2:00 PM - 2:15 PM
[MIS20-01] Distribution of redeposited beds by cuttings generated by non-riser drilling, off Sakata and Joetsu, Sea of Japan.
Keywords:shallow methane hydrate, assessment for drilling impact,, distribution of redeposited bed, off Sakata, off Joetsu, The D/V Chikyu
Since 2013, marine geological and environmental survey for shallow methane gas hydrate production are conducting by AIST. Since shallow methane hydrate is contained in relatively shallow unconsolidated sediments below the seafloor, direct drilling of the sediments has been proposed as a method of recovery and production of methane hydrate. Therefore, there is a need to understand the impact of excavation debris on the seafloor environment. A drilling survey to geomechanical study in the shallow methane hydrate-bearing zones conducted in 2022 and 2023 using the D/V Chikyu (JAMSTEC) (Satoh et al., JpGU2024 MIS22-03) was considered as a simulation of drilling for shallow methane hydrate recovery and production, and the impact on the surrounding seafloor was evaluated by comparing the results of seafloor surveys conducted immediately before and after the drilling and one to two years later. In this presentation, we report on “distribution of re-deposited layers by drilling cuttings,” which provides basic data for these evaluations.
The driiling impact study targeted seven wells at six locations in Sakata Knoll off Sakata, Joetsu Knoll and Umitaka Spur off Joetsu, Sea of Japan. All holes were drilled on non-riser drilling and without casings. The depth of excavation was 160 m below the seafloor at the Sakata Knoll MH site (SK-MH; 532.5 m depth) and the Sakata Knoll Reference site (SK-RE; 556.0 m depth), 155 m below the seafloor at the Joetsu Knoll MH site (JK-MH; 981 m depth), 144 m and 200 m below the seafloor at two wells at the Joetsu Knoll Reference site (JK-RE; 984 m depth), 122 m below the seafloor at the Unitaka Spur North MH site (UTN-MH; 916 m depth) and 144 m below the seafloor at the Unitaka Spur North Reference site (UTN-RE; 925 m depth).
To identify the redeposited layers by excavation debris, following methods and data were used : seafloor images acquired using the hovering AUV and acoustic survey (MBES, SSS, SBP) data acquired by the ROV to identify the distribution and extent of redeposited layers, and push cores acquired using the ROV to estimate the thickness of the redeposited layer. SSS mosaic images acquired by the cruising AUV are used to estimate the distribution in sites where is no seafloor images. The detailed bathymetry data (grid spacing 1-2 m) obtained from the acoustic survey (MBES) acquired by the cruising AUV were used for the bathymetry around these six sites.
At the SK-MH, the redeposited layer was widely distributed to 50 m away to east and northeast side of the drilled pit and 80 m away to the west side, both along the lower part of the submarine topography. The thickness of the redeposited layer was 5 to 7 cm at a distance of about 15 m from the drilled pit. At the JK-MH, the distribution of the redeposited layer was observed to a point about 30 m to the southwest of the drilled pit, and again, it was considered to be distributed toward the lower part of the topography. Push cores at a distance of about 15 m from the drilled pit showed that the thickness of the redeposited layer was 5 to 7 cm. At the UTN-MH, the redeposited layer was found to be distributed along the northeast and southwest sides of the drilled pit toward the lower topography, reaching up to about 30 m from the drilled pit on the southwest side and at least 20 m from the drilled pit on the northeast side. At the two reference sites (JK-RE and UTN-RE), the SSS mosaic image shows a circular distribution with a diameter of about 26 m and 18 m, respectively, where the backscatter intensity is stronger than the surrounding area around the drilled holes.
In summary, the following results were obtained: 1) debris generated by the excavation was distributed toward the lower topography at the location with topographic undulations, and was circularly distributed around the drill holes at the location without undulations; 2) the maximum distance of the redeposited layer was about 80 m; and 3) the thickness of the redeposited layer was about 5-7 cm at 15 m from the drill holes.
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 driiling impact study targeted seven wells at six locations in Sakata Knoll off Sakata, Joetsu Knoll and Umitaka Spur off Joetsu, Sea of Japan. All holes were drilled on non-riser drilling and without casings. The depth of excavation was 160 m below the seafloor at the Sakata Knoll MH site (SK-MH; 532.5 m depth) and the Sakata Knoll Reference site (SK-RE; 556.0 m depth), 155 m below the seafloor at the Joetsu Knoll MH site (JK-MH; 981 m depth), 144 m and 200 m below the seafloor at two wells at the Joetsu Knoll Reference site (JK-RE; 984 m depth), 122 m below the seafloor at the Unitaka Spur North MH site (UTN-MH; 916 m depth) and 144 m below the seafloor at the Unitaka Spur North Reference site (UTN-RE; 925 m depth).
To identify the redeposited layers by excavation debris, following methods and data were used : seafloor images acquired using the hovering AUV and acoustic survey (MBES, SSS, SBP) data acquired by the ROV to identify the distribution and extent of redeposited layers, and push cores acquired using the ROV to estimate the thickness of the redeposited layer. SSS mosaic images acquired by the cruising AUV are used to estimate the distribution in sites where is no seafloor images. The detailed bathymetry data (grid spacing 1-2 m) obtained from the acoustic survey (MBES) acquired by the cruising AUV were used for the bathymetry around these six sites.
At the SK-MH, the redeposited layer was widely distributed to 50 m away to east and northeast side of the drilled pit and 80 m away to the west side, both along the lower part of the submarine topography. The thickness of the redeposited layer was 5 to 7 cm at a distance of about 15 m from the drilled pit. At the JK-MH, the distribution of the redeposited layer was observed to a point about 30 m to the southwest of the drilled pit, and again, it was considered to be distributed toward the lower part of the topography. Push cores at a distance of about 15 m from the drilled pit showed that the thickness of the redeposited layer was 5 to 7 cm. At the UTN-MH, the redeposited layer was found to be distributed along the northeast and southwest sides of the drilled pit toward the lower topography, reaching up to about 30 m from the drilled pit on the southwest side and at least 20 m from the drilled pit on the northeast side. At the two reference sites (JK-RE and UTN-RE), the SSS mosaic image shows a circular distribution with a diameter of about 26 m and 18 m, respectively, where the backscatter intensity is stronger than the surrounding area around the drilled holes.
In summary, the following results were obtained: 1) debris generated by the excavation was distributed toward the lower topography at the location with topographic undulations, and was circularly distributed around the drill holes at the location without undulations; 2) the maximum distance of the redeposited layer was about 80 m; and 3) the thickness of the redeposited layer was about 5-7 cm at 15 m from the drill holes.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).