Observation of the Relationship between Ocean Bathymetry and Acoustic Bearing-Time Record Patterns Acquired during a Reverberation Experiment in the Southwestern Continental Margin of the Ulleung Basin, Korea

We observed a distinct drop-off region in the bearing-time record of acoustic reverberation data acquired from the south-western continental margin of the Ulleung Basin, East Sea, in the summer of 2015. 3 kHz continuous waves with pulse lengths of 0.1, 0.3, and 1.0 s were used as source pulses, with an R/V Cheonghae vessel towing a variable depth source and a triplet towed array toward the deep sea from shallow water. The observed pattern changed as the R/V Cheonghae moved across the continental slope further into the sea. This pattern arises as a result of the downward-refracted beams in the 1/2 convergence zone interacting with the soft bottom. In addition, the boundary of the drop-off region was modeled with the two-way maximum travel time of the first bottom-reflected rays using the bathymetry model of the General Bathymetric Chart of the Oceans, 2020. Some discrepancies were observed when comparing the modeled curve to the measured results, and the inaccuracy of the bathymetry model on the continental slope could be the main cause of these discrepancies. This pattern could be useful for bathymetry mapping, as well as estimations of source and receiver configurations.

Numerical Simulation of Gas Hydrate Production Using the Cyclic Depressurization Method in the Ulleung Basin of the Korea East Sea

The depressurization method is known as the most productive and effective method for successful methane recovery from hydrate deposits. However, this method can cause considerable subsidence because of the increased effective stress. Maintenance of geomechanical stability is necessary for sustainable production of gas from gas hydrate deposits. In this study, the cyclic depressurization method, which uses changing the bottomhole pressure and production time during primary and secondary depressurization stage, was utilized in order to increase stability in the Ulleung Basin of the Korea East Sea. Various case studies were conducted with alternating bottomhole pressure and production time of the primary and secondary depressurization stages over 400 days. Geomechanical stability was significantly enhanced, while cumulative gas production was relatively less reduced or nearly maintained. Specially, the cumulative gas production of the 6 MPa case was more than three times higher than that of the 9 MPa case, while vertical displacement was similar between them. Therefore, it was found that the cyclic depressurization method should be applied for the sake of geomechanical stability.

Characterization of Thin Gas Hydrate Reservoir in Ulleung Basin with Stepwise Seismic Inversion

Natural gas hydrates (GHs) filling sand layer pores are the most promising GHs that can be produced via conventional mechanisms in deep-sea environments. However, the seismic tracking of such thin GH-bearing sand layers is subject to certain limitations. For example, because most GH-bearing sand layers are thin and sparsely interbedded with mud layers, conventional seismic data with a maximum resolution of ~10 m are of limited use for describing their spatial distribution. The 2010 Ulleung Basin drilling expedition identified a relatively good GH reservoir at the UBGH2-6 site. However, the individual GH-bearing sand layers at this site are thin and cannot therefore be reliably tracked using conventional seismic techniques. This study presents a new thin layer tracking method using stepwise seismic inversion and 3D seismic datasets with two different resolutions. The high-resolution acoustic impedance volume obtained is then used to trace thin layers that cannot be harnessed with conventional methods. Moreover, we estimate the high-resolution regional GH distribution based on GH saturation derived from acoustic impedance at UBGH2-6. The thin GH layers, previously viewed as a single layer because of limited resolution, are further subdivided, traced, and characterized in terms of lateral variation.

Effect of Permeability on Hydrate-Bearing Sediment Productivity and Stability in Ulleung Basin, East Sea, South Korea

Methane hydrate has attracted attention as a next-generation resource, and many researchers have conducted various studies to estimate its productivity. Numerical simulation is the optimal method for estimating methane gas productivity. Meanwhile, using a reasonable input parameter is essential for obtaining accurate numerical modeling results. Permeability is a geotechnical property that exhibits the greatest impact on productivity. The permeability of hydrate-bearing sediment varies based on the sediment pore structure and hydrate saturation. In this study, an empirical permeability model was derived from experimental data using soil specimens from the Ulleung Basin, and the model was applied in numerical analysis to evaluate the sediment gas productivity and ground stability. The gas productivity and stability of hydrate-bearing sediments were compared by applying a widely used permeability model and the proposed model to a numerical model. Additionally, a parametric study was performed to examine the effects of initial hydrate saturation on the sediment gas productivity and stability. There were significant differences in the productivity and stability analysis results according to the proposed permeability model. Therefore, it was found that for accurate numerical analysis, a regional permeability model should be applied.