Prediction of hydrate formation and plugging in the trial production pipes of offshore natural gas hydrates

Hydrate plugging in pipes is a serious issue that affects flow safety in the trial production of offshore natural gas hydrates. Current research on hydrate flow assurance has primarily focused on conventional oil-and-gas production and transportation processes. Therefore, models for predicting the hydrate formation region and hydrate deposition in the trial production pipes of offshore natural gas hydrate were developed in the present study, which can be used to estimate the risk of hydrate plugging in the production pipes. The results revealed that hydrate formation conditions could be satisfied in the water production pipe at a low water production rate, and hydrate generation was possible in a certain region in the gas production pipe. The hydrate deposition layer formed in the gas production pipe grew in a non-uniform manner; the longer the production time, the more obvious was the non-uniform distribution; this resulted in a high risk of hydrate plugging after a certain process time. Additionally, as the rate of gas production increased, the hydrate formation region in the gas production pipe gradually moved upward and decreased in size, and the degree of hydrate deposition also slightly decreased. Finally, the installation of a heater at the bottom of the gas pipe at a high production rate was proposed to increase the gas temperature, thereby decreasing the possibility of hydrate formation and plugging. For the water production pipe, a low dose of kinetic inhibitors or anti-agglomerants could be injected to avoid the risk of hydrate plugging and reduce the corresponding prevention costs. This study can provide a theoretical basis and technical support for the efficient prevention of hydrate plugging during production trials or long-term production of offshore natural gas hydrates.