[M-IS22_29PM1] Gas hydrates in environmental-resource sciences

[MIS22-01] Formation evaluation and production interval determination at the 1st offshore methane hydrate production test site

*Tetsuya FUJII¹, Tokujiro TAKAYAMA¹, Kiyofumi SUZUKI¹, Koji YAMAMOTO¹ (1.Japan Oil, Gas and Metals National Corporation)

Keywords:methane hydrate, offshore production test, formation evaluation, production interval, eastern Nankai Trough, Daini-Atsumi Knoll

In order to evaluate productivity of gas from marine methane hydrate (MH) by the depressurization method, on March 2013, the first offshore production test form MH concentrated zone (MHCZ) was conducted by the Research Consortium for Methane Hydrate Resource Development in Japan (MH21) at the AT1 site located in the north-western slope of Daini-Atsumi Knoll in the eastern Nankai Trough, Japan.Before the production test, during the pre-drilling campaign conduced in 2012, extensive geophysical logging and pressure coring using Hybrid Pressure Coring System were conducted at monitoring well (AT1-MC) and coring well (AT1-C), in order to obtain fundamental information about reservoir properties of MH bearing formation for reservoir characterization, and also to decide on the production interval. The MHCZ confirmed by the geophysical logging at AT1-MC has a thin-turbidite assemblage (from several tens of centimeters to a few meters) with 60 m of gross thickness; it is composed of lobe/sheet type sequences in the upper part, and relatively thick channel sand sequences in the lower part. The MHCZ at AT1-MC is thicker than those found in wells drilled in 2004 (β1, 45 m), which were located about 150 m northeast of MT1-MC. This fact indicates that the predictions provided by a seismic interpretation and an inversion analysis were reasonable. Moreover, we confirmed that the silt-dominant formation just above the MHCZ was more than 20 m thick ; this was expected to be a seal formation. The well-to-well correlation between two monitoring wells (AT1-MC and MT1) in a 40 m distance shows fairly good lateral continuity of these sand layers (upper part of MHCZ), indicating an ideal reservoir for the production test. In the upper part of the MHCZ, hydrate pore saturation (Sh) estimated from resistivity log showed distinct difference in value between sand and mud layers, compared to Sh from Nuclear Magnetic Resonance (NMR) log. Resistivity log has higher vertical resolution than NMR log, so it is favorable for these kinds of thin bed evaluation. In this part, 50 to 80% of Sh was observed in sandy layer. On the other hand, lower part of the MHCZ, Sh estimated from both resistivity and NMR log showed higher background value and relatively smoother curve than upper part. In this part, 50 to 80% of Sh was observed in sandy layer as well.On the basis of the above observations, a production interval was planed. When we consider an effective depressurization, the existence of sealing layers is critical both above and below the interval. We expect that thin silty layers within the lower part of MHCZ will serve as a sealing layer that will prevent water coning from water-bearing layers. Therefore, we stopped drilling the production well at about 20 m above BSR, and decided to produce from approximately 40 m from the top of the MHCZ.Our future (ongoing) work is to integrate reservoir characterizations based on well logs and pressure core data for the history matching of production test results.This study is a part of the program of the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium).