution within the thin brine film. To this end, a gap space, with a length of 4 nm along z dimension, was placed above the oleic phase to avoid organic particles contacting with the periodic image of the calcite layer. The water film was built of 850 molecules, albeit with varied ionic compositions in three distinct simulations. The various compositions include deionized water (DW), LS and HS brines which respectively represent ion-free water, injecting seawater and connate water. The ionic content of the brines is as follows: 15 NaCl molecules in LS brine and 50 NaCl, 9 CaCl2 & 2 MgCl2 molecules in HS brine (see Table 1). Such ionic content of the brines led to approximate salinities of 60000 and 230000 ppm of LS and HS, respectively. The negative charge of decanoate molecules was compensated by addition of 15 extra counterions of Na+ to fulfill the pre-requirement of solving Ewald summation during the simulation.62 Interatomic interactions are defined by classical force field including Lennard-Jones (LJ) 12-6 and Columbic potentials (equation 1). 𝑈𝑖𝑗 = 4𝜀𝑖𝑗

[( ) ( ) ] 𝜎𝑖𝑗 𝑟𝑖𝑗

12

𝜎𝑖𝑗 ― 𝑟𝑖𝑗

6

+

𝑞𝑖𝑞𝑗 4𝜋𝜀 ∘ 𝑟

(1)

The partial charges and LJ parameters for calcite and calcite-water interactions are those introduced by Shen et al.63, a fitted form of Raiteri’s force field63 combined with SPC/E potential64 for water. The atom-wise potentials proposed by Smith and Dang65, Åqvist66 and Williams67 were impacted for Na+/Cl¯, Mg2+/Ca2+ and SO42- ions, respectively. The OPLS-AA force field68,69 was applied to describe interactions of hydrocarbons together with mixing rules. Long-range electrostatic interactions are calculated by means of Ewald summation method70 11 ACS Paragon Plus Environment

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and periodic boundary conditions were applied in all directions. A cut-off radius of 1.95 nm was assigned to compute LJ interactions. We defined such large cut-off to ensure the potential screening of oil/brine and brine/rock interfaces. For investigating physical phenomena within the brine film under the limit of available computational resources, we must consider the particles in the encompassing phases, oil and calcite, to have effective interaction with the brine media. This practically requires the lying of those particles within the cut-off zone around the brine system. MD simulation were executed using DL_POLY simulation package71 in canonical ensemble (NVT) along with the Nosé-Hoover thermostat.72,73 MD simulations were performed at T=300 K for a total simulation time of 5 ns following 1 ns of equilibration using a time step of 1 fs. Most analysis were done within the last 1 ns of simulation during which the system has reached a state of equilibrium (see the appendix A in Supporting Information for determination of equilibrium).

3. RESULTS AND DISCUSSION We pursued analysis of our simulations by concentrating on the impact of salinity on oil/brine/rock interactions. Therefore, the distribution of ions within interfaces and brine is scrutinized as an important factor in studying mechanisms associated with thin brine film stability. After 5 ns of simulation, the distribution of ions is completely different within thin brine films (Figure 2). The ions of LS brine dominantly appear within areas closer to interfaces, whereas those of HS brine are mainly present in the mid-film regions (Figure 2; a-b). Such diversity is highlighted in Figure 2; c-d in which LS brine ions uniformly distribute as individual particles, whereas those of HS brine form aggregations of ions within the brine. 12 ACS Paragon Plus Environment

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Such dissimilar distribution must be the consequence of different regimes governing thin brine film stability in different salinities. This is in agreement with the idea of Papavasileiou et al.23 who showed that polar hydrocarbons hardly influence ionic distribution within the vicinity of minerals. Therefore, the chemistry of brine is the most effective parameter in thin brine film stability. Note that although higher number of ions in HS brine occupy an extra volume, the equilibrated thickness of the HS thin brine film practically comparable to that of LS brine, i.e. ca. 2 nm. The charge distribution of ions reveals that the presence of ions of LS brine in both oil/brine and brine/rock interfaces leads to the development of two electrical double layers; EDL (Figure 3, main chart). However, these EDLs are intrinsically inconvenient and hence expose different features. The EDL at calcite/brine interface is built of a tight array of Na+ cations in close proximity to the mineral and Cl¯ anions distributed within the next layer in its adjacency. This well-ordered arrangement of ions over the substrate forms a positive compact Stern layer together with a negative diffuse layer which satisfies the classical definition of an EDL.74 Santos et al.40 validated this fact that the first layer of ions in the EDL consists of cations. On the other hand, there exists an EDL at oil/brine interfacial region due to the dissociation of carboxylic acids assembled within the interface. This EDL is composed of counter charged layers formed by Na+ cations and ionized −COO¯ anions. These charged layers overlap and broadly spread within oil/brine interface to construct an EDL that fits the characteristics of fluid/fluid interactions. However, well-organized and distinguished Stern and diffuse layers at calcite/brine interface form an EDL clearly influenced by high discipline of the substrate. This well-ordered arrangement of ions has also been validated in other minerals as a characteristics of solids.30–32 The development of 13 ACS Paragon Plus Environment

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these two EDLs at each interface well fits the criteria of DLE mechanism proposed in LSW. Likewise, Zhao et al.41 attributed the oppositely-charged ionic distribution within EDL to the validity of DLE mechanism. The repulsive/attractive forces between these EDLs are known to impact the stability of thin brine film. These forces are the likely consequences of the screening effect of the adsorbed counterions at both interfaces recently noticed by Anvari and Choi.24 A very surprising observation in the advent of these EDLs is the role of Cl¯ anions. We noticed that in the absence of Cl¯ anions, charge-compensating Na+ ions in DW mostly tend to interact with carboxylates at oil/brine interface rather than existing in proximity to calcite (Figure S1). However, Cl¯ anions in LS brine exclusively appear within calcite/brine interface and moreover motivate a major fraction of Na+ cations towards substrate. This confirms our recent result74 and that observed by Kirch et al.34 that Na+ and Cl⁻ ions of brine occupy the available sites near the calcite/brine interface. Therefore, the presence of anions is crucial for the impact of DLE mechanism in carbonates. Cl¯ anions motivate the development of EDL at calcite/brine interface which promotes the hydrophilicity of the mineral in favor of oil recovery as verified by Zhang et al.25 Further, the chlorine anions inspire the dissociation of carboxylic acids to carboxylates (as a result of characteristic basic pH of the reservoir), expected to intensify the advent of EDL within oil/brine interface. The ions trajectory and density distribution are affected by simultaneous interaction to both neighboring phases, oil and calcite. We verified this statement by performing an additional simulation in which LS brine is in contact with calcite surface without any overlying hydrocarbon. By comparing the charge density of the given system, Figure S2, to its counterpart with oil phase, Figure 3, one simply 14 ACS Paragon Plus Environment

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notices the more pronounced positive Stern and negative diffuse layers in absence of oil phase in the system. Decanoate molecules carry negative charge and their presence at oil/brine region will strongly capture Na+ in that interface. As such, sodium ions would less likely come to the calcite surface and the accompanying EDL is attenuated in response to the presence of overlying polar organic phase. Based on this observation, we believe that the ions’ trajectory and distribution within the thin brine film is governed by their affinities to the characteristics of encompassing phases, which was notified for the first time in this research. As previously observed in Figure 2, the distribution of ions in HS brine is totally different from that of LS. The charge distribution of ions in HS brine (Figure 3; inset) is in agreement with this observation. It denies the development of EDLs at both interfaces but nevertheless verifies approximate uniform distribution of counter ions within the brine. This uniform distribution of cations and anions is an indication of strong ionic interactions confirmed by radial distribution function (RDF) in Figure 4. The importance of these ionic interactions is specified by comparing Na-Cl RDFs in LS and HS brine. The higher peak of RDF in HS brine indicates that ionic interactions dominantly govern the ionic characteristics of thin brine film. While those interactions are much less probable in LS brine and influenced by interfacial interactions. This difference is also observed in entirely different patterns of ions’ trajectories in the final 2ns of simulation for LS and HS brine (Figure 5). As expected, the ions of LS brine mostly exist within interfaces. The ions at calcite/brine interface form a well-exposed EDL but the ions close to oil/brine interface outspread over a broad region fitting the characteristics of fluid/fluid interface. The gap between the first layer of ions and the topmost layer 15 ACS Paragon Plus Environment

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of calcite (at z=0) in Figure 5 is filled by water molecules. Ricci et al.37 reported that the water layers above the calcite surface avoid the ions from direct interaction with the surface. On the other hand, the ions of HS brine dominantly appear as aggregates within the brine rather than interfaces. The xy-projection of the same analysis for the ions of HS brine in Figure 6 validates the formation of such ionic aggregates. The size alteration of those ionic aggregates is small during simulation, especially in the last 2ns (Figure S3). The features of these aggregates beyond interfaces is obviously distinct from distribution of ions within interfaces in LS brine. The appearance of ions in aggregate formation is also in accordance with more likely ionic interactions among ions of HS than LS brine verified by RDF analysis in Figure 4. As well, this participation of ions in aggregate formation validates approximate uniform distribution of counter ions within HS brine as shown in the inset of Figure 3 by charge distribution. These diversities in ionic distributions of two brines influence the interactions of ions within interface and brine. For instance, the RDF between Na+ ions and decanoate molecules reveals stronger ion-carboxylate interactions in LS brine (Figure S4). The less probable ion-carboxylate interaction in HS brine is a result of ion restriction as a consequence of formation of ionic aggregates within the brine. This restriction is reflected in the mobility of common ions which is represented by lower values of mean square displacement (MSD) of ions within HS brine (Figure 7). The trend of MSD plots of ions changes after about 2 ns of simulation in both salinities. In LS brine, the steeper slope of MSD up to 1 ns is due to the high affinity of ions to adsorb onto calcite surface. It is expected that the slowdown of the increasing trend of ions’ MSD beyond 1 ns is due to the fact that they have approached the surface and look for a room to reside on the mineral. By then, when sufficient 16 ACS Paragon Plus Environment

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ions find room over the substrate, the rest of ions are still mobile within the brine film. As a result, the MSD recovers its increasing trend within the rest of the simulation. On the other hand, in HS brine, the affinity of counter ions of Na⁺ and Cl⁻ to pair with each other in order to form aggregates is much more than adsorption onto surface. Therefore, a linear trend of MSD is observed for ions from the beginning of the simulation. Again after 2 ns of simulation, the increasing trend of MSD is retarded because the formation of ionic aggregates is almost at equilibrium. From this point until the end of simulation a new approximately linear trend of MSD of ions is observed. The somehow sluggish trend of MSD plot in HS brine is due to the anchoring effect75 of clusters with carboxylates that reduces their mobility. Notice the linear trend of MSD plots after 2 ns of simulation at both salinities that verifies the equilibrium state of the system prior to the half of simulation time. The ionic distribution within thin brine film also affects the distribution of carboxylate polar species at oil/brine interface. As shown in Figure 8, carboxylate functional groups, more or less, evenly distribute within oil/brine interface in DW. This distribution slightly changes within oil/LS brine interface due to likely interactions between Na+ cations and −COO¯ anions. However, the distribution pattern of carboxylate functional groups represents local accumulation of decanoates within oil/HS brine interface rather than equal distribution of them. The different distributions of these polar components affect the values and trends of their MSD in Figure 9. The values of MSD for carboxylates within oil/HS brine interface is lower than that of LS brine. More interestingly, the MSD of polar components within oil/LS brine interface follows an increasing trend. This is an illustration of steady mobilization of these components within the interface. However, the MSD of decanoate molecules 17 ACS Paragon Plus Environment

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reaches a nearly constant value within the mid-term of simulation. This is an indication of confined mobility of these polar components recently demonstrated by local accumulation of these groups. This confinement might be attributed to the impact of ionic aggregate formation. To scrutinize this hypothesis, we defined the center of mass of local accumulations verified in Figure 6 to represent each ionic aggregate. Then we calculated the RDF between these points, as representatives of aggregates, and carboxylates. We performed the same calculation between these centers of mass and calcite surface. The RDFs in Figure 10 illustrate simultaneous interaction of aggregates with both calcite and oil components. The RDFs reveal strong interaction of aggregates with oil components in close distance. This justifies the confinement of oil polar components as verified in MSD results and local accumulation of carboxylates. The superposition of distribution pattern of carboxylates and ionic aggregates in Figure 11 clearly validates such interactions. This strong interaction of ionic aggregates with polar components of oil phase leads to mobility hindrance of these components and their non-uniform distribution within oil/brine interface. The stability of thin brine film can be discussed based on the mentioned observations. The ionic aggregates take role of a cross-linking agent between calcite surface and oil polar components due to simultaneous interactions with both materials. This leads to indirect interaction of polar oil components with calcite surface via bridging of ionic aggregates. This adhesion of oil components to surface occurs despite a persisting thin brine film separating them. This finding is an extension to the theory of pinning points previously speculated by Mahani et al.76,77 They introduced discrete pinning points inside the thin brine film which serve as a bond between oil and mineral surface, a kind of non-DLVO interaction. 18 ACS Paragon Plus Environment

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Moreover, Ricci et al.78 reported that this pinning point effect retards dissolution process of calcite. The polar oil components pinned by ionic aggregates have a great hydrophobic interaction with other oil component as well, which act like “anchors”60 to localize them at certain spots. This phenomenon is coined anchoring effect by pinning points which affects stabilization of the thin brine film. This is validated by the observation of Kim et al.38 who pointed out the role of polar oil components in establishment of interaction between the nonpolar fraction of the oil and polar calcite surface through an adsorbed layer of water. Upon LSW, the ionic aggregates will disintegrate and as a result, the attractions between oil/brine and calcite/brine interfaces established by pinning points are weakened, hence oil is released. Further, as observed in case of LS brine, EDLs are developed at both interfaces and DLE mechanism favors oil recovery. Concisely, we presented an atomistic insight to explain the initial oil-wet state of carbonate rocks and their wettability alteration to more water-wetting state after changing the brine characteristics of the reservoir. We proposed the development of ion clusters in thin brine film at high salinity conditions of carbonate reservoir that act as anchors for connecting hydrocarbon phase to the calcite surface. This is an improvement on widely accepted EDL expansion because our MD simulations suggest that upon diluting brine salinity to the sea water level those cluster disintegrate and neat EDL at oil/brine and calcite/brine interfaces are created and consequently oil components attached to the surface via ion clusters would release. Finally, as a suggestion for further improvements in the theory of EDL expansion of thin brine film (to be pursued in our group or other research 19 ACS Paragon Plus Environment

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groups), huge, large-scale system subjected to advanced statistical techniques, like umbrella sampling is proposed. Through many trials, we came to the conclusion that it’s not technically plausible for directly observing expansion/contraction of thin brine film by typical MD simulations, like present study. For this purpose, one may devise a new system and scenario to deal with cohesion of water molecules, owing to strong H-bond network, which acts against the variation of brine film thickness. The system taken in the current work best represents microscopic characteristics of nano-film in terms of salinity; however, it is not energetically favorable for water film to expand at the expense of distorting water H-bond structure.

4. CONCLUSIONS The ionic composition of the long-stablished thin brine film within reservoir pores affects the wetting state of the rock. The injection of low salinity brine is expected to disturb this equilibrium within the pores. Unfortunately, there is not a general consensus on interfacial interaction of oil/brine and rock/brine before and after injection of low salinity brine. Molecular dynamics simulation, as a favorable choice, can help to gain molecular understanding of the phenomena. The results of MD simulations verified the formation of ionic aggregates within high salinity thin brine film confined between oil phase and carbonate mineral. The ionic aggregates experience a couple of concurrent interactions. They interact with polar oil components and result in the localization of organic compounds within oil/brine interface. The aggregates simultaneously interact with calcite surface and connect the oil components to the mineral. This connection facilitates oil adhesion to the surface despite a persisting thin brine film. Ion aggregation has 20 ACS Paragon Plus Environment

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been observed in the bulk phase of high salinity aqueous solutions. Ionic clusters grow up rapidly by approaching to the saturation point and keeps ions in associated state.79 In this research, for the first time, we introduced the potential of ion clustering for affecting preferred wetting state of calcite mineral via oil phase on the mineral surface. The concomitant interplay between enclosing interfaces is the key to the clustering process, which stems from the tight separation of oil and calcite phases across the confined brine film. This observation is in line with the postulate of pinning points by Mahani et al.76 and the phenomena is coined as anchoring effect of pinning points because the aggregates act as anchors for connecting hydrocarbon phase to the calcite surface. The formation of even small ionic aggregates was not observed within the brine film at low salinity. Therefore, we inferred that the injection of low salinity brine breaks up the aggregates and oil is released due to the breakage of connections between oil/brine and rock/brine interfaces afforded by those aggregates. The development of electrical double layers at both interfaces was observed and the validation of the known double layer expansion phenomena is plausible as a result of low salinity effect.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Snapshots of ensembles at the end of simulation, Charge density within the ensemble, The size fluctuation of ionic aggregates during simulation, RDF of Nacarboxylate functional group interaction, validations over the equilibrium state of the simulation. 21 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *Email: [email protected] & [email protected] ORCID Mohammad Mehdi Koleini: 0000-0001-7950-951X Mohammad Hasan Badizad: 0000-0001-8144-6896 Shahab Ayatollahi: 0000-0001-7561-6393 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors appreciate the financial supports by the Research and Technology Deputy of the Sharif University of Technology. Close collaboration among the members of Computational Nanoscience and Engineering (CNE) group in SUPRI is acknowledged.


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Colloids Surfaces A Physicochem. Eng. Asp. 2018, 537. (75) Koleini, M. M.; Badizad, M. H.; Kargozarfard, Z.; Ayatollahi, S. The Impact of Salinity on Ionic Characteristics of Thin Brine Film Wetting Carbonate Minerals: An Atomistic Insight. Colloids Surfaces A Physicochem. Eng. Asp. 2019, 571, 27–35. (76) Mahani, H.; Berg, S.; Ilic, D.; Bartels, W.-B.; Joekar-Niasar, V.; others. Kinetics of Low-Salinity-Flooding Effect. SPE J. 2015, 20 (01), 8–20. (77) Mahani, H.; Keya, A. L.; Berg, S.; Bartels, W. B.; Nasralla, R.; Rossen, W. R. Insights into the Mechanism of Wettability Alteration by Low-Salinity Flooding (LSF) in Carbonates. Energy and Fuels 2015, 29 (3), 1352–1367. (78) Ricci, M.; Segura, J. J.; Erickson, B. W.; Fantner, G.; Stellacci, F.; Voïtchovsky, K. Growth and Dissolution of Calcite in the Presence of Adsorbed Stearic Acid. Langmuir 2015, 31 (27), 7563–7571. (79) Lanaro, G.; Patey, G. N. Birth of NaCl Crystals: Insights from Molecular Simulations. J. Phys. Chem. B 2016.


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Figure 1. Schematic representation of thin brine film separating oil phase and rock surface within the reservoir.


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z-axis

z =0

Figure 2. Snapshots of ensembles after 5 ns of simulation; top: side view of ensembles containing a. LS brine, b. HS brine; bottom: top view of ensembles containing c. LS brine, d. HS brine (Water molecules are shown tiny-sized in both views to avoid confusion. Atoms of calcite slab and hydrocarbon film are color-faded in top view figures to clearly observe ionic distributions). See Figure S1 in Supplementary material for more images.


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Figure 3. Charge distribution of ions and carboxylate functional groups of oil within ensembles containing LS (main chart) and HS (inset chart) brines.


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Figure 4. RDF of Na-Cl interaction in LS and HS brines.


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Figure 5. Trajectory analysis of ions of LS (left) and HS (right) brines in the final 2 ns of simulation; a side view representation of the ensembles.


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Figure 6. Projection on the xy-plane of ionic distribution in HS brine in the last 2ns of simulation illustrates the formation of ionic aggregates.


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Figure 7. MSD of common ions in LS and HS brines.


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Figure 8. Distribution of carboxylate functional groups within oil/brine interface at different brine salinities. Data are gathered for the late 2ns of simulation.


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Figure 9. MSD of carboxylate oil components at oil/brine interface at salinities relevant to LS and HS brines.


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Figure 10. RDFs illustrating ionic aggregates interaction with polar oil components and calcite surface.


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Figure 11. Superposition of distribution pattern of oil polar components and ionic aggregates in the late 2ns of simulation.


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Table 1. The composition and salinity of brine films. Abbreviated brine name

Na+

DW 0a LS brine 15 HS brine 50 a number of species

Cl¯

Mg2+

Ca2+

H2O

0 0 0 850a 15 0 0 850 72 2 9 850 b concentration in ppm

Total salinity

Comment

0b 60,000 230,000

Deionized water Low salinity brine High salinity brine


Brine Interfaces within Thin

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