(60d) Hydrate Formation Experiments in a Lab-Scale Vertical Flowloop

The formation of gas hydrates in subsea oil and gas flowlines remains a significant operational risk, due to their rapid growth rates and ability to form large aggregates. Much of our understanding of hydrate blockage formation to date has been derived for oil-continuous pipes, where water droplets may be dispersed in the oil phase. However, knowledge of hydrate growth rates and particle transportability in water-continuous systems is less well-understood. In this presentation, we employ the use of a vertical flowloop to investigate hydrate growth behavior using mineral oil and water under steady-state and transient operations.

A high-pressure water tunnel (HPWT), initially constructed to examine hydrate growth behavior on rising gas bubbles following a subsea blowout, was converted for use as a vertical flowloop. Changes included the installation of an ultrasonic flowmeter, differential pressure transducer and viscometer. More recently, the flowloop has been adapted to allow the introduction of mineral oil, as well as the installation of a high-pressure wire feedthrough to measure the conductivity of the flowing fluids, thereby determining the continuous phase.

Experiments were performed using water and mineral oil at water volume fractions of 100, 80, and 50%, at a constant mixture velocity of approximately 1 m/s. Gas addition, hydrate dissociation and flowloop restart tests (with and without hydrates present) were conducted to investigate the effects of hydrate formation under both steady-state and transient operations. A transparent ‘bulls-eye’ pipe segment, on one of the vertical sections of the loop, allowed for visual observation of hydrate growth behavior. Coupled with a high-speed camera, this allowed for detailed investigation of interfacial phenomena under flow. Contrary to the formation of solid shells on water droplets in oil-continuous systems, we present new evidence suggesting hydrates do not always form complete shells around methane-saturated oil droplets in water-continuous systems. Instead, hydrate crystals were sheared from the gas-water interface soon after formation, and aggregated in the continuous water phase.

These tests have enabled a quantitative comparison of the potential mass transport limitations to hydrate growth, including gas dissolution in the aqueous phase and gas diffusion to the growing hydrate particle. The measured hydrate slurry viscosity increased directly with hydrate volume fraction, increasing the momentum energy required to restart flow following a shut-in. The results from this investigation provide new insight into hydrate growth behavior in high watercut systems, which may be used in the validation of predictive models.