Kiyofumi Suzuki

New Method of Assessing Absolute Permeability of Natural Methane Hydrate Sediments by Microfocus X-ray Computed Tomography

The structure of natural-gas hydrate sediments was studied using a microfocus X-ray computed-tomography (CT) system. The free-gas spaces, sand particles, and hydrates or ices were identified from the obtained three-dimensional (3-D) images. We used CT data to analyze a continuous pore, which allows gas and water flow. The absolute permeability of sediment samples correlated well with horizontal-channel density in terms of direction. The grain-size distribution in sediment samples depended on the spread of flow channels. The average area and length of a channel evidently have little effect on absolute permeability. We determined that absolute permeability increased with the ratio of horizontal- to vertical-channel numbers. It was clear that the number ratio of the horizontal to vertical channels is a predominant factor that determines absolute permeability in similar porosity ranges. These results indicate that the pore network in sediments can be useful for assessing permeability.

Three-dimensional paleomorphologic reconstruction and turbidite distribution prediction revealing a Pleistocene confined basin system in the northeast Nankai Trough area

Integrated three-dimensional (3-D) paleomorphologic and sedimentary modeling was used to predict the basin architecture and depositional pattern of Pleistocene forearc basin turbidites in a gas hydrate field along the northeast Nankai Trough, off central Japan. Structural unfolding and stratigraphic decompaction of the targeted stratigraphic unit resulted in successful modeling of the paleobathymetry at the time of deposition. This paleobathymetry was characterized by a simple U-shaped paleominibasin. Subsequent turbidity current modeling on the reconstructed paleobathymetric surface demonstrated morphologically controlled turbidity current behavior and selective turbidite sand distribution within the minibasin, which strongly suggests the development of a confined turbidite system. Among three candidate inflow patterns, a northeasterly inflow pattern was determined as most likely. In this scenario, flow reflection and deflection caused ponding and a concentration of sandy turbidite accumulation in the basin center, which facilitated filling of the minibasin. Such a sedimentary character is undetected by seismic data in the studied gas hydrate reservoir formation because of hydrate-cementation–induced seismic anomalies. Our model suggests that 3-D horizon surfaces mapped from 3-D seismic data along with well-log data can be used to predict paleobasin characteristics and depositional processes in deep-water turbidite systems even if seismic profiles cannot be determined because of the presence of gas hydrates.

Morphological and Compositional Characterization of Self-Preserved Gas Hydrates by Low-Vacuum Scanning Electron Microscopy

Gas hydrates (also called clathrate hydrates) are non-stoichiometric solid compounds that form when small guest molecules of suitable size and shape are enclathrated by host cage structures comprising hydrogen-bonded water molecules under appropriate conditions (pressure, temperature, and concentration). The decomposition rates of gas hydrates are known to be suppressed considerably at temperatures below the ice melting point: this is the so-called self-preservation effect. Selfpreservation is not only scientifically interesting; it is also important for the application of gas hydrates as energy-storage materials. For instance, a project for the large-scale transport of natural gas using self-preserved hydrates is underway. A general explanation for self-preservation is that coating of dissociating hydrates by ice products prevents further decomposition either by maintaining internal gas pressure at or near the equilibrium pressure or by limiting gas diffusion through the reaction boundary 8, 9] (note that ice formation is not necessary for decomposition of certain double hydrates; for example, CO2 in the small cages of dimethyl ether-CO2 hydrates can be released without disruption of the basic hydrate structure). However, this simple idea is insufficient to account for the observed complexity of preservation effect. Preservation behaviors are known to vary nonlinearly with temperature and pressure. They also depend strongly on the guest-gas composition. Although the detailed mechanism remains unclear, previous studies have indicated that the texture of ice from hydrate dissociation can be modified according to P-T conditions and according to the type of guest species, engendering different degrees of ice-shielding effects. 11, 14] Scanning electron microscopy (SEM) has been used to investigate microstructures of gas hydrates and hydrate–ice mixtures. 16] Stern et al. conducted SEM observations of partly dissociated methane hydrate particles. They reported a lack of evidence for development of ice-coating around individual hydrate grains. In contrast, SEM studies by Kuhs and co-workers, from their observations of both natural and synthetic specimens, have revealed the evolution of ice films on decaying hydrates. 17] Nevertheless, details of the microstructure of selfpreserved gas hydrates remain unclear because of methodological limitations of previous SEM experiments, such as specimen sublimation in a high-vacuum atmosphere necessary for normal SEM observations, and charging artifacts (abnormal image contrast unrelated to actual surface topology) incurred by negative charge of insulating samples. The most severe problem is how to differentiate hydrate and ice phases. For phase identification, earlier studies 12, 16, 17] relied mainly on surface appearances because a conventional secondary-electron image fundamentally does not contain compositional information, but the difference in textures between the two components is often unclear. Although X-ray analyses are useful for this purpose in some cases, they require a considerable amount of samples for detection (e.g. submicron hydrate inclusions shown in Figures 2 f, 2 h, 3 f and 3 h are too small to analyze with energy-dispersive spectra). Additionally, X-ray scans often give rise to beam damage of the analyzed surfaces. To overcome these difficulties, we use low-vacuum SEM (LVSEM). At a low vacuum, the sample sublimation is negligible (see Figure S1 of the Supporting Information). Interaction between ionized atmospheric gases and sample surfaces neutralizes the negative specimen charge. More importantly, for LVSEM, micrographs are imaged from backscattered electrons, which provide compositional image contrasts because atomic composition dominantly influences the intensity of backscattered electrons (see Supporting Information and Figure S2). Ar and Kr hydrates were investigated in this work for comparison with our latest report : we have reported that different modes of hydrate dissociation to ice were observed between the two systems using microfocus X-ray computed tomography (MFXCT). Figure 1 portrays LVSEM images of as-grown samples. Synthesized hydrates were pure crystals of clathrate structure II, as described in a previous report. Ar hydrates exhibited dimpled surfaces with scattered hollows up to a few tens of micrometers (Figures 1 a and 1 b). Observations of inner surfaces that had been exposed accidentally during sample preparation indicated that crystals were dense, although some voids were included (Figures 1 c and 1 d). Kr hydrates presented a similar pitted surface topology (Figure 1 e), but magnified images [a] Dr. H. Ohno , Dr. J. Nagao Production Technology Team, National Institute of Advanced Industrial Science and Technology 2-17-2-1 Tsukisamu-Higashi Toyohiraku, Sapporo 062-8517 (Japan) Fax: (+ 81) 11-857-8985 E-mail : hiroshi-ohno@aist.go.jp jiro.nagao@aist.go.jp [b] Dr. O. Nishimura , Dr. K. Suzuki Reservoir Characterization Team, National Institute of Advanced Industrial Science and Technology 2-17-2-1 Tsukisamu-Higashi Toyohiraku, Sapporo 062-8517 (Japan) [c] Dr. H. Ohno , Dr. O. Nishimura , Dr. K. Suzuki , Dr. H. Narita, Dr. J. Nagao Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology 2-17-2-1 Tsukisamu-Higashi Toyohiraku, Sapporo 062-8517 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201100079.

Modeling gas hydrate petroleum systems of the Pleistocene turbiditic sedimentary sequences of the Daini-Atsumi area, eastern Nankai Trough, Japan

We have performed 2D and 3D gas hydrate (GH) petroleum systems modeling for the Pleistocene turbiditic sedimentary sequences distributed in the Daini-Atsumi area in the eastern Nankai Trough to understand the accumulation mechanisms and their spatial distribution related to geologic and geochemical processes. High-resolution seismic facies analysis and interpretations were used to define facies distributions in the models. We have created a new biogenic methane generation model based on the biomarker analysis using core samples and incorporated it into our model. Our 2D models were built and simulated to confirm the parameters to be used for 3D modeling. Global sea level changes and paleogeometry estimated from 3D structural restoration results were taken into account to determine the paleowater depth of the deposited sedimentary sequences. Pressure and temperature distributions were modeled because they are the basic factors that control the GH stability zone. Our 2D modeling results suggested that the setting of biogenic methane generation depth is one of the most important controlling factors for GH accumulation in the Nankai Trough, which may be related to the timing of methane upward migration (expulsion) and methane solution process in pore water. Our 3D modeling results suggested that the distribution of sandy sediments and the formation dip direction are important controlling factors in the accumulation of GHs. We also found that the simulated amount of GH accumulation from the petroleum systems modeling compares well with independent estimations using 3D seismic and well data. This suggests that the model constructed in this study is valid for this GH system evaluation and that this type of evaluation can be useful as a supplemental approach to resource assessment.

Sequence stratigraphy and controls on gas hydrate occurrence in the eastern Nankai Trough, Japan

AbstractThe 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 fal...