(201d) Multiphase Flowloop Investigation of Transportability and Flow Properties of Highly Concentrated Hydrate Slurries

Authors: 
Bbosa, B., University of Tulsa
Volk, M., University of Tulsa
Hydrate formation, if not properly monitored and controlled, may lead to pipeline blockage. In order to avoid pipeline blockage and other hydrate formation risks, chemical additives are added to the system. Additives such as anti-agglomerants (AA) help improve hydrate transportability by dispersing the formed hydrates into slurries and preventing them from sticking to the pipe wall. This enables transportation of highly concentrated slurries. However, the high hydrate volume fractions (HVF) slurries may exhibit complex rheology. There is therefore a great need to correlate flow properties such as friction factor and viscosity to HVF. Hydrate slurry transport is important whether hydrates are deliberately generated for energy storage purposes or hydrates formed because of the prevailing flow conditions.

In this work, a multiphase flowloop was use to generate gas hydrate slurries with models oils of different viscosities. Tests were performed at high liquid loading (+75%) and at intermediate watercuts (30 to 70%). Pressure drop was monitored with increasing HVF. AA dosage was maintained at the recommended dosage, i.e., about 2 vol% with respect to aqueous phase for the AA used.

The first part of the manuscript presents viscosity predictions using the non-Newtonian multiphase friction factor model developed by Garcia et al. 2003. Results indicate that viscosity increases almost linearly with increasing HVF up to 18% HVF but exponentially above 25%HVF. This behavior may suggest onset of heterogeneous flow, i.e., AA fails at high HVF.

The second part of the paper presents a comparative study of viscosity models including those developed on benchtop scale. It was observed that models that account for carrier fluid viscosity in predicting slurry viscosity agreed better with experimental data. Presence of free gas phase presented the greatest challenge to viscosity and friction factor models.

Lastly, transportability assessment showed that low viscosity systems were greatly affected by increase in HVF whereas high viscosity systems in addition to HVF, they were significantly affected by temperature changes. Low viscosity flow systems exhibited heterogeneous flow behavior and even plugging at higher HVF. On the other hand, high viscosity oils formed emulsion and were characterized by high shear heat buildup.