Exploration for Natural Hydrate in Nankai-Trough Wells Offshore Japan

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.

Large gas hydrate accumulations on the eastern Nankai Trough inferred from new high‐resolution 2‐D seismic data

“…[5] An exploratory well was drilled on the Nankai slope ( Figure 1) at 945m water depth [Takahashi et al, 2001]. The top 100 m unit is flat lying mudstone-siltstone with occurrences of ash beds.…”

Toward Production From Gas Hydrates: Current Status, Assessment of Resources, and Simulation-Based Evaluation of Technology and Potential

“…This area has probably experienced the largest investment and most advanced field research activity because of the intensive Japanese effort to evaluate the potential and feasibility of gas production from hydrates. Following the discovery of hydrates and the drilling of an exploration well at a water depth of 945 m in 2000 (Takahashi et al, 2001), a multi-well exploration program was conducted in 2004 at 16 locations in three different sites (Kumano Basin, Second Atsumi Knoll and Offshore Tokai) at water depths of 720 m to 2,033 m ( Figure 12) that had been selected on the basis of the BSR signature (Takahashi and Tsuji, 2005).…”

Gas Hydrates Research Programs: An International Review

“…METI organized in 1995 a 5 years research plan to study the area. The resultant research program 'Nankai Trough' led to the drilling in the area of the first exploratory wells for methane hydrates in Japan 57 . The wells drilled at a water depth of 945 m established the presence of hydrate bearing formations at a depth range from 1135 to 1213 m. Analyses of the samples taken during the research program produced many scientific data that proved without doubt the presence of a big accumulation of gas hydrates in the ocean floor.…”

“…Abundant presence of gas hydrates has been inferred from Bottom Simulated Reflector (BSR) analysis 55,56 and determined by drilling 57 . It has been estimated that convergent margins may contain up to two-thirds of the total gas hydrate volume on the earth 58 .…”