a b s t r a c tNatural 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 neverdepressurized 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 < 0.4, the state of effective stress and stress history are significant factors determining sediment stiffness. Controlled depressurization tests show that hydrate dissociation occurs too quickly to maintain thermodynamic equilibrium, and pressureetemperature conditions track the hydrate stability boundary in pure-water, rather than that in seawater, in spite of both the in situ pore water and the water used to maintain specimen pore pressure prior to dissociation being saline. Hydrate dissociation accompanied with fines migration caused up to 2.4% vertical strain contraction. The firstever direct shear measurements on never-depressurized pressure-core specimens show hydratebearing sediments have higher sediment strength and peak friction angle than post-dissociation sediments, but the residual friction angle remains the same in both cases. Permeability measurements made before and after hydrate dissociation demonstrate that water permeability increases after dissociation, but the gain is limited by the transition from hydrate saturation before dissociation to gas saturation after dissociation. In a proof-of-concept study, sediment microbial communities were successfully extracted and stored under high-pressure, anoxic conditions. Depressurized samples of these extractions were incubated in air, where microbes exhibited temperature-dependent growth rates.Published by Elsevier Ltd.

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No abstract available. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.14.06.2012" target="_blank">10.2204/iodp.sd.14.06.2012</a>

The estimation of gas production rates from hydrate bearing sediments requires complex numerical simulations. This manuscript presents a set of simple and robust analytical solutions to estimate the maximum depressurization-driven recoverable gas. These limiting-equilibrium solutions are established when the dissociation front reaches steady state conditions and ceases to expand further. Analytical solutions show the relevance of (1) relative permeabilities between the hydrate free sediment, the hydrate bearing sediment, and the aquitard layers, and (2) the extent of depressurization in terms of the fluid pressures at the well, at the phase boundary, and in the far field. Close form solutions for the size of the produced zone allow for expeditious financial analyses; results highlight the need for innovative production strategies in order to make hydrate accumulations an economically-viable energy resource. Horizontal directional drilling and multiwellpoint seafloor dewatering installations may lead to advantageous production strategies in shallow seafloor reservoirs.

Free gas in sediments results from the microbial decomposition of organic matter (CH 2 O, solid) into carbon dioxide and methane (methanogenesis), which can escape the seafloor as gas flares, accumulate as free gas and as hydrate, or be consumed in the sulphate reduction zone. It has been estimated that the accumulation rate of organic carbon in oceans is in the order of 0.1 GtC•yr −1 , while methane trapped as hydrate is in the order of 455 GtC up to 1,800 GtC (Ruppel & Kessler, 2017; Wallman et al., 2012 -see also;Boswell & Collett, 2011). Methane is a potent greenhouse gas. Even low amounts of gas released to the atmosphere can irreversibly affect global climate and the biosphere (Judd, 2003;Kennett & Stott, 1991;Walter et al., 2006).Gas flares are the telltale of gas activity in the seafloor. They have been observed in several oceans and water bodies (