Gas hydrate deposition from water saturated vapor in deadlegs

Abstract

Deadlegs are pipe sections with no through flow which contribute to the complexity in gas and oil production systems. Deadlegs commonly face flow assurance challenges related to hydrate deposition. Despite the challenges, very few reports or studies are publicly available about this topic. The related studies concentrate on either hydrate deposition in a flowing system, or deadlegs with no hydrate. The understanding of hydrate deposition in a deadleg is limited. The focus of this thesis is to better understand the hydrate deposition in a deadleg by qualitatively and quantitatively studying the process. A model system is established to mimic vertical gas-filled deadlegs. The experiments are run at constant wall temperature, constant header temperature, and constant pressure. All experiments use methane/ethane (75/25%) mixture as a hydrate gas former. The thesis has studied various variables that are important to hydrate deposition, including the header temperature, the wall temperature, the pipe size, the water vapor content, and the flow pattern. When the wall temperature is lower than the HET, the hydrate deposition may not be avoidable. The hydrate can accumulate and plug the system at a location Lp when the pipe length is large enough. At given gas components and pressure, Lp is a function positively correlated to header temperature and wall temperature. The hydrate growth rate, increases with increasing header temperature due to increasing condensation, and decreases with increasing wall temperature due to less subcooling. When the pipe is short than Lp, the hydrate deposition is limited due to high temperature, and the plugging risk is reduced. However, a certain amount of deposit can still exists, and raise concerns. The effect of the water vapor content is studied by adding glycerol to the liquid. Glycerol in the header is observed to slow the growth rate approximately proportional to its concentration at low header temperature, but not at high header temperature. The results indicate that the condensation is the rate limiting step at low header temperature. A blower is used to generate forced convection to test the hydrate deposition under a mixed convection. The added flow is observed to increase to the mixing in the system and changes the temperature field. However, no significant change of the deposition is observed at the tested conditions. By analyzing the heat and mass transfer in the system, the temperature profiles without hydrate are found. The correlation between temperature and Lp is also found. The heat and mass transfer coefficients are calculated based on a set of assumptions.