Prediction and management of hydrate reformation risk in pipelines during offshore gas hydrate development by depressurization

Abstract

The hydrate reformation in marine gas hydrate production system can significantly affect the accuracy control of the production pressure differential and even cause pipeline blockage problems. In this paper, a transient temperature and pressure calculating model was established to predict the risk of hydrate reformation in production pipelines during offshore gas hydrate development, which considering the coupling interactions between the real-time drainage of electric submersible pump (ESP), the downhole preheating of electric induction heater (EIH), the wellbore multiphase flow and heat transfer. Using the model, the dynamic evolution laws of gas–liquid two-phase flow behaviors in different pipelines of mixed transportation, drainage and gas production were studied. Furthermore, the region of hydrate reformation in production pipelines was predicted quantitatively. The results indicate the risk of hydrate reformation in the drainage pipe is the highest relative to other pipes, and the main flow pattern in this pipe is bubbly flow. Besides, the conventional hydrate management method through inhibitor injection was applied to prevent the hydrate reformation. It can be found that the required Ethylene glycol (MEG) minimum concentration reaches 22% in order to completely eliminate the risk of hydrate reformation. To this end, a new hydrate management strategy by adding additional pumps and heaters was proposed, and the prevention effect of hydrate reformation in different scenarios was discussed in detail. The simulated results demonstrate that this method can effectively address the potential hydrate reformation risks without the use of inhibitors. This work will provide theoretical references for the hydrate flow assurance operations in marine gas hydrate production testing.