Xing Huang ,Ran Zhu ,Liwei Cheng ,Yijian Zhu ,Peng Xiao ,Xiaohui Wang ,Bei Liu,Changyu Sun,*,Weixin Pang,Qingping Li,Guangjin Chen,*,Xinjing Xu,Jinfeng Ji

1 State Key Laboratory of Natural Gas Hydrates/College of Chemical Engineering and Environment,China University of Petroleum,Beijing 102249,China

2 State Key Laboratory of Natural Gas Hydrate,CNOOC,Beijing 100010,China

3 College of Chemistry and Chemical Engineering,Neijiang Normal University,Neijiang 641100,China

4 Faculty of Engineering,China University of Geosciences,Wuhan 430074,China

5 Hainan Longpan Oilfield Technology Co.,Ltd,Hainan 571900,China

Keywords:Hydrate Kinetics Inhibitor Poly (N-vinyl aprolactam)-co-tert-butyl acrylate Synergist Synthesis

ABSTRACT Low dosage kinetic hydrate inhibitors (KHIs) are a kind of alternative chemical additives to high dosage thermodynamic inhibitors for preventing gas hydrate formation in oil&gas production wells and transportation pipelines.In this paper,a new KHI,poly (N-vinyl caprolactam)-co-tert-butyl acrylate (PVCapco-TBA),was successfully synthesized with N-vinyl caprolactam (NVCap) and tert-butyl acrylate.The kinetic inhibition performances of PVCap-co-TBA on the formations of both structure I methane hydrate and structure II natural gas hydrate were investigated by measuring the onset times of hydrate formation under different conditions and compared with commercial KHIs such as PVP,PVCap and inhibex 501.The results indicated that PVCap-co-TBA outperformed these widely applied inhibitors for both structure I and structure II hydrates.At the same dosage of KHI,the maximum tolerable degree of subcooling under which the onset time of hydrate formation exceeded 24 hours for structure I hydrate was much lower than that for structure II hydrate.The inhibition strength increased with the increasing dosage of PVCap-co-TBA;The maximum tolerable degree of subcooling for the natural gas hydrate is more than 10 K when the dosage was higher than 0.5% (mass) while it achieved 12 K when that dosage rose to 0.75% (mass).Additionally,we found polypropylene glycol could be used as synergist at the dosage of 1.0 % (mass) or so,under which the kinetic inhibition performance of PVCap-co-TBA could be improved significantly.All evaluation results demonstrated that PVCap-co-TBA was a very promising KHI and a competitive alternative to the existing commercial KHIs.

1.Introduction

Gas hydrates are ice-like crystalline compounds in which guest gas molecules like methane and ethane are trapped in the polyhedral cages formed by hydrogen-bonded host water molecules under suitable pressure/temperature conditions [1-3].There exist three types of hydrate structures,i.e.,structure I,structure II and structure H,depending on the size of the guest molecules [4].The plugging accident caused by the formation of hydrate could take place in both the production and transportation of oil &gas[5,6].In fact,gas hydrate prevention and control has been a major challenge for oil and gas industry since Hammerschmidt [7] discovered hydrate formation may bring the plugging of pipelines in 1934.Different mitigation measures and remediation strategies have been developed to deal with the risk of hydrate blockage in pipelines such as depressurization[8-10],chemical inhibitor injection[11-14],heating[15,16],etc.,among which chemical inhibitor injection is the most frequently used method.The traditional hydrate inhibitors mainly refer to alcohols like methanol and ethylene glycol,called thermodynamic hydrate inhibitors (THIs)[17,18],because they are usually used in large dosage (＞20 %(mass)) and can change the thermodynamic formation conditions of hydrates significantly.The high dosage of THI may bring high cost and environmental risk,hence the development of low dosage hydrate inhibitors (LDHIs) has drawn more and more attention in recent decades.Up to now,LDHIs mainly include kinetic hydrate inhibitors (KHIs) [19-21] and anti-agglomerates (AAs) [22-25].The KHIs disturb the nucleation process and delay the growth of hydrate nuclei and therefore guarantee no hydrate formed in the desired time.

Typical KHIs,e.g.,poly (N-vinylpyrrolidone) (PVP) and polyvinylcaprolactam (PVCap),are water-soluble polymers containing hydrophilic amide and hydrophobic functional groups which bind to the interfaces between hydrate nuclei and aqueous solution[26].Several KHIs such as Gaffix V-713,Luvicap 55 W and Inhibex 501 have achieved commercial application.However,they usually exhibit poor inhibition performance at higher subcooling surpassing 10-12 K.One way to overcome the drawback is to introduce active functional groups into polymeric structures.For example,Qinet al.[27] synthesized two new KHIs,which exhibited better inhibition performance than PVP and Inhibex 55 W in structure I and II hydrate,by introducing both t-butyl group and phenyl group into PVP molecules.Chenget al.[28]found that the length of alkyl chain on ester group grafted in PVP molecules played an critical role in improving the inhibition performance,by combining molecular dynamic simulation and experimental methods.Longet al.[29] synthesized acrylamide-terminated PVCap which was of better kinetic inhibitory performance on CH4hydrate formation than PVP and PVCap under the same conditions.Another way is to apply some additives into KHIs to act as synergists [26].The three most studied types of synergists are quaternary ammonium salts,glycol ether compounds,and polyethylene oxide(PEO)[30,31].Additionally,some THIs (monoethylene glycol,alcohols) [32-34],ionic liquids[35,36]also showed remarkable synergic effect.The inhibition performance of KHIs could be improved by adding synergists,i.e.,the hydrate onset time can be further prolonged due to synergistic effect [37].Some polymers such as PEO also exhibit synergistic effects to KHIs.Jianget al.[38] found that the hydrate nucleation and aggregation was remarkably inhibited after a small amount of PVP was added to the PEG drilling fluid.

With the purpose of discovering more powerful KHIs,a novel block copolymer poly(N-vinyl caprolactam)-co-tert-butyl acrylate(PVCap-co-TBA) was synthesized withN-vinyl caprolactam(NVCap)andtert-butyl acrylate in this work.The inhibition performances of PVCap-co-TBA on methane and natural gas were investigated and compared with commercial KHIs such as PVP,PVCap and Inhibex 501.Besides,the synergistic effect of polypropylene glycol (PPG) with PVCap-co-TBA was evaluated.The results showed that the new polymer PVCap-co-TBA exhibited inhibition performance outperforming existing commercial KHIs under the same conditions;Synergist PPG could enhance the kinetic inhibition performance of PVCap-co-TBA significantly.

2.Experimental

2.1.Materials

The Inhibex 501 was purchased from Ashland Corporation.PVP was obtained from Sigma-Aldrich.Deionized water was produced in our laboratory.Analytical pure ethanol and isopropanol were supplied by Beijing Chemical Works and Energy Chemical,respectively.N-vinyl caprolactam,2,2′-azobisisobutyronitrile(AIBN),PPG were purchased from Energy Chemical.The average molecular weight of PPG was 400 g·mol-1.Analytical grade pure CH4(99.99 %) and natural gas were supplied by the Beijing Haipu Gas Industry Corporation,the composition of which was listed in Table 1.

Table 1 Composition of different hydrate forming gas.

2.2.Synthesis method of KHIs

Both homo-polymers PVCap and PVCap-co-TBA were synthesized in a 100 ml three-neck round bottom flask.Correspondingreaction equations are shown in Figs.1 and 2 respectively.For the synthesis of PVCap,NVCap and initiator AIBN were mixed together in isopropanol at 353.15 K under nitrogen atmosphere for 12 h.After the polymerization,the solvent was evaporated and a certain quantity of ethanol was added to the flask and evaporated again to purify the product.Afterwards,the product was dried at 333.15 K in a vacuum oven for several hours until the weight did not change.The procedures for preparing PVCap-co-TBA were similar to those for PVCap except that certain tertbutyl acrylate was added to the solvent.The purification process was repeated for three times and then we got the product.The functional groups of produced polymers were determined with Fourier Transform Infrared Resonance (FTIR) spectroscopy to further ascertain the structure of polymerization.

2.3.Experimental apparatus and procedures for testing the performance of KHIs

Schematic diagram of the apparatus used in this work was shown in Fig.3.The experiment was carried out in a highpressure transparent sapphire cell which was 2.54 cm in diameter with an effective volume 59 cm3and a maximum working pressure of 40 MPa.The device has been used in the evaluation of hydrate inhibitors in our group’s previous works[27,28,32,39].The temperature of the cell was controlled by an air bath with a viewable window.A magnetic stirrer was also equipped inside the cell.The hydrate formation process could be observed directly outside the air bath.The temperature and pressure in the cell were measured by Pt-100 resistance and pressure transducer with an accuracy±0.1 K and ± 0.001 MPa,respectively.The air bath temperature was stabilized with a fluctuation within ± 0.1 K.

Fig.1.The synthesis of PVCap.

Fig.2.The synthesis of PVCap-co-TBA.

Fig.3.Schematic diagram of experimental apparatus.

Before the experiments,the transparent sapphire cell was washed by ethanol for three times to remove the possible residual impurities.Afterwards it was purged with nitrogen to ensure the cell was cleaned and dried thoroughly.The isothermal method used in our previous works [27,28,32],was employed for testing the inhibition performance of KHI in this work.The main procedures were described as follows.Firstly,about 10 g aqueous solution of inhibitor was added to the transparent sapphire cell and the blind cell was charged with hydrate forming gas up to 10 MPa or so.Subsequently,the sapphire cell was vacuumed to guarantee the absence of air;And then the air bath was powered on and the temperature was set to the desired value.When the temperature inside the sapphire cell became constant for 1 hour or so,the gas was injected into the sapphire cell from the blind cell until the desired pressure was achieved.Finally,the inlet valve of the sapphire cell was closed and the stirrer was turned on at a constant speed of 51 r·min-1.The change of pressure and phenomena inside the cell through the whole experimental run was recorded by Monitor and Control Generated System(MCGS).Fig.4 showed a typical change of pressure with the elapsed time during an experimental run,where the hydrate formation system was composed with methane gas and aqueous solution of 1.0 % (mass) PVCap-co-TBA;the temperature and initial pressure was set to 275.69 K and 6.2 MPa respectively.The induction time for hydrate formation was defined as the time period between the end of gas injection and the onset of the hydrate formation.In this work,two kinds of onset times for hydrate formation were defined according to Chenet al.’s work [39,40],i.e.,‘‘TVO”,the time when hydrate crystals could be initially observed by naked eyes,and‘‘TPD”,the time when the obvious and continuous dropping of system pressure began.The onset time TVO could be taken as induction time proximately.The magnitude of the difference between TVO and TPD represented the initial growth rate of hydrate crystals;The bigger difference,the slower initial growth rate [27].If there was no hydrate formation observed in 24 h (1440 min),it was assumed that the inhibitor was able to tolerate the current subcooling.

3.Results and Discussion

3.1.Characterization of the KHIs

Fig.4.The change of pressure with the elapsed time during a typical experimental run for evaluating the inhibition performance of KHI,where the hydrate formation system was composed with methane and aqueous solution of 1.0 % (mass) PVCapco-TBA;The temperature and initial pressure was set to 275.69 K and 6.2 MPa respectively.

Fig.5.FTIR spectra of PVCap and PVCap-co-TBA.

The structure and functional groups of block polymer PVCap-co-TBA were characterized by FTIR spectra,and compared with PVCap.As shown in Fig.5,there were no peaks at～3100 cm-1indicating the C=C stretching vibration of vinyl group from monomers VCap andtert-butyl acrylate disappeared due to the successful polymerization of PVCap-co-TBA.The strong peaks at 1617 cm-1,1670 cm-1belonged to the characteristic C=O stretching vibration absorption associated with lactam amide group from PVCap and copolymer PVCap-co-TBA,respectively.There existed tertiary butyl groups in the polymer which resulted in peaks at～2800-2900 cm-1and～1450 cm-1.Meanwhile,the peak at 1780 cm-1was assigned to the C=O stretching vibration in ester,indicating the proof of tertiary butyl groups taking part in the unit of block copolymer PVCap-co-TBA thoroughly.The number average molecular weight of PVCap-co-TBA measured by gel permeation chromatograph is 11499 g·mol-1,and the polydispersity index is 3.318.The cloud point of PVCap-co-TBA is 308.8 K.

3.2.Inhibition performance of KHIs

A series of hydrate formation experiments were carried out to evaluate the inhibition performance of PVCap-co-TBA as KHI.For comparison,several parallel experiments were also performed with several famous KHIs,PVCap,PVP and Inhibex 501.The subcooling was set to 6 K when methane was used as hydrate forming gas while it was set to 10-12 K when the natural gas was used to form hydrate.The subcooling was defined as the difference between the equilibrium hydrate formation temperature under present experimental pressure,which was calculated by Chen-Guo model [41,42],and the present experimental temperature.The morphologies inside the sapphire cell were recorded by a camera.Several typical screenshots corresponding to four different stages of an experimental run on the methane hydrate formation were shown in Fig.6,where the concentration of PVCap-co-TBA and the subcooling were set to 1.0% (mass) and 6 K respectively.In the first stage,i.e.,the time period before the gas being injected into sapphire cell,there was remarkable foam occurring above the aqueous solution(Fig.6(a)),indicating PVCap-co-TBA was a kind of surfactant.After the injection of methane gas,the quantity of foam increased significantly (Fig.6(b)) and kept in the following stages.The second stage was the induction period,i.e.,the time period between the gas injection and onset of hydrate formation,TVO.Fig.6(c) showed a trace of hydrate particles occurring in the wall of sapphire cell at TVO.The red circle in Fig.6(c),6(d)represented the morphology of hydrate at the first emergence moment and massive formed time.The third stage was the time period between TVO and TPD,in which hydrate grew very slowly and remarkable pressure dropping could not be observed yet.The last stage was the fast hydrate growth period,in which remarkable pressure dropping occurred.Fig.6(d)showed that large quantity of hydrate formed in the sapphire cell wall.

KHI may exhibit different inhibition performance to the formation of hydrate with different crystalline structures [43].In usual cases,methane formed structure I hydrate while natural gas formed structure II hydrate because of the presence of molecules with larger sizes like propane [44,45].Thus methane and natural gas were used as hydrate forming gas respectively for investigating the inhibition performances of KHI synthesized in this work on the formation kinetics of hydrate with different structures.The composition of the natural gas was given in Table 1.The experimental results and corresponding experimental conditions were displayed in Tables 2 and 3 as well as Figs.7 and 8.Because the nucleation process of hydrate exhibited certain randomness,the evaluation experiment under the same conditions was repeated three times in order to determine the uncertainties of the measurements on TVO and TPD.The values of TVO and TPD given in all following Tables were the average of three repeated measurements.The uncertainties of the measurements were also given in these Tables correspondingly.In order to provide the same comparison basis,the dosages of each KHI were specified to 0.5% (mass) and 1.0%(mass)uniformly for each KHI in these two groups of experiments as shown in Tables 2 and 3.

Fig.6.The morphologies inside the sapphire cell at different formation stages of methane hydrate.(a)before gas injection,T=0 min;(b)after the injection of methane gas,T=3 min;(c) occurrence of a trace of hydrate, T=550 min;(d) occurrence of mass hydrate, T=650 min.

Table 2 Onset time of hydrate formation in the presence of different KHIs with methane as hydrate forming gas.

Table 3 The onset time of hydrate formation for different kinds of KHIs with natural gas as hydrate forming gas.

Fig.7.Pressure-time profiles of formation process of methane hydrate at subcooling of nearly 6 K: (a) the whole hydrate formation process;(b) the enlarged curve of the enclosed part by a dotted frame in (a).

As shown by Tables 2 and 3,when the dosages of KHIs,subcooling and hydrate forming gas were specified,the inhibition performance of new KHI,PVCap-co-TBA,was obviously superior to other KHIs,PVP,PVCap and Inhibex 501,indicated by much longer TVO and TPD.PVP worked worst,the gap between TVO and TPD was too small to be distinguished with each other because of the very fast growth rate of hydrates,which was shown by Figs.7 and 8.The reason why PVP exhibited little inhibition effect might be that the subcooling set in this work was too large,especially for the cases when natural gas was used as hydrate forming gas.On the contrary,the rest of the tested inhibitors all exhibited certain inhibition effect.These results were in accordance with the earlier findings on poly (N-alkyl-N-vinylamide)s and poly (N-vinyl lactam)s that the water-soluble polymers with larger hydrophobic groups performed better on inhibiting hydrate formation[27].The results tabulated in Tables 2 and 3 also demonstrated that larger dosage of KHI usually corresponded to stronger inhibition effect,especially for cases when natural gas was used as hydrate forming gas and PVCap was used as KHI.TVO could increase from 667 to more than 1440 minutes when dosage of PVCap increased from 0.5% (mass) to 1.0% (mass).Similar results were also found in the previous study [46,47].

Fig.8.Pressure-time profiles of natural gas hydrate formation at subcooling 10 K or so: (a) the whole methane hydrate formation process;(b) the enlarged curve of the enclosed part by a dotted frame in (a).

The subcooling was set to only 6 K or so for cases when methane was used as hydrate forming gas,much smaller than 10 K or so set in cases when natural gas was used as forming gas.However,both TVO and TPD measured in former cases were much shorter than those measured in later cases.Especially when PVCap-co-TBA used as KHI,natural gas could not form hydrate in 24 h at both dosages,0.5%(mass)and 1.0%(mass),while methane formed in 270 min and 550 min respectively.These results were consistent with the reported finding that PVCap and its derivatives gave a weaker inhibition performance to structure I methane hydrates than to structure II natural gas hydrates[26].Fortunately,structure II natural gas hydrate formed in most practical cases where KHI was required.Meanwhile,it was found that the TPD was close to TVO for PVCap-co-TBA,which implied that the new inhibitor mainly inhibited the nucleation of hydrate but not the growth of hydrate nuclei.Once the hydrate crystals appeared,the growth rate of hydrate was rapid.

A series of molecular dynamic simulations have demonstrated that hydrogen bonding between KHI molecules and hydrate nuclei as well as the steric hindrance effect originated from the larger molecular size of KHI (usually polymer) made most contribution to the kinetic inhibition performance [46,48].Because the strong hydrophobic tertiary butyl functional groups were introduced to PVCap-co-TBA,stronger barrier between liquid water molecules and hydrate nuclei or particles originated.Additionally,there were N and O atoms on PVCap monomers which could easily form hydrogen bonds with water molecules and facilitated the adsorption of the PVCap-co-TBA molecules on the hydrate crystal surface[49-51].Those might be the reason why PVCap-co-TBA worked well.

In order to determine the most suitable dosage of PVCap-co-TBA for practical application,the influence of content of the KHI upon the onset time of hydrate formation was investigated in more details by using the natural gas as hydrate forming gas.The subcooling was set to higher values,11 K or so and 12 K or so in this group of experiments.Five experiments,corresponding to five different KHI dosages,0.250%,0.375%,0.500%,0.750% and 1.00%(mass)respectively,were performed for each specified subcooling.The experimental results were displayed in Table 4 and Fig.9.One could see that both the onset time of TVO and TPD increased rapidly with the increasment of KHI dosages in the range of 0.250% to 0.750% (mass) while the increasing rate declined when the dosage was higher than 0.750% (mass).The dosage of 0.750%(mass) was high enough to tolerate the subcooling up to 12 K.0.750% (mass) was then suggested as the suitable dosage of the new KHI for practical application when making economic consideration.It is very promising that the new KHI PVCap-co-TBA could tolerate a subcooling up to 12 K,which will be a competitive alternative to current commercial KHI.According to the reported literature,the maximum subcooling of PVCap and commercial Luvicap EG was 8.3 K and 6.8 K for structure II hydrate when they were applied as inhibitors,respectively [43].The comparison of the maximum subcooling in the present study with the results in the literature made the newly synthesized KHI more potential for industrial application.

3.3.Performance of synergist

It has been reported that some surfactants or low molecular weight polymers could work as synergist to improve the inhibition performance of KHIs [43].PPG is a polymer with relatively low price and its monoester is a kind of nonionic surfactant.Theproperty of PPG makes it a potential synergist for PVCap-co-TBA.In order to verify whether PPG was of synergic effect or not and determine the proper working dosage,the onset time of hydrate formation with the presence of different contents of PPG in aqueous solution were measured by using the natural gas as hydrate forming gas,where the subcooling and dosage of PVCap-co-TBA were specified to 12 K or so and 0.5% (mass) respectively.The experimental results were displayed in Table 5 and Fig.10.

Table 4 Onset time of hydrate formation under different dosages of PVCap-co-TBA and two different subcooling,11 K or so and 12 K or so,with the natural gas as hydrate forming gas.

Table 5 Onset time of hydrate formations from aqueous solutions with fixed 0.5% (mass) PVCap-co-TBA and different contents of PPG under 12 K subcooling with the natural gas as hydrate forming gas.

Fig.9.The variation of onset time,TVO and TPD,of the natural gas hydrate formation with the dosage of KHI PVCap-co-TBA under two specified subcoolings 11 K and 12 K.(The error bars represented the uncertainties of the measurements).

One could see that the onset time TVO and TPD did not increase with the increasing dosage of PPG monotonously;PPG exhibited significant synergetic effect in the higher dosage range (＞1.0%(mass))while was of negative effect on the inhibition performance of PVCap-co-TBA in the lower dosage range(＜0.5%(mass)).Even in the higher dosage range,the strength of the synergetic effect did not increase with the increasing dosage of PPG still.It was indicated that 1.0% (mass) or so was the most suitable dosage for PPG as a synergist of KHI PVCap-co-TBA.It has been reported that PEG could acted as a thermodynamic inhibitor[39]and defoaming agent.Its thermodynamic inhibition characteristics should be positive for strengthening the inhibition behavior of PVCap-co-TBA.However this effect was not obvious or negligible when its dosage was very low [52].On the contrary,its defoaming performance may have negative effect on the kinetic inhibition behavior of PVCap-co-TBA.Defoaming process usually results in the increase of the gas/liquid interfacial tension.However,it was found that a higher interfacial tension corresponded to shorter hydrate onset time and a weaker kinetic inhibition performance [32].The negative effect originating from the defoaming characteristic of PPG might be the reason why the strength of the synergistic effect did not increase with the increasing dosage of PPG monotonously.Surely,the mechanism of synergistic effect is very complicated,it deserves further research.

Fig.10.The variation of onset time,TVO and TPD,of the natural gas hydrate formation with the dosage of PPG,where dosage of PVCap-co-TBA and the subcooling were specified to 0.5%(mass)and 12 K or so.(The error bars represented the uncertainties of the measurements).

4.Conclusions

The block polymer PVCap-co-TBA has been successfully synthesized in isopropanol medium and evaluated as a gas hydrate kinetic inhibitor.The isothermal method was employed for KHI performance testing in a sapphire transparent apparatus.The inhibition performance of PVCap-co-TBA was investigated by measuring the onset times of hydrate formation under different conditions and compared with commercial KHIs such as PVP,PVCap and inhibex 501,with respect to both structure I methane hydrate and structure II natural gas hydrate.The experimental results indicated that PVCap-co-TBA was of kinetic inhibition performance outperformed these commercial inhibitors.For structure I hydrate,the maximum tolerable subcooling,under which no hydrate appeared in 24 h,was much lower than that for structure II hydrate.Besides,the inhibition strength increased with the increasing dosage of KHIs.When the dosage of PVCap-co-TBA was greater than 0.5%(mass),the maximum tolerable subcooling for natural gas hydrate achieved 10 K while it achieved 12 K when the dosage rose to 0.75% (mass).PVCap-co-TBA mainly inhibited hydrate nucleation but not the growth of hydrate nuclei.This excellent inhibition behavior is possibly attributed to the stronger steric hindrance effect originated by the strong hydrophobic tertiary butyl func-tional groups in PVCap-co-TBA molecules.In addition,PPG was found to be a good synergist to improve the performance of PVCap-co-TBA significantly when the corresponding concentration was 1.0% (mass) or so.

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 the National Natural Science Foundation of China (U20B6005) and Hainan Province Science and Technology Special Fund (ZDKJ2021026),which were gratefully acknowledged.