For the purpose of evaluating methane hydrate occurrence offshore central Japan, 32 wells were drilled through the BSR (Bottom Simulating Reflector) horizon at the water depth of 720 m to 2,033 m over a 122-day period from January 2004 to May 2004. This was the second methane hydrate exploration campaign in the Nankai-Trough as a national project led by Ministry of Economy, Trade and Industry(METI, re-structured from MITI (Ministry of International Trade and Industry) in 2001) to seek a new energy source, based on the technologies developed through the first campaign of 1999–2000 1. It was organized by Japan National Oil Corporation (JNOC, re-structured to JOGMEC (Oil, Gas and Metals National Corporation) in 2004) in collaboration with Japan Petroleum Exploration Co., Ltd. (JAPEX) as the drilling operator and with Teikoku Oil Co., Ltd. (TOC). Sixteen (16) wells were LWD-logged and two of them were installed with the high accuracy bore hole temperature sensors, 2 wells were wireline-logged, 12 wells were cored, and 2 wells were cased and experimented to verify technologies for the future possible stage including 1 horizontal well. All scheduled programs were performed successfully with a deepwater research vessel the "JOIDES Resolution" operated by Transocean. This paper introduces its general program.
Japan has been seeking for the potential of natural hydrate as a future energy source and has developed systematic research programs in the last decade. In 1995, when Ministry of International Trade and Industry (MITI) established the initial program and declared to drill an exploration well offshore Japan in 5 years, Japan National Oil Corporation (JNOC) immediately organized the research teams leading ten (10) Japanese private companies, including JAPEX, to develop the most efficient methodology for the challenging exploration drilling program 2. Based on the result of the research, the world-wide first offshore natural hydrate exploration wells were successfully drilled from November 1999 to February 2000 in the Nankai-Trough offshore Japan at a water depth of 945 m where BSR was clear on the seismic section 1. The program brought extremely valuable results regarding how hydrate occurred at the wells 3, 4, 5. And one of the biggest accomplishments of the program was to establish the methodology to evaluate hydrate occurrence by drilling a well. Thus, the next program was planned to apply the methodology to many wells drilled at many locations to explore the extensive area effectively. 2D and 3D seismic surveys were conducted in 2001 and 2002 respectively in order to select locations for the multi wells, and the multi-well exploration program was scheduled in 2004. In this paper, the previous exploration program conducted at the single location of the Nankai-Trough in 1999–2000 is described as "the 1999–2000 program", while the multi-well exploration program conducted in 2004 is described as "the 2004 program" which is the theme of this paper.
Based on the comprehensive interpretation of 2D and 3D seismic surveys,