Prediction of Pressure and Temperature Profiles and Hydrate Formation Region in ESP-Lifted Natural Gas Hydrate Wells

Abstract Depressurization is one of the most effective methods in natural gas hydrate (NGH) production trails. Meanwhile, electric submersible pumps (ESP) are mostly employed to depressurize natural gas hydrate reservoirs. The profiles of temperature and pressure in the wellbore have an important impact on the secondary formation of hydrate. The pressure increase caused by ESP system and the low temperature on seabed will increase the risk of secondary hydrate formation in gas hydrate production wells. In contrast, the temperature rise generated by motor and the heater added to the ESP string will decrease the risk of secondary hydrate formation. Therefore, it is necessary to evaluate the risk of gas hydrate formation in ESP-Lifted gas hydrate production wells. Based on the world's first offshore methane-hydrate production-test system in Nankai Trough comprised of an ESP with a gas-separation system and a heater, a gas-liquid two-phase flow model coupling temperature and pressure is established to predict the formation region of hydrate in dedicated gas/water lines and mixing-delivery line. The influences of operating frequency of ESP and power of heater on temperature and pressure are analyzed in the proposed model in this research. The results of the proposed model are verified by comparing with the first offshore gas hydrate production test in the Nankai Trough. The temperature limit for gas hydrate formation inside the well can be estimated with the proposed model and phase equilibrium model to predict the risk of secondary hydrate formation in ESP-lifted wells. In this paper, it is demonstrated that the highest hydrate formation risk is above the discharge location of the pump and the middle and lower part of the seawater section of the water drainage line. Increasing the operating frequency of ESP and power of heater can reduce gas hydrate formation in gas hydrate wells. Besides, injection of chemical inhibitor can eliminate the risk of hydrate reformation. The results of this research can lay the foundation for flow assurance, ESP designs and production optimization of natural gas hydrate production wells.