Is subcooling the right driving force for testing low-dosage hydrate inhibitors?

The degree of subcooling is usually used as the driving force for hydrate formation; however, it does not encompass the effect of pressure. A comprehensive driving force for hydrate formation is a function of pressure, temperature, and gas composition; however, its calculation is not as simple as that of subcooling. In this work, by application of the two latest driving force expressions for hydrate formation, the relationships between subcooling and the true driving force at different conditions for pure gas-water and natural gas-water systems are analysed. The effect of pressure on the induction time in the presence and absence of a kinetic inhibitor have been tested at similar degrees of subcooling.The results show that for pure gas-water systems subcooling is proportional to the driving force, with a good approximation over a wide pressure range at isothermal conditions. However, for multicomponent systems (e.g., natural gases), the driving force is more than that suggested by subcooling at some pressures. Changes of driving force with pressure at a constant degree of subcooling for the above systems have been presented. The results show that the pressure has no significant effect on the driving force (at a constant degree of subcooling) above a certain pressure range. The experimental results show that in a natural gas-water system at constant degree of subcooling the induction time is not significantly affected by pressure. However, in the presence of the kinetic inhibitor tested in this study, high-pressure conditions decreased the induction time.     

Studies of hydrate nucleation with high pressure differential scanning calorimetry

Current models for hydrate formation in subsea pipelines require an arbitrary assignment of a subcooling criterion for nucleation. In reality hydrate nucleation times depend on both the degree of subcooling and the amount of time the fluid has been subcooled. In this work, differential scanning calorimetry was applied to study hydrate nucleation for gas phase hydrate formers. Temperature ramping and isothermal approaches were combined to explore the probability of hydrate nucleation for both methane and xenon. A system-dependent subcooling of around 30 K was necessary for hydrate nucleation from both guest molecules. In both systems, hydrate nucleation occurred over a narrow temperature range (2-3 K). The system pressure had a large effect on the hydrate nucleation temperature but the ice nucleation temperature was not affected over the range of pressures investigated (3-20 MPa). Cooling rates in the range of (0.5-3 K/min) did not have any statistically significant effect on the nucleation temperature for each pressure investigated. In the isothermal experiments, the time required for nucleation decreased with increased subcooling.     

Methane Hydrate Formation and Dissociation in the presence of Bentonite Clay Suspension

The present work reports the effect of bentonite clay on methane hydrate formation and dissociation in synthetic seawater of salinity 3.55 % of total dissolved salts. Extensive observations of pressure‐temperature equilibrium during formation and decomposition of methane hydrate under different conditions have been made. It is observed that phase equilibrium conditions of hydrate are affected on changing the concentration of bentonite clay in synthetic seawater. Induction time for hydrate nucleation has been measured under different concentrations of clay and subcooling conditions. The presence of bentonite clay in synthetic seawater reduces the induction time of hydrate formation. Enthalpy of hydrate dissociation is calculated by Clausius‐Clapeyron equation using measured phase equilibrium data. The amount of gas consumed during hydrate formation has been calculated using real gas equation. It is found that a larger amount of gas is consumed upon addition of bentonite clay in synthetic seawater.     

Study on methane hydrate formation using ultrasonic waves

Methane hydrate is a kind of gas hydrate and has the crystal structure I. 1 m³ of methane hydrate can be decomposed to a maximum of 172 m³ of methane gas in standard conditions. If this characteristic of methane hydrate is reversely utilized, natural gas, which mainly consists of methane gas, is fixed into water in the form of hydrate solid. However, when methane hydrate is formed artificially by simply reversing its process of natural generation, the amount of methane gas consumed owing to hydrate formation is fairly low, which would be problematic for its massive synthesis and application. In this study, experiments are carried out with the goal of increasing the amount of gas consumed by using ultrasonic waves. The power for maximum gas consumption was observed at 150 W, and the amount of gas consumed was four times higher than that at 0 W at the subcooling temperature of 0.5 K. The ultrasonic waves are more effective at the subcooling temperature of 5.7 K than at the subcooling temperature of 0.5 K, and are another effective method for enhancing methane hydrate formation and reducing the hydrate formation time.     

丁二酸二异辛酯磺酸钠对甲烷水合物生长动力学特性的影响

通过改变添加量（600mg/L、900mg/L、1200mg/L）、过冷度（3.5℃、5.5℃、7.5℃）以及压力（4.90MPa、6.0MPa、7.31MPa）的方式,考察了在静态体系下绿色促进剂丁二酸二异辛酯磺酸钠（AOT）对甲烷水合物生长动力学特性的影响。实验结果表明,3种浓度下AOT均能够有效缩短诱导时间,并且浓度越大,诱导时间越小（1200mg/L时为0.21h）,但储气量随着添加量的增加,先增大后减小,最终确定最佳添加量为900mg/L,水合物储气量为55.76m³/m³;另外,过冷度越大,实验压力越高,水合物成核速度越快,诱导时间越短,耗气速率越高。当过冷度为7.5℃时,诱导时间最小为0.31h,耗气速率最大为0.275mol/h,储气量最大为63.95m³/m³;但压力过大,釜内气液界面会快速生成水合物层,阻碍水合物继续生成,导致水合物储气量减少为46.84m³/m³。所以,在静态体系下,合理选择促进剂的浓度和驱动力的大小,可显著促进水合物生成。     

Catalysis of Gas Hydrates by Biosurfactants in Seawater‐Saturated Sand/Clay

Biosurfactants catalyzed natural gas hydrate formation in sand/clay packs saturated with seawater. Representative samples from the five possible biosurfactant classifications enhanced hydrate formation rate and decreased hydrate induction time. Biosurfactants increased rates 96% to 288% and decreased induction times 20% to 71% relative to the control. Micellar‐forming rhamnolipid reached a critical micellar concentration at 13 ppm at hydrate‐forming conditions; these micelles migrated readily through a seawater‐saturated sand pack to catalyze hydrate formation in another zone. The type of biosurfactant, in conjunction with specific porous media, help determine massive, dispersed, nodular, or stratified forms of hydrates. Results suggested that minimal microbial activity in ocean‐floor sands can greatly influence gas hydrate formation.     

Experimental Study on the Influence of Hydrogel on CO2 Hydrate Formation

The influence of two differently cross‐linked polyacrylate particles on CO₂ hydrate formation was investigated. A series of up‐scaling experiments from small (high‐pressure differential scanning calorimetry, HP‐DSC) over medium (glass reactor) to large scale (HP‐reactor) was carried out. It was found out that there is a low influence on the induction time, which is an essential key parameter of the hydrate formation. The results show the same trends: with a low degree of cross‐linker used in low concentration CO₂ hydrate formation could be enhanced.     

Prediction of induction time for natural gas components during gas hydrate formation in the presence of kinetic hydrate inhibitors in a flow mini‐loop apparatus

The objective of this work is the prediction of induction time (t_i) for simple gas hydrate formation in the presence or absence of kinetic hydrate inhibitors at various conditions based on the Kashchiev and Firoozabadi model in a flow mini‐loop apparatus. For this purpose, the t_i model is developed for simple gas hydrate formation in batch system for natural gas components during hydrate formation in a flow mini‐loop apparatus. A laboratory flow mini‐loop apparatus is designed and built up to measure the t_i for simple gas hydrate formation when a hydrate former (such as C₁, C₃, CO₂ and i‐C₄) is contacted with water in the absence or presence of dissolved inhibitor, such as poly vinylpyrrolidone, PVCap and L ‐tyrosine. In each experiment, a water blend saturated with pure gas is circulated up to a required pressure. Pressure is maintained at a constant value during experimental runs by means of the required gas make‐up. The average absolute deviation (AAD) of the predicted t_i values from the corresponding experimental data are found to be about 11% and 9.4% for gas hydrate formation t_i in the presence or absence of kinetic hydrate inhibitors, respectively. © 2012 Canadian Society for Chemical Engineering     

Initial thickness measurements and insights into crystal growth of methane hydrate film

The initial thickness of methane hydrate film was directly measured by suspending a single methane bubble in water at 274.0, 276.0, and 278.0 K. The results show that the initial hydrate film thickness decreases from tens of micrometers to about 10 µm with the subcooling increased from 0.5 K to about 3 K. When subcooling is higher than 1.0 K, all initial film thickness data measured under different temperatures vary inversely with the subcooling. Notable three‐dimensional growths of hydrate crystals of different sizes and shapes at film front and emergence of new crystal were clearly observed at lower subcooling that resulting in the rougher surface of hydrate film and uncertainty of initial thickness measurement under lower subcooling. The hydrate film growth was dominated by film growth in thickness, not by lateral growth at low subcooling. The growth in thickness of hydrate shell covering one whole bubble surface was also investigated. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2145–2154, 2013     

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(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 hy...     

Characteristics of methane hydrate formation in carbon nanofluids

When methane hydrate is formed artificially by simply reversing its process of natural generation, the amount of methane gas consumed by hydrate formation is fairly low, which is problematic for its large scale synthesis and application. Therefore, this study examined methods for increasing the amount of gas consumed by adding MWCNTs (multi-walled carbon nanotubes) and OMWCNTs (oxidized multi-walled carbon nanotubes). The surfaces of the MWCNTs were oxidized chemically, and dispersed uniformly in distilled water after a dispersion operation. The amount of methane gas consumed during the formation of methane hydrate in the oxidized carbon nanofluid was approximately 4.5 times higher than that in distilled water. The hydrate-nanocarbon fluid phase boundary line was shifted to the right side of the hydrate-pure water phase boundary line in the pressure–temperature phase diagram. The carbon nanofluid system accelerated the rate of methane hydrate formation at low subcooling temperatures (<8 K).     

An investigation into the laboratory method for the evaluation of the performance of kinetic hydrate inhibitors using superheated gas hydrates

Kinetic hydrate inhibitors (KHIs) are used to prevent gas hydrate formation in gas and oilfield operations. Recently, a new KHI test method was reported in which hydrates are formed and re-melted just above the equilibrium temperature, before the fluids are re-cooled and the performance of the chemical as a KHI is determined. The method, which we have called the superheated hydrate test method, is claimed to be more reliable for KHI ranking in small equipment, giving less scattering in the hold time data due to avoiding the stochastic nature of the first hydrate formation. We have independently investigated this superheated hydrate test method in steel and sapphire autoclave tests using a gas mixture forming Structure II hydrates and a liquid hydrocarbon phase, which was necessary for satisfactory results. Our results indicate that hold times are shorter than using non-superheated hydrate test methods, but they are more reproducible with less scattering. The reduced scattering occurs in isothermal or slow ramping experiments even when the hydrates are melted at more than 10 °C above the equilibrium temperature (T_eq). However, if a rapid cooling method is used, the improved reproducibility is retained when melting hydrate at 2.4 °C above T_eq but lost when warming to 8.4 °C above T_eq. Using the ramping test method, most, but not all the KHIs tested agreed with the same performance ranking obtained using traditional non-superheated hydrate test methods. This may be related to the variation in the dissociation temperature of gas hydrates with different KHIs and different KHI inhibition mechanisms. Results also varied between different size autoclave equipments.     

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.     

过冷壁面液滴中四丁基溴化铵水合物生成的可视化研究

使用高速摄影技术对2 μl液滴中四丁基溴化铵（TBAB）水合物晶体的成核与生长进行了实验研究。对不同过冷度与不同浓度（10%,20%,30%,质量分数）的液滴中TBAB水合物晶体的生长特性进行了分析并建立了相应的数学模型,推导出TBAB水合物形成活化能E_a为-14.99 kJ/mol。研究结果表明,通过以液滴滴落过冷固体表面的方式可以有效缩短水合物成核的诱导时间,促进水合物的快速生成。为解决水合物在工业中大规模应用的难题提供了新方法。     

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.     

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.     

Consolidation of sand formation using freon'll gas hydrate

Freon 11 gas hydrate was used to block the pores of four size ranges of sand from 24 mesh to 60 mesh. A 50.8 mm deep bed of sand when thus “frozen” with hydrate could sustain a dfferential water pressure of at least 6895 kPa. A subcooling of about 5 to 6°C below its thermodynamic formation temperature was required to cause the hydrate to form such a plug. Once formed, the hydrate remained stable at temperatures up to its decomposition temperature. The time required for the hydrate crystal to grow to a size large enough to block the pores of the bed was about two hours. The amount of hydrate forming agent required to block the sand pores was found to be approximately that calculated from the ideal composition of the hydrate.     

An investigation on gas hydrate formation and slurry viscosity in the presence of wax crystals

Clarifying the interaction effect between hydrate and wax is of great significance to guarantee operation safety in deep water petroleum fields. Experiments in a high‐pressure hydrate slurry rheological measurement system were carried out to investigate hydrate formation and slurry viscosity in the presence of wax crystals. Results indicate that the presence of wax crystals can prolong hydrate nucleation induction time, and its influence on hydrate growth depends on multiple factors. Higher stirring rate can obviously promote hydrate growth rate, while its influence on hydrate nucleation induction time is complicated. Higher initial pressure will promote hydrate formation. Gas hydrate slurry shows a shear‐thinning behavior, and slurry viscosity increases with the increase of wax content and initial pressure. A semiempirical viscosity model showing a well‐fitting is established for hydrate slurry with wax crystals by considering the aggregation and breakage of hydrate particles, wax crystals, and water droplets. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3502–3518, 2018     

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.     

Performance Improvement of Various Kinetic Hydrate Inhibitors Using 2-Butoxyethanol for Simple Gas Hydrate Formation in a Flow Mini-Loop Apparatus

The effect of 2-butoxyethanol as an additive on simple gas hydrate formation in the presence of kinetic hydrate inhibitors such as modified starch, polyvinylcaprolactam, and Gaffix VC-713 under various conditions in a flow mini-loop apparatus has been studied. A laboratory flow mini-loop apparatus has been designed and manufactured to measure the induction time of simple gas hydrate formation. Hydrate formers (such as C1, C2, C3, i-C4, and CO₂) are contacted with water containing dissolved inhibitor in the presence of 2-butoxyethanol as an additive at desired temperature and pressure. The effect of 2-butoxyethanol on the induction time during gas hydrate formation was investigated in the presence and absence of modified starch, polyvinylcaprolactam, and Gaffix VC-713 as kinetic inhibitors. Results show that the induction time is prolonged in the presence of Gaffix VC-713 compared to polyvinylcaprolactam and modified starch as inhibitors. Moreover, the induction time in the presence of 2-butoxyethanol is greater than in the absence of this additive for simple gas hydrate formation. As a result, the performance of kinetic hydrate inhibitors was enhanced in the presence of 2-butoxyethanol for natural gas components during gas hydrate formation.