Rheological Characterisation of Hydrate-in-Oil Slurries

Gas hydrates are ice-like solids that may form in crude oil flowlines under high pressure and at low temperature, resulting in high frictional pressure loss and increasing the risk of a non-flowing condition. The viscosity of hydrate slurries has been suggested as a primary criterion to determine blockage risk, but limited rheology data are available to inform the predictive viscosity models. The rheology of hydrate slurries formed from water-in-oil emulsions was measured using a high-pressure controlled-stress rheometer, where gas saturation in the slurry as maintained with a vane-blade rotor. Tests were conducted under isochoric conditions, where the apparent viscosity and pressure of the system were recorded throughout the hydrate growth process, enabling the first comparison of hydrate slurry viscosity as a function of hydrate volume fraction in the liquid phase. The results show that, for 5 to 30 % watercut systems, hydrate formation was repeatedly identified by a significant decrease in pressure and a corresponding increase in slurry viscosity; the maximum and steady-state viscosity values recorded during the hydrate formation process deviated by a maximum of 10% through independent trials. As vane blade rotor was able to maintain saturation of the crude oil phase, these measurements represent the first insight into the worst-case slurry viscosity condition achievable in crude oil flowlines.