Rheology of Cyclopentane sII Hydrate Slurry in Water-in-Model Oil Emulsions: Effect of Surfactant Concentration and Water Droplet Size Relevant for Flow Assurance

Abstract

Investigation on the effect of surfactant concentration and droplet size distribution in a water-in-oil emulsion system on the hydrate suspension rheology is vital to delineate the antiagglomeration role and formulate a suitable mitigation strategy for flow assurance issues. Though several studies have been reported on the effect of surfactants on hydrate kinetics, rheological characterization of hydrate systems with surfactants and their role as antiagglomerant are scarce. In this study, rheological measurements are carried out to understand the effect of a mixed surfactant system (Span80 and Tween80 with HLB value of 5.9) at varying concentrations (0.5, 1, 1.5, 2, and 3 wt %) on the structure sII cyclopentane hydrate slurry formed in a water-in-model oil emulsion with varied droplet size distribution at 265.15 K, atmospheric pressure, and γ̇ = 100 s–1. The model oil consists of light paraffin oil and toluene to represent the saturate and aromatic fractions of the crude oil. Viscosity profiles are reported during formation and dissociation of hydrate slurries, while the flow curve and viscoelasticity tests are reported for the stable hydrate slurries. The presence of a mixed surfactant system is observed to increase the maximum peak and steady-state viscosities of the hydrate slurries up to 1.5 wt % surfactant concentration due to an enhanced oil–water interface and the water bridging between hydrate particles and droplets. At higher surfactant concentration (>1.5 to 3 wt %), capillary bridge between hydrate particles in contact is diminished due to steric hindrance provided by the hydrophobic group of the surfactant adsorbed on the hydrate surface, thus reducing the hydrate slurry viscosity and acting as antiagglomerant. At higher surfactant concentration, the yield stress reduction is observed in the range of ∼44–87% from the base value. The dissociation experiments show a small depression in the hydrate dissociation temperature from 0.5 wt % to 2 wt %. For 3 wt %, the higher dissociation temperature could be due to metastable hydrate formation in addition to the surfactant molecules surrounding the hydrate crystals, delaying their dissociation.