Nankai Trough In Japan Research Articles

Study of Mechanical Properties of Different Hydrate Reservoirs

Based on the triaxial shear tests of mud sediments in the South China Sea, mud chalk-type sediments in the South China Sea, and sandy sediments in the Nankai Trough, Japan, at different hydrate saturation and enclosing pressures, the stress-strain relationships of hydrate-bearing clays, mud chalk-type soils, and sandy soils were analyzed. The test results show that:(1)The stress-strain curves of the South China Sea clay sediments show three stages of elasticity, plastic deformation and strain hardening, which are significantly different from those of the hydrate-free clays; the stress-strain relationships of the hydrate-containing clays before and after hydrate decomposition are significantly different, and the undrained strength of the clays decreases by up to 50% after hydrate decomposition compared with that before hydrate decomposition; the above results indicate that the presence of hydrate enhances the linkage or cementation between the clay particles. The above results indicate that the presence of hydrate enhances the linkage or cementation between the clay particles.(2)For the mechanical properties of the hydrate specimens of the muddy chalky sand type in the South China Sea, the hydrate endowment, effective stresses and pore pressures all have some influence on the strength and deformation properties. The inclusion of hydrate in the pore space enhances the shear expansion characteristics of the sediments. For the effect of effective stress on the mechanical properties of hydrate sediments, the increase of effective peritectic pressure reduces the strain softening properties of hydrate sediments. The destructive strength of the sediments increases with the increase of effective confining pressure.(3)For the hydrate specimens in the Nankai Trough of Japan under the same effective circumferential pressure conditions, with the increase of hydrate saturation, the stress-strain law of the sediments showed a tendency to transform from strain hardening to strain softening, and the morphology of sandy sediment axial strain-lateral strain curves was controlled by the factors of hydrate saturation, and the absolute value of the slope of the curves expressed the rule of change of the tangent Poisson's ratio.

Overpressure Estimation Based on Velocity Data Analysis in The Subduction Zone of Hyuga-nada at Western Nankai Trough, Japan

Subduction zone is one of the areas which has great potential of earthquake. Nankai trough in Japan is such an area with subduction zone where earthquake records are well archived, despite its complexity. Also, slow earthquakes have been detected in this zone over the past 20 years, so that there are many opportunities to understand the dynamics and deformation in such an active setting. To expand the knowledge about slow earthquake which might occur and trigger a bigger earthquake, this study wants to infer the relationship between overpressure, one of the states and properties of seismogenic zone, and tremors. To carry out this research, 4 velocity models of seismicity line taken in Hyuga-nada are used. And the recorded tremor data placed around the lines came from Yamashita. In correlating the overpressure and tremors, this research is started by converting the Vp model to porosity model using Wyllie equation to predict the pore pressure conditions in the area. Then, the model will be used for fitting the actual data. Assuming there is an exact location following the normal compaction curve as a function of effective stress, it can be used for estimating the overpressure. Overpressure is economized by comparing the observed porosity and the reference curve at the same depth below seafloor. Potential overpressure occurred may be associated with areas of low Vp. The estimated overpressure is in the range of 1.2 to 12 MPa in the depth range of 700 to 4000mbsf. This study also concludes that the overpressure zone overlaps with the tremor location, although the overpressure value estimated is also still uncertain due to the uncertainty of the density value used.

Offshore near-fault pulse-like ground motions: An insight into the DONET1 seafloor ground motions from the 2016 Mw6.0 Off-Mie earthquake in Japan

Offshore near-fault ground motions were obtained by the DONET1 seafloor seismic network during the 2016 Off-Mie Mw6.0 earthquake that occurred in the Nankai Trough of Japan. These data provide an opportunity for exploring the characteristics of offshore near-fault ground motions, especially pulse-like ground motions. A comparison of the offshore and onshore near-fault ground motions indicates that the recorded offshore spectral acceleration (SA) in the period of 0.5–5.0 s and peak ground velocity (PGV) for this earthquake are higher than those of the onshore values, and the observed maximum spectral value (βmax) of the offshore near-fault ground motions in this event is underestimated by the design spectra in the seismic codes of China, the United States, and Japan. Based on the Hilbert-Huang transform method, eight offshore pulse-like ground motion records are identified and extracted in the Off-Mie earthquake. The primary analysis indicates that the pulse-like ground motion may be caused by the directivity effect and the radiation pattern. Comparing the pulse parameters, time-frequency parameters, and response spectra of offshore and onshore pulse-like ground motions, the findings demonstrate that within the context of this earthquake event, approximately half of the offshore pulse amplitude (Vp) exceeds the predicted values of onshore models; the low-frequency components of offshore pulse-like ground motions of this event are more pronounced, constituting a larger portion of the total ground motion energy; offshore pulse-like ground motions exhibit a tendency to possess higher maximum instantaneous energy (Einst) compared to onshore pulse-like ground motions with similar rupture distances. Moreover, the βmax of offshore pulse-like ground motions is found to be greater than that of onshore pulse-like ground motions in medium to long periods (>0.5 s). The identified and characteristic recognition of offshore near fault pulse-like ground motion revealed by this earthquake event raises new concerns about seismic design for offshore engineering.

Numerical Simulation of Optimized Step-Wise Depressurization for Enhanced Natural Gas Hydrate Production in the Nankai Trough of Japan

The utilization of natural gas hydrates as an alternative energy source has garnered significant attention due to their proven potential. Despite the successful offshore natural gas hydrate production tests, commercial exploitation has not been achieved. This study aims to enhance the understanding of gas production behavior through simulations from a single vertical well in the Nankai Trough and assess the effectiveness of the step-wise depressurization method for gas production using TOUGH + HYDRATE. The simulation results showed that the effective permeability for the water phase decreased as the hydrates were decomposed, and the invasion of the pore water from the underburden eliminated this effect. Compared with the direct depressurization method, the step-wise depressurization method significantly increased the cumulative gas production by more than 10% and mitigated the rapid generation of gas and water production during the moment of depressurization. The results also indicated that the depressurization gradient was more sensitive to the cumulative gas production than the maintenance time of depressurization. In view of the gas and water production characteristics coupled with the challenges in carrying out the step-wise depressurization method, it is suggested that a depressurization gradient of 1 MPa and a maintenance time of 1 day should be employed.

Multi-Lateral Well Productivity Evaluation Based on Three-Dimensional Heterogeneous Model in Nankai Trough, Japan

Widely employed in hydrate exploitation, the single well method is utilized to broaden the scope of hydrate decomposition. Optimizing the well structure and production strategy is necessary to enhance gas recovery efficiency. Complex wells represented by the multilateral wells have great application potential in hydrate mining. This study focused on the impact of multilateral well production methods on productivity, taking the Nankai Trough in Japan as the study area. The spatial distribution of physical parameters such as porosity, permeability, and hydrate saturation in the Nankai Trough has significant heterogeneity. For model accuracy, the Sklearn machine learning and Kriging interpolation methods were used to construct a three-dimensional heterogeneous geological model to describe the structure and physical property parameters in the study area of the hydrate reservoir. The numerical simulation model was solved using the TOUGH + Hydrate program and fitted with the measured data of the trial production project to verify its reliability. Finally, we set the multilateral wells for hydrate high saturation area to predict the gas and water production of hydrate reservoir with different exploitation schemes. The main conclusions are as follows: ① The Sklearn machine learning and Kriging interpolation methods can be used to construct a three-dimensional heterogeneous geological model for limited site data, and the fitting effect of the heterogeneous numerical simulation model is better than that of the homogeneous numerical simulation model. ② The multilateral well method can effectively increase the gas production rate from the hydrate reservoir compared with the traditional single well method by approximately 8000 m3/day on average (approximately 51.8%). ③ In the high saturation area, the number of branches of the multilateral well were set to 2, 3, and 4, and the gas production rate was increased by approximately 51.8%, 52.5%, and 53.5%. Considering economic consumption, the number of branching wells should be set at 2–3 in the same layer.

Thermal Properties of Pressure Core Samples Recovered From Nankai Trough Wells Before and After Methane Hydrate Dissociation

AbstractThe thermal properties of methane hydrate (MH)‐bearing sediments are necessary for developing gas production technologies (e.g., gas production rate prediction). The direct thermal property measurement of a general MH reservoir is difficult because it demands an advanced coring and measuring technology. Therefore, a good thermal property value estimation model for a system containing sediment grains, water, MH, and gas is necessary. In 2018, the pressure core samples were recovered from the AT‐CW1 and AT‐CW2 wells in Nankai Trough of Japan. Using a single‐sided transient plane source method, we measure herein the thermal property values of those core samples and estimate their thermal conductivity by improving existing models. The measured thermal conductivity (λ), specific heat (ρCp), and thermal diffusivity (α) of the MH‐bearing sediments before and after MH dissociation are obtained. The de Vries model agrees well with the measured thermal conductivity values before and after the MH dissociation. For the random distribution (geometric mean) model, the difference between the estimation values and the data after the MH dissociation increases as the gas saturation Sg′ increases. In the worst case, the distribution model shows an 87.6% error, while the de Vries model yields 5.2% of the same. We propose and use an estimation method of the thermal property values in an MH reservoir, which requires classical thermophysical models and well log data. The in‐situ thermal property values can be estimated when the log data are highly accurate.

Characteristics of High Saturation Hydrate Reservoirs in the Low-Angle Subduction Area of the Makran Accretionary Prism

To delineate the spatial distribution of high saturation gas hydrate reservoirs in the low-angle subduction areas of the Makran Accretionary Prism, we conducted seismic data interpretation and impedance inversion of gas hydrates in the Makran Accretionary Prism and comprehensively analyzed the characteristics of the high saturation gas hydrate reservoirs in the Nankai Trough in Japan and the Shenhu Area in the South China Sea. The results show that the Makran Accretionary Prism features thick sediments, developed transport systems of “two-way gas supply” (i.e., thrust fault and normal fault, thrust fault and high permeable strata), and clear and continuous bottom simulating reflector (BSR). Meanwhile, strong-amplitude reflectors and strong-impedance anomalies coexist in the anticline wing above the BSR. Combined with the proven characteristics of high saturation gas hydrate reservoir, the high saturation gas hydrate reservoirs in the Makran Accretionary Prism are probably mainly distributed in the anticline wings immediately above the BSR. These results provide useful information for the exploration and development of gas hydrate in the low-angle subduction area of the Makran Accretionary Prism.

Numerical Simulation of Natural Gas Hydrate Exploitation in Complex Structure Wells: Productivity Improvement Analysis

About 90% of the world’s natural gas hydrates (NGH) exist in deep-sea formations, a new energy source with great potential for exploitation. There is distance from the threshold of commercial exploitation based on the single well currently used. The complex structure well is an efficient and advanced drilling technology. The improvement of NGH productivity through various complex structure wells is unclear, and there is no more complete combing. Thus, in order to evaluate their gas production characteristics, we establish a mathematical model for exploitation of NGH, and then 13 sets of numerical models based on the geological parameters of the Nankai Trough in Japan are developed and designed, including a single vertical well, a single horizontal well, 1~4 branch vertical wells, 1~4 branch horizontal wells, and 2~4 branch cluster horizontal wells. The research results indicate that wells with complex structures represented by directional wells and multilateral wells can significantly increase the area of water and gas discharge, especially cluster wells, whose productivity can be increased by up to 2.2 times compared with single wells. Complex structural wells will play an irreplaceable role in the future industrialization of NGH.

Estimation of fault models for short-term slow\xa0slip events from groundwater pressure in soft sedimentary layers

Deep low-frequency tremors and short-term slow slip events (SSEs) are known to occur along the Nankai Trough in Japan. Although short-term SSEs are thought to occur repeatedly in the northern part of Ise Bay, deep low-frequency tremors are not observed there. Because the area around the northern part of Ise Bay is characterized by soft sedimentary layers not suitable for strainmeter or tiltmeter installation, there are no high-quality crustal deformation observation sites in the area. Accordingly, the local extent of slip of short-term SSEs has not been confirmed. We significantly improved the crustal strain sensitivity of the groundwater level (pressure) in the inner pipe of the observation well at the Hokusei observation site (HKSi, northwest of Ise Bay) by sealing it with a packer. Consequently, we were able to observe groundwater pressure changes related to slow slip at the plate boundary. We estimate that slip of short-term SSEs during July 2016, November 2017, and April 2018 extended to the northern part of Ise Bay. Furthermore, groundwater pressure fluctuations at HKSi in April 2018 greatly improved the SSE fault model, confirming that slow slip occurs in the area, despite the absence of deep low-frequency tremor. The slips without tremors are a characteristic of the northern area of Ise Bay, differing from other areas of the Nankai Trough. Our results demonstrate that groundwater level/pressure fluctuations are a powerful tool for crustal deformation observations in areas characterized by soft sedimentary layers.

A novel type of neural networks for feature engineering of geological data: Case studies of coal and gas hydrate-bearing sediments

The nature of the measured data varies among different disciplines of geosciences. In rock engineering, features of data play a leading role in determining the feasible methods of its proper manipulation. The present study focuses on resolving one of the major deficiencies of conventional neural networks (NNs) in dealing with rock engineering data. Herein, since the samples are obtained from hundreds of meters below the surface with the utmost difficulty, the number of samples is always limited. Meanwhile, the experimental analysis of these samples may result in many repetitive values and 0s. However, conventional neural networks are incapable of making robust models in the presence of such data. On the other hand, these networks strongly depend on the initial weights and bias values for making reliable predictions. With this in mind, the current research introduces a novel kind of neural network processing framework for the geological that does not suffer from the limitations of the conventional NNs. The introduced single-data-based feature engineering network extracts all the information wrapped in every single data point without being affected by the other points. This method, being completely different from the conventional NNs, re-arranges all the basic elements of the neuron model into a new structure. Therefore, its mathematical calculations were performed from the very beginning. Moreover, the corresponding programming codes were developed in MATLAB and Python since they could not be found in any common programming software at the time being. This new kind of network was first evaluated through computer-based simulations of rock cracks in the 3DEC environment. After the model’s reliability was confirmed, it was adopted in two case studies for estimating respectively tensile strength and shear strength of real rock samples. These samples were coal core samples from the Southern Qinshui Basin of China, and gas hydrate-bearing sediment (GHBS) samples from the Nankai Trough of Japan. The coal samples used in the experiments underwent nuclear magnetic resonance (NMR) measurements, and Scanning Electron Microscopy (SEM) imaging to investigate their original micro and macro fractures. Once done with these experiments, measurement of the rock mechanical properties, including tensile strength, was performed using a rock mechanical test system. However, the shear strength of GHBS samples was acquired through triaxial and direct shear tests. According to the obtained result, the new network structure outperformed the conventional neural networks in both cases of simulation-based and case study estimations of the tensile and shear strength. Even though the proposed approach of the current study originally aimed at resolving the issue of having a limited dataset, its unique properties would also be applied to larger datasets from other subsurface measurements.

Complete Genome Sequence of Bacillus sp. Strain KH172YL63, Isolated from Deep-Sea Sediment.

Bacillus sp. strain KH172YL63 is a Gram-positive bacterium isolated from the deep-sea floor surface sediment at 3,308 m below sea level in the Nankai Trough in Japan. Here, we report the complete genome sequence of Bacillus sp. strain KH172YL63, which has a genome size of 4,251,700 bp and a G+C content of 44.8%.

Advanced tsunami detection and forecasting by radar on unconventional airborne observing platforms

Sustaining an accurate, timely, and global tsunami forecast system remains a challenge for scientific communities. To this end, various viable geophysical monitoring devices have been deployed. However, it is difficult to implement new observation networks in other regions and maintaining the existing systems is costly. This study proposes a new and complementary approach to monitoring the tsunami using existing moving platforms. The proposed system consists of a radar altimeter, Global Navigation Satellite Systems receiver, and an adequate communication link on airborne platforms such as commercial airplanes, drones, or dedicated high-speed aircraft, and a data assimilation module with a deterministic model. We demonstrated, through twin-data experiment, the feasibility of the proposed system in forecasting tsunami at the Nankai Trough of Japan. Our results demonstrated the potential of an airborne tsunami observation as a viable future technology through proxy observations and rigorous numerical experiments. The wide coverage of the tsunamigenic regions without a new observation network is an advantage while various regulatory constraints need to be overcome. This study offered a novel perspective on the developments in tsunami detection and forecasting technology. Such multi-purpose observation using existing platforms provides a promising and practical solution in establishing sustainable observational networks.

Effect of multiphase flow on natural gas hydrate production in marine sediment

Natural gas hydrates (NGHs), regarded as an alternative future energy source. Currently, tests for hydrate exploitation from marine sediment have been performed in the Nankai Trough of Japan and the Shenhu area of the South China Sea. Hydrate exploitation is influenced by water-gas flow in the sediment, and considering the huge seawater reserves in hydrate accumulation areas, an experiment of seawater-gas flow was performed to dissociate hydrate. The effects of seawater-gas flow rates and initial hydrate saturation on methane hydrate (MH) production were analyzed. The results showed that seawater-gas flow efficiently promotes hydrate dissociation and inhibits hydrate reformation. Moreover, there was a faster heat and mass transfer with increasing seawater flow rates and decreasing gas flow rates, which enhanced the average MH dissociation rate. In addition, the variation time of the flow channel increased with higher initial hydrate saturation. Additionally, seawater-gas flow promotes MH dissociation stronger than deionized water-gas flow.

Using GIS to simulate tsunami evacuation guidance signs for the hearing impaired.

The Nankai Trough in Japan has been identified as a geological feature that could cause extensive damage in the event of a major earthquake. In this study, we investigated the impact of effective guidance signs for hearing-impaired individuals requiring special care when escaping to a tsunami evacuation building (emergency evacuation location) using geographical information system (GIS) and viewshed analysis. We selected an area we considered would suffer severe damage following a major earthquake and tsunami; we identified difficulties in the provision of escape routes. Using GIS, we determined the time required for escape and tsunami arrival time if effective signs were installed; we undertook such analysis using the height data of buildings in the target area. With effectively installed guidance signs, the average required evacuation time was 36.88 minutes; without such signs (which is currently the case in the target area), the average time was 47.10 minutes: that would result in citizens getting caught by the tsunami. Installing effective guidance signs would allow hearing-impaired individuals to escape to an evacuation building before being hit by the tsunami.

Production performance and numerical investigation of the 2017 offshore methane hydrate production test in the Nankai Trough of Japan

Since the first offshore methane hydrate production test was conducted in the Nankai Trough of Japan in 2013, the second production test was also conducted at the same test site in 2017, but information about this field test is limited. Thus, in this study, we aimed to report detailed information about the 2017 production test for the first time, and conducted short-term simulations of the oceanic methane hydrate production from the multilayered reservoir models for model validation. Then, we predicted the long-term gas production behavior for a production period of 5 y, and investigated the effect of the production intervals on the processes of hydrate dissociation and gas production. The simulation results indicated that most of the released gas from hydrate dissociation was actually produced indirectly in the aqueous phase rather than directly in the gas phase, and the proper utilization of the packers on the wellbore could effectively promote gas production, but also caused excess water production. Therefore, high-performance gas-water separation devices and water production management are necessary for future production tests. Furthermore, the 5-y average gas production rates for each case designed in this study were estimated as 1.01 × 104–1.21 × 104 ST m3/d, which were far less than the commercial exploration level of 3.0 × 105 ST m3/d, so improvements in the production strategies should be considered for future production tests.

Complete Genome Sequence of Psychrobacter sp. Strain KH172YL61, Isolated from Deep-Sea Sediments in the Nankai Trough, Japan.

Psychrobacter sp. strain KH172YL61 is a Gram-negative bacterium isolated from deep-sea sediment in the Nankai Trough in Japan. Here, we report the complete genome sequence of this strain, which has a genome size of 3.19 Mb, with a G+C content of 44.0%.

Numerical analysis of gas production from layered methane hydrate reservoirs by depressurization

Many natural methane hydrate (MH) reservoirs are heterogeneous and are characterized by a layered structure. In this study, we numerically investigate gas production from a multi-layered hydrate reservoir by depressurization through a single vertical and a single horizontal well. This layered MH reservoir is constructed based on field test data at the AT1 site of the Eastern Nankai Trough in Japan, and involves three hydrate-bearing layers (HBLs): Upper HBL-1, middle HBL-2, and lower HBL-3. The simulation results indicate that the horizontal well shows a better gas production performance in comparison to the vertical well. Over a production duration of two years, the average gas production rate by using the horizontal well reached 7.3 × 104 ST m3/d, which is 5.7 times higher than that by using the vertical well. However, the gas-to-water ratio for both the vertical and horizontal well cases is low in absolute terms. Sensitivity analysis of gas production by the horizontal well indicates that both the very higher and lower levels of permeability in HBL-2 and hydrate saturation in HBL-3 are unfavorable for long-term gas production. In addition, decrease of vertical permeability in HBL-1 and HBL-3 can lead to lower gas production efficiency.

Oceanic methane hydrate utilization system design and reservoir scale numerical modeling

In recent years, the exploration and utilization of methane hydrate from permafrost and off-coast regions has become a very hot topic. Aiming at its potential as a vast energy resources in future use, the current study is focused on the low emission extraction and utilization system for oceanic methane hydrate and its economic and technological feasibility. In this study, recent real extraction tests in both China and Japan in 2017 are firstly discussed and compared, where the challenges in production period and production rate are explained. In those tests, depressurization method is used to dissociate the methane hydrate for gas production form the unconsolidated reservoir layers below the seabed. Based on the conditions of Japan, a systematic utilization system is proposed for oceanic methane hydratewhich is consisted of extraction system, power generation system, Carbon Capture and Sequestration (CCS) system and hot fluid injection system. Vertical well systems are adopted in this design and the strategic warming up process is used before depressurization extraction, so as to obtain optimal production rate of methane gas. The inclusion of CCS makes it possible to utilize methane hydrate with very low carbon emission. It is estimated that the electricity price obtained by this system can be comparable with the current civil electric price in Japan if the production rate is around 1.0×105 m3/d. Also in this study, the real extraction scenarios and possible strategies of Nankai Trough in Japan and Shenhu Area in China are re-visited and discussed by numerical models. It is found that the only depressurization or thermal stimulation method have the problems of wellbore deformation, pressure problem and energy efficiency problems, while combined strategic modes show the possibility of long-term continuous production. For the basic dissociation parameter analysis, the conditions near the equilibrium curve show higher production rate, which is due to higher initial reservoir temperature. Such results indicate the importance of heat supply inside the reservoir layer as the methane hydrate dissociation is endothermic. It is shown that with the increase of reservoir pressure, such temperature effects become more critical as the optimal initial temperature increases according to equilibrium curve. This results support the system design with strategic warming-up process in a sister well during methane hydrate extraction. In addition, the effects of reservoir layer thermal management, hot reservoir fluid flow effects on the dissociation behaviors are analyzed in this study. For vertical well analysis, the production rates under different permeability parameters show the same trend with real production, which in turn help confirm the in-lab permeability tests. Horizontal well simulation show the importance of overburden and under burden. If the confining layers are less permeable, the production rate becomes higher due to the limited intrusive water flow from the confining layers. In sum, this study summarized the advantages and disadvantages of oceanic methane hydrate extraction/utilization methods and put forward a low-carbon emission system. It is hoped that this study will contribute to the future development of both production prediction and real extraction tests.

Study of methane hydrate as a future energy resource: low emission extraction and power generation

With the fast increase of world energy consumption in recent years, new and sustainable energy sources are becoming more and more important. Methane Hydrate is one promising candidate for the future energy supply of humankind, due to its vast existence in permafrost regions and near-coast seabed. This study is focused on the effective low emission utilization of methane hydrate from deep seabed. The Nankai Trough of Japan is taken as the target region in this study for methane hydrate extraction and utilization system design. Low emission system and power generation system with CCS (Carbon Capture and Sequestration) processes are proposed and analyzed for production rate and electricity generation efficiency problem study. It is found that the gas production price can reach the current domestic natural gas supply price level if the production rate can be improved. The optimized system is estimated to have power efficiency about 35%. In addition, current development and analysis from micro-to-macro scale methane hydrate production and dissociation dynamics are also discussed into detail in this study.

A new hybrid pressure-coring system for the drilling vessel <i>Chikyu</i>

Abstract. Retrieving core samples without releasing the in situ hydrostatic pressure during core recovery is one of the many technical challenges in scientific drilling. We report here a newly developed hybrid pressure-coring system for the use on the drilling vessel Chikyu and its successful use during expeditions 906 and 802 in the Nankai Trough of Japan. The system is gas-tight and hence enables researchers to study in situ geophysical and geochemical characteristics of sediments containing gaseous components, such as methane hydrates that cannot be reliably recovered with nonpressure coring systems. In addition, pressure coring is a powerful tool, not only for scientific but also for hydrocarbon resources research.