Dissociation behavior of C2H6 hydrate at temperatures below the ice point: melting to liquid water followed by ice nucleation

The dissociation of C(2)H(6) hydrate particles by slow depressurization at temperatures slightly below the ice melting point was studied using optical microscopy and Raman spectroscopy. Visual observations and Raman measurements revealed that ethane hydrates can be present as a metastable state at pressures lower than the dissociation pressures of the three components: ice, hydrate, and free gas. However, they decompose into liquid water and gas phases once the system pressure drops to the equilibrium boundary for supercooled water, hydrate, and free gas. Structural analyses of obtained Raman spectra indicate that structures of the metastable hydrates and liquid water from the hydrate decay are fundamentally identical to those of the stable hydrates and supercooled water without experience of the hydration. These results imply a considerably high energy barrier for the direct hydrate-to-ice transition. Water solidification, probably induced by dynamic nucleation, was also observed during melting.     

Gas hydrate fast nucleation from melting ice and quiescent growth along vertical heat transfer tube

Gas hydrates, or clathrate hydrates, are ice-likecrystal, composed of host lattice (cavities) formed byhydrogen-bonded water molecules, and other guestmolecules called guest molecules. The guest mole-cules act with host lattice in weak van der Waals force…     

Experimental and theoretical investigation of simple gas hydrate formation with or without presence of kinetic inhibitors in a flow mini-loop apparatus

Gas hydrates are ice-like crystalline compounds, which form through a combination of water and suitably sized guest molecules under low temperature and elevated pressure conditions. These solid compounds give rise to problems in the natural gas oil industry because they can plug pipelines and process equipment. Low dosage hydrate inhibitors are a recently developed hydrate control technology, which can be more cost-effective than traditional practices such as methanol and glycols. The kinetics of hydrate growth has been modeled by numerous authors who have measured the gas consumption rate during hydrate formation in batch agitator reactors.     

Formation and decomposition of gas hydrates of natural gas components

Based on our theoretical and experimental work carried out during the last decade, our understanding of the thermodynamics and the kinetics of formation and decomposition of gas hydrates is presented. Hydrate formation is modelled as a crystallization process where two distinct processes (nucleation and growth) are involved. Prior to the nucleation the concentration of the gas in the liquid water exceeds that corresponding to the vapor-liquid equilibrium. This supersaturation is attributed to the extensive structural orientation in the liquid water and is necessary for the phase change to occur. The growth of the hydrate nuclei or the decomposition of a hydrate particle are modelled as two-step procedures. Only one adjustable parameter for each hydrate forming gas is required for the intrinsic rate of formation or decomposition. In addition the inhibiting effects of electrolytes or methanol on hydrate formation are discussed and experimental data on methane gas hydrate formation in the presence of aqueous solutions of 3% NaCl and 3% NaCl + 3% KCI, are presented along with the predicted values. Finally, the relevence of the ideas to the technological implications of gas hydrates as well as areas where future research is needed are discussed.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Comparing effectiveness of rhamnolipid biosurfactant with a quaternary ammonium salt surfactant for hydrate anti-agglomeration

Natural gas is projected to be the premium fuel of the 21st century because of availability, as well as economical and environmental considerations. Natural gas is coproduced with water from the subsurface forming gas hydrates. Hydrate formation may result in shutdown of onshore and offshore operations. Industry practice has been usage of alcohols--which have undesirable environmental impacts--to affect bulk-phase properties and inhibit hydrate formation. An alternative to alcohols is changing the surface properties through usage of polymers and surfactants, effective at 0.5-3 wt % of coproduced water. One group of low-dosage hydrate inhibitors (LDHI) are kinetic inhibitors, which affect nucleation rate and growth. A second group of LDHI are anti-agglomerants, which prevent agglomeration of small hydrate crystallites. Despite great potential, reported work on hydrate anti-agglomeration is very limited. In this paper, our focus is on the use of two vastly different surfactants as anti-agglomerants. We use a model oil, water, and tetrahydrofuran as a hydrate-forming species. We examine the effectiveness of a quaternary ammonium salt (i.e., quat). Visual observation measurements show that a small concentration of the quat (0.01%) can prevent agglomeration. However, a quat is not a green chemical and therefore may be undesirable. We show that a rhamnolipid biosurfactant can be effective to a concentration of 0.05 wt %. One difference between the two surfactants is the stability of the water-in-oil emulsions created. The biosurfactant forms a less stable emulsion, which makes it very desirable for hydrate application.     

Detecting gas hydrate behavior in crude oil using NMR

Because of the associated experimental difficulties, natural gas hydrate behavior in black oil is poorly understood despite its grave importance in deep-water flow assurance. Since the hydrate cannot be visually observed in black oil, traditional methods often rely on gas pressure changes to monitor hydrate formation and dissociation. Because gases have to diffuse through the liquid phase for hydrate behavior to create pressure responses, the complication of gas mass transfer is involved and hydrate behavior is only indirectly observed. This pressure monitoring technique encounters difficulties when the oil phase is too viscous, the amount of water is too small, or the gas phase is absent. In this work we employ proton nuclear magnetic resonance (NMR) spectroscopy to observe directly the liquid-to-solid conversion of the water component in black oil emulsions. The technique relies on two facts. The first, well-known, is that water becomes essentially invisible to liquid state NMR as it becomes immobile, as in hydrate or ice formation. The second, our recent finding, is that in high magnetic fields of sufficient homogeneity, it is possible to distinguish water from black oil spectrally by their chemical shifts. By following changes in the area of the water peak, the process of hydrate conversion can be measured, and, at lower temperatures, the formation of ice. Taking only seconds to accomplish, this measurement is nearly direct in contrast to conventional techniques that measure the pressure changes of the whole system and assume these changes represent formation or dissociation of hydrates - rather than simply changes in solubility. This new technique clearly can provide accurate hydrate thermodynamic data in black oils. Because the technique measures the total mobile water with rapidity, extensions should prove valuable in studying the dynamics of phase transitions in emulsions.     

Effect of the Degree of Oil Biodegradation on the Crystallization of Methane Hydrate and Ice in Water-Oil Emulsions

To study the influence exerted by oxidized oil components on the nucleation and growth of gas hydrates the nucleation of methane hydrate and ice in 50 wt % emulsions of oil in native oil and two samples of the same oil subjected to biodegradation for 30 and 60 days (samples N, V30, and V60, respectively) were examined. In the course of measurements, the samples were cooled to–15°C at a constant rate of 0.14 deg min^–1 and then heated to the initial temperature. The initial methane pressure in the system was 15 MPa at 20°C. In the process, the temperatures were recorded at which heat effects corresponding to the formation of hydrate/ice and the melting of these. In the case of emulsion N, no exothermic effects were manifested in the cooling stage. In the heating stage, the endothermic effects of ice melting were found in half of the samples. No effects corresponding to the decomposition of the hydrate were observed. In experiment with V30 samples, the formation of the hydrate and ice was manifested as strong exothermic effects. Ice was formed in all the experiments, and the hydrate, only in 21% of the samples. Finally, in experiments with V60, ice and the hydrate were formed in 54 and 13% of cases, respectively. Their formation was manifested as weak exothermic effects in the cooling stage. Thus, it was demonstrated that the biodegradation level of oil samples affects the nucleation of methane hydrate and ice in emulsions formed on the basis of these samples.

     

Kinetic hydrate inhibitor performance of new copolymer poly(N-vinyl-2-pyrrolidone-co-2-vinyl pyridine)s with TBAB

In oil and gas field, the application of kinetic hydrate inhibitors (KHIs) independently has remained problematic in high subcooling and high water-cut situation. One feasible method to resolve this problem is the combined use of KHIs and some synergists, which would enhance KHIs’ inhibitory effect on both hydrate nucleation and hydrate crystal growth. In this study, a novel kind of KHI copolymer poly(N-vinyl-2-pyrrolidone-co-2-vinyl pyridine)s (HGs) is used in conjunction with TBAB to show its high performance on hydrate inhibition. The performance of HGs with different monomer ratios in structure II tetrahydrofuran (THF) hydrate is investigated using kinetic hydrate inhibitor evaluation apparatus by step-cooling method and isothermal cooling method. With the combined gas hydrate inhibitor at the concentration of 1.0 wt%, the induction time of 19 wt% THF solution could be prolonged to 8.5 h at a high subcooling of 6℃. Finally, the mechanism of HGs inhibiting the formation of gas hydrate is proposed.     

Accelerated nucleation of tetrahydrofuran(THF)hydrate in presence of ZIF-61

Clathrate hydrate can be used in energy gas storage and transportation,CO 2 capture and cool storage etc.However,these technologies are difficult to be used due to the low formation rate and long induction time of hydrate formation.In this paper,ZIF-61(zeolite imidazolate framework,ZIF) was first used in hydrate formation to stimulate hydrate nucleation.As an additive of clathrate hydrate,ZIF-61 promoted obviously the acceleration of tetrahydrofuran(THF) hydrate nucleation.It shortened the induction time of THF hydrate formation from 2-5 h to 0.3-1 h mainly due to the template function of ZIF-61 by which the nucleation of THF hydrate has been promoted.     

Experimental studies of the formation and dissociation of methane hydrate in loess

In order to study the nature of gas hydrate in porous media, the formation and dissociation processes of methane hydrate in loess were investigated. Five cooling rates were applied to form methane hydrate. The nucleation times of methane hydrate formation at each cooling rate were measured for comparison. The experimental results show that cooling rate is a significant factor affecting the nucleation of methane hydrate and gas conversion. Under the same initial conditions, the faster the cooling rate, the shorter the nucleation time, and the lower the methane gas conversion. Five dissociating temperatures were applied to conduct the dissociation experiment of methane hydrate formed in loess. The experimental results indicated that the temperature evidently controlled the dissociation of methane hydrate in loess and the higher the dissociating temperature, the faster the dissociating rates of methane hydrate.     

The effect of the water/methane interface on methane hydrate cages: the potential of mean force and cage lifetimes

Molecular dynamics simulations were used to determine the influence of a methane-water interface on the position and stability of methane hydrate cages. A potential of mean force was calculated as a function of the separation of a methane hydrate cage and a methane-water interface. The hydrate cages are found to be strongly repelled from the methane gas into the water phase. At low enough temperatures, however, the most favorable location for the hydrate cage is at the interface on the water side. Cage lifetime simulations were performed in bulk water and near a methane-water interface. The methane-water interface increases the cage lifetime by almost a factor of 2 compared to cage lifetimes of cages in bulk water. The potential of mean force and the cage lifetime results give additional explanations for the proposed nucleation of gas hydrates at gas-water interfaces.     

Effect of cooling rate on methane hydrate formation in media

In order to study gas hydrate in media, formation of methane hydrate in three different media including loess, fine and coarse sands were studied. Five cooling rates were applied to form the methane hydrate. The nucleation time of the formation of methane hydrate with each cooling rate were measured for comparison. The experimental results show that the cooling rate is a significant factor affecting nucleation of methane hydrate and gas conversion. Under the same initial conditions, the faster the cooling rate, the shorter the nucleation time and the lower the methane gas conversion rate. The media also affect the formation process of methane hydrate within it. In loess, the gas conversion rate is lowest; in coarse sand, the gas conversion rate is the greatest; and in fine sand, it is in between. According to the study, it is found that the smaller the particle size of the media, the harder the methane hydrate forms within it.     

Alcohol cosurfactants in hydrate antiagglomeration

Because of availability, as well as economical and environmental considerations, natural gas is projected to be the premium fuel of the 21st century. Natural gas production involves risk of the shut down of onshore and offshore operations because of blockage from hydrates formed from coproduced water and hydrate-forming species in natural gas. Industry practice has been usage of thermodynamic inhibitors such as alcohols often in significant amounts, which have undesirable environmental and safety impacts. Thermodynamic inhibitors affect bulk-phase properties and inhibit hydrate formation. An alternative is changing surface properties through usage of polymers and surfactants, effective at 0.5 to 3 weight % of coproduced water. One group of low dosage hydrate inhibitors (LDHI) are kinetic inhibitors, which affect nucleation rate and growth. A second group of LDHI are antiagglomerants, which prevent agglomeration of small hydrate crystallites. Despite great potential, work on hydrate antiagglomeration is very limited. This work centers on the effect of small amounts of alcohol cosurfactant in mixtures of two vastly different antiagglomerants. We use a model oil, water, and tetrahydrofuran as a hydrate-forming species. Results show that alcohol cosurfactants may help with antiagglomeration when traditional antiagglomerants alone are ineffective. Specifically, as low as 0.5 wt. % methanol cosurfactant used in this study is shown to be effective in antiagglomeration. Without the cosurfactant there will be agglomeration independent of the AA concentration. To our knowledge, this is the first report of alcohol cosurfactants in hydrate antiagglomerants. It is also shown that a rhamnolipid biosurfactant is effective down to only 0.5 wt. % in such mixtures, yet a quaternary ammonium chloride salt, i. e., quat, results in hydrate slurries down to 0.01 wt. %. However, biochemical surfactants are less toxic and biodegradable, and thus their use may prove beneficial even if at concentrations higher than chemical surfactants.     

钻井液添加剂对形成天然气水合物的影响

针对深水钻井中水基钻井液易形成天然气水合物从而导致钻井作业无法正常进行的问题,利用自行设计研制的气体水合物反应装置,模拟深水钻井温度压力条件,对水基钻井液添加剂进行了天然气水合物形成的实验研究。分析了各实验体系形成水合物的过冷度。以过冷度为评价指标,评价了各种钻井液添加剂在深水钻井水合物形成过程中的作用。结果表明,在钻井液使用的加量范围内,阳离子聚丙烯酰胺CPAM、两性离子聚合物FA367等对天然气水合物的形成有抑制作用,且随着加量的增加抑制作用增强;磺甲基丹宁SMT、木质素磺酸盐FCLS对天然气水合物的形成有微弱的促进作用,但影响不大。聚合物添加剂的离子类型对天然气水合物的形成影响不大。     

Formation process and fractal growth model of HCFC-141b refrigerant gas hydrate

The microscopic visualization experiment on the formation process of HCFC-141b refrigerant gas hydrate has been investigated, and the morphological photos of hydrate formation process have been obtained. The results show that gas hydrate originally nucleated on the interface of refrigerant HCFC-141b and water under the condition of supercooling, then the hydrate grows continually due to the inducement of formed nucleation and diffusion of refrigerant. The formation of gas hydrate presents an arboreous phenomenon. The fractal dimension of the hydrate formation morphology on different stages was calculated. The calculating results indicate that the initial stage of the hydrate formation belongs to fractal growth, and the dimension is about 1.52. Based on the fractal theory, an RIN-DLA (random inducement nucleation-diffusion limited aggregation) model for the HCFC-141b hydrate growth was developed. The hydrate growth process was simulated with the developed model, and the fractal dimension for the simulated     

Advances in the Study of Gas Hydrates by Dielectric Spectroscopy

The influence of kinetic hydrate inhibitors on the process of natural gas hydrate nucleation was studied using the method of dielectric spectroscopy. The processes of gas hydrate formation and decomposition were monitored using the temperature dependence of the real component of the dielectric constant ε′(T). Analysis of the relaxation times τ and activation energy ΔE of the dielectric relaxation process revealed the inhibitor was involved in hydrogen bonding and the disruption of the local structures of water molecules.     

Comparison and application of different empirical correlations for estimating the hydrate safety margin of oil-based drilling fluids containing ethylene glycol

As the oil and gas industries continue to increase their activity in deep water, gas hydrate hazards will become more serious and challenging, both at present and in the future. Accurate predictions of the hydrate-free zone and the suitable addition of salts and/or alcohols in preparing drilling fluids are particularly important both in preventing hydrate problems and decreasing the cost of drilling operations. In this paper, we compared several empirical correlations commonly used to estimate the hydrate inhibition effect of aqueous organic and electrolyte solutions using experiments with ethylene glycol (EG) as a hydrate inhibitor. The results show that the Najibi et al. correlation (for single and mixed thermodynamic inhibitors) and the Ostergaard et al. empirical correlation (for single thermodynamic inhibitors) are suitable for estimating the hydrate safety margin of oil-based drilling fluids (OBDFs) in the presence of thermodynamic hydrate inhibitors. According to the two correlations, the OBDF, composed of 1.6 L vaporizing oil, 2% emulsifying agent, 1% organobentonite, 0.5% SP-1, 1% LP-1, 10% water and 40% EG, can be safely used at a water depth of up to 1900 m. However, for more accurate predictions for drilling fluids, the effects of the solid phase, especially bentonite, on hydrate inhibition need to be considered and included in the application of these two empirical correlations.     

Gas-hydrate formation, agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

One of the main challenges in deep-water drilling is gas-hydrate plugs, which make the drilling unsafe. Some oil-based drilling fluids (OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation, agglomeration and inhibition by an experimental system under the temperature of 4 ℃ and pressure of 20 MPa, which would be similar to the case of 2000 m water depth. The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF. The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles. The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later. Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.     

Gas-hydrate formation,agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

One of the main challenges in deep-water drilling is gas-hydrate plugs,which make the drilling unsafe.Some oil-based drilling fluids(OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation,agglomeration and inhibition by an experimental system under the temperature of 4 ?C and pressure of 20 MPa,which would be similar to the case of 2000 m water depth.The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF.The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles.The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later.Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.     

Acceleration of methane hydrate nucleation by crystals of hydrated sodium dodecyl sulfate

It was shown that hydrated crystals of sodium dodecyl sulfate (SDS), which precipitate from dilute SDS solutions sharply accelerate nucleation of methane gas hydrate. This finding adds significant details to the available information on the mechanisms of hydrate formation from SDS solutions and can form the basis for the development of a new class of kinetic promoters of hydrate formation.