Physical properties of sediments from the Ulleung Basin, East Sea: Results from Second Ulleung Basin Gas Hydrate Drilling Expedition, East Sea (Korea)

• Pressure core samples were utilized to characterize hydrate occurrences.
• P-wave velocity of grain-displacing GH shows reversed trend with the bulk density.
• Grain-displacing GH dissociates in stages upon depressurization.
• Geometry of grain-displacing GH is mainly governed by stress and flow condition.

Physical properties, including basic index properties, undrained shear strength, and thermal conductivities were measured on conventional cores retrieved from the Ulleung Basin, during the second Ulleung Basin Gas Hydrate expedition (UBGH2). The UBGH2 logged 13 sites and cored 10 sites to locate potential sites for an offshore test production. A total of 211 conventional cores were recovered. Undrained shear strength, thermal conductivity, and index properties, including porosity and grain density, were measured onboard. Mineral composition analyses and grain size analyses were completed onshore after the expedition.The averaged porosity at each site ranges from 65% to 71%, the averaged grain density at each site ranges from 2.57 to 2.66 g/cm3, and the mean grain size mostly lies between 4 μm and 25 μm, with sparsely scattered coarse grained intervals. The relatively high porosity and low grain density are due to the large portion of diatomaceous sediments. The thermal conductivities average around 0.8 W/mK, and the low porosity and the abundance of clay mineral and OAPL-A may have caused the relatively low thermal conductivity in the Ulleung Basin.The geomechanical analyses revealed a few relevant findings important for sediment physical behaviors during gas hydrate production. The particle migration in coarse grained layers during production necessitates proper measure for sand productions while fine grained layers are mostly self-filtering. The vertical deformation estimated from the compression index suggested that the subsidence induced by the pore pressure change from the depressurization is much higher than those induced by the lost of gas hydrate particles from dissociation. Massive vertical deformation in fine-grained layers has been predicted to occur due to the pore pressure change from depressurization induced gas hydrate production. The drilling mud weight induced pressure window is mostly determined by the water depth in deep sea drilling, but caution should be taken in determination of fracture pressures in hydrate-bearing sediments, because gas hydrate saturations alter Poisson's ratio.