Productivity from Eastern Nankai Trough Methane-Hydrate Reservoirs
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 19382, "Prediction of Gas Productivity From Eastern Nankai Trough Methane-Hydrate Reservoirs," by Masanori Kurihara, SPE, Akihiko Sato, and Hisanao Ouchi, Japan Oil Engineering Company; Hideo Narita, National Institute of Advanced Industrial Science and Technology; Yoshihiro Masuda, SPE, the University of Tokyo; Tatsuo Saeki and Tetsuya Fujii, Japan Oil, Gas, and Metals National Corporation, prepared for the 2008 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed.
The MH21 Research Consortium of Japan evaluated methane-hydrate (MH) reservoirs in the eastern Nankai trough from the viewpoints of geology, geophysics, petrophysics, and reservoir/production engineering. The applicability of several MH-dissociation/-production methods to these MH reservoirs and the feasibility of future development in terms of gas production and economics were examined. Simple economic analysis on the basis of those simulation results indicated promise that some MH reservoirs in the eastern Nankai trough could be developed economically if well spacing and MH-dissociation/-production methods were designed and applied appropriately.

Introduction
A numerical simulator was developed for rigorous prediction of MH-dissociation and -production behaviors, at both core and field scales. Use of this numerical simulator enabled studies to predict MH-reservoir performance to study the effectiveness of several production methods in terms of the gas producibility from MH reservoirs with diverse characteristics. These studies, however, were conducted targeting imaginary reservoirs as well as those roughly mimicking eastern Nankai trough reservoirs by simplifying reservoir properties.
Exploratory drilling was carried out in 1999 and 2004, and a 3D-seismic survey was conducted in 2002, which clarified the characteristics of the reservoirs. In this study, the performance of MH dissociation and production were forecast by use of more-realistic reservoir models that rigorously reflect the geological/petrophysical properties analyzed through those exploration activities.
MH Reservoirs in the Eastern Nankai Trough. Six exploration wells were drilled in which logging-while-drilling (LWD) data, wireline-logging data, and core samples were acquired offshore Tokai in 1999. These wells confirmed the presence of an interval with relatively high MH saturations. Additional exploratory drilling targeted a broader area in 2004. A total of 32 exploration wells were drilled to obtain well-log data and core samples at 16 locations in the Kumano basin, Second Atsumi Knoll, and offshore Tokai, as shown in Fig. 1. Japan Oil, Gas, and Metals National Corporation has been analyzing these data, aiming at estimating the initial volume of methane in place in MH reservoirs in the eastern Nankai trough. During these analyses, it was revealed that the MH reservoirs are a sequence of thin layers (most thinner than 0.4 m) of sand, silt, and clay within turbidite beds. The MH saturation is negligibly small in clay layers, even if MH is contained in these layers.