It is well known that methane hydrate undergoes several phase transformations at high pressures. At room temperature and low to moderate pressure, methane and water form a stable cubic structure I (sI) hydrate that is also known as MH-I. The structure is transformed to a hexagonal phase (sH/MH-II) above 1.0GPa. Another phase transformation occurs above 1.9GPa where the filled ice structure (MH-III) is stable up to 40 GPa before a new high pressure phase transition occurs. Experiments at such high pressures have to be performed in a diamond anvil cell (DAC). Our main interest, though, is to form sH methane hydrate at a lower pressure than reported in previous studies but with some methane in the large cages consequently increasing the methane content. This can be accomplished by introducing the molecules of the large hydrate forming substance (tert-butyl methyl ether/TBME) at a concentration slightly below the stoichiometric amount as suggested by molecular dynamics simulations. In this study we have synthesized mixed methane hydrate of sI and sH and loaded the clathrate with methane into several DACs. Raman spectra were collected at room temperature and pressures in the range of 0.1 to 11.3 GPa. The existence of sH methane hydrate was observed down to 0.2 GPa. However, the existence of methane in the large cages was visible only at pressure higher than 1.0 GPa. The excess methane in the system apparently destabilizes the sH clathrate at pressure below 1.0 GPa as it transforms to sI clathrate.

Recently, by performing molecular dynamics simulations in the methane-water system, we have measured the static lifetimes of cagelike water clusters (CLWC) immersed in bulk liquid water, during which the member-water molecules of CLWCs are not allowed to exchange with their surrounding water molecules [J. Phys. Chem. C, 2007, 111, 2595]. In this study, we measure the dynamic lifetimes of CLWCs with permitting such water exchanges. It is found that the dynamic lifetimes of CLWCs are not less than the static lifetimes previously obtained, and their ratio increases with the lifetime values. The results strengthen that CLWCs are metastable structures in liquid water and the occurrence probability of long-lived CLWCs will increase if one uses the dynamic lifetimes instead of the static lifetimes. The implications of this study for hydrate nucleation are discussed.

The chemical structure of polyether amines (PEA), mainly electron donating multiple oxygen and nitrogen atoms as well as active hydrogen atoms, make such compounds actively participating in the formation of hydrogen bonds with surrounding molecules. Hydrophobic polypropylene glycol functionality gives PEA's properties of multi-headed surfactants having hydrophilic amine groups. These groups have a strong affinity for water molecules, ice and hydrate crystals. Such PEA compounds have been known for several years. However, the hydrate inhibition properties of PEA’s were only discovered about twelve years ago. The first discovery stimulated more research in laboratories and led to practical applications for hydrate inhibition in gas fields. An interesting property of PEAs is their synergistic effect on hydrate inhibition when applied concurrently with polymeric kinetic hydrate inhibitors (KHI) or thermodynamic inhibitors (THI). The combination inhibitors are better inhibitors than a single component one. Quaternized polyether diamines are efficient antiagglomerant (AA) hydrate inhibitors while different derivatization can produce dual functionality compounds, i.e. corrosion inhibitors/gas hydrate inhibitors (CI/GHI). With all of this versatility, PEAs found application for hydrate inhibition in oil and gas fields onshore and offshore in production, flowlines and completion. The PEAs have an excellent record in protecting gas-producing wells from plugging with hydrates. (Final corrected copy of ICGH paper 5347)

The use of Kinetic Hydrate Inhibitors (KHI) is one of the optimum methods employed to control gas hydrate formation issues and provide flow assurance in oil and gas production systems. The application of this technology has several advantages to operators, including significant cost savings and extended life of oil and gas systems. This paper will highlight a specific case where a Major operator in the North Sea (UK sector) significantly reduced the cost of well intervention operations by applying a KHI in a subsea gas lift line. Considerable cost savings were realized by reducing volume of chemical required and this enabled the application to be performed from the FPSO eliminating the need for a dedicated Diving Support Vessel (DSV). Furthermore, the application of KHI also reduced manual handling and chemical logistics usually associated with this particular treatment. In order to prevent mineral scale deposition occurring in downhole tubing and near well bore and in the formation; scale inhibitor squeeze applications are standard practice. For subsea wells the fluids can be pumped down in to the well via gas lift lines. However, upon completion of previous scale squeeze operations at this particular location, hydrate formation was observed when a mixture of MEG and water was used following interventions via the gas lift line. By applying 1% KHI with a mixture of MEG and Water, the well was brought back into production following scale squeeze operations without hydrate formation occurring.

In this paper we report a low symmetry version of structure H hydrate that results from the hexagonal form on cooling below 167 K. Phase changes with temperature in the common clathrate hydrates structural families I, II and H have not been observed before, except in doped systems where ordering transitions take place or in the structure I hydrate of trimethylene oxide where the guest molecule dipoles are known to order. Since there is an inverse relationship between the effect of temperature and pressure on ices, it may well be that the low symmetry form reported at low temperature can also be reached by applying high pressure, and that in fact some of the observed high pressure phases are lower symmetry versions of hexagonal sH.

Previously we have suggested the concept of tuning hydrate compositions which makes it possible to increase the gas storage capacity of binary hydrates. Herein, we report for the first time the existence of a critical guest concentration (CGC) and establish the complete tuning pattern that appears to exist in binary hydrates, including the water-soluble hydrate formers (promoters) and water insoluble guests,. The first attempt to verify the new features of clathrate hydrate compositions is executed on the binary hydrate of CH4 + THF and involves a detailed examination of the guest distribution by spectroscopic methods. THF molecules by themselves form sII hydrate from a completely miscible aqueous solution, and in this structure, because of their size, THF molecules occupy only the large 51264 cages. The CGC value appears to depend largely on the chemical nature of the liquid guest component participating in the binary hydrate formation. The present experimental findings on the existence of critical guest concentration and the complete tuning phenomenon can be expected to make a meaningful contribution to both inclusion chemistry and a variety of hydrate-based fields.

Raman spectroscopic observations of the characteristics and dissociation of methane hydrate were carried out on hydrates synthesized in silica sands with particle sizes of 53-75 μm, 90-106 μm, 106-150 μm, and 150-180 μm. The results obtained indicate that methane hydrates formed in silica sands had similar characteristics regarding cage occupancy and hydration number (5.99) to bulk hydrate, indicative of no influence of particle size on hydrate composition. During hydrate dissociation, the change in average intensity ratio of large to small cages were generally consistent with that of bulk hydrate but dropped dramatically after a certain time, and this turning point seems to be related to the particle size of silica sands.

Recently, clathrate hydrate has attracted much attention because of its energy gas enclathration phenomenon. Since energy gas such as methane, ethane, and hydrogen could be stored in solid hydrate form, clathrate hydrate research has been considerably focused on energy gas storage and transportation medium. Especially, methane hydrate, which is crystalline compound that are formed by physical interaction between water and relatively small sized guest molecules, can contain about as much as 180 volumes of gas at standard pressure and temperature condition. To utilize gas hydrate as energy storage and transportation medium, two important key features: storage capacity and storage condition must be considered. Herein, we report the inclusion phenomena of methane occurred on tert-butyl alcohol hydrate through thermodynamic measurement and spectroscopic analysis by using powder X-ray diffractometer, and 13C solidstate NMR. From spectroscopic analysis, we found the formation of sII type (cubic, Fd3m) clathrate hydrate by introducing methane gas into tert-butyl alcohol hydrate whereas tert-butyl alcohol hydrate alone does not form clathrate hydrate structure. Under equilibrium condition, pressure-lowering effect of methane + tert-butyl alcohol double hydrate was also observed. The present results give us several key features for better understanding of inclusion phenomena occurring in the complex hydrate systems and further developing methane or other gas storage and transportation technique.

A hydrate plugging and formation model for oil and gas pipelines is becoming increasingly important as producers continue to push flow assurance boundaries. A key input for any hydrate plugging model is the rate of hydrate growth and the volume fraction of hydrate at a given time. This work investigates a fundamental approach toward predicting hydrate growth and volume fraction in a water saturated gas pipeline. This works suggests that, in the absence of free water, hydrate volume fraction can be predicted using a wall growth and sloughing model. Wall growth can be predicted using a one-dimensional, moving boundary, heat and mass transfer model. It is hypothesized that hydrate sloughing can be predicted when a coincident frequency exists between hydrate natural frequency and flow induced vibrations over the hydrate surface.

The avoidance of hydrate blockages in deepwater subsea tiebacks presents a major technical challenge with severe implications for production, safety and cost. The successful prediction of when and where hydrate plugs form could lead to substantial reductions in the use of chemical inhibitors, and to corresponding savings in operational expenditure. The diffusivity of the gas hydrate former (methane) or the host molecule (water), through a hydrate film is a key property for such predictions of hydrate plug formation. In this paper, a novel application of Differential Scanning Calorimetry is described in which a hydrate film was allowed to grow at a hydrocarbon-water interface for different hold-times. By determining the change in mass of the hydrate film as a function of hold-time, an effective diffusivity could be inferred. The effect of the subcooling, and of the addition of a liquid hydrocarbon layer were also investigated. Finally, the transferability of these results to hydrate growth from water-in-oil emulsions is discussed.