Guangchun Song, Yuanxing Ning, Yuxing Li, Wuchang Wang

1 Shandong Key Laboratory of Oil-Gas Storage and Transportation Safety, College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, China

2 State Key Laboratory of Natural Gas Hydrates, Beijing 100028, China

Keywords:Hydrate Asphaltene Interface Growth Shape

ABSTRACT Natural gas hydrates can readily form in deep-water oil production processes and pose a great threat to the oil industry.Moreover, the coexistence of hydrate and asphaltene can result in more severe challenges to subsea flow assurance.In order to study the effects of asphaltene on hydrate growth at the oil-water interface,a series of micro-experiments were conducted in a self-made reactor,where hydrates nucleated and grew on the surface of a water droplet immersed in asphaltene-containing oil.Based on the micro-observations,the shape and growth rate of the hydrate shell formed at the oil-water interface were mainly investigated and the effects of asphaltene on hydrate growth were analyzed.According to the experimental results, the shape of the water droplet and the interfacial area changed significantly after the formation of the hydrate shell when the asphaltene concentration was higher than a certain value.A mechanism related to the reduction of the interfacial tension caused by the absorption of asphaltenes on the interface was proposed for illustration.Moreover,the growth rate of the hydrate shell decreased significantly with the increasing asphaltene concentration under experimental conditions.The conclusions of this paper could provide preliminary insight how asphaltene affect hydrate growth at the oil-water interface.

1.Introduction

Natural gas hydrates (NGHs) are crystalline compounds composed of water and hydrocarbon gas molecules[1].NGHs are readily formed under the low-temperature and high-pressure conditions in oil and gas industry and could pose a great threat to subsea flow assurance by agglomerating, jamming, bedding and depositing in the pipelines [2-6].Up till now, NGHs in subsea flow assurance have been extensively investigated and several hydrate prevention/management strategies have been proposed[7-10].Except for NGHs,another big concern in subsea flow assurance is about asphaltene.When the pipeline operating temperature and pressure decrease below the upper asphaltene precipitate envelope, asphaltene molecules dissolved in the crude oil will gradually precipitate due to the reduction in the solvation effectiveness of oil and then deposit on the pipewall [11], which may further lead to pipeline plugging.Considering that the conditions of asphaltene precipitation are usually above the hydrate phase boundary [12], NGHs and asphaltenes could coexist in the subsea pipelines and result in a more severe challenge to subsea flow assurance.In one estimate, the remediation of a partially plugged subsea pipeline can cost as much as $1 million dollars per mile and more enormous economic losses will be caused if a completely plug occurs [13].Moreover, as natural surfactants,asphaltene molecules and precipitated asphaltenes tend to absorb on the oil-water interfaces and could change the interfacial properties[14],which would further affect the formation of hydrates at the interface.Therefore, it is of great importance to study hydrate issues in the presence of asphaltene and vice-versa.

Up till now, only a few researches have been conducted to investigate the interaction between hydrate and asphaltene.In terms of formation,Ivanovaet al.[15]found by performing experiments that the equilibrium pressure of hydrate formation increased in the presence of asphaltene.This indicated that asphaltene may prevent the formation of hydrates.Similarly,Zhanget al.[16]found in experiments that asphaltenes could absorb on the surface of water droplet and reduce the nucleation rate of hydrates by hindering the diffusion-induced mass transfer.Morrissyet al.[17]measured the growth rate of hydrate shell and found that asphaltene could significantly reduce the growth rate of the hydrate shell.They also pointed out that the presence of asphaltene could change the wettability of the hydrate particle surface.Daraboinaet al.[18]also found in experiments that asphaltene could inhibit hydrate formation.However, Leporcheret al.[19]pointed out that precipitated asphaltenes could work as the nucleation sites of hydrates and thereby promote hydrate formation.Through molecular simulation,Ziet al.[20,21]found that asphaltenes at the gas-liquid interface could accelerate hydrate formation,but asphaltenes in the bulk liquid would prevent hydrate formation.In terms of agglomeration, Morrissyet al.[12]measured the adhesive force between hydrate particles and asphaltene particles in liquid cyclopentane using a micromechanical force apparatus and found the measured hydrate-asphaltene adhesive force had the same order of magnitude as that of hydrate particle cohesion.This indicated that hydrates and asphaltenes might agglomerate together during flowing.Lachanceet al.[22]pointed out through experiments that asphaltenes in crude oil could absorb on the surface of hydrates and droplets and therefore prevent the agglomeration between them.Chenet al.[14]also got the same conclusion by doing experiments.Recently, Salminet al.[23]and Delgado-Linareset al.[24]found in experiments that the aggregation state of asphaltenes had a key impact on the anti-agglomeration effect of asphaltenes.To be detailed,dissolved asphaltenes had a poor antiagglomeration effect but precipitated asphaltenes had an excellent anti-agglomeration effect.However, Gaoet al.[25]and Zhaoet al.[26]pointed out that asphaltenes in the crude oil could reduce the anti-agglomeration effect of hydrate anti-agglomerations.In addition, in terms of deposition, Ivanovaet al.[27]found in experiments that hydrates could deposit together with asphaltenes and the deposits were harder to decompose compared with purehydrates.

In this scenario, a series of micro-experiments were performed by our group in a self-made reactor to specially investigate hydrate growth at the oil-water interface in the presence of asphaltene.Based on the micro-observations, the shape and growth rate of the hydrate shell formed at the oil-water interface were studied and the effects of asphaltene on hydrate growth were analyzed.The conclusions of this paper could provide preliminary insight how asphaltene affect hydrate growth at the oil-water interface.

2.Experimental

2.1.Experimental apparatus

In this paper, hydrate growth at the oil-water interface was investigated in a self-made atmospheric visual reactor system(AVRS).As can be seen in Fig.1, the AVRS mainly consisted of a reactor with a visual window,a microscope(S9 D,Leica,Germany)with a CCD camera(Leica MC170 HD),a chiller and a data acquisition system.In experiments, hydrate nucleation was induced on the surface of a water droplet placed on the stainless steel plate installed in the reaction cell of the reactor.The water droplets were produced and placed on the plate using a microsyringe with a maximum volume of 4 μl.The reaction cell had a size of 4 cm×4 cm×8.7 cm and a quartz visual window was mounted in front of it.The temperature of the reaction cell was controlled by a chiller (DC-3015, Shanghai Hengping Instrument and Meter Factory, China) with a precision of ±0.1°C, which was connected to the cooling jacket welded behind the reaction cell.During the experiments, the temperature of the reaction cell was collected by a temperature transducer(W2PT-187,Shanghai Huixin Thermal Instrument Factory, China) and the micro-observations of hydrate nucleation and growth were recorded by the microscope.A PCbased data acquisition system was used to record all the data collected in real time.

2.2.Experimental materials

Cyclopentane (CP, 99.0% purity, Aladdin, China), deionized water,n-heptane(99%purity,Macklin,China),methylnaphthalene(98% purity, Macklin) and laboratory-precipitated asphaltenes were the main materials used in the experiments.In this work,CP was selected as the oil phase and hydrate former because its relatively mild equilibrium temperature (7.7°C) under atmospheric pressure[30].Moreover,CP is immiscible with water and can form hydrates of structure II,which is the structure of most of the NGHs formed in pipelines.In oil-water emulsions, hydrates usually first nucleate and grow at the oil-water interface.Therefore, although the differences between cyclopentane and gaseous hydrocarbons in the dispersion behaviors could change their accessibility to the interface, it is plausible that the differences may only affect the nucleate rate or growth rate of the hydrate shell.However,further experiments conducted using gaseous hydrocarbons under more realistic conditions are still needed to validate this deduction.Considering that asphaltenes are insoluble inn-heptane but soluble in methylnaphthalene, thusn-heptane and methylnaphthalene were used in the precipitation and dissolution of asphaltenes, respectively.In this work,asphaltenes were extracted from Shengli crude oil and the detailed procedures are introduced in Section 2.3.

2.3.Experimental method

In this work, experiments of hydrate nucleation and growth at the oil-water interface were performed in the AVRS.Before experiments, laboratory-precipitated asphaltenes should be prepared first as follows.(1) A liquid mixture composed of 10 g Shengli crude oil and 300 mln-heptane was processed by ultrasonics for 30 min in a ultrasonic homogenizer (JY92-IIN, SCIENTZ, China) to ensure the complete dispersal of the crude oil inn-heptane.(2)Subsequently, the ultrasonic-processed mixture was transferred to a centrifuge (LG10-2.4A, BeijingLab, China) using centrifuge tubes and was then centrifuged at a rate of 10,000 r·min-1for 30 min.After centrifugation,solid asphaltenes had already precipitated and settled down to the bottom of the centrifuge tubes.(3)Afterwards, the asphaltene solid suspension was filtered under vacuum through a 0.22 μm pore-size nylon membrane with the collected asphaltene solids being allowed to dry in air for 12 h at ambient temperature prior to use.

Fig.1. Schematic diagram of the atmospheric visual reactor system.

Then, asphaltene-containing solutions could be prepared using the following procedures.(1) A desired amount of solid asphaltenes were added into 10 ml methylnaphthalene and the mixture was then processed using ultrasonic for 5 min in order to got a fully dissolution state.(2) 60 ml CP was added into the asphaltenecontaining methylnaphthalene.Homogenized the liquid mixture by shaking and then added it into the reaction cell.In experiments,the volume of the asphaltene-containing solutions was chose to be 70 ml because the corresponding liquid level in the reaction cell was much higher than the top surface of the stainless steelplate.

Afterward,place a deionized water droplet on the stainless steel plate through the microsyringe.Then, reduce the temperature in the reaction cell to 2°C.During this process,CP hydrate can hardly form [31]although the subcooling for CP hydrate formation is about 5.7°C at the temperature of 2°C.Consequently, hydrate nucleation was initiated using seed hydrates [32]in this work and the detailed procedures are as follows.(1) Immerse a glass fiber (with a inner diameter of 0.3 mm) moistened by deionized water into liquid nitrogen (-196°C) for ice formation.(2) Transfer the glass fiber rapidly from the liquid nitrogen to the reaction cell and touch the deionized water droplet.(3) The deionized water droplet will soon convert to a hydrate particle right after the contact with the glass fiber.(4)Remove the converted hydrate particle and fine seed hydrates (usually invisible in the view field of the microscope) will then leave at the place where the converted hydrate particle used to be.(5) Place a new deionized water droplet with a temperature of 2°C where there are seed hydrates.Subsequently, hydrate nucleation will start at the CP-water interface under the induction of seed hydrates.

In this work, all the experiments were conducted at 2°C under static conditions.At least three deionized water droplets were used to form hydrate in one case in order to ensure the repeatability of the experiments.Detailed experimental conditions are listed in Table 1.In experiments,asphaltenes only existed in the dissolutionstate without precipitation.The concentration of asphaltene was calculated based on the total volume of CP and methylnaphthalene.

Table 1Experimental conditions

According to previous researches [28,31,33], hydrate formation at the oil-water interface usually includes lateral growth and radial growth.Lateral growth refers to the formation process of a hydrate shell at the oil-water interface and radial growth refers to the thicken process of the hydrate shell.In our experiments,lateral growth was mainly focused on.During the process of lateral growth, the morphologies and growth behaviors of the hydrate shell were recorded using the microscope.All the discussions in this paper were based on the micro-observations recorded.

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3.Results and Discussion

3.1.Hydrate growth in the absence of asphaltene

In this work, experiments on hydrate growth at the oil-water interface in the absence of asphaltene were conducted first as blank tests.

Fig.2 shows the typical process of the lateral growth of hydrate shell.As can be seen in Fig.2, hydrate crystals first formed at the two poles of the water droplet and then gradually grew towards the equator of the water droplet.As a result,a smooth hydrate shell formed along the oil-water interface eventually by lateral growth.In addition, it can also be seen from Fig.2 that the shape of the water droplet changed slightly from spherical to oblate ellipsoidal after coated by the hydrate shell.This indicated that in water-in-oil(W/O) emulsions, an appropriate modification on the particle size is necessary before directly use the size distribution of the water droplets as the size distribution of the initially formed hydrate particles [34].

The agglomeration tendency of two hydrate-coated water droplets in contact with each other were also studied.According to our previous work [31], liquid bridge could form immediately between two water droplets coated by crumpled hydrate shells after contact, as can be seen in Fig.3(a).However, in this work,as shown in Fig.3(b), liquid bridge still could not form between the two water droplets coated by smooth hydrate shells even a long time after the contact.Therefore, it can be deduced that the smooth hydrate shell formed in this work was much denser than the crumpled hydrate shell formed in our previous work[31].Consequently, the smooth hydrate shell had a very low porosity and the unconverted water trapped inside could hardly penetrate.Under these circumstances, the smooth hydrate shells were very dry with a low wetness and liquid bridge could not form between them,which indicated a low tendency of agglomeration.Moreover,CP was also hard to penetrate the smooth hydrate shell due to the low porosity.Thus, the water conversion of the initial water droplet could be quite low considering the significant mass transfer resistance, leaving much unconverted water trapped inside the hydrate shell.Therefore,although the smooth hydrate shell formed in this work was denser, its thickness was also very small due to the limited radial growth.In this scenarios, rupture of the hydrate shell could easily happen during the contact when the loading force was relatively large, as can be seen in Fig.4.To sum up, in pipeline flow assurance,this kind of smooth hydrate shell covering the water droplet would lead to a low water conversion and the hydrate-coated water droplet formed accordingly has a low tendency of agglomeration but a high tendency of breakage after collision, which may result in further hydrate formation from the released and unconverted water core.

Fig.2. Lateral growth of the hydrate shell along the oil-water interface in the absence of asphaltene.The white circle in the figure represents for the initial outline of the water droplet.The golden color of the bulk liquid comes from the methylnaphthalene.

Fig.3. The contact between two water droplets coated by crumpled hydrate shells (a) and smooth hydrate shells (b).

Fig.4. Rupture of the smooth hydrate shell: (a) before contact, (b) the moment of contact, (c) puff off after contact.

In addition to the morphology, shape and other properties of the hydrate shell, the growth rate of the hydrate shell was also investigated in this work.Detailed discussions on the growth rate of the hydrate shell formed in the absence/presence of asphaltene will be given in Section 3.2.

3.2.Hydrate growth in the presence of asphaltene

This work mainly focused on hydrate growth at the oil-water interface in the presence of asphaltene.In experiments, it was found that asphaltene could affect hydrate growth at the oil-water interface in terms of the shape and growth rate of the hydrate shell.As for the porosity, wetness and agglomeration tendency of the hydrate shell, no significant difference was observed between the cases with and without asphaltene.Therefore, the following discussions will only focus on the shape and growth rate of the hydrate shell.

Fig.5 shows the shape of the hydrate shell formed in the presence of asphaltene and Fig.6 illustrates the typical formation process of the hydrate shell in the presence of asphaltene.As can be seen from Figs.5 and 6,the shape of the water droplet changed significantly from spherical to cocoon-like after coated by the hydrate shell formed in the presence of asphaltene.Treat the water droplets with and without hydrate shells as spheres and prolate spheroid respectively,the changes in the droplet size could be calculated and are listed in Table 2.As shown in Table 2,in water/asphaltenecontaining oil systems,the size of the initially formed hydrate particles could be quite different from the size of the water droplets.Thus, the conclusion that the droplet size distribution does not change significantly during the conversion from water to hydrate[34]needs to be further discussed in water/asphaltenecontaining oil systems.In addition, as can be seen from Table 2 and Fig.7, the interfacial area increased by 12.4%-21.4% after the water droplet was coated by hydrate shell when the concentration of asphaltene was higher than 0.0114 g·L-1.Consequently, considering that the size distribution of the initially formed hydrate particles and the interfacial area are critical important in hydrate growth models [35]and hydrate agglomeration models [36,37], a modified prediction model of the size distribution of the initially formed hydrate particles used in the cases where asphaltenes exist is needed to ensure the accuracy and reliability of relevant hydrate models.Details of the modified prediction model will be mainly focused on in another paper of our group.

Fig.5. Cocoon-like hydrate shells formed in the presence of asphaltene.The color of the bulk liquid darkens gradually due to the increasing asphaltene concentration.

Fig.6. Formation process of the cocoon-like hydrate shell observed in case #8 (0.06 g·L-1 asphaltene).The white circle in the figure represents for the initial outline of the water droplet.

In this work,changes in the shape of the water droplets resulted from the changes in the oil-water interfacial tension.A mechanism illustrated in Fig.8 was proposed and explained as follows.As a kind of natural surfactant in crude oil, asphaltenes are known to have strong interfacial activity [23].Therefore, asphaltenes in the bulk liquid tended to absorb to the oil-water interface in experiments,as shown in Fig.8(a).According to the Gibbs adsorption isotherm given in Eq.(1) [38], where γ is the interfacial tension,Cis the interfacial concentration, Γ is the amount of the absorbed asphaltene,Ris the gas constant, andTis the temperature, the interfacial tension of the oil-water interface would decrease due to the absorption of asphaltene, which could soften the oil-water interface.Then, the Young-Laplace relation given by Eq.(2) [38]can be used to explain the shape change of the water droplet,where ΔPis the difference between the pressure inside (P1) and the pressure outside (P2) the water droplet, andris the radius of curvature of the water droplet.In experiments,rwould not change at the moment when γ decreased due to the absorption of asphaltenes.Therefore, ΔPwould decrease according to Eq.(2).Under these circumstances, the pressure balance inside and outside the water droplet would be disturbed according to Eq.(3),considering that the pressure inside and outside the droplet were almost constant during the experiments.As a result, the unconverted water trapped inside the incomplete hydrate shell tended to force their way out under the action of the excess pressure inside the droplet.During this process,the spherical shape of the water droplet would gradually change to cocoon-like, with the top of the water droplet gradually tapered,as shown in Fig.8(b)and(c).As a result,rgradually decreased in the meantime due to the shape change of the water droplet.Finally, according to Eqs.(2) and (3), the pressure balance inside and outside the water droplet would establish again whenrdecreased to a certain degree and then the shape change of the water droplet stopped accordingly, as can be seen in Fig.8(d).

It should be noticed that, although the shape and size of the water droplet changed after the formation of hydrate shell in every case, significant changes only took place in the cases where the concentration of asphaltene was higher than 0.0114 g·L-1, as can be seen in Table 2 and Fig.7.This was because in experiments when the concentration of asphaltene was lower than 0.0114 g·L-1, there was no sufficient asphaltene molecule absorbing tothe oil-water interface.As a result,the reduction of the interfacial tension caused by the absorption of asphaltene was too small to induce a significant change in the droplet size.

Fig.7. Increase in the interfacial area after the formation of hydrate shell.

Table 2Changes in the droplet size after the formation of hydrate shell

The effects of asphaltene on the growth rate of the hydrate shell were also focused on in this work.In this work,the growth rate of the hydrate shell was estimated according to the interfacial area after hydrate shell formed and the time required for the formation of a complete hydrate shell.As can be seen in Fig.9, the growth rate of the hydrate shell was significantly reduced in the presence of asphaltene.Moreover, under our experimental conditions, the higher the concentration of asphaltene, the slower the growth of the hydrate shell.This was mainly because the asphaltene molecules absorbed at the oil-water interface could work as a physical barrier of the heat and mass transfer process during hydrate formation.In addition, asphaltene molecules in the bulk liquid could form hydrogen bonds with the water molecules[38],which would weaken the local hydrogen-bonded networks between water molecules and thereby inhibit the formation of the cavities in hydrate structures.Consequently, in water/asphaltene-containing oil systems, hydrate growth may be inhibited due to the presence of asphaltene and this could contribute to the natural-hydrateinhibiting property of the asphaltene-containing crude oil.In the future, macro-experiments with a wider range of asphaltene concentration and different asphaltene aggregation states should be performed to further investigate the effects of asphaltene on hydrate formation in oil-water systems.

4.Conclusions

In the present work, a series of micro-experiments were conducted using a self-made reactor to study hydrate growth at the oil-water interface in the presence of asphaltene.Based on the micro-observations, the morphology, shape, porosity, wetness and growth rate of the hydrate shell formed at the oil-water interface were investigated and the effects of asphaltene on hydrate growth were analyzed.The main conclusions of this work are as follows.

Smooth hydrate shells gradually formed through lateral growth along the oil-water interface both in the cases with and without asphaltene.In experiments, the porosity, wetness and agglomeration tendency of the smooth hydrate shell were low and asphaltene showed no significant effect on these properties.The shape and size of the water droplet changed slightly after the formationof the hydrate shell when the concentration of asphaltene was lower than 0.0114 g·L-1.Nevertheless, when the concentration of asphaltene was higher than 0.0114 g·L-1, the shape of the water droplet gradually changed from spherical to cocoon-like during the formation of the hydrate shell and the interfacial area increased by 12.4%-21.4% accordingly after the formation of the hydrate shell.This experimental phenomenon was closely related to the reduction of the interfacial tension caused by the absorption of asphaltene molecules at the oil-water interface and a mechanism was proposed for illustration.Thus,in future modeling works,an appropriate modification on the particle size is necessary before directly use the size distribution of the water droplets as the size distribution of the initially formed hydrate particles.In addition,it was found under our experimental conditions that the growth rate of the hydrate shell was significantly reduced in the presence of asphaltene and the higher the concentration of asphaltene, the slower the growth of the hydrate shell.This indicates an inhibition effect of asphaltene on hydrate growth at the oil-water interface.In the future,macro-experiments with a wider range of asphaltene concentration and different asphaltene aggregation states should be performed to further investigate the effects of asphaltene on hydrate formation in oil-water systems.

Fig.8. Proposed mechanism for the shape change of the water droplet happened during the formation of hydrate shell.

Fig.9. Estimated growth rate of the hydrate shell in experiments.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (U19B2012, 51974349, 51991363), Fundamental Research Funds for the Central Universities (20CX06098A), and State Key Laboratory of Natural Gas Hydrates(CCL2020RCPS0225ZQN), which are gratefully acknowledged.