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It is of great significance to study the mechanical behavior and permeability properties of hydrate-bearing sediments for a safe, efficient, and sustainable exploitation of hydrate. However, most of the studies conducted so far have focused only on a single stress field or seepage field, which is detached from practical engineering. In this paper, a new integrated experimental system (IES) was proposed, which realizes the coupling study of stress and seepage. The main body of IES is a triaxial subsystem and a seepage subsystem. The triaxial subsystem can realize in situ synthesis and triaxial shear of hydrate-bearing sediments (HBS). Stable seepage can be effectively formed using a constant pressure infusion pump and a back pressure valve. A series of shear–seepage coupling tests were carried out to verify the effectiveness of the IES and explore the stress–seepage coupling characteristics of HBS. The results show that stress has a significant influence on permeability, and its essence is the stress compression on the seepage channel. The stress–strain relationship, volume response, and permeability are related to each other. The permeability will be affected by the coupling of hydrate saturation (pore plugging), effective confining pressure (pore compression), and shear (fracture generation). [ABSTRACT FROM AUTHOR]

Dynamic mechanical properties of natural gas hydrate (NGH) reservoirs are extremely important owing to their critical role in drilling safety and reservoir stability during NGH development. However, the comprehensive influence of reservoir skeleton and hydrate status on the dynamic mechanical properties of hydrate‐bearing sediments (HBS) has rarely been reported. In this study, three types of sandy sediments with different particle size were simulated. A customized resonant column system was used to synthesize HBS and to test the dynamic mechanical properties of the HBS under the conditions of different confining pressures and shear strains. In addition, the dynamic mechanical responses of the hydrate‐free and hydrate‐dissociated specimens were comprehensively studied to further demonstrate the dominant role of hydrate status and skeleton‐hydrate interactions on the dynamic mechanical properties of HBS. The results show that the shear moduli of the HBS specimens increase exponentially with an increase in confining pressure and decrease in amplitude strain. However, the damping ratios decreases with an increase in confining pressure and decrease in amplitude strain. With an increase in the median particle size (D50) of the simulated sediments, the shear moduli tend to increase, whereas the damping ratios decreas. In addition, the hydrate occurrence decreases differences in moduli between fine‐, medium‐ and coarse‐skeleton specimens, and the enhancement effect of hydrate on the mechanical properties of the specimen is the maximum for the fine sediment specimens. Interestingly, we find that the modulus of the hydrate‐dissociated specimen is relatively smaller than that of the hydrate‐free specimen, which implies that the structure of the sediments could be rearranged and weakened due to the hydrate formation‐dissociation processes. [ABSTRACT FROM AUTHOR]

Anaerobic oxidation of methane and sulfate reduction was studied in the pore waters of four cores at two stations of the middle Okinawa Trough. Pore water vertical distributions of sulfate, methane, sulfide, total alkalinity, ammonium, and phosphate were determined in this study. Our results show strong linear sulfate concentration gradients of 6.83 mmol m in Core A and 5.96 mmol m in Core C, which were collected from two stations. Concurrent variations of methane, total alkalinity and hydrogen sulfide all exhibit steep increases with depth at both cores, which indicate active methane seep activities around two stations. Pore water ammonium and phosphate concentrations reveal minor influences of organic matter degradation on sulfate reduction at two stations. Sulfate methane interface (SMI) was extrapolated from linear sulfate profiles in methane seep cores. Shallower SMI depths (A: 4.9 mbsf; C: 5.4 mbsf) indicate strong methane fluxes and active anaerobic oxidation of methane in the underlying sediments. [ABSTRACT FROM AUTHOR]

Gas hydrate has been recognized as a potential energy resource in South China Sea (SCS). Understanding the acoustic response of gas hydrate formation in the SCS sediments is essential for regional gas hydrate investigation and quantification. The sediments were obtained from gravity core sampling at E 115°12.52363′ N 19°48.40299′. Gas hydrate was formed within a “gas + water-saturated SCS sediments” system. Combination of a new bender element technique and coated time domain reflectometry (TDR) was carried out to study the acoustic response of hydrate occurrence in SCS sediments. The results show the acoustic signal becomes weak when hydrate saturation (S h ) is lower than 14%. The acoustic velocities (Vp, Vs) of the sediments increase with S h during hydrate formation, and Vs increases relatively faster when S h is higher than 14%. These results indicate that tiny hydrate particles may firstly float in the pore fluid, which causes a significant acoustic attenuation, but has little influence on shear modulus. As time lapses and S h approaches 14%, numerous particles coalesce together and contact with sediment particles. As a result, Vs has a sharp increase when hydrate saturation exceeds 14%. Several velocity models were validated with the experimental data, which suggests a combination of the BGTL (Biot–Gassmann Theory modified by Lee) model and the Weighted Equation is suitable to estimate S h in SCS. [ABSTRACT FROM AUTHOR]

It is of great importance to investigate the mechanical properties and constitutive models of hydrate-bearing sediments (HBS) for the safe extraction of hydrate. However, there are few experiments and constitutive models for fine-grained HBS to date. In this work, a range of triaxial tests were first performed to reveal the mechanical properties of hydrate-bearing clayey-silty sediments (HBCSS). Subsequently, the compression properties and shear behaviors of HBS were summarized in depth. Finally, a constitutive model that could describe the mechanical behaviors of clayey sediments and sandy sediments uniformly was established by proposing the compressibility parameter (reflecting compression characteristics), cementation factor (reflecting cementation effect), and dilatancy factor (reflecting filling effect). The results show that increasing saturation can significantly improve the stiffness and strength of HBCSS. While strain softening can be induced by high hydrate saturation. Strain hardening often produces volume contraction, and strain softening produces volume dilatation. The established constitutive model can not only effectively reflect strain softening and hardening, volume dilatation and contraction, but also describe the characteristics of isotropic compression and one-dimensional consolidation. Meanwhile, it has a good prediction on the effects of confining pressure and hydrate saturation. The model is suitable for both coarse-grained sandy HBS and fine-grained clayey HBS. • The mechanical behaviors of HBCSS are revealed. • A strength prediction formula considering hydrate saturation and confining pressure is proposed. • The compression characteristics of HBS can be reflected by a compressibility parameter. • A constitutive model suitable for clayey HBS and sandy HBS is established. [ABSTRACT FROM AUTHOR]

Experiments are performed to study the closed-loop effect of gas hydrate formation and decomposition on the flow properties of a fine-grained quartz sand specimen. The high resolution X-ray CT images of the test specimen at different experimental stages are acquired. In order to elucidate the changes in pore structure of the test specimen, topologically representative pore networks are established. The evolution of the flow properties during gas hydrate formation and decomposition is further evaluated. The results show that, gas hydrates occupancy in pore space exhibit different modes; they grow mainly as the grain-cementing mode except some intermediate stages, where pore-filling or load-bearing hydrates are observed. It is also found that the formation and decomposition of gas hydrates can cause the pore structure and flow properties changed. Increase of gas hydrate saturation results in a sharp decline in water relative permeability, larger irreducible water saturation and smaller gas-water percolation zone, while gas relative permeability does not exhibit obvious changing law. The decomposition of gas hydrates will exert a greater influence on the flow properties described above than gas hydrate formation does. The increase in frequency percentage of 10–20 μm pores within the fine-grained test specimen after experiments might be caused by gas hydrate decomposition induced damage of pore structure. [ABSTRACT FROM AUTHOR]

During the last decade, gypsum has been discovered widely in marine methane hydrate-bearing sediments. However, whether this gypsum is an in-situ authigenic precipitate remains controversial. The GMGS2 expedition carried out in 2013 by the Guangzhou Marine Geological Survey (GMGS) in the northern South China Sea provided an excellent opportunity for investigating the formation of authigenic minerals and, in particular, the relationship between gypsum and methane hydrate. In this contribution, we analyzed the morphology and sulfur isotope composition of gypsum and authigenic pyrite as well as the carbon and oxygen isotopic compositions of authigenic carbonate in a drillcore from Site GMGS2–08. These methane-derived carbonates have characteristic carbon and oxygen isotopic compositions (δ 13 C: −57.9‰ to −27.3‰ VPDB; δ 18 O: +1.0‰ to +3.8‰ VPDB) related to upward seepage of methane following dissociation of underlying methane hydrates since the Late Pleistocene. Our data suggest that gypsum in the sulfate-methane transition zone (SMTZ) of this core precipitated as in-situ authigenic mineral. Based on its sulfur isotopic composition, the gypsum sulfur is a mixture of sulfate derived from seawater and from partial oxidation of authigenic pyrite. Porewater Ca 2+ ions for authigenic gypsum were likely generated from carbonate dissolution through acidification produced by oxidation of authigenic pyrite and ion exclusion during methane hydrate formation. This study thus links the formation mechanism of authigenic gypsum with the oxidation of authigenic pyrite and evolution of underlying methane hydrates. These findings suggest that authigenic gypsum may be a useful proxy for recognition of SMTZs and methane hydrate zones in modern and ancient marine methane hydrate geo-systems. [ABSTRACT FROM AUTHOR]

Evaluating deformation characteristics and predicting strength parameters of hydrate-bearing sediments (HBS) are indispensable for natural gas hydrate exploration and development. A series of triaxial shearing tests were conducted in this paper to analyze the mechanical behaviors and failure mechanisms of hydrate-bearing sediments. The failure strength is discussed comprehensively based on the Mohr-Coulomb criterion, the Drucker-Prager criterion, and the Lade-Duncan criterion. Failure mechanisms of the specimens during shearing are discussed at the micro-level. The results indicate that specimens with relatively high hydrate saturation exhibit strain-softening brittle failure mode, whereas those with relatively low hydrate saturation exhibit obvious strain-hardening ductile failure mode. The cohesion of HBS is enhanced from 0.42 MPa to 1.0 2 MPa, and the friction angle increases from 26.4° to 34.2° with hydrate saturation rising. Additionally, the modified criteria based on the Drucker-Prager criterion can predict the strength with the highest accuracy. The results provide a theoretical and experimental reference for developing the natural gas hydrate reservoirs efficiently. • Mechanical properties of hydrate-bearing sediments based on triaxial shearing tests. • Failure strength is discussed comprehensively based on the Mohr-Coulomb criterion, the Drucker-Prager criterion, and the Lade-Duncan criterion. • Failure mechanisms of the specimens during shearing are discussed at the micro-level. • The cohesion is enhanced from 0.42 MPa to 1.0 2 MPa, and the friction angle increases from 26.4° to 34.2°. [ABSTRACT FROM AUTHOR]