Simulations of hydrate reformation in the water production line of the second offshore methane hydrate production test in Japan's Nankai trough

Natural gas hydrates are a potential new energy resource, however, challenges for stable, long-term gas hydrate production remain. This study examines the flow assurance risk of hydrate re-formation in production lines during gas hydrate production, which is a key issue in designing reliable production systems and operations. The focus of this study is the second offshore methane hydrate production test conducted in the Nankai trough area of Japan in 2017, where the depressurization method with a pump, wellbore gas separation, and two production lines for gas and water was applied. In this field test, separation inefficiencies caused significant quantities of gas to be produced from the water production line, posing a risk of hydrate re-formation. To understand and aid in management of this risk, we have developed models of hydrate growth and dissociation in water-dominant flow and conducted flow simulations that were compared to field data from the water production line in this field case. The simulation predictions suggest that the hydrate re-formation was not significant in this production test due to use of flexible hoses with a low overall heat transfer coefficient. The thermal insulation of the flexible hoses contained the heat generated by hydrate formation inside the production lines, resulting in a heat transfer limited system with low hydrate volume fractions (<5.2 vol%). Further simulations were conducted to estimate the sensitivity of such systems to insulation and materials choice, where a system with the flexible hoses replaced with carbon steel pipes was predicted to experience volume fractions four times greater (<19.8 vol%). Given hydrate production test systems typically take the form of short vertical lines, this indicates that pipe insulation may be critical in managing the risk of plugging due to re-formation. This insight is particularly important given the widespread use of carbon steels in subsea production facilities. Simulations enabled by the algorithm presented in this work will inform design choices for future methane hydrate production facilities. The use of thermal insulators will likely limit the degree of hydrate re-formation in such systems, and enable the deployment of management strategies that minimize or eliminate chemical injection.