Risk and preventive strategies of hydrate reformation in offshore gas hydrate production trials: A case study in the Eastern Nankai Trough

Hydrate reformation in offshore gas hydrate production trials is an important issue that can readily cause flow blockage accidents, resulting in the termination of production trials. Previous studies on hydrate flow obstacle were mostly focused on conventional oil and gas production or transportation. However, there is a knowledge gap on prediction and management of hydrate reformation risk in gas hydrate production trials. In this study, a fully coupled thermal and multiphase flow model was developed to evaluate the risk of hydrate reformation in gas hydrate production wells, considering its specially designed well configuration and depressurization strategy. Model predictions were in good agreement with field monitoring data. A thorough analysis on the issues of hydrate reformation in twice offshore production trials of the Eastern Nankai Trough was conducted for the first time. The results indicate that in AT1-P and AT1-P3 wells, there is a greater risk of hydrate reformation in the water line (WL) and no risk exists in the gas line (GL); however, the hydrate reformation risk is also encountered in the GL of AT1-P2 well because wellhead backpressure and lower depressurization amplitude were implemented. Moreover, a series of sensitivity analyses were conducted with different well design and production parameters. It reveals that the hydrate reformation in the WL is inevitable unless complete downhole gas-liquid separation is achieved, but that in the GL can be completely prevented by raising the depressurization amplitude to above 6 MPa. Additionally, a new hydrate management strategy by rationally configuring wellbore pressure and improving thermal utilization efficiency was proposed. It has proven that new method enables to fully eliminate the risk of hydrate reformation in the production well without the used of the hydrate inhibitors. This work provides valuable references and practical implications for future short-term or long-term offshore gas hydrate production trials.