Morphology and Rheology of Cyclopentane-Hydrate Emulsions

Clathrate hydrates generate great interest and concern in the petroleum industry because of the potentially severe consequences of pipeline blockage by hydrate plug formation. In this study, the mechanical properties of cyclopentane-hydrate forming emulsions will be examined, seeking primarily to relate the morphology of the hydrate formed for different parameters to the bulk rheological and solid mechanical properties. Cyclopentane forms hydrates under atmospheric conditions, and these structure II hydrate formed is stable in a temperature window above the melting point. These features allow ready experimentation under standard laboratory conditions with clear connection of hydrate crystal morphology to the observed properties.

A combination of single-drop visualizations and rheometric studies using density matched emulsions (to avoid sedimentation/creaming effects) will be described which show that surfactant plays a key role in the development of the hydrate layer formed on water drops. The work focuses on the oil-soluble surfactant sorbitan monooleate (Span 80), and will illustrate that depending on its loading it can have a range of effects; the hydrate layer can vary from a robust polycrystalline layer at small loadings of surfactant to a mushy, hairy structure at large loadings. The impact of this morphology on the rheology of the mixture includes cases of large viscosification and jamming, although the influence of the surfactant is found to be dependent on the water fraction.