5:15 PM - 6:45 PM
[MIS22-P05] Geomechanical properties of gas hydrate reservoir sediments in Japan Sea (Off Joetsu and Sakata)
Keywords:gas hydrate, mechanical property, physical property, methane
Gas hydrates, widely distributed in continental margin and permafrost regions, are often composed of methane gas, hence also known as methane hydrates. Not only anticipated as a non-conventional energy resource, methane hydrates, involving the greenhouse gas methane, attract attention for their formation and dissociation. Marine gas hydrates are classified into two types: the pore-filling type hydrate in sandy horizon and the massive nodular type of hydrate in shallow sediments (shallow type), have been identified in the Pacific Ocean and the Japan Sea/around Hokkaido, respectively. While research on shallow-type methane hydrates has been actively conducted for the past two decades, studies on the geomechanical properties of sediments containing hydrates are scarce. In this presentation, we report the results of physical and geomechanical property tests of seafloor sediments collected from the eastern margin of the Japan Sea (off Joetsu, Niigata Prefecture and off Sakata, Yamagata Prefecture), where the presence of the gas hydrate was suggested. These areas include survey points where presence of gas hydrate have been estimated or collected through previous studies and preliminary investigations.
The survey points include the Sakata Knoll (tentative name) off Yamagata, the Joetsu Knoll and the Umitaka Spur off Niigata,. These were designated as reference sites (RE) outside the distribution range of the bottom simulating reflector (BSR) and the presence of gas hydrate sites (MH) within BSR. Seafloor ground strength surveys were conducted in the summers of 2021 (PS21), 2022 (CK22-03C), and 2023 (CK23-02C). In 2021, the survey was conducted by 'Poseidon-1,' and in 2022 and 2023, by 'Chikyu'. The Sakata Knoll was surveyed in 2021 and 2023, and the Joetsu Knoll/Umitaka Spur was surveyed in 2022 and 2023. During the PS21 cruise, sediment samples were stored on board and transported to the laboratory after the cruise for XCT imaging. Undisturbed sediment samples were determined from XCT images, and physical and geomechanical property tests were conducted. For the CK22-03C and CK23-02C cruises, undisturbed sample intervals suitable for geomechanical tests were determined from XCT images of sediment cores taken on board. Samples for geomechanical tests were brought back as whole-round cores and subjected to tests. From sediment cores not provided for geomechanical tests, undisturbed intervals were chosen for simplified geomechanical tests (vane shear test and penetration resistance test). After simplified tests, samples for physical tests were taken from the test locations. Samples for geomechanical tests were also subjected to physical tests.
The results of physical tests (soil particle density and water content, etc.) showed depth profiles dependent on sediment depth regardless of the survey area, gas hydrate sites, reference sites, etc. It was not possible to extract clear features based on the survey area or presence of gas hydrate and reference sites. On the other hand, the results of geomechanical tests showed differences in depth profiles based on the survey area, reference sites, and gas hydrate sites. From the results of the simplified tests conducted during the CK cruise, it was observed that the increase in shear strength with the increase in sediment depth was greater for the Joetsu Knoll than for the Umitaka Spur. This was considered to be partly due to the proximity of the Umitaka Spur to the land and the higher sedimentation rate compared to the Joetsu Knoll. We are in the process of estimating sedimentation rates.
The shear strength obtained by simplified geomechanical tests is generally considered synonymous with cohesion. Therefore, the shear strength obtained by simplified tests was considered as cohesion strength, and a comparison was made with cohesion strength obtained from geomechanical tests. The trends in depth profiles of simplified tests and geomechanical tests matched well. While not all measurements can be replaced by simplified tests, the results suggest that it is an effective combination to increase the number of measurement points and understand depth profiles.
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 survey points include the Sakata Knoll (tentative name) off Yamagata, the Joetsu Knoll and the Umitaka Spur off Niigata,. These were designated as reference sites (RE) outside the distribution range of the bottom simulating reflector (BSR) and the presence of gas hydrate sites (MH) within BSR. Seafloor ground strength surveys were conducted in the summers of 2021 (PS21), 2022 (CK22-03C), and 2023 (CK23-02C). In 2021, the survey was conducted by 'Poseidon-1,' and in 2022 and 2023, by 'Chikyu'. The Sakata Knoll was surveyed in 2021 and 2023, and the Joetsu Knoll/Umitaka Spur was surveyed in 2022 and 2023. During the PS21 cruise, sediment samples were stored on board and transported to the laboratory after the cruise for XCT imaging. Undisturbed sediment samples were determined from XCT images, and physical and geomechanical property tests were conducted. For the CK22-03C and CK23-02C cruises, undisturbed sample intervals suitable for geomechanical tests were determined from XCT images of sediment cores taken on board. Samples for geomechanical tests were brought back as whole-round cores and subjected to tests. From sediment cores not provided for geomechanical tests, undisturbed intervals were chosen for simplified geomechanical tests (vane shear test and penetration resistance test). After simplified tests, samples for physical tests were taken from the test locations. Samples for geomechanical tests were also subjected to physical tests.
The results of physical tests (soil particle density and water content, etc.) showed depth profiles dependent on sediment depth regardless of the survey area, gas hydrate sites, reference sites, etc. It was not possible to extract clear features based on the survey area or presence of gas hydrate and reference sites. On the other hand, the results of geomechanical tests showed differences in depth profiles based on the survey area, reference sites, and gas hydrate sites. From the results of the simplified tests conducted during the CK cruise, it was observed that the increase in shear strength with the increase in sediment depth was greater for the Joetsu Knoll than for the Umitaka Spur. This was considered to be partly due to the proximity of the Umitaka Spur to the land and the higher sedimentation rate compared to the Joetsu Knoll. We are in the process of estimating sedimentation rates.
The shear strength obtained by simplified geomechanical tests is generally considered synonymous with cohesion. Therefore, the shear strength obtained by simplified tests was considered as cohesion strength, and a comparison was made with cohesion strength obtained from geomechanical tests. The trends in depth profiles of simplified tests and geomechanical tests matched well. While not all measurements can be replaced by simplified tests, the results suggest that it is an effective combination to increase the number of measurement points and understand depth profiles.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan)