Exploration for Natural Hydrate in Nankai-Trough Wells Offshore Japan
Authors: Takahashi Hideaki , Yonezawa Tetsuo , Takedomi Yoshikazu 
Published: October 2008

Abstract
Exploration for natural methane hydrate was carried out in the Nankai-Trough offshore Japan at a water depth of 945m over an 88 day period, from November 1999 to February 2000. This was a national project led by Ministry of International Trade and Industry (MITI) to seek a new energy source. It was organized by Japan National Oil Corporation (JNOC) in collaboration with Japan Petroleum Exploration Co., Ltd. (JAPEX) as the drilling operator. The Nankai-Trough well was drilled with R&B Falcon's deepwater semisubmersible, the "M. G. Hulme, Jr.". The location was selected where BSR (Bottom Simulating Reflector) is the clearest on the seismic section. Six wells within 100 m distances were drilled through the BSR horizon and the hydrate rich formation was confirmed between 1135 m to 1213 m BMSL (below mean sea level). This paper describes the planning, preparatory phase and the unique operations.

Introduction
Methane hydrate accumulations in deepwater sediments and Arctic permafrost have been expected as one of the future energy resources since its amount is estimated as twice that of conventional hydrocarbon resources1. It has heightened the interest of Japan which produces little oil and is surrounded by deepwater. MITI included its evaluation project into the 8th 5- year Plan for Development of Japan's Domestic Oil & Gas Resources starting from 1995, and declared to drill an exploration well offshore Japan in 1999, the last year of the 5- year Plan. The well was intended to explore not only the hydrate, but also the conventional oil and gas at a deeper Tertiary prospect, should prospects overlap.
The drilling location was selected in the Nankai Trough area where existence of BSR has been pointed out by scientific seismic program2, and cold seepage with biogenic hydrocarbon has been reported by submersible and ROV (Remotely Operated Vehicle) investigation3. Conventional and high resolution seismic surveys were carried out in the area by JNOC in 1996. Another high resolution seismic survey targeting only shallow zones was done in 1997. The drilling point was finally selected at 50 km south of Shizuoka prefecture at 950 m water depth, where the BSR is clearly developed on the seismic section, and there is Tertiary prospect. The seismic interpretation indicated that the BSR was lined at 1240 m (290 m below sea floor) and that the Tertiary prospect needed to be drilled down to 2800m. Fig. 1 shows the location map. Fig. 2 shows the high resolution shallow seismic section crossing the location.
Fig. 3 shows the equilibrium curve of methane hydrate and the estimated in-situ temperature of the well. The two curves crosses at depth of 1240 m. Thus, the BSR at 1240 m on the seismic section was expected to be the base of the hydrate zone.

Collaborative R & D Studies
When the project was established in 1995, JNOC's Technology Research Center (TRC) immediately organized 5- year collaborative R & D studies with ten (10) Japanese private companies, including JAPEX, to perform the challenging exploration efficiently.