(26c) Hydrate Deposit Formation and Dissociation in Gas-Dominant Systems: Experiments and Modelling Using a High-Pressure Miniloop

The design and operation of subsea oil and gas flowlines requires the capacity to predict the formation and transport of hydrate particles, which remain one of the largest flow assurance concerns due to the short operating period in which they can result in flowline blockage. While significant research has focused on the development of a mechanistic model to describe hydrate blockage in crude oil systems, this behavior has not been well-established in gas-dominant flowlines.

Previous experiments using a single-pass, gas-dominant flowloop operating at high velocities have reported rapid stenosis at the wall supported by the solidification of water droplets entrained in the gas phase and subsequent deposition on the wall. When the same flowloop was operated without liquid entrainment in the gas phase, the rate of hydrate growth was reduced by one order of magnitude and the impact of hydrate formation on frictional pressure drop decreased. While the deposition of hydrate particles from the gas phase on the wall may represent a primary pathway to hydrate blockage formation, fundamental studies have suggested that the growth of a solid hydrate crystal film at the wall may also contribute to stenosis-based blockage mechanism.

To better characterize the growth of a solid hydrate film, a high-pressure, laboratory-scale miniloop was used to characterize the growth rate and morphology of this hydrate film as a function of subcooling and fluid velocity. The miniloop incorporated a specially-designed “deposition test section,” which was cooled below the remainder of the flowloop to encourage hydrate film growth. The test section contained four viewing windows in which to visually confirm hydrate formation, a differential pressure transducer, and temperature sensors at the inlet and outlet of the test section to better quantify hydrate deposition. Experiments were conducted using superficial gas and liquid velocities between 0.1 and 1m/s, and test section subcoolings of up to 11 °C. Additionally, hydrate dissociation tests were performed by increasing the bulk fluid temperature, whilst maintaining the temperature of the deposition section; this experimental procedure provided insight into the difficulty of removing an annealed hydrate deposit from the wall.

Preliminary tests were used to investigate hydrate deposition rates on the wall in both the gas and liquid phases. Deposits formed in the gas phase were supported by the condensation and crystallization of water at the wall, which provided an annealing mechanism over time. The formation of a deposit in the liquid phase was limited by the diffusion of gas molecules to the growing deposit, and appeared to be less porous than the deposit formed in the gas phase. The rate of deposit growth in the liquid phase was modelled considering transport limitations for gas molecules. The prediction of hydrate deposit porosity in the gas phase requires additional experimental investigation, as there are currently limited direct experimental studies available to inform this property.