京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20324号 / 工博第4261号 / 新制||工||1660(附属図書館) / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 木村 亮, 教授 三村 衛, 准教授 木元 小百合 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Methane derived authigenic carbonate (MDAC) precipitation occurs within marine sediments as a byproduct of the microbial anaerobic oxidation of methane (AOM). While these carbonates form in chemical and isotopic equilibrium with the fluids from which they precipitate, burial diagenesis and recrystallization can overprint these signals. Plane polarized light (PPL) and cathodoluminescent (CL) petrography have allowed for detailed characterization of carbonate phases and their subsequent alteration. Modern MDACs sampled offshore in northern Cascadia (n =33) are compared with paleoseep carbonates (n =13) uplifted on the Olympic Peninsula in order to elucidate primary vs. secondary signals, with relevance to interpretations of the carbonate record. The modern offshore environment (S. Hydrate Ridge and Barkley Canyon) is dominated by metastable acicular and microcrystalline aragonite and hi-Mg calcite (HMC) that with time will recrystallize to low-Mg calcite (LMC). The diagenetic progression is accompanied by a decrease in Mg/Ca and Sr/Ca ratios while variation in Ba/Ca depends upon the Ba-concentration of fluids that spur recrystallization. CL images discern primary carbonates with high Mn/Ca from secondary phases that reflect the Mn- enrichment that characterizes deep sourced fluids venting at Barkley Canyon. Methane along the Cascadia continental margin is mainly of biogenic origin, where reported strontium isotopic values reflect a mixture of seawater with fluids modified by reactions with the incoming Juan de Fuca plate. In contrast, the Sr-isotopic composition of carbonates and fluids from Integrated Ocean Drilling Program (IODP) Site U1329 and nearby Barkley Canyon point to a distinct endmember (lowest ⁸⁷Sr/⁸⁶Sr = 0.70539). These carbonates also show elevated Mn/Ca and δ¹⁸O values as low as -12‰, consistent with a deep-source of fluids feeding thermogenic hydrocarbons to the Barkley Canyon seeps. Two paleoseep carbonates sampled from the uplifted Pysht/Sooke Fm. have ⁸⁷Sr/⁸⁶Sr values similar to those of the anomalous Site U1329 and Barkley Canyon carbonates (⁸⁷Sr/⁸⁶Sr = 0.70494 and 0.70511). We postulate that the ⁸⁷Sr-depleted carbonates and pore fluids found at Barkley Canyon represent migration by the same type of deep, exotic fluid as is found in high permeability conglomerate layers down to 190 mbsf at Site U1329, and which fed paleoseeps in the Pysht/Sooke Fm. These exotic fluids likely reflect interaction with the 52-57 Ma igneous Crescent Terrane, which is located down-dip from both Barkley Canyon and Site U1329. This previously unidentified endmember fluid in northern Cascadia may have sourced cold seeps in this margin since at least the late Oligocene. / Graduation date: 2012

The mass of carbon trapped in methane hydrates exceeds that in conventional fossil fuel reservoirs. While methane in coarse-grained hydrate-bearing sediments is technically recoverable, most methane hydrates are found in fine-grained marine sediments where gas recovery is inherently impeded by very low gas permeability. Using experimental methods and analyses, this thesis advances the understanding of fine-grained sediments in view of gas production from methane hydrates. The research scope includes: a new approach for the classification of fines in terms of electrical sensitivity, the estimation of the sediment volume contraction during hydrate dissociation, a pore-scale study of gas migration in sediments and the self-regulation effect of surfactants, the formation of preferential gas migration pathways at interfaces during gas production, pressure core technology for the characterization of hydrate bearing sediments without causing hydrate dissociation, and the deployment of a bio-sub-sampling chamber in Japan.

The aim of this work is to model the thermal evolution inside a hydrate forming system which is submitted to an imposed steady cooling. The study system is a cylindrical thin film of aqueous solution at 19 Mpa, the methane is the hydrate forming molecule and it is assumed that methane is homogeneously dissolved in the aqueous phase. The model in this work takes into account two factors involved in the hydrate crystallization: 1) the stochastic nature of crystallization that causes sub-cooling and 2) the heat source term due to the exothermic enthalpy of hydrate formation. The model equation is based on the resolution of the continuity equation in terms of a heat balance. The crystallization of the methane hydrate occurs at supercooling conditions (Tcryst < TF), besides, the heat released during crystallization interferes with the imposed condition of steady decrease of temperature around the system. Thus, the inclusion of the heat source term has to be considered in order to take into account the influence of crystallization. The rate of heat released during the crystallization is governed by the probability of nucleation J(T ). The results provided by the model equation subjected to boundary conditions allow depict the evolution of temperature in the dispersed phase. The most singular point in the temperature–time curve is the onset time of hydrate crystallization. Three time intervals characterize the temperature evolution during the steady cooling: (1) linear cooling, (2) hydrate formation with a release of heat, (3) a last interval of steady cooling.

Raman spectroscopic observations of the characteristics and dissociation of methane hydrate were carried out on hydrates synthesized in silica sands with particle sizes of 53-75 μm, 90-106 μm, 106-150 μm, and 150-180 μm. The results obtained indicate that methane hydrates formed in silica sands had similar characteristics regarding cage occupancy and hydration number (5.99) to bulk hydrate, indicative of no influence of particle size on hydrate composition. During hydrate dissociation, the change in average intensity ratio of large to small cages were generally consistent with that of bulk hydrate but dropped dramatically after a certain time, and this turning point seems to be related to the particle size of silica sands.

The dedicated gas hydrate coring/drilling program was carried out under National Gas Hydrate Program (NGHP) in four Indian offshore areas (Kerala-Konkan, Krishna- Godavari, Mahanadi and Andman) during 28th April to 19th August, 2006. During NGHP Expedition 01, 2006, total of 39 holes were drilled/cored at 21 sites in these areas. The gas hydrates have been found to be present in large quantities in Indian offshore areas particularly in KG basin. More than 130 confirmed solid gas hydrate samples were recovered during this hydrate coring/drilling program. The laboratory analysis was carried out on the 34 natural gas hydrate samples recovered from offshore India. The gas hydrate characterization was carried out using the microscopic techniques such as Raman, 13C NMR and XRD for its structure, cavity occupancy and hydration number. The gas hydrates occur in grayish green fine sediments, gray medium sands and white volcanic ash as pore-filling hydrate and massive hydrates in fractured shale/clay. The visible massive gas hydrates developed especially at Site NGHP 1-10B, 10C, 10D and 21A in K G area. The structures of the gas hydrates in the studied samples are all sI, with methane as the dominant guest molecule. The occupancy of methane in large cage is almost complete, while it is variable in the small cage (0.75 to 0.99). The hydration number is 6.10 ± 0.15 for most of the hydrates in the samples studied. This paper presents the results of the laboratory analysis on the structural characterization of natural gas hydrates in core samples from offshore India.

The effects of porous medium heterogeneity on methane hydrate formation, water flow through the heterogeneous hydrate-bearing sand, and hydrate dissociation were observed in an experiment using a heterogeneous sand column with prescribed heterogeneities. X-ray computed tomography (CT) was used to monitor saturation changes in water, gas, and hydrate during hydrate formation, water flow, and hydrate dissociation. The sand column was packed in several segments having vertical and horizontal layers with two distinct grain-size sands. The CT images showed that as hydrate formed, the water and hydrate saturations were dynamically redistributed by variations in capillary strength of the medium (the tendency for a material to imbibe water), which changed with the presence and saturation of hydrate. Water preferentially flowed through fine sand near higher hydrate-saturation regions where the capillary strength was elevated relative to the lower hydrate saturation regions. Hydrate dissociation initiated by depressurization varied with different grain sizes and hydrate saturations.

Depressurization is thought to be a promising method for gas recovery from methane hydrate reservoirs, but considerable water production is expected when this method is applied to the hydrate reservoir of high initial water saturation. In this case, the prediction of water production is a critical problem. This study examined relative permeability curves during hydrate dissociation by comparing numerical simulations with laboratory experiments. Data of gas and water volumes produced during depressurization were taken from gas recovery experiments using sand-packed cores containing methane hydrates. In each experiment, hydrates were dissociated by depressurization at a constant pressure. The surrounding temperature was held constant during dissociation. The volumes of gas and water produced, the temperatures inside of the core, and the pressures at the both ends of the core were measured continuously. The experimental results were compared with numerical simulations by using the simulator MH21-HYDRES (MH21 Hydrate Reservoir Simulator). The experimental results showed that considerable volume of water was produced during hydrate dissociation, and the simulator could not reproduce the large water production when we used typical relative permeability curves such as the Corey model. To obtain good matching for the volumes of gas and water produced during hydrate dissociation, the shape of relative permeability curves was modified to express the rapid decrease in gas permeability with increasing water saturation. This result suggests that the connate water can be easily displaced by hydrate-dissociated gas and move forward in the hydrate reservoir of high initial water saturation.

The system H₂O-CH₄ is found in a variety of geological environments in the earth’s crust, from sedimentary basins to low grade metamorphic terrains. Knowledge of the PressureVolume-Temperature-Composition (PVTX) properties of the H₂O-CH₄ system is necessary to understand the role that these fluids play in different geological environments. In this study the properties of the H₂O-CH₄ fluid system at elevated temperatures and pressures has been investigated experimentally to determine the PVTX properties of H₂O-CH₄ fluids in the P-T range equivalent to late diagenetic to low grade metamorphic environments, and XCH₄≤4mol%. A study has also been conducted to determine methane hydrate stability over the temperature range of -40~20°C. Synthetic fluid inclusions were employed in both studies as miniature autoclaves. Experimental data for the PVTX properties of H₂O-CH₄ fluids under late diagenetic to low grade metamorphic conditions was used to calculate the slopes of isoTh lines (the line connecting the P-T conditions of the inclusions at formation and at homogenization) at different PTX conditions. An empirical equation to describe the slope of iso-Th line as a function of homogenization temperature and fluid composition was developed. The equation is applicable to natural H₂O-CH₄ fluid inclusions up to 500°C and 3 kilobars, for fluid compositions ≤4 mol% CH₄. The Raman peak position of CH₄ gas is a function of the pressure and temperature. This relationship was used to determine the pressure along the methane hydrate stability curve in the H₂O-CH₄ system. The combined synthetic fluid inclusion, microthermometry and Raman spectroscopy method is a novel experimental approach to determine the P-T stability conditions of methane hydrates. The method is fast compared to conventional methods, and has the potential to be applied to study other gas hydrate systems. / Ph. D.

Hydrocarbon deposits in the form of petroleum, natural gas, and natural gas hydrates occur offshore worldwide. Electromagnetic methods that measure the electrical resistivity of sediments can be used to map, assess, and monitor these resistive targets. In particular, quantitative assessment of hydrate content in marine deposits, which form within the upper few hundred meters of seafloor, is greatly facilitated by complementing conventional seismic methods with EM data. The North-East Pacific Time-series Undersea Network Experiment (NEPTUNE) is an underwater marine observatory that provides power and network connection to a host of instruments installed on the seafloor on the Cascadia Margin offshore Vancouver Island. The observatory’s aim is to provide a platform for very long-term studies in which access to data is available on a continuous basis. For this thesis project, a transient dipole-dipole time-domain EM system was constructed and deployed on the NEPTUNE network with the goal of long-term monitoring of a well-studied hydrate deposit in the area. The instrument includes a source transmitter of electrical current and individual receivers to measure small electric field variations. The instrument is powered by the NEPTUNE observatory and data can be collected remotely by connecting to the instrument through the web. Data collected so far from the instrument are consistent with a resistive structure. The best fitting model from 1D inversion is a 36 ± 3 m thick layer of 5.3 ± 0.3 Ωm resistivity, overlaying a less resistive 0.7 ± 0.1 Ωm halfspace. Average hydrate concentration, deduced with the aid of ODP-889 well-log derived Archie’s parameters, is approximately 72% of pore space in the resistive layer, consistent with the very high concentration of gas hydrates (~80%) recovered from seafloor cores. The weekly collection of data from the instrument shows that the resistive structure has changed little since monitoring began in October of 2010.