Surface Wettability of Basal Surfaces of Clay Minerals: Insights from

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Surface Wettability of Basal Surfaces of Clay Minerals: Insights from Molecular Dynamics Simulation Lihu Zhang, Xiancai Lu,* Xiandong Liu, Kan Yang, and Huiqun Zhou State Key Laboratory for Ore Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, P. R. China S Supporting Information *

ABSTRACT: Understanding the wettability of clay mineral surfaces is crucial for enhancing oil recovery, investigating primary migration of hydrocarbon, and evaluating the performance of sealing rocks in a petroleum system. On the basis of molecular dynamics simulations, we investigated the interactions between four typical clay minerals (i.e., pyrophyllite, montmorillonite, illite, and kaolinite) and confined pore fluids (i.e., water/alkane/salts). The influences of surface group, layer charge, and salts on the wettability of clay surfaces were revealed. As the layer charge increases, the hydrophilicity of the montmorillonite basal surface gradually increases. The basal surface of 2:1-type pyrophyllite is completely alkane-wet independent of salts. However, for 1:1type kaolinite, the presence of salts makes the siloxane surface completely water-wet, whereas it is partially alkane-wet at the absence of salts. In general, the salt ions adsorbed onto clay surfaces promote the surface hydrophilicity. By using nonequilibrium molecular dynamics, we explored the hydrodynamics of the water/alkane/salts fluid confined in slit nanopores with pore walls made up of montmorillonite and kaolinite. Both montmorillonite and kaolinite surfaces remarkably restrain the movement of the water confined in nanopores. Decane molecules tend to aggregate together and transport as a cluster. Moreover, the migration of the decane cluster is faster than that of water molecules. These findings are helpful for understanding the primary migration of hydrocarbon in clayey source rocks and the geological sealing of oil by clayey cap rocks in petroleum systems. exhibit various surface wettability.37−39 However, it is hard to reveal the contribution of each surface to the apparent wettability of a clay sample and hard to understand the subtle difference in surface wettability from one clay sample to another. With dramatic advances in high-performance computers and continuous developments of force filed potentials in last decades, molecular dynamics (MD) simulation is widely used to obtain detailed insights into the interactions between clay minerals and organic matters at an atomistic level.40−42 Underwood and coworkers performed MD simulations to investigate montmorillonite-organic matter interactions under varying salinity.43 They observed that the nonpolar decane molecules tended to coalesce together and scarcely interacted with Na- and Ca-montmorillonite surfaces. Using ab initio MD simulations, Tunega et al.44−46 revealed that the octahedral surface of kaolinite formed strong hydrogen bonds (HBs) with water layer, while only weak HBs occurred between the tetrahedral surface of kaolinite and water molecules. Thus, the octahedral and tetrahedral surfaces presented as hydrophilic and hydrophobic ones, respectively. Such different wettability of kaolinite surfaces were also proved by a MD simulation of microscopic contact angle of water nanodroplets.47 Another MD simulation showed that a fully water-wet kaolinite system was thermodynamically preferred over a fully cyclohexane-wet kaolinite system.48 Rotenberg and co-workers49 revealed the environmental humidity-dependence of the wettability of talc. At low relative humidity the talc surface behaved as hydrophilic one, whereas it changed into hydro-

1. INTRODUCTION Understanding the wettability of clay mineral surfaces is crucial for not only oil recovery engineering by water-flooding in conventional oil reservoirs but also geological sealing of oil and gas by cap rocks.1−6 In conventional reservoirs, the wettability transition from oil-wet to water-wet can significantly enhance the oil recovery through spontaneous imbibition. A great number of mechanisms and corresponding engineering have been postulated to alter the wettability of reservoir rocks in order to promote the oil recovery, such as chemically adding surfactant,7−11 low-salinity brine,3,12,13 and gas (e.g., carbon dioxide) injection.14−17 Moreover, the wettability of the surface of clayey source rocks and caprocks also greatly affects the migration of oil and gas18,19 and, thus, determines the sealing capacity of cap rocks and expulsive efficiency of hydrocarbon from source rocks. Especially, the rapid development of exploitation of shale-gas and shale-oil requires comprehensive studies on the wettability of clay minerals in organic matter-rich mudstones.20−23 However, it is seriously challenging to get insights into such an issue. For instance, by experimental methods it is commonly hard to distinguish and characterize the interactions between minerals and each component of extremely complex oil fluids in petroleum reservoirs. Many surface characterization techniques have been employed to investigate the interactions between organics and clay minerals, such as Fourier transform infrared spectroscopy,24−26 X-ray photoelectron spectroscopy,24,27,28 scanning electron microscopy,29−31 and atomic force microscopy,28,32,33 which are supplement to contact angle measurement.34−36 However, many essential questions remain unsolved. For instance, different surfaces of a clay particle (i.e., basal surface and edge surface) © XXXX American Chemical Society

Received: September 19, 2015 Revised: December 25, 2015

A

DOI: 10.1021/acs.energyfuels.5b02142 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Detailed Information of All Systems Simulated in This Study

number of species in pore fluid clay type pyrophyllite illite montmorillonite

kaolinite

system name Pyro Pyro-NaCl Illite Na0.25-Mont Na0.50-Mont Na0.75-Mont Mont-NaCl Mont-CaCl2 Mont-Super-NaCl Kao Kao-NaCl Kao-CaCl2 Kao-Super-NaCl

chemical formula

layer charge (e/unit cell)

[Si8][Al4]O20(OH)4 K1.50[Si7Al1][Al3.50Mg0.50]O20(OH)4 Na0.25[Si8][Al3.75Mg0.25]O20(OH)4 Na0.50[Si8][Al3.50Mg0.50]O20(OH)4 Na0.75[Si8][Al3.25Mg0.75]O20(OH)4 Na1.00[Si8][Al3.00Mg1.00]O20(OH)4 Ca0.50[Si8][Al3.50Mg0.50]O20(OH)4 Na1.00[Si8][Al3.00Mg1.00]O20(OH)4 Al2Si2O5(OH)4

−1.50 −0.25 −0.50 −0.75 −1.00 −1.00 −1.00

decane

water

100 100 100 100 100 100 100 100 800 200 200 200 1200

2900 2900 2900 2900 2900 2900 2900 2900 24000 4600 4600 4600 27600

Na+ or Ca2+

Cl−

32 32 48 64 80 96 48 768

32 32 32 32 32 32 32 256

64 32 384

64 64 384

2. MODELS AND SIMULATION DETAILS

phobic surface as they were immersed into water, because the interaction between water and talc surface is weaker than that between water and water. The surface properties of pyrophylite were similar. Clay minerals are the dominated components of mudstone and shale which lead to abundant micropores and mesopores.36 Thus, the wettability of clay surfaces controls the hydrodynamics of the hydrocarbon-bearing fluids confined in these pores. Moreover, it also determines the sealing ability of cap rocks and recovery efficiency of shale oil/gas reservoir. At macroscopic scale, the fluid transfer satisfies the Darcy’s law. In porous media, it is usually described by Navier−Stokes equation, providing that the confined fluid has a uniform density.50,51 However, the fluid confined in nanopores, taking water as an example, strongly interacts with pore walls (mainly clay surfaces in mudstone) and exhibits inhomogeneous density. Hence, it is questionable to describe such confined fluid as a continuous state by Navier− Stokes equation. Only water locating in the middle of the nanopore can be described by Navier−Stokes equation, while the water close to clay surfaces shows great deviations.50 Moreover, in geological environment, the fluids confined in nanopores are generally composed by multiple phases (e.g., water and oil). It is necessary to study the effects of clay surfaces on the hydrodynamics of such complex fluids. At nanoscale, it is hard to directly observe and measure the wettability of clay surfaces at conditions with relative high temperature and pressure. Classical MD simulations and nonequilibrium molecular dynamics (NEMD) simulations are proven methods for investigating the density distribution and hydrodynamics of fluid confined in nanopores.50−55 This study focuses on the wettability of clay surfaces which are exposed to the water/alkane/salts fluids. The hydrodynamics of the fluids confined in the nanopores has also been investigated. Montmorillonite, illite, and kaolinite are selected as representative clay minerals, and pyrophyllite is selected as a reference. Decane is used to represent the nonpolar hydrocarbon in geofluids. Classical MD simulations provided insights into the influence of surface group, layer charge, and salts on the wettability of these clay minerals. And NEMD simulations acquired more detailed information about the hydrodynamics of water/alkane/salts fluid confined in nanopores composed by montmorillonite and kaolinite.

2.1. Models. Pyrophyllite, montmorillonite, and illite are all typical 2:1-type dioctahedral clay minerals. In this study, pyrophyllite (Al4Si8O20(OH)4) is used as a reference as it is a neutral clay. Each pyrophyllite layer is made up of one octahedral sheet sandwiched between two tetrahedral sheets. Octahedral and tetrahedral sheets are connected together by bridging oxygen atoms. Montmorillonite and illite have the same framework but include certain isomorphic substitutions in octahedral sheets (e.g., Al3+ replaced by Mg2+) and/or tetrahedral sheets (e.g., Si4+ replaced by Al3+). Counterions are introduced to compensate the permanent negative charges of the clay sheets (e.g., K+ in illite and Na+/Ca2+ in montmorillonite). Moreover, the layer charge of illite is much higher than that of montmorillonite. In our study, the layer charge of the illite model is 1.5 times higher than the Mont-NaCl model. The unit cell parameters for montmorillonite systems are a = 5.18 Å, b = 8.98 Å, c = 15.0 Å; α = β = γ = 90°.56 The cell parameters are also applied to build pyrophyllite and illite systems only with some modifications to the isomorphic substitutions (details in Table 1). The basal area of 2:1-type clay systems (pyrophyllite, montmorillonite, and illite) is 82.88 Å × 35.92 Å. Unlike 2:1-type clay minerals, kaolinite (Al2Si2O5(OH)4), is a typical 1:1-type clay mineral and consists of alternating sheets of silica tetrahedral and aluminum hydroxide octahedral. In the octahedral sheet, each aluminum atom is coordinated by two oxygen atoms of the tetrahedral sheet and shares four hydroxyls with neighboring aluminum atoms. The hydrogen bonds between the hydroxyl ions of the octahedral sheet of one bilayer and the tetrahedral oxygen atoms of the adjacent silica sheet of the next bilayer hold the kaolinite layers together. Although a few impurity substitutions may induce local charged sites, kaolinite is typically neutral and no charge-balancing hydrated ions occur in the interlayer region. The unit cell parameters of kaolinite is a = 5.1535 Å, b = 8.9419 Å, c = 7.3906 Å; α = 91.926°, β = 105.046°, γ = 89.797°.57 A triclinic supercell kaolinite model was obtained by enlarging the unit cell (specifically, 10 × 10 × 2 times in a, b, and c directions, respectively). Then, we modified the triclinic supercell model by cutting and pasting to get the final orthorhombic kaolinite model. The basal area of orthorhombic kaolinite systems is 51.535 Å × 89.419 Å. In the geological environment, there are abundant micro-meso pores ranging from 3 to 8 nm in shale and mudstone.58−60 Hence, the pore scale in this study were set to about 4.5 nm to satisfy such a consequence. For mimicking the hydrocarbon-bearing geofluids, neutral nonpolar decane (C10H22) was selected as the representative nonpolar oil phase in this study. Two kinds of salts (NaCl and CaCl2) were added to represent real geological fluids and were employed to investigate their influences onto the wettability of basal surfaces of clay minerals. Hence, the fluid in the pore region of different clay minerals consisted of decane, water and salts. Initially, all species in the pores distributed randomly and were generated by packmol source.61 To eliminate the influence of periodic image model, two tetrahedral-octahedral (TO)-type kaolinite sheets B


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Figure 1. Final snapshots of the Mont-NaCl and Kao-CaCl2 systems. without interlayer water were constructed for kaolinite systems. Similarly, two tetrahedral-octahedral-tetrahedral (TOT)-type sheets were used for montmorillonite, illite, and pyrophyllite systems. Detailed information of all systems modeled in this study is summarized in Table 1. The schematic representations of montmorillonite and kaolinite systems considered in this study are illustrated in Figure 1. All configurational snapshots are rendered by VESTA software.62 2.2. Force Fields. The clay minerals and decane are described by CLAYFF63 and OPLS-AA force fields,64 respectively. The flexible version of the simple point charge water model (SPC-Fw)65 is incorporated to describe water and hydroxyl group of clays due to its known compatibility with the CLAYFF force field. The nonbonding intermolecular interactions in both CLAYFF and OPLS-AA force fields are represented by a short-range Lennard-Jones (LJ) potential, and a long-range Coulombic potential expressed in a form of pairwise interacting atomic charges. In the CLAYFF force field, the clay framework is not fixed. All atoms are free to adjust their positions to reach their equilibrium structures. All parameters used in this study are collected in Tables S1 and S2 in the Supporting Information. 2.3. Equilibrium Molecular Dynamics Simulation (EMD). All equilibrium molecular dynamics calculations were performed using the DL_POLY CLASSIC simulation package.66 In calculations, the Verlet velocity integration algorithm was used with a time step of 1.0 fs. The potential energy was evaluated with a 10.0 Å cutoff for the short-range van der Waals interaction, and the Ewald summation for the Coulombic interaction was calculated with a precision of 1.0 × 10−6. The Lorentz− Berthelot combination rule was employed to describe the LJ potential between different atomic species. All simulations were first performed 5 ns in NσT ensemble to obtain equilibrium structures. The constant temperature and stress (NσT) ensemble is a generalization of the constant temperature and pressure (NPT) ensemble, in which the simulation boxes are allowed to change anisometrically. And then a subsequent 10 ns simulation was performed in NVT ensemble where the number of particles (N), the simulation box volume (V), and the temperature (T) were kept constant. The periodic boundary condition was applied to all three dimensions. During the 5 ns NσT ensemble calculation of each model, the energy and volume data were monitored and they both rapidly converged. The trajectory of simulation was also checked and the separation of decane and water occurs less than 500 ps. Hence, it can be concluded that our simulation time is long enough for each system to reach its equilibrium structure. 2.4. Nonequilibrium Molecular Dynamics Simulation (NEMD). We used NEMD to investigate the hydrodynamics behaviors of water/decane/salts fluid confined in the slit pore walled by montmorillonite or kaolinite under a uniform pressure gradient. More precisely, we built two large systems to carry out NEMD simulations. The x−y planes of these two systems were enlarged based on previous Mont-NaCl and Kao-NaCl models and, here, are named as Mont-SuperNaCl and Kao-Super-NaCl (Figure 2) respectively. The specific basal area is 165.76 Å × 143.68 Å for Mont-Super-NaCl and 154.605 Å × 178.838 Å for Kao-Super-NaCl (Table 1), respectively. All NEMD simulations were performed using LAMMPS package.67 The specific strategy is: First, a 10 ns NPT ensemble EMD simulation was carried out for both systems to obtain their equilibrium

Figure 2. Snapshots of Mont-Super-NaCl and Kao-Super-NaCl systems. configurations, and then a 6 ns NVT ensemble NEMD simulation with a constant external x-directional force (Fx) applied on each atom of the pore fluid. Three external forces for each model were selected to reveal the influences of different pressure gradients onto the hydrodynamics of the confined fluid. The used external forces (Fx) are 1.3647 × 10−3, 6.8235 × 10−3, 1.3647 × 10−2 (kcal/mol)/Å for Mont-Super-NaCl and 1.23458 × 10−3, 6.17290 × 10−3, 1.23458 × 10−2 (kcal/mol)/Å for Kao-Super-NaCl systems. The forces Fx = 1.3647 × 10−3 and 1.23458 × 10−3 correspond to the pressure gradient of about 150 MPa for Mont-Super-NaCl and Kao-Super-NaCl models, respectively. The other two forces applied for each model are just enlarged 5 times and 10 times, respectively. Due to the differences in the system size and fluid species in systems of Mont-Super-NaCl and KaoSuper-NaCl, the applied forces are slightly different. These large pressure gradient is necessary because the fluid flow through pores is slow, and it will take extremely long simulations for measuring at geological pressure gradients. The x-directional velocity of each water molecule confined in the slit pore region during the final 1.0 ns was sampled every 0.5 ps in bins of width 0.25 Å in the z-direction and then averaged. As decane molecules rapidly aggregated together and then moved as a cluster, the velocity of all decane was averaged for the final 1 ns to represent the velocity of the decane cluster. 2.5. Simulation Analysis. The geometric criteria applied to define a hydrogen bond are those frequently used in the analysis of bulk water, i.e., intermolecular O···H distance less than 2.40 Å and angle between the O···O and covalent O−H bond less than 30°. The cutoff distance of 2.40 Å adopted here corresponds to the first minimum in radial distribution function (RDF) of O···H using the SPC-Fw water model. The cutoff angle of 30° is used, because it includes 90% of the angular distribution of hydrogen bond in bulk liquid water under ambient C


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Energy & Fuels conditions.68−70 The hydrogen bonds were analyzed using “Hbonds” plugin of VMD software.71 Here, it is worthy to note that the number of hydrogen bond has been normalized to a surface area of 1000 Å2 for montmorillonite, illite, and kaolinite systems due to their different basal surface areas.

3. SURFACE WETTABILITY OF BASAL SURFACES 3.1. 2:1-Type Clay. 3.1.1. Montmorillonite and Illite. In Mont-CaCl2, Mont-NaCl, and illite systems, decane molecules all aggregate into a cluster and are shielded away from the basal surfaces by water and solvated cations (i.e., Na+ and Ca2+) (Figure 3), which indicates the hydrophilic basal surfaces of these three clays. Similar aggregation of oil was observed in experimental studies24,72,73 and molecular dynamics simulations.43 It is energetically reasonable for decane molecules to coalesce together and keep away from the hydrophilic pore walls. In all systems, water molecules exhibit symmetric layered arrangements in which high density at about 2.6 and 5.8 Å to the pore walls of both montmorillonite and illite (Figure 3). However, the distribution of cations in montmorillonite systems shows great differences from that in illite system. In montmorillonite systems, an obvious dominated density peak of Ca2+ and Na+ at 4.3 Å to the basal surface can be observed and both ions tend to be hydrated as outer-sphere complexes, which has also been observed in our previous simulation.74 However, in the illite system, there are three Na+ density peaks at about 0.3, 1.9, and 4.0 Å from the basal surface. Based on the analysis of configurations, the first peak corresponds to the position where Na+ falls into six-member rings on layer surface, the second one represents those Na+ ions with inner-sphere complexes, and the third one are those outer-sphere Na+ complexes. No obvious density peak of Cl− can be found in the montmorillonite systems suggests that anions tend to reside in the aqueous solution due to the electrostatic repulsion by negative charged oxygen atoms of clay surfaces. However, a clear Cl− density peak locates at about 3.1 Å from the illite basal surface. These Cl− anions are paired with the surface complexing Na+ ions. This differences may be caused by different amounts and positions of isomorphic substitutions in montmorillonite and illite. In montmorillonite systems, only octahedral isomorphic substitution were imposed, while both octahedral and tetrahedral substitutions exist in the illite system with tetrahedral ones dominating. Moreover, the amount of substitutions in the illite system is much higher than that in montmorillonte systems (Table 1). Therefore, the illite basal surface is much more negatively charged and afterward more attractive to cations. Meanwhile the adsorbed Na+ cations onto illite surfaces pair with some Cl− anions, which results in a clear Cl− density peak near illite surfaces. 3.1.2. Pyrophyllite. In the density profiles of water/decane/ salts across the pyrophyllite pore region (Figure 4), two obvious decane peaks at about 3.4 and 7.6 Å from the surface clearly indicate the strong adsorption of decane distributed as layers onto pyrophyllite surfaces. Ions are shielded by decane layers and cannot directly interact with pyrophyllite surfaces, which is distinctly different from the cases of charged montmorillonite and Illite. This indicates that the hydrophobic propriety of pyrophyllite surface is independent of the presence of salts, which can be directly observed in the snapshots shown in Figure 4. Thus, it is reasonable to speculate that pyrophyllite surface will be fully covered by decane if there is sufficient amount of decane molecules present. Previous DFT studies75−77 indicates that the water−water interaction is much stronger than the water−

Figure 3. Number density profiles of the species in the pore region of Mont-CaCl2, Mont-NaCl, and illite systems. The vertical dashed lines correspond to the pore boundaries. (inset) Final configurations. “Cdecane” represents the carbon atoms of decane. Ow and Hw respectively denote the oxygen and hydrogen atom of water. The peak intensity of Cl− is 10 times magnified for clarity.

pyrophyllite basal surface interaction, which demonstrates that the basal surface of pyrophyllite is strongly hydrophobic. 3.2. Influences of the Layer Charge. The different wettabilities of the basal surfaces of pyrophyllite, montmorillonite, and illite can be attributed to their different layer charges. Here we illustrated the final snapshots of four Ari-type montmorillonite with different layer charges, named Na0.25Mont, Na0.50-Mont, Na0.75-Mont, and Na1.00-Mont (Figure 5). As the layer charge increases (Table 1), the contact area between D


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Figure 4. Number density profiles of species in the pore region of Pyro and Pyro-NaCl systems. The vertical dashed lines correspond to the lower and upper boundary of the pore. (inset) Final configuration of each system. “Cdecane” represents the carbon atoms of decane molecules. Ow and Hw respectively denote the oxygen and hydrogen atom of water. The peak intensity of Na+ and Cl− is 10 times magnified for clarity.

Figure 5. Final snapshots of Na0.25-Mont, Na0.50-Mont, Na0.75-Mont, and Na1.00-Mont systems.

decane and montmorillonite surface decreases, and finally, the surface transforms to completely water-wet. Similar to the results of pyrophyllite systems, layered distribution of alkane molecules can be observed in the systems with low layer charge (i.e., Na0.25Mont and Na0.50-Mont). The increasing of surface charge deservedly needs more counterions (i.e., Na+ and Ca2+). Both Na+ and Ca2+ have great hydration capacity and induce water molecules close to the montmorillonite surfaces. Hence, the increasing of layer charge leads to the increasing of counterions, and then the increasing of counterions results in the enhancement of surface hydrophilicity. Previous molecular dynamics simulations of uncharged talc78 and charged muscovite79 also implied the influence of layer charge on the wettability of clay basal surfaces. Our simulation is also consistent with a capillary rise test of organo-montmorillonite revealing that the hydrophobicity of montmorillonite increases with a decrease in the layer charge.80 3.3. 1:1-Type Clay: Kaolinite. Being different from 2:1-type clay minerals (e.g., montmorillonite, illite, and pyrophyllite),

kaolinite has two different basal surfaces: siloxane surface and hydroxylated surface. Difference affinity of water to such two surfaces has been previously observed:44,45,47 the siloxane surface is quantified as hydrophobic surface while the hydroxylated surface is hydrophilic. In the number density profiles (Figure 6), the shape and position of water peaks are almost the same in Kao, Kao-NaCl, and Kao-CaCl2 systems. However, the distribution of water near the siloxane surface and hydroxylated surface is obviously distinct and no symmetrical feature appears. Three obvious water peaks locate at 2.6, 3.9, and 6.0 Å from the hydroxylated surface, whereas only two peaks appear at 2.5 and 5.7 Å from the siloxane surface. It is illustrated Na+/Ca2+ and Cl− are respectively adsorbed by the siloxane surface and hydroxylated surface, which is consistent with previous MD simulations.81,82 The distribution of the cations and anions is influenced by the negative-charged oxygen atoms of siloxane surface and the positive-charged hydrogen atoms of hydroxylated surface, respectively. Specifically, Na+ and Ca2+ ions are predominantly fully coordinated by E


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of salts, the hydroxylated surface is completely hydrophilic while the siloxane surface exhibits partially alkane-wettability. However, in the presence of salts, both siloxane and hydroxylated surfaces are characterized as hydrophilic surfaces.

4. HYDROGEN BONDS BETWEEN WATER AND THE CLAY SURFACE The distinct high water density at clay−water interface indicates a strong water affinity which may be attributed to the formation of hydrogen bonds (HBs). Because the Si−O−Si on siloxane surface can serve as HB acceptors while the O−H group on hydroxylated surface act as both HB acceptors and donors. 4.1. HBs at Water-Montmorillonite/Illite Interface. The surface oxygen atoms of montmorillonite and illite can serve as HB acceptors and form abundant HBs with water molecules close to the interface. The HB number is almost the same for Mont-CaCl2 (42.8) and Mont-NaCl (42.6), whereas it is less for illite (35.8) (Table 2), which agrees well with the density profiles (Figure 3). In the illite system, most of the Na+ ions fall into the surface cages or form inner-sphere hydration complexes. Thus, the basal surface of illite is occupied by these Na+ ions complexes and partly excludes water molecules, which results in the relative weaker water density peak and less HBs. The calculation also indicates less HBs of single water in the first water layer close to the illite surface (i.e., 1.4) comparing with montmorillonite surface (i.e., 1.6 and 1.7 shown in Table 2). More than 20% of the HBs of the first layer water molecules are formed between water and clay surfaces, which demonstrates that both montmorillonite and illite siloxane surfaces can form strong HB networks. Figure 7 shows the RDF profiles of Ow···Hw and Oclay···Hw of Mont-CaCl2, Mont-NaCl, and illite systems. For all systems, the first dominated peak of Ow···Hw appears at about 1.74 Å, which clearly indicates the formation of HBs among water molecules. In Mont-CaCl2 and Mont-NaCl systems, the first obvious peak of Oclay···Hw locating at around 1.82 Å demonstrates that montmorillonite basal surface can form HBs with water and act as HB acceptors. We can conclude that the intensity of HB of water−water is a little greater than montmorillonite−water, which can be proven by comparing their respective HB distances (i.e., 1.74 vs 1.82 Å). The illite basal surface also forms HBs with water and acts as HB acceptors. However, the HB distance of illite-water (i.e., 1.70 Å) is a little smaller comparing with that of water−water (1.74 Å) and montmorillonite−water (1.82 Å). Meanwhile, its intensity is also much greater and the peak is much sharper than that of Mont-CaCl2 and Mont-NaCl systems (Figure 7). All these clearly reveal that the HBs formed with the illite surface are much stronger than those with montmorillonite surface, even though the HB number is the smallest for illite (Table 2). Comparing with montmorillonite, the illite basal surface has abundant Lewis bases attributed to the tetrahedral substitutions (e.g., Si4+ substituted by Al3+), which leads to the formation of stronger HBs between water and the illite basal surface. Previous studies also demonstrated that the surface oxygen atoms of clays with tetrahedral substitutions could form very strong HBs with water.83,84 Hence, we can conclude that both montmorillonite with high layer charges and illite are completely hydrophilic, and the hydrophilicity of the illite basal surface is obviously stronger than that of montmorillonite. 4.2. HBs at the Water−Kaolinite Interface. Similar to the basal surface of montmorillonite and illite, the siloxane surface oxygen atoms of kaolinite only act as HB acceptors. However,

Figure 6. Number density profiles of species in the pore region of KaoNaCl, Kao-CaCl2, and Kao systems. The vertical dashed lines correspond to the pore boundaries. (inset) Final configuration of each system. “Cdecane” represents the carbon atoms of decane molecules, Ow and Hw denote the oxygen and hydrogen atom of water. The peak intensity of Na+, Ca2+, and Cl− is 5 times magnified for clarity.

water molecules as outer-sphere complex near the siloxane surface, whereas Cl− mainly form inner-sphere complexes with the hydroxylated surface. In Kao-NaCl and Kao-CaCl2 systems, decane molecules aggregate together and are separated from kaolinite surfaces by water layers (Figure 6). However, in the Kao system, decane molecules directly interact with the siloxane surface and exhibit layered arrangements, whereas water shields them away from the hydroxylated surface. Thus, the wettability of kaolinite basal surfaces is dependent on salinity of the pore fluids. At the absence F


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Table 2. Average Number of Hydrogen Bonds (HBs) between Water and Basal Surface of Montmorillonite, Illite, and Kaolinite Systemsa clay systems 2:1-type

1:1-type

Mont-NaCl Mont-CaCl2 Illite Kao-NaCl

surface type

HB type

siloxane surface

OB as acceptor

siloxane surface hydroxylated surface

OB as acceptor OH− as acceptor OH− as donor OB as acceptor OH− as acceptor OH− as donor OB as acceptor OH− as acceptor OH− as donor

Kao-CaCl2


Kao


NHB/103 Å2

NHB‑average

NHB‑clay/NHB‑all (%)

42.6 42.8 35.8 30.4 11.8 73.0 31.2 11.6 72.7 26.6 17.5 73.2

1.6 1.7 1.4 1.6 2.3

28.4 29.8 22.2 18.3 51.8

1.6 2.3

18.4 51.4

1.7 2.4

16.0 46.4

The value is averaged from the final 500 ps NVT simulation of each system. OB denotes surface bridge oxygen atom on siloxane surface. OH− represents the hydroxyl group on a hydroxylated surface. NHB‑average means the average HB number of single water in the first water layer close to the corresponding clay surface. NHB‑clay/NHB‑all means the ratio of HBs formed between water and corresponding clay surface to all HBs related to water in first water layer. a

Kao system is slightly higher than that of Kao-NaCl and KaoCaCl2 systems (Figure 6), which indicates the relatively stronger layered arrangements of water close to the kaolinite hydroxylated surface at the absence of salts. The location of the first peak of Cl− anion density profile is very close to the first peak of water. Hence, the number of water molecules that can interact with the hydroxylated surface is reduced due to the occupation of the part of the surface by Cl− anions. The RDF of Cl···HOH and Cl···Hw of Kao-CaCl2 and Kao-NaCl (Figure 9) clearly demonstrates that Cl− anions can interact with both water molecules and the kaolinite hydroxylated surface through weak hydrogen bonding. The HB distance is about 2.25 Å which is consistent with previous computational and experimental studies.90−95 This is the possible reason why the number of hydrogen bonds at the kaolinite hydroxylated surface slightly decreases in the presence of Cl− anions. The formation of HBs determines the surface wettability of clays. The presence of salts influences the surface wettability through changing the interface structure. More HBs and more intense HBs lead to stronger water affinity of the clay basal surface. The addition of salts favors for the formation of a strong HB network at the interfaces and makes basal surfaces of illite, montmorillonite, and kaolinite hydroxylated surface completely hydrophilic. An MD simulation on the HBs formed at water−talc and water−mica interfaces indicates that a low degree of interfacial hydrogen bonding for water molecules indicates a relatively hydrophobic surface while a high degree of hydrogen bonding reflects a strong hydrophilic surface.96

abundant hydroxyl groups lead to the strong interaction between the hydroxylated surface and interfacial water molecules through hydrogen bonding. Figure 8 illustrates the RDF of OB···Hw, OOH···Hw, and Ow··· HOH of Kao-CaCl2, Kao-NaCl, and Kao systems. In all three systems, a strong peak of Ow−HOH and a weak peak of OOH−Hw are both located at about 1.75 Å. A relatively broad peak located at about 1.83 Å appears in the RDF of OB···Hw of all kaolinite systems independent of salts. This clearly indicates the formation of HBs between water and kaolinite surfaces. At the presence of salts, The HB number of Kao-CaCl2 and Kao-NaCl systems are almost same (Table 2). Taking the KaoNaCl system as an example, the average HB number (84.8) of the hydroxylated surface is apparently greater than that of the siloxane surface (30.4). In more detail, the HB number donated by hydroxyl groups (73.0) is much greater than that accepted by hydroxyl groups (11.8). Hence, the results demonstrate that the HBs formed at the hydroxylated surface is both stronger and more than that formed at the siloxane surface, which is consisting with previous ab initio MD simulations.44−46 The hydroxylated surface mainly acts as HB donors, which also agrees well with previous simulations.85,86 Hence, it can be concluded that the kaolinite hydroxylated surface is more hydrophilic than kaolinite siloxane surface. The average HB number of single water in the first water layer close to the siloxane surface is about 1.6, while the number is about 2.3 related to the hydroxylated surface (Table 2), which also proves that the kaolinite hydroxylated surface is more hydrophilic. Moreover, the percentage of HBs formed between the first layer water and hydroxylated surface goes over than 45%, which clearly demonstrates that the kaolinite hydroxylated surface is extremely hydrophilic. Previous MD simulations considering interfacial water in contact with hydroxylated silica surface87−89 clearly revealed that water molecules were strongly adsorbed on the fully hydroxylated surface comparing with the nonhydroxylated one, which also proves that hydroxylated surface is much more hydrophilic than siloxane surface. At the absence of salts (i.e., Kao model), the HB number on siloxane surface is 26.2, whereas it increases to 90.7 on a hydroxylated surface (Table 2). The hydroxylated surface still mainly acts as an HB acceptor. The intensity of the first water peak of the density profile close to the hydroxylated surface of the

5. HYDRODYNAMICS OF FLUIDS CONFINED IN CLAY SLIT PORES The surface wettability of clays influences the hydrodynamics of pore fluids due to the complex interface structure. Using NEMD simulations, we investigate the hydrodynamics of water/decane/ salts fluid confined in the slit pore with pore walls made up of montmorillonite and kaolinite. 5.1. Hydrodynamics of Fluids Confined in the Montmorillonite Slit Pore. It is clear that decane molecules aggregate together and move collectively as a cluster under the external forces. However, assuming a uniform density and viscosity of water, the Navier−Stokes equation predicts a parabolic velocity profile of water, which is obtained in the G


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Figure 8. Radial distribution function of OB···Hw, OOH···Hw, and Ow··· HOH of Kao-CaCl2, Kao-NaCl, and Kao systems. Ow and Hw represent the oxygen and hydrogen atom of water, respectively. OB, OHO, and HOH denote the bridge oxygen atom, hydroxyl oxygen, and hydroxyl hydrogen atom of kaolinite. The blue dashed and solid vertical lines indicate the position where the first peak is located.

Figure 7. Radial distribution function of Ow···Hw and Oclay···Hw of Mont-CaCl2, Mont-NaCl, and illite systems. Ow and Oclay respectively represent the oxygen atom of water and clay. Hw denotes the hydrogen atom of water. The blue dashed and solid vertical lines indicate the position where the first peak is located.

middle pore region of Mont-Super-NaCl model. More specifically, the distance ranges roughly from 12.5 to 32.5 Å (Figure 10). In this range, the density of water is homogeneous and approximately 1.0 cm3/g and, thus, can be taken as bulk water. However, the water close to the clay surface shows layered arrangements and exhibits much lower velocity. It deviates from the continuous parabolic profile and results in two apparent inflection points near the pore boundaries (denoted by white dashed circles locating at about 3.75 and 40.5 Å, Figure 10).

Similar parabolic velocity profile has also been obtained in other NEMD studies.50,51 This indicates that the hydrophilic surface of montmorillonite greatly influences the hydrodynamics of the pore fluid. Conversely, the velocity of decane cluster is much faster than that of water molecules in the pore. Thus, it indicates that hydrocarbon is easily expulsed from slit pore systems with hydrophilic pore walls. H


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Figure 9. Radial distribution function of Cl−···HOH and Cl−···Hw of Kao-CaCl2 and Kao-NaCl systems. HOH and Hw represent the hydrogen atom of water and kaolinite, respectively. The blue dashed and solid vertical lines indicate the position where the first peak is located.

Figure 10. Velocity and density profiles of fluids confined in Mont-Super-NaCl system. The vertical dashed gray lines represent the pore boundaries. The inserted table is the average velocities of decane cluster. The gray rectangles denote the water layers.

5.2. Hydrodynamics of Fluids Confined in the Kaolinite Slit Pore. In the slit pore with kaolinite walls, decane molecules also tend to aggregate and move as a cluster. The velocity profile of confined water is apparent unsymmetrical, which corresponds to the different nature of the hydroxylated surface and siloxane surface (Figure 11). The locations of inflection points (white dashed circles in Figure 11) of water velocity profiles reveal different constrains of such two different surfaces on the movement of the confined water. The hydroxylated surface more greatly reduces the velocity of water than the siloxane surface, which is attributed to the stronger HB network at the hydroxylated surface. In Kao-Super-NaCl system, the transport of a decane cluster is also faster than that of water. In summary, the movement of water molecules in slit nanopores with pore walls composed of montmorillonite or kaolinite basal surfaces is remarkably restrained due to the hydrophilic property of pore walls. However, decane molecules tend to aggregate together and migrate faster than water molecules. This finding can be used to explain the remarkable sealing ability of clayey cap rocks and high efficiency of primary migration of hydrocarbon from clayey source rocks97−99 because slit clay pores are generally well developed in these rocks.

6. CONCLUSIONS The MD simulations of four clay minerals (i.e., pyrophyllite, montmorillonite, illite, and kaolinite) interacted with the water/ decane/salts fluid provide information on the influences of surface group, layer charge and salts on the surface wettability. As the layer charge increases, the basal surface gradually transform from completely alkane-wet (uncharged pyrophyllite) to mainly alkane-wet (montmorillonite with low layer charge), to mainly water-wet (montmorillonite with moderate layer charge), and finally to completely water-wet (montmorillonite with high layer charge and illite). In general, the salt ions (i.e., Na+ and Ca2+) adsorbed onto clay surface also promote to the surface hydrophilicity. However, the influence of salts on the basal surface of uncharged 2:1-type clays is different from that of 1:1-type ones. The basal surface of 2:1-type pyrophyllite is completely alkanewet independent of salts. But, for 1:1-type kaolinite, the presence of salts makes the siloxane surface completely water-wet which is partially alkane-wet at the absence of salts. The NEMD simulations demonstrate the hydrodynamics of the water/decane/salts fluid confined in the slit nanopores with I


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Figure 11. Velocity and density profiles of fluids confined in Kao-Super-NaCl system. The vertical dashed gray lines represent the pore boundaries. The inserted table is the average velocities of decane cluster. The gray rectangles denote the water layers.

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pore walls made up of montmorillonite and kaolinite. Both montmorillonite and kaolinite surface remarkably inhibit the movement of the confined water on the hydrophilic clay surfaces. Alkane molecules tend to aggregate together and transport as a cluster. Moreover, the migration of the alkane cluster is faster than that of water. These findings are helpful for understanding the primary migration of hydrocarbon in clayey source rocks and the geological sealing of oil by clayey cap rocks in petroleum systems and, also, will facilitate technology developments for enhancement of recovery efficiency of conventional and unconventional petroleum.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02142. Table S1. Nonbonding parameters of the CLAYFF force field, OPLS-AA force field, SPC-Fw water model, and aqueous ions. Table S2. Parameters of bond stretch, angle bend, and torsional energy (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-25-89681065 (X.L.). Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

We acknowledge the China National Basic Research Program (973) of China (No. 2012CB214803) and the National Science Foundation of China (Nos. 41103029 and 41425009). We also acknowledge the High Performance Computing Center of Nanjing University for the calculations carried out on the IBM Blade cluster. J


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