Evaluation of the Gas Production Potential of Marine Hydrate Deposits in the Ulleung Basin of the Korean East Sea

Evaluation of the Gas Production Potential of Marine Hydrate Deposits in the Ulleung Basin of the Korean East Sea George J. Moridis, SPE, Matthew T. Reagan, SPE, Lawrence Berkeley National Laboratory, Se-Joon Kim, SPE, Korea Institute of Geoscience and Mineral Resources, Yongkoo Seol, and Keni Zhang, SPE, Lawrence Berkeley National Laboratory estimate of the resource is used as a basis of evaluation, its magnitude is sufficient large to command attention as a potential energy source 4,5 . This interest is further fueled by dwindling conventional hydrocarbon supplies, the rapidly expanding global demand for (and the corresponding rises in the cost of) energy, and the environmental desirability of CH 4 as a “clean” fuel. The emerging importance of hydrates as a potential gas resource was the impetus behind the proliferation of recent studies evaluating the technical and economic feasibility of gas production from hydrate deposits 5-11 , and provided the motivation for this study. The Ulleung Basin. This study focuses on the evaluation of the gas production potential from marine hydrate deposits in the Ulleung Basin of the Korean East Sea. The East Sea is a semi-closed marginal sea enclosed between the Eurasian continent and the Japanese Islands. The East Sea consists of three deep basins: the Ulleung, the Japan, and the Yamato (Figure 1). The Ulleung Basin, located at the southwestern corner of the East Sea, is a bowl-shaped pull-apart basin formed by extension of continental crust during the Late Oligocene to Early Miocene and by compression at the Middle Miocene 12 . The west side of the basin is bounded by a narrow and steep sloped continental shelf, and the north side by a plateau with numerous ridges and troughs. The south and east sides of the basin are broad and gently sloped (Figure 1). The basin has a water depth of 1500-2300 m, and gradually deepens toward the north and the northeast 13 . The sediment thickness at the center of the basin is about 5 km 14 , and increases to 10 km in its southern part 15 . Seismic stratigraphic analysis showed that the sediments in the Ulleung Basin consist of four distinctive subdivisions deposited in early Miocene to Quaternary 16 . Hydrates in the Ulleung Basin. Preliminary surveys conducted by the Korea Institute of Geoscience and Mineral Resources (KIGAM) between 2000 and 2004 suggest that there is a significant potential for gas hydrate occurrence in the Ulleung Basin 17 . The potential presence of gas hydrates in the basin has been suggested by several gas-related features identified by geophysical explorative analysis including (1) a shallow gas zone in the southwestern part of the basin, identified by high-resolution Chirp sub-bottom profiles and echo-sounding images, (2) gas-charged sediments and upward fluid migration, implied by acoustic turbidity and columnar structure of acoustic blanking in surveys of the area, (3) gas seepages on the continental slope, recognized by highly reflective, hyperbolic signals in the water column in echo- sounding images, (4) gas-related structures (pockmarks and domes) on the continental slope of the Ulleung Basin, detected by echo-sounding images 17 . Analysis of piston core samples recovered from the western Ulleung Basin 13 showed rapid sedimentation rates, Abstract Although significant hydrate deposits are known to exist in the Ulleung Basin of the Korean East Sea, their survey and evaluation as a possible energy resource has not yet been completed. However, it is possible to develop preliminary estimates of their production potential based on the limited data that are currently available. These include the elevation and thickness of the Hydrate-Bearing Layer (HBL), the water depth, and the water temperature at the sea floor. Based on this information, we developed estimates of the local geothermal gradient that bracket its true value. Reasonable estimates of the initial pressure distribution in the HBL can be obtained because it follows closely the hydrostatic. Other critical information needs include the hydrate saturation, and the intrinsic permeabilities of the system formations. These are treated as variables, and sensitivity analysis provides an estimate of their effect on production. Based on the geology of similar deposits, it is unlikely that Ulleung Basin accumulations belong to Class 1 (involving a HBL underlain by a mobile gas zone). If Class 4 (disperse, low saturation accumulations) deposits are involved, they are not likely to have production potential. The most likely scenarios include Class 2 (HBL underlain by a zone of mobile water) or Class 3 (involving only an HBL) accumulations. Assuming nearly impermeable confining boundaries, this numerical study indicates that large production rates (several MMSCFD) are attainable from both Class 2 and Class 3 deposits using conventional technology. The sensitivity analysis demonstrates the dependence of production on the well design, the production rate, the intrinsic permeability of the HBL, the initial pressure, temperature and hydrate saturation, as well as on the thickness of the water zone (Class 2). The study also demonstrates that the presence of confining boundaries is indispensable for the commercially viable production of gas from these deposits. Introduction Background. Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) occupy the lattices of ice crystal structures (called hosts). The hydration reaction of methane, the main gas ingredient of natural hydrates in geological systems, is described by the equation CH 4 + N H H 2 O = CH 4 •N H H 2 O,…………………(1) where N H is the hydration number that varies between 5.75 (for complete hydration) and 7.2 1 , with an average value of N H = 6. Such hydrates occur at locations in the permafrost and in deep ocean sediments where the necessary conditions of low T and high P exist for their formation and stability. Current estimates of the size of the hydrocarbon resource trapped in hydrates vary widely 1,2,3 (ranging between 10 15 to 10 18 ST m 3 ), but the consensus is that it is vast, exceeding the total energy content of the known conventional fossil fuel resources. Even if only a fraction of the most conservative