Blockage of natural gas and crude oil subsea transport pipelines due to hydrate plug formation is a major concern in the petroleum industry as it directly aspects the goal of flow assurance. A proper analysis of hydrates requires knowledge of the dynamic processes of formation and dissociation as well as understanding of their rheological and morphological properties. This work addresses the issue of clathrate hydrate formation in oil-dominated systems relevant to subsea crude oil transport. Cyclopentane, which forms hydrate at atmospheric pressure, is used as a component of the oil phase in our model emulsions.
Differential scanning calorimetry (DSC) is applied to investigate the formation of cyclopentane hydrates in a water-in-oil emulsion. A novel method based on comparing the heat flow measured by DSC for samples of identically prepared hydrate-forming and non-hydrate (ice-forming) emulsions is developed to obtain the rate of cyclopentane hydrate growth. Experimental results lead to the conclusion that the hydrate formation is an interfacial phenomenon and occurs primarily at the interface between water drops and the continuous oil phase. A three-step mechanism — nucleation, lateral surface growth and radial growth — is described to capture the main features of the hydrate formation process. Mechanical stresses developed in the hydrate shell due to volume expansion upon hydrate formation (a liquid-solid transition) are analyzed.
The proposed three-step mechanism for the hydrate formation is supplemented by direct visualization of hydrate growth at a single water drop suspended in the oil phase. The crystal morphology study probes the effect of an oil-soluble surfactant sorbitan monooleate (Span 80) on the crystal behavior at the water-oil interface. In the absence of surfactant, a faceted polycrystalline hydrate shell develops around the water drop. This limits transport of hydrate former to the free (liquid) water which remains trapped inside the hydrate layer. The presence of Span 80 at concentrations greater than 0.01% by volume in the oil phase leads to a hairy or mushy hydrate morphology. A unique hollow-conical crystal is observed during the lateral surface growth at these Span 80 concentrations. A detailed analysis is presented to explain the role of Span 80 on driving force and transport for the morphological development. The effect of a commercially available anti-agglomerant on the hydrate morphology is investigated. Preliminary results for the high-pressure propane hydrate morphology are reported.
The rheological properties of hydrate-forming emulsions are investigated over a wide range of water volume fractions, both for oil- and water-continuous emulsions. The hydrate formation leads to an irreversible increase in the mechanical properties; high dispersed phase fraction emulsions, especially 30% to 60% water volume fraction, show a rapid viscosification resulting in a rheometer jamming. A mechanism in which the hairy and porous hydrate growth combined with enhanced agglomeration due to liquid bridges formed by wetted water films leads to development of a porosity, resulting in greater effective dispersed phase fraction is proposed. This is supported by experiments performed at variable shear rates, temperatures and surfactant concentrations. In order to probe the hydrate morphological properties under flow, we describe the development of a flow rheometer providing the capability of measurement of mechanical properties together with morphological characteristics through visualization. The results are reported for a 40% water-in-oil emulsion and it is observed that mushy, cotton-like hydrate balls form a complex and porous network developing into a hydrate plug.
|Advisor:||Morris, Jeffrey F., Lee, Jae W.|
|Commitee:||Couzis, Alexander, Denn, Morton, Maldarelli, Charles, Subramani, Hariprasad|
|School:||City University of New York|
|School Location:||United States -- New York|
|Source:||DAI-B 73/09(E), Dissertation Abstracts International|
|Keywords:||Calorimetry, Clathrate hydrates, Crystal morphology, Emulsions, Kinetics, Rheology|
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