Energy & Fuels, Vol.32, No.2, 1012-1023, 2018
Simulating Hydrate Growth and Transport Behavior in Gas-Dominant Flow
The current hydrate kinetics model implemented in the multiphase flow simulator OLGA treats hydrate growth in oil-continuous systems by considering the solidification of emulsified water droplets to form a hydrate-in-oil slurry that is assumed to be stable. To date, the validity of this model has not been established for gas-dominant systems, where gas void fractions can exceed 90 vol %. Here, six experimental data sets, collected using a 40-m single-pass gas-dominant flowloop operating in the annular-flow regime, were compared with predictions made using the current hydrate kinetics model. The comparison identified discrepancies in the predicted flow regime and the gas water interfacial area that significantly affect kinetic hydrate-growth-rate calculations; these discrepancies might be due, in part, to differences in dynamic similarity between flowloop experiments and industrial-scale simulations. By adjusting only the kinetic rate scaling factor, it was not possible to match the pressure drop observed experimentally, illustrating that the formation of a viscous hydrate slurry alone cannot account for the resistance to flow observed in gas-dominant systems. We demonstrate that it is possible to emulate deposition in the current model by adjusting the slip ratio between the hydrate particles and the condensed phases; this approach allowed stenosis-type restrictions to occur in the simulation, as well as pressure-drop behavior similar to that observed experimentally. Utilizing a simple in-house model with empirical correlations to predict the hydrodynamics, it is possible to match relatively closely the measured growth rate and pressure drop simultaneously. Such agreement could not be reached using the current hydrate implementation available in OLGA, highlighting the need for a gas-specific hydrate growth model that is capable of capturing both hydrate growth from suspended droplets in the gas phase and solid growth at the flowline wall, as well as the extent of hydrate deposition on the wall.