microbially induced cementation to control sand response to undrained shear current methods to improve the engineering properties of sands are diverse with respect to methodology treatment uniformity cost environmental impact site accessibility requirements etc all of these methods have benefits and drawbacks and there continues to be a need to explore new possibilities of soil improvement particularly as suitable land for development becomes more scarce this paper presents the results of a study in which natural microbial biological processes were used to engineer a cemented soil matrix within initially loose collapsible sand microbially induced calcite precipitation micp was achieved using the microorganism bacillus pasteurii an aerobic bacterium pervasive in natural soil deposits the microbes were introduced to the sand specimens in a liquid growth medium amended with urea and a dissolved calcium source subsequent cementation treatments were passed through the specimen to increase the cementation level of the sand particle matrix the results of both micp and

estimation of model parameters and properties for numerical simulation on geomechanical stability of gas hydrate production in the ulleung basin east sea korea abstract the process of hydrate dissociation and production induced by depressurization incorporates intricate hydraulic thermal and mechanical phenomena thus coupled thermal hydraulic mechanical t h m simulation is critically necessary to evaluate the geomechanical stability of hydrate production in hydrate bearing sediments hbs however methods of estimating the input model parameters and properties of the target reservoir in particular in unconsolidated marine sediments have received limited attention compared to studies on production simulators the t h m properties of the marine sediments change considerably with depth geological strata and soil type of each layer therefore it is important that representative layers and their corresponding t h m properties should be properly estimated to analyze the stability and productivity of methane gas recovery in the field this study provides a comprehensive estimation for the model parameters and properties of unconsolidated marine sediments based on vast data from field seismic surveys and laboratory experimental results with core samples investigates empirical correlations between model parameters and methane hydrate saturation and finally summarizes the estimated model parameters and properties which can possibly be applied to on going numerical research into stability assessment of the pilot gas hydrate gh production test which is soon to be performed in the ulleung basin

numerical simulations of sand migration during gas production in hydrate bearing sands interbedded with thin mud layers at site nghp 02 16 abstract the geomechanical behavior of gas hydrate bearing sands during gas production is complex involving changes in hydrate dependent mechanical characteristics and also redistribution of stresses carried by the hydrate when an interbedded mud layer is present the complex response of the hydrate bearing sediments is more pronounced because hydrate dissociation tends to occur preferentially in the sands adjacent to the hydrate free mud layer which can act as a heat source this leads to non uniform sediment deformation and potentially to shearing deformation along the sand mud layers in addition since the hydrate bearing sand permeability becomes far greater than the mud permeability with hydrate dissociation a greater hydraulic gradient may exist along the interface between the sand mud layers these two behaviors may contribute to sand migration and thus a thorough study on the effect of a thin interbedded mud layer on the extent of sand migration is necessary this paper presents numerical sand migration modeling of sand mud interbedded hydrate bearing sediments found at site nghp 02 16 one of 25 sites drilled as a part of national gas hydrate program expedition 02 nghp 02 in the offshore of india subjected to depressurization for a comparison and better understanding simulation of homogeneous hydrate bearing sediments is also conducted it is found that greater sand migration occurs when the interbedded mud layer is present because of greater shearing deformation and greater pressure gradients at sand mud interface

shear strength characteristics of an unsaturated expansive clay the shear strength characteristics of an expansive clay from china were studied by performing suction controlled direct shear tests on both natural and compacted specimens the tested soil was a silty clay with intermedi ate plasticity and medium expansion potential a modified direct shear apparatus with a newly developed water volume indicator was used for this laboratory study the experimental results clearly show that the dilatancy of the expansive clay increases with an increase in the applied suction for both the natural and compacted specimens matric suction contributes to the shear strength of the expansive clay via two different mechanisms the contribution of capillary force to interparticle normal stress and the effect of suction on soil dilatancy as a result of the second mechanism the con tribution of suction to peak shear strength for the clay is more significant than that to post failure shear strength par ticularly at a high suction range the contribution of suction to post failure shear strength for the natural specimen is basically consistent with that for the compacted specimen the higher peak shear strength and dilatancy for the natural specimen are related to the cementation effect of the iron and manganese oxides the contribution of suction to shear strength for the compacted expansive clay is more significant than that for a compacted kaolin at suctions less than 100 kpa

influence of gas hydrate morphology on the seismic velocities of sands this paper reports the results of a series of resonant column tests on specimens where gas hydrate has been formed in sands using an excess water technique in these specimens the amount of hydrate formed is restricted by the amount of gas in the specimen and with an excess of water being present in the pore space results of resonant column tests carried out to determine compressional and shear wave velocities suggest that gas hydrate formed in this way are frame supporting in contrast the behavior observed in sands where the hydrate is formed from finite water where the remaining pore space is saturated with methane gas termed in this paper the excess gas method exhibits a cementing behavior while tetrahydrofuran hydrate sands or where the hydrate is formed from dissolved methane within the pore water exhibit a pore filling behavior for hydrate saturations less than 40 for sands where the hydrate is formed using the excess water method much larger volumes of hydrate are required before a significant increase in shear wave velocity occurs although increases in compressional wave velocity are seen at lower hydrate contents these results suggest that hydrate interaction with the sediment is strongly dependent on morphology and that natural hydrate may exhibit contrasting seismic signatures depending upon the geological environment in which it forms

mechanical properties of sand silt and clay containing tetrahydrofuran hydrate 1 the mechanical behavior of hydrate bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes drilling and coring operations and the analysis of certain small strain properties of these sediments for example seismic velocities this study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 mpa confining pressure on sand crushed silt precipitated silt and clay specimens with closely controlled concentrations of synthetic hydrate the results show that the stress strain behavior of hydrate bearing sediments is a complex function of particle size confining pressure and hydrate concentration the mechanical properties of hydrate bearing sediments at low hydrate concentration probably 50 of pore space the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination cementing particles together and filling the pore space the cementation contribution to the shear strength of hydrate bearing sediments decreases with increasing specific surface of soil minerals the lower the effective confining stress the greater the impact of hydrate formation on normalized strength

on the critical state characteristics of methane hydrate bearing sediments abstract reasonable prediction of marine ground stability is vital for the safe exploitation of methane gas from the seabed and development of relevant technologies for gas production constitutive models based on a critical state soil mechanics framework for methane hydrate bearing sediments have been proposed in recent decades they required careful calibration of laboratory results to obtain accurate information on model parameters this study presents a comprehensive analysis of triaxial shear test data to examine the effects of hydrate saturation on the geomechanical characteristics of methane hydrate bearing sediments particularly at the critical state multiple critical state lines are identified for methane hydrate bearing sediments with different levels of hydrate saturation on the specific volume and logarithm of mean stress plane a rise in hydrate saturation shifts the critical state line of methane hydrate bearing sediments upward and slightly increases the gradient of critical state line in the v ln p plane non unique critical state lines are observed for methane hydrate bearing sediments under plane strain loading conditions considering methane hydrate saturation as an extra dimension of critical states the critical state line in the plane of specific volume and logarithm of mean stress becomes a three dimensional critical state surface

degradation of stiffness of cemented calcareous soil in cyclic triaxial tests the deformation characteristics of artificially cemented calcareous soil subjected to undrained cyclic triaxial loading are investigated at different confining pressure and cyclic stress levels the influence of cementation on the shear stiffness is investigated by comparing the behavior of cemented and uncemented soils with similar initial conditions it is observed that the deviator stress and the deviatoric strain at yield reduced with increasing number of cycles for cemented sand due to progressive degradation of bond which results in significant decrease in stiffness on the other hand a strain hardening effect is observed in uncemented sand and this results in increasing yield stress and strain with progressive number of cycles a linear relationship between degradation index and number of cycles is observed for cemented sand this relationship has been synthesized in the form of an empirical equation by modifying a previously proposed equation for cohesive soils this empirical equation was further used to evaluate the fatigue life of soils by adopting a failure criterion

yielding and pre failure deformation of structured sands many natural sands have a structure that is imparted to them during their geological life and that significantly modifies their pre failure behaviour when compared with that of the reconstituted material unlike clays for which emphasis has been given to both the bonding and fabric components of structure for sands structure has generally been simply identified with bonding whereas the effects of fabric have often been overlooked an understanding of the way in which these components of structure influence the yielding and stiffness of these materials is required to formulate constitutive models that can adequately predict their response the paper examines the pre failure behaviour of two structured sands in triaxial compression performed over a wide range of pressures since yielding occurred at small strains laboratory techniques were developed to produce reliable and accurate determinations of stiffness ranging from 1 microstrain to failure the influence of structure on the shear stiffness was in

testing a thermo chemo hydro geomechanical model for gas hydrate bearing sediments using triaxial compression laboratory experiments natural gas hydrates are considered a potential resource for gas production on industrial scales gas hydrates contribute to the strength and stiffness of the hydrate bearing sediments during gas production the geomechanical stability of the sediment is compromised due to the potential geotechnical risks and process management issues the mechanical behavior of the gas hydrate bearing sediments needs to be carefully considered in this study we describe a coupling concept that simplifies the mathematical description of the complex interactions occurring during gas production by isolating the effects of sediment deformation and hydrate phase changes central to this coupling concept is the assumption that the soil grains form the load bearing solid skeleton while the gas hydrate enhances the mechanical properties of this skeleton we focus on testing this coupling concept in capturing the overall impact of geomechanics on gas production behavior though numerical simulation of a high pressure isotropic compression experiment combined with methane hydrate formation and dissociation we consider a linear elastic stress strain relationship because it is uniquely defined and easy to calibrate since in reality the geomechanical response of the hydrate bearing sediment is typically inelastic and is characterized by a significant shear volumetric coupling we control the experiment very carefully in order to keep the sample deformations small and well within the assumptions of poroelasticity the closely coordinated experimental and numerical procedures enable us to validate the proposed simplified geomechanics to flow coupling and set an important precursor toward enhancing our coupled hydro geomechanical hydrate reservoir simulator with more suitable elastoplastic constitutive models

coupled deformation flow analysis for methane hydrate extraction methane hydrate is estimated to be present in substantial amounts below deep sea floors particular scientific and engineering interests that encourage studies of mechanical behaviour of methane hydrate soils include submarine geohazards such as the initiation of marine landslides through hydrate dissociation wellbore stability and estimation of future gas production from wells to study these problems a formulation of a multi physics model of methane hydrate flow coupled to soil deformation is developed by assuming deformable porous media soil matrix that accommodate non movable but dissociable hydrate a two phase flow formulation of water and methane gas is suggested according to darcy s law and capillary pressure law a single phase elastic perfectly plastic constitutive model for hydrate soil sediments based on the concept of effective stress is developed to account for the effect of hydrate saturation on mechanical strength and stiffness the formulation is incorporated into the explicit sch

sand production model in gas hydrate bearing sediments abstract this paper provides a comprehensive analytical formulation that entails the features of sand production in gas hydrate bearing sediments including grain detachment migration sediment deformation and hydrate dissociation the formulation is thermo hydro mechanically coupled such that grain detachment causes stress reduction sediment shear deformation induces grain detachment and grain flow alters multiphase fluid pressure and temperature profiles through a series of analyses the sensitivity of sand production related parameters on resulting solid volume changes is evaluated furthermore the effect of various operational methods on mitigating sand production in hydrate bearing sediments during gas production is numerically investigated it is found that out of the different operational methods investigated lowering depressurization rate was the most effective in reducing sand production for a given gas production

hydrate occurrence in europe a review of available evidence abstract large national programs in the united states and several asian countries have defined and characterised their marine methane hydrate occurrences in some detail but european hydrate occurrence has received less attention the european union funded project marine gas hydrate an indigenous resource of natural gas for europe migrate aimed to determine the european potential inventory of exploitable gas hydrate to assess current technologies for their production and to evaluate the associated risks we present a synthesis of results from a migrate working group that focused on the definition and assessment of hydrate in europe our review includes the western and eastern margins of greenland the barents sea and onshore and offshore svalbard the atlantic margin of europe extending south to the northwestern margin of morocco the mediterranean sea the sea of marmara and the western and southern margins of the black sea we have not attempted to cover the high arctic the russian ukrainian and georgian sectors of the black sea or overseas territories of european nations following a formalised process we defined a range of indicators of hydrate presence based on geophysical geochemical and geological data our study was framed by the constraint of the hydrate stability field in european seas direct hydrate indicators included sampling of hydrate the presence of bottom simulating reflectors in seismic reflection profiles gas seepage into the ocean and chlorinity anomalies in sediment cores indirect indicators included geophysical survey evidence for seismic velocity and or resistivity anomalies seismic reflectivity anomalies or subsurface gas escape structures various seabed features associated with gas escape and the presence of an underlying conventional petroleum system we used these indicators to develop a database of hydrate occurrence across europe we identified a series of regions where there is substantial evidence for hydrate occurrence some areas offshore greenland offshore west svalbard the barents sea the mid norwegian margin the gulf of cadiz parts of the eastern mediterranean the sea of marmara and the black sea and regions where the evidence is more tenuous other areas offshore greenland and of the eastern mediterranean onshore svalbard offshore ireland and offshore northwest iberia we provide an overview of the evidence for hydrate occurrence in each of these regions we conclude that around europe areas with strong evidence for the presence of hydrate commonly coincide with conventional thermogenic hydrocarbon provinces

a comprehensive review on the geomechanical properties of gas hydrate bearing sediments abstract natural gas hydrates ngh that form in sediments beneath the ocean beds are a potential source to meet growing global energy demand gas hydrate bearing sediments ghbs exhibit improved geomechanical properties viz shear strength stiffness compressibility and poisson s ratio that are responsible for their greater stability as compared to the native sediments however dissociation of gas hydrates during extraction of gas instability in the ghbs could trigger submarine slope failures seabed subsidence and failure of the foundations of seafloor installations in order to reduce the risk of occurrence of such geohazards it is imperative to ascertain the stability of ghbs by understanding the variations in their geomechanical properties through field and laboratory investigations which are time consuming and prohibitively expensive for many researchers with this in view the present study attempts to decipher the variation of geomechanical properties with respect to various parameters that affect them viz hydrate saturation sh hydrate morphology and confining pressure temperature conditions by critically synthesizing the existing data in the literature it has been noticed that the shear strength and stiffness are the major parameters influencing the stability of ghbs and both share power law relationship with sh further it is noticed that the cohesion is the predominant component of the shear strength of ghbs and shows significant improvement for sh values greater than 15 50 the variability of cohesion and stiffness with respect to sh have been analysed in tandem with the volumetric deformation during the shearing process to propose particle level mechanism hypothesis that provides an insight in to the stress strain behavior of ghbs the proposed relationships and mechanisms would aid in understanding the behavior and development of improved modeling of geomechanical properties of ghbs

a structured cam clay model a theoretical study of the behaviour of structured soil is presented a new model referred to as the structured cam clay model is formulated by introducing the influence of soil structure into th

submarine landslides triggered by destabilization of high saturation hydrate anomalies submarine landslides occur along continental margins at depths that often intersect the gas hydrate stability zone prompting suggestions that slope stability may be affected by perturbations that arise from changes in hydrate stability here we develop a numerical model to identify the conditions under which the destabilization of hydrates results in slope failure specifically we focus on high saturation hydrate anomalies at fine grained to coarse grained stratigraphic boundaries that can transmit bridging stresses that decrease the effective stress at sediment contacts and disrupt normal sediment consolidation we evaluate slope stability before and after hydrate destabilization hydrate anomalies act to significantly increase the overall slope stability due to large increases in effective cohesion however when hydrate anomalies destabilize there is a loss of cohesion and increase in effective stress that causes the sediment grains to rapidly consolidate and generate pore pressures that can either trigger immediate slope failure or weaken the surrounding sediment until the pore pressure diffuses away in cases where failure does not occur the sediment can remain weakened for months in cases where failure does occur we quantify landslide dynamics using a rate and state frictional model and find that landslides can display either slow or dynamic i e catastrophic motion depending on the rate dependent properties size of the stress perturbation and the size of the slip patch relative to a critical nucleation length scale our results illustrate the fundamental mechanisms through which the destabilization of gas hydrates can pose a significant geohazard

experimental behaviour and modelling of an unsaturated compacted soil this paper reports the experimental study and modelling of the mechanical response of a silty sand used in the core of the metramo dam italy specimens were prepared by compacting the soil at optimum water content conditions using the modified proctor technique tests were performed under suction controlled conditions by a stress path triaxial cell and an oedometer the experimental program consists of 23 tests carried out in the suction range of 0 400 kpa the findings indicate the strong influence of suction on compressibility stiffness and shear strength the mechanical properties of the soil improve with suction following an exponential law with decreasing gradient furthermore the soil exhibited collapsible behaviour upon wetting even at low stress levels interesting results were also achieved in elastoplastic modelling as well the results led to characterization of soil behaviour with reference to widely accepted modelling criteria for unsaturated soils providing noteworthy suggestions about

an international code comparison study on coupled thermal hydrologic and geomechanical processes of natural gas hydrate bearing sediments abstract geologic reservoirs containing gas hydrate occur beneath permafrost environments and within marine continental slope sediments representing a potentially vast natural gas source numerical simulators provide scientists and engineers with tools for understanding how production efficiency depends on the numerous interdependent coupled processes associated with potential production strategies for these gas hydrate reservoirs confidence in the modeling and forecasting abilities of these gas hydrate reservoir simulators ghrss grows with successful comparisons against laboratory and field test results but such results are rare particularly in natural settings the hydrate community recognized another approach to building confidence in the ghrs comparing simulation results between independently developed and executed computer codes on structured problems specifically tailored to the interdependent processes relevant for gas hydrate bearing systems the united states department of energy national energy technology laboratory doe netl sponsored the first international gas hydrate code comparison study ighccs1 in the early 2000s ighccs1 focused on coupled thermal and hydrologic processes associated with producing gas hydrates from geologic reservoirs via depressurization and thermal stimulation subsequently ghrss have advanced to model more complex production technologies and incorporate geomechanical processes into the existing framework of coupled thermal and hydrologic modeling this paper contributes to the validation of these recent ghrs developments by providing results from a second ghrs code comparison study ighccs2 also sponsored by doe netl ighccs2 includes participants from an international collection of universities research institutes industry national laboratories and national geologic surveys study participants developed a series of five benchmark problems principally involving gas hydrate processes with geomechanical components the five problems range from simple geometries with analytical solutions to a representation of the world s first offshore production test of methane hydrates which was conducted with the depressurization method off the coast of japan to identify strengths and limitations in the various ghrss study participants submitted solutions for the benchmark problems and discussed differing results via teleconferences the ghrss evolved over the course of ighccs2 as researchers modified their simulators to reflect new insights lessons learned and suggested performance enhancements the five benchmark problems final sample solutions and lessons learned that are presented here document the study outcomes and serve as a reference guide for developing and testing gas hydrate reservoir simulators

critical state soil constitutive model for methane hydrate soil 1 this paper presents a new constitutive model that simulates the mechanical behavior of methane hydrate bearing soil based on the concept of critical state soil mechanics referred to as the methane hydrate critical state mhcs model methane hydrate bearing soil is under certain geological conditions known to exhibit greater stiffness strength and dilatancy which are often observed in dense soils and also in bonded soils such as cemented soil and unsaturated soil those soils tend to show greater resistance to compressive deformation but the tendency disappears when the soil is excessively compressed or the bonds are destroyed due to shearing the proposed model represents these features by introducing five extra model parameters to the conventional critical state model it is found that for an accurate prediction of ground settlement volumetric yielding plays an important role when hydrate soil undergoes a significant change in effective stresses and hydrate saturation which are expected during depressurization for methane gas recovery

estimating pore space gas hydrate saturations from well log acoustic data relating pore space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment in the present study sonic velocities of gas hydrate bearing sands are modeled using a three phase biot type theory in which sand gas hydrate and pore fluid form three homogeneous interwoven frameworks this theory is developed using well log compressional and shear wave velocity data from the mallik 5l 38 permafrost gas hydrate research well in canada and applied to well log data from hydrate bearing sands in the alaskan permafrost gulf of mexico and northern cascadia margin velocity based gas hydrate saturation estimates are in good agreement with nuclear magneto resonance and resistivity log estimates over the complete range of observed gas hydrate saturations

effect of gas hydrates melting on seafloor slope instability we present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature pressure pore water chemistry and the mean pore size distribution the model uses a new formulation based on the enthalpy form of the law of conservation of energy the developed model shows that due to a temperature and pressure increase hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water this original result confirms what has been already shown through experiments the second part of the paper presents an application of the model through a back analysis of the giant storegga slide on the norwegian margin two of the most important changes during and since the last deglaciation hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature were considered in the calculation simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the storegga slope once again the developed model leads to predictions which are supported by laboratory experiment results but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone

coupling of hydraulic and mechanical behaviour in unsaturated compacted clay an experimental study was carried out to investigate the shear strength of a compacted non active clay speswhite kaolin a total of 33 statically compacted specimens were prepared at vertical stresses of 300 600 and 1200 kpa and water contents ranging from 0 24 to 0 34 this made it possible to explore a broad range of initial compaction induced conditions and hence to examine whether differences in water retention characteristics and shear strength are due to differences in initial compaction induced state or are to be attributed to fundamentally different microfabrics a shearbox was modified to allow the direct measurement of negative pore water pressure using trento high suction tensiometers tests were carried out at constant water content by monitoring suction changes in these tests it was possible to investigate the transition from unsaturated to saturated states in the first stage of the test the specimen was compressed at constant water content the specimen was therefore wetted in the s

the structure of baroclinic modes in the presence of baroclinic mean flow abstractthis contribution seeks to understand the vertical structure of linearized quasigeostrophic baroclinic modes when they are modified by the presence of a baroclinic mean flow and associated

study on the mechanical properties of hydrate bearing silty clay the mechanical properties are important for the evaluation of stratum deformation and instability in hydrate exploitation by using an integrated test apparatus for synthesis of hydrate sediment and triaxial tests a series of compression tests is conducted on the silty clay containing tetrahydrofuran hydrate scth similar to that of south china sea the stress strain curves and mohr circles are obtained at different degree of hydrate saturations and confining pressures the silty clay containing tetrahydrofuran hydrate shows typical ductile behavior the shear strength increases linearly with the increase of hydrate saturation and confining pressure the secant modulus increases with the increase of hydrate saturation the shear strength is analyzed based on the drucker prager criterion mohr coulomb criterion lade duncan criterion while the secant modulus is analyzed using the mixed law of composite materials it is shown that the strength can be well described by the drucker prager criterion and mohr coulomb criterion and the secant modulus is close to the harmonic average of the modulus of soil skeleton and hydrate in scth c 2015 elsevier ltd all rights reserved

an experimental investigation of the mechanical behaviour of an unsaturated gneiss residual soil the paper presents compression and triaxial tests conducted under controlled suction in horizons b and c of a residual soil gneissic rock profile from south east brazil horizon b found at the surface consists of a 2 m thick reddish lateritic clayey soil layer whereas horizon c is a saprolitic soil layer that reaches great depths the block samples representing horizons b and c were manually taken from depths of 1 m and 5 m respectively the test programme carried out consisted of isotropic and anisotropic compression tests as well as drained and constant water content triaxial tests tests were performed under saturated and unsaturated conditions at controlled suction strength parameters c and were shown to increase with matric suction the critical state parameters were shown to vary with suction which also expanded the yield curve data obtained may be useful for the development of soil constitutive models for unsaturated soils

strain rate dependent hardening softening characteristics of gas hydrate bearing sediments the presence of gas hydrates ghs increases the stiffness and strength of marine sediments in elasto plastic constitutive models it is common to consider gh saturation sh as key internal variable for defining the contribution of ghs to composite soil mechanical behavior however the stress strain behavior of gh bearing sediments ghbs also depends on the microscale distribution of gh and on gh sediment fabrics a thorough analysis of ghbs is difficult because there is no unique relation between sh and gh morphology to improve the understanding of stress strain behavior of ghbs in terms of established soil models this study summarizes results from triaxial compression tests with different sh pore fluids effective confining stresses and strain histories our data indicate that the mechanical behavior of ghbs strongly depends on sh and gh morphology and also on the strain induced alteration of gh sediment fabrics hardening softening characteristics of ghbs are strain rate dependent which suggests that gh sediment fabrics dynamically rearrange during plastic yielding events we hypothesize that rearrangement of gh sediment fabrics through viscous deformation or transient dissociation and reformation of ghs results in kinematic hardening suppressed softening and secondary strength recovery which could potentially mitigate or counteract large strain failure events for constitutive modeling approaches we suggest that strain rate dependent micromechanical effects from alterations of the gh sediment fabrics can be lumped into a nonconstant residual friction parameter we propose simple empirical evolution functions for the mechanical properties and calibrate the model parameters against the experimental data plain language summary gas hydrates ghs are crystalline like solids which are formed from natural gas molecules and water at high pressure and low temperature ghs and particularly methane hydrates are naturally abundant in marine sediments it is known that the presence of gh increases the mechanical stiffness and strength of sediments and there is strong effort in analyzing and quantifying these effects in order to understand potential risks of sediment destabilization or slope failure based on our experimental results from high pressure geotechnical studies we show that not only the initial amount and distribution of gh are important for the increased strength of gh bearing sediments but also the dynamic rearrangement of gh sediment fabrics during deformation characterizes the stress strain response and enables strength recovery after failure we propose that different microstructural mechanisms contribute to this rearrangement and strength recovery of gh sediment however we consider these complicated processes in a simplified manner in an improved numerical model which can be applied for geotechnical risk assessment on larger scales

the effects of hydrate cement on the stiffness of some sands cementing of sediment occurs naturally in many soils and weak rocks during both the early and late stages of diagenesis this paper reports the results of a series of resonant column tests carried out on a range of sand sized geomaterials to explore the effects of different hydrate cement morphologies on the very small strain stiffness of the materials it is shown that the proportion of void space filled by hydrate cement cement location sand size and grain shape all have a significant effect on shear modulus the change in stiffness between different host materials is affected by the density specific surface and grading of the geomaterial

the general and congruent effects of structure in natural soils and weak rocks the engineering properties of naturally occurring sedimentary and residual deposits which are usually treated in geotechnical engineering as soils are reviewed and it is shown that usually they have characteristics due to bonded structure which are similar to those of porous weak rock while this structure can arise from many causes its effects follow a simple general pattern that involves stiff behaviour followed by yield this yield can be described in a similar way to that occurring due to overconsolidation although it is a separate phenomenon the effects of structure are as important in determining engineering behaviour as are the effects of initial porosity and stress history which are the basic concepts of soil mechanics as it can be described in a general way it is concluded that structure and its effects should be treated as a further basic concept of equal importance l article passe en revue les proprietes des depots sedimentaires et residuels naturels qui sont nor malement traites comm

a particulate scale investigation of cemented sand behavior in this paper triaxial tests and numerical simulations using the discrete element method dem are combined to explore the underlying mechanisms of the unique behavior of artificially cemented sands the experimental results show that strength enhancement volumetric dilation and the shear banding associated failure mode are observed in portland cement sand these features become more pronounced with increasing cement content different responses are found in gypsum cemented sand even though both types of cemented sand specimens were prepared under very similar void ratios before shearing the dem simulations on the portland cement sand were carried out under two particular arrangements i e the use of small cementing particles and flexible membrane boundaries the simulation results reveal that particles in the bonding network jointly share the loading and many micro force chains associated with cementation are created compared with uncemented sand a more stable and stronger force chain complex su

role of critical state framework in understanding geomechanical behavior of methane hydrate bearing sediments a proper understanding of geomechanical behavior of methane hydrate bearing sediments is crucial for sustainable future gas production there are a number of triaxial experiments conducted over synthetic and natural methane hydrate mh bearing sediments and several soil constitutive models have been proposed to describe their behavior however the generality of a sophisticated model is questioned if it is tested only for a limited number of cases furthermore it is difficult to experimentally determine the associated parameters if their physical meanings and significance are not elucidated the objective of this paper is to demonstrate that a simple extension of the critical state framework is sufficient to capture the geomechanical behavior of mh bearing soils from various sources around the world while the significance of each parameter is quantified through variance based global sensitivity analyses our results show that the influence of hydrates can be largely represented by one hydrate dependent parameter pcd which controls the expansion of the initial yield surface this is validated through comparisons with shearing and volumetric response of mh bearing soils tested at various institutes under different confining stresses and with varying degrees of hydrate saturation our study suggests that the behavior of mh bearing soils can be reasonably predicted based on pcd and the conventional critical state parameters of the host sediments that can be obtained through typical geotechnical testing procedures

influence of fines content on the mechanical behavior of methane hydrate bearing sediments methane hydrate bearing sediments with different amounts of fines content and at three densities were artificially prepared under controlled temperature and pressure conditions the void ratios of specimens after isotropic consolidation tend to decrease with a rise in fines content the fines particles enter into the pore space between sand grains and densify the specimens a series of triaxial compression tests were performed to systematically investigate the influences of fines content and density on the shear properties of hydrate free sediments and methane hydrate bearing sediments the test results demonstrate that a rise in fines content within methane hydrate bearing sediments significantly enhances peak shear strength and promotes dilation behavior these influences are particularly prominent for specimens at loose packing state a decrease in void ratio increases the shear strength and stiffness of hydrate free sediments and methane hydrate bearing sediments containing fines content of 0 and 8 9 it is noted that the formation of methane hydrate in samples with varying amounts of fines content increases the stress ratios at the critical state the addition of fines particles into coarse grained sand grains alters the internal microstructure of sand matrix and the hydrate formation pattern in the pore space between sand grains and fines particles

isotropic yielding in an artificially cemented soil cured under stress the work simulates in the laboratory the formation of a cemented sedimentary deposit in which cement bonding occurs after burial and under geostatic stresses isotropic compression tests were carried out on artificially cemented specimens made with variable cement contents after consolidating the samples to the uncemented normal compression line the specimens were allowed to cure for 48 h before testing the curing confining stresses ranged from 50 to 2000 kpa and were intended to represent soil elements at different depths in the fictitious sedimentary deposit when the cementing occurred the contribution of the cement bonds to soil compression and the changes in the isotropic yield stress as a function of void ratio and cement content were analysed the results showed the importance of the void ratio during the formation of cement bonds and also of the degree of cementation for the compressive behaviour of the cemented soil and demonstrated that the variation in yield stress with void ratio and ce

formation history and physical properties of sediments from the mount elbert gas hydrate stratigraphic test well alaska north slope abstract the synthesis of available geological information and surface temperature evolution in the alaska north slope region suggests that biogenic and deeper thermogenic gases migrated through fault networks and preferentially invaded coarse grained layers that have relatively high hydraulic conductivity and low gas entry pressures hydrate started forming before the beginning of the permafrost eventually the permafrost deepened and any remaining free water froze so that ice and hydrate may coexist at some elevations the single tested specimen depth 620 47 620 62 m from the d unit consists of uncemented quartzitic fine sand with a high fraction of fines 56 by mass finer than sieve 200 the as received specimen shows no evidence of gas present the surface texture of sediment grains is compatible with a fluvial deltaic sedimentation environment and shows no signs of glacial entrainment tests conducted on sediments with and without thf hydrates show that effective stress porosity and hydrate saturation are the major controls on the mechanical and geophysical properties previously derived relationships between these variables and mechanical geophysical parameters properly fit the measurements gathered with mount elbert specimens at different hydrate saturations and effective stress levels we show that these measurements can be combined with index properties and empirical geomechanical relationships to estimate engineering design parameters volumetric strains measured during hydrate dissociation vanish at 2 4 mpa therefore minimal volumetric strains are anticipated during gas production at the mount elbert well however volume changes could increase if extensive grain crushing takes place during depressurization driven production strategies if the sediment has unexpectedly high in situ porosity associated to the formation history or if fines migration and clogging cause a situation of sustained sand production

excess pore pressure resulting from methane hydrate dissociation in marine sediments a theoretical approach 1 this study quantifies the excess pore pressure resulting from gas hydrate dissociation in marine sediments the excess pore pressure in confined pore spaces can be up to several tens of megapascals due to the tendency for volume expansion associated with gas hydrate dissociation on the other hand the magnitude of excess pore pressure in well connected sediment pores is generally smaller depending primarily on the hydrate dissociation rate and the sediment permeability volume expansion due to gas hydrate dissociation in well connected pore spaces is related via darcy s law to an increase in pore pressure and its gradient in sediment which drives an additional upward fluid flow through the sediment layer overlying the gas hydrate dissociation area the magnitude of this excess pore pressure is found to be proportional to the rate of gas hydrate dissociation and the depth below seafloor and inversely proportional to sediment permeability and the depth below sea level the excess pore pressure is the greatest at low initial pressures and decreases rapidly with increasing initial pressure excess pore pressure may be the result of gas hydrate dissociation due to continuous sedimentation tectonic uplift sea level fall heating or inhibitor injection the excess pore pressure is found to be potentially able 1 to facilitate or trigger submarine landslides in shallow water environments 2 to result in the formation of vertical columns of focused fluid flow and gas migration and 3 to cause the failure of a sediment layer confined by low permeability barriers in relatively deep water environments

mechanical behaviour of unsaturated kaolin with isotropic and anisotropic stress history part 1 wetting and compression behaviour over the last 40 years considerable progress has been made in understanding the complex behaviour of unsaturated soils research using constitutive modelling has extended the critical state framework and the concept of yielding in saturated soils to encompass unsaturated soils experiencing suction however validation testing of the framework for unsaturated soils has shown disagreement with the basic propositions the main reason for this disparity is the anisotropic properties of the soil specimens tested as a result of preparation using one dimensional compaction the paper describes the detailed testing carried out to justify this statement as part of the work presented samples of unsaturated kaolin were prepared using isotropic compression the suctions in these samples were reduced to predefined values by wetting under low isotropic loading the pore size distributions the pressure volume relationships and yielding under subsequent isotropic loading are compared with tests on samples prepared by

assessing the geomechanical stability of interbedded hydrate bearing sediments under gas production by depressurization at nghp 02 site 16 abstract establishing the geomechanical stability of marine sediments in the vicinity of a production well is one of the key design considerations in planning offshore gas production from marine hydrate reservoirs this paper presents an assessment of the sediment stability at india s national gas hydrate program expedition 2 nghp 02 site 16 area b offshore eastern india for which gas production is to be carried out by depressurization one important feature of the study is that extensive calibration of constitutive model parameters has been conducted based on laboratory test data from pressured core samples from analysis perspective the site is challenging because the hydrate reservoir consists of thin layers of hydrate bearing sands interbedded with mud moreover depressurization at the depth of a reservoir more than 2750 m below sea surface will lead to a pore pressure drop and accordingly an effective confining stress increase as high as 25 mpa in dealing with thin interbedded hydrate bearing strata meshing requirements for flow and geomechanical analysis are quite different from those for reservoirs with thicker massive layers an axisymmetric model and one way coupling simulations were thus adopted for this study in which the geomechanical study utilizes pore pressure and hydrate saturation output from the flow study but the flow study does not takes the porosity changes from the geomechanical analysis instead the reduction of porosity due to sediment deformation in the flow study is based on a pressure dependent pore compressibility relationship derived from geomechanical modeling the rationality is validated through back computing the pore compressibility from the geomechanical deformation results the study shows that large compression in the reservoir will result in movement of the sediments from above and below as well as laterally in smaller magnitudes and the sediment is deemed stable during the gas production period

base and precious metal resources in seafloor massive sulfide deposits abstract interest in seafloor massive sulfide deposits as a possible source of metals has been strong since the discovery of submarine hydrothermal vents in 1977 to aid decision makers an assessment of undiscovered seafloor massive sulfide deposits is made here estimates of number of undiscovered deposits and their metal contents are made and presented in probabilistic form to represent the inherent uncertainties in such estimates the three part form of assessment is used as a framework for this assessment because it can provide unbiased and reasonable estimates provided that consistent mineral deposit models are used four kilometer wide zones centered on a 256 000 km2 tract associated with spreading centers and a tract of 100 000 km2 associated with volcanic arcs and back arc basins are delineated as permissive for undiscovered deposits well explored volcanogenic massive sulfide deposits on land serve both as models of contained metals and as density models of deposit numbers density models provide probabilistic estimates of number of deposits in each tract frequency distributions of zn cu pb ag and au contents in land deposits are used to represent metal contents in undiscovered seafloor deposits metal distributions were grouped into a mafic related group to apply to the spreading center tract and a felsic and bimodal group to apply to the back arc and volcanic arc tract tests of dependency of resource estimates demonstrate for the first time that independence can be assumed to combine such estimates using monte carlo simulation simulation is used to combine probabilistic number of deposits estimates for the two tracts and each of the five linked metal content distributions median estimates of undiscovered zn cu pb ag and au are 6 4 million 4 6 million 1 2 million 10 000 and 390 t respectively although there is considerable uncertainty in estimated amounts of zn cu pb ag and au in undiscovered seafloor massive sulfide deposits located in the 4 km wide zones even the most optimistic estimates suggest that these deposits cannot make a substantial contribution to global metal production

energy resource potential of natural gas hydrates the discovery of large gas hydrate accumulations in terrestrial per mafrost regions of the arctic and beneath the sea along the outer continental margins of the world s oceans has heightened interest in gas hydrates as a possible energy resource however significant to potentially insurmountable technical issues must be resolved be fore gas hydrates can be considered a viable option for affordable supplies of natural gas the combined information from arctic gas hydrate studies shows that in permafrost regions gas hydrates may exist at subsurface depths ranging from about 130 to 2000 m the presence of gas hydrates in offshore continental margins has been inferred mainly from anomalous seismic reflectors known as bottom simulating reflectors that have been mapped at depths below the sea floor ranging from about 100 to 1100 m current estimates of the amount of gas in the world s marine and permafrost gas hydrate accumulations are in rough accord at about 20 000 trillion m3 disagreements over fundamental issues such as the volume of gas stored within delineated gas hydrate accumulations and the concentration of gas hydrates within hydrate bearing strata have demonstrated that we know little about gas hydrates recently however several countries including japan india and the united states have launched ambitious national projects to further examine the resource potential of gas hydrates these projects may help answer key questions dealing with the properties of gas hydrate reservoirs the design of production systems and most important the costs and economics of gas hydrate production

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

a multidimensional spectral description of ocean variability abstractan updated empirical analytical model for the frequency and wavenumber distribution of balanced motion in the ocean is presented the spectrum model spans periods longer than the inertial but shorter than a decade and wavelengths between 100 and 10 000 km assuming geostrophic dynamics a spectrum model for the streamfunction is constructed to be consistent with a range of observations including sea surface height from satellite altimetry velocity from moored and shipboard instruments and temperature from moorings first order characteristics of the observed spectra including amplitude and spectral moments vary slowly geographically the spectrum model is horizontally anisotropic accommodating observations that zonal wavenumber frequency spectra are dominated by a nondispersive line qualitative and quantitative agreement is found with one dimensional frequency and wavenumber spectra and observed vertical profiles of variance illustrative application is made of the model spectrum to obse