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.     

Methane hydrate formation and decomposition: structural studies via neutron diffraction and empirical potential structure refinement

Neutron diffraction studies with hydrogen/deuterium isotope substitution measurements are performed to investigate the water structure at the early, medium, and late periods of methane clathrate hydrate formation and decomposition. These measurements are coupled with simultaneous gas consumption measurements to track the formation of methane hydrate from a gas/water mixture, and then the complete decomposition of hydrate. Empirical potential structure refinement computer simulations are used to analyze the neutron diffraction data and extract from the data the water structure in the bulk methane hydrate solution. The results highlight the significant changes in the water structure of the remaining liquid at various stages of hydrate formation and decomposition, and give further insight into the way in which hydrates form. The results also have important implications on the memory effect, suggesting that the water structure in the presence of hydrate crystallites is significantly different at equivalent stages of forming compared to decomposing. These results are in sharp contrast to the previously reported cases when all remaining hydrate crystallites are absent from the solution. For these systems there is no detectable change in the water structure or the methane hydration shell before hydrate formation and after decomposition. Based on the new results presented in this paper, it is clear that the local water structure is affected by the presence of hydrate crystallites, which may in turn be responsible for the "history" or "memory" effect where the production of hydrate from a solution of formed and then subsequently melted hydrate is reportedly much quicker than producing hydrate from a fresh water/gas mixture.     

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.     

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.     

Dissociation enthalpies of synthesized multicomponent gas hydrates with respect to the guest composition and cage occupancy

This study presents the influences of additional guest molecules such as C2H6, C3H8, and CO2 on methane hydrates regarding their thermal behavior. For this purpose, the onset temperatures of decomposition as well as the enthalpies of dissociation were determined for synthesized multicomponent gas hydrates in the range of 173-290 K at atmospheric pressure using a Calvet heat-flow calorimeter. Furthermore, the structures and the compositions of the hydrates were obtained using X-ray diffraction and Raman spectroscopy as well as hydrate prediction program calculations. It is shown that the onset temperature of decomposition of both sI and sII hydrates tends to increase with an increasing number of larger guest molecules than methane occupying the large cavities. The results of the calorimetric measurements also indicate that the molar dissociation enthalpy depends on the guest-to-cavity size ratio and the actual concentration of the guest occupying the large cavities of the hydrate. To our knowledge, this is the first study that observes this behavior using calorimetrical measurements on mixed gas hydrates at these temperature and pressure conditions.     

Simulation of thermobaric conditions of the formation,composition, and structure of mixed hydrates containing xenon and nitrous oxide

Structural, dynamic, and thermodynamic features of double hydrates of xenon and nitrous oxide are calculated. Thermodynamic stability regions of these hydrates are found. At the atmospheric pressure the xenon hydrate is in the equilibrium with the gas phase at temperatures up to 263 K, whereas at these pressures the nitrous oxide hydrate decomposes already at 218 K. A strong dependence of the equilibrium temperatures and pressures of the formation/decomposition of double nitrous oxide and xenon hydrates on the composition of their mixture in the gas phase is shown.     

Numerical Simulation on the Dissociation,Formation, and Recovery of Gas Hydrates on Microscale Approach

Investigations into the structures of gas hydrates, the mechanisms of formation, and dissociation with modern instruments on the experimental aspects, including Raman, X-ray, XRD, X-CT, MRI, and pore networks, and numerical analyses, including CFD, LBM, and MD, were carried out. The gas hydrate characteristics for dissociation and formation are multi-phase and multi-component complexes. Therefore, it was important to carry out a comprehensive investigation to improve the concept of mechanisms involved in microscale porous media, emphasizing micro-modeling experiments, 3D imaging, and pore network modeling. This article reviewed the studies, carried out to date, regarding conditions surrounding hydrate dissociation, hydrate formation, and hydrate recovery, especially at the pore-scale phase in numerical simulations. The purpose of visualizing pores in microscale sediments is to obtain a robust analysis to apply the gas hydrate exploitation technique. The observed parameters, including temperature, pressure, concentration, porosity, saturation rate, and permeability, etc., present an interrelationship, to achieve an accurate production process method and recovery of gas hydrates.     

Gas hydrates in low water content gases: Experimental measurements and modelling using the CPA equation of state

In this communication, new experimental data are reported for the water content of methane and two synthetic gas mixtures in equilibrium with hydrates at pressures range from 5 to 40 MPa and temperature down to 251.65 K. The measurements have been made on equilibrated samples taken from a high-pressure variable volume hydrate cell using a new analyser based upon tuneable diode laser absorption spectroscopy (TDLAS) technology. A statistical thermodynamic approach, with the Cubic-Plus-Association equation of state, is employed to model the phase equilibria. The hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The thermodynamic model was used to predict the water content of methane and synthetic gases in equilibrium with gas hydrates.     

Kinetic and thermodynamic behaviour of CF4 clathrate hydrates

This study presents experimental kinetic and thermodynamic data for CF₄ clathrate hydrates. Experimental measurements were undertaken in a high pressure equilibrium cell with a 40 cm³ inner volume. The measurements of experimental hydrate dissociation conditions were performed in the temperature range of (273.8 to 278.3) K and pressures ranging from (4.55 to 11.57) MPa. A thermodynamic model based on van der Waals and Platteeuw (vdW–P) solid solution theory was used for prediction and comparison of hydrate dissociation conditions and the Langmuir constant parameters for CF₄ based on Parrish and Prausnitz equation are reported. For the kinetics, the effect of initial pressure and temperature on the induction time, CF₄ hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion during the hydrate formation were studied. Kinetic experiments were performed at initial temperatures of (275.3, 276.1 and 276.6) K and initial pressures of (7.08, 7.92, 9.11, 11.47 and 11.83) MPa. Results show that increasing the initial pressure at constant temperature decreases the induction time, while CF₄ hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion increase. The same trends are observed with a decrease in the initial temperature at constant pressure.     

Experimental and modeling study on decomposition kinetics of methane hydrates in different media

The decomposition kinetic behaviors of methane hydrates formed in 5 cm3 porous wet activated carbon were studied experimentally in a closed system in the temperature range of 275.8-264.4 K. The decomposition rates of methane hydrates formed from 5 cm3 of pure free water and an aqueous solution of 650 g x m(-3) sodium dodecyl sulfate (SDS) were also measured for comparison. The decomposition rates of methane hydrates in seven different cases were compared. The results showed that the methane hydrates dissociate more rapidly in porous activated carbon than in free systems. A mathematical model was developed for describing the decomposition kinetic behavior of methane hydrates below ice point based on an ice-shielding mechanism in which a porous ice layer was assumed to be formed during the decomposition of hydrate, and the diffusion of methane molecules through it was assumed to be one of the control steps. The parameters of the model were determined by correlating the decomposition rate data, and the activation energies were further determined with respect to three different media. The model was found to well describe the decomposition kinetic behavior of methane hydrate in different media.     

Towards a fundamental understanding of natural gas hydrates

Gas clathrate hydrates were first identified in 1810 by Sir Humphrey Davy. However, it is believed that other scientists, including Priestley, may have observed their existence before this date. They are solid crystalline inclusion compounds consisting of polyhedral water cavities which enclathrate small gas molecules. Natural gas hydrates are important industrially because the occurrence of these solids in subsea gas pipelines presents high economic loss and ecological risks, as well as potential safety hazards to exploration and transmission personnel. On the other hand, they also have technological importance in separation processes, fuel transportation and storage. They are also a potential fuel resource because natural deposits of predominantly methane hydrate are found in permafrost and continental margins. To progress with understanding and tackling some of the technological challenges relating to natural gas hydrate formation, inhibition and decomposition one needs to develop a fundamental understanding of the molecular mechanisms involved in these processes. This fundamental understanding is also important to the broader field of inclusion chemistry. The present article focuses on the application of a range of physico-chemical techniques and approaches for gaining a fundamental understanding of natural gas hydrate formation, decomposition and inhibition. This article is complementary to other reviews in this field, which have focused more on the applied, engineering and technological aspects of clathrate hydrates.     

Formation conditions and thermodynamic model predictions of carbon monoxide hydrates

With a fine accuracy and conciseness, Chen-Guo hydrate model has been widely applied to predict the hydrates formation conditions of different systems, including inhibitor containing systems and salt containing systems. However, the model could not predict the formation condition of carbon monoxide (CO) hydrates as the parameter values of CO required in the calculation are not available. In this work, CO hydrate formation pressures were measured at different temperatures in tetrahydrofuran (THF) solution first, then the parameter values of CO required in Chen-Guo model were fitted completely for the first time. On that basis, the hydrates formation conditions of different systems including CO were predicted by the model to verify the accuracy of the fitted values. The comparison between the predicted results and our experimental data (or literature data) shows that the absolute average deviation percentage (AADP) of structure I hydrates is no more than 1.481%, and the AADP of structure II hydrates is less than 6.796%. It is proved that the fitted parameter values of CO are credible, and Chen-Guo model is capable of predicting the formation conditions of CO hydrates. The experimental results and model modifications extend the applied range of Chen-Guo model and promote the development of CO hydrates thermodynamics research.     

Applicability of the Vogel-Tammann-Fulcher type asymptotic exponential functions for ice, hydrates, and polystyrene latex

The validity of the Vogel-Tammann-Fulcher type asymptotic exponential functions for representing the temperature dependence of various properties, such as the particle-particle pull-off adherence forces in ice and hydrates and the annealing effect on polystyrene latex films, is demonstrated. The parameters of this equation are determined using the reported experimental data. The applications show that such functions yield accurate correlations of the experimental property data measured at various temperatures.     

Double clathrate hydrates of cross-linked tetrabutylammonium polyacrylate and noble gases at high pressures

Temperatures of hydrate decomposition were measured by means of the differential thermal analysis at a pressures up to 800–900 Mpa in the systems: cross-linked tetrabutylammonium polyacrylate–water and cross-linked tetrabutylammonium polyacrylate–water–noble gas (He, Ne, Ar, Kr, Xe). The effect of the deformation of D- cavities of the hydrates on the temperature of their decomposition is discussed on the basis of the experimental data.     

Experimental study and thermodynamic modelling of methane clathrate hydrate dissociation conditions in silica gel porous media in the presence of methanol aqueous solution

In this work, the phase equilibria of clathrate hydrates of methane in the presence of pure water and 0.035 mass fraction of methanol aqueous solution in confined silica gel pores with (10 and 15) nm mean diameters are measured and reported. A thermodynamic model is also developed for prediction of the obtained experimental hydrate dissociation data. The Valderrama–Patel–Teja (VPT-EoS) equation of state (EoS) accompanied with the non-density dependent (NDD) mixing rules coupled with a previously developed activity model are applied to evaluate the fugacity of the species present and the activity coefficient of water in methanol aqueous solution. Acceptable agreement between the reported data and the predicted results using the proposed model and an existing method reported in the literature demonstrates the reliability of the presented model.     

Phase equilibria of clathrate hydrates of methane + carbon dioxide: New experimental data and predictions

In this communication, we report experimental dissociation conditions for region clathrate hydrates of methane + carbon dioxide in gas–liquid water–hydrate (G–Lw–H) equilibrium. The temperature and pressure conditions are in the range of (279.1–289.9) K and (2.96–13.06) MPa, respectively. Concentrations of carbon dioxide in the feed gas are also varied. An isochoric pressure-search method was used to perform the measurements. The dissociation data generated in this work along with the literature data are compared with the predictions of a thermodynamic model and a previously reported empirical equation. A discussion is made on the deviations between the experimental and predicted data.     

Double clathrate hydrates of tetrabutylammonium fluoride + helium, neon, hydrogen and argon at high pressures

Decomposition curves of double ionic clathrate hydrates of tetrabutylammonium fluoride with helium, neon, hydrogen and argon were studied at pressures up to 800 MPa. Formation of double hydrates with helium, neon and hydrogen does not lead to any significant increase of the temperatures of decomposition of these hydrates; at high temperatures the hydrates may decompose even at lower temperatures than the hydrate of pure tetraalkylammonium salt does. Decomposition temperatures of double hydrates with argon in all cases were 4–8 °C higher in comparison with the decomposition temperature of ionic clathrate hydrates of tetrabutylammonium fluoride. We suppose that this behavior is caused by simultaneous effect of three factors on hydrate decomposition temperature: (1) partial filling of the small cavities in the framework of the hydrate with water molecules, (2) weakness of the van der Waals interactions between the gas molecules and the host water molecules, and (3) dissolution of helium, hydrogen and neon in the solution of tetrabutylammonium salt causing a decrease of melting temperatures of the hydrates formed from these solutions.     

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.     

Molecular Dynamics Simulation of Methane Hydrate Decomposition in the Presence of Alcohol Additives

The decomposition process of methane hydrate in pure water and methanol aqueous solution was studied by molecular dynamics simulation. The effects of temperature and pressure on hydrate structure and decomposition rate are discussed. The results show that decreasing pressure and increasing temperature can significantly enhance the decomposition rate of hydrate. After adding a small amount of methanol molecules, bubbles with a diameter of about 2 nm are formed, and the methanol molecules are mainly distributed at the gas-liquid interface, which greatly accelerates the decomposition rate and gas-liquid separation efficiency. The radial distribution function and sequence parameter analysis show that the water molecules of the undecomposed hydrate with ordered ice-like configuration at a temperature of 275 K evolve gradually into a long-range disordered liquid structure in the dynamic relaxation process. It was found that at temperatures above 280 K and pressures between 10 atm and 100 atm, the pressure has no significant effect on hydrate decomposition rate, but when the pressure is reduced to 1 atm, the decomposition rate increases sharply. These findings provided a theoretical insight for the industrial exploitation of hydrates.