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Change mechanism of transient gas-liquid two-phase flow in wellbore during marine natural gas hydrate reservoir drilling

Wei Na (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China)
Sun Wan-Tong (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China)
Meng Ying-Feng (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China)
Liu An-Qi (Geological Explorations and Development Institute, Chuan Qing Drilling Engineering Company Limited, Chengdu, China)
Zhao Jin-Zhou (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China)
Xu Chao-Yang (Directional Drilling Service Company, CNPC Bohai Drilling Engineering Company Limited, Tianjin, China)
Li Hai-Tao (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China)
Jiang Lin (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China)
Xu Han-Ming (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China)

In the drilling process of a marine natural gas hydrate reservoir, the original temperature and pressure condition of the hydrate will change due to the influence of construction, and gas will be released from the hydrate. The decomposition gas makes the wellbore flow into a complex gas-liquid-solid multiphase flow after entering into the wellbore, and thus poses a great threat to the well control security. Accordingly a transient gas-liquid two-phase flow drift flux mathematical model is established in this paper. The numerical method is proposed and verified, and an example analysis is carried out. Finally the change mechanism of transient gas-liquid two-phase flow in wellbore is obtained. This study provides an important foundation for the safe design of an offshore gas hydrate. Meanwhile, it is of great positive significance to find the decomposition gas and to optimize drilling parameters in the drilling process of a marine natural gas hydrate reservoir.

Keywords: marine natural gas hydrate, transient gas-liquid two-phase flow, drilling, numerical calculation, wellbore pressure