Effect of brine salinity on the rheological properties of hydrate-in-oil slurries

Abstract

As oil and gas production moves into deep-water, the effect of brine salinity in water produced with petroleum is an important factor to be considered for hydrate management in transport pipelines. In this work, methane hydrate formation was conducted in water-in-oil emulsions with various salinities in the aqueous phase by using a high-pressure rheometer. The viscosity evolution during hydrate formation was analyzed. The results suggested that the increase in salt concentration could significantly reduce the hydrate slurry viscosity, and improve the stability of the slurry. As the salinity increased from 0.3 wt% to 5 wt%, the relative viscosity of the final slurry after hydrate formation decreased from more than 300 to only 42. When the salinity was greater than 1 wt%, hydrate slurries could keep stable flow for a long time, and the relative viscosity was less than 100. Meanwhile, a shear ramp was conducted. The hydrate slurries exhibited shear-thinning, and the rheological properties were described with an established viscosity model. Good agreement was observed between the experiments and model calculations under different brine salinity, and the aggregation process of hydrate particle and viscosity change at different shear rates were analyzed. In addition, a rest and restart process was conducted, and the yield stress of time-dependent was analyzed. With increasing rest time, the yield stress increased gradually. A high brine salinity of 5 wt% in the aqueous phase could significantly decrease this yield stress value and slow its increase with the rest time. After 1280 min of rest, the yield stress increased to 83 Pa, which was only one third of the yield stress value of the hydrate slurry with 1 wt% NaCl. Particularly, with a high brine salinity, when the rest time was over 160 min, two-step yielding behaviors were first observed for the hydrate slurry in the stress sweep, and a possible mechanism for the two-step yielding in the hydrate slurry system was proposed.