Salt effects on thermodynamic and rheological properties of hydrate forming emulsions

Abstract

Results from an experimental study of the effect of aqueous phase salinity on a density-matched 40% (v v) aqueous fraction hydrate-forming emulsion are reported. Using micro-differential scanning calorimetry and rheometry, a correlation of the rheological properties to thermodynamic driving forces is obtained. The cyclopentane hydrate-brine system liquidus line is determined from the calorimetric study. Equilibrium temperature and thermodynamically allowed conversion of water to hydrate are computed from the hydrate-brine phase diagram. Good agreement was found between the experiments and calculations by an established thermodynamic simulation tool. Rheological properties, including the viscosity evolution time, final viscosity and its shear rate dependence, and yield stress of the hydrate-forming emulsion are determined; the material viscosity increases markedly as hydrate forms and the mixture develops a yield stress. The viscosity evolution time is smaller at lower temperature, i.e., at higher subcooling relative to the hydrate equilibrium dissociation temperature. The final hydrate slurry exhibits shear thinning effects describable by the Ostwald-de Waele model, and displays very little hysteresis in the shear rate dependence. Maximum viscosity is attained when the thermodynamic water to hydrate conversion is 61-85% while the yield stress was found to be a maximum at about 80% conversion; this non-monotonic rheological property dependence on the thermodynamically allowed water to hydrate conversion demonstrates that both liquid (brine) and solid hydrate content are important to the mechanical properties of the network structure developed when hydrate is present. (c) 2013 Elsevier Ltd. All rights reserved.