Quantitative Evaluation of Hydrate Risk and Prevention Strategies in Deepwater Ultra-Shallow Wellbore Testing

Multiple hydrate deposits and underlying gas reservoirs have been successfully drilled in the deepwater ultra-shallow Qiongdongnan Basin, which is located in the western segment of the northern South China Sea, a achievement that demonstrates excellent potential for natural gas exploration. Due to the extremely high risk of hydrates forming and blocking the pipe string under complex test conditions (which can easily lead to test failure), operators must implement rigorous preventive measures. In this paper, the phase equilibrium conditions in the multi-phase environment of gas, water, ice and sand in the target reservoir are defined through laboratory experiments. Solid phases such as ice and sand promoted hydrate crystal growth, accelerated the formation rate, and shortened the clogging time. Based on the wellbore temperature and pressure fields and the phase equilibrium conditions, a model for assessing the risk of hydrate formation was established. Owing to the combined effects of overpressure and low-temperature conditions, the entire wellbore in deepwater shallow formations lies within the hydrate stability zone. Moreover, higher production rates further intensify the risk of hydrate formation. For instance, at a natural gas production rate of 20 × 104 m3/d, the maximum hydrate formation rate reaches 0.28 g/(m3·s). The mudline is identified as the point of highest risk; therefore, the injection rate of the hydrate inhibitor should be no less than 5m3/d. Based on the previous analysis, a comprehensive hydrate prevention and control strategy has been formulated. Hydrate inhibitor injection valves have been installed at strategic depths to prevent hydrate formation. These valves enable the real-time monitoring of actual water production and wellbore temperature-pressure conditions. Furthermore, emergency mitigation measures have been implemented, including methanol soaking for blockage removal, coiled tubing milling, and the deployment of breakable thin-walled tubing. These measures have successfully ensured flow assurance during the formation testing of two deepwater shallow gas wells. This research provides a critical reference for flow assurance planning in future analogous deepwater shallow gas well tests.