Molecular Dynamics Simulations of Methane Hydrate Formation in

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Molecular Dynamics Simulations of Methane Hydrate Formation in Model Water-in-Oil Emulsion Containing Asphaltenes Mucong Zi, Guozhong Wu, Lei Li, and Daoyi Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06746 • Publication Date (Web): 20 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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The Journal of Physical Chemistry

Molecular Dynamics Simulations of Methane Hydrate

1 2

Formation in Model Water-in-Oil Emulsion Containing Asphaltenes

3 Mucong Zi †,‡,1, Guozhong Wu†,‡,1, Lei Li†, Daoyi Chen*,†,‡

4 5 6

†

7 8

Division of Ocean Science and Technology, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

‡

School of Environment, Tsinghua University, Beijing 100084, China

9 10

(1 These authors contributed equally to this work)

11 12 13 14

*

Corresponding Author

E-mail: [email protected] Tel: +86 0755 2603 0544

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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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ABSTRACT

16

Gas hydrate formation and asphaltene aggregation are two major concerns for

17

the flow assurance in petroleum industry, while the atomistic understanding of their

18

interactions remains limited especially in the oil-dominated systems. Microsecond

19

molecular dynamics simulations were performed to investigate the combined effects

20

of solvent type, water droplet size and asphaltenes on methane hydrate formation in

21

the model water-in-oil emulsion by characterizing the four-body structural order

22

parameter, molecular configurations and the evolution of hydrate cages. Results

23

indicated the faster hydrate formation in small water droplet with n-heptane because

24

of the decreased interfacial curvature. Meanwhile, hydrate growth was promoted in

25

large water droplet with toluene, due to the occurrence of a vertical water channel

26

which provided an extra growth site. Results also demonstrated the inhibition effect of

27

asphaltenes on hydrate formation, which was more pronounced in small droplet with

28

n-heptane or large droplet with toluene. This was attributed to two main processes that

29

were closely related to the surface concentration of asphaltene at oil-water interface,

30

including the prevention of methane solution by the formation of an asphaltene shell,

31

and the disruption on local hydrogen-bonded networks by the formation of hydrogen

32

bonds between asphaltene and water. Overall results provided theoretical support for

33

better

34

asphaltene-rich water-in-oil emulsion, which was ubiquitous during the emulsification

35

process of hydrate blockage in offshore subsea pipelines.

understanding

the

formation

mechanisms

2


of

methane

hydrates

in

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36

1. INTRODUCTION

37

Gas hydrates are non-stoichiometric crystalline compounds composed of water

38

and small gas molecules such as methane. It is common to form gas hydrates in the

39

offshore subsea pipelines at low temperature and high pressure, which becomes one

40

of the major concerns for the flow assurance in petroleum industry because it

41

contributes to severe blockages in the multi-phase transportation pipelines.1

42

Meanwhile, asphaltene in the crude oil is another flow assurance issue, because it is

43

the heaviest and most polar fraction of crude oil and has strong tendency to aggregate

44

by a series of mechanisms including acid−base and hydrogen-bonding interactions,

45

hydrophobic pocket, metal coordination complex, and π−π stacking interactions.2

46

Disturbance of pressure, temperature and solvent composition will significantly affect

47

the solubility of asphaltene, resulting in asphaltene aggregation and precipitation

48

which eventually blocks the offshore subsea pipelines.3, 4 Neglecting the interactions

49

between gas hydrates and asphaltenes during flow assurance assessment may lead to

50

misleading laboratory data and costly design decisions.3

51

To date, there are only a few works on the interactions between the above two

52

issues. For example, Daraboina et al. reported the stronger inhibition effect of crude

53

oil on hydrate formation with high ratio of asphaltenes and resins.5 However, it

54

remained unclear about how much asphaltene indeed contributed to the above

55

inhibition, since all fractions of crude oil were integrally considered. Additionally, it

56

was hypothesized that the presence of asphaltene in crude oil was adverse for 3



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57

anti-blocking treatment of gas hydrate, since asphaltenes near the surface of hydrate

58

particle might reduce the contact opportunity between hydrate and anti-agglomerate

59

inhibitors.6 Our recent works particularly focused on the influences of asphaltenes on

60

the hydrate formation and dissociation at the gas-water interface, in the bulk water

61

and on the metal surface, which theoretically demonstrated the inhibition effects of

62

asphaltenes on the kinetics of methane hydrate formation in the oil-water mixture.7, 8

63

Nevertheless,

the

above

studies

were

primarily

performed

in

the

64

water-dominated systems, while little works have been reported in the oil-dominated

65

systems with water-in-oil emulsion. Four-step mechanism was previously proposed

66

for the gas hydrate formation in the blockage process of pipeline including (i) water

67

emulsification in the oil phase, (ii) hydrate nucleation and growth at the water-oil

68

interface, (iii) particle aggregation and (iv) blockage formation from aggregate

69

jamming.9,

70

water-in-oil emulsion, which is closer to reality in petroleum industry. It should be

71

noted that hydrate blockage in the water-in-oil emulsion is influenced by several

72

factors such as water droplet size and droplet spreading process.9, 11, 12 For example, a

73

hydrate shell initially formed at the surface of water droplet, which then grew into a

74

solid hydrate particle according to the shell model.9 Such conversion process to

75

hydrate particle was reported to be limited for large water droplet, because the dense

76

hydrate shell would prevent the mass transport into the inner water phase.10,

77

Moreover, micromechanical force-studies suggested that the interactions between

10

This indicated the significance in studying hydrate formation in

4


13

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78

unconverted water droplets and hydrate particles provided enough cohesive forces

79

between hydrate particles, which played an important role at the early stage of

80

agglomeration.14, 15 Hence, it was hypothesized that changes in water droplet size,

81

solvent type and specific compounds with interfacial properties (e.g. surfactant)

82

would have a complex influence on the contact procedure between free water droplet

83

and hydrate particles, which further affected its conversion to hydrate structure and

84

the agglomeration process. Particularly, asphaltenes were also reported to have

85

interfacial properties and would perform as emulsification agents to increase emulsion

86

stability, which was expected to prevent hydrate-induced blocking.16 Nevertheless,

87

recent studies more likely focused on the common surfactants, while the influence of

88

asphaltene on hydrate formation in water-in-oil emulsion still remained limited.

89

Accordingly, we performed MD simulations in this study to investigate the

90

methane hydrate formation in model water-in-oil emulsion containing asphaltenes.

91

Specific objectives were to clarify (i) the molecular mechanisms of the spreading

92

process of water droplet on the hydrate surface, (ii) the effect of droplet size and

93

solvent type on hydrate formation and (iii) the effect of asphaltenes on hydrate

94

formation in water-in-oil emulsion.

5



95

2. METHODOLOGY

96

2.1 Modeling

97

MD simulations were performed with GROMACS (version 5.0.5).17 The unit

98

cell of the structure I (sI) methane hydrate was obtained from Lenz et al.18, which was

99

used to build a hydrate layer by creating a 6 × 6 × 2 supercell. It was placed at the

100

bottom of a simulation box to serve as a template for hydrate formation. Water droplet

101

models with two different sizes were created by placing 837 and 2024 water

102

molecules in cubic box with the dimension of 3 nm × 3 nm × 3 nm and 4 nm × 4 nm ×

103

4 nm, respectively. Spherical water droplets with radius of 3.3 and 4.5 nm,

104

respectively, spontaneously formed after running MD simulation to equilibrate the

105

box. The simulation system was then constructed by placing the water droplet on the

106

top of the hydrate layer. It should be noted that water droplet size in real experiments

107

generally ranges from nanometer to micrometer, depending on various factors such as

108

oil composition, emulsification method and emulsification degree. The water droplet

109

sizes in this study were comparable to the size of the nanoemulsions stabilized by

110

surfactants in experiments.19-21 Our preliminary test also indicated that the average

111

diameter of water droplets in the n-heptane emulsion samples were 14 ~ 18.4 nm in

112

the n-heptane w/o emulsions (experimental protocols are detailed in the Supporting

113

Information), suggesting that the water droplet sizes in the simulations were

114

reasonable.

115

To study the influence of asphaltenes on the hydrate formation, 24 asphaltene

116

molecules were symmetrically added surrounding the water box with their 6


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117

poly-aromatic cores parallel with one another. Asphaltene structure was simplified

118

using the Violanthrone-79 (VO-79, C50H48O4) model shown in Fig. S1. This model

119

has structure similarities with the continental type of asphaltenes, where one aromatic

120

region per molecule is connected with side chains.8 It was reported to resemble the

121

adsorption behavior of C7 asphaltene from the Athabasca bitumen, which was used in

122

our laboratory.22 Additionally, VO-79 model has been widely used to simulate the

123

aggregation, adsorption and emulsion behaviors of asphaltenes.23-25

124

To study the influence of organic solvent on the hydrate formation, n-heptane

125

(designated with “Hep-” in the simulation system) and toluene (designated with “Tol-”

126

in the simulation system) were added into the simulation box, which were regarded as

127

“bad solvent” and “good solvent” for asphaltene, respectively. Methane molecules

128

were then averagely dissolved into the small water droplet and large water droplet,

129

respectively, with an initial aqueous methane concentration of 0.06 mol mol-1.

130

Methane molecules were also added into the organic solvent with an initial methane

131

concentration of 0.24 mol mol-1. Details of the simulation systems are summarized in

132

Table 1.

133 134

2.2 Simulation Details

135

The OPLS-AA force field was adopted to model VO-79, toluene and

136

n-heptane.26 Water and methane was modeled by TIP4P-ICE model and united-atom

137

Lennard-Jones model,27 respectively. Long-range Coulombic interactions were

138

calculated by the particle mesh ewald method with a Fourier spacing of 0.12 nm, 7



139

while the cut-off value of short-range interactions was set at 1.2 nm.28 Cross

140

interactions between different species were calculated using the standard

141

Lorentz−Berthelot mixing rules.29 All simulations were performed using the leapfrog

142

algorithm with a time step of 1 fs,30 while three-dimensional periodic boundary

143

conditions (PBC) were applied throughout the simulations. The motivation to use

144

PBC was to gain insights into the interactions between water and two adjacent hydrate

145

particles. This is relevant to the aggregation of hydrate particles during the blockages

146

in the real situation, because it is an important process highly associated with the

147

adjacent water. As demonstrated by Turner et al.,9 the aggregation of hydrate crystals

148

formed from oil-water emulsion was driven by the capillary cohesion induced from

149

the water droplet between particles.

150

The initial configuration was energy-minimized using the steepest descent

151

algorithm, followed by a 50 ps-NPT equilibration (constant temperature and pressure)

152

at 250 K and 500 bar. Subsequently, production runs (1.5 µs) were performed at 250

153

K and 500 bar, while the temperature and pressure was controlled by V-rescale

154

thermostat and Berendsen barostat, respectively. Semi-isotropic pressure coupling was

155

used to independently control the fluctuation of z direction of simulation system.

156 157 158 159

2.3 Data Analysis. The four-body structural order parameter F4φ was used to quantify the degree of hydrate growth, which was defined as follows:31

8


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‫ܨ‬ସఝ

௡

1 = ෍ cos 3߮௜ n ௜ୀଵ

160

where n is the total number of water pairs with the oxygen atoms within 3.5 Å, and φi

161

is the H-O…O-H torsion angle between two oxygen atoms and two outermost

162

hydrogen atoms in the ith water pair. The average F4φ for hydrate, liquid water and ice

163

are 0.7, -0.04 and -0.4, respectively.32

164

The face-saturated incomplete cage analysis (FSICA) was applied to quantify the

165

variation of different species (e.g. number of aqueous methane molecules) and the

166

evolution of various cage types.33 Particularly, the sI structure of hydrate was traced

167

by calculating the total amounts of corresponding 512 and 51262 cages.

9



168

3. RESULTS AND DISCUSSION

169

3.1 Adsorption and spreading of water droplet on hydrate surface

170

Fig. 1 and 2 show the morphology evolution of water droplet on the surface of

171

methane hydrates. As expected, the water droplet spontaneously moved towards the

172

hydrate surface and gradually spread to form a water layer. The spreading dynamics

173

were influenced by the size of water droplet, the type of organic solvent and the

174

presence of asphaltenes.

175

For example, the water droplet spreading was obviously faster in n-heptane than

176

in toluene (Fig. 1A and 2A). To quantitatively describe the spreading process, the

177

oil-water interfacial curvature was calculated following the procedures illustrated in

178

Fig. S2 in the Supporting Information.34 As shown in Fig. 3, the radius of interfacial

179

curvature (reciprocal of interfacial curvature) for the Tol-S system increased from 3.5

180

nm to 4.1 nm between 150 ns to 500 ns, while it increased from 7.8 nm to 11.0 nm for

181

the Hep-S system. It indicated that small water droplet tended to quickly form a flat

182

layer on the surface of solid hydrates in n-heptane, but remained more spherical in

183

toluene. Meanwhile, the increased radius of interfacial curvature suggested that the

184

asphaltenes also promoted the spreading process of small water droplet (Fig. 3 and

185

Fig. 1B). Similar phenomenon was also found in our previous study where asphaltene

186

migrated to the gas/oil-water interface and thus decreased the interfacial curvature.8

187

This indicated that the model asphaltene showed interfacial activity in both water- and

188

oil-dominated systems, which accorded with the experimental and simulation results

189

from Jian et al.35 When the droplet size increased, the promotion effect of n-heptane 10


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190

on the water spreading was also observed (Fig. 1C and 2C), but the aforementioned

191

role of asphaltenes became less pronounced for large droplet in n-heptane (Fig. 1C

192

and 1D).

193

Results further demonstrated that the spreading rate of large droplet was higher

194

than that of small droplet in n-heptane, since the interfacial curvature of the former

195

decreased faster (Fig. 1A and 1C). However, opposite result was observed in the

196

toluene (Fig. 2A and 2C). This was due to the fact that the asphaltenes in toluene

197

prevented the formation of the vertical water channel between the bottom and top

198

hydrate layers (existence of top hydrate layer was due to the setting of periodic

199

boundary conditions in simulations). As shown in Fig. 2C, the large water droplet in

200

toluene was absorbed on both the bottom and top hydrate layers due to the relatively

201

short distance between the edge of droplet and the surface of the top hydrate layer. By

202

contrast, the small water droplet directly contacted with only the bottom hydrate layer

203

(Fig. 2Aand 2B). This resulted in the appearance of a vertical water channel in the

204

Tol-L system, which became thinner with time and finally fractured at about 540 ns.

205

Similar configurations of water channel were also observed by Bagherzadeh et al.36

206

and Ji et al..34 The presence of water channel in this case significantly slowed the

207

spreading process of water droplet. Nevertheless, it disappeared when the water

208

droplet was surrounded by asphaltene molecules, which performed like a cover

209

protecting the water droplet from contacting with the two hydrate layers at the same

210

time (Fig. 2D). It should be noted that all simulations were performed under NPT

211

ensemble, therefore, the simulation box size of different systems varied with time and 11



212

was affected by the total number of molecules. For example, the length of simulation

213

box in the z-axis at 50 ns was 7.9 and 8.4 nm for Tol-L and Asp+Tol-L system,

214

respectively. In order to gain insights into the molecular mechanisms, we performed

215

an extra simulation by initially adding more toluene molecules in the Tol-L system so

216

that the corresponding box size was same to that of the Asp+Tol-L system. As shown

217

in Fig. S3, water channel was also observed when the water droplet needed to migrate

218

a longer distance to reach both hydrate layers. This proved that it was the role of the

219

asphaltenes instead of the box size that prevented the formation of water channel in

220

the Asp+Tol-L system. Accordingly, it was inferred the presence of asphaltene was

221

able to prevent the cohesion between hydrate particles, which was adverse for hydrate

222

aggregation.

223 224

3.2 Hydrate formation from water droplet without asphaltenes

225

It was clearly observed that the new hydrates started growing from the hydrate

226

layer towards the oil-water interface (Fig. 4). The solvent type and the size of water

227

droplet did not influence the growing direction but had obvious effects on the growing

228

kinetics. As shown in Fig. 5, the F4φ curve for the Hep-S system was much steeper

229

than that for the Tol-S system, indicating faster kinetics for the conversion of the

230

small water droplet to hydrates in n-heptane than in toluene. This tendency was

231

consistent with previous experimental results that hydrate formation in the pure

232

n-heptane was faster than in the heptane-toluene mixture.5 It should be noted that the

233

water-in-oil emulsion in the present study was different from the water-dominated 12


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234

system in the above experiment, while our results highlighted the role of solvent type

235

in the oil-water interface when no obvious change occurred in the interfacial

236

properties.

237

Additionally, it was found that hydrate formation in the n-heptane was slowed

238

when increasing the size of the water droplet (Fig. 6B). This might be attributed to the

239

variation in the oil-water interfacial curvature after changing the droplet size. As

240

aforementioned, the spreading process of small droplet was slower than that of the

241

large droplet, which helped the former remain a more curved interface with larger

242

contact area for mass transfer. This was supposed to enhance the solubility of methane

243

in water and facilitated the conversion of methane to hydrates which agreed with

244

Walsh et al. .37

245

It was interesting to find the opposite tendency in the hydrate formation rate

246

when changing the size of the water droplet or the type of the organic solvent. For

247

example, the F4φ curve of the Tol-L system was higher than that of the Hep-L system,

248

which indicated a faster rate of hydrate formation from a large water droplet in

249

toluene than in n-heptane (Fig. 5). This finding was opposite to that from small water

250

droplet. Meanwhile, it was observed that hydrate formation rate from large water

251

droplet was initially same with from small water droplet in toluene and the former

252

became higher after 540 ns (Fig. 6A). This trend was also opposite to that found in

253

n-heptane. A closer examination of the water morphology suggested that these

254

variances were associated with the phenomenon of vertical water channel as

255

aforementioned. For instance, during the hydrate formation from large water droplet 13



256

in toluene, the fracture of the water channel at 540 ns provided an extra site for

257

hydrate formation. This facilitated the hydrate formation compared with that from

258

small water droplet, because such evolution of the water channel did not take place in

259

the systems with small water droplet. The above results suggested that changes in the

260

adsorption and spreading mode of water droplet on the hydrate surface had

261

remarkable influences on the hydrate formation rate, which should also be taken into

262

account rather than only focusing on the effects of solvent type or droplet size.

263 264

3.3 Hydrate formation from water droplet with asphaltenes

265

Results demonstrated that the presence of asphaltenes inhibited the kinetics of

266

methane hydrate formation in all the simulation systems in this study, which was

267

evidenced by the evolution of the F4φ value and the number of sI hydrate cages during

268

hydrate formation from water droplet (Figs. 5 and 6). This tendency was similar to

269

that in our previous simulation in water-dominated systems.8 In both studies,

270

asphaltenes showed the main inhibition effects when they were located at the

271

oil-water interface. Accordingly, it was inferred that the role of asphaltene on

272

decreasing hydrate formation rate was mainly attributed to the interfacial activities of

273

asphaltenes in both water- and oil-dominated (e.g. water-in-oil emulsion) systems.

274

In order to gain insights into the molecular mechanism of asphaltene-induced

275

inhibition on hydrate formation, we first checked its influence on the methane

276

dissolution. As shown in Fig. 7, it was clearly observed that the asphaltenes weakened

277

the solubility of methane in the water droplet. This was partly attributed to the 14


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278

decrease of the interfacial curvature as aforementioned. The asphaltenes near the

279

small water droplet would significantly affect its spreading process and the resulted

280

almost flat interface was adverse for dissolving methane in water (Fig. 3). Moreover,

281

the asphaltene molecules absorbed on the water droplet were similar as a shell, which

282

covered on the newly-formed hydrate layer and prevented further penetration of

283

methane. This was confirmed by computing the diffusion coefficient and the mean

284

square distribution of methane along z-axis. As shown in Fig. S4, the decreased

285

diffusivity of methane after the addition of asphaltenes in the systems suggested the

286

increased difficulty for the methane to directly penetrate through the asphaltene shell

287

to react with the water. Another attributable factor for the inhibited hydrate formation

288

by asphaltenes was the hydrogen bonds formed between asphaltene and water

289

molecules, which was adverse for the construction of the local hydrogen-bonded

290

networks among water molecules and therefore hindered its conversion to hydrate

291

structure (Fig. 8)

292

Additionally, the degree of inhibition from asphaltenes on the kinetics of hydrate

293

formation varied with the solvent type and the size of water droplet. For the small

294

water droplet, the inhibition effects were more pronounced in n-heptane than in

295

toluene. For example, it took about 150 ns to finish hydrate growth in the n-heptane

296

but the corresponding time was 800 ns when asphaltenes were present. The number of

297

new formed sI hydrate cages decreased by 82% during the initial 150 ns when

298

asphaltenes were added into n-heptane (Fig. 6B). By contrast, little difference was

299

observed in the number of sI hydrate cages during the initial 300 ns between the Tol-S 15



300

and ASP + Tol-S systems (Fig. 6A). Compared with the system with toluene, the

301

higher degree of inhibition from asphaltenes in the system with n-heptane was mainly

302

attributed to the higher “surface concentration” of asphaltenes at the oil-water

303

interface. In other words, more asphaltenes preferred to locate at the n-heptane-water

304

interface than at the toluene-water interface, because the solubility of asphaltenes was

305

much smaller in the n-heptane. This was confirmed by calculating the radial

306

distribution functions of asphaltene around the center of mass of the water droplet,

307

which indicated that asphaltenes were much closer to the water droplet in n-heptane

308

(Fig. S5). Hence, the higher surface concentration of asphaltene in the n-heptane

309

provided a denser shell at the oil-water interface, which further decreased the

310

solubility of methane in water (Fig. 7A and 7B) and offered more opportunities to

311

form hydrogen bonds between asphaltenes and water (Fig. 8).

312

For the large water droplet, the kinetic inhibition effects from asphaltenes were

313

more pronounced in toluene than in n-heptane. For example, a distinct inhibition on

314

the hydrate formation in the toluene was observed at 300 ns, but it was not observed

315

until 700 ns when asphaltenes were added into n-heptane (Fig. 6). This was mainly

316

due to the prevention for the formation of the aforementioned water channel when

317

asphaltenes were added into the Tol-L system (Fig. 4). This finding highlighted the

318

possibility to enhance the hydrate inhibition by changing the spreading process of

319

water droplet on the surface of hydrate particles in the water-in-oil emulsion systems.

16


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320

4. CONCLUSIONS

321

Results demonstrated that hydrate formation in small water droplet with

322

n-heptane would be slowed either by changing the solvent into toluene or increasing

323

the droplet size. It was attributed to the fact that increasing droplet size in n-heptane

324

would result in decreasing the curvature of oil-water interface, which was adverse for

325

the dissolution of methane in water and its conversion to hydrate structure. It also

326

indicated that hydrate formation in large water droplet with toluene would be retarded

327

either by changing the solvent into n-heptane or decreasing the droplet size. This was

328

due to the appearance of a vertical water channel between hydrate layers, which

329

remarkably increased the solubility of methane and provided an extra site for hydrate

330

growth.

331

Result further highlighted the ubiquitous inhibition effect of asphaltenes on

332

hydrate growth, while the inhibited degree varied with the solvent composition and

333

droplet size. The asphaltene-induced inhibition was due to the fact that (i) asphaltenes

334

served as a shell near oil-water interface, preventing the solution of methane into

335

water, and (ii) the formation of hydrogen bonds between asphaltene and water

336

weakened the local hydrogen-bonded networks between water molecules. These were

337

more pronounced in n-heptane because a lower solubility of asphaltene would

338

contribute to a higher surface concentration at oil-water interface. Nevertheless, the

339

above mechanisms became less conspicuous when hydrate formed in the large water

340

droplet with the presence of asphaltene and toluene, because asphaltene prevented the

341

occurrence of the aforementioned water channel, which displayed a much stronger 17



342

inhibition effect. Overall results provided theoretical support for better understanding

343

the formation mechanisms of methane hydrates in asphaltene-rich water-in-oil

344

emulsion, which was ubiquitous during the emulsification process of hydrate

345

blockage in offshore subsea pipelines. It should be noted that the microsecond-scale

346

of simulations is long enough to enable observation of the formation and aggregation

347

of a couple of hydrate crystals, but conclusions in the real hydrate blockage process is

348

waiting to validate by statistical analysis and macroscopic experiments at larger time-

349

and length-scale. We are now working towards this direction.

350 351

ASSOCIATED CONTENT

352

Supporting Information

353

Procedures in preparation and measurement of n-heptane w/o emulsion, molecular

354

structure of the Violanthrone-79 model (Fig. S1), schema for calculating the oil-water

355

interfacial curvature (Fig. S2), snapshots of the extra simulation (Fig. S3), mean

356

square displacement of methane along z-axis (Fig. S4) and radial distribution

357

functions of asphaltene around the center of mass of water droplet (Fig. S5).

358 359

AUTHOR INFORMATION

360

Corresponding Author

361

*Email: [email protected].

362

Tel: +86 0755 2603 0544

18


Page 18 of 34

Page 19 of 34 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


363

Author Contributions

364

1

365

Notes

366

Mucong Zi and Guozhong Wu contributed equally to this work.

The authors declare no competing financial interest.

367 368

ACKNOWLEDGEMENTS

369

This study was financially supported by the Fundamental Research Project of

370

Shenzhen, China (JCYJ20160513103756736), the Shenzhen Peacock Plan Research

371

Grant

372

Commission of Shenzhen Municipality (DCF-2018-64).

(KQJSCX20170330151956264),

and

the

Development

19


and

Reform


373

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374

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477

Table 1 Details of simulation systems *

System

Initial system box 3

Initial water box 3

No. of water

No. of toluene

No. of n-heptane

No. of asphaltene

No. of methane

molecules

molecules

molecules

0

898

size (nm )

size (nm )

molecules

molecules

Tol-S

7×7×8.8

3×3×3

4149

1070

Tol-L

7×7×10.5

4×4×4

5336

1200

0

987

Asp+Tol-S

7×7×8.8

3×3×3

4149

1070

24

898

Asp+Tol-L

7×7×10.5

4×4×4

5336

1200

24

987

EXTRA

7×7×10.5

4×4×4

5336

1310

0

987

Hep-S

7×7×8.8

3×3×3

4149

786

0

898

Hep-L

7×7×10.5

4×4×4

5336

786

0

987

Asp+Hep-S

7×7×8.8

3×3×3

4149

895

24

898

Asp+Hep-L

7×7×10.5

4×4×4

5336

895

24

987

478

*

479

toluene and asphaltenes; Asp+Tol-L: large water droplet in the system with toluene and asphaltenes; Hep-S: small water droplet in the system with n-heptane; Hep-L:

480

large water droplet in the system with n-heptane; Asp+Hep-S: small water droplet in the system with n-heptane and asphaltenes; Asp+Hep-L: large water droplet in

481

the system with n-heptane and asphaltenes.

Tol-S: small water droplet in the system with toluene; Tol-L: large water droplet in the system with toluene; Asp+Tol-S: small water droplet in the system with

25



Page 26 of 34

A

B

C

D

0 ns

150 ns

500 ns

1000 ns

1500 ns

482

Fig. 1 Snapshots of systems (A) Hep-S, (B) Asp+Hep-S, (C) Hep-L and (D)

483

Asp+Hep-L. Water: blue lines; hydrogen bonds: blue dashed lines; asphaltene: green

484

sticks; methane: red balls, n-heptane molecules are not shown for clarity.

26


Page 27 of 34 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

B

C

D

0 ns

75 ns

150 ns

500 ns

1500 ns

485

Fig. 2 Snapshots of systems (A) Tol-S, (B) Asp+Tol-S, (C) Tol-L and (D) Asp+Tol-L.

486

Water: blue lines; hydrogen bonds: blue dashed lines; asphaltene: green sticks;

487

methane: red balls, toluene molecules are not shown for clarity. 27


The Journal of Physical Chemistry 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

150 ns

Page 28 of 34

R ≈ 3.5 nm

R ≈ 5.0 nm

R ≈ 7.8 nm

R ≈ 8.0 nm

R ≈ 4.1 nm

R≈∞

R ≈ 11.0 nm

R≈∞

Tol-S

Asp+Tol-S

500 ns

Hep-S

Asp+Hep-S

488

Fig. 3 Selected snapshots and curvature of the oil-water interface (green, asphaltene; blue, water). Water molecules at the oil-water interface are

489

highlighted by stick display, while toluene and n-heptane are not shown for clarity.

28


Page 29 of 34 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


A

C

E

G

B

D

F

H

500 ns

540 ns

150 ns

1000 ns

490

Fig. 4 Snapshots of hydrate cages during early stage of hydrate formation in systems (A) Tol-S, (B) Asp+Tol-S, (C) Tol-L, (D) Asp+Tol-L, (E)

491

Hep-S, (F) Asp+Hep-S, (G) Hep-L and (H) Asp+Hep-L. Water: blue lines; asphaltene: green sticks; methane: red balls. Hydrate cages (512, 51262,

492

51263 and 51264) are highlighted in blue sticks. Solvent molecules are not shown for clarity. 29



0.7

0.6

0.5

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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

0.4 Tol-S Hep-S Tol-L Hep-L

0.3

Asp+Tol-S Asp+Hep-S Asp+Tol-L Asp+Hep-L

0.2 0

500

1000

1500

Time (ns) 493

Fig. 5 Evolution of F4φ order parameter during hydrate formation.

30


Page 31 of 34

600

600 Tol-S Asp+Tol-S Tol-L Asp+Tol-L

Number of cages

Number of 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 47


500

400

Hep-S Asp+Hep-S Hep-L Asp+Hep-L

500

400

A

B

300

300 0

500

1000

1500

0

Time (ns) 494

500

1000

1500

Time (ns)

Fig. 6 Number of sI hydrate cages during hydrate formation in the systems with (A) toluene and (B) n-heptane.

31


650

Page 32 of 34

650

Number of aqueous methane

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

Number of aqueous methane


Tol-S Asp+Tol-S Tol-L Asp+Tol-L

600

550

500

450

Hep-S Asp+Hep-S Hep-L Asp+Hep-L

600

550

500

450

B

A 400

400

0

500

1000

1500

0

Time (ns) 495

500

1000

1500

Time (ns)

Fig. 7 Number of aqueous methane during hydrate formation in the systems with (A) toluene and (B) n-heptane.

32


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


Number of hydrogen bonds

Page 33 of 34

Asp+Tol-S Asp+Tol-L Asp+Hep-S Asp+Hep-L

60

40

20

0 0

500

1000

1500

Time (ns) 496

Fig. 8 Number of hydrogen bonds between water and asphaltenes during hydrate

497

formation.

33



700

Number of hydrate 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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

without asphaltene

500

with asphaltene 300 0

500

1000

Time (ns) 498

TOC graphic

34


1500

Molecular Dynamics Simulations of Methane Hydrate Formation in

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