Well Log Evaluation Of Gas Hydrate Saturation In The Miti Nankai-Trough Well, Offshore South East Japan

The MITI Nankai-Trough is the first methane hydrate research well drilled in offshore Tokai, Japan, which was planned to investigate the potential of in-situ natural gas hydrate resources below deep-sea water. The research well consist of group holes, and each hole was drilled to play independent role for the complete data acquisition. These group wells penetrated the level of BSR (bottom simulating reflector) observed at 1,242 m. sub-sea, and are located at the adjacent site along seismic survey lines. LWD, including density, neutron, induction and azimuthal laterolog measurements, was carried out in one of the pilot wells. Open-hole wireline loggings, including dipole shear- and compressional-wave slowness, array induction and laterolog were recorded in the main well after continuous coring, and acoustic wave slowness, laterolog, NMR (nuclear magnetic resonance) log and electrical formation image were additionally measured in the two post survey wells. By using the suite of log data, lithology, porosity and hydrate saturation were evaluated. The logged intervals are interpreted as alternation of sand and silty shale beds. Shale content was estimated by using natural gamma ray. Due to lack of reliable density and neutron data in rugose hole, the trend of NMR total porosity was referred to obtain sandstone porosity. Concentration of methane hydrate are interpreted in the sands at limited depth interval. Hydrate saturation was evaluated by applying methods using resistivity, acoustic transit time, and NMR total porosity as saturation indices. Among them, resistivity derived saturation was found to show best consistency with hydrate concentration estimated from chlorinity anomaly of the pore fluid in core samples. NMR porosity, which was measured first among worldwide hydrate research wells, was also found to be good index for the hydrate saturation. The maximum hydrate saturation was estimated at around 80% of total pore in sandstone beds. Hydrate bearing beds were indicated between 1,135 m. sub-sea (192 m sub-bottom or mbsf) and 1,212 m. sub-sea (269 m sub-bottom), and they can be correlated between the wells with 94m distance. Hydrate distribution was interpreted to be controled by the development of sand beds and the existence of free gas supply mechanism. Hydrate/water contact was supposed to exist near the base of hydrate stable zone (or phase equilibrium region) that can be theoretically predicted based on pressure-temperature condition.