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Gas hydrates are expected to be an energy resource in this century. Many countries, such as the United States, China, India, and Japan, have explored potential reservoirs and promising production m...

Gas hydrates deposited in subseafloor sediments are considered to primarily consist of biogenic methane. However, little evidence for the occurrence of living methanogens in subseafloor sediments has been provided. This study investigated viable methanogen diversity, population, physiology and potential activity in hydrate-bearing sediments (1–307 m below the seafloor) from the eastern Nankai Trough. Radiotracer experiments, the quantification of coenzyme F430 and molecular sequencing analysis indicated the occurrence of potential methanogenic activity and living methanogens in the sediments and the predominance of hydrogenotrophic methanogens followed by methylotrophic methanogens. Ten isolates and nine representative culture clones of hydrogenotrophic, methylotrophic and acetoclastic methanogens were obtained from the batch incubation of sediments and accounted for 0.5–76% of the total methanogenic sequences directly recovered from each sediment. The hydrogenotrophic methanogen isolates ofMethanocalculusandMethanoculleusthat dominated the sediment methanogen communities produced methane at temperatures from 4 to 55 °C, with an abrupt decline in the methane production rate at temperatures above 40 °C, which is consistent with the depth profiles of potential methanogenic activity in the Nankai Trough sediments in this and previous studies. Our results reveal the previously overlooked phylogenetic and metabolic diversity of living methanogens, including methylotrophic methanogenesis.

Marine methane hydrate in sands has huge potential as an unconventional gas resource; however, no field test of their production potential had been conducted. Here, we report the world’s first offshore methane hydrate production test conducted at the eastern Nankai Trough and show key findings toward future commercial production. Geological analysis indicates that hydrate saturation reaches 80% and permeability in the presence of hydrate ranges from 0.01 to 10 mdarcies. Permeable (1–10 mdarcies) highly hydrate-saturated layers enable depressurization-induced gas production of approximately 20,000 Sm3/D with water of 200 m3/D. Numerical analysis reveals that the dissociation zone expands laterally 25 m at the front after 6 days. Gas rate is expected to increase with time, owing to the expansion of the dissociation zone. It is found that permeable highly hydrate-saturated layers increase the gas–water ratio of the production fluid. The identification of such layers is critically important to increase the en...

報告, Reports, 本研究は、看護師の筋・骨格系のフィジカルアセスメントに関する知識と実践について質問紙による実態調査を行った。対象は、総合病院勤務の看護師322 名とし、回収数は120 名（37.3％）であった。その結果、112 名（93.3％）の看護師は、フィジカルアセスメントを学んだ経験があった。必要な形態・機能の知識では、「四肢の動脈」を除くすべての項目で「まったくわからない」または「なんとなくわかる」と回答した者が４割以上を占めていた。フィジカルアセスメントの実践では、「臨床での活用の頻度」と「検査：MMT」などに関連を認めた。フィジカルアセスメントを活用している者の方が知識を持ち、実践できる傾向にあった。「実践できない」が最も多かった項目は、「検査：筋トーヌス」27.5％であった。このことから、フィジカルアセスメントに必要な形態・機能の知識を教授する研修が必要である。「臨床での活用の頻度」と関連を認めた「MMT の実践」などは、臨床場面で正確に修得できる可能性がある。

Accurate reservoir potential evaluation requires reliable 3D reservoir models. Geostatistical simulation techniques can reproduce the heterogeneity and quantify the uncertainty in a reservoir. We have applied sequential Gaussian simulation with collocated cokriging to generate the spatial distribution of gas hydrate (GH) saturation around a gas production test site in the eastern Nankai Trough. The simulation was performed using well-log data obtained from the exploration and production tests as a primary variable and inversion-derived seismic impedance data as a secondary variable under the good correlations between two variables. The integrated model adequately described the reservoir heterogeneity and effectively interpolated the seismic trend with respect to the well data. To confirm the usability of the seismic data for the accurate representation of the GH saturation distribution, we ran two model simulations: one using well data only and the other using well and seismic data. Each model was validated using the well-log data obtained at the production test site that were not included during the simulation. The model generated using well and seismic data appropriately reproduced the trend of well-log data at the production test site, especially for the low-GH-saturation unit within the reservoir. However, the model generated using well data only was insufficient to predict the trend of the well data. The results demonstrated that the seismic data were effective for the prediction of the GH saturation distribution, and integration of the well and seismic data could improve the accuracy of the reservoir model.

The first offshore gas hydrate production test was conducted within the gas-hydrate-concentrated zone (reservoir) of the eastern Nankai Trough, which is considered to be a stratigraphic accumulation. However, the accumulation mechanism for this concentrated zone was not yet well understood. We used core and geophysical log data sets to determine the subsurface geologic architecture and stratigraphic evolution most likely responsible for the stratigraphic accumulation of gas hydrate in the eastern Nankai Trough. Seven depositional sequences were identified based on grain size, bed thickness, sedimentary structure, and stacking patterns. The sequence boundaries were also identified by terminations of seismic reflection. These sequences were attributed to a fourth to fifth-order eustatic sea-level changes because the stacking pattern cycle was in phase with global oxygen isotope curves; the cycle was also identified in the onshore formation during the same period. The reservoir was interpreted as falling-stage systems tract (FSST) and lowstand systems tract (LST). FSST and LST consisted mostly of trough-fill channel deposits. The deposits were represented by alternations of very fine- to fine-grained sand and silt. The reservoir is located in association with the structural wing of the Daini-Atsumi Knoll. The uplift of the knoll was strongly controlled by tectonic events associated with subduction of the pacific plate during Pleistocene time. The muddy deposits above the reservoir were interpreted as condensed section. We identified channel facies pinched out against structural highs, and together, these result in stratigraphic traps. Consequentially, the gas hydrate trapping system was constrained by sedimentary facies, systems tracts, and geographic and tectonic setting. Concepts and data generated in this study can be used for gas hydrate petroleum system analysis such as basin simulation.

BACKGROUND There is no clear information about the optimal bed reclining angle for promoting efficient and safe defecation in bedfast patients. OBJECTIVE The aim of this study was to examine the optimal bed reclining angle for facilitating increases in intrarectal pressure without causing marked cardiovascular changes in order to develop an efficient and safe defecation position for bedfast patients. METHODS Twelve healthy men participated in this study. The subjects were required to strain for 15 seconds at the end stage of inspiration while their bed was reclined at 0° (supine), 15°, 30°, or 60°. During straining, the subjects were asked to maintain (a) an intrarectal pressure of 20 mm Hg or (b) the maximal intrarectal pressure. Intrarectal pressure, blood pressure, heart rate, and abdominal muscle activity (electromyographic activity) were recorded continuously throughout the study period. RESULTS During straining, intrarectal pressure increased with the reclining angle, and a significant linear correlation was detected between the sine of the reclining angle of the bed and intrarectal pressure (η = .57, p < .01). When subjects were straining with the aim of maintaining maximal intrarectal pressure, the extent of the observed changes (delta) in blood pressure and heart rate did not differ significantly across the reclining angles. When subjects were straining with the aim of maintaining an intrarectal pressure of 20 mm Hg, the delta blood pressure decreased as the reclining angle increased 0°: M = 23.7, SD = 15.3 mm Hg, 95% CI [11.9, 35.4]; 15°: M = 25.9, SD = 10.8 mm Hg, 95% CI [17.6, 34.2]; 30°: M = 17.7, SD = 9.4 mm Hg, 95% CI [10.4, 24.9]; 60°: M = 15.5, SD = 9.5 mm Hg, 95% CI [8.1, 22.8]; 15° versus 30°: p < .05; 15° versus 60°: p < .05. The amount of muscle activity observed during straining decreased as the reclining angle increased. DISCUSSION In bedfast patients, it is suggested that higher reclining angles may enable safer and more efficient defecation, because it decreases the amount of muscle activity required to increase the intrarectal pressure and reduces the potentially deleterious effects of straining on the cardiovascular system to develop an efficient and safe defecation position for bedfast patients.

To understand the paleocurrent directions and depositional processes in the forearc basin along the active convergent margin of the eastern Nankai Trough, analysis of the anisotropy of magnetic susceptibility (AMS) has been applied to Pleistocene turbidite sediments in a borehole core obtained in the vicinity of the gas hydrate production test site. Sixty-one core specimens mainly sampled from silty and sandy sediments were measured, and the AMS results show generally oblate magnetic fabrics characterized by girdle-distributed weakly-clustered maximum and intermediate susceptibility axes. Diagnostic magnetic fabric parameters show that the sediments generally preserve primary sedimentary structures without significant sediment disturbance due to events such as coring and bioturbation. The characteristic remanent magnetization was also measured to determine the reorientation of each core. Paleocurrent directions estimated from the offset of the minimum susceptibility axes in the stratigraphic coordinates show two predominant directions: one toward the southwest (SW) and the other toward the northwest (NW). On the basis of the interpretations of 3D seismic data, the principal SW direction is generally parallel to the trough-shaped basin axis; this suggests that the sediments were carried by axial turbidity currents within the channel. The subordinate NW direction corresponds to the downslope direction of the Daini–Atsumi Knoll; this may be due to a sediment supply from the southeast associated with a submarine slope failure and landslide evolution or it may be due to flow reflections from the slope toward the basin center. The results of the paleocurrent analysis suggest that the Pleistocene depositional system around the Daini–Atsumi Knoll is affected by the basin configuration. These results are in good agreement with previous studies of the coeval depositional system for the adjacent forearc minibasin around the eastern Nankai Trough.

This study describes the chemical and crystallographic properties of natural gas hydrates recovered from a methane production test site in the eastern Nankai Trough. Gases released from the hydrate-bearing sediments contain methane as the main hydrocarbon component. The hydrate-bound gas includes small amounts of ethane and heavier hydrocarbons (less than ∼300 ppm). Concentrations of minor hydrocarbon components decrease in sediment cores recovered from shallower subseafloor depths. Molecular and isotopic analyses suggest a microbial origin for the natural gas distributed at this site. The 13C NMR and Raman spectra provide evidence that methane molecules are encaged in two distinct polyhedral cages of the structure I hydrate with a hydration number of 6.1. The powder X-ray diffraction profile shows that the crystal type of the gas hydrate is structure I (sI), with lattice constants estimated at 1.1841(2) nm at 83 K. At widely varying temperatures, the lattice constants of the pore-space natural gas hydrate crystals agree well with those of massive natural gas hydrate and artificial methane hydrate, suggesting that the mode of hydrate occurrence does not significantly affect the physical dimensions of the crystal lattice. The small amounts of ethane and heavier hydrocarbons that form sI hydrate have no influence on the lattice expansion of the pore-space hydrate. The density of the natural gas hydrate crystals in the hydrate-bearing sediment sample is estimated at 0.95 g/cm3 at 83 K.

The permeability of methane hydrate sediments is controlled by the lithology of the sediments and the saturation of methane hydrate in pore spaces. Thus, estimates of porosity in the reservoir are important. However, sediments in gas hydrate reservoirs are soft; thus, disturbances during drilling easily degrade the quality of data, despite the sediment-reinforcing effects of gas-hydrate crystals. Hence, methods for correcting the measured properties of sediments are quite important. In this study, we obtain the density (porosity) structure of sediments within and above the methane hydrate reservoir. Due to uncertainties in the density logs, we corrected the logs using borehole caliper data, which is sometimes negatively correlated with density. The corrected density data agreed well with results inferred from analysis of sediments recovered in pressure cores. On these samples, we conducted mercury injection porosimetry on frozen depressurized samples from 2004 to 2012 and also determined density from the onboard multisensor core logger (MSCL) data on conventional cores, especially in methane hydrate concentrated zone (MHCZ). Our results allow us to divide the sediments above the gas hydrate stability zone (GHSZ) on Daini-Atsumi Knoll into four zones. The zone boundaries correspond to facies boundaries. This implies that the sedimentary facies strongly influenced not only the lithology, but also physical properties such as density (porosity).

From June to July 2012, the Ministry of Economy, Trade and Industry completed a pressure core-sampling well (AT1-C) for determining reservoir properties at the first offshore methane hydrate production test site on the north slope of the Daini–Atsumi Knoll off the Atsumi and Shima peninsulas. Coring through the targeted gas-hydrate-bearing reservoir was completed using the hybrid pressure coring system and the extended shoe coring system. The coring program achieved 61% recovery through 60 m of the hole. The recovered core consists of alternating clayey to sandy siltstones and very fine to fine sandstones. The stratigraphic pattern indicates upward thinning of the sand layer. Grain-size distributions indicate that the transition to mud facies is marked by an increase in fine particles and more poorly sorted sediments. Sedimentary structures and bioturbation characteristic of conventional cores are absent in the pressure cores due to the effects of fluidization attributed to the release of water from dissociation of gas hydrates. The core sediments were subdivided into four facies: Facies 1 (hemipelagic setting or slope) structureless silt; Facies 2 (abandonment surface mud drape) silt-dominated alternating beds of sand and silt; Facies 3 (non-amalgamated channel deposit) proportionally interbedded sand and silt; and Facies 4 (semi-amalgamated channel deposit) sand-dominated, with alternating beds of sand and silt. The results aid the interpretation of subsurface data and to quantitatively constrain geological models, thereby reducing uncertainties in the development of reservoirs.

To obtain basic information for methane hydrate (MH) reservoir characterization at the first offshore production test site (AT1) located on the northwestern slope of the Daini-Atsumi Knoll in the eastern Nankai Trough, extensive geophysical logging and pressure coring using a hybrid pressure coring system were conducted in 2012 at a monitoring well (AT1-MC) and a coring well (AT1-C). The MH-concentrated zone (MHCZ), which was confirmed by geophysical logging at AT1-MC, has a 60-m-thick turbidite assemblage with sublayers ranging from a few tens to hundreds of centimeters thickness. The turbidite assemblage is composed of lobe/sheet-type sequences in the upper part and relatively thick channel-sand sequences in the lower part. Well-to-well correlations of sandy layers between two monitoring wells within 40 m of one another exhibited fairly good lateral continuity of sand layers in the upper part of the reservoir. This suggests an ideal reservoir for the production test. The validity of MH pore saturation (Sh) evaluated from geophysical logging data were confirmed by comparing with those evaluated by pressure core analysis. In the upper part of the MHCZ, Sh values estimated from resistivity logs showed distinct differences between the sand and mud layers, compared with Sh values from nuclear magnetic resonance (NMR) logs. Resistivity logs have higher vertical resolution than NMR logs; therefore, they are favorable for these types of thin-bed evaluations. In the upper part, Sh values of 50%–80% were observed in sandy layers, which is in fairly good agreement with core-derived Sh values. In the lower part of the MHCZ, Sh values estimated from both resistivity and NMR logs showed higher background values and relatively smoother curves than those for the upper part. In the lower part, Sh values of 50%–80% were also observed in sandy layers, and they showed good agreement with the core-derived Sh values.

Natural hydrate-bearing sediments from the Nankai Trough, offshore Japan, were studied using the Pressure Core Characterization Tools (PCCTs) to obtain geomechanical, hydrological, electrical, and biological properties under in situ pressure, temperature, and restored effective stress conditions. Measurement results, combined with index-property data and analytical physics-based models, provide unique insight into hydrate-bearing sediments in situ. Tested cores contain some silty-sands, but are predominantly sandy- and clayey-silts. Hydrate saturations S h range from 0.15 to 0.74, with significant concentrations in the silty-sands. Wave velocity and flexible-wall permeameter measurements on never-depressurized pressure-core sediments suggest hydrates in the coarser-grained zones, the silty-sands where S h exceeds 0.4, contribute to soil-skeletal stability and are load-bearing. In the sandy- and clayey-silts, where S h

Gas hydrate-bearing sediments from the eastern Nankai Trough, Japan, are characterized in terms of their lithology, interpreted processes and paleoenvironments of deposition, and various geometric parameters of their grain size distribution. These data are used to determine the relative influence of each characteristic on gas hydrate saturation within the sedimentary column. Four lithologies have been identified in a single turbidite sequence that can be attributed to hyperpycnal flow deposits, Tc or Td divisions of a turbidite sequence, a Te division of a turbidite sequence, and hemipelagic mud. Facies association indicates that the sediment core can be vertically divided into units that are characteristic of three depositional environments: a lowermost channel-fill turbidite sequence, an intervening sheet-like turbidite sequence, and an uppermost basin floor sequence. The channel-fill turbidite and sheet-like turbidite sequences are the best hydrate reservoirs, as evidenced by the high levels of gas hydrate contained within them. The relationships between gas hydrate saturation and the grain size distribution parameters of median grain size, sand content, and skewness show that the latter can be useful tools with which to assess the quality of the gas hydrate reservoir in the eastern Nankai Trough area. This result provides useful criteria for assessing reservoir quality in the eastern Nankai Trough area.

Effective and absolute permeability are among the most important factors affecting the productivity of hydrate-bearing sediments during gas recovery operations. In this study, effective and absolute permeability have been measured using natural sediment cores obtained from a methane hydrate reservoir in the Eastern Nankai Trough off the shore of Japan. The cores were recovered under pressure and shaped cylindrically with liquid nitrogen spray after rapid pressure release. The cylindrical core was inserted into a core holder for flooding tests in order to apply a near in situ effective stress. The effective permeability of water in the hydrate-bearing sandy sediment was 47 millidarcies (md) with a hydrate saturation of 70%. After hydrate dissociation, the absolute permeability was estimated to be 840 md. Other test results showed that the absolute permeability of the hydrate-free sediments was estimated to be tens of microdarcies for clayey sediments, tens of md for silty sediments, and up to 1.5 darcy for sandy sediments. Absolute permeability showed a strong correlation with sediment grain size in log–log plots. In addition, the effective permeability of hydrate-bearing sandy sediments and the absolute permeability of hydrate-free sandy sediments correlated with the effective porosity. We compared measured data to other experimental data using pressure cores recovered from the same well and wireline pressure tests from a well near the coring well. The results are consistent with each other. At this location, we found that the effective permeability for hydrate-bearing sandy sediments was in the range of 1–100 md, which was 2–3 orders of magnitude higher than conventional estimates. Finally, the change of permeability, potentially caused by depressurization-induced gas production, was analyzed. It was found that the high effective stress owing to depressurization and freshwater generation originating from hydrate dissociation caused reduction in absolute permeability.

Although the methane in marine methane hydrates is mainly of microbial origin, information about the distribution of methanogens in subseafloor sediments is limited. To address this issue, we analyzed sediment core samples from two sites in the Nankai Trough, off the Pacific coast of central Japan, including those bearing methane hydrates from depths > 100 m below the seafloor (mbsf), for isopranyl ether-linked polar lipids (i.e. with polar head groups of phosphate, sugar, or both) as biomarkers of archaea, including methanogens. In most samples, including the deepest (381 mbsf), archaeol, and sn -2- and sn -3-hydroxyarchaeols were detected as their hydrolyzed derivatives. Concentrations of these three archaeal lipids correlated strongly with each other, suggesting a common biological source. The δ 13 C values of phytane derived from the phytanyl groups in the archaeal lipids were distinctly higher than those of methane, indicating that methanogens rather than anaerobic methanotrophic archaea were the major biological source. Depth profiles of polar sn -2-hydroxyarchaeol concentration were consistent with those of the potential methane production activity previously estimated from incubation of core sediments from the same sites. This observation, together with results of previous studies showing the presence of sn -2-hydroxyarchaeol mainly in shallow young sediments, strongly suggests that this polar lipid is a valid biomarker for in situ methanogens in sediments. There was a strong correlation between the concentration of polar sn -2-hydroxyarchaeol and that of total organic carbon, suggesting that bulk organic matter concentration is a primary control on the distribution of methanogens in sediments.

報告, Reports, 本研究の目的は、分散型基礎看護学実習Ⅱにおいて学生が経験した看護技術の現状を明らかにし、学生が興味・関心を持って主体的・積極的に看護技術を実地に学ぶ授業形態について検討することである。2011 年度に基礎看護学実習Ⅱを履修した学生を対象に調査を実施した。看護基本技術経験は、多く学生が「バイタルサイン測定」「スタンダードプリコーションに基づいた手洗い」「病床環境整備」「ベットメイキング」「清拭」を実施し、「薬物療法」「車椅子移乗」「口腔内吸引」「陰部洗浄」「転倒・転落・外傷予防」を見学していた。臨地実習評価は、多くの学生が「実習に興味・関心をもって積極的に取り組んだ」「臨地実習を満足した内容であった」と回答した。講義と演習と臨地実習が同時進行する分散型基礎看護学実習Ⅱは、既習の日常生活援助の技術を繰り返し活用でき、学習したばかりの診療補助技術を見学できる。これにより、実習に興味・関心を抱きやすくなることが推察された。

報告, Reports, 本研究は、看護師養成機関における性同一性障害（gender identity disorder : 以下GID とする）学生の演習・実習指導などについて、受け入れ経験者から得た具体例を検討した。対象は、身体的性は女性であるが自己認識は男性であるFTM（Female to Male Transsexual）学生の受け入れ経験のある教員３名である。結果、GID 学生の治療状況により、演習などのグループやトイレ、更衣室の使用が変化した。特に身体の外観的が男性と認識できれば、男子学生として受け入れ、女性であれば女子学生として受け入れていた。女子学生として受け入れた場合は、本人の意向によりユニフォームの考慮や演習時のペアを特定するなどの対応が必要であった。今回の３事例に関わった教員は、全てGID 学生の相談などを担当し、演習や学内行事の前にGID 学生と話し合いを行うことで、最善な対策を選択することを心がけていた。

報告, Reports, 本研究の目的は、模擬患者（Simulated or Standardized Patient: 以下SP）養成において、SP が熟達する過程ごとの特徴を明らかにすることである。 対象は、2002 年から2012 年までSP 養成者が作成したSP トレーニング時、SP 参加型授業時のSP の発言・行動記録、SP 養成者の感想および介入の記録である。SP 養成過程は、ドレイファス・モデルを参考に、初心者、新人、一人前、中堅、達人のレベルとし、得られたデータを各レベルに分類し、特徴をカテゴリー化した。 その結果、一般市民であったSP が熟達する過程の特徴として、以下のカテゴリーが抽出された。初心者レベルでは＜SP として不確実な自分＞を感じ、新人レベルでは＜SP として不安な自分＞を感じる。さらに一人前レベルでは＜SP として自覚が芽生える自分＞を認識し、中堅レベルでは＜SP として成長を感じる自分＞を実感する。そして、達人レベルに到達すると＜SPを極める自分＞の存在を認識する。

The reservoir architecture of methane hydrate (MH) bearing turbidite channels in the eastern Nankai Trough, offshore Japan is evaluated using a combination of 3-D seismic and well data. On the 3-D seismic section, the MH-bearing turbidite channels correspond to complex patterns of strong seismic reflectors, which show the 3-D internal architecture of the channel complex. A seismic-sequence stratigraphic analysis reveals that the channel complex can be roughly classified into three different stages of depositional sequence (upper, middle, and lower). Each depositional sequence results in a different depositional system that primarily controls the reservoir architecture of the turbidite channels. To construct a 3-D facies model, the stacking patterns of the turbidite channels are interpreted, and the reservoir heterogeneities of MH-bearing sediments are discussed. The identified channels at the upper sequence around the β1 well exhibit low-sinuosity channels consisting of various channel widths that range from tens to several hundreds of meters. Paleo-current flow directions of the turbidite channels are typically oriented along the north-northeast-to-south-southwest direction. High-amplitude patterns were identified above the channels along the north-to-south and north-northeast-to-south-southeast directions. These roughly coincide with the paleo-current flow of the turbidite channels. An interval velocity using high-density velocity analysis shows that velocity anomalies (>2000 m/s) are found on the northeastern side of the turbidite channels. The depositional stage of the northeastern side of the turbidite channels exhibits slightly older sediment stages than the depositional stages of the remaining channels. Hence, the velocity anomalies of the northeastern side of the channels are related to the different stages of sediment supply, and this may lead to the different reservoir architectures of the turbidite channels.

症例は42歳，女性．突然の心窩部痛を主訴に他院を受診し，急性腹症との診断で当院紹介となった．右下腹部から側腹部にかけて圧痛を認め，同部位にやや硬い腫瘤を触れた．腹部CT検査にて回腸の一部にtarget sign様の層構造とその内部に均一な脂肪濃度の腫瘤影を認めたため，腸重積と診断し緊急手術を施行した．手術所見では，回腸末端より40cmの部位から肛門側20cmにわたり回腸回腸型腸重積が認められた．Hutchinson手技により重積解除すると，先進部は軟な腫瘤様で，肉眼的には内翻したMeckel憩室と思われた．憩室部分を含む回腸部分切除術を施行した．病理組織検査では異所性胃粘膜や膵組織の迷入は認めず，小腸型粘膜であった．以上より最終的に内翻したMeckel憩室による腸重積症と診断した．術後経過は良好で，術後9日目に退院となった．成人における腸重積症例ではMeckel憩室も原因のひとつとして念頭に置くべきであると思われた．

MH 21 Research Consortium in Japan is planning to implement a gas production test targeting a methane hydrate (MH) reservoir located in MH concentrated zone in the Eastern Nankai Trough (ENT). Two areas surrounding the exploration wells drilled in different MH concentrated zones in ENT were selected as candidates of the production test area because of the availability of well log analysis and seismic interpretation results. The evaluation studies for these areas have been conducted from the various points of view to implement the test successfully. As one of these studies we constructed the 3D reservoir models for the areas consulting the well log analysis and the seismic interpretation results, and predicted the production test performances through numerical simulation. This paper shows how we constructed the 3D models and what results we obtained thorough numerical simulation.First, the frames of the 3D reservoir models for the vicinity of the candidate wells were constructed by consulting seismic interpretation results. The flames were then divided into multiple layers to reproduce the alternating beds structure of sand and mud reflecting the well log interpretation results. The distributions of reservoir properties of each grid layer were estimated by geostatistical techniques using well log interpretation results as hard data and seismic attributes as soft data. Finally the reservoir models were completed by specifying initial pressure and temperature.The well production test performances were predicted through numerical simulation assuming the application of depressurization method. In addition, case studies were conducted to investigate the effect of various scale heterogeneities on the production test performances. The followings were reveled through these studies :(1) Average gas production rate during 30 day test would be over 50,000 m3/d.(2) Large scale heterogeneities such as faults and reservoir dip would affect production test performances significantly.

An integrated sedimentological workflow was applied to the methane-hydrate exploration project in the eastern Nankai Trough area, central Japan, as a useful tool to investigate the distributions and actual volume of methane hydrates. Since previous research revealed that most of the methane hydrates in the eastern Nankai Trough area occur in matrix pores of submarine-fan turbidite sandstones, the facies distribution of turbidite sandstones may be one of the important keys to evaluate the occurrence of methane hydrates. Accordingly, this study conducted a) core facies analysis, b) log facies analysis mainly using FMI, c) depositional sequence division, d) seismic facies analysis on the 2D/3D seismic sections, and e) seismic geomorphological analysis on the 3D seismic survey data to map submarine-fan turbidite facies distributions for 17 depositional sequence horizons in the targeted Pleistocene interval. The obtained facies maps reveal that submarine-fan depositional styles changed throughout Pleistocene from a braided channel type, through small radial fan, trough-fill fan, and muddy sheet fan types, to a channel-levee system type. As the next step, the facies maps of each depositional sequence were overlaid with bottom simulating reflector (BSR) distributions as a proxy of methane hydrates. 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. The facies maps were also used for calculation of average net-to-gross ratio of the methane hydrate occurrence zones, as the maps provide information on the spatial distributions of seismic facies class, which has individual value of net-to-gross ratio. Finally, this study analyzed reservoir-scale geologic bodies, such as depositional lobes and braided channels of a submarine fan, using 3D seismic survey data, and constructed a detailed geologic model for simulations of methane-hydrate generation and production.

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.

The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium), which was organized to attain the exploration and exploitation of methane hydrate (MH) resources, has been evaluating 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 producibility of gas from MH reservoirs located in the Eastern Nankai Trough by a variety of MH dissociation/production methods and on the feasibility of the future development of MH 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 as well as of initial pressure, temperature, permeability and MH saturation. Some of these reservoirs contain MH of high saturation at a certain interval (MH concentrated reservoirs), while in the others MH is deposited sparsely (MH non-concentrated reservoirs). First, the numerical near well model for one of the typical MH reservoirs was constructed mimicking this alteration of sand, silt and clay layers by simplified grid layer system, which was used for the simulation to clarify the mechanisms of MH dissociation and production by the application of the depressurization method.The detailed numerical reservoir models were then constructed for the vicinity of the wells located both in MH concentrated and in non-concentrated reservoirs, consulting the well log and core interpretation results. MH dissociation/production performances were predicted through numerical simulation assuming the application of various MH dissociation methods such as depressurization, wellbore heating, hot water huff'n'puff and hot water flooding. The simulation studies revealed the difference in the gas production between MH concentrated and non-concentrated reservoirs. These studies also suggested that the methane recovery from MH concentrated reservoirs by depressurization methods ranged from 30 to 60% and that the energy efficiency by thermal methods was very small despite high methane recovery. Furthermore, simple economic analyses on the basis of these simulation results exhibited the promise that some MH reservoirs in the Eastern Nankai Trough could be economically developed if the well spacing and MH dissociation/production methods were appropriately designed.

Gas hydrates and associated high methane concentrations are widely distributed along the boundary of the Nankai Trough off Japan. Radiotracer activity measurements were conducted to estimate current methane production rates in sediment cores at two sites in the eastern Nankai Trough off Tokai containing gas hydrates. Low rates of methanogenesis from CO2 reduction and acetate fermentation were detected within the gas hydrate-stability zone and below the Bottom Simulating Reflector (BSR), but not in sediments from shallow depths (

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.

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.