The possibility of commercial natural gas production from gas hydrates has been tested by researchers and industry for more than ten years. Depressurisation of gas hydrates in porous and permeable sandstones has successfully produced water and natural gas. However long term sustainable production is still elusive. Catastrophic sand production into the wellbore has terminated at least three of the significant depressurisation trials including the 2013 trial at the Daini-Atsumi knoll, Nankai Trough, offshore Japan. Sand production is generally thought to be the result of mechanical and hydrodynamic instability, however it appears the failure mechanism is not the same for all reservoirs and the location of reservoir porosity and pressure on the normal compression line for sands could be a controlling factor. Sand production in reservoirs at shallow depths and low confining stresses (less than 10 MPa) are likely to be influenced by fluid flow effects like those described by the Shields (1936) diagram. The relative density of the formation may also affect the nature of the sand production in these reservoirs. The Daini-Atsumi knoll is a structural high on the outer ridge of the Kumano forearc basin, offshore Japan. Hydrate saturations of 50 to 80 % occur within three geological units of the Middle Pleistocene Ogasa group. This group is made up of deep water sediments including sediment gravity flow deposits distinguished by alternating silt and sand layers. The presence of these alternating layers could have influenced the sand production seen during the trial. This reservoir heterogeneity at the 2013 Daini-Atsumi knoll gas hydrate production trial site was characterised using the descriptions of geological units, analogues and statistical techniques. Scenarios of this heterogeneity were tested in a high pressure plane-strain sand production apparatus. The results of these tests suggest the boundary shear stress of the fluid on the grains is a significant control on sand production for the Daini-Atsumi Knoll reservoir and the layering and grainsize structure of the sediments encourages sand production. Relative density of the sediments appears to impact the nature of the sand production where denser sediments show more localised movement. These results indicate that even minor weaknesses in sand control devices will result in uncontrollable sand production rates from the Daini-Atsumi Knoll gas hydrate reservoir. Managing the fluid flow rate in the reservoir and selectively completing coarser grained zones at the base of sand layers could help limit sand production in future trials.

This thesis studies methane hydrate crystal formation and decomposition at 1.0, 4.0 and 7.0°C in a new apparatus. Hydrate was formed in the interstitial space of a variable volume bed of silica sand particles with an average diameter equal to 329μm (150 to 630μm range). The initial pressure inside the reactor was 8.0MPa for all the formation experiments. Three bed sizes were employed in order to observe the effects of the silica sand bed size on the rate of methane consumption (formation) and release (decomposition). The temperature at various locations inside the silica sand bed was measured with thermocouples during formation and decomposition experiments. For the decomposition experiments, two different methods were employed to dissociate the hydrate: thermal stimulation and depressurization. It was found that more than 74.0% of water conversion to hydrates was achieved in all hydrate formation experiments at 4.0°C and 1.0°C starting with a pressure of 8.0MPa. The dissociation of hydrate was found to occur in two stages when thermal stimulation was employed whereas three stages were found during depressurization. In both cases, the first stage was strongly affected by the changing bed size whereas it was not found to depend on the bed size afterwards.

Thesis (M.S. in E.A.S.)--School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 2004. Directed by Daniel Lizarralde. / Includes bibliographical references (leaves 69-73).

Mathematical model, which depicts on macroscopic scale the physical phenomena occurring during the decomposition of gas hydrate, was set up and applied to the spherical methane hydrate pellet decomposing into ice. Initially, porous hydrate pellet is at uniform temperature and pressure within hydrate stable region. The pressure starts to decrease at t=0 with a fixed rate down to the final pressure and is kept constant afterwards. The bounding surface of pellet is heated by convection. Governing equations are based on the conservation principles, the phase equilibrium relation, equation of gas state and phase change kinetics. The single-domain approach and volume average formulation are employed to take into account transient change of local pressure, volumetric liberation of latent enthalpy, and convective heat and mass transfer accompanied by the decomposed gas flow through hydrate/ice solid matrix. The algorithm called “enthalpy method” is extended to deal with non-equilibrium phase change and utilized to determine local phase volume fractions. Predicted results suggest that the present numerical implementation is capable of predicting essential features of heat and mass transfer during non-equilibrium decomposition of hydrate pellet.

It is becoming increasingly recognized that the flood of anthropogenic CO2 into the atmosphere should be reduced in order to mitigate the Earth’s atmospheric greenhouse and slow climate change. If immediate action is required, then a number of greenhouse gas reduction strategies may need to be implemented even before complete study of their impacts can be fully understood. Energy production through combustion produces large amounts of CO2 in a relatively small number of locations at which CO2 capture and compression to a liquid, transportable form can be achieved. Physical disposal offers the best option for sequestering this waste CO2. Because of the costs of transportation, geological sequestration will be most applicable for one set of power plants, deep ocean sequestration may be most applicable for some others. In both cases, the sequestration processes can provide some economic benefits. Ocean CO2 disposal can produce desalinated, treated water as a byproduct.

Understanding when hydrates will nucleate has notable importance in the area of flow assurance. Attempts to model hydrate formation in subsea pipelines currently requires an arbitrary assignment of a nucleation subcooling. Previous studies showed that sII hydrate containing a model water-soluble former, tetrahydrofuran, would nucleate over a narrow temperature range of a few degrees with constant cooling. It is desirable to know if gas phase hydrate formers, which are typically more hydrophobic and hence have a very low solubility in water, also exhibit this nucleation behavior. In this study, differential scanning calorimetry has been applied to determine the hydrate nucleation point for gas phase hydrate formers. Constant cooling ramps and isothermal approaches were combined to explore the probability of hydrate nucleation. In the temperature ramping experiments, methane and xenon were used at various pressures and cooling rates. In both systems, hydrate nucleation occurred over a narrow temperature range (2-3°C). Using methane at lower pressures, ice nucleated before hydrate; whereas at higher pressures, hydrate formed first. A subcooling driving force of around 30°C was necessary for hydrate nucleation from both guest molecules. The cooling rates (0.5-3°C/min) did not show any statistically significant effect on the nucleation temperature for a given pressure. The isothermal method was used for a methane system with pure water and a water-in-West African crude emulsion. Two isotherms (-5 and -10°C) were used to determine nucleation time. In both systems, the time required for nucleation decreased with increased subcooling.

Here, we present a dissolution study of exposed hydrate from outcrops at Barkley Canyon. Previously, a field experiment on synthetic methane hydrate samples showed that mass transfer controlled dissolution in under-saturated seawater. However, seafloor hydrate outcrops have been shown to have significant longevity compared to expected dissolution rates based upon convective boundary layer diffusion calculations. To help resolve this apparent disconnect between the dissolution rates of synthetic and natural hydrate, an in situ dissolution experiment was performed on two distinct natural hydrate fabrics. A hydrate mound at Barkley Canyon was observed to contain a “yellow” hydrate fabric overlying a “white” hydrate fabric. The yellow hydrate fabric was associated with a light condensate phase and was hard to core. The white hydrate fabric was more porous and relatively easier to core. Cores from both fabrics were inserted to a mesh chamber within a few meters of the hydrate mound. Time-lapse photography monitored the dissolution of the hydrate cores over a two day period. The diameter shrinkage rate for the yellow hydrate was 45.5 nm/s corresponding to a retreat rate of 0.7 m/yr for an exposed surface. The white hydrate dissolved faster at 67.7 nm/s yielding a retreat rate of 1.1 m/yr. It is possible these hydrate mounds were exposed due to the fishing trawler incident in 2001. If these dissolution experiments give a correct simulation, then the exposed faces should have retreated ~ 3.5 m and 5.5 m, respectively, from 2001 to this expedition in August 2006. While the appearance of the hydrate mounds appeared quite similar to photographs taken in 2002, these dissolution experiments show natural hydrate dissolves rapidly in ambient seawater. The natural hydrate dissolution rate is on the same order as the synthetic dissolution experiment strongly implying another control for the dissolution rates of natural hydrate outcrops. Several factors could contribute to the apparent longevity of these exposed mounds from upward flux of methane-rich fluid to protective bacterial coatings.

The dissociation behavior of the CH4+CO2 binary gas hydrate has been investigated using Nuclear Magnetic Resonance (NMR) spectroscopy. This technique allows us to distinguish the hydrate structure present, as well as to quantify phase concentrations. Single-pulse excitation was used in combination with magic-angle spinning (MAS). Time-resolved in situ decomposition experiments were carried out at different compositions in sealed, pressurized samples. The decomposition profiles of the CH4+CO2 binary gas hydrate system obtained at various compositions suggest that the decomposition rate is a strong function of the fractional cage occupancy and temperature. An unexpected CH4 hydrate reformation was observed during our decomposition experiments when the temperature reached the ice melting point. A decrease on the CO2 content in the hydrate phase was found during the decomposition experiment, as the pressure and temperature of the system increases.

The oil and gas industry is often plagued by the formation of clathrate hydrates in oil pipelines. While the industry originally had a heuristic of avoidance of clathrate hydrates they are moving to a heuristic of risk management. To successfully implement a risk management heuristic, time dependent phenomena of clathrate hydrate formation and flowline plugging must be known. The study of time dependent phenomena of formation and agglomeration are investigated using a TA Instruments AR-G2 rheometer with a pressure cell capable of operating at up to 13.8 MPa. Pressurized rheological experiments examine clathrate hydrates formed in situ. Both shear and oscillatory experiments have been conducted on the samples, giving flow and viscoelastic parameters. Shear experiments show sharp increases in viscosity upon clathrate hydrate formation indicating rapid aggregation. Transient oscillation experiments show a sharp increase in the elastic and loss moduli followed by a decrease in the loss moduli. Thus, both in situ clathrate hydrate formation and annealing are quantified. In addition these oscillatory measurements provided a novel technique for non-destructive investigation of clathrate hydrate aggregation over time.

As the oil industry moves from a heuristic of avoidance of hydrates to a heuristic of risk management time dependent phenomena of hydrate formation and plugging must be known. One of the key parameters to this process is the aggregation of hydrate particles, the fractal networks they form, and the effect these two parameters have on flow. Unfortunately the aggregation and fractal structure information is extremely difficult to acquire experimentally, for this reason a three-dimension discrete element method (3D-DEM) model has been implemented. The 3D-DEM model calculates detailed solutions to Newton's equations of motion for individual particles. In addition these particles are coupled with the surrounding fluid through computational fluid dynamics (CFD). This coupled 3D-DEM can be used to investigate what the effects of shear, suspending viscosity, attractive forces, and other relevant variables have on the structure, stresses, and positions of the hydrate particles over time. In addition, the effect on viscosity has been calculated using CFD and compared back to basic hard sphere theory.