(70c) Development of a Predictive Model for Hydrate-in-Oil Slurry Viscosity

The cooling of high-pressure oil and gas flowlines enables the formation of ice-like solids known as gas hydrates, which may be suspended in the oil phase, forming viscous, hydrate-laden slurries. As the slurry viscosity is a critical parameter to assess the probability of hydrate blockage, the development of a predictive viscosity model is critical for multiphase flow simulations. However, the current model used throughout the industry has received limited experimental validation. In this work, a controlled-stress high-pressure rheometer with a vane blade rotor was used to measure the viscosity of hydrate-in-oil slurries with two crude oils at watercuts of 5 to 30%. The dynamic viscosity profiles during hydrate formation and the steady-state flow curves were measured and compared to the current industry-standard viscosity model. The results show that hydrate slurries are shear thinning, and the magnitude of the flow curve increases directly with hydrate volume fraction. At steady-state, the deviation between experiment and the current viscosity model exceeds 50%, where the deviation increased directly with watercut. This result illustrates that a new generation of viscosity model is required to improve the accuracy of multiphase flow simulations. The yield stress of the fully-converted hydrate-in-oil slurries was measured after an eight-hour annealing process that was designed to simulate shut-in for a subsea flowline; the results showed that the yield stress increased exponentially with hydrate volume fraction.