Summary The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) has been evaluating methane-hydrate (MH) reservoirs located in the eastern Nankai trough from the viewpoints of geology, geophysics, petrophysics, and reservoir/ production engineering. As one of these studies, we have been predicting gas/water production performances from these MH reservoirs showing diverse characteristics. This paper presents the results of our examinations on the applicability of a variety of MH dissociation/production methods to these MH reservoirs and on the feasibility of future development in terms of gas production and economics. Eastern Nankai trough MH reservoirs, which are composed of alternating beds of sand, silt, and clay in turbidite sediments, have various conditions of clay distribution and of initial pressure, temperature, permeability, and MH saturation. Some of these reservoirs contain MH of high saturation at a certain interval (MH concentrated reservoir), while in the others, MH is deposited sparsely (MH-nonconcentrated reservoir). Detailed numerical reservoir models were constructed for both MH-concentrated and -nonconcentrated reservoirs, consulting the well-log and seismic-interpretation results. MH-dissociation/-production performances were then predicted through numerical simulation, assuming the application of various MH-dissociation methods (such as depressurization, well-bore heating, hot-water huff'n'puff, and hot waterflooding. The simulation studies clarified the difference in the gas production between MH-concentrated and -nonconcentrated reservoirs. These studies also revealed that the permeability not only of sand layers but also of clay layers has a significant effect on the gas productivity from MH-concentrated reservoirs. Furthermore, it was suggested that the hot-water injection was effective when it was applied as a secondary-recovery method after depressurization. Simple economic analyses on the basis of these simulation results exhibited the promise that some MH reservoirs in the Eastern Nankai trough could be developed economically if the well spacing and MH-dissociation/-production methods were designed appropriately.
Resource assessment of methane hydrate (MH) in the eastern Nankai Trough was conducted through probabilistic approach using 2D/3D seismic data and drilling survey data from METI exploratory test wells " Tokai-oki to Kumano-nada??. We have extracted more than 10 prospective " MH concentrated zones?? characterized by high resistivity in well log, strong seismic reflectors, seismic high velocity, and turbidite deposit delineated by sedimentary facies analysis. The amount of methane gas contained in MH bearing layers was calculated using volumetric method for each zone. Each parameter, such as gross rock volume (GRV), net-to-gross ratio (N/G), MH pore saturation (Sh), porosity, cage occupancy, and volume ratio was given as probabilistic distribution for Monte Carlo simulation, considering the uncertainly of these evaluations. The GRV for each hydrate bearing zones was calculated from both strong seismic amplitude anomaly and velocity anomaly. Time-to-depth conversion was conducted using interval velocity derived from Seismic Vision While Drilling (SVWD). Risk factor was applied for the estimation of the GRV in 2D seismic area considering the uncertainty of seismic interpretation. The N/G was determined based on the relationship between LWD resistivity and grain size in zones with existing wells. Seismic facies map created by sequence stratigraphic approach was also used for the determination of the N/G in zone without well controls. Porosity was estimated using density log, together with calibration by core analysis. The Sh was estimated by the combination of density log and NMR log, together with the calibration by observed gas volume from onboard MH dissociation tests using Pressure Temperature Core Sampler (PTCS). The Sh in zone without well control was estimated using relationship between seismic P-wave interval velocity and Sh from NMR log at well location. Total amount of methane gas in place contained in MH within survey area in the eastern Nankai Trough was estimated to be 40 tcf as Pmean value. Total gas in place for MH concentrated zone was estimated to be 20 tcf (Half of total amount) as Pmean value. Sensitivity analysis indicated that the N/G and Sh have higher sensitivity than other parameters, and they are important for further detail analysis. Introduction Seismic data from the Nankai Trough, offshore central Japan, indicates widespread distribution of bottom simulating reflectors (BSR) 1 that are interpreted to represent lower boundary of methane hydrate (MH) bearing zones. MH in the Nankai Trough is a potential natural gas resource, however, the volume, distribution, and occurrence of MH in this area is poorly understood. In 1999, MH-bearing sand-rich intervals in turbidite fan deposits were recognized from the eastern Nankai Trough based on results of MITI (Ministry of International Trade and Industry) " Nankai Trough?? wells2, 3. Based on this exploration result, the Japanese government inaugurated a 16-year MH exploration program in 2001. As a part of this program, the Ministry of Economy, Trade and Industry (METI), Japan, drilled the " Tokai-oki to Kumano-nada?? exploratory test wells from January to May 2004, in order to obtain data for understanding the occurrence of MH and estimating resource potential4, 5. In this campaign we carried out logging-while-drilling (LWD) at 16 sites, coring at four sites, wireline logging at two sites, and long-term monitoring of formation temperature at single site5,6,7 (Figure 1). Using following data set, we conducted MH resource assessment in the survey area in 2005 and 2006. Figure 2 shows the flow chart of the resource assessment.
To investigate characteristics of a seismogenic out‐of‐sequence thrust (OOST) imaged as a strong reflection on seismic profiles in the Nankai accretionary prism, we determined acoustic properties of discrete samples from an fossil Nobeoka OOST outcrop under confining pressures, and compared the acoustic properties with those of an active OOST in the Nankai accretionary prism. We observed anisotropy of velocity and attenuation in the hanging wall of Nobeoka OOST attributed to foliation of pelitic‐phyllite. In contrast, the footwall is composed of brittlely deformed, chaotic shales and fine sandstones, and velocities in the footwall are lower than those in the hanging wall. Amplitude variation with offset (AVO) modeling utilizing contrasts in P‐ and S‐wave velocities and densities between the hanging wall and footwall of the Nobeoka OOST indicates that fractures filled with overpressured fluid likely account for angle‐dependent reflection amplitudes of the active OOST in the Nankai Trough.
As gas hydrate energy assessment matures worldwide, emphasis has evolved away from confirmation of the mere presence of gas hydrate to the more complex issue of prospecting for those specific accumulations that are viable resource targets. Gas hydrate exploration now integrates the unique pressure and temperature preconditions for gas hydrate occurrence with those concepts and practices that are the basis for conventional oil and gas exploration. We have aimed to assimilate the lessons learned to date in global gas hydrate exploration to outline a generalized prospecting approach as follows: (1) use existing well and geophysical data to delineate the gas hydrate stability zone (GHSZ), (2) identify and evaluate potential direct indications of hydrate occurrence through evaluation of interval of elevated acoustic velocity and/or seismic events of prospective amplitude and polarity, (3) mitigate geologic risk via regional seismic and stratigraphic facies analysis as well as seismic mapping of amplitude distribution along prospective horizons, and (4) mitigate further prospect risk through assessment of the evidence of gas presence and migration into the GHSZ. Although a wide range of occurrence types might ultimately become viable energy supply options, this approach, which has been tested in only a small number of locations worldwide, has directed prospect evaluation toward those sand-hosted, high-saturation occurrences that were presently considered to have the greatest future commercial potential.
Since previous research revealed that most of the methane hydrates in the eastern Nankai Trough area occur in matrix pores of turbidite sandstones, the facies distribution of turbidite sandstones may be one of the important keys to evaluate the distributions and actual volume of methane hydrates in the eastern Nankai Trough area. This paper attempts to reconstruct depositional processes of submarine-fan turbidites, and examines the relationship between turbidite facies distributions and bottom simulating reflector (BSR) occurrence as a proxy of methane hydrate using sedimentologic and sequence stratigraphic methodology. First, 2D/3D seismic survey data and well data including cores and logs were used to identify turbidite facies, seismic facies, and depositional sequences. The targeted Plio-Pleistocene Kakegawa and Ogasa Groups can be divided into 17 depositional sequences, and include six seismic facies indicating submarine-fan elements and surrounding slope to basin-floor environments. Next, facies maps for each depositional sequence unit were created by plotting all information on seismic facies, 3D seismic geomorphology, and well facies data. The obtained facies maps reveal that 11 major submarine canyons functioned as positionally fixed sediment supply systems from main land Japan, along which submarine fans were formed in the forearc basins. Submarine-fan depositional styles changed through Plio-Pleistocene from a braided channel type, through small radial fan, troughfill fan, and muddy sheet fan types, to a channel-levee system type. Finally, the facies maps of each depositional sequence were overlaid with the BSR distribution. The overlaid maps indicate that the BSRs occur on feeder channels, distributary channels, and proximal lobes of submarine fans, suggesting that methane hydrates selectively occur in coarser grained portions of a submarine fan. Because the lower part of the Kakegawa Group is mainly composed of braided channeltype submarine fan turbidites, the lower Kakegawa horizon serves one of the major horizons bearing methane hydrates in the eastern Nankai Trough area.
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