Considerable research has been done to improve hydrate formation rate. One of the ideas is to introduce mechanical mixing which later tend to complicate the design and operation of the hydrate formation processes. Another approach is to add surfactant (promoter) that will improve the hydrate formation rate and also its storage capacity to be closer to the maximum hydrate storage capacity. Surfactant is widely known as a substance that can lower the surface or interfacial tension of the water when it is dissolved in it. Surfactants are known to increase gas hydrate formation rate, increase storage capacity of hydrates and also decrease induction time. However, the role that surfactant plays in hydrate crystal formation is not well understood. Therefore, understanding of the mechanism through morphology studies is one of the important aspects to be studied so that optimal industrial processes can be designed. In the present study the effect of three commercially available anionic surfactants which differ in its alkyl chain length on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. The surfactants used were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). Memory water was used and the experiments for SDS were carried out at three different degrees of under-cooling and three different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. In addition, extensive hydrate crystal growth on the crystallizer walls is observed. Also a “mushy” hydrate instead of a thin crystal film appears at the gas/water interface. Finally, the addition of SDS with concentration range between 242ppm – 2200ppm (ΔT =13.10C) was found to increase the mole consumption for hydrate formation by 14.3 – 18.7 times. This increase is related to the change in hydrate morphology whereby a more porous hydrate forms with enhanced water/gas contacts.

This thesis is aimed at investigating the possibilities of using vertical line array (VLA) data to image the gas hydrate stability zone (GHSZ) in the northern Gulf of Mexico. The presence of gas hydrate can be inferred from seismic evidence such as bottom simulating reflectors (BSRs) or changes in seismic velocity. The petroliferous northern Gulf of Mexico is noted for its obvious absence of BSRs, a characteristic it shares with other active passive margins with mobile salt and/or shale, which have high propagation velocities for seismic waves. This makes the imaging and the identification of the gas hydrates a challenging process with conventional seismic techniques. Therefore. new techniques in data acquisition. processing and analysis are sought to improve the imaging of complex areas. The new, unconventional seismic data acquisition technique used here is the VELA. This work defines a seismic processing flow that has been developed to extract velocity, travel-time and amplitude information from VLA data to predict the hydrate distribution over the surveyed area. Specialized amplitude versus offset analysis and inversion is applied to the VLA data using a Bayesian inversion approach to provide estimates and uncertainties of the viscoelastic physical parameters at an interface. This thesis will compare the inversion of the 2-D seismic reflection data collected in 1998 by the USGS and in 2002 by the Center for Marine Resources and Environmental Technology (CMRET) to the VLA data collected in 2002 and 2003 by the CMRET to assess the value of a VLA in monitoring changes in the near-surface sediments that can be associated with the presence of gas hydrate.

This dissertation work applies seismic tomographic inversion methods to two different datasets - one to address the earthquake hazard within the Strait of Georgia and the other to estimate hydrate concentration and distribution in the continental slope off Vancouver Island. In the first part of the study, seismic refraction/wide-angle reflection data from onshore-offshore experiments in 1998 and 2002 were inverted for a smooth three-dimensional (3D) velocity structure down to depths of 6-7 km beneath the Strait of Georgia, a seismically active region where an earthquake swarm (with magnitude up to 5) occurred in 1995-1997. The objectives were to map structures that contribute to seismic hazard evaluation in the Georgia Basin. The main structural features obtained from the inversion are: a northeast-southwest trending hinge line at the location of the earthquake swarm, where the basin deepens rapidly to the southeast; a northwest-southeast trending velocity discontinuity that correlates well with the surface expression of the shallow Outer Island fault; sediment thickening from north to south; and basement uplift at the San Juan Islands, possibly caused by a thrust fault. In the second part of the dissertation, seismic single channel and wide-angle reflection data collected in September 2005 were analyzed for a 2D profile of ocean bottom seis¬mometers (OBSs) on the continental slope region off Vancouver Island, near ODP Site 889 and IODP Site 1327. The objectives were to determine the shallow sediment velocity structure associated with marine gas hydrates and to estimate the hydrate concentration in the sediment pore space. Combined inversion of single channel and OBS data produced a P-wave velocity model down to the depth of the BSR at 230 m below seafloor. Strong attenuation of P-waves below the BSR indicates the presence of free gas. To investigate structures below the BSR, forward modelling of S-waves was carried out using the data. from the OBS horizontal components. Both the P- and S-wave models match very well with the sonic log data from ODP Site 889 and IODP Site 1327. The increase in P-wave velocity of the hydrate bearing sediments relative to the background no-hydrate velocity was utilized to estimate the hydrate concentration by using a simple porosity-reduction equation. An average concentration of 15% was estimated from the P-wave velocity model. Prestack depth migration was applied to the OBS data to image the structure along the 2D profile containing the OBSs. The primary and multiple arrivals were migrated separately. Conventional migration of the primary arrivals produced an image with a very narrow illumination and the shallow subsurface layers including the seabottom were not imaged. However, migration of the OBS multiples, using a mirror imaging technique, pro¬duced a continuous structural image of the subsurface including the shallowest layers. The lateral illumination is much wider with a quality comparable to that of vertical incidence reflection data.

At the frontal ridge near the base of the slope off Vancouver Island, wide-angle ocean bottom seismometer (OBS) data were acquired in summer 2005, in support of the Integrated Ocean Drilling Program (IODP) Expedition 311. Marine gas hydrate is present beneath the ridge based on the observation of the 'Bottom Simulating Reflector' (BSR) that is interpreted to coincide with the base of the methane hydrate stability zone. Hydrate was also observed in downhole logs and drilling by IODP. The BSR has been identified on single-channel seismic data at -250-260 m depth beneath the ridge crest and on its seaward slope. The OBS data have been analyzed with the objective of determining the velocity structure in the upper portion of the accretionary wedge especially the hydrate stability zone and underlying free gas. As identified by a clear refracted phase, the velocity structure above the BSR shows anomalous high velocities of about 1.95 (±0.5) km/s at shallow depths of 80 - 110 m. On vertical incidence data, high amplitude reflectors are observed near this depth. Below the BSR, the velocities increase to -2.4 km/s at sub-seafloor depths of about 600 m. A strong refracted phase with a velocity of 4.0 km/s is generated at a depth of about 1700 mbsf. Velocities from traveltime inversion of OBS data are in general agreement with the Integrated Ocean Drilling Program (IODP) X311 downhole sonic velocities. In particular, on the log data, a layer with low porosity and high velocities of 2.4 - 2.8 km/s was observed at depths of 50 - 75 m. This probably corresponds with the 1.95 km/s layer at depths of 80-110 m interpreted from the OBS data. The refraction data thus suggest that this high-velocity layer varies laterally through the frontal ridge region, out to distances of at least 4 km from the drillhole. BSR depths (250-280 m) estimated in the present work also agree with the IODP X311 depths. From the velocity structure, we can make estimates of hydrate concentration in a region close to the deformation front, where fluid flow velocities are expected to be large. The gas hydrates concentrations vary from -35% for the shallow phase to -22% for the layer above the BSR. The deep refracted phase with a velocity of 4.0 km/s at 1700 m depth indicates the presence of highly compacted accreted wedge sediments. On the SW side of the frontal ridge, a collapse structure is observed in newly acquired multi-beam bathymetry data from the University of Washington and in seismic reflection data. The BSR is present in the region surrounding the slump. There are only weak indications of its presence within the slide region. Since hydrates may prevent normal sediment compaction, their dissociation in sediment pores is thought to decrease seafloor strength, potentially facilitating submarine landslides on continental slopes. The head wall of the frontal ridge slide is -250 m high, extending close to the BSR depth, and the slump has eroded a -2.5 km long section into the ridge, along strike. Migrated seismic reflection data image a set of normal faults in the frontal ridge striking NE-SW, perpendicular to the strike of the ridge and the direction of plate convergence. These faults outcrop at the seafloor and can be traced from the surface through the sedimentary section to depths well below the BSR in some locations. Seafloors scarps show that fault seafloor displacements of -25 m to 75 m are generated. The two faults with the largest seafloor scarps bound the region of slope failure on the frontal ridge, suggesting that the lateral extent of slumping is fault-controlled. The triggering mechanism for the slope failure may have been a combination of various effects. The possible mechanisms explored include gas hydrate dissociation, high pore pressure fluid expulsion along the faults, and salinity elevation in faults which would inhibit the formation of gas hydrates along the faults. However, an earthquake may induce initial slope failure, which can not only start gas hydrate dissociation but also increase fluid expulsion and pore pressure.

Hydrate-bearing sediments may contribute to the availability of energy resources, affect climate change, or cause seafloor instability. The comprehensive study of hydrate-bearing sediments documented in this manuscript includes physicochemical aspects of hydrate nucleation near mineral surfaces, the validity of THF as a substitute guest molecule for the study of hydrate-bearing sediments, and the effects of hydrate formation on the electromagnetic and the mechanical properties of various soils with a wide range of specific surface. Natural marine sediments are included as part of this investigation to explore the effects of inherent fabric, salts, organic matter, and stress history on the geophysical properties of hydrate-bearing sediments. Experiments are designed to reproduce the state of effective stress in the field at the time of hydrate formation. A comprehensive set of instruments is deployed in this study, and the unprecedented development of electrical resistivity tomography for the study of hydrate formation and dissociation is also documented in detail. Results from this research have important implications for geophysical field characterization and monitoring processes such as production.

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008. / Committee Chair: Dr. Boris Mizaikoff; Committee Member: Dr. Andrew Lyon; Committee Member: Dr. Donald R. Webster; Committee Member: Dr. Facundo M. Fernandez; Committee Member: Dr. Joseph Montoya. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Considerable research has been done to improve hydrate formation rate. One of the ideas is to introduce mechanical mixing which later tend to complicate the design and operation of the hydrate formation processes. Another approach is to add surfactant (promoter) that will improve the hydrate formation rate and also its storage capacity to be closer to the maximum hydrate storage capacity. Surfactant is widely known as a substance that can lower the surface or interfacial tension of the water when it is dissolved in it. Surfactants are known to increase gas hydrate formation rate, increase storage capacity of hydrates and also decrease induction time. However, the role that surfactant plays in hydrate crystal formation is not well understood. Therefore, understanding of the mechanism through morphology studies is one of the important aspects to be studied so that optimal industrial processes can be designed. In the present study the effect of three commercially available anionic surfactants which differ in its alkyl chain length on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. The surfactants used were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). Memory water was used and the experiments for SDS were carried out at three different degrees of under-cooling and three different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. In addition, extensive hydrate crystal growth on the crystallizer walls is observed. Also a “mushy” hydrate instead of a thin crystal film appears at the gas/water interface. Finally, the addition of SDS with concentration range between 242ppm – 2200ppm (ΔT =13.10C) was found to increase the mole consumption for hydrate formation by 14.3 – 18.7 times. This increase is related to the change in hydrate morphology whereby a more porous hydrate forms with enhanced water/gas contacts. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Research into hydrate production technologies has increased in the past years. While many technologies have been presented, there is no consensus on which reactor design is best for each potential application. A direct experimental comparison of hydrate production technologies has been carried out in between a variety of reactor configurations at similar driving force conditions. Three main reactor types were used: a stirred tank, a fixed bed and a bubble column and compared different phase contacting patterns for the stirred tank and bubble column. In the initial phase of hydrate formation in a stirred tank, formation was mass and heat transfer limited at the lower stirring speed, and heat transfer limited at the higher stirring speed. After more than 10% of the water had been converted to hydrate, formation was mass transfer limited regardless of the other conditions. Neither the use of a gas inducing impeller, nor a 10 wt% particle slurry significantly affected hydrate formation rates; however, the particle slurry did lower the induction time. Due to the poor scale-up of impeller power consumption in a stirred tank, a semi-batch fixed bed was studied since it does not require any power input for mixing. The significantly slower rates of formation observed in the semi-batch fixed bed, as well as the lost reactor capacity to particles, mean that this type of system would require a much larger reactor. Faster volume and power normalized rates of hydrate formation were observed in the bubble column than in a stirred tank at similar mass transfer driving force conditions. Higher conversions of water to hydrate were observed in the bubble column because mixing was accomplished by bubbling gas from the bottom rather than by an impeller. The highest conversions of water and gas were achieved during a later stage of accelerated hydrate formation, indicating an optimal hydrate fraction for continuously operated bubble column reactors. The second stage of hydrate formation occurred more frequently at higher gas flowratess. Therefore, the increased water conversion and single-pass gas conversion justify the increased energy input required by the higher gas flowrate. Balancing the rates of mass transfer and heat removal was also critical for optimal bubble column as insufficient mass transfer would result in a lower rate of formation and insufficient heat transfer would cause previously formed hydrates to dissociate. The addition of 10wt% glass beads to the reactor promoted hydrate formation; however, it did not do so sufficiently to make up for the loss in reactor capacity or the increased energy requirement.

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