Effects of Asphaltenes on the Formation and Decomposition of

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Effects of asphaltenes on the formation and decomposition of methane hydrate#a molecular dynamics study Mucong Zi, Daoyi Chen, Haoqing Ji, and Guozhong Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01040 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 12, 2016

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Effects of asphaltenes on the formation and decomposition of

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methane hydrate：：a molecular dynamics study

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Mucong Zi, Daoyi Chen, Haoqing Ji, Guozhong Wu *

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

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University, Shenzhen 518055, China

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ACS Paragon Plus Environment

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GRAPHICAL ABSTRACT:

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ABSTRACT:

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Oil asphaltene and gas hydrate are two major challenges for the flow assurance in the

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oil-gas industry, but it remains unclear about their interactions as they were often

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investigated separately. Molecular dynamic (MD) simulations were performed to gain

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insights into the role of asphaltenes on the hydrate formation in bulk water and at the

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water-gas interface, respectively. The promotion and inhibition effects were

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elaborated by quantifying the partitioning of water and methane, the distribution of

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different types of hydrate cages, and the linkage and crystallization of hydrate cages.

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The influence of asphaltenes on the hydrate decomposition was also investigated,

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which suggested the theoretical feasibility to employ temperature-programmed

20

heating to initially destroy the hydrate cages at low temperature and then disassociate

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the asphaltene aggregates at high temperature. To our knowledge, this was the first

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study on the atomic interactions between asphaltenes and gas hydrate, which was of

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added knowledge for a realistic understanding of the oil-gas flow assurance issues

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when they coexisted in the multi-phase system.

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1. INTRODUCTION

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Gas hydrates are non-stoichiometric crystalline compounds with gas molecules

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encapsulated in water cages formed by hydrogen-bonded water molecules.1 Gas

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hydrate formation and agglomeration have long been plaguing the oil and gas industry

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by causing severe risks of pipeline blockages both onshore and offshore.2 During the

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last decades, considerable works have been carried out for understanding the gas

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hydrate nucleation, growth, agglomeration, deposition and plugging from laboratory

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measurements to field applications.3 To date, the kinetic and thermodynamic

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evolution of gas hydrate in complicated engineering systems are still not completely

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understood due to the complex compositions of crude oil 4 and the intricate interfacial

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behavior of gas hydrate and crude oil.5, 6

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For the purpose of simplification, light oil such as aliphatic hydrocarbons with less

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than 21 carbons was widely used previously,7-9 while the influences of heavy fractions

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of crude oil such as asphaltene were less taken into account. Asphaltenes are the

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largest, densest, most polar and surface-active fraction of crude oil, which has a strong

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tendency for aggregation driven by the acid−base and hydrogen bonding interactions,

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metal coordination complex, hydrophobic pocket and π−π stacking interactions.10 The

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flocculation and precipitation of asphaltenes in flow lines also sparked the interests of

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oil-gas industry for the operational risks. Assessment of gas hydrate flow assurance

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neglecting its association with asphaltenes will result in misleading laboratory data

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and lead to costly design decisions for industrial application.11

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Early evidence suggested that the asphaltene flocculation might provide gas

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hydrates nucleation sites and promote hydrate plugging during multi-phase

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transportation.12 The presence of asphaltenes could also potentially jeopardize the

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hydrate prevention strategy.11 However, recent study demonstrated that the crude oil

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with high content of asphaltenes showed stronger inhibition of hydrate formation

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compared with that contained lower asphaltenes concentration.4 The hydrate

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nucleation rate was obviously higher in the oil without asphaltenes than in that with

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asphaltenes, but a direct correlation was not observed between the hydrate nucleation

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rate and the asphaltene concentration.13 Additionally, the asphaltenes may also affect

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the hydrate growth by influencing the gas diffusion.14 The properties analogues to

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surfactants made it possible for asphaltenes to compete for the surface of hydrate

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particles and consequently interfere the linkage of hydrate cages and the adsorption of

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hydrate inhibitors.15 These conflicting conclusions suggested the coexistences of

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different mechanisms for the role of asphaltenes on hydrate evolution. Moreover, the

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crude oils from different originations varied greatly, and it remains unclear about how

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much the asphaltene indeed contributed to the variance in the gas hydrate formation

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observed in previous studies. It demands systematic investigations to particularly

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focus on the interactions between asphaltenes and gas hydrate, which is essential for a

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realistic understanding of the flow assurance issues when the oil asphaltenes and gas

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hydrate coexisted in the multi-phase system. 5


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Accordingly, we performed molecular dynamic (MD) simulations in this study with

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specific objectives to (i) evaluate the influences of asphaltenes on the overall and

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regional tendency of hydrate formation in bulk water and at the water-gas interface,

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respectively; (ii) identify the mechanisms for the observed phenomena by quantifying

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the partitioning of water and methane, the distribution of different types of hydrate

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cages, and the linkage and crystallization of hydrate cages; and (iii) assess the role of

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asphaltenes on the hydrate decomposition and in turn the influence of hydrate

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decomposition on the disassociation of asphaltene aggregates.

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2. METHODOLOGY

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2.1. Model construction. MD simulation was performed using Gromacs, an open

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source software, with the OPLSAA force field.16, 17 The Violanthrone-79 (VO-79,

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C50H48O4) model was selected to model asphaltenes (Fig. S1), which has been

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successfully used to simulate the asphaltene aggregation process.18 The methane

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molecules were represented by the united-atom model while the TIP4P/Ice model was

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used for water.19 The cross interactions between different molecules were computed

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using the Lorentz–Berthelot combining rules. 20

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Two groups of simulations were designed for evaluating the influences of

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asphaltenes on the hydrate formation at the water-gas interface (S1 and S2) or in the

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bulk water (S3 and S4), respectively. The S1 simulation system (5 nm × 5 nm × 11 nm)

6


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was constructed by adding a gas phase (1971 methane) on the top of a water phase

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(5857 water, 260 methane, and 12 asphaltenes in the form of aggregates). The

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structure of asphaltene aggregates was obtained from our preliminary simulation

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detailed in the supporting information. In order to facilitate the simulation, a seed of sI

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methane hydrate containing 2 × 2 × 2 unit cells was located in the water phase close

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to the water-gas interface. The S3 simulation system was built by packing 317

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methane, 3687 water, and 12 asphaltenes in the form of aggregates in a cubic box with

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an edge length of 5 nm. In order to provide control, simulations were also established

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following the same procedures except that asphaltenes were not added at the

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water-gas interface (S2) or in the bulk water (S4).

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2.2. Molecular dynamic simulation. The simulation of hydrate formation process

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was carried out by 2 µs MD simulation for S1 - S4. The pressure and temperature

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were set as 50 MPa and 250 K, respectively. The resulted structures were then used to

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investigate the hydrate decomposition process by running 50 ns MD simulation with a

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temperature of 273 K, 283 K, 293 K and 303 K, respectively. All simulations were

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performed at NPT ensemble (constant number of atoms, pressure, and temperature)

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with 3D periodic boundary conditions. The pressure and temperature were controlled

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using the Parrinello–Rahman barostat and the modified Berendsen thermostat,

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respectively.21, 22

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2.3. Data analysis. The aggregation degree of asphaltenes was quantified by

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computing the number of intermolecular contacts, which were counted if the 7


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minimum distance between the center of mass of each two asphaltene molecules were

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less than 0.5 nm. This value was widely used in previous simulation works,18, 23 which

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was also consistent with the minimum interlayer separation between stacked

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poly-aromatic cores (~ 0.35 nm) determined by X-ray diffraction experiments. 24, 25

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To quantify the degree of methane hydrate formation and decomposition, H2O-H2O

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pairs were only selected when the distance between two oxygen atoms was less than

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3.5 Å. The four-body structural order parameter (F4ϕ) was calculated as follows:26, 27

Fସ஦

௡

1 = ෍ cos3φ௜ ݊ ௜ୀଵ

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where n is the total number of the selected H2O-H2O pairs, φ௜ is the torsional angle

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between oxygen atoms and the outermost hydrogen atoms of the ith H2O-H2O pair.28

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The average value of F4ϕ for liquid water, ice, and hydrate is 0, -0.4, 0.7, respectively.

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27, 29

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The face-saturated incomplete cages analysis (FSICA) method was applied to

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evaluate the specific cages and composition variation of methane and water.30 The

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FSICA was able to identify all possible face-saturated cages (FSCs) in a system and

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give an overall description of hydrate evolution. FSCs are defined when every edge of

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a cage is shared with no less than two faces. It consists of complete cages (CCs) such

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as D-cages (512 cages) and T-cages (51262 cages) in standard sI hydrate structure, and

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face-saturated incomplete cages (FSICs) which would shortly occur during hydrate

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formation process.20 Quantification of FSCs is important in terms of two facts that (i) 8


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each of FSCs is free of holes making it possible to absorb the dissolved methane

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molecules on its faces and prevent the absorbed methane from contacting with the

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encaged methane. This phenomenon is beneficial for the occurrence of hydrate

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structures rather than methane bubbles, which makes it the precursors of hydrate

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nucleation, and (ii) FSCs were regarded as the building blocks of amorphous hydrate.

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Since their surface area and volume can be clearly defined, it is possible to calculate

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linking relationship between cages without ambiguity.30 Therefore, the FSICA can be

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used to distinguish different types of hydrate cages and study crystallinity and

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aggregation degree of methane hydrate.31

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3. RESULTS AND DISCUSSION

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3.1. Overall tendency of hydrate formation. Selected snapshots of the simulation

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boxes are shown in Fig. 1 and 2. In the bulk water, little changes were noted in the

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structure of asphaltene aggregates during hydrate formation (Fig. 1). It was evidenced

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by the number of intermolecular contacts between asphaltenes and asphaltenes that

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fluctuated very slightly around an average value of 45 (Fig. 3A). The percentage of

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the hydrate phase in the bulk water was low as the maximum overall F4ϕ was less than

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0.25, which was stable and unlikely to increase at the end of the simulation (Fig. 4A).

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It also demonstrated that the presence of asphaltenes had little influence on the

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formation of methane hydrate at an early stage since the F4ϕ curves for S3 and S4

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overlapped with each other during the first 900 ns. Slight inhibition effects were

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observed from the asphaltenes on the hydrate formation during the last 1000 ns. In

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order to gain insights into the local growth of hydrate around the asphaltene

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aggregates, the regional F4ϕ was obtained by dividing the whole system into three

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regions including the internal region (< 1 nm), surface region (1 - 1.7 nm) and

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external region (> 1.7 nm). The data in brackets represents the distance to the mass

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center of asphaltene aggregates (Fig. S3). It demonstrated that the fastest growth was

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initially occurred in the surface region, while the hydrate mainly grew in the internal

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region during 700 and 1100 ns (Fig. 4B). The external region became the predominant

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region for hydrate growth at the end of the simulation.

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When an obvious gas phase was present, the asphaltenes moved from the center of

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water phase towards the water-gas interface spontaneously (Fig. 2). The spherical

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structure turned to be an inverted cone-shaped structure consisted of parallel stacked

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asphaltene molecules, which was then dispersed by molecular rotation and eventually

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formed a film flat lying at the water-gas interface. Accordingly, the intensity of

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asphaltene aggregation decreased by 44% at 125 ns, and the intermolecular contacts

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declined to 20 at 200 ns. This transformation was mainly driven by the attractive force

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from the gaseous methane due to the hydrophobic nature of the asphaltenes.

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Additionally, the frame of methane hydrate cages became much clearer and appeared

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fully occupied the water phase at the end of the simulation (Fig. 2). This was

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confirmed by the continuous rising of the F4ϕ curve with a maximum value of 0.65

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(Fig. 4A), which was close to the average F4ϕ value (0.7) for a pure hydrate phase.32 It

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also clearly demonstrated the promotion effects from asphaltenes on the formation of

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methane hydrate, because the F4ϕ curve in the system without asphaltenes was

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obviously lower than that with asphaltenes.

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The above results showed different phenomena in the water with or without a gas

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phase, suggesting different behaviors of asphaltenes in these two scenarios. The F4ϕ

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parameter was useful for evaluating the overall tendency for the dynamics of hydrate

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growth, however, information remains limited on the mechanisms of how the

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asphaltenes influenced the methane hydrate in the bulk water and at the water-gas

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interface. In order to gain insights into this issue, we characterized the partitioning of 11


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water and methane and then analyzed the distribution and linkage of specific water

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cages, which were elaborated in the following subsections.

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3.2. Partitioning of water and methane during hydrate formation. In order to

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clarify the patterns of molecular partitioning during the hydrate growth, cage water

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was defined as the fraction of water in the hydrate cages while the rest was identified

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as free water. Similarly, methane was divided into the free and restricted fraction,

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while the latter was consisted of guest methane (encapsulated within hydrate cages)

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and absorbed methane (with less than 3Å distance to water cage faces). As expected,

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the tendency for the evolution of cage water was consistent with that of the F4ϕ

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parameter (Fig. 5A). For example, up to 90% cage water was observed in S1

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suggesting a thorough transformation of free water to hydrate phase. A closer

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examination of the mean square distribution (MSD) curves of methane indicated that

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the asphaltenes located at the water-gas interface increased the diffusion coefficient of

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methane by 71% (Fig. S2A). This finding suggested that the asphaltenes promoted the

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hydrate growth by enhancing the mass transport of methane from the gas phase to the

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water-gas interface and increasing its opportunity for association with water.

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The diffusivity enhancement by asphaltene addition was not obviously noticed

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when the methane entered the bulk water (Fig. S2B). In this case, only 30% methane

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was entrapped inside the hydrate cages although adequate water was present in the

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system (Fig. 5B). By contrast, the methane molecules were predominated by the

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adsorbed fraction which accounted for 58% of the total number of methane (Fig. 5B). 12


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Particularly, the degree of methane adsorption was 47% higher than that in the water

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without asphaltenes. It should be noted that the calculated adsorption fraction

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contained a large portion of methane strongly binding with asphaltenes, because our

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previous experimental results suggested that asphaltenes would form porous networks

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with small hydrocarbon molecules.33 In order to confirm this, we calculated the radial

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distribution function (RDF) for the distance between methane and the center of mass

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of asphaltenes. As shown in Fig. 3C, the first characteristic peak was located at 0.5

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nm on the RDF curves, suggesting the adsorption of methane on the asphaltenes. It

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was inferred that the majority of these methane located in the surface region instead of

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the internal region from the asphaltene aggregates according to the density profiles

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(Fig. S3). This might contribute to the fast growth of hydrate in the surface region

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revealed by the regional F4ϕ (Fig. 4B).

211

3.3 Distribution and linkage of specific cages during hydrate formation.

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Overall, the evolution of total cages and CCs (Figs. 6A and B) was consistent with the

213

changes in the F4ϕ parameter (Fig. 4A) and the number of water and methane in

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hydrate cages (Fig. 5). For instance, the presence of asphaltenes at the water-gas

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interface increased the total cages and the CCs by 29% and 59%, respectively. It also

216

changed the proportion of sI hydrate structure by increasing the ratio of T-cages to

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D-cages (Fig. 6B).

218

Additionally, the number of FSICs decreased by 58% during the last 400 ns (S1 in

219

Fig. 6C). It was inferred that the lost FSICs mainly transformed into the CCs because 13


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(i) the FSICs were predominated by empty cages, and (ii) the empty CCs increased by

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1.4-fold during the last 400 ns (Fig. 6D). Such transformation was also reported by

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Guo et al..34 In context of this study, it was attributed to the presence of asphaltenes

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because the FSICs lost was not observed when asphaltenes were removed from the

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water-gas interface.

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In addition to changing the distribution of individual cages, the asphaltenes also

226

influenced the degree of connections between cages which was characterized by the

227

links per cage and the standard hydrate crystallinity. A link was counted when two

228

cages shared at least one cage face, while the standard hydrate crystallinity was

229

defined as the ratio of the links between standard complete cages (composition of sI,

230

sII and sH structure) to the total cage links. Results indicated that the presence of

231

asphaltenes at the water-gas interface led to an increase in these two parameters (Fig.

232

7). The practical implication derived from this finding was that the co-existence of

233

asphaltenes with natural gas in the pipeline would facilitate the blockage process.

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Although it appeared that the asphaltenes tend to weaken the links per cage in the

235

bulk water, a slight promotion to the standard hydrate crystallization was also

236

observed (Fig. 7). The decline in the links per cage was attributed to the fact that the

237

cages adsorbed on the surface of asphaltene aggregates shared the cage face with

238

asphaltenes rather than with other hydrate cages.

239

14


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3.4. Decomposition of methane hydrate. The dynamics of hydrate decomposition

241

under various temperatures are shown in Fig. 8, which was consistent with the

242

changes in the number of total cages (Fig. S4). The asphaltenes were demonstrated to

243

promote the hydrate decomposition in the bulk water especially at relatively high

244

temperatures (Fig. 8B). For instance, more than 55% of the hydrate was still

245

remaining in the bulk water at 293 K, which turned to be completely disassociated

246

when asphaltenes were present in the water. One possible reason for this phenomenon

247

was the adsorption of the released methane to the asphaltenes, which reduced the

248

concentration of the dissolved methane and resulted in the formation of methane

249

bubbles around the asphaltene aggregates (Fig. 2D). When the temperature increased

250

to 303 K, the hydrate cages in the bulk water were almost lost leading to the

251

formation of methane bubbles irrespective of whether asphaltenes were added in the

252

water. Nevertheless, the speed of cages lost was obviously facilitated by the addition

253

of asphaltenes (Fig. S4B).

254

By contrast, the effects of asphaltenes on the decomposition of hydrate was not

255

obvious at the water-gas interface, because the F4φ curves in the presence of

256

asphaltenes were parallel to those without asphaltenes at given temperatures (Fig. 8A).

257

The hydrates in both scenarios started to disassociate until 283 K and a complete

258

decomposition was observed at 303 K. The promotion effects on hydrate

259

disassociation from asphaltene adsorption aforementioned was less pronounced in

260

these cases, which attributed to the fact that the released methane would 15


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spontaneously transport from water to gas phase.

262

Another important finding was that the decomposition of hydrate would in turn

263

promote the disassociation of asphaltene aggregates. This was evidenced by the facts

264

that the number of intermolecular contacts between asphaltenes and asphaltenes (i)

265

remained stable at 273 and 283 K when the hydrates were hardly decomposed, (ii)

266

declined by up to 38% at 293 and 303K when the hydrates started to disassociate, and

267

(iii) stopped decreasing after 20 - 30 ns when the hydrates were completely

268

disassociated (Fig. 3B). Generally, heating up to 303 K was not strong enough for

269

the disaggregation of asphaltenes.

270

Two implications derived from the above findings in the bulk water were that (i)

271

the gas hydrate has the priority to be artificially decomposed when attempting to

272

address the issue of co-deposition between asphaltene and gas hydrate, and (ii) in

273

order to reduce the energy cost, it is theoretically feasible to employ

274

temperature-programmed heating to initially destroy the hydrate cages at relatively

275

low temperature and then disassociate the asphaltene aggregates at higher

276

temperature.

277 278

4. CONCLUSIONS

279

The methane hydrate formation and decomposition with the coexistence of asphaltene

280

aggregation were conceptualized in Fig. 9. It demonstrated that the asphaltenes

16


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located at the water-gas interface promoted the hydrate formation, which was mainly

282

attributed to the increased methane diffusion and the facilitated transformation from

283

face-saturated incomplete cages to completed cages. By contrast, the asphaltenes in

284

bulk water showed slight inhibition on the hydrate growth. It was mainly attributed to

285

the methane adsorption on the asphaltene aggregates and on the hydrate cages close to

286

asphaltenes, which impeded the encapsulation of methane in water cages. In both

287

cases, the presence of asphaltenes increased the hydrate crystallization suggesting

288

high risks of hydrate co-deposition with asphaltenes. Results also suggested the

289

priority for treating hydrate rather than asphaltenes when co-deposition took place,

290

because the elimination of hydrate cages could potentially alleviate the risks

291

associated with asphaltene aggregation. This is awaiting to confirm by flow loop test

292

where the dynamic interaction between asphaltene and gas hydrate become more

293

complicated. It is not able to show these data in the present study due to the limitation

294

of MD simulation. Our ongoing works are to determine the operational conditions

295

using asphaltenes from various originations.

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AUTHOR INFORMATION

298

Corresponding Author

299

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

300

Notes

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The authors declare no competing financial interest.

302 303

ACKNOWLEDGEMENT

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This research was financially supported by National Natural Science Foundation of

305

China (No. 21307069), Science Foundation for Youths, Graduate School at Shenzhen,

306

Tsinghua University (No. QN201400010), and Shenzhen Gas Hydrate Research

307

Center (No. HYCYPT20140507010002). Partial support of this research by the

308

Special

309

NSFC-Guangdong Joint Fund (the second phase) is also greatly acknowledged.

Program

for

Applied

Research

on

Super

Computation

of

the

310 311

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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

A

B

Page 24 of 32

C

D

409 410

Fig. 1 Snapshots of S3 simulation at different stages. (A) initial setting, (B) hydrate formation at 250 K, (C) hydrate decomposition at 283 K, and

411

(D) hydrate decomposition at 293 K (Red: hydrogen bonds, Silver: water, Green: asphaltene, Blue: methane)

412

24


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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

Energy & Fuels

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0 ns

600 ns

125 ns

2000 ns

414

Fig. 2 Snapshots of S1 simulation at different time during hydrate formation (Red: hydrogen bonds, Silver: water, Green: asphaltene, Blue:

415

methane) 25


A

S1 S3

50 40 30 20 10 0 0

500

1000

Time (ns)

1500

2000

Page 26 of 32

60

273K 283K 293K 303K

B

50 40 30 20 10 0 0

10

20

30

40

Time (ns)

50

1.5

Radial distribution functions g(r)

60

Number of intermolecular contacts

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

Number of intermolecular contacts

Energy & Fuels

S3

C 1.0

0.5

0.0 0.0

0.5

1.0

1.5

2.0

r (nm)

416

Fig. 3 Changes in the number of intermolecular contacts between asphaltene and asphaltene during the (A) formation and (B) decomposition of

417

methane hydrate. An example of the radial distribution function for the distance between methane and the center of mass of the asphaltenes in S3

418

is shown in C.

26


2.5

Page 27 of 32

0.7

700

250

0.4

A

0.6

1100

B 0.3

0.5 0.4

0.2

F4ϕ

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

Energy & Fuels

0.3

0.1

0.2 0.1

S1 S2 S3 S4

0.0 -0.1 0

500

1000

1500

2000

0.0 Internal region Surface region External region

-0.1 0

1000

Time (ns)

Time (ns) 419

500

Fig. 4 Changes in the (A) overall F4φ in S1-S4 and (B) local F4φ in S3 during hydrate formation

27


1500

2000

Energy & Fuels

1.0

0.8

Methane fractions

S1 S2 S3 S4

0.6

0.4

0.2

1.0

S1 S2 S3 S4

0.8

Free methane

1.0

Cage water

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 34420 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 28 of 32

0.5

0.0

0

1000

2000

Time (ns)

0.6

0.4

0.2

A

0.0 0

500

1000

1500

B

0.0

2000

0

Time (ns)

500

1000

1500

2000

Time (ns)

Fig. 5 Fractionation of water and methane during hydrate formation (Solid circles: guest methane. Open circles: adsorbed methane)

28


700

B

S1 S2 S3 S4

600 500

Number of CCs

Number of total cages

A

400 300 200

0 0

500

1000

1500

500 400

4

2 1 0

0

1000

200 0

Time (ns)

200

0

500

Time (ns)

150

D

S1 S2 S3 S4

100

50

0

500

1000

1500

2000

1000 Time (ns)

1500

2000

1000 Time (ns)

1500

2000

S1 S2 S3 S4

0.5

0.2

0.1

0.0

0

Time (ns)

421 422

S1 S2 S3 S4

3

300

0

2000

Empty CCs ratio

C

600

100

100

Number of FSICs

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

Energy & Fuels

T/D cage ratio

Page 29 of 32

500

Fig. 6 Number of (A) total cages, (B) complete cages, (C) face-saturated incomplete cages, and (D) ratio of empty complete cages to total cages during hydrate formation 29


Energy & Fuels

6

1.0

Standard hydrate crystallinity

S1 S2 S3 S4

5

Links per cage

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

4 3 2 1

A

0 0

Page 30 of 32

500

1000

1500

S1 S2 S3 S4

0.8

0.6

0.4

0.2

B

0.0

2000

0

Time (ns)

500

1000

1500

Time (ns)

423 424

Fig. 7 Changes in the (A) links per cage and (B) standard hydrate crystallinity during hydrate formation

30


2000

Page 31 of 32

0.7

273K

283K

293K

303K

0.3

273K

283K

293K

303K

0.6 0.5

0.2

0.4

F4ϕ

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

Energy & Fuels

0.3

0.1

0.2

0.0

0.1

B

A

0.0

-0.1 0

10

20

30

40

0

50

Time (ns)

10

20

30

40

50

Time (ns)

425

Fig. 8 Influence of temperature on the overall F4φ during hydrate decomposition in water with (A) and without (B) an obvious gas phase

426

(Solid circles: system in presence of asphaltenes; Open circles: system in absence of asphaltenes)

427 31


Energy & Fuels

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428 429

Fig. 9 Conceptualization of hydrate formation and decomposition in the gas-water-asphaltene system

32


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Effects of Asphaltenes on the Formation and Decomposition of

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