Production performance and numerical investigation of the 2017 offshore methane hydrate production test in the Nankai Trough of Japan

Since the first offshore methane hydrate production test was conducted in the Nankai Trough of Japan in 2013, the second production test was also conducted at the same test site in 2017, but information about this field test is limited. Thus, in this study, we aimed to report detailed information about the 2017 production test for the first time, and conducted short-term simulations of the oceanic methane hydrate production from the multilayered reservoir models for model validation. Then, we predicted the long-term gas production behavior for a production period of 5 y, and investigated the effect of the production intervals on the processes of hydrate dissociation and gas production. The simulation results indicated that most of the released gas from hydrate dissociation was actually produced indirectly in the aqueous phase rather than directly in the gas phase, and the proper utilization of the packers on the wellbore could effectively promote gas production, but also caused excess water production. Therefore, high-performance gas-water separation devices and water production management are necessary for future production tests. Furthermore, the 5-y average gas production rates for each case designed in this study were estimated as 1.01 × 104–1.21 × 104 ST m3/d, which were far less than the commercial exploration level of 3.0 × 105 ST m3/d, so improvements in the production strategies should be considered for future production tests.