Hydrate management in Deadlegs: Effect of water vapor content on hydrate deposition

Abstract

Deadlegs are pipe sections with no through flow. In the oil and gas industry, they often impose flow assurance challenges in gas dominated systems due to gas hydrate deposition. Gas hydrate formation depends on temperature, pressure, water, and guest molecules. This paper discusses the effect of water vapor content on hydrate deposition in a gas dominated system. A deadleg is mimicked using a vertical upward pipe of 5.0 cm (2 in.) in inner diameter and 121 cm in length. The pipe wall temperature (Tw) is at 4 °C. The header temperature (Tr) is either at 30 or 80 °C. Hydrates are formed with a methane/ethane (3:1) mixture at 100 bar for 168 h. The water content in the vapor phase is controlled by adding glycerol (C3H8O3) to the water in the header. The study improves the understanding in the hydrate deposition mechanism in deadlegs by confirming the importance of water condensation. This study shows that the low-volatility thermodynamic hydrate inhibitors (THIs) can affect water content in the vapor, which subsequently affects hydrate deposition rate rather than preventing hydrate formation. The growth rates are compared via the visual observation and the water recovery from hydrate dissociation. At Tr = 30 °C, the reduction of the total deposit is found to be correlated to the initial glycerol fraction. The reduction is less significant at Tr = 80 °C. The differences of glycerol effects at different Tr should be related to the growth stages of the hydrate deposition.