Depending on imports for 98.6% for oil and natural gas requirements, it has been a long-cherished dream for Japan to have domestic hydrocarbon energy resources. Methane hydrate, an ice-like white solid composed of water and methane gas, is believed to be widely distributed in the sea area off Japan. Therefore, Japan's Methane Hydrate R & D Program was established by the Ministry of Economy, Trade and Industry in FY2001. The organization established to execute this program is the Research Consortium for Methane Hydrate Resources in Japan (MH21). This program is divided into three phases, extending over 18 years. Phase 1 ended in FY2008. The main results of Phase 1 are as follows: (1) World's first successful continuous production of methane gas from methane hydrate-bearing layers in onshore production tests, (2) For the first time in the world, methane hydrate concentrated zones, which have development potential, were identified in the eastern Nankai Trough area, (3) Experimental in-situ testing methods of artificial and natural core samples of methane hydrate-bearing sediments were established, (4) Probabilistic resource assessment method based on well and seismic data was established, and the amount of methane gas trapped in the eastern Nankai Trough area was estimated, (5) Japan's own simulator dedicated to evaluating the production behavior of methane gas from methane hydrate-bearing layers was developed. Besides the above-mentioned achievements, fundamental studies were conducted on the Environmental Impact Assessment and Methane Hydrate System. The following technological issues have been identified by a sensitivity analysis of the Economic Evaluation: (1) increasing gas production rate (2) improving recovery factor, (3) reducing rate of sand-induced problems, and (4) reducing cost of subsea system. Phase 2 began in April 2009 to resolve these issues. The most important issue in Phase 2 is to conduct two offshore production tests in the sea off Japan. This paper introduces detailed results of Phase 1 and issues to be tackled during and after Phase 2.
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, trough-fill 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 channel-type submarine fan turbidites, the lower Kakegawa horizon serves one of the major horizons bearing methane hydrates in the eastern Nankai Trough area.
From January to May of 2004, METI/JOGMEC-MH21 prepared the METI Exploratory Test Wells Tokai-oki to Kumanonada, including drillings, loggings, and long-term borehole temperature monitoring by DTS/FBG. A high-resolution geochemical study of interstitial water and sediments was conducted at the Dai-ichi Tenryu Knoll and the Daini-Atsumi Knoll. Cl- baselines (original in situ Cl- concentration) at both sites show contrasting and characteristic patterns. The oxygen isotope compositions of the interstitial water seem to show a symmetrical pattern mirror image similar to the Cl- concentration. The fluctuations of oxygen and the Cl- baseline can generally be explained by dilution/enrichment mechanisms caused by gas-hydrate formation and dissociation. The formation and dissociation history of gas hydrate at the two sites is summarized as follows: (1) Sea level had fallen toward the Last Glacial Maximum, and BGHS had traveled upward. Methane and heavy oxygen-enriched water traveled upward in response to gas-hydrate dissociation: (2) Released methane was again trapped above the new BGHS, and gas hydrates were concentrated within sandy sediments. The upper BSR in the Dai-ichi Tenryu Knoll area was formed at this time: (3) BGHS has migrated downward following the transgression over the last 18,000 yrs. In the Daini-Atsumi area, relic-BSR corresponding to the upper-BSR in the Dai-ichi Tenryu Knoll area would have disappeared due to rapid accumulations of gas hydrate generated by a high methane flux, whereas in the Dai-ichi Tenryu Knoll area, it would have remained long after BGHS migration due to a lower methane supply. Eustatic sea-level change has brought about a hydrostatic pressure change, and gas hydrate stability zone would also have changed. However, the amount of additional gas hydrate accumulation would have obliterated or facilitated the development of the relic BSR.
A resource assessment of methane hydrate (MH) in the eastern Nankai Trough was conducted through a probabilistic approach using 2D/3D seismic reflection data and drilling survey data obtained from Ministry of Economy, Trade and Industry (METI) Tokai-oki to Kumano-nada exploratory test wells. We extracted more than 10 prospective MH-concentrated zones characterized by high resistivity in well log, strong reflectors and high velocity on seismic profiles (data), and turbidite deposits delineated by sedimentary facies analyses. The amount of methane gas contained in MH-bearing layers was calculated using a volumetric method for each zone. Each parameter, such as gross rock volume (GRV), net-to-gross ratio (N/G: ratio of sandy layer divided by gross thickness), MH pore saturation (Sh), porosity, cage occupancy, and volume ratio, was given as a probabilistic distribution for a Monte Carlo simulation, considering the uncertainty of these values. The GRV of each hydrate-bearing zone was calculated from both strong seismic amplitude and velocity anomalies. The N/G was determined from the relationship between LWD resistivity and grain size in zones with existing wells. A seismic facies map created by a sequence stratigraphic approach was also used for determining the N/G in zone without well controls. Sh was estimated from a combination of density and NMR logs, together with calibration by gas volume measured from onboard MH dissociation tests using a pressure temperature core sampler (PTCS). Sh in a zone without well control was estimated from the relationship between seismic P-wave interval velocity and Sh from NMR log at a well site. A total area of 4687 km2 of Bottom Simulating Reflectors (BSRs) was interpreted within the survey area in the eastern Nankai Trough (12000 km2). The total amount of methane gas in place contained in MH in the survey area was estimated to be 40 tcf as a Pmean value (10 tcf as P90, 83 tcf as P10). Total gas in place at the MH-concentrated zone (767 km2) was estimated to be 20 tcf (Half of the total amount) as Pmean value. Sensitivity analysis indicated that N/G and Sh have higher sensitivities than other parameters.
It is important to understand the relations between the formation of methane hydrate in shallow sediments and seafloor manifestations accompanied by methane discharges to delineate the following issues concerning the exploitation of methane hydrate: (1) establishing methane hydrate exploration method through geological and geochemical surveys of the seafloor; (2) understanding methane hydrate system; (3) clarifying relations between methane hydrate-bearing formations and global warming; and, (4) understanding production problems during methane hydrate development. As a preliminary study to solve the above-mentioned issues, we attempted to clarify the relations between methane hydrate-bearing formations and various seafloor manifestations accompanied by methane releases from the seafloor, such as pockmarks and carbonate precipitations, using 3D seismic data in the three survey areas of the eastern Nankai Trough. Bathymetric and seafloor amplitude maps constructed from high-resolution 3D data provided extensive information on the seafloor. We also constructed BSR depth anomaly maps to interpret geothermal gradient and structure of the P-wave velocity in shallow formations because methane hydrate is sensitive to temperature change. It is likely that methane hydrate-bearing formations and seafloor manifestations have a strong relationship with geological migration conduits of methane-bearing fluid in the eastern Nankai Trough, e.g., permeable sandy sediments, and large and shallow faults. New geological and geochemical surveys of the seafloor are required to clarify the relationship.
Studies of naturally existing methane hydrate are classified according to their aims; 1. Roles of methane hydrate in the global environment related mainly to global warming, 2. Potential as a natural resource, and 3. Drilling hazards and flow assurance of gas pipelines in oil and natural gas field developments. This report focuses on one of the field operations carried out in the eastern Nankai trough off Japan in 2004, which was part of the Japanese national project for methane hydrate study. The aim of this project is to assess the potential of methane hydrates off Japan as natural resources.
Accurately estimating natural gas hydrate resources is one of the most important issues when assessing the energy potential of natural gas hydrate, which relies largely on the precision of data on the hydrate saturation level in sediments of a reservoir. The primary mode of occurrence of gas hydrate recognized in the eastern Nankai Trough area is in sediment pores. It is suggested that the distribution of coarse-grained sands is one of the most important factors controlling the occurrence of natural gas hydrates. This research aims to elucidate the particle size and clay mineral effects on hydrate saturation in sediments through an experimental approach. The specimens tested include sand, silty sand, and silt, representing the main sediment types recovered from the gas hydrate distribution region of the eastern Nankai Trough. The results obtained from the experiments clearly indicate a particle size and clay content-dependent trend of low saturation in fine sediment but high saturation in coarse sediment. These results are generally consistent with NMR logging results for high-saturation samples, but are somewhat different for samples with low or medium saturation levels. To obtain a better understanding of the mechanism of these two factors, studies were carried out to investigate the saturation level of methane hydrate in a series of silica powders and clay. The results indicate that particle size and clay contents are the two key factors determining the saturation level of gas hydrate in sediments—the finer the particle size and/or the higher the clay content, the lower the hydrate saturation.
Seafloor displacements need to be measured continuously to monitor seafloor stability during methane hydrate production. Ground displacement at an onshore landslide site is usually monitored using instruments such as slide sensors. These instruments measure relative displacements between a fixed point and measuring points in the landslide mass. However, they cannot be used for monitoring displacement at the seafloor where it is difficult to establish a fixed reference point. In this study, we propose a new method for monitoring seafloor displacement using a 3-component servo-accelerometer system. The basic concept of the proposed method is to calculate displacements from double integrals of acceleration waveform records. Theoretically, a velocity waveform is obtained from a single integral of an acceleration waveform, and displacement is obtained from double integrals of an acceleration waveform. One of the most important points in data processing is to reduce noise, especially long-period trends, which produce many errors in integral calculations.
Methane hydrate-bearing sediments taken by a Pressure Temperature Core Sampler (PTCS), were recovered from the NE-Nankai Trough to clarify the accumulating mechanisms whereby methane hydrate accumulates in pore spaces. The sediments were of the sand and mud alternation layer, which were turbidities and hemi-pelagic mud, respectively. Their sediment features such as grain size, porosity, and methane hydrate saturation of pore spaces were analyzed systematically. Relationships among grain size distribution, Bouma's sequence and hydrocarbon gas volume produced by methane hydrate dissociation were confirmed, thus we understood the correspondence of the features of turbidities features and methane hydrate saturation; i.e., we understood that the part of high methane hydrate saturation had a porosity of 50-55%, median of 1.9-3.3φ, and grain sorting of from -1.4 to -0.4. These features indicate the lower section of turbidite called as Ta and/or Tb. Other sections of turbidite had small amounts of methane hydrate. From these results, we estimated that only permeable sections could maintain the accumulation of methane hydrate from first to last stage. If their pores had been filled with gas-saturated water and the pressure/temperature condition was sufficient for methane hydrate, the methane hydrate would precipitate homogeneously in sediments from its initial accumulation. However, methane hydrate precipitation reduces permeability because methane hydrate growth in pores causes a reduction of pore volume. During accumulation, it is inevitable that gas-rich fluid could not reach into low permeable layers, although it could still be supplied into high permeable layers. This mechanism does not need any cap rock or some trap system in sediments, and it is quite a different mechanism from conventional oil and gas accumulation.
The oil industry has gradually accepted methane clathrate hydrate (MH) as an unconventional hydrocarbon resource. However, resource assessments and investment decisions for such new natural resources require field verifications of production technologies. Under the government-supported Japan's Methane Hydrate R & D Program, the MH21 consortium for MH resource development has carried out two onshore gas hydrate production tests in the Mackenzie Delta in the Canadian arctic with international collaboration, and plans offshore production tests in Japanese waters. The exploration drilling campaign in the Eastern Nankai Trough in 2004 revealed the resource size of hydrate deposits as well as the physical properties of hydrate-bearing sediments in this area. Information on formation properties such as pressure, temperature, and permeabilities taken from the first onshore production test in 2002 and the Nankai Trough exploration data, along with laboratory and numerical study results, suggest that simple depressurization, an energy efficient gas production method that is comparable to the primary recovery of conventional oil, is more feasible than previously considered. Thus, verification of the depressurization method and technologies were defined as objectives of the second production test undertaken by Japan Oil, Gas and Metals National Corporation (JOGMEC) and Natural Resources Canada (NRCan) in 2007 and 2008. Six-day gas production by depressurization produced 13000 m3 of gas from the hydrate reservoir in 2008, and led to the phase 2 study of MH21 in which the first offshore production test is planned for 2012.
Marine electromagnetic (EM) sounding methods were developed originally for imaging the deep mantle, partial melt below mid-oceanic ridges, active faults in the crust around subduction zones, etc. Recently, marine EM sounding has been applied extensively for oil, gas, and methane hydrate exploration. In this paper, we introduce various marine EM sounding methods and case studies. Natural fluctuations of EM fields from the ionosphere can be observed on the seafloor using ocean-bottom electromagnetometers (OBEMs), and the marine magnetotelluric sounding method can be applied for imaging the deep structure (> several km below the seafloor). However, the natural signal is less powerful in the higher frequency band and provides less resolution for near-seafloor structures. A high-frequency controlled electromagnetic source on/near/below the seafloor is necessary for evaluating the shallow structure. Marine controlled-source EM (CSEM) sounding using a horizontal electric dipole, towed near the seafloor and sending an electric signal, and OBEMs as receivers settled on the seafloor is widely used for oil and gas exploration, and occasionally for methane hydrate explorations. A vertical electric dipole is also used for magnetometric resistivity (MMR) sounding when exploring hydrothermal systems around ridges. On the other hand, both sources and receivers attached to a cable can be towed near the seafloor for a continuous survey. Towed-CSEM sounding with a time-domain analysis is used especially for methane hydrate exploration. Another type of towed-CSEM with magnetic signal obtains porosity information on near-seafloor sediments. A deep-towed marine DC resistivity survey has been applied near the seafloor, and successfully imaged shallow methane hydrate distributions in sedimentary layers. Thus, various marine EM soundings can illuminate sub-seafloor methane hydrate, and are useful for discussing the accumulation/dissolution process of methane hydrate and the potential of methane hydrate as a new energy resource.
To produce methane gas from methane hydrate safely and without damaging the environment, we need to address many wide-ranging environmental issues. One is to assess seabed deformation during methane gas production. We are investigating if deformation of seabed ground occurs during the production of methane gas from methane hydrate. Geotechnical properties of seabed ground have significant effects on deformation behavior. Soil samples were recovered from the East Nankai Trough where there is expected to be a large amount of methane hydrate, based on the extensive distribution and high amplitude BSRs. This paper presents the geotechnical properties of samples based on the results of laboratory tests. Soil index tests, consolidation tests and triaxial compression tests were conducted to obtain the geotechnical parameters that are necessary for a deformation analysis of seabed ground in deep seas.
Plantonic and benthic foraminifera are analyzed with 11 sediment cores recovered from the Umitaka Spur area of the Joetsu Basin off Joetsu, Niigata Prefecture. The area is characterized by active methane seeps and methane hydrates. We recognize 12 foraminiferal biozones (Biozone I to XII in descending order) in the last 32000 years based on three selected cores (two well-dated and one longest), and apply them to another 8 cores for correlation. Sediment cores are divided into five lithologic units as massive to bioturbated mud (lithologic unit 1), thinly laminated mud (unit 2), gray massive mud (unit 3), thinly laminated dark mud (unit 4), and bioturbated mud (unit 5) from upper to lower. Lithologic units 2 and 4 correspond to basin-wide thinly laminated layers, previously reported as TL-1 and TL-2, respectively. The Japan Sea became a closed inland basin during the lowest sea level period of the last glacial maximum (LGM) at 27-26cal kyr BP (Biozone VIII). The surface water reached the lowest salinity level, while the bottom water was strongly anoxic due to reduced vertical circulation. An expulsion of a large amount of methane occurred on the Umitaka Spur during the LGM due to a massive dissociation of subsurface methane hydrate. Biozones VIII, VII, and VI at around 27-17 cal kyr BP with planktonic foraminiferal maximum and benthic foraminiferal minimum are found in a dark layer of TL-2, which was formed during the period of the lowest sea level in the LGM. Biozone IV, 12-11 cal kyr BP, is characterized by low oxygen tolerant benthic species of Bolivina pacifica, and correlates with dark layer TL-1, which implies that the deep circulation of Japan Sea was severely reduced for a short period during (or soon after) the Younger Dryas Cooling Event. B III represents the planktonic foraminiferal minimum zone, which marks the transition from cool water species to warm water species in planktonic foraminifera. Foraminiferal stratigraphy reveals that the sedimentation rate of the Umitaka spur sediments varied significantly depending on topography such as pockmarks or mounds.
Methane seep activity around the Joetsu Gas Hydrate Field of the western Joetsu Basin, eastern margin of the Japan Sea, was investigated in detail using heat flow measurements. Heat flow was obtained by Ewing-type heat flow probe and SAHF probe with five thermistors at 11-cm intervals using a ROV during nine research cruises in 2004-2008. Average heat flow value obtained on a normal muddy seafloor in this area is 98 ± 13 mW/m2, which is consistent with the ambient heat flow in the Japan Sea. Based on the results of three day's monitoring, temperature fluctuations (> 0.02 K) of bottom water influence sub-bottom temperature at around a depth of 20 cm. Heat flow values greater than 300 mW/m2 were measured not only at the methane venting sites but also in the some areas covered by bacterial mats. This high heat flow value (> 150 mW/m2) is confined to certain areas (several meters to a few tens of meters scale) on the mounds in the Umitaka Spur and the Joetsu Knoll. Therefore, methane migration from the deep subsurface to seafloor occurs on a very local scale, although seismic profiles show the presence of many small faults through gas chimneys just below the mounds. Convex temperature profiles around the gas venting sites indicate the presence of fluid discharges with Darcy's flow velocity of 1.3 × 10-6 m/s and 5.0∼8.6 × 10-7 m/s, respectively. On the other hand, concave temperature profiles, obtained in the “collapsed hydrate zone” on the mounds, may indicate the presence of a recharge zone. Some temperature reversal profiles in areas covered by bacterial mats were probably caused by a lateral fluid movement from a fluid conduit or by the presence of a methane fluid pool. Some apparent negative geothermal gradient anomalies were obtained only in the “collapsed hydrate zone”. Most of these apparent negative anomalies are possibly explained by the influence of bottom water temperature fluctuations. There seem to be some different hydrological regimes in the high methane flux area of the Joetsu Gas Hydrate Field.