Modeling Oilfield Emulsions: Comparison of Cyclopentane Hydrate and Ice

A comparison of cyclopentane hydrate- and ice-forming emulsions is reported. Density-matched 40% (v/v) water emulsions are studied at various salt concentrations, using experimental tools of rheometry and direct visualization of the morphology under formation of hydrate or ice from the liquid water. Viscosity and yield stress were measured at similar subcoolings (temperature differential between dissociation or melting temperature and the system temperature, with the latter lower) and water conversions, which was controlled by the initial salt concentration in the aqueous phase of the emulsions at a fixed temperature. Large differences in viscosity and yield stress of the final structure were observed between the hydrate- and ice-forming emulsions. At water conversions of X-w = 58%-81%, the ratio of final relative viscosity (final slurry viscosity, relative to the continuous phase viscosity) of hydrate- to ice-forming emulsions is given as eta(rel-hyd)/eta(rel-ice) = O(10(2)); at complete water conversion (X-w 100%), the ratio is eta(rel-hyd)/eta(rel-ice) = O(10). At a water conversion of X-w >= 58%, the yield stress ratio of hydrate-forming emulsions to ice-forming emulsions is given as tau(y-hyd)/tau(y-ice) >= O(10(2)). Morphological results show that, with no salt, the hydrate seed surface punctures the drop in the hydrate-forming emulsion; consequently, as the drop wets the seed, hydrate is formed, whereas in the ice-forming emulsion, the entire water drop freezes as a bulk and its spherical shape is retained. When brine is present, in the hydrate-forming emulsion, hydrate crystals form at the oil/aqueous phase interface of the drop and, as they grow, they puncture adjacent drops, causing coalescence and wetting of the newly generated hydrate; in the ice-forming emulsion, ice growth occurs inside the drop and the growing ice remains completely wetted by residual brine, so no significant change in the oil/aqueous phase interface is observed. These observations indicate that the ice-forming emulsions do not capture the rheological properties and mechanism of morphology evolution of hydrate-forming emulsions.