Deposition processes of gas hydrate-bearing sediments in the inter-canyon area of Shenhu Area in the northern South China Sea

The Shenhu Submarine Canyon Group on the northern slope of the South China Sea consists of 17 slope-confined canyons,providing a good example for investigating their hosting sediments.Three drilling sites,including W07,W18,and W19,have proven the occurrence of gas hydrate reservoirs in the inter-canyon area between canyons C11 and C12.Whereas,variations of the geomorphology and seismic facies analyzed by high-resolution 3D seismic data indicate that the gas hydrate-bearing sediments may form in different sedimentary processes.In the upper segment,a set of small-scale channels with obvious topographic lows can be identified,revealing fine-grained turbidites supplied from the shelf region during a very short-term sea-level lowstand.In the middle part,gas hydrate units at Site W07 showing mounded or undulation external configuration are interpreted as sliding sedimentary features,and those features caused by gravity destabilization were the main formative mechanism of gas hydrate-bearing sediments that were sourced from the upper segments.In contrast,for the canyon transition zone of lower segments between C11-C12 inter-canyon and C12 intra-canyon areas,where W18 and W19 sites are located,the gas hydratebearing sediments are deposited in the channelized feature in the middle to lower segment and slide erosive surface.Gas hydrate-bearing sediments of the lower segment were migrated through channelized features interconnecting with the middle to lower slope by gravity-driven flows.The majority of deposits tended to be furtherly moved by lateral migration via erosive surface created by sediment failed to intra-canyon area.The conclusion of this study may help better understand the interaction between the formation mechanism of gas hydrate-bearing sediments and the geomorphologic effects of inter-canyon areas.

Mechanical properties of gas hydrate-bearing sediments during hydrate dissociation

The changes in the mechanical properties of gas hydrate-bearing sediments(GHBS) induced by gas hydrate(GH) dissociation are essential to the evaluation of GH exploration and stratum instabilities. Previous studies present substantial mechanical data and constitutive models for GHBS at a given GH saturation under the non-dissociated condition. In this paper, GHBS was formed by the gas saturated method, GH was dissociated by depressurization until the GH saturation reached different dissociation degrees. The stress–strain curves were measured using triaxial tests at a same pore gas pressure and different confining pressures. The results show that the shear strength decreases progressively by 30%–90% of the initial value with GH dissociation, and the modulus decreases by 50% –75%. Simplified relationships for the modulus, cohesion, and internal friction angle with GH dissociated saturation were presented.

Theoretical analysis and numerical simulation of acoustic waves in gas hydrate-bearing sediments

Based on Carcione-Leclaire model,the time-splitting high-order staggered-grid finite-difference algorithm is proposed and constructed for understanding wave propagation mechanisms in gas hydrate-bearing sediments.Three compressional waves and two shear waves,as well as their energy distributions are investigated in detail.In particular,the influences of the friction coefficient between solid grains and gas hydrate and the viscosity of pore fluid on wave propagation are analyzed.The results show that our proposed numerical simulation algorithm proposed in this paper can effectively solve the problem of stiffness in the velocity-stress equations and suppress the grid dispersion,resulting in higher accuracy compared with the result of the Fourier pseudospectral method used by Carcione.The excitation mechanisms of the five wave modes are clearly revealed by the results of simulations.Besides,it is pointed that,the wave diffusion of the second kind of compressional and shear waves is influenced by the friction coefficient between solid grains and gas hydrate,while the diffusion of the third compressional wave is controlled by the fluid viscosity.Finally,two fluid-solid(gas-hydrate formation)models are constructed to study the mode conversion of various waves.The results show that the reflection,transmission,and transformation of various waves occur on the interface,forming a very complicated wave field,and the energy distribution of various converted waves in different phases is different.It is demonstrated from our studies that,the unconventional waves,such as the second and third kinds of compressional waves may be converted into conventional waves on an interface.These propagation mechanisms provide a concrete wave attenuation explanation in inhomogeneous media.

Experimental Study on Mechanical Properties of Gas Hydrate-Bearing Sediments Using Kaolin Clay

A triaxial system is designed with a temperature range from -20 ℃ to 25℃ and a pressure range from 0 MPa to 30 MPa in order to improve the understanding of the mechanical properties of gas hydrate-bearing sediments. The mechanical properties of synthetic gas hydrate-bearing sediments （gas hydrate-kaolin clay mixture） were measured by using current experimental apparatus. The results indicate that： （1） the failure strength of gas hydrate-bearing sediments strongly depends on the temperature. The sediment＇s strength increases with the decreases of temperature. （2） The maximum deviator stress increases linearly with the confining pressure at a low-pressure stage. However, it fluctuates at a high-pressure stage. （3） Maximum deviator stress increases with increasing strain rate, whereas the strain-stress curve has no tremendous change until the axial strain reaches approximately 0.5%. （4） The internal friction angles of gas hydrate-bearing sediments are not sensitive to kaolin volume ratio. The cohesion shows a high kaolin volume ratio dependency.

Saturation Estimation with Complex Electrical Conductivity for Hydrate-Bearing Clayey Sediments:An Experimental Study

Clays have considerable influence on the electrical properties of hydrate-bearing sediments.It is desirable to understand the electrical properties of hydrate-bearing clayey sediments and to build hydrate saturation(S_(h))models for reservoir evaluation and monitoring.The electrical properties of tetrahydrofuran-hydrate-bearing sediments with montmorillonite are characterized by complex conductivity at frequencies from 0.01 Hz to 1 kHz.The effects of clay and Sh on the complex conductivity were analyzed.A decrease and increase in electrical conductance result from the clay-swelling-induced blockage and ion migration in the electrical double layer(EDL),respectively.The quadrature conductivity increases with the clay content up to 10%because of the increased surface site density of counterions in EDL.Both the in-phase conductivity and quadrature conductivity decrease consistently with increasing Sh from 0.50 to 0.90.Three sets of models for Sh evaluation were developed.The model based on the Simandoux equation outperforms Archie’s formula,with a root-mean-square error(E_(RMS))of 1.8%and 3.9%,respectively,highlighting the clay effects on the in-phase conductivity.The fre-quency effect correlations based on in-phase and quadrature conductivities exhibit inferior performance(E_(RMS)=11.6%and 13.2%,re-spectively)due to the challenge of choosing an appropriate pair of frequencies and intrinsic uncertainties from two measurements.The second-order Cole-Cole formula can be used to fit the complex-conductivity spectra.One pair of inverted Cole-Cole parameters,i.e.,characteristic time and chargeability,is employed to predict S_(h) with an E_(RMS) of 5.05%and 9.05%,respectively.

Mechanical Properties of Methane Hydrate-Bearing Interlayered Sediments

The complex distribution of gas hydrate in sediments makes understanding the mechanical properties of hydrate-bearing sediments a challenging task.The mechanical behaviors of hydrate-bearing interlayered sediments are still poorly known.A series of triaxial shearing tests were conducted to investigate the strength parameters and deformation properties of methane hydrate-bearing interlayered sediments at the effective pressure of 1 MPa.The results indicate that the stress-strain curves of hydrate-bearing interlayered sediments are significantly different from that of hydrate-bearing sediments.The peak strength,Young's modulus,initial yielding modulus,and failure mode are deeply affected by the methane hydrate distribution.The failure behaviors and mechanism of strain softening and hardening patterns of the interlayered specimens are more complicated than those of the integrated specimens.This study compares the different mechanical behaviors between integrated and interlayered specimens containing gas hydrate,which can serve as a reference for the prediction and analysis of the deformation behaviors of natural gas hydrate reservoirs.

Experimental study on mechanical properties of methane-hydrate-bearing sediments

Mechanical properties of methane hydrate- bearing-sediments （MHBS） are basic parameters for safety analysis of hydrate exploration and exploitation. Young＇s modulus, cohesion, and internal friction angle of hydrate- bearing sediments synthesized in laboratory, are investigated using tri-axial tests. Stress-strain curves and strength parameters are obtained and discussed for different compositions and different hydrate saturation, followed by empirical expressions related to the cohesion, internal friction angle, and modulus of MHBS. Almost all tested MHBS samples exhibit plastic failure. With the increase of total saturation of ice and methane hydrate （MH）, the specimens＇ internal friction angle decreases while the cohesion increases.

Drucker-Prager Elasto-Plastic Constitutive Model for Methane Hydrate-Bearing Sediment

A constitutive model for methane hydrate-bearing sediment(MHBS)is essential for the analysis of mechanical response of MHBS to the change of hydrate saturation caused by gas extraction. A new elasto-plastic constitutive model is built in order to simulate the mechanical behavior of MHBS in this paper. This model represents more significant mechanical properties of MHBS such as bonding, higher stiffness, softening and stress-strain nonlinear relationship. The bonding behavior can be described by use of a parameter related to mechanical hydrate saturation. Higher stiffness can be modeled by the introduction of hydrate saturation into traditional expression of soil stiffness. Softening can be controlled by a function describing the relationship between cohesion and bonding structure factor. Dilatancy can be estimated by establishing the relationship between the lateral strain and axial strain. Meanwhile, the hypothesis of isotropic expanding is applied to the calculation of the volumetric strain. The stress-strain curves under different hydrate saturation conditions predicted by the proposed model are in good agreement with the test data. All the coefficients can be easily obtained by the triaxial test of MHBS.

An improved specimen preparation method for marine shallow gasbearing sand sediments and its validations

Instabilities of shallow gas-charged seabed are potential geological hazards in ocean engineering.In practice,the conventional field sampling techniques failed to obtain undisturbed gas-bearing sediments from the seabed for laboratory mechanical testing because of sensitive gas exsolution and escape from sediments.However,preparation of representative remoulded gas-charged specimens is a challenging issue,because it is rather difficult to quantitatively control the gas content and obtain uniform distribution of gas bubbles within the specimen.Given the above problems,this work proposes a reliable approach to reconstitute the high-saturation specimen of gas-charged sediments in the laboratory by an improved multifunction integrated triaxial apparatus(MITA).This apparatus is developed based on an advanced stress path triaxial system by introducing a temperature-controlled system and a wavemonitoring system.The temperature-controlled system is used to accurately mimic the in situ environments of sediments in the seabed.The wave-monitoring system is utilized to identify exsolution point of free gas and examine the disturbance of gas to specimens during gas exsolution.The detailed procedure of gassy specimen preparation is introduced.Then,the quality of prepared specimens using our improved apparatus is validated by the high-resolution micro-X-ray computed tomography(mCT)scanning test,from which bubble occurrence and size distribution within the gassy sand specimen can be obtained;and preliminary mechanical tests on gassy sand specimens with various initial saturation degrees are performed.The proposed specimen preparation procedure succeeds in proving the postulated occurrence state of gas bubbles in coarse-grained sediments and accurately controlling the gas content.

Investigation of the hydrate formation process in fine sediments by a binary CO2/N2 gas mixture

To obtain the fundamental data of CO2/N2 gas mixture hydrate formation kinetics and CO2 separation and sequestration mechanisms,the gas hydrate formation process by a binary CO2/N2 gas mixture(50:50)in fine sediments(150–250μm)was investigated in a semibatch vessel at variable temperatures(273,275,and 277 K)and pressures(5.8–7.8 MPa).During the gas hydrate reaction process,the changes in the gaseous phase composition were determined by gas chromatography.The results indicate that the gas hydrate formation process of the binary CO2/N2 gas mixture in fine sediments can be reduced to two stages.Firstly,the dissolved gas containing a large amount of CO2 formed gas hydrates,and then gaseous N2 participated in the gas hydrate formation.In the second stage,all the dissolved gas was consumed.Thus,both gaseous CO2 and N2 diffused into sediment.The first stage in different experiments lasted for 5–15 h,and>60%of the gas was consumed in this period.The gas consumption rate was greater in the first stage than in the second stage.After the completion of gas hydrate formation,the CO2 content in the gas hydrate was more than that in the gas phase.This indicates that CO2 formed hydrate easily than N2 in the binary mixture.Higher operating pressures and lower temperatures increased the gas consumption rate of the binary gas mixture in gas hydrate formation.

Fluid-solid coupling model for studying wellbore instability in drilling of gas hydrate bearing sediments

As the oil or gas exploration and development activities in deep and ultra- deep waters become more and more, encountering gas hydrate bearing sediments （HBS） is almost inevitable. The variation in temperature and pressure can destabilize gas hydrate in nearby formation around the borehole, which may reduce the strength of the formation and result in wellbore instability. A non-isothermal, transient, two-phase, and fluid-solid coupling mathematical model is proposed to simulate the complex stability performance of a wellbore drilled in HBS. In the model, the phase transition of hydrate dissociation, the heat exchange between drilling fluid and formation, the change of mechanical and petrophysical properties, the gas-water two-phase seepage, and its interaction with rock deformation are considered. A finite element simulator is developed, and the impact of drilling mud on wellbore instability in HBS is simulated. Results indicate that the re- duction in pressure and the increase in temperature of the drilling fluid can accelerate hydrate decomposition and lead to mechanical properties getting worse tremendously. The cohesion decreases by 25% when the hydrate totally dissociates in HBS. This easily causes the wellbore instability accordingly. In the first two hours after the formation is drilled, the regions of hydrate dissociation and wellbore instability extend quickly. Then, with the soaking time of drilling fluid increasing, the regions enlarge little. Choosing the low temperature drilling fluid and increasing the drilling mud pressure appropriately can benefit the wellbore stability of HBS. The established model turns out to be an efficient tool in numerical studies of the hydrate dissociation behavior and wellbore stability of HBS.

An experimental study on microscopic characteristics of gas-bearing sediments under different gas reservoir pressures

Gas-bearing sediments are widely distributed in five continents all over the world.Most of the gases exist in the soil skeleton in the form of discrete large bubbles.The existence of gas-phase may increase or decrease the strength of the soil skeleton.So far,bubbles’structural morphology and evolution characteristics in soil skeleton lack research,and the influence of different gas reservoir pressures on bubbles are still unclear.The micro characteristics of bubbles in the same sediment sample were studied using an industrial CT scanning test system to solve these problems.Using the image processing software,the micro variation characteristics of gas-bearing sediments in gas reservoir pressure change are obtained.The results show that the number and volume of bubbles in different equivalent radius ranges will change regularly under different gas reservoir pressure.With the increase of gas reservoir pressure,the number and volume of tiny bubbles decrease.In contrast,the number and volume of large bubbles increase,and the gas content in different positions increases and occupies a dominant position,driving the reduction of pore water and soil skeleton movement.

Lipid Distribution in Marine Sediments from the Northern South China Sea and Association with Gas Hydrate

The distributions of lipids in surface and subsurface sediments from the northern South China Sea were determined. The n-alkanes were in bimodal distribution that is characterized by a centre at n-C16 –n-C20 with maximum at C18（or C19） and n-C27 –n-C31 as well as at C29（or C31）. The short-chain alkanes suffered from significant losses due to their slow deposition in the water column, and their presence with a slight even carbon predominance in shallow seafloor sediments was ascribed mainly to the direct input from the benthos. The long-chain alkanes with odd predominance indicate transportion of terrigenous organic matter. Immature hopanoid biomarkers reflect the intense microbial activity for bacteria–derived organic matter and the gradual increase of maturity with burial depth. Abundant n-fatty acid methyl esters（n-FAMEs） that are in distributions coincident with fatty acids were detected in all samples. We proposed that the observed FAMEs originated from the methyl esterification of fatty acids; methanol production by methanotrophs and methanogenic archaea related to the anaerobic oxidation of methane, and sulfate reduction provided an O–methyl donor for methylation of fatty acids. The CH4 released from hydrate dissociation at oxygen isotope stage II of Cores ZD3 and ZS5, which had been confirmed by the occurrence of negative δ13C excursion and spherical pyrite aggregates, could have accelerated the above process and thus maximized the relative content of FAMEs at ZD3-2（400–420 cm depth） and ZS5-2（241–291 cm depth）.

Evidence for pore-filling gas hydrates in the sediments through morphology observation

To provide an evidence of natural gas hydrate occurrence state,a series of experiments on multiple growth and dissociation of 90.0%methane/10.0%propane hydrates at 1.3 MPa and 270.15 K were carried out in two sediments for morphology observation via a visible jacketed-reactor.The gas hydrate crystals were observed to form and grow on the surface of sediments at the initial growth.During the thermal decomposition,gas and liquid products had an unceasingly impact on the sediments,then gas/liquid–solid migration occurred,and a large number of cavitation appeared.In the later growth and dissociation experiments,the gas hydrate particles were in suspension or supporting states in the interstitial pore space between the sediment particles,indicating that the gas hydrate displayed a pore-filling characteristics.Through analyzing the distribution of gas hydrates and bubbles,it was found that the amount of gas hydrates distributed in the sediments was improved with multiple growth-dissociation cycle proceedings.Gas migration enhanced the sediment movement,which led to the appearance of the increasing quantity of gas bubbles in the sediments during cycles.Salts affected the growth of the gas hydrates and the migration of sediment grains,which also restricted the accumulation of gas bubbles in the sediments.According to the Raman analysis,the results showed that sII hydrates were formed for CH4 and C3H8 gas mixtures in different sediments and solutions with hydration number of 5.84–6.53.The Salt restricted the access of gas into the hydrate cages.

Determination of butyltin compounds in sediments by gas chromatography with flame photometric detector

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Research on Pressure Tight Sampling Technique of Deep-Sea Shallow Sediment—A New Approach to Gas Hydrate Investigation

Analyzed and calculated are pressure changes and body deformation of the sample inside of the corer in the process of sampling of deep-sea shallow sediment with a non-piston corer for gas hydrate investigation, Two conclusions are drawn： （1） the stress increments associated with the corer through the sampling process do not affect the stabilization of the gas hydrate; （2） the body deformation of the sample is serious and the ＂incremental filling ratio＂ （IFR） is less than unit, For taking samples with in-situ pressure and structure, combining with the design theories of the pressure tight corer, we have designed a kind of piston corer, named the gas hydrate pressure tight piston corer, Several tests on the sea have been conducted. Test results indicate that the piston corer has a good ability of taking sediment samples on the seafloor and maintaining their original in-situ pressure, meeting the requirement of exploration of gas hydrate in deep-sea shallow sediment layers.

Sediment permeability change on natural gas hydrate dissociation induced by depressurization

The permeability of a natural gas hydrate reservoir is a critical parameter associated with gas hydrate production.Upon producing gas from a hydrate reservoir via depressurization,the permeability of sediments changes in two ways with hydrate dissociation,increasing with more pore space released from hydrate and decreasing due to pore compression by stronger effective stress related to depressurization.In order to study the evolution of sediment permeability during the production process with the depressurization method,an improved pore network model(PNM)method is developed to establish the permeability change model.In this model,permeability change induced by hydrate dissociation is investigated under hydrate occurrence morphology of pore filling and grain coating.The results obtained show that hydrate occurrence in sediment pore is with significant influence on permeability change.Within a reasonable degree of pore compression in field trial,the effect of pore space release on the reservoir permeability is greater than that of pore compression.The permeability of hydrate containing sediments keeps increasing in the course of gas production,no matter with what hydrate occurrence in sediment pore.

Identification of functionally active aerobic methanotrophs and their methane oxidation potential in sediments from the Shenhu Area,South China Sea

Large amounts of gas hydrate are distributed in the northern slope of the South China Sea,which is a potential threat of methane leakage.Aerobic methane oxidation by methanotrophs,significant methane biotransformation that occurs in sediment surface and water column,can effectively reduce atmospheric emission of hydrate-decomposed methane.To identify active aerobic methanotrophs and their methane oxidation potential in sediments from the Shenhu Area in the South China Sea,multi-day enrichment incubations were conducted in this study.The results show that the methane oxidation rates in the studied sediments were 2.03‒2.36μmol/gdw/d,which were higher than those obtained by sediment incubations from other areas in marine ecosystems.Thus the authors suspect that the methane oxidation potential of methanotrophs was relatively higher in sediments from the Shenhu Area.After the incubations family Methylococcaea(type I methanotrophs)mainly consisted of genus Methylobacter and Methylococcaea_Other were predominant with an increased proportion of 70.3%,whereas Methylocaldum decreased simultaneously in the incubated sediments.Collectively,this study may help to gain a better understanding of the methane biotransformation in the Shenhu Area.

Laminae characteristics of gas-bearing shale fine-grained sediment of the Silurian Longmaxi Formation of Well Wuxi 2 in Sichuan Basin,SW China

Based on various test data, the composition, texture, structure and lamina types of gas-bearing shale were determined based on Well Wuxi 2 of the Silurian Longmaxi Formation in the Sichuan Basin. Four types of lamina, namely organic-rich lamina, organic-bearing lamina, clay lamina and silty lamina, are developed in the Longmaxi Formation of Well Wuxi 2, and they form 2 kinds of lamina set and 5 kinds of beds. Because of increasing supply of terrigenous clasts and enhancing hydrodynamics and associated oxygen levels, the contents of TOC and brittle mineral reduce and content of clay mineral increases gradually as the depth becomes shallow. Organic-rich lamina, organic-rich + organic-bearing lamina set and organic-rich bed dominate the small layers 1-3 of Member 1 of the Longmaxi Formation, suggesting anoxic and weak hydraulic depositional setting. Organic-rich lamina, along with organic-bearing lamina and silty lamina, appear in small layer 4, suggesting increased oxygenated and hydraulic level. Small layers 1-3 are the best interval and drilling target of shale gas exploration and development.

Simulation experiments on gas production from hydrate-bearing sediments

Experiments were made on 58 sediment samples from four sites(1244,1245,1250 and 1251) of ODP204 at five temperature points(25,35,45,55 and 65℃) to simulate methane production from hydrate-bearing sediments.Simulation results from site 1244 show that the gas components consist mainly of methane and carbon dioxide,and heavy hydrocarbons more than C2+ cannot be detected.This site also gives results,similar to those from the other three,that the methane production is controlled by experimental temperatures,generally reaching the maximum gas yields per gram sediment or TOC under lower temperatures(25 and 35 ℃).In other words,the methane amount could be related to the buried depth of sediments,given the close relation between the depth and temperature.Sediments less than 1200 m below seafloor are inferred to still act as a biogenic gas producer to pour methane into the present hydrate zone,while sedimentary layers more than 1200 m below seafloor have become too biogenically exhausted to offer any biogas,but instead they produce thermogenic gas to give additional supply to the hydrate formation in the study area.