A water droplet size distribution dependent modeling of hydrate formation in water/oil emulsion

Experimental data on chord length distributions and growth rate during methane hydrate formation in water‐in‐oil emulsions were obtained in a high pressure stirring reactor using focused beam reflectance measurement and particle video microscope. The experiments were carried out at 274.2 K for 10–30% water cuts and agitation rates ranging from 200 to 500 rpm initially at 7.72 MPa. Rapid growth was accompanied by gradually decrease in rate. Free water was observed to become depleted during rapid growth while some water remained encapsulated inside hydrate layers constituting a mass transfer barrier. The apparent kinetic constants of methane hydrate formation and free‐water fractions were determined using a newly developed kinetic model independent of the dissolution rate at the gas–oil interface. It was illustrated that continued growth depends on distribution and transfer of water in oil‐dominated systems. This perception accords with observations of hydrate film growth on suspended water droplet in oil and clarifies transfer limits in kinetics. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1010–1023, 2017     

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.     

Experimental study of hydrate formation in oil–water systems using a high-pressure visual autoclave

To investigate the characteristics of hydrate formation in oil–water systems, a high-pressure visual autoclave equipped with visual windows was used where a series of hydrate formation experiments were performed from natural gas + diesel oil + water systems at different water cuts (30 and 70%), rotation rates (100, 200, 300 r/min) and thermodynamic conditions (temperature, pressure). According to the temperature and pressure profiles in test experiments, the processes of hydrate formation under two kinds of experimental procedures were analyzed first. Then, based on the experimental phenomenon observed through the visual windows, hydrate morphologies and hydrate morphological evolvements throughout the experiments were mainly investigated. In experiments, the growth and annealing of hydrate films on the wall, the agglomeration and deposition of hydrate coated water droplets, flocculent-like hydrate deposition with water trapped in and the Pickering effect of hydrates were identified. Simultaneously, based on the experimental data of thermodynamic parameters, the kinetics of hydrate formation was studied by calculating the variations of hydrate film area and gas consumption in different experiments. In addition, the influences of temperature, pressure, and rotation rate on hydrate morphologies, hydrate morphological evolvements, and hydrate formation kinetics were also focused on.     

Calorimetric investigation of cyclopentane hydrate formation in an emulsion

Differential scanning calorimetry (DSC) is applied to investigate the formation of cyclopentane hydrates in a water-in-oil emulsion. Protocols of cooling below the ice formation temperature and warming to a temperature above the ice and hydrate melting temperatures are applied. Cyclopentane, which forms hydrates at atmospheric pressure, is a component of the continuous oil phase in the hydrate-forming emulsion and is replaced by iso-octane to obtain a comparable ice-forming emulsion. A method based on comparing the heat flow measured by DSC for samples of identically prepared hydrate-forming and non-hydrate (ice-forming) emulsions is developed to obtain the rate of cyclopentane hydrate growth. Results are reported for a 40% water volume fraction emulsion. Experimental results lead to the conclusion that the hydrate formation takes place primarily at the interface between water drops and the continuous oil phase. In the absence of surfactants, a robust hydrate “shell” develops around the water drop limiting transport of hydrate former to the free water which remains trapped inside the hydrate layer. Direct visualization of hydrate formation in larger water drops under the influence of oil-soluble surfactants shows that the hydrate crystals have much smaller features and the appearance is hairy or mushy. A three-step mechanism – nucleation, surface growth and radial growth – is described to capture the main features of the hydrate formation process. Mechanical stresses developed in the hydrate shell due to volume expansion upon hydrate formation (a liquid–solid transition) are analyzed.     

天然气水合物形成与生长影响因素综述

天然气水合物(NGH)是水分子和天然气分子形成的一种复杂的笼型晶体,其在油气管道输送、天然气储存和制冷等行业中都具有重要的研究意义和利用价值,但天然气水合物的形成是一个多组分、多阶段的复杂过程,不同因素对于天然气水合物形成和生长的影响尚有待明确。本文介绍了天然气水合物形成的物理过程以及水合物成核的3种机理假说;详细梳理了基质两亲性、添加剂、多孔介质环境和杂质、液体组成、温度压力以及流动条件等因素对于天然气水合物形成和生长的影响,并对其作了简要分析。同时指出,原油组成对于水合物抑制效果的定量化、蜡晶结构对于水合物形成过程中传质和传热的影响以及微观化的动力学抑制剂抑制机理等都是水合物相关研究中需要进一步深入探究和明确的问题。     

油水体系内水合物的生成:温度、压力和搅拌速率影响

水合物在管道内的生成对流动安全保障构成了极大威胁。为研究水合物在油水体系内的生成特性,本文以天然气、柴油、水为实验介质,在高压可视反应釜内开展了一系列不同温度、压力和搅拌速率的水合物生成实验。根据测试实验中温度、压力的变化趋势,首先分析了两种不同实验步骤下水合物的生成过程。然后,基于从反应釜可视窗处观察到的实验现象,研究了温度、压力和搅拌速率对水合物生成和分布位置、水合物生成形态及水合物形态演化过程的影响。实验中,可以观察到水合物的聚集、沉积和壁面膜生长现象。同时,实验还研究了温度、压力和搅拌转速对诱导时间、壁面水合物膜生长速率及气体消耗速率等水合物生成动力学参数的影响。本文研究成果可为油气管道水合物防治技术的发展提供理论支持。     

Quantification of the risk for hydrate formation during cool down in a dispersed oil-water system

Gas hydrates are considered a nuisance in the flow assurance of oil and gas production since they can block the flowlines, consequently leading to significant losses in production. Hydrate avoidance has been the traditional approach, but recently, hydrate management is gaining acceptance because the practice of hydrate avoidance has become more and more challenging. For better management of hydrate formation, we investigated the risk of hydrate formation based on the subcooling range in which hydrates form by associating low, medium, and high probability of formation for a gas+oil+water system. The results are based on batch experiments which were performed in an autoclave cell using a mixture gas (CH₄: C₃H₈=91.9 : 8.1 mol%), total liquid volume (200 ml), mineral oil, watercut (30%), and mixing speed (300 rpm). From the measurements of survival curves showing the minimum subcooling required before hydrate can form and hydrate conversion rates for the initial 20 minutes, we developed a risk map for hydrate formation.     

Direct conversion of water droplets to methane hydrate in crude oil

Water droplets suspended in a crude oil were converted to methane hydrate by pressurization in an autoclave cell. Droplet size distributions were monitored using a focused beam reflectance method (FBRM) particle size analyzer as the water converted to hydrate. The droplet size distribution did not change significantly during conversion of nearly all the water to hydrate. The preservation of the distribution during conversion indicates that water droplets act as individual reactors and supports a hydrate shell formation model. Water droplet size distributions were measured with the FBRM probe at multiple shear rates in four crude oils (Albacora Leste, Conroe, Petronius, and a West African oil) with various surface tensions and viscosities. The water droplet size distributions, and thus hydrate particle distributions, were found to be lognormal with breadth increasing with mean. A correlation model has been developed to predict the entire size distribution of water droplets in these oils as a function of viscosity, interfacial tension, and shear rate. The model has been extended to represent gas hydrate particle size distributions in oil after conversion.     

Hydrate formation in oil–water systems: Investigations of the influences of water cut and anti-agglomerant

To investigate the characteristics of hydrate formation in oil–water systems, a high-pressure cell equipped with visual windows was used where a series of hydrate formation experiments were performed from natural gas + diesel oil + water systems at different water cuts and anti-agglomerant concentrations. According to the temperature and pressure profiles in test experiments, the processes of hydrate formation under two kinds of experimental procedures were analyzed first. Then, based on the experimental phenomena observed through the visual windows, the influences of water cut and anti-agglomerant on the places of hydrate formation and distribution, hydrate morphologies and hydrate morphological evolvements were investigated. Hydrate agglomeration, hydrate deposition and hydrate film growth on the wall were observed in experiments. Furthermore, three different mechanisms for hydrate film growth on the wall were identified. In addition, the influences of water cut and anti-agglomerant on the induction time of hydrate formation were also studied.     

Investigation on the effect of oxalic acid,succinic acid and aspartic acid on the gas hydrate formation kinetics

Gas hydrate reserves are potential source of clean energy having low molecular weight hydrocarbons trapped in water cages. In this work, we report how organic compounds of different chain lengths and hydrophilicities when used in small concentration may modify hydrate growth and either act as hydrate inhibitors or promoters. Hydrate promoters foster the hydrate growth kinetics and are used in novel applications such as methane storage as solidified natural gas, desalination of sea water and gas separation. On the other hand, gas hydrate inhibitors are used in oil and gas pipelines to alter the rate at which gas hydrate nucleates and grows. Inhibitors such as methanol and ethanol which form strong hydrogen bond with water have been traditionally used as hydrate inhibitors. However, due to relatively high volatility a significant portion of these inhibitors ends up in gas stream and brings further complexity to the safe transportation of natural gas. In this study, organic additives such as oxalic acid, succinic acid and L-aspartic acid (all three) having—COOH group(s) with aspartic acid having an additional—NH₂ group, are investigated for gas hydrate promotion/inhibition behavior. These compounds are polar in nature and thus have significant solubility in liquid water; the presence of weak acidic and water loving (carboxylic/amine groups) moieties makes these organic acids an excellent candidate for further study. This study would pave ways to identify a novel(read better) promoter/inhibitor for gas hydrate formation. Suitable thermodynamic conditions were generated in a stirred tank reactor coupled with cooling system; comparison of gas hydrate formation kinetics with and without additives were carried out to identify the effect of these acids on the formation and growth of hydrates. The possible mechanisms by which these additives inhibit or promote the hydrate growth are also discussed.     

油气管道水合物堵塞机理研究进展

国内外对油气管道水合物堵塞机理的实验研究虽然较多,但一直缺少系统的总结归纳。本文根据水合物堵塞实验开展条件的不同,首先将油气管输体系分为油基体系、水基体系（纯水体系及水主导体系）、气主导体系和部分分散体系。文章分析表明,管道水合物堵塞机理众多,具体包括水合物的聚集和沉积、水合物大量聚集阻塞管道流通截面及油水相分离等。其中,水合物的聚集和沉积是油基体系水合物堵塞的主要机理,水合物颗粒的着床沉积是纯水体系水合物堵塞的主要机理,水合物大量聚集阻塞管道流通截面则是气主导体系水合物堵塞的主要机理。水主导体系和部分分散体系的水合物堵塞机理,目前尚无统一定论,有待进一步深入研究。文章指出对环状流液滴分布、油水分散状态、乳状液稳定性及未乳化自由水层等的量化研究则是未来水合物堵塞机理的研究重点。     

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.     

四氢呋喃水合物浆液黏度影响因素敏感性分析

在深水油气开发过程中,注入水合物防聚剂可以防止水合物聚积,使其以水合物浆液的形式输送,是一种很好的防治水合物替代方法。文中以四氢呋喃、-10号柴油、渤海JZ20-2凝析油和水为工质进行水合物浆液实验,测量了不同实验条件下四氢呋喃水合物浆液的黏度。着重归纳了初始含水质量分数、剪切率、油相黏度等因素对水合物浆液黏度的影响规律,并利用多因素分析法分析了各个影响因素的敏感性大小。对今后水合物浆液混输管线的设计与安全运行具有较好的指导意义。     

Crystallization Mechanisms and Rates of Cyclopentane Hydrates Formation in Brine

Clathrate hydrates most often grow at the interface between liquid water and another fluid phase (hydrocarbon) acting as a provider for the hydrate guest molecules, and some transfer through this shell is required for the hydrate growth to proceed, thus self‐limiting the reaction rate. An optical microscope and a horizontal reaction cell are utilized to capture the shell growth phenomenology and to estimate the hydrate layer growth rates from sequential pictures. Cyclopentane (CP) is chosen as the hydrate‐forming molecule to obtain hydrates at low pressure. Experimental hydrate layer growth rates are provided for the CP+brine system, using various combinations of salts and degrees of subcooling.     

石英砂中甲烷水合物的溶解开采实验研究

天然气水合物是天然气与水在低温高压的条件下形成的一种冰状物质,广泛分布于海底和冻土区的沉积物中,资源量巨大,有望成为未来接替能源。在已发现的资源中,有一种类型的天然气水合物位于海底浅表层或裸露于海底,其形成过程和稳定性规律尚不明确。为揭示其稳定性规律,实验研究了石英砂中甲烷水合物的溶解过程。结果表明,水和白油均能有效溶解石英砂中的甲烷水合物,注水溶解的气水体积比约为2,注油溶解的气液体积比约为10,溶解速率主要受液流-水合物的接触情况影响,随水合物饱和度升高而升高。水/油易在石英砂中窜进,形成优势渗流通道,随后气液比逐渐降低。实验结果为深入研究海底浅表层或裸露的天然气水合物的稳定机理提供了基础。     

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.     

多相流管输体系中水合物沉积机理研究进展

国内外对多相流管输体系中水合物沉积的研究虽然很多，但水合物沉积机理仍有待进一步研究。本文根据水合物沉积实验开展条件的不同，将多相流管输体系分为气体主导体系、油基体系、部分分散体系、水主导体系，总结了各体系的水合物沉积的主要机理，并提出了未来的发展方向。管输体系中水合物沉积机理包括水润湿沉积表面、水合物颗粒聚并、水合物的管壁膜生长、水合物颗粒的管壁粘附和水合物的颗粒着床沉积等。大多数学者认为：水合物的管壁膜生长是气体主导体系水合物沉积的主要机理；油基体系水合物沉积的主要机理是水合物颗粒的着床沉积；而部分分散体系和水主导体系的水合物沉积机理尚无统一定论，需进一步研究。多相流管输体系中水合物沉积研究未来的发展方向如下。①搭建全透明的流动环路，观测水合物在管路内实际的形成过程及沉积过程，对水合物沉积机理进行深入研究。②量化研究油水分层、油包水(或水包油)乳状液、自由水层对水合物沉积、堵塞的影响。③对于气体主导体系，除环状流和分层流外，有必要对段塞流、气泡流等其他常见的流型下沉积机理进行研究，重点在于开发一个综合模型来描述水合物沉积过程。④对于水主导体系，水合物形成过程出现的油水破乳的具体机理应是未来水合物沉积过程进行定量研究的方向。⑤国内外对垂直管、弯管及管阀件处水合物沉积堵塞理论研究较少，未来应着重这方面。     

Molecular dynamics study on growth of carbon dioxide and methane hydrate from a seed crystal

In the current work, molecular dynamics simulation is employed to understand the intrinsic growth of carbon dioxide and methane hydrate starting from a seed crystal of methane and carbon dioxide respectively. This comparison was carried out because it has relevance to the recovery of methane gas from natural gas hydrate reservoirs by simultaneously sequestering a greenhouse gas like CO₂. The seed crystal of carbon dioxide and methane hydrate was allowed to grow from a super-saturated mixture of carbon dioxide or methane molecules in water respectively. Two different concentrations (1:6 and 1:8.5) of CO₂/CH₄ molecules per water molecule were chosen based on gas–water composition in hydrate phase. The molecular level growth as a function of time was investigated by all atomistic molecular dynamics simulation under suitable temperature and pressure range which was well above the hydrate stability zone to ensure significantly faster growth kinetics. The concentration of CO₂ molecules in water played a significant role in growth kinetics, and it was observed that maximizing the CO₂ concentration in the aqueous phase may not result in faster growth of CO₂ hydrate. On the contrary, methane hydrate growth was independent of methane molecule concentration in the aqueous phase. We have validated our results by performing experimental work on carbon dioxide hydrate where it was seen that under conditions appropriate for liquid CO₂, the growth for carbon dioxide hydrate was very slow in the beginning.     

Methane-ethane and methane-propane hydrate formation and decomposition on water droplets

Gas hydrate formation and decomposition on water droplets using an 89.4% methane—10.6% ethane mixture, and a 90.1% methane—9.9% propane mixture were carried out in a new apparatus suitable for morphology studies. As expected the induction time was found to be much shorter when the water had hydrate memory. All droplets nucleated simultaneously and the droplet size and shape had no noticeable effect on induction time and macroscopic crystal growth morphology for hydrates from the methane-ethane mixture. However, the surface of the hydrate crystals from methane-propane had a “hairy-like” appearance which changed to a smooth surface over time. Moreover, the smaller droplets during hydrate reformation showed an extensive hydrate growth and looked like snow-flakes. Sequential pictures generated by time-lapse videos showed that the time required for hydrate to cover the water droplet surface ranged from 10 to 23 s and was shorter when there was gas-phase agitation (mixing). The growth is postulated to occur in two stages. The first stage lasts about 10-23 s and growth takes place laterally. Growth takes place at the hydrate/gas and the hydrate/water interfaces during the second stage. The implication of the findings for process design of hydrate formation vessels is also discussed.     

The initial stage of hydrate formation in liquid volume on impurity particles upon contact of gas and water

A concept and appropriate theoretical construction have been proposed to describe initial stage of hydrate layer formation at the interface between water and hydrate-forming gas. The model presented indicates that this stage (or induction period) is accompanied by the dissolution of gas in water, as well as the formation and growth of hydrate in the bulk zone on impurity particles near the contact boundary. An analytical solution was obtained for the characteristic time during which the volume content of the hydrate phase at the contact boundary reaches one and, thus, nuclei form as a film prior to a hydrate layer at the gas–water boundary. This characteristic time is accepted as the induction time. According to the obtained formula, the induction period depends inversely on static pressure and in inverse two-thirds proportion on the number of impurity particles per unit volume of water. The problem of the formation and growth of hydrate at the interface between the hydrate layer and aqueous gas solution has been considered and solved. The temperature fields caused by heat generated during hydrate formation on the contact surface of hydrate massif and gas solution are analyzed.