Laboratory tests have been performed using a stirred cell where SI and SII gas hydrates have been formed under the presence of the kinetic inhibitor Poly Vinyl Caprolactam (PVCap) and INHIBEX. The latter is a mixture containing 50wt% PVCap 2k and 50wt% butyl glycol. The effect of PVCap is enhanced by the presence of butyl glycol; the latter acts as a synergist for the former. Dissociation temperatures were obtained and compared for hydrates formed 1) in presence of PVCap and 2) in presence of INHIBEX. The effect of INHIBEX concentration on the temperature of dissociation was also investigated. Systems containing INHIBEX dissociated at lower temperatures than the corresponding systems with only PVCap present. Furthermore, 3000 ppm INHIBEX mixtures were found to have higher dissociation temperatures than 1500 ppm INHIBEX mixtures.

Inhibitor containing systems were investigated for hydrate structures I and II. The kinetic inhibitor PVCap was added to the water phase for each hydrate structure. Dissociation temperatures were determined for various formation temperatures and PVCap concentrations. Obtained dissociation temperatures were compared with corresponding values calculated with CSMHYD. Differences between experimental and calculated values were compared for various formation temperatures and inhibitor concentrations. Comparison revealed that these parameters (formation temperature and concentration) had an effect on the dissociation temperature. Dissociation temperatures for hydrates formed at low degrees of subcooling were higher than for hydrates formed at large subcooling. The effect depended on the system pressure; apparently decreasing or vanishing with increasing pressure. Furthermore, the temperature of dissociation increased with the inhibitor dose.

The kinetic inhibitor Poly Vinyl Caprolactam (PVCap) was added as a kinetic inhibitor to the gas-water system. Different hydrate formers were used in order to obtain formation of the different hydrate structures (sI, sII and sH). All hydrate structures were formed with PVCap. The effect of applying different melting rates was investigated. The isochoric technique was used to obtain dissociation temperatures and corresponding pressures. The melting rate was found to be a parameter influencial for the dissociation temperature. Even for very slow melting rates such as 0.0125 Kelvin per hour, the final dissociation temperature was significantly higher that the dissociation temperature for the corresponding non-inhibited system.

In this work the nucleation kinetics of propane gas hydrate has been investigated experimentally using a stirred batch reactor. The experiments have been performed isothermally recording the pressure as a function of time. Experiments were conducted at different stirring rates, but in the same supersaturation region. The experiments showed that the gas dissolution rate rather than the induction time of propane hydrate is influenced by a change in the stirring rate. This was especially valid at high stirring rates when the water surface was severely disturbed. Addition of polyvinylpyrrolidone to the aqueous phase was found to reduce the gas dissolution rate slightly, however the induction times were prolonged quite substantially. The induction time data were correlated using a newly developed induction time model based on crystallization theory also capable of taking into account the presence of additives. In most cases reasonable agreement between the data and the model could be obtained. The results revealed that especially the effective surface energy between propane hydrate and water is likely to change when the stirring rate varies from very high to low. The prolongation of induction times according to the model is likely to be due to a change in the nuclei-substrate contact angle.

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 reformation of methane-ethane hydrate was observed in situ using 13C MAS NMR spectroscopy. In all reformation experiments, structure I (stable state for the reformation conditions) reformed, and the hydrate cage occupancy ratios were found to be almost the same as those predicted by a statistical thermodynamics program CSMGem, suggesting that there is no preferential formation of large or small cages on the relatively long time scale of this NMR experiment. It was also found that the reformation rate of the sample with PVCap is several times faster compared with the pure system, indicating that the presence of PVCap promotes the hydrate reformation at a high subcooling though this chemical is well-known as a good hydrate inhibitor.

Fundamental understanding of the structural changes occurring during hydrate formation and inhibition is important in the development of new strategies to control hydrates in flowlines and in inhibitor design. Neutron diffraction coupled with Empirical Potential Structure Refinement (EPSR) simulation has been used to determine the hydration structure around propane molecules before and during sII hydrate formation. The EPSR simulation results were generated by fitting neutron data (with H/D isotopic substitution) obtained from the SANDALS diffractometer at ISIS. Using this combination of techniques, the structural transformations of water around propane can be studied during propane (sII) hydrate formation. The hydration structure was found to be different in the liquid phases of the partially formed propane hydrate compared to that before any hydrate formation. The effect of a kinetic hydrate inhibitor, poly-N-pyrrolidone on the hydration structure was also examined. No significant effect was observed on the water structure in the presence of this inhibitor.

The inter-conversion of methane-ethane hydrate from metastable to stable structures was studied using Raman spectroscopy. To investigate factors controlling the inter-conversion, the rate of structural transition was measured at 59% and 93% methane in ethane. The observed slower structural conversion rate in the lower methane concentration atmosphere can be explained in terms of the differences in kinetics (mass transfer of gas and water rearrangement). Also, the effect of kinetic hydrate inhibitors, poly-N-vinylpyrrolidone (PVP) and polyethylene-oxide (PEO), on the hydrate metastability was investigated at 65% and 93% methane in ethane. PVP increased the conversion rate at 65% methane in ethane (sI is thermodynamically stable), but retarded the rate at 93% methane in ethane (sII is thermodynamically stable), indicating that the function of PVP depends on hydrate structure. PEO did not affect the structural transition considerably for either methane-ethane compositions.

A HP micro DSC-VII from SETARAM was used to study the efficiency and mechanism of action of commercial kinetic inhibitors for gas-hydrate formation in drilling fluids (OBM). The main objective was to find a suitable and reliable method of screening for these chemicals. The DSC technique consists in monitoring the heat exchanges, due to phase changes (here hydrate formation or dissociation), either versus time at constant temperature or versus temperature during a heating or cooling program. All products showed a gas hydrate dissociation temperature (at a given pressure) that matched with theoretical and previously published data. Nevertheless, for some additives two thermal signals were observed on the thermograms, one that corresponds to the theoretical value and another at a higher temperature (about +4°C). This second peak is insensitive to the heating rate applied for the dissociation, but the areas ratio (1stpeak/2nd peak) changes with the additive concentration and with the driving force applied during the hydrate formation. Additionally, additive/water and additive/water/THF systems were tested. In each case, two dissociation peaks were also measured. The results allow us to disregard any kinetic effects bonded to this thermal phenomenon, and lead us to infer that some additives may induce non-classical crystalline structures of gas hydrates. To verify these results, crystallographic and spectroscopic experiments must be performed. The stabilities of these new compounds are under study.

The economic and safety hazards associated with the ability of gas hydrates to form in pipelines have prompted our interest in the inhibition of hydrate growth. Antifreeze proteins (AFPs) adsorb to ice surfaces and certain AFPs can also inhibit the growth of hydrates formed from water molecules organized in cage-like formations around a central small gas molecule. A Monte Carlo computational method for simulating the growth of ice crystals has been developed and it has proved useful in the understanding of the inhibition mechanism of these proteins. We have modified this crystal growth software in order to simulate the growth of large structure II gas hydrates, consisting of millions of water and gas molecules. This represents a first step towards investigating the effectiveness of novel compounds to inhibit hydrate growth in silico. Here, we describe these software modifications, and our efforts to incorporate type I AFP molecules into the hydrate growth simulations. Because both the docking interaction and inhibition mechanism for AFP towards hydrates remains unknown, we have set up a number of inhibitor screens to investigate possible AFP-hydrate docking models. Our goal is to reproduce the changes to gas hydrate morphology that have been observed in the presence of AFP, which will guide our choices for the binding alignment between AFPs and hydrates. This alignment will be instrumental for determining the AFPI-inhibition mechanism and should prove invaluable for the development of novel, hyperactive hydrate inhibitors.