Hydro-bio-geomechanical properties of hydrate-bearing sediments from Nankai Trough

Abstract

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.

Characterizing electrical properties and permeability changes of hydrate bearing sediments using ERT data

“…Because the sediment's pore space in our experiments is completely water saturated at all times throughout hydrate formation and hydrates are formed from methane dissolved in the pore water, we assume to mimic the formation of natural, marine gas hydrates. Recent studies (Konno et al 2015;Santamarina et al 2015) carried out on natural sediment cores obtained from a methane hydrate reservoir in the Eastern Nankai Trough indicated from permeability and velocity measurements that natural, marine hydrates form and accumulate in the centre of the pores, justifying the assumption made by eq. (9).…”

Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough

“…Relation between initial effective water permeability and hydrate saturation at the Eastern Nankai Trough. Laboratory experiments include a pressure relaxation test using pressure-preserved cores treated with LN 2 (Yoneda et al, 2015, same sample preparation) and two different flooding tests using pressure-preserved cores without pressure release and LN 2 treatment (Priest et al, 2014;Santamarina et al, 2015). All samples were recovered from the same well (AT1-C).…”

Geomechanical, Hydraulic and Thermal Characteristics of Deep Oceanic Sandy Sediments Recovered during the Second Ulleung Basin Gas Hydrate Expedition

“…Due to a higher sand fraction, the sediment sample in this study, classified as SM, has a higher peak friction angle than other clay-or silt-dominant sediments (i.e., fine-grained sediments) from the Ulleung Basin, Krishna-Godavari Basin, and Mahanadi Basin. The peak friction angles of coarse-grained sediments from hydrate-occurring layers are predominantly in the range of 27 • -31 • for oceanic sediments [8,36,37], 34 • -44 • for permafrost sediments [37], and 30 • -37 • for the sandy sediments of the eastern Nankai Trough [25]. In comparison, it appears that the studied sediment has a relatively low peak friction angle (26 • ).…”