Growth kinetics of hydrate formation from water–hydrocarbon system

Gas hydrates have drawn global attentions in the past decades as potential energy resources. It should be noted that there are a variety of possible applications of hydrate-based technologies, including natural gas storage, gas transportation, separation of gas mixture, and seawater desalination. These applications have been critically challenged by insufficient understanding of hydrate formation kinetics. In this work, the literatures on growth kinetic behaviors of hydrate formation from water-hydrocarbon were systematically reviewed. The hydrate crystal growth, hydrate film growth and macroscopic hydrate formation in water system were reviewed, respectively. Firstly, the hydrate crystal growth was analyzed with respect to different positions, such as gas/liquid interface, liquid–liquid interface and gas–liquid–liquid system. Secondly, experimental and modeling studies on the growth of hydrate film at the interfaces between guest phase and water phase were categorized into two groups of lateral growth and thickening growth considering the differences in growth rates. Thirdly, we summarized the promoters and inhibitors reported (biological or chemical, liquid or solid and hydrophobic or hydrophilic) and analyzed the mechanisms affecting hydrate formation in bulk water system. Knowledge gaps and suggestions for further studies on hydrate formation kinetic behaviors are presented.     

水合物储气促进技术研究进展

气体水合物是一种笼形晶体化合物，单位体积的水合物可包含标准状况下160～180（v/v）的天然气，是一种潜在的固态天然气储运方法，受到广泛关注。由于天然气在水中溶解度小，天然气水合物在纯水中通常难以形成，形成的水合物中天然气含量也不高。为提高水合物储存天然气的密度，提高水合物生长速度，研究者探索了多种促进水合物形成的方法，如物理强化以及热力学与动力学促进剂等化学强化方法。本文总结了搅拌、喷雾、鼓泡等机械方法和向水合体系中添加热力学促进剂、动力学促进剂等方法对水合物形成和储气能力的影响，讨论了这些技术措施影响水合物形成与储气能力的机理。指出表面活性剂与其他促进技术的协同是改善水合物生长和储气密度的有效方法，其复合作用机理有待进一步研究。     

A short review on natural gas hydrate,kinetic hydrate inhibitors and inhibitor synergists

Gas hydrate-caused pipeline plugging is an industrial nuisance for petroleum flow assurance that calls for technological innovations. Traditional thermodynamic inhibitors such as glycols and inorganic salts suffer from high dosing, environmental unfriendliness, corrosiveness, and economical burden. The development and use of kinetic hydrate inhibitors (KHIs), mostly polymeric compounds, with their inhibiting effects on hydrate nucleation and growth are considered an effective and economically viable chemical treatment for hydrate prevention. However, the actual performance of a KHI candidate is dependent on various factors including its chemical structure, molecular weight, spatial configuration, effective concentration, pressure and temperature, evaluation methods, use of other additives, etc. This review provides a short but systematic overview of the fundamentals of natural gas hydrates, the prevailing categories of polymeric kinetic hydrate inhibitors with proposed inhibition mechanisms, and the various synergists studied for boosting the KHI performance. Further research endeavors are in need to unveil the KHI working modes under different conditions. The conjunctive use of KHIs and synergists may facilitate the commercial application of effective KHIs to tackle the hydrate plugging problem in the oil and gas flow assurance practices.     

Dual function inhibitors for methane hydrate

The performance of five imidazolium-based ionic liquids as a new class of gas hydrate inhibitors has been investigated. Their effects on the equilibrium hydrate dissociation curve in a pressure range of 30-110 bar and the induction time of hydrate formation at 114 bar and a high degree of supercooling, i.e., about 25 °C, are measured in a high-pressure micro differential scanning calorimeter. It is found that these ionic liquids, due to their strong electrostatic charges and hydrogen bond with water, could shift the equilibrium hydrate dissociation/stability curve to a lower temperature and, at the same time, retard the hydrate formation by slowing down the hydrate nucleation rate, thus are able to act as both thermodynamic and kinetic inhibitors. This dual function is expected to make this type of inhibitors perform more effectively than the existing inhibitors.     

Synthesis and evaluation of poly (N-vinyl caprolactam)-co-tert-butyl acrylate as kinetic hydrate inhibitor

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 (PVCap-co-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.     

The combination of 1-octyl-3-methylimidazolium tetrafluorborate with TBAB or THF on CO2 hydrate formation and CH4 separation from biogas

[C₈min] BF₄ was used in this work to combine with TBAB or THF for the investigation about thermodynamic and kinetic additives on CO₂ and CH₄/CO₂ hydrates. The results show that[C₈min] BF₄ has the inhibition effect on the equilibrium of hydrate formation. About the kinetic study,[C₈min] BF₄ could improve the rate of CO₂ hydrate formation and increase the gas uptake in hydrate phase. At the same time, the combination of TBAB and[C₈min] BF₄ could increase the mole friction of CH₄ in residual gas comparing with the data in THF solution. CH₄ separation efficiency was strongly enhanced. Since that the size of CO₂ and CH₄ molecules are similar, CH₄ and CO₂ could form the similar hydrate, so the recovery of CH₄ from biogas decreases lightly. The CH₄ content in biogas can purified from 67 mol% to 77 mol% after one-stage hydrate formation. In addition, the combination of THF and[C₈min] BF₄ do not have obvious promoting effect on CH₄ separation comparing with the gas separation results in pure THF solution.     

流动体系天然气水合物生长研究进展

结合国内外研究者的相关研究,系统地总结了流动体系中水合物生长的实验及相关模型的研究进展:水合物在相界面处开始形成、生长和发展;动力学、传质和传热是水合物生长的关键因素;水合物生长的理论模型应用于流动体系中的不多;将动力学/传质/传热三者有机结合起来的模型研究还不成熟。并提出了未来研究的方向及建议。     

Modelling hydrate formation kinetics of a hydrate promoter-water-natural gas system in a semi-batch spray reactor

Hydrate formation kinetic modelling studies reported so far mainly concentrates on pure water-gas systems in stirred-tank batch environments. This work proposes a model for gas hydrate formation kinetics of a hydrate promoter-water-natural gas system in a semi-batch reactor assuming steady-state, isothermal and isobaric conditions. The hydrate formation kinetics was modelled after extending the recent method proposed by Kashchiev and Firoozabadi (J. Crystral Growth 241 (2002a) 220; J. Crystal Growth 243 (2002b) 476; J. Crystal Growth 250 (2003) 499) for a single component gas-water system to a multi-component gas-water-additive system. The extended Kashchiev and Firoozabadi model was applied for a semi-batch spray reactor here for the first time. The hydrate formation experiments were carried out in a pilot plant spray reactor at three different pressure-temperature regimes to determine the actual hydrate formation kinetics in the spray reactor. The experiment results were then used to finetune the adjustable parameters to facilitate accurate model predictions.     

Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter

Methane hydrate equilibrium has been studied upon continuous heating of the water-hydrate-gas system within the temperature range of 275-300 K. This temperature range corresponds to equilibrium pressures of 3.15-55 MPa. The hydrate formation/dissociation experiments were carried out in a high-pressure reactor under isochoric conditions and with no agitation. A small amount of surfactant (0.02 wt% sodium dodecyl sulfate, SDS) was added to water to promote hydrate formation. It was demonstrated that SDS did not have any influence on the gas hydrate equilibrium, but increased drastically both the hydrate formation rate and the amount of water converted into hydrate, when compared with the experiments without surfactant. To understand and clarify the influence of SDS on hydrate formation, macroscopic observations of hydrate growth were carried out using gas propane as hydrate former in a fully transparent reactor. We observed that 10^-3 wt% SDS (230 times less than the Critical Micellar Concentration of SDS) were sufficient to prevent hydrate particles from agglomerating and forming a rigid hydrate film at the liquid-gas interface. In the presence of SDS, hydrates grew mainly on the reactor walls as a porous structure, which sucked the solution due to capillary forces. Hydrates grew with a high rate until about 97 wt% of the water present in the reactor was transformed into hydrate.Our data on methane hydrate equilibrium both confirm already published literature data and complement them within the pressure range of 20-55 MPa.     

水合物法分离混合物技术研究进展

综述了水合物生成法在溶液浓缩和混合气分离两大领域的研究进展。分析表明,水合物法在海水淡化、生物工程、油样分离等溶液分离过程均表现出了一定的应用价值,但仍存在浓缩液夹带、分离效率较低或分离压力高等问题而未能实现工业化应用。针对混合气的分离,与单独水合分离过程相比,包含有水合物生成的吸收-水合和吸附-水合双机制耦合分离技术表现出了气体处理量大、分离效率高、或可实现连续气体分离等优势。但与它们相关的一些基础性问题如水合物浆液的实际流动特性、水合物晶粒与多孔介质之间的作用关系等,需要进一步的研究或确认。在此基础上,对这些分离过程的继续研究给出了参考意见：溶液浓缩分离过程可着眼于开发更有效的水合物生成促进剂或耦合其他分离技术等;混合气分离方面可以在寻找更有效的水合物生成促进剂、明确采用乳液体系分离所得水合物浆液的实际流动特性、揭示采用含水多孔介质分离所得不同相之间的作用机制等方面展开。     

THF hydrate crystal growth inhibition with small anionic organic compounds and their synergistic properties with the kinetic hydrate inhibitor poly(N-vinylcaprolactam)

Small, cationic tetraalkylammonium ions (particularly for alkyl=butyl or pentyl) are known to inhibit tetrahydrofuran (THF) and natural gas hydrate crystal growth and have been used as synergists for commercial kinetic hydrate inhibitor polymers (KHIs), such as N-vinylcaprolactam polymers, for a number of years. The ability for small, organic anionic molecules to inhibit (THF) hydrate crystal growth and their potential as KHI synergists in blends with poly(N-vinylcaprolactam) have been investigated. Several series of sodium alkyl carboxylates, sulphates and sulphonates were synthesised. It was found that none of these molecules were capable of inhibiting THF hydrate crystal growth as well as the best tetraalkylammonium salts. Alkyl carboxylates appeared to be more effective as inhibitors than the sulphonates or sulphates. The most effective anionic THF hydrate crystal growth inhibitors had butyl or pentyl groups, with alkyl branching at the tail (i.e. iso- rather than n-isomers) being advantageous. Anionic carboxylate molecules, particularly with isopentyl or isobutyl groups, showed some kinetic inhibition synergy with poly(N-vinylcaprolactam) lowering the onset and catastrophic hydrate formation temperatures in high pressure (78 bar) constant cooling experiments with Structure II hydrates by 1–2 °C when dosed at 2500 ppm compared with using 2500 ppm polymer alone. This synergism was however less than the best tetraalkylammonium salts (alkyl=n-butyl or n-pentyl) at the same test conditions. Sodium butyl sulphonate and sodium 4-methylpentanoate did not prevent hydrate agglomeration with 3.6% brine and decane at 25% water cut in stirred sapphire cells when dosed at 20,000 ppm based on the aqueous phase, whereas 10,000–20,000 ppm active material of several commercially available anti-agglomerants gave fine transportable slurries and no hydrate deposits at the same conditions.     

Effect of different surfactants on methane hydrate formation rate, stability and storage capacity

The effects of anionic surfactants sodium dodecyl sulfate (SDS) and linear alkyl benzene sulfonate (LABS), cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and non-ionic surfactant ethoxylated nonylphenol (ENP) on the formation, dissociation and storage capacity of methane hydrate have been investigated. Each surfactant was tested with 3 concentrations 300, 500 and 1000 ppm and it has been found that SDS, when prepared with these three concentrations speeds up the hydrate formation rate effectively. LABS increases the hydrate formation rate at 500 and 1000 ppm but decreases it at 300 ppm. CTAB and ENP have promotion effect on hydrate formation rate at 1000 ppm but decrease it at 300 and 500 ppm. Hydrate stability tests have been performed at three temperatures 268.2, 270.2 and 272.2 K with and without surfactant promoters. The results show that all tested additives increase the dissociation rate of methane hydrate below the ice point. CTAB has the minimum and LABS the maximum effect on the methane hydrate dissociation rate. Experimental results on hydrate gas content revealed that maximum storage capacity of 165 V/V is obtained with 1000 ppm of CTAB in water.     

Experimental and Theoretical Investigation of Double Gas Hydrate Formation in the Presence or Absence of Kinetic Inhibitors in a Flow Mini‐Loop Apparatus

Double gas hydrate formation in the presence or absence of kinetic inhibitors in a flow mini‐loop apparatus was investigated. For the prediction of the gas consumption rate during hydrate formation in this system, the rate equation based on the Kashchiev and Firoozabadi model for simple gas hydrate formation in a batch system was developed for double gas hydrate formation in a flow mini‐loop apparatus. To complete the theoretical evaluation of gas hydrate formation through the mini‐loop apparatus in the presence or absence of kinetic hydrate inhibitors (KHI), a laboratory flow mini‐loop apparatus was set up to measure the induction time for hydrate formation and the uptake rate when a gaseous mixture (such as 75 % C1/25 % C3, 25 % C1/75 % C3, 75 % C1/25 % i‐C4, and 25 % C1/75 % i‐C4) is contacted with water containing or not containing dissolved inhibitor under suitable temperature and pressure conditions. In each experiment, a water blend saturated with gas mixture was circulated up to the required pressure. The pressure was maintained at a constant value during the experimental runs by means of a required gas mixture make‐up. The effect of pressure on gas consumption during hydrate formation was investigated in the presence or absence of polyvinylpyrrolidone (PVP) and L ‐tyrosine as kinetic inhibitors at various concentrations. A good agreement was found between the predicted and experimental data in the presence or absence of KHI. The total average absolute deviation percents between the experimental and predicted values of gas consumption were found to be 16.4 and 17.5 % for the double gas hydrate formation in the presence or absence of the kinetic inhibitors, respectively.     

In situ injection of THF to trigger gas hydrate crystallization: Application to the evaluation of a kinetic hydrate promoter

This paper investigates an original method to efficiently trigger gas hydrate crystallization. This method consists of an in situ injection of a small amount of THF into an aqueous phase in contact with a gas-hydrate-former phase at pressure and temperature conditions inside the hydrate metastable zone. In the presence of a CO₂–CH₄ gas mixture, our results show that the THF injection induces immediate crystallization of a first hydrate containing THF. This triggers the formation of the CO₂–CH₄ binary hydrate as proven by the pressure and temperature reached at equilibrium. This experimental method, which “cancels out” the stochasticity of the hydrate crystallization, was used to evaluate the effect of the anionic surfactant SDS at different concentrations, on the formation kinetics of the CO₂–CH₄ hydrate. The results are discussed and compared with those published in a recent article (Ricaurte et al., 2013), where THF was not injected but present in the aqueous phase from the beginning and at much higher concentrations.     

Development and application of gas hydrate reservoir simulator based on depressurizing mechanism

Natural gas hydrates are known to occur in vast quantities at the ocean floor or in permafrost regions. In-situ hydrate contains great volumes of methane gas, which indicates a potential future energy resource. In this study, we have developed a three-dimensional, multi-phase (gas, water, and hydrate) flow finite-difference model by using implicit pressure explicit saturation technique in order to investigate simultaneous flow through ice-liked hydrate reservoir. The developed model is based on the depressurizing method as producing mechanism. The model evaluates local gas generation dissociated from the hydrate with the aid of kinetic dissociation theory proposed by Kim-Bishnoi. The computation of kinetic dissociation uses the empirical dissociation rate as a function of specific surface area between phases and pressure difference. With the developed model, a one-dimensional system has been simulated for analyzing the production performance of a hydrate reservoir and for investigating the effect of hydrate saturation on absolute permeability and relative permeability characteristics. Also, for the three-dimensional field-scaled reservoir system, a number of numerical exercises have been conducted to understand the effect of mass transfer and to characterize the flowing mechanism under the conditions of increased permeability resulting from the dissociation hydrate.     

Kinetic Modeling of Methane Hydrate Formation in the Presence of Low‐Dosage Water‐Soluble Ionic Liquids

The kinetic and thermodynamic effects of three typical low‐dosage imidazolium‐based ionic liquids (ILs) on methane hydrate formation and dissociation were investigated, considering the anion nature and subcooling and/or overpressure driving forces. Isochoric hydrate formation and dissociation data were obtained by the modified slow step‐heating method. ILs proved to have a dual effect on both formation and dissociation of methane hydrate including thermodynamic and kinetic inhibition. Kinetic modeling of methane hydrate inhibition by low‐dosage ILs was performed. Kinetic analysis showed that IL inhibitors mainly cause a delay in the nucleation or hydrate growth step. The related inhibition mechanism was resolved regarding the ionic nature and electrostatic interactions of ILs with water molecules. Two binomial exponential kinetic relations were derived and used for simple methane hydrate formation in the presence of ILs as kinetic hydrate inhibitors. The proposed relations can serve for a quick estimation of the nature, extent, strength, and effectiveness of ILs on various gas hydrates.     

凝析气井井筒水合物生成位置预测

凝析气井井筒水合物是生产过程中经常遇到的问题,本文针对某凝析气井,对水合物的形成条件进行了论述,采用水合物P-T图回归公式法对该凝析气井在不同压力条件下水合物形成的温度进行了预测,计算出了该井在不同产量条件下的井筒温度分布和压力分布,根据计算结果得知：产气量为11.98×10^4m^3/d时水合物生成温度曲线与井筒温度曲线在距离井口175m处相交,此时温度为22.8℃。若存在自由水,则从此深度到井口的油管段形成水合物。针对该凝析气井情况,建议采用加注甲醇（己二醇）和地面加热升温措施以防止水合物的形成。     

Study of the Kinetics and Morphology of Gas Hydrate Formation

The kinetics and morphology of ethane hydrate formation were studied in a batch type reactor at a temperature of ca. 270–280 K, over a pressure range of 8.83–16.67 bar. The results of the experiments revealed that the formation kinetics were dependant on pressure, temperature, degree of supercooling, and stirring rate. Regardless of the saturation state, the primary nucleation always took place in the bulk of the water and the phase transition was always initiated at the surface of the vortex (gas‐water interface). The rate of hydrate formation was observed to increase with an increase in pressure. The effect of stirring rate on nucleation and growth was emphasized in great detail. The experiments were performed at various stirring rates of 110–190 rpm. Higher rates of formation of gas hydrate were recorded at faster stirring rates. The appearance of nuclei and their subsequent growth at the interface, for different stirring rates, was explained by the proposed conceptual model of mass transfer resistances. The patterns of gas consumption rates, with changing rpm, have been visualized as due to a critical level of gas molecules in the immediate vicinity of the growing hydrate particle. Nucleation and decomposition gave a cyclic hysteresis‐like phenomena. It was also observed that a change in pressure had a much greater effect on the rate of decomposition than it did on the formation rate. Morphological studies revealed that the ethane hydrate resembles thread or is cotton‐like in appearance. The rate of gas consumption during nucleation, with different rpm and pressures, and the percentage decomposition at different pressures, were explained precisely for ethane hydrate.     

Investigation on hydrate growth at the oil–water interface: In the presence of asphaltene

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.     

Phase equilibria and kinetic behavior of co<Subscript>2</Subscript> hydrate in electrolyte and porous media solutions: application to ocean sequestration of CO<Subscript>2</Subscript>

Understanding the phase behavior and formation kinetics of CO₂ hydrate is essential for developing the sequestration process of CO₂ into the deep ocean and its feasibility. Three-phase equilibria of solid hydrate, liquid water, and vapor were determined for aqueous mixtures containing CO₂ and NaCl/clay to examine the effect of both ocean electrolytes and sediments on hydrate stability. Due to the capillary effect by clay pores and inhibition effect by NaCl the corresponding hydrate formation pressure appeared to be a little higher than that required for simple and pure hydrate at specified temperature. In addition, the hydrate formation kinetics of carbon dioxide in pure water and aqueous NaCl solutions with or without clay mineral were also measured at various conditions. The formation kinetic behavior was found to be strongly influenced by pressure, temperature and electrolyte concentration. A simplified kinetic model having two adjustable parameters was proposed and the estimated results agreed well with the experimental data. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.