1. Methane Hydrate Production Technology Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan
2. Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Onogawa, Tsukuba, Ibaraki 305-8569, Japan