Hydrate-bearing Marine Sediments Research Articles

Upward migration of the shallow gas enhances the production behavior from the vertical heterogeneous hydrate-bearing marine sediments

Low gas production rates and insufficient gas yield hinder the large-scale production from natural gas hydrates; a joint production of gas hydrate and its underlying shallow gas is expected to address this limitation. This study focuses on the interaction between heterogeneous hydrate reservoirs and the upward migration of the shallow gas. The results indicated a 38.4 % increase in production efficiency with the assistance of the shallow gas. Specially, the melt water generated from extensive hydrate decomposition could potentially induce a lateral migration of the shallow gas at the interfacial zones of the heterogeneous reservoirs. Further investigation revealed that a lower production pressure would help the release of the shallow gas as well as the hydrate decomposition, thereby contributing to a 64.55 % shorter t90. A faster depressurization rate could facilitate the temperature recovery by intensifying the lateral movement of the shallow gas in the junction layer. Consequently, a proper control of the upward channeling of the shallow gas was suggested in the field test for a successive and secure gas production. Our results could be of help in elucidating the interlayer interference mechanisms and the selection of the depressurization strategy for a better recovery efficiency from the multi-gas source reservoirs.

The locally varying thermodynamic driving force dominates the gas production efficiency from natural gas hydrate-bearing marine sediments

The locally varying thermodynamic driving force dominates the gas production efficiency from natural gas hydrate-bearing marine sediments

Microbial communities associated with thermogenic gas hydrate-bearing marine sediments in Qiongdongnan Basin, South China Sea

Biogenic and thermogenic gas are two major contributors to gas hydrate formation. Methane hydrates from both origins may have critical impacts on the ecological properties of marine sediments. However, research on microbial diversity in thermogenic hydrate-containing sediments is limited. This study examined the prokaryotic diversity and distributions along a sediment core with a vertical distribution of thermogenic gas hydrates with different occurrences obtained from the Qiongdongnan Basin by Illumina sequencing of 16S rRNA genes as well as molecular and geochemical techniques. Here, we show that gas hydrate occurrence has substantial impacts on both microbial diversity and community composition. Compared to the hydrate-free zone, distinct microbiomes with significantly higher abundance and lower diversity were observed within the gas hydrate-containing layers. Gammaproteobacteria and Actinobacterota dominated the bacterial taxa in all collected samples, while archaeal communities shifted sharply along the vertical profile of sediment layers. A notable stratified distribution of anaerobic methanotrophs shaped by both geophysical and geochemical parameters was also determined. In addition, the hydrate-free zone hosted a large number of rare taxa that might perform a fermentative breakdown of proteins in the deep biosphere and probably respond to the hydrate formation.

Characterizing Gas Hydrate-Bearing Marine Sediments Using Elastic Properties—Part 2: Seismic Inversion Based on a Pore-Filling–Solid Matrix Decoupling Scheme

Characterizing gas hydrate-bearing marine sediments using seismic methods is essential for locating potential hydrate resources. However, most existing pre-stack seismic inversion methods estimate the properties of sediments containing gas hydrates without considering specific characteristics associated with gas hydrate occurrences. In the present study, a pore-filling–solid matrix decoupling amplitude variation with offset (AVO) formula is proposed to represent seismic reflectivity in terms of properties associated with gas hydrates. Based on the rock physics relationships of solid substitution, the parameters introduced into the decoupling AVO equation estimate the concentration of gas hydrates with different occurrences, including pore fillings mixed with water and solid components forming part of the dry sediment frame. A theoretical model test indicates that seismic attributes obtained with the decoupling AVO inversion are superior to the conventional wave velocities-related properties in predicting gas hydrate saturations. A realistic model test further validates the applicability of the proposed method in characterizing a gas hydrate system with varying concentrations and layer thickness. By adjusting the tuning parameters, the configurations and concentrations of the gas hydrate system can be identified using the obtained attributes. Therefore, the presented method provides a useful tool for the characterization of gas hydrate-bearing sediments.

Strength behaviours of methane hydrate-bearing marine sediments in the South China Sea

Understanding the mechanical response of hydrate-bearing marine sediments (HBMSs) prior to gas production enables engineers to forecast risks associated with large-scale methane extraction from silty hydrate reservoirs in the South China Sea. To this end, this study remoulded HBMSs containing hydrate with pore-spacing or cementing morphologies and then designed a series of drained shear tests on these materials. The results show that the HBMSs containing hydrate in cementing morphology present greater strength behaviours than and similar deformation characteristics to those containing hydrate in pore-spacing morphology. The increase in effective confining pressure magnifies the enhancement effect of hydrate on the stiffness and strength of marine sediments, which is more pronounced for the hydrate in pore-spacing morphology. The cohesion of marine sediments decreases and the internal friction angle increases as the hydrate forms, while the cohesion and internal friction angle show a slight decrease when the hydrate morphology transforms from cementing to pore-spacing. The results are expected to motivate future studies of the strength behaviours of silty hydrate reservoirs.

Velocity-porosity relationships in hydrate-bearing sediments measured from pressure cores, Shenhu Area, South China Sea

Evaluating velocity-porosity relationships of hydrate-bearing marine sediments is essential for characterizing natural gas hydrates below seafloor as either a potential energy resource or geohazards risks. Four sites had cored using pressure and non-pressure methods during the gas hydrates drilling project (GMGS4) expedition at Shenhu Area, north slope of the South China Sea. Sediments were cored above, below, and through the gas-hydrate-bearing zone guided with logging-while-drilling analysis results. Gamma density and <i>P</i>-wave velocity were measured in each pressure core before subsampling. Methane hydrates volumes in total 62 samples were calculated from the moles of excess methane collected during depressurization experiments. The concentration of methane hydrates ranged from 0.3% to 32.3%. The concentrations of pore fluid (25.44% to 68.82%) and sediments (23.63% to 54.28%) were calculated from the gamma density. The regression models of <i>P</i>-wave velocity were derived and compared with a global empirical equation derived from shallow, unconsolidated sediments data. The results were close to the global trend when the fluid concentration is larger than the critical porosity. It is concluded that the dominant factor of <i>P</i>-wave velocity in hydrate-bearing marine sediments is the presence of the hydrate. Methane hydrates can reduce the fluid concentration by discharging the pore fluid and occupying the original pore space of sediments after its formation.

Characterization of gas hydrate systems on the Hikurangi margin (New Zealand) through geostatistical seismic and petrophysical inversion

Quantitative characterization of gas hydrate systems on continental margins from seismic data is challenging, especially in regions where no well logs are available. However, probabilistic seismic inversion provides an effective means for constraining the physical properties of subsurface strata in such settings and analyzing the variability related to the results. We have applied a workflow for the characterization of two deepwater gas hydrate reservoirs east of New Zealand, where high concentrations of gas hydrate have been inferred previously. We estimate porosity and gas hydrate saturation in the reservoirs from multichannel seismic data through a two-step procedure based on geostatistical seismic and Bayesian petrophysical inversion built on a rock-physics model for gas hydrate-bearing marine sediments. We find that the two reservoirs together host between [Formula: see text] and [Formula: see text] of gas hydrate, with the best estimate at [Formula: see text]. This estimate provides a first-order assessment for further gas hydrate evaluations in the region. The two-step statistically based seismic inversion method is an effective approach for characterizing gas hydrate systems from long-offset seismic reflection data.

Strength and Deformation Behaviors of Methane Hydrate-Bearing Marine Sediments in the South China Sea during Depressurization

Strength and Deformation Behaviors of Methane Hydrate-Bearing Marine Sediments in the South China Sea during Depressurization

Study on the Influence of Sand Production on Seepage Capacity in Natural Gas Hydrate Reservoirs

Sand production has become a common phenomenon in the exploitation of unconsolidated natural gas hydrate reservoirs, which will hinder the long-term production of natural gas hydrate reservoirs. However, there are few literatures reported on the influences in reservoir physical properties such as permeability and porosity, and production laws caused by sand production. This paper provides a numerical model, coupled with reservoir sand-gas-water multiphase flow processes, which is capable to simulate the process of sand production in natural gas hydrate reservoirs. The simulation results indicate that sand settlement is mainly concentrated near the wellbore due to the high concentration of migrated sand. The decrease in reservoir porosity and permeability caused by sand settlement has a significant impact on production. The impact of sand production on reservoir fluid fluidity shows that fluid flow is inhibited near the wellbore, while fluid flow performance increases far away from the wellbore. The numerical model and analysis presented here could provide useful insight into changes in reservoir physical properties and production laws caused by sand production in the natural gas hydrate-bearing marine sediments using depressurization method.

Authigenic Greigite as an Indicator of Methane Diffusion in Gas Hydrate-Bearing Sediments of the Hikurangi Margin, New Zealand

Authigenic ferrimagnetic iron sulfides, essentially greigite (Fe3S4), are commonly found in gas hydrate-bearing marine sediments of active accretionary prisms. Greigite is a by-product, either intracellular or extracellular, of microbial activity, and therefore provides good indication of microbial processes which are closely related to the occurrence of gas hydrate. A high-resolution rock magnetic study was conducted at Site U1518 of International Ocean Discovery Program Expedition 375, located in the frontal accretionary wedge of the Hikurangi Margin, offshore New Zealand. Samples were collected throughout the entire recovered stratigraphic sequence, from the surface to ∼492 m below seafloor (mbsf) which includes the Pāpaku fault zone. This study aims to document the rock magnetic properties and the composition of the magnetic mineral assemblage at Site U1518. Based on downhole magnetic coercivity variations, the studied interval is divided into five consecutive zones. Most of the samples have high remanent coercivity (above 50 mT) and first-order reversal curves (FORC) diagrams typical of single-domain greigite. The top of the hanging wall has intervals that display a lower remanent coercivity, similar to lower coercivities measured on samples from the fault zone and footwall. The widespread distribution of greigite at Site U1518 is linked to methane diffusion and methane hydrate which is mainly disseminated within sediments. In three footwall gas hydrate-bearing intervals, investigated at higher resolution, an improved magnetic signal, especially a stronger FORC signature, is likely related to enhanced microbial activity which favors the formation and preservation of greigite. Our findings at the Hikurangi Margin show a close linkage between greigite, methane hydrate and microbial activity.

Formation and production characteristics of methane hydrates from marine sediments in a core holder

The abundant methane hydrates stored in marine sediments have been widely evaluated as a potential energy source. Understanding the gas and water production characteristics of methane hydrate-bearing marine sediments is critical for hydrates commercial exploitation. In this study, confining pressure was applied to simulate a sub-seafloor environment. Methane was repeatedly injected into cores to remold hydrate-bearing marine sediments with different hydrate saturation. The hydrate saturation increased from 10.6% to 21.6% as the water content increased from 8.9% to 22.2%. The results indicate that the higher the core water content, the greater the hydrate saturation and the longer the hydrate dissociation time. The gas production characteristics of hydrate-bearing sediments were severely affected by water and hydrates in pores. The results indicated that some hydrates and free gas were easily trapped by the surrounding soil, meaning that the hydrates were isolated and disconnected with the pore channels under confining pressure during depressurization. Thus, a second depressurization was conducted to achieve further gas production. For cores with different water contents, their water conversion percentage is approximately 20%. When the water content exceeded 16.7%, the water production was observed. The results of this study are meaningful for further related research and field production of marine hydrates.

Numerical analysis of sand production during natural gas extraction from unconsolidated hydrate-bearing sediments

Sand production, which poses a great challenge for sustainable methane gas production, is commonly encountered during natural gas extraction from unconsolidated hydrate-bearing sediments. However, studies on the subject are still limited. This paper provides a numerical model, coupled with thermo-hydro-mechanical (THM) processes, which is capable to characterize the process of sand production in gas hydrate-bearing sediments. The numerical simulator is developed by introducing the newly developed particle migration module into the existing framework of the THM coupled simulator HydrateBiot. The dynamic of sand production was investigated numerically, and the influence of various parameters on sand, gas and water production was studied. The simulation results show that particle detachment and migration mainly occurs in the vicinity of wellbore, while lowering the depressurization rate can mitigate sand production to some extent without significantly inhibiting the gas production. The using of the sand control device can effectively inhibit sand production, but its blockage will cause a significant drop in gas productivity. Sensitivity analyses of the critical fluidization velocity and solid-liquid velocity show that the increase of solid particle concentration will significantly affect the fluidity of water, thus affecting the depressurization process and gas productivity. It is also found that the massive particle detachment and migration may cause the appearance of clogging area in the vicinity of the wellbore and severely restrict the sustainable gas production. The numerical model and analysis presented here could provide useful insight into sand detachment, migration and production processes induced by methane gas production using depressurization method in the hydrate-bearing marine sediments.

Dielectric behaviors of marine sediments for reliable estimation of gas hydrate saturation based on numerical simulation

The accurate evaluation of hydrate saturation, in marine sediments, not only can provide references for hydrate resources assessment but also help to monitor hydrate decomposition that might cause disasters. The dielectric properties can be employed to efficiently estimate hydrate saturation. However, the correlations between dielectric properties and hydrate saturation are still poorly understood. To obtain such a reliable relationship, we first adopt a numerical simulation method based on the three-dimensional finite difference model and digital core technology to calculate the dielectric properties of hydrate-bearing marine sediments. After testing and validating the numerical method by comparing the simulation results with published experimental data, we further apply the simulation technique to calculating the dielectric properties of hydrate-bearing sediments with varying porosities, hydrate saturations and micro-distributions in a broaden frequency. It is found that the dependence of dielectric behaviors on hydrate saturation is significantly affected by hydrate micro-distributions, especially in lower frequency range. We also find that relative permittivity at the frequency of 1 GHz decreases exponentially with increasing hydrate saturation, and the two coefficients that determine the exponential correlation are both dependent on the porosity of the sediments. Using the established relationships, the hydrate saturation in the Mount Elbert Well is successfully estimated from the measured dielectric data. The study provides a reliable tool for the accurate estimation of hydrate saturation in marine sediments.

Strength Behaviors of Remolded Hydrate-Bearing Marine Sediments in Different Drilling Depths of the South China Sea

The mechanical behaviors of hydrate-bearing marine sediments (HBMS) drilled from the seafloor need to be understood in order to safely exploit natural gas from marine hydrate reservoirs. In this study, hydrates were prepared using ice powder and CH4 gas, and HBMS from the Shenhu area in the South China Sea were remolded using a mixed sample preparation method. A series of triaxial tests were conducted on the remolded HBMS to investigate the effects of soil particle gradation and the existence of hydrate on the mechanical properties of hydrate reservoirs. The results show that the stiffness and failure strength of HBMS decrease along with the decrease of mean particle size and soil aggregate morphology change at different drilling depths, and the reduction of failure strength is more than 20% when the drilling depth drops by 30 m. A better particle gradation of marine sediments may boost the stiffness and failure strength of HBMS. In addition, the existence of hydrate plays an important role in the strength behaviors of HBMS. The reduction of failure strength of HBMS with 30% initial hydrate saturation is more than 35% after complete hydrate dissociation.

Methane source linked to gas hydrate system at hydrate drilling areas of the South China Sea: Porewater geochemistry and numerical model constraints

Methane of various origins is widespread in the sediments of continental margins, representing a key component of the carbon cycle in the ocean. Its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column. However, this type of methane is difficult to track due to complex reactions occurred during its long distance migration. Porewater geochemical analyses of two ∼7 m piston cores (D17-15 and W01-16) combined with a reaction-transport model were applied to quantify methane-related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of Shenhu and Dongsha in the South China Sea (SCS). The model results revealed that anaerobic oxidation of methane (AOM) is the predominant biogeochemical process in both cores, consuming approximately 85% and 74% of sulfate in the modeled domain, respectively. The crossplot of dissolve inorganic carbon (DIC) accumulation versus sulfate depletion of the system suggests that methane consumed during the AOM is mainly originated from external methane sources rather than local methanogenesis. Using a δ13CDIC mass balance calculation together with model-derived fractions of different DIC sources, we estimated that the δ13C value of the external methane in core D17-15 is −66.6‰, which is in accordance with the previous reported δ13C values of hydrate-bound methane in the area. These results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the SCS is mainly originated from local gas hydrate reservoir. The proposed approach has the potential to be used to differentiate the sources of methane, which will provide constraints on dynamics of methane in gas hydrate-bearing marine sediments.

Triaxial experiments on the mechanical properties of hydrate-bearing marine sediments of South China Sea

In this paper, a series of experiments was conducted on remoulded hydrate-bearing marine sediments from a gas hydrate core drilling site located in the Pearl River Estuary of the South China Sea. The composition and particle size distribution of marine sediments were analysed, and triaxial loading experiments were performed to study the mechanical properties of hydrate-bearing marine sediments. The strength properties of hydrate-bearing marine sediments were compared with those of hydrate-bearing kaolin clay, ice-marine sediment mixtures and hydrate-dissociated marine sediments under the same porosity (40%), temperature (8 °C), confining pressures (2, 3, and 4 MPa), and strain rate (1%/min) conditions. Several important results were obtained: The composition and particle size distribution analysis indicated the marine sediment and kaolin clay are both sandy clays, and the stress-strain behaviours and strength properties of hydrate-bearing marine sediments are similar to those of hydrate-bearing kaolin clay. Under different confining pressures, the failure strengths of all ice-marine sediment mixtures were lower than those of methane hydrate-bearing sediments, confirming that hydrate particles enhance the cementation of sediment particles. Hydrate dissociation leads to strength reduction of gas hydrate-bearing marine sediments, as was demonstrated in our previous research of hydrate-bearing kaolin clay. Through the analysis of Mohr circles and failure envelopes of three types of mixtures under different confining pressures, it can be inferred that the cohesive strength played an important role in the failure strength of hydrate-bearing marine sediments following hydrate dissociation.

Rock magnetic characterization of ferrimagnetic iron sulfides in gas hydrate-bearing marine sediments at Site C0008, Nankai Trough, Pacific Ocean, off-coast Japan

A high-resolution rock magnetic study was carried out in Integrated Ocean Drilling Program (IODP) Expedition 316 Hole C0008A located in the Megasplay Fault Zone of the Nankai Trough, SW offshore Japan, in order to document changes in magnetic properties throughout gas hydrate-bearing horizons. A total of 169 Pleistocene discrete samples were collected from ~110 to 153 m core depth below sea floor (CSF), and their magnetic minerals concentration, grain size, composition, and rock magnetic parameters were estimated. Results showed the presence of iron oxides ((titano)-magnetite), iron sulfides (greigite and pyrrhotite), and their mixture, among which single-domain greigite is the most major magnetic mineral present in the samples. Two horizons containing ferrimagnetic iron sulfides (114.5–127.5 and 129.5–150 m CSF) covering almost the entire studied interval were identified, both associated with slight local pore water anomalies, suggesting occurrence of gas hydrates and anoxic conditions. These results are different from the neighboring Hole C0008C (215 m away from Hole C0008A) where four pore water anomalies and six iron sulfide-rich intervals were identified for the same time slice. Comparison of the lithology, physical properties, and geochemical data of the two boreholes at Site C0008 suggests that a combination of processes (e.g., availability of reactive iron, microbial activity) is responsible for such laterally varying distribution of the ferrimagnetic iron sulfides.

Methane Flux and Authigenic Carbonate in Shallow Sediments Overlying Methane Hydrate Bearing Strata in Alaminos Canyon, Gulf of Mexico

In June 2007 sediment cores were collected in Alaminos Canyon, Gulf of Mexico across a series of seismic data profiles indicating rapid transitions between the presence of methane hydrates and vertical gas flux. Vertical profiles of dissolved sulfate, chloride, calcium, magnesium, and dissolved inorganic carbon (DIC) concentrations in porewaters, headspace methane, and solid phase carbonate concentrations were measured at each core location to investigate the cycling of methane-derived carbon in shallow sediments overlying the hydrate bearing strata. When integrated with stable carbon isotope ratios of DIC, geochemical results suggest a significant fraction of the methane flux at this site is cycled into the inorganic carbon pool. The incorporation of methane-derived carbon into dissolved and solid inorganic carbon phases represents a significant sink in local carbon cycling and plays a role in regulating the flux of methane to the overlying water column at Alaminos Canyon. Targeted, high-resolution geochemical characterization of the biogeochemical cycling of methane-derived carbon in shallow sediments overlying hydrate bearing strata like those in Alaminos Canyon is critical to quantifying methane flux and estimating methane hydrate distributions in gas hydrate bearing marine sediments.

Stress-dependent and strength properties of gas hydrate-bearing marine sediments from the Ulleung Basin, East Sea, Korea

This paper presents a comprehensive laboratory experimental characterization of the host sediment from natural hydrate-bearing sediments recovered during the Ulleung Basin Gas Hydrate Expedition 2 (UBGH2) in 2010. The basic geotechnical properties are characterized and compared with previously reported data. The mineralogy and microstructure analyses confirm that the sediments mainly consist of illite, kaolinite, chlorite, calcite, and montmorillonite, with a high concentration of microfossils (diatoms). The presence of microfossils may affect many of the geotechnical properties, including plasticity, activity, specific surface, and grain size. The tested specimens have a relatively high initial void ratio, with a high compression index (e.g., Cc ranging from 0.6 to 1.0) and swelling index (e.g., Cs ranging from 0.1 to 0.25). The shear wave velocities measured during loading indicate a high sensitivity to the stress state, which demonstrates the critical role of effective stress in controlling the stiffness of the sediments. The peak friction angles are determined to be 17.6°–24.6° for clay and silt sediments, and 27.1° for high sand fraction specimens. The stress dependent and strength parameters obtained in this study may be very useful for performing stability analyses during hydrate dissociation and production.

Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations

Molecular dynamic simulations have been carried out to investigate the behavior of methane hydration in Na-smectite interlayers with different layer-charge distributions and water contents under certain pressure–temperature (P–T) conditions, which is analogous to the methane hydrate-bearing marine sediments. It was found that sufficient interlayer water is necessary for coordinating with methane and forming hydrate-like structures. Methane molecules are solvated by nearly 12–13 water molecules and coordinated with six oxygen atoms from the clay surface in the interlayer of nontronite as well as in montmorillonite. The mobility of the interlayer water of smectite, which is determined by the layer-charge amount and distribution of smectite, also influences the stability of hydrate methane complexes. The tetrahedral negative charge site is closer to the surface than the octahedral charge site and is more effective in confining water than methane water molecules.