To obtain basic information for methane hydrate (MH) reservoir characterization at the first offshore production test site (AT1) located on the northwestern slope of the Daini-Atsumi Knoll in the eastern Nankai Trough, extensive geophysical logging and pressure coring using a hybrid pressure coring system were conducted in 2012 at a monitoring well (AT1-MC) and a coring well (AT1-C).
The MH-concentrated zone (MHCZ), which was confirmed by geophysical logging at AT1-MC, has a 60-m-thick turbidite assemblage with sublayers ranging from a few tens to hundreds of centimeters thickness. The turbidite assemblage is composed of lobe/sheet-type sequences in the upper part and relatively thick channel-sand sequences in the lower part. Well-to-well correlations of sandy layers between two monitoring wells (AT1-MC and MT1) within 40 m of one another exhibited fairly good lateral continuity of sand layers in the upper part of the reservoir. This suggests an ideal reservoir for the production test. On the other hand, in the lower part of the MHCZ, each sand layer varies in thickness and there are difficult to correlate individually. This means sand continuity in the lower MHCZ is not necessarily as good as in the upper MHCZ. In the MHCZ, MH pore saturation (Sh) values of 50% -80% were observed from geophysical logging in sandy layers, which is in fairly good agreement with core-derived Sh values.
These reservoir information were utilized to determine production interval and history matching as well. We are expecting to use these data and knowledge not only for next offshore MH production test preparation but also other conventional oil and gas projects.
