10:45 AM - 12:15 PM
[MIS18-P04] Geomechanical properties of sediments retrieved from off-Joetsu gas hydrate reservoir in Japan Sea
Keywords:physical properties, mechanical properties, shallow gas hydrate
Shallow gas hydrate is known to be widely distributed along the eastern margin of the Japan Sea. Although strenuous investigations and research have been conducted, there have been few reports on the mechanical behavior of sediments containing shallow gas hydrate. In this presentation, we report the results of physical and geomechanical tests of seafloor sediments sampled off Joetsu, Niigata Prefecture, on the eastern margin of the Japan Sea.
The geotechnical research cruise (CK22-03C) was conducted in September 2022, combining core drilling and well logging. A total of four survey sites were investigated from the Umitaka Spur (reference site US-RE: 144 m, the presence of gas hydrate site US-MH: 155 m) and the Joetsu Knoll (reference site JK-RE: 144 m, the presence of gas hydrate site JK-MH: 122 m) off Joetsu.
Simplified soil tests (vane shear test and penetration resistance test) were conducted on the collected sediment samples on board. After the tests, samples for physical property tests were collected from the test points. The samples for laboratory soil tests were determined from XCT images of sediment cores taken on board. Samples for laboratory soil tests were brought back to the laboratory in whole-round-core condition. Physical property tests were conducted as well as samples from the simplified soil test.
The depth profile of particle densities was almost constant regardless of sediment depth or site. The fine-grain content did not show any tendency to change with depth but was slightly lower in JK-MH. Water content decreased rapidly from the seafloor to about 10 m below the seafloor (mbsf), then decreased slowly to 60 mbsf, and almost the same at depths deeper than 60 mbsf. The trend was almost the same regardless of the survey site.
From the results of the simplified test, the ratio of increase in shear strength with increasing sediment depth was greater in the Joetsu Knoll than in the Umitaka Spur. This may be due to the higher sedimentation rate of the Umitaka Spur because of its close to the land. The sedimentation rate is currently being calculated from sediment samples. Comparison of the results of the simple onboard test and the laboratory soil mechanics test showed good agreement. However, the laboratory test results for samples taken from a depth of approximately 140 mbsf were far apart from the results of the simple tests. The cause of this discrepancy needs to be examined in with the results of other analyses.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).
The geotechnical research cruise (CK22-03C) was conducted in September 2022, combining core drilling and well logging. A total of four survey sites were investigated from the Umitaka Spur (reference site US-RE: 144 m, the presence of gas hydrate site US-MH: 155 m) and the Joetsu Knoll (reference site JK-RE: 144 m, the presence of gas hydrate site JK-MH: 122 m) off Joetsu.
Simplified soil tests (vane shear test and penetration resistance test) were conducted on the collected sediment samples on board. After the tests, samples for physical property tests were collected from the test points. The samples for laboratory soil tests were determined from XCT images of sediment cores taken on board. Samples for laboratory soil tests were brought back to the laboratory in whole-round-core condition. Physical property tests were conducted as well as samples from the simplified soil test.
The depth profile of particle densities was almost constant regardless of sediment depth or site. The fine-grain content did not show any tendency to change with depth but was slightly lower in JK-MH. Water content decreased rapidly from the seafloor to about 10 m below the seafloor (mbsf), then decreased slowly to 60 mbsf, and almost the same at depths deeper than 60 mbsf. The trend was almost the same regardless of the survey site.
From the results of the simplified test, the ratio of increase in shear strength with increasing sediment depth was greater in the Joetsu Knoll than in the Umitaka Spur. This may be due to the higher sedimentation rate of the Umitaka Spur because of its close to the land. The sedimentation rate is currently being calculated from sediment samples. Comparison of the results of the simple onboard test and the laboratory soil mechanics test showed good agreement. However, the laboratory test results for samples taken from a depth of approximately 140 mbsf were far apart from the results of the simple tests. The cause of this discrepancy needs to be examined in with the results of other analyses.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).