[1] Methane gas hydrates, crystalline inclusion compounds formed from methane and water, are found in marine continental margin and permafrost sediments worldwide. This article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate-bearing sediments. Formation phenomena include pore-scale habit, solubility, spatial variability, and host sediment aggregate properties. Physical properties include thermal properties, permeability, electrical conductivity and permittivity, small-strain elastic P and S wave velocities, shear strength, and volume changes resulting from hydrate dissociation. The magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate-bearing sediments to changes in mechanical, thermal, or chemical boundary conditions. These predictions are vital for mitigating borehole, local, and regional slope stability hazards; optimizing recovery techniques for extracting methane from hydrate-bearing sediments or sequestering carbon dioxide in gas hydrate; and evaluating the role of gas hydrate in the global carbon cycle.

Respiratory defective mutants of Saccharomyces cerevisiae previously assigned to complementation group G178 are characterized by an abnormally low ratio of FAD/FMN in mitochondria. A nuclear gene, designated FLX1, was selected from a yeast genomic library, based on its ability to confer wild-type growth properties to a representative G178 mutant. Genetic evidence has confirmed that the flavin nucleotide imbalance of G178 mutants is caused by mutations in FLX1. The sequence of FLX1 is identical to a reading frame recently reported to be present on yeast chromosome IX (GenBank Z47047). The sequence and tripartite repeat structure of the FLX1 product (Flx1p) indicate it is a member of a protein family consisting of mitochondrial substrate and nucleotide carriers. In yeast, FAD synthetase is present in the soluble cytoplasmic protein fraction but not in mitochondria. Riboflavin kinase, the preceding enzyme in flavin biosynthesis, is present in both subcellular fractions. The absence of FAD synthetase in mitochondria implies that FAD is imported from the cytoplasm. The lower concentration of mitochondrial FAD in flx1 mutants suggests that Flx1p is involved in flavin transport, a role that is also supported by biochemical evidence indicating more efficient flux of FAD across mitochondrial membrane vesicles prepared from wild-type strains than membrane vesicles from flx1 mutants.

Even a small fraction of fine particles can have a significant effect on gas production from hydrate-bearing sediments and sediment stability. Experiments were conducted to investigate the role of fine particles on gas production using a soil chamber that allows for the application of an effective stress to the sediment. This chamber was instrumented to monitor shear-wave velocity, temperature, pressure, and volume change during CO 2 hydrate formation and gas production. The instrumented chamber was placed inside the Oak Ridge National Laboratory Seafloor Process Simulator (SPS), which was used to control the fluid pressure and temperature. Experiments were conducted with different sediment types and pressureÀtemperature histories. Fines migrated within the sediment in the direction of fluid flow. A vuggy structure formed in the sand; these small cavities or vuggs were precursors to the development of gas-driven fractures during depressurization under a constant effective stress boundary condition. We define the critical fines fraction as the clay-to-sand mass ratio when clays fill the pore space in the sand. Fines migration, clogging, vugs, and gasdriven fracture formation developed even when the fines content was significantly lower than the critical fines fraction. These results show the importance of fines in gas production from hydrate-bearing sediments, even when the fines content is relatively low.

The water retention curve and relative permeability are critical to predict gas and water production from hydrate‐bearing sediments. However, values for key parameters that characterize gas and water flows during hydrate dissociation have not been identified due to experimental challenges. This study utilizes the combined techniques of micro‐focus X‐ray computed tomography (CT) and pore‐network model simulation to identify proper values for those key parameters, such as gas entry pressure, residual water saturation, and curve fitting values. Hydrates with various saturation and morphology are realized in the pore‐network that was extracted from micron‐resolution CT images of sediments recovered from the hydrate deposit at the Mallik site, and then the processes of gas invasion, hydrate dissociation, gas expansion, and gas and water permeability are simulated. Results show that greater hydrate saturation in sediments lead to higher gas entry pressure, higher residual water saturation, and steeper water retention curve. An increase in hydrate saturation decreases gas permeability but has marginal effects on water permeability in sediments with uniformly distributed hydrate. Hydrate morphology has more significant impacts than hydrate saturation on relative permeability. Sediments with heterogeneously distributed hydrate tend to result in lower residual water saturation and higher gas and water permeability. In this sense, the Brooks‐Corey model that uses two fitting parameters individually for gas and water permeability properly capture the effect of hydrate saturation and morphology on gas and water flows in hydrate‐bearing sediments.

A model for water permeability reduction in hydrate‐bearing sediments is presented by considering capillary effect in hydrate nucleation. Both grain‐coating and pore‐filling cases are considered. The model is developed from a series of lattice Boltzmann flow simulations. Results show that the permeability decreases quasi‐linearly with increasing hydrate saturation during grain‐coating nucleation and that the permeability tends to be higher than predicted by previous analytical models, in which capillarity is not taken into account. The permeability follows unique reduction curve and is not so sensitive to initial sediment bulk density and grain size distribution. Simulations further show that there is a transition zone at Shyd = 0.3~0.4, where permeability reduction trend switches from grain‐coating model to pore‐filling model. Analyses of tortuosity and surface area confirm that the permeability reduction mechanisms result from pore‐channel blocking in grain‐coating case and pore size reduction in pore‐filling case.

We aimed to present the study design of an independent validation cohort from the Korean Brain Aging Study for the Early Diagnosis and Prediction of Alzheimer’s disease (AD) (KBASE-V) and to investigate the baseline characteristics of the participants according to the AD clinical spectrum. We recruited 71 cognitively normal (CN) participants, 96 with subjective cognitive decline (SCD), 72 with mild cognitive impairment (MCI), and 56 with AD dementia (ADD). The participants are followed for three years. The Consortium to Establish a Registry for AD scores was significantly different between all of the groups. The logical memory delayed recall scores were significantly different between all groups, except between the MCI and ADD groups. The Mini-Mental State Examination score, hippocampal volume, and cerebrospinal fluid (CSF) amyloid-β42 level were significant difference among the SCD, MCI, and ADD groups. The frequencies of participants with amyloid pathology according to PET or CSF studies were 8.9%, 25.6%, 48.3%, and 90.0% in the CN, SCD, MCI, and ADD groups, respectively. According to ATN classification, A+/T+/N+ or A+/T+/N− was observed in 0%, 15.5%, 31.0%, and 78.3% in the CN, SCD, MCI, and ADD groups, respectively. The KBASE-V showed a clear difference according to the AD clinical spectrum in neuropsychological tests and AD biomarkers.

S U M M A R YThe hydraulic conductivity can control geotechnical design, resource recovery and waste disposal. We investigate the effect of pore-scale spatial variability on flow patterns and hydraulic conductivity using network models realized with various tube size distributions, coordination number, coefficient of variation, correlation and anisotropy. In addition, we analyse flow patterns to understand observed trends in hydraulic conductivity. In most cases, the hydraulic conductivity decreases as the variance in pore size increases because flow becomes gradually localized along fewer flow paths; as few as 10 per cent of pores may be responsible for 50 per cent of the total flow in media with high pore-size variability. Spatial correlation reduces the probability of small tubes being next to large ones and leads to higher hydraulic conductivity while focused fluid flow takes place along interconnected regions of high conductivity. A pronounced decrease in tortuosity is observed when pore size and spatial correlation in the flow direction are higher than in the transverse direction. These results highlight the relevance of grain size and formation history dependent pore size distribution and spatial variability on hydraulic conductivity, related geo-process and engineering applications.

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