Methane hydrate exists at specific conditions of high pressure and low temperature due to its instability at normal sea-level pressure and temperature. This naturally occurring resource is estimated to exist in abundance in nature especially in shallow sediments of deep-water continental shelves thus attracting significant research attention. In particular, Japan has made a breakthrough in offshore methane hydrate exploration and production by being the first world country to conduct production tests in the years 2013 and 2017 at sites located in the eastern Nankai Trough region.
The 2013 test was conducted using a single vertical well by depressurization. Despite the short test duration, the test provided partial verification of the technical feasibility of depressurization-induced gas production from oceanic reservoirs thus proving the basis for future methane hydrate production tests. In 2017, a second test was conducted at the same site but this time round using two single wells. Considering that the two tests only lasted for a short duration, numerical simulation methods have been used to predict mid-term and long-term gas production behaviors. However, the proposed numerical systems are mainly focused on the 2013 production test with little attention on the 2017 test.
To this end, Hirosaki University scholars: Dr. Tao Yu, Professor Guoqing Guan, and Professor Abuliti Abudula presented, for the first time, a detailed report regarding the 2017 production test. First, they conducted short-term numerical simulations of the offshore methane hydrate production from the multilayered methane hydrate reservoir to obtain replication of real field test data obtained during the 2017 production test. This data was used in validating the model. Next, they predicted the long-term gas production behavior based on numerical simulation for a period of 5 years. Finally, they investigated the effects of production intervals on the hydrate dissociation and gas production processes. The work is published in the journal, Applied Energy.
Results showed some discrepancies in the simulation results and real field test data obtained in 2017 regarding water production attributed to the numerical simplification of the reservoir models and production processes. On the other hand, gas production at the wells agreed well with the field test data thus validating the feasibility of the multilayered reservoir models. For mid- and long-term predictions of the methane hydrate production trend, the gas production rate was observed to decrease with time while the water production rate increased. However, water production could be reduced by the configuration of the production intervals at the upper part of the reservoir and proper utilization of the packers on the wellbore. Consequently, most of the gas released from the hydrate dissociation was indirectly produced in the aqueous phase rather than directly in the gas phase leading to excess water production. This necessitated the need for high-performance gas-water separation devices and effective water production management systems.
It was worth noting that the order of magnitude of the average long-term gas production rate even though the same as that confirmed in the 2013 test and larger than that confirmed in the 2017 test, was significantly less than the commercial exploration level. Therefore, this study proposed production improvement strategies for future production tests. According to Dr. Tao Yu first author in a statement to Advances in Engineering: these strategies included but not limited to the application of dual vertical wells, horizontal wells, and combined depressurization and hot water injection methods. The authors also hope that this study can provide a reference for the commercial production of the offshore methane hydrate resources in the Nankai Trough in the near future.
Yu, T., Guan, G., & Abudula, A. (2019). Production performance and numerical investigation of the 2017 offshore methane hydrate production test in the Nankai Trough of Japan. Applied Energy, 251, 113338.