Effects of Salt Ions on the Methane Hydrate Formation and

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Effects of salt ions on the methane hydrate formation and dissociation in the clay pore water and bulk water Guozhong Wu, Haoqing Ji, Linqing Tian, and Daoyi Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03486 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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Effects of salt ions on the methane hydrate formation and dissociation in

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the clay pore water and bulk water

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Guozhong Wu †, ‡, 1, Haoqing Ji †, ‡, §, 1, Linqing Tian †, ‡, Daoyi Chen †, ‡, *

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† Division of Ocean Science and Technology, Graduate School at Shenzhen,

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Tsinghua University, Shenzhen 518055, China

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‡ School of Environment, Tsinghua University, Beijing 100084, China

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§ College of Energy, Soochow University, Suzhou 215006, China

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(1 These authors contributed equally to this work)

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* Corresponding Author

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E-mail: [email protected]

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Tel: +86 0755 2603 0544

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1

ACS Paragon Plus Environment

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ABSTRACTS

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Gas hydrates in marine sediments are promising energy resources, while an effective

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recovery of methane from clay pores relies on a comprehensive appreciation of the

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hydrate evolution inside and outside the pore especially at saline environment.

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Molecular dynamics simulations were conducted to investigate the methane hydrate

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formation and dissociation in the sodium montmorillonite interlayer (Na-MMT) with

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fresh water and saline water, respectively, by characterizing the distribution and

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transportation of methane and ions (K+, Na+, Ca2+), the overall and local four-body

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structural order parameter, and the radial distribution functions. Results indicated that

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it was much easier to form methane hydrates in the bulk water than in the pore water,

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while the hydrates in the pore region were more readily dissociated than in the bulk

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region. The effects of salt ions on the hydrate formation were opposite in these two

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regions, which highlighted the role of the salting-out effect and the ion exchange

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between bulk water and pore water on the hydrate formation dynamics. It also

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demonstrated the priority for the hydrate dissociation from the contact area between

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MMT edge and bulk water, because this region is more favorable for the distribution of

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salt ions and is more susceptible to be perturbed by the diffusion of salt ions from the

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pore water. Overall results provided theoretical supports for better understanding the

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microscopic mechanisms for the methane hydrates evolution at the heterogeneous

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environment with salt ions.

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1. INTRODUCTION

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Natural gas hydrates (NGH) in marine sediments are promising energy resources in

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which the energy stored is estimated twice of that in all other fossil fuels. Since there

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are abundant salt ions (e.g., Na+, K+, Ca2+, Mg2+, Cl-, SO42-) in the seawater, it is of

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particular interests to investigate the effects of these naturally occurred salt ions on the

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formation and dissociation dynamics of NGH. Recent evidences demonstrated that the

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salt diffusion through sediment affected the gas hydrate destabilization zone and caused

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the gas hydrate decomposition close to the seafloor.1 It suggested that the gas hydrate

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dissociation may occur in the future due to the salinity changes of the sea water. For

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better understanding the reservoir formation mechanisms and developing energy

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recovery technologies, insights into the effects of salt ions transports between sediment

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pore water and bulk water on the hydrate evolution are highly demanded.

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Previous studies revealed that salt ions could act as thermodynamic inhibitors being

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able to decrease the stability of gas hydrates by altering the hydrate phase equilibrium.2

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Recent studies demonstrated that salt ions could also inhibit the kinetics of methane

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hydrate formation and facilitate the hydrate dissociation. However, Sowa et al. found

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that some salt solutions such as LiI and KI could promote hydrate formation kinetics

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when present at concentrations below 1 M.3 Nguyen et al. also observed the

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concentration-dependent effects of sodium halide (especially NaI) solutions on

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methane hydrate formation (i.e., promotion at low concentration but inhibition at high

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concentrations).4 They attributed such observation to the hydrophobic hydration of

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large and polarizable anions, which was similar to that of methane and facilitated the 3


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process of entropy change during hydrate nucleation. Moreover, Lu et al. suggested that

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anions were more important than cations in affecting hydrate stability in electrolyte

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solutions, because anions have stronger ability to influence the ambient water structure

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than cations.5 However, Sabil et al. argued that there was no conclusive evidence to

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support the above speculation by Lu et al., because they demonstrated that the hydrate

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equilibrium was shifted to a lower temperature when the Na+ was substituted by Mg2+

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in chloride solution which was similar to the case when the Cl- was substituted by F- in

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sodium solution.6 Recently, Sun et al. demonstrated that cations had less effect on the

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hydrate dissociation enthalpy than anions due to the different ability in affecting the

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ambient water networks.7 These findings suggested the demands for future works

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gaining insights into the underlying mechanisms of gas hydrate formation and

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dissociation in presence of different types of salt ions. Accordingly, Lv et al.

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investigated the phase equilibrium of cyclopentane-methane hydrates formed in the

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mixture of four cations (K+, Na+, Mg2+, Ca2+) and two anions (Cl-, SO42-) at different

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concentrations using the orthogonal test method, which suggested that the hydrate

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inhibition strength of an ion depended on the charge and radii of ion. 8

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More importantly, majority of previous studies focusing on the effects of salt ions on

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hydrate evolution were performed in salt solution with less consideration of the clay

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minerals. It is known that the presence of solid particles will change the pathway of

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hydrates by providing heterogeneous reaction environment, so a lot of efforts have been

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devoted to investigate the hydrate formation and dissociation in sandy sediments.9-11

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However, geological data indicated that the most abundant but also the most 4


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challenging hydrate resources are stored in the clay-rich “silty” sediments. Although

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some experimental and theoretical works were also carried out to investigate the

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hydrate formation in the clay, these studies were more focused on the effects of the

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organic matters, pore size and experimental conditions such as gas release rate.

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Some clays, such as the sodium montmorillonite (Na-MMT) abundant in the marine

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hydrate-bearing sediments, possesses negative charges on the surface, which are

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supposed to associate with the surrounding salt ions and affect their distributions. It

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would eventually influence the distribution of water and methane due to the hydration

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capability and “salting-out” effect of ions. It remains unclear how the ions with different

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hydration ability can drive the partitioning of methane between clay pore water and

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bulk water and how the ion exchange process can influence the dynamics of hydrate

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formation and dissociation at different regions in the clay environment.

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Accordingly, microsecond molecular dynamics (MD) simulations were performed in

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this study to investigate the methane hydrate evolution in the MMT pore and bulk water

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in presence of three typical salts (NaCl, KCl and CaCl2). Specific objectives were to (i)

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identify the distribution of salt ions inside and outside the MMT interlayer and clarify

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its effects on the overall tendency of hydrate formation, (ii) quantify the mobility and

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ion exchange capability of ions between pore water and bulk water and clarify its role

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on the regional dissociation of methane hydrates, and (iii) compare the differences in

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the above processes at different temperature.

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2. METHODOLOGY

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2.1 Molecular dynamics simulation

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MD simulations were performed with an open-source software Gromacs (version 5.0.5).

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16

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from the American Mineralogist Crystal Structure Database. 17 MMT surface was built

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by replicating the unit cell along the xy-plane (7 × 4 × 1), which was then truncated to

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obtain the (0 1 0) edges. The dangling bonds at the edges were saturated by adding

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terminal H atoms or –OH groups using the method proposed by Suter et al.,

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the x-dimension of the MMT surface remained continuous. An MMT interlayer

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structure was constructed by sandwiching a water box between two MMT surfaces.

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Subsequently, a bulk water box was stacked to get in contact with the (0 1 0) edge of

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the MMT surface (Fig. 1). The height of the simulation box (equals the interlayer space

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of the MMT interlayer) was initially set at 4.0 nm, while the length and width of the

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simulation boxes were set at 3.6 nm and 9.7 nm, respectively. To compensate for the

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negative charges on the MMT surface, 28 Na+ were added to the MMT interlayer. Three

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types of salts (NaCl, KCl, CaCl2) were added in the water at a concentration of 3.5 wt.%,

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respectively, to construct the saline-clay models. Accordingly, four scenarios were

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established including the MMT with fresh water (28 Na+), with NaCl brine (70 Na+ +

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42 Cl-), with KCl brine (28 Na+ + 33 K+ + 33 Cl-) and with CaCl2 brine (28 Na+ + 22

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Ca+ + 44 Cl-), respectively. For each scenario, the number of methane and water

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molecules was 330 and 3750, respectively.

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The MMT and salt ions were modeled by the CLAYFF force field.

The MMT unit cell with a stoichiometry of [Al3Mg1][Si8O20][OH]4 was obtained

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while

Water and

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methane were described by the TIP4P/Ice model

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model,

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using the standard Lorentz−Berthelot mixing rules.

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truncated with a cut-off value of 1.2 nm, while long-range electrostatic interactions

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were calculated using the particle mesh ewald method with a Fourier spacing of 0.12

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nm. 23 The leapfrog algorithm with time step of 1 fs was used to integrate the equations

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of motion. 24

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The initial configurations were energy-minimized using the steepest descent algorithm,

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followed by isobaric−isothermal (NPT) equilibration at 250 K and 500 bar for 200 ps.

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MD simulations of methane hydrate formation were performed using the NPT

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ensemble (250 K, 500 bar) with durations of 3 μs. The final configurations of hydrate

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formation processes were used as starting structures for the subsequent hydrate

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dissociation simulations (NPT ensemble, 50 ns). The dissociation pressure was fixed at

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50 bar, while the dissociation temperature was set as 293 K and 303 K, respectively.

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The positions of atoms in the MMT surface were restrained with a force constant of

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1000 kJ mol-1 nm2 during MD simulations. Temperature and pressure were controlled

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by the Nose-Hoover thermostat

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Semi-isotropic pressure coupling was used to allow the z dimension of the simulation

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box to fluctuate independently from the x and y dimensions. Periodic boundary

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conditions were applied in all the three directions.

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and united-atom Lennard-Jones

respectively. Cross interactions between different species were calculated

25

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Short-range interactions were

and Parrinello-Rahman barostat,

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respectively.

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2.2 Data analysis

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The four-body structural order parameter F4φ was used to characterize the evolution of

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methane hydrate, which was defined as follows

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1 𝐹4𝜑 = 𝑛

𝑛

∑ cos 3𝜑

𝑖

𝑖 =1

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where φi is the H−O···O−H torsion angle in the ith water pair, and n is the total number

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of water pairs with the distance between oxygen atoms less than 3.5 Å. The F4φ values

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for ice, liquid water and hydrate are -0.4, -0.04 and 0.7, respectively. 27

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The face-saturated incomplete cage analysis (FSICA) developed by Guo et al.

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used to identify the guest methane (encapsulated in hydrate cages) and gas methane

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(less than 16 water molecules within a sphere of 0.54 nm radius) during methane

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hydrate formation and dissociation. The number density profiles, radial distribution

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functions (RDF) and mean-square displacements (MSD) were calculated by the

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standard modules integrated in Gromacs.

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was

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3. RESULTS AND DISCUSSIONS

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3.1 Effects of salts on the methane hydrate formation

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The trajectories of the salt ions during the last 100 ns of hydrate formation are shown

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in Fig. 2. Cations were tightly distributed near the electronegative MMT surface, while

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the Cl- were distributed a little farther. The K+ was closer to the MMT surface than the

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Na+ (Fig. 2C), while similar distribution was observed between Na+ and Ca2+ (Fig. 2D).

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This finding was attributed to the different hydration properties of the selected ions. Ion

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hydration in solution is a normal phenomenon,

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which was evidenced by the first

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peaks located from 0.24 to 0.32 nm in the RDFs of oxygen in water around the ions

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(Fig. 3A). The intensity of the first peak in the RDF curves was ranked as Ca2+ > Na+ >

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K+. It suggested stronger hydration ability of Na+ and Ca2+ than K+, which was

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consistent with previous experimental results that the ion hydration was more

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pronounced for the ions with higher charge density.

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molecules were required to hydrate one K+, resulting in the smaller distance to the

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MMT surface compared with the other two cations.

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The overall tendency of hydrate formation was evaluated by the F4φ parameter (Fig.

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4A). It demonstrated that the presence of salts facilitated the hydrate formation during

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the initial 500 ns. This might be attributed to the salt ion hydration phenomenon

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aforementioned. Ion hydration had salting-out effect that could exclude the surrounding

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methane, which was supported by the decrease in the intensity of RDF peaks for

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methane around ions with time (Figs. 3B - D). This process was kinetically favorable

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for methane hydrate formation, because it resulted in the local accumulation of methane,

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increased the corresponding liquid concentration, and therefore provided higher driving

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forces for hydrate formation compared to the system without salts. Meanwhile, the ion

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hydration process also reduced the number of water molecules being able to participate

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in the methane hydrate formation. It should have decreased the total amount of hydrate

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formation, but this was not observed as little difference was found in the overall F4φ

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between fresh and saline water at the end of hydrate formation (Fig. 4A). For better

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interpretation, snapshots of the main types of hydrate cages (i.e., 512, 51262, 51263 and

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51264 cages) at the end of hydrate formation are shown in Fig. 5. It demonstrated that

30-32

9


Accordingly, less water

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the number of hydrate cages inside the MMT interlayer was obviously less than that in

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the bulk water, which was most pronounced in fresh water system.

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This implied different mechanisms for hydrate formation inside and outside the MMT

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interlayer, which motivated us to divide the simulation box into two regions and

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calculate the regional F4φ parameter corresponding to the pore water and bulk water,

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respectively (Fig. 1). Results demonstrated that it was more favorable for hydrate

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formation in the bulk water than in the pore water, which was evidenced by the 3 - 4

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folds higher F4φ in the former (Fig. 4B). The slow kinetics of hydrate formation in the

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pore water might be attributed to the MMT surface where ions were concentrated and

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inhibited hydrate formation due to their thermal motions. Another attributable fact was

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the transport of methane from pore water to bulk water resulting from the salting-out

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effects aforementioned. As shown in Fig. 6, the number of methane inside the interlayer

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region decreased by 20 ~ 55% during hydrate formation. In the simulation system with

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fresh water, such observation was due to the salting-out effects from the

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counterbalancing cations on the Na-MMT surface. When salts were added in the system,

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the methane transport towards bulk water was hindered by the movement of salt ions

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along the opposite direction, while the latter was driven by the negatively charged

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MMT surface. The trend of methane transport was in good agreement with the

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corresponding amount of hydrate formed in different systems (Fig. 4).

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The above findings highlighted the role of salt ions exchange between bulk water and

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pore water on the variance of hydrate formation in these two regions. It meant that the

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methane molecules in the pore water were expelled from the pore by the salt ions on 10


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the MMT surface, but this process was resisted by the salt ions in the bulk water through

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converse movement. Correspondingly, the ions added in the water promoted hydrate

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formation in pore water but inhibited hydrate formation in bulk water (Fig. 4B).

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Moreover, the exchange capability varied with the ion types. For example, 14 Na+ in

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the pore was exchanged by the same amount of K+ from bulk water (Fig. 6C), while

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only 6 Na+ was exchanged by 3 Ca2+ (Fig. 6D). The K+ had the strongest ability to

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transport from bulk water to pore water due to its highest self-diffusivity among the

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three types of ions (Fig. 7A), which led to the least amount of methane transport from

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pore water to bulk water and therefore the amount of hydrate formation inside the MMT

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interlayer was highest among the four scenarios (Fig. 4B). The strongest mobility of K+

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was due to its weakest hydration ability aforementioned, because ion diffusion in

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solution was accompanied by the simultaneous diffusion of the hydration shell.

228 229

3.2 Effects of salts on the methane hydrate dissociation

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3.2.1 Overall tendency of methane hydrate dissociation

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Overall tendency of methane hydrate dissociation was also evaluated by the F4φ

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parameter (Fig. 8A). Noticeable dissociation was not observed in any simulation system

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at 293 K. When the temperature increased to 303 K, hydrates in saline water were

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completely dissociated but only about 50% was decomposed in fresh water. It suggested

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that salt ions had promotion effects on the hydrate dissociation. Results further

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demonstrated that the slow dissociation rate at 293 K was attributed to the difficulty in

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collapsing the hydrate structures outside the MMT interlayer (Fig. 8B). As suggested 11


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by the fluctuations of the F4φ curves, hydrates in the bulk water region underwent the

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structure break-up and re-formation processes at 293 K. By contrast, all hydrates inside

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the MMT interlayer were completely decomposed at 293 K and 303 K (Figs. 8B and

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8C).

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Above results suggested that hydrates dissociated more readily in the pore water than

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in the bulk water. To gain insights into the dissociation characteristics of hydrates in

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each region, the interlayer space was divided into five horizontal slices with equal

245

thickness of 0.6 nm, while the region outside the interlayer was divided into five

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vertical slices with equal thickness of 1.2 nm (Fig. 1). For each slice, we calculated and

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discussed the corresponding F4φ parameter in the following subsections.

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3.2.2 Methane hydrate dissociation inside the MMT interlayer

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Evolution of F4φ inside the MMT interlayer during methane hydrate dissociation are

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shown in Fig. 9. Due to the symmetry of the interlayer space, only the results of the

252

first three slices are reported. As expected, faster hydrate dissociation was observed at

253

higher temperature. For example, hydrates inside the MMT interlayer with NaCl brine

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dissociated incompletely at 15 ns at 293 K (Fig. 9B), while it only took 5 ns for

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complete dissociation at 303 K (Fig. 9F). This was ascribed to the less stable hydrate

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structure and the increased mobility of ions resulted from heating. The latter was

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evidenced by the MSD curves for ions during the last 5 ns of dissociation (Fig. 7B).

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The increased mobility of ions would strengthen their ability to break up the

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surrounding hydrate structures and lead to faster hydrate dissociation. 12


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At 293 K, little change was observed in the F4φ value for Slice 1 in all cases due to the

261

lack of hydrate cages in this region (Fig. 9). It worth noting that the hydrates in the

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second slice dissociated ahead of that in the third slice in the MMT interlayer with KCl,

263

which was different from the systems with the other two ions (Fig. 9). For example, the

264

F4φ value for Slice 2 decreased by about 38% during the first 2.5 ns, but obvious

265

decrease was not observed in the corresponding value for Slice 3 during this period

266

(Fig. 9C). This might be associated with the distribution of ions at the beginning of

267

dissociation. As shown in Fig. 2, K+ in the pore region were more tightly distributed

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near the MMT surface than Na+ or Ca2+. Consequently, the stronger perturbation of K+

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resulted in the hydrate dissociation from the vicinity of MMT surface to the middle area

270

of pore. When the temperature increased to 303 K, the hydrates in the third slice became

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more readily to dissociate due to the increased mobility of ions and the decreased

272

stability of hydrate cages (Fig. 9G).

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3.2.3 Methane hydrate dissociation outside the MMT interlayer

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Evolution of F4φ outside the MMT interlayer during methane hydrate dissociation at

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293 K is shown in Figs. 10A - 10D. It demonstrated that the hydrate structures near the

277

MMT edge (Slices 1 and 5) were most susceptible to break up. For example, the F4φ

278

values in these two slices decreased by 70% and 36%, respectively (Fig. 10A). Similar

279

tendency was observed when salt ions were present (Figs. 10B - 10D). This was

280

attributed to two facts that (i) the salt ions outside the MMT interlayer were mainly

281

located at the vicinity of the MMT slab rather than at the center of the bulk water, which 13


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were adverse for the stability of hydrate structure in these regions, and (ii) the fast

283

dissociation of the hydrates inside the MMT interlayer resulted in more free water,

284

which increased the diffusivity of the ions in pore water and perturbed the hydrates in

285

the adjacent region. By contrast, little dissociation was observed for the hydrates at the

286

center region of the bulk water (Slices 2 to 4) from which even new hydrates were

287

formed as evidenced by the increased F4φ values. The newly formed hydrates were

288

resulted from the methane molecules released from the partially dissociated hydrates in

289

the neighbor regions. Results also indicated that it was hard to destroy all the hydrate

290

structures outside the MMT interlayer at 293 K, because the F4φ values in all scenarios

291

at the end of dissociation was larger than 0.07 which was larger than the corresponding

292

value for liquid water (-0.04). This was further confirmed by the fact that over 20% of

293

the methane molecules remained trapped in hydrate cages while the percentage of the

294

methane released from hydrate dissociation was less than 10% (Fig. 11).

295

The above findings, including the collapse of the hydrate structure near the boundary

296

between pore water and bulk water and the hydrate re-formation at the center region of

297

the bulk water, were also noted in the MMT with fresh water when the temperature

298

increased to 303 K (Fig. 10E). When the salt ions were present, all the hydrates outside

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the MMT interlayer disappeared during the first 25 ns (Figs. 10F - 10H). An interesting

300

finding was that there were distinct turning points in the F4φ curves for the saline water

301

at 303 K. For example, the F4φ value in Slice 3 of the NaCl brine system sharply

302

decreased from 9 to 12.5 ns (Fig. 10F). Similar phenomenon was observed for the F4φ

303

value in Slice 2 of the KCl or CaCl2 brine systems at around 17 ns. A closer examination 14


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of the hydrate cages evolution suggested that such sharp decreasing points were

305

triggered by the methane bubbles resulted from the dissociation of the hydrates from

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neighbor regions. For better clarifying this process, we selected some snapshots from 9

307

to 12.5 ns during hydrate dissociation in the NaCl brine as an example, which clearly

308

showed the bubble evolution and fast collapse of the hydrate cages in the Slice 3 (Fig.

309

12). As can be seen, a small methane bubble was formed at the boundary between pore

310

water and bulk water at 9 ns. During the next 1 ns, this bubble slightly moved rightwards

311

while the methane released from Slices 4 and 5 accumulated to form another bubble at

312

the bottom right corner. Subsequently, the left bubble kept on moving rightwards which

313

destabilized the nearby hydrate structures and markedly accelerate the hydrate

314

dissociation in Slice 3. Eventually, the methane molecules released from Slice 3

315

integrated into the two bubbles at 12.5 ns (Fig. 12D).

316 317

4. CONCLUSIONS

318

This study demonstrated that the presence of salt ions initially facilitated the methane

319

hydrate formation, because the salting-out effect led to the local accumulation of

320

methane. Hydrate formation in bulk water was 3 – 4 folds higher than in the pore water,

321

which was attributed to the transport of methane from pore water to bulk water driven

322

by the cations inside the pore. Compared with the fresh water, the addition of salt ions

323

increased the amount of hydrate formed in pore water but was adverse for hydrate

324

formation in bulk water. This was attributed to the fact that the aforementioned methane

325

transport was hindered by the movement of salt ions along the opposite direction. It 15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

326

highlighted the role of salt ion exchange between bulk water and pore water on the

327

varied dynamics of hydrate formation between these two regions, which was more

328

pronounced in the system with K+ due to the stronger ion diffusivity than Na+ and Ca2+.

329

Moreover, the salt ions promoted the hydrate dissociation, while the hydrate dissociated

330

more readily in the pore water than in the bulk water. At relatively low temperature, the

331

hydrate structure near the interface between MMT and bulk water was more susceptible

332

to break up than that in the central region of bulk water. At relatively high temperature,

333

the increased mobility of salt ions strengthen their capability to break up the

334

surrounding hydrate structures, while the formation of gas bubbles due to methane

335

release further facilitated the dissociation of the hydrates in the neighbor regions.

336 337

Author information

338

1

339

Corresponding Author:

340

* E-mail: [email protected]. Telephone/Fax: +86-0755-26030544.

341

Notes:

342

The authors declare no competing financial interest.

343

ACKNOWLEDGEMENTS

344

This study was financially supported by the Shenzhen Peacock Plan Research Grant

345

(No. KQJSCX20170330151956264), Guangdong Natural Science Foundation (No.

346

2018A030313899), and the Development and Reform Commission of Shenzhen

347

Municipality (No. DCF-2018-64).

Guozhong Wu and Haoqing Ji contributed equally to this work

16


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348

17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Energy & Fuels

34, 173-199.

21


Energy & Fuels

MMT Surface Slice 5, width: 1.2 nm

Slice 1, thickness: 0.6 nm

Slice 4, width: 1.2 nm





MMT Surface Slice 1, width: 1.2 nm

439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454

Pore water region

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 22 of 33

MMT Surface

MMT Surface Bulk water region

455

Fig. 1 Schema for the definition of pore water region and bulk water region and the division of each region.

456

(MMT surfaces at right are shown due to the periodic boundary conditions)

457

22


Page 23 of 33

4

MMT

Na+ 2

0

0

MMT 2

MMT

(A) Control

4

6

Box-height (nm)

Box-height (nm)

4

Na+ Cl-

2 1 0

8

0

MMT 2

0

4

0

4

6

Box-lengh (nm)

Box-height (nm)

(C) KCl Na+ ClK+ MMT 2

4

6

8

Box-length (nm)

MMT

2

(B) NaCl

3

Box-length (nm) 4

Box-height (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

(D) CaCl2 Na+ ClCa2+

2

0

8

MMT

0

MMT 2

4

6

8

Box-length (nm)

Fig. 2 Trajectories of salt ions during the last 100 ns of hydrate formation in the systems with (a) fresh water, (b) NaCl brine, (c) KCl brine, and (d) CaCl2 brine. Trajectories were extracted at 1-ns interval. Water and methane are hidden to highlight the ions 23


Energy & Fuels

1.2

A

+

Ow- Na (NaCl) Ow- Cl- (NaCl)

15

g (Na+ - CH4)

g (Ow - ion)

20

+

Ow- K (KCl) Ow- Ca2+ (CaCl2)

10 5 0

0.2

0.4

0.6

0.8

B

0.9 0.6 0.3

0 - 100 ns 100 - 200 ns 200 - 300 ns

0.0 0.0

1.0

0.4

r (nm) 2.0

2.0

C

1.5 1.0 0.5 0.0 0.0

0 - 100 ns 100 - 200 ns 200 - 300 ns

0.4

0.8

0.8

1.2

1.6

r (nm)

1.2

g (Cl- - CH4)

g (Ca2+ - CH4)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

D

1.5 1.0

0.0 0.0

1.6

0 - 100 ns 100 - 200 ns 200 - 300 ns

0.5

0.4

0.8

1.2

r (nm)

r (nm)

Fig. 3 Radial distribution functions for (a) oxygen in water around ions, and (b - d) methane around the Na+, Ca2+ and Cl- in the system with CaCl2.

24


1.6

Page 25 of 33

0.4

0.6

A

B

0.3

F4

0.2 0.1 0.0 -0.1

NaCl (b) CaCl2 (b)

0

pore water

NaCl KCl CaCl2

0.2

Control

0.0

1

2

3

KCl (b) Control (b)

bulk water

0.4

F4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

NaCl (p) CaCl2 (p)

0

Time (s)

1

KCl (p) Control (p)

2

3

Time (s)

Fig. 4 Evolution of (A) overall F4φ and (B) local F4φ during hydrate formation (“b” in the bracket represents “bulk water”, “p” in the bracket represents “pore water”, “control” represents the system with fresh water)

25


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(A) Control

(B) NaCl

(C) KCl

(D) CaCl2

Fig. 5 Snapshots of hydrate cages (512, 51262, 51263, 51264 cages) at the end of hydrate formation (red: methane, yellow: Na+, green: Cl-, purple: K+, black: Ca2+, blue: water).

26


Page 26 of 33

Page 27 of 33

120

(A) Control

(B) NaCl

+

Na CH4

Na+ ClCH4

90

90

Number

Number

120

60

60 30

30 0

1

2

0

3

0

1

Time (s) 120

(C) KCl

100

Na K+

+

120

-

Cl CH4

(D) CaCl2

100

80 60 20

0

2

3

Time (s)

Number

Number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Na+ Ca2+

ClCH4

80 60 20

0

1

2

3

0

0

Time (s)

1

2

Time (s)

Fig. 6 Number of salt ions and methane molecules inside the MMT interlayer

27


3

Energy & Fuels

5

(A)

(B) NaCl Ca2+(CaCl2)

Na+(CaCl2)

3.0

K+(KCl)

+

Na (KCl)

2.4

4

Cl-(CaCl2) Cl-(KCl)

MSD (nm2)

3.6

MSD (nm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Cl-(NaCl)

Na+(NaCl) Na+(Control)

1.8 1.2

+

3K 0 3

3

0

10

20

30

40

+

0 45

50

,

Na

2 1

0.6 0.0

Page 28 of 33

293

46

47

48

a K, N

49

Time (ns)

Time (ns)

Fig. 7 MSD of ions during the (A) first 50 ns of hydrate formation and (B) last 5 ns of hydrate dissociation

28


50

Page 29 of 33

(A)

0.3

NaCl (293 K) CaCl2 (293 K)

KCl (293 K) Control (293 K)

NaCl (303 K) CaCl2 (303 K)

KCl (303 K) Control (303 K)

0.4

0.1

0.0

0.0

0

10

20

30

Time (ns)

40

50

NaCl (b) CaCl2 (b) NaCl (p) CaCl2 (p)

0.3 NaCl (b) CaCl2 (b) NaCl (p) CaCl2(p)

0.2

0.1

(C) 303K, 50 bar

0.4

0.3

F4

0.2

0.5

(B) 293 K, 50 bar

KCl (b) Control(b) KCl (p) Control (p)

F4

0.4

F4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

KCl (b) Control (b) KCl (p) Control (p)

0.2 0.1 0.0

0

10

20

30

40

50

0

10

Time (ns)

Fig. 8 Evolution of (A) overall F4φ and (B and C) local F4φ during hydrate dissociation (“b” in the bracket represents “bulk water”, “p” in the bracket represents “pore water”)

29


20

30

Time (ns)

40

50

Energy & Fuels

0.6

0.2

0.6

(B) 293 K, NaCl

(A) 293 K, Control 0.1

Order parameter F4φ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 30 of 33

0.6

(C) 293 K, KCl

(D) 293 K, CaCl2

0.4

0.4

0.4

0.2

0.2

0.2

0.0

0.0

0.0

0.0

-0.1

0.2

0

5

10

0

15

5

10

15

0.6

5

10

15

0.6

(F) 303 K, NaCl

(E) 303 K, Control 0.1

0

0.6

(G) 303 K, KCl

0

5

0.4

0.4

0.2

0.2

0.2

0.0

0.0

0.0

10

0

5

10

0

5

10

0

Fig. 9 Evolution of F4φ at different positions in the pore water region during hydrate dissociation


10

15

Slice 1 Slice 2 Slice 3

Time (ns)

30

5

(H) 303 K, CaCl2

0.4

0.0

-0.1

0

5

10

Page 31 of 33

0.8

0.8

0.8

(A) 293 K, Control

Order parameter F4φ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

0.8

(B) 293 K, NaCl

(C) 293 K, KCl

0.6

0.6

0.6

0.6

0.4

0.4

0.4

0.4

0.2

0.2

0.2

0.2

0.0

0.0

0.0

0.0

0

10

20

30

40

0.8

50

0

10

20

30

40

50

0 0.8

0.8

(E) 303 K, Control

(F) 303 K, NaCl

10

20

30

40

50

(G) 303 K, KCl

0.6

0.6

0.6

0.4

0.4

0.4

0.4

0.2

0.2

0.2

0.2

0.0

0.0

0.0

0.0

10

20

30

40

50

0

10

20

0

30

40

50

10

20

30

40

50

0.8

0.6

0

(D) 293 K, CaCl2

0

10

20

30

40

50

(H) 303 K, CaCl2 Slice 1 Slice 2 Slice 3 Slice 4 Slice 5 0

Time (ns) Fig. 10 Evolution of F4φ at different positions in the bulk water region during hydrate dissociation

31


10

20

30

40

50

Energy & Fuels

50

100

A Gaseous methane (%)

Methane in cages (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

40 30 20 10 0

0

Page 32 of 33

10

20

30

40

80 60

NaCl (303 K) KCl (303 K) CaCl2 (303 K)

40

Control (303 K) NaCl (293 K) KCl (293 K) CaCl2 (293 K)

20

Control (293 K)

0

50

B

0

10

20

30

Time (ns)

Time (ns)

Fig. 11 Percentage of methane molecules in the (A) hydrate cages and (B) gaseous phase

32


40

50

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Energy & Fuels

bubble 1

bubble 1

bubble 2 bubble 1

bubble 1

bubble 2

bubble 2

Fig. 12 Evolution of methane bubbles during hydrate dissociation in the system with NaCl brine at 303 K

33


Effects of Salt Ions on the Methane Hydrate Formation and

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