Viscosity and yield stresses of ice slurries formed in water-in-oil emulsions

Abstract

Molecular inclusion compounds called clathrate hydrates are a common concern in oil and gas pipelines, as they cause disruption to production. These crystalline compounds are over 80 mol% water and are often only stable at high pressures and low temperatures. As a means to understand the rheology of clathrate hydrates, we investigated ice slurries, in crude oil, as a simple analogy to clathrate hydrates. A series of water-in-oil emulsions were prepared at different volume fractions of water, ranging from 0.10 to 0.70. Water used in the samples was deionized watger or a 3.5 wt% NaCl brine solution. The emulsions were cooled to - 10 ° C and the viscosity and yield stress were analyzed as a function of time after nucleation. No yield stresses were observed at volume fractions below 0.2 for fresh water and 0.3 for brine solution. In the fresh water system, the yield stress varied with increasing volume fraction. Between volume fractions of 0.25–0.55, yield stresses were on the order of 300 Pa, and at larger volumer fractions (0.6–0.7) yield stress quickly increased to an unmeasurable value (greater than 3000 Pa, the instrument’s limit). In the brine system, yield stress increased with volume fraction of water. After formation of ice, flow was stopped and the system was “annealed”. During the “annealing” period, the magnitude of complex viscosity of the fresh water system reached a peak value after two hours, decreased for approximately four hours, and then changed little for the next forty hours. The yield stress during “annealing” mimicked the trend of the magnitude of complex viscosity. In the brine system, the magnitude of complex viscosity increased over the first three hours, then changed little. However, the yield stress decreased as the “annealing” time increased. Following the measurements of yield stress, the slurry was conditioned at 500 s - 1 and the apparent viscosity was analyzed as a function of shear rate. At volume fractions greater than 0.10 the slurry was found to be shear thinning and exhibited a viscosity increase compared to the initial emulsion.