| Summary: | <p> Flow blockage of subsea petroleum pipelines due to clathrate hydrate formation and deposition is a major challenge experienced by the oil and gas industry. Plugging due to hydrate formation in pipelines leads to unexpected shut down and economic losses. This problem becomes serious and a focus of interest when deep and ultra-deep sea fields are drilled and explored. A thorough analysis of hydrate formation requires understanding of associated evolving and dynamic processes. This work probes the properties of hydrate-forming emulsions and the resulting hydrate slurries using rheology, crystal morphology, calorimetry and flow loop. In this work, primarily two guests namely liquid cyclopentane and propane gas are used to study structure II hydrates. </p><p> The rheological properties of hydrate-forming emulsions investigated over a wide range of water volume fractions for oil-continuous emulsions are reported in Chapter 2. A hydrate slurry is prepared under shear using liquid cyclopentane (CP) as the hydrate former at atmospheric conditions from a density-matched water-in-oil emulsion, by quenching it to a lower temperature at a fixed shear rate. Hydrate slurries were found to exhibit shear thinning and thixotropic behavior. Direct visualization of hydrate formation on water droplets in water-in-oil emulsions is also presented. The influence on morphology of the crystalline structure in the presence of surfactant is analyzed, and correlated to observations of the evolution of rheological properties as the reaction to form hydrate proceeds. For 30 vol% water fraction, flocs of mushy/hairy hydrate particles agglomerating together and creating a strong networked structure is observed. </p><p> The effects of hydrophobic fumed silica particles (of average primary particle size 7 nm) on the rheology of hydrate-forming emulsion are reported in Chapter 3. In this work, the possibility of using hydrophobic silica nanoparticles at a surfactant-stabilized water-oil interface as a method of hydrate inhibition and its limitations are studied. The performance of these particles at different levels of subcoolings is also evaluated. Oscillatory measurements are carried out to further test the role of particles at the interface in the hydrate formation process. </p><p> The mechanistic aspects of gas hydrate crystal effects on rheology of the system can be in part understood by studying its morphology, which involves a direct visualization of hydrate formation at fluid/fluid interfaces. For this, a single suspended water droplet exposed to a second phase containing a hydrate guest molecule at significant concentration is used and is presented in Chapter 4. In this work, a systematic comparison of morphologies of structure II hydrates synthesized using two different guests is carried out. At all water fractions, the propane hydrate slurry was found to be shear thinning. Increasing and decreasing shear rate ramps on the propane hydrate slurry obtained at a fixed shear rate showed a small hysteresis. </p><p> The effect of salt, a thermodynamic inhibitor of hydrate, on a 40 vol% aqueous phase density-matched cyclopentane hydrate-forming water-in-kerosene emulsion is presented in Chapter 5, based on studies by micro-differential scanning calorimetry (μDSC) and rheometry. Kerosene is a candidate material for flow loop studies. Micro-differential scanning calorimetry data are presented on the equilibrium temperature (liquidus line); from this information, thermodynamically allowed hydrate conversions are determined. Hydrate-forming water-in-kerosene emulsion is used. The evolution of pressure drop during the hydrate formation in the emulsion is recorded. The overall results from flow loop shows a similar trend as obtained from a rheometer. (Abstract shortened by UMI.)</p>
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