Pore Habit Research Articles

Effect of Hydrate on Gas/Water Relative Permeability of Hydrate-Bearing Sediments: Pore-Scale Microsimulation by the Lattice Boltzmann Method

As a clean energy source with ample reserves, natural gas hydrate is studied extensively. However, the existing hydrate production from hydrate deposits faces many challenges, especially the uncertain mechanism of complex multiphase seepage in the sediments. The relative permeability of hydrate-bearing sediments is key to evaluating gas and water production. To study such permeability, a set of pore-scale microsimulations were carried out using the Lattice Boltzmann Method. To account for the differences between hydrate saturation and hydrate pore habit, we performed a gas-water multiphase flow simulation that combines the fluids’ fundamental properties (density ratio, viscosity ratio, and wettability). Results show that the Lattice Boltzmann Method simulation is valid compared to the pore network simulation and analysis models. In gas and water multiphase flow systems, the viscous coupling effect permits water molecules to block gas flow severely due to viscosity differences. In hydrate-bearing sediments, as hydrate saturation increases, the water saturation S w between the continuous and discontinuous gas phase decreases from 0.45 to 0.30 while hydrate saturation increases from 0.2 to 0.6. Besides, the residual water and gas increased, and the capillary pressure increased. Moreover, the seepage of gas and water became more tedious, resulting in decreased relative permeability. Compared with different hydrate pore habits, pore-filling thins the pores, restricting the gas flow than the grain-coating. However, hydrate pore habit barely affects water relative permeability.

Numerical Study on Seepage Characteristics of Hydrate-Bearing Sediments: A Pore-Scale Perspective

Natural gas hydrate is one of the most promising energy resources. The permeability of hydrate-bearing sediments (HBS) is a critical parameter that affects gas production efficiency. In this study, by carrying out two-dimensional seepage models of HBSs with uniformly round particles, the effect of factors including pore habit, hydrate saturation, and particle size on the permeability of HBS is investigated. Two pore habits, seven hydrate saturations, and three matrix particle sizes were considered to complete the numerical simulation. The results show that the permeability of HBSs decreases with increasing hydrate saturation and decreasing matrix particle size because of the formation of smaller flow channels. The pore-filling HBSs present a considerable reduction of permeability at low hydrate saturation than that of grain-coating HBSs. This can be attributed to the fact that hydrates formed in the center of pores restrict fluid flow more than those on the surface of particles. As a result, the permeability assumes a zero value at hydrate saturation close to 68%, though it is supposedly only possible at 100%. This discrepancy may account for the variations of pore spaces between the two-dimension and three-dimension, the ignorance of heterogeneity in HBSs, and the shapes of hydrates and soils. For coarse-grained HBSs, models developed to account for features as pore geometry can capture the primary trend of HBSs permeability and reveal its seepage characteristics.

Combining Optical Microscopy and X-ray Computed Tomography Reveals Novel Morphologies and Growth Processes of Methane Hydrate in Sand Pores

Understanding the mechanisms involved in the formation and growth of methane hydrate in marine sandy sediments is crucial for investigating the thermo-hydro-mechanical behavior of gas hydrate marine sediments. In this study, high-resolution optical microscopy and synchrotron X-ray computed tomography were used together to observe methane hydrate growing under excess gas conditions in a coarse sandy sediment. The high spatial and complementary temporal resolutions of these techniques allow growth processes and accompanying redistribution of water or brine to be observed over spatial scales down to the micrometre—i.e., well below pore size—and temporal scales below 1 s. Gas hydrate morphological and growth features that cannot be identified by X-ray computed tomography alone, such as hollow filaments, were revealed. These filaments sprouted from hydrate crusts at water–gas interfaces as water was being transported from their interior to their tips in the gas (methane), which extend in the µm/s range. Haines jumps are visualized when the growing hydrate crust hits a water pool, such as capillary bridges between grains or liquid droplets sitting on the substrate—a capillary-driven mechanism that has some analogy with cryogenic suction in water-bearing freezing soils. These features cannot be accounted for by the hydrate pore habit models proposed about two decades ago, which, in the absence of any observation at pore scale, were indeed useful for constructing mechanical and petrophysical models of gas hydrate-bearing sediments.

A Study on the Electrical Characteristics of Fractured Gas Hydrate Reservoirs Based on Digital Rock Technology

Abstract The electrical characteristics of fractured gas hydrate reservoirs were investigated through the diffusion-limited aggregation model, digital rock technology, and the finite element method. The results show that the fracture and gas hydrate have a significant effect on the electrical characteristics of rock partially saturated with gas hydrate, where the matrix pore and fracture mixed gas hydrate form a dual-porosity system. Due to the fracture and gas hydrate effect, the electrical characteristics of fractured gas hydrate reservoirs cannot be described well by traditional Archie equations. The resistivity index vs. water saturation curve of fractured gas hydrate reservoirs shows a nonlinear relationship for different gas hydrate pore habits (pore-filling, cementing, and grain-coating types), and this curve consists of two parts with different gas hydrate saturation exponents for pore-filling and cementing gas hydrate and presents a curve without a fixed water saturation exponent for grain-coating gas hydrate. Fractured gas hydrate reservoirs with different fracture apertures, different gas hydrate pore habits, and saturation features will lead to macroscopic electrical anisotropy. The results of theoretical analysis and numerical simulation show that the electrical anisotropy coefficient of fractured gas hydrate reservoirs is a function of gas hydrate saturation. The function curve consists of three segments with the turning point for pore-filling and cementing gas hydrate, and this curve can be divided into two parts through the turning point. The findings of this study can help for a better understanding of the electrical characteristics of fractured gas hydrate reservoirs, which have great significance for the exploration and development of gas hydrate resources.

Influence of gas hydrate saturation and pore habits on gas relative permeability in gas hydrate-bearing sediments: Theory, experiment and case study

Gas relative permeability characterization is of key significance to model the behavior of gas flow in gas hydrate-bearing sediments. The present study proposes a novel model to relate gas relative permeability to gas hydrate saturation based on X-ray micro-CT imaging information of xenon hydrate pore-scale distribution in sand sediments. Lattice Boltzmann method (LBM) was used to obtain permeability values of xenon hydrate-bearing sediments via micro-CT data. The results showed that gas relative permeability (Kr) versus gas hydrate saturation (Sh) data are consistent with the new model and the imitative effect is relatively better than that of the simple Corey model. Besides, we calculated gas relative permeability versus gas hydrate saturation curves for various pore habits via idealized models. Experimental measurements and simulation results showed that the grain-coating gas hydrate exhibits the highest gas relative permeability, while pore-filling gas hydrate exhibits the lowest values of gas relative permeability, and the cementing gas hydrate ranges in between. We validated the new gas relative permeability calculation model by applying it to the well logging data of gas hydrate reservoirs. Our results showed that the novel model is beneficial for permeability characterization of gas hydrate reservoirs and gas relative permeability calculations.

MRI investigation of hydrate pore habits and dynamic seepage characteristics in natural gas hydrates sand matrix

A clear understanding of methane (CH4) hydrate pore habits and gas seepage behaviors in hydrate bearing sediments is critical for the exploitation of natural gas hydrate (NGH) reservoir. In this paper, we presented the experimental data of CH4 hydrate formation and decomposition in sand packs with the aid of magnetic resonance imaging (MRI) technique. The results showed that in homogeneous sand packs CH4 hydrates preferentially formed in the places where the water saturation was lower than the others, whereas in heterogeneous sand packs, the small grain size parts were the preferred locations for hydrate nucleation and the final hydrate saturation was much higher than the other parts. In this study, the CH4 hydrate decomposition process was found to consist of three stages, the initiation stage, the rapid decomposition stage and the post-decomposition stage. The relationship between gas permeability and hydrate saturation was obtained by analyzing the MRI scanned images during percolation experiments. The dynamic gas permeability indicated that the hydrate occurrence mode shifted from grain-coating and pore-filling co-existing to grain-coating predominant with the decrease of hydrate saturation. Moreover, the flow obstruction caused by the presence of hydrate barrier and sand migration was much severer in NGH reservoir.

Pore Habit of Gas in Gassy Sediments

AbstractGas bubbles are widespread in seafloors and lakebeds and typically found in shallow and fine‐grained sediments. Sediment properties control gas nucleation and gas migration. Gas migration and pathways have been studied mostly in clean coarse particles or fine‐grained matrices. Nevertheless, both cases show very distinct geo‐behaviors. Pore habit is defined by the counteracting effects of effective stress and pore‐throat‐dependent capillary pressure. In this article, we explore gas nucleation by CO2 gas exsolution and its consequent gas‐driven fractures (open‐mode discontinuities) or pore invasion in binary sediments as a function of fines content (FC). We conducted physical test analogies for different FC subjected to gas exsolution. Our results highlight that the pore habit of gas in gassy sediments depends on its capability to invade a neighboring pore (capillarity) and burial depth (effective stress). We show that the load dominant fraction in binary soils can be used to estimate the dominant pore throat size. We then proposed a robust methodology to predict the pore habit of gassy sediments from its properties as defined in recent developments in soil behavior and characterization. Finally, we applied it to a real case offshore Vancouver Island.

An Analytical Model for the Permeability in Hydrate‐Bearing Sediments Considering the Dynamic Evolution of Hydrate Saturation and Pore Morphology

AbstractHydrate‐saturation‐dependent permeability in sediments is largely affected by the pore morphology of hydrate. Hydrate pore morphology evolves from a single pore habit, which depends on its formation conditions, toward a sophisticated hybrid habit with increased hydrate saturation. Resulted permeability reduction curves in hydrate‐bearing sediments are diverse and lack universal models. By using a two‐parameter logistic function to link microscale hydrate pore habit evolution with macroscale permeability variations, this study establishes a pore‐morphology‐weighted permeability model that well captures the permeability evolution in hydrate‐bearing sediments at various conditions. The universality of this model is validated and the values of the two parameters in the model are calibrated using published laboratory and pressure core data. This newly proposed model offers a mechanistic understanding of the permeability in hydrate‐bearing sediments and an elegant expression that can be implemented in reservoir simulators for field‐scale gas production estimation.

Tools for pressure core sub-coring and pore-scale micro-CT (computed tomography) scans

Abstract. The pore habits of gas hydrate in natural sediment matrices provide essential clues for understanding physical (mechanical, thermal, hydraulic, and electrical) properties of hydrate-bearing sediments, yet there are no tools that can directly visualize the pore habits of natural gas hydrate other than indirect interpretation based on core-scale or field-scale observations. A significant challenge is to obtain a mini-core from pressure cores retrieved from natural reservoirs for high-resolution micro-CT (computed tomography) scans while maintaining pressure and temperature conditions required for stability of gas hydrate during all operational steps including manipulation, cutting, transferring, sub-coring and CT scanning. We present a new set of tools for pore-scale micro-CT imaging of natural hydrate-bearing sediments while maintaining pressure and temperature control. The tests with laboratory-prepared cores and pressure cores successfully demonstrate the capability of this set of tools to subsample a mini-core from pressure cores, transfer the mini-core to an X-ray transparent core holder, and conduct micro-CT scans. Successfully obtained CT images prove the functionality of this set of tools.

Pore-scale influence of methane hydrate on permeability of porous media

The influence of methane hydrate on the permeability of hosting sediments is critical to understand natural hydrate formation and gas production. Reported experimental data from tests on laboratory synthesized hydrate-bearing specimens and hydrate-bearing cores retrieved from natural reservoirs spread in a large range, and often deviates from predictions of a few conceptual models. This is very likely due to the mismatch between the actual hydrate pore habits and distribution and the overly simplified geometric assumptions used in those models. This study simulates single phase flow through hydrate-bearing sediments with a numerical approach to explore influences of hydrate on fluid flow, based on real 3D pore structures of methane hydrate-bearing sediments obtained via micro-CT scans. Pore-scale analysis and observations show (1) hydrate particles, at low hydrate saturation, protrude into the flow channels and efficiently inhibit the flow; (2) at high saturation or in small pores, hydrate particles block some pores and pores without hydrate determine permeability. Influence of pore habit in single pores on permeability is not obvious; by contrast, distribution of hydrate in the overall pore network has a higher impact and can cause drastic permeability anisotropy. Heterogeneity in both hydrate distribution and the sediments can explain the widespread range of relative permeability as a function of hydrate saturation.

A mechanistic model for permeability in deformable gas hydrate-bearing sediments

Accurate characterization of permeability behavior in hydrate-bearing sediments (HBS) is crucial for sustainable gas production. During gas exaction from HBS, permeability is strongly affected by the geomechanical conditions, which change pore structure of HBS. Moreover, hydrate dissociation will also change pore structure and geomechanical behavior of HBS. Therefore, the predictive modeling of permeability behavior in HBS under stress condition poses great challenges. In this work, a new analytical model is derived to study the effect of hydrate saturation on stress-dependent permeability behavior of HBS. The proposed permeability model solves the steady-state Navier-Stokes equations for fluid flow in HBS with hydrates. It considers hydrate saturation, retained water and hydrate-growth pattern, and is adequately validated with the experimental results in existing literatures. The model demonstrates that the permeability in wall coating (WC) hydrates is larger than that in pore filling (PF) hydrates. Pore habits of gas hydrates transfer from WC pore habit to PF pore habit when hydrate saturation increases. This work constitutes a comprehensive investigation of stress-dependent permeability in deformable HBS, which is a key issue for sustainable gas production. It not only provides theoretical foundations for quantifying permeability in HBS, but also can be used to estimate pore-scale parameters and other relevant parameters (e.g., rock lithology) of HBS using inverse modeling.

An experimental investigation on methane hydrate morphologies and pore habits in sandy sediment using synchrotron X-ray computed tomography

As pore-scale morphologies and spatial distribution (pore habits) of natural gas hydrates in marine sediments considerably affect their physical/mechanical properties, they have extensively been investigated by X-ray computed tomography (XRCT) and especially synchrotron X-Ray computed tomography (SXRCT). While both image spatial and scan temporal resolutions are being improved over time, it is still challenging to distinguish water from methane hydrate in an image due to their low absorption contrast. In this study, methane hydrate formation and growth in wet sand were observed at submicron/micron scale using SXRCT. Saline water (Potassium iodide - KI) was used in order to improve the image contrast. Evolutions of methane hydrate morphologies and distribution at both the pore and sample scales were observed. Results are discussed based on various mechanisms related to material behaviors and experimental conditions, e.g. suction variation during methane hydrate formation, local temperature gradient in the sample, and particularly the interaction of X-rays with the sample. Methane hydrate formation is interpreted as a dynamic process, favoring the Ostwald ripening. Furthermore, morphologies and pore habits of methane hydrates under excess-gas and excess-water conditions are discussed. Some recommendations are finally given for further studies on methane hydrates-bearing sediments via XRCT or SXRCT.

Maximum Sizes of Fluid-Occupied Pores within Hydrate-Bearing Porous Media Composed of Different Host Particles

Hydraulic properties of hydrate-bearing sediments are largely affected by the maximum size of pores occupied by fluids. However, effects of host particle properties on the maximum size of fluid-occupied pores within hydrate-bearing sediments remain elusive, and differences in the maximum equivalent, incircle, and hydraulic diameters of fluid-occupied pores evolving with hydrate saturation have not been well understood. In this study, numerical simulations of grain-coating and pore-filling hydrate nucleation and growth within different artificial porous media are performed to quantify the maximum equivalent, incircle, and hydraulic diameters of fluid-occupied pores during hydrate formation, and how maximum diameters of fluid-occupied pores change with hydrate saturation is analyzed. Then, theoretical models of geometry factors for incircle and hydraulic diameters are proposed based on fractal theory, and variations of fluid-occupied pore shapes during hydrate formation are discussed. Results show that host particle properties have obvious effects on the intrinsic maximum diameters of fluid-occupied pores and introduce discrepancies in evolutions of the maximum pore diameters during hydrate formation. Pore-filling hydrates reduce the maximum incircle and hydraulic diameters of fluid-occupied pores much more significantly than grain-coating hydrates; however, hydrate pore habits have minor effects on the maximum equivalent diameter reduction. Shapes of fluid-occupied pores change little due to the presence of grain-coating hydrates, but pore-filling hydrates lead to much fibrous shapes of fluid-occupied pores.

Load-bearing characteristic of methane hydrate within coarse-grained sediments – Insights from isotropic consolidation

Gas hydrate, as a solid pore constituent, can potentially share external load with sediment matrix, and critically impact geological evolution of its hosting sediments, well integration and sediment settlement during gas production depending on its contribution to support external loading. We aim to address the question of whether hydrate shares the load with the soil skeleton through isotropic consolidation experiments. Our results highlight the formation of methane hydrate does not affect the integrity of sediment skeletons if the effective stress previously applied on the skeleton remains constant, which indicates that methane hydrate would not actively support the load. Subsequent external load applied to the sediments after hydrate formation can be supported by hydrate. Secondly, hydrate as well as hydrate-bearing sediments exhibit creep behavior. As hydrate creeps, the load transfers to the sediment skeleton with time and hydrate gradually retreats from the status of load-bearing. When hydrate saturation is low and the coexisting pore fluid can percolate through sediments, the local deformation of hydrate due to sediment consolidation induces excess pore pressure, which dissipates by pushing out the pore fluid. When the coexisting pore fluid is enclosed in hydrate mass in high hydrate saturation environments, the dissipation of excess pore pressure relies on the viscous flow of hydrate. This pressure dissipation could last for millions of years according to analytical analysis based on Terzaghi's consolidation theory, during which hydrate cannot completely escape the status of load-bearing. These observations conclude that load-bearing is a dynamic status rather than a stagnant pore habit, countering conventional thinking that load-bearing joins grain-coating, cementing and pore-filling as a pore habit of gas hydrate.

Simultaneous measurement of water permeability and methane hydrate pore habit using a two-dimensional glass micromodel

In this study, we developed an in situ chamber to measure water permeability as a function of the methane hydrate (MH) habit in pore space using a two-dimensional glass micromodel by adding a methane-dissolved water source unit to prevent MH dissociation. This unit enabled us to adjust the amount of methane dissolved in the water by controlling the pressure and temperature. We simultaneously observed MH crystallization behavior and measured the water permeability of the porous media. The growth rate of MH and the pore habit formation changed with subcooling, ΔT. As ΔT increased, the shape of the pattern transformed from a hexagonal plate to a dendrite and hexagonal plate, and finally, to a dendrite. The glass pillar played a role in inhibiting the growth of dendritic patterns. The plugging process of the porous media following MH formation was also observed. We compared our results with various water permeability models of MH sediments and found that the nonempirical relative permeability (NRP) models showed relatively good agreement. This was likely because the NRP models enhanced the effect of the gas phase. However, a gap remains between the NRP model predictions and our experimental results. On the other hand, there was no possibility of agreement other than the NRP models because the gas phase was not included in the equation.

Pore-scale evidence of ion exclusion during methane hydrate growth and evolution of hydrate pore-habit in sandy sediments

Hydrate-bearing sediments constitute complex hydrological systems, within which the pore habit of hydrate significantly affects all physical properties. However, there is scarce pore-scale evidence of the evolution of hydrate growth in porous media and the influence of water salinity in the micro-morphology of hydrate. In this paper, we used X-ray computed micro-tomography (CT) to monitor three experiments of methane hydrate growth in sand partially saturated with either NaBr or KI brine under excess-gas conditions. Gas, brine and hydrate are observed to co-exist at the pore-scale during the evolution of hydrate pore-habit towards three-phase equilibrium. The experiments and data analysis reveal that (1) hydrate forms as a porous medium mixed with inclusions of brine and gas, where hydrate and brine evolve gradually into separate phases as hydrate cages exclude salt ions, (2) hydrate growth mobilizes water over fairly long distances resulting in heterogeneous hydrate distribution, and (3) hydrate can exhibit interconnecting pore-habit at local hydrate saturations higher than ~50%, even if grown under excess-gas conditions. The results imply that hydrate micro-morphology and pore habit evolve during hydrate formation and, we suggest, so do the physical properties of hydrate-bearing sediments. Our findings provide new insights into the micro-morphology of hydrate and evolution of the properties of hydrate-bearing sediments synthesized in the laboratory and in natural systems.

Methane hydrate-bearing sediments: Pore habit and implications

Abstract Hydrate-bearing sediments are relevant to the organic carbon cycle, seafloor instability, and as a potential energy resource. Sediment characteristics affect hydrate formation, gas migration and recovery strategies. We combine the physics of granular materials with robust compaction models to estimate effective stress and capillary pressure in order to anticipate the pore habit of methane hydrates as a function of the sediment characteristics and depth. Then, we compare these results to an extensive database of worldwide hydrate accumulations compiled from published studies. Results highlight the critical role of fines on sediments mechanical and flow properties, hydrate pore habit and potential production strategies. The vast majority of hydrate accumulations (92% of the sites) are found in fines-controlled sediments at a vertical effective stress between σ′z = 400 kPa and 4 MPa, where grain-displacive hydrate pore habit prevails in the form of segregated lenses and nodules. While permeation-based gas recovery by depressurization is favored in clean-coarse sediments, gas recovery from fines-controlled sediments could benefit from enhanced transmissivity along gas-driven fractures created by thermal stimulation.

3D measurements of hydrate surface area during hydrate dissociation in porous media using dynamic 3D imaging

A better understanding of the kinetics of hydrate dissociation is essential to reliably predict gas production potential from natural hydrate reservoirs. Most hydrate dissociation models assume hydrates to be a constant number of equal-sized spheres dissociating at a constant rate. This paper uses dynamic 3D synchrotron micro-computed tomography (SMT) imaging to study hydrate surface area evolution during Xenon hydrate dissociation. Hydrates are formed inside a high-pressure low-temperature cell filled with partially saturated ASTM 20-30 Ottawa sand. Hydrate dissociation is initiated through depressurization in the first experiment and through thermal stimulation in the second experiment. During dissociation, continuous full 3D SMT images were acquired where each scan took 45 s to complete. A combination of cementing, pore-filling, and surface coating pore habits were observed for the depressurization experiment and pore-filling for the thermal stimulation experiment. Surface coating hydrates dissociate faster than hydrates with pore-filling pore habit due to the higher specific area which allows for more surface for hydrates to dissociate it. Direct measurements of hydrate volume and hydrate surface area suggest that even with a combination of hydrate pore habits formed within the 3D porous media, estimation of hydrate surface area as a linear relation with (hydrate volume)2/3 is best for hydrate saturation less than a threshold value depending on the dissociation method and driving force. (hydrate volume)2 and (hydrate volume)3 were found to better estimate hydrate interfacial area in comparison to (hydrate volume)2/3 for the depressurization experiment and thermal stimulation experiment, respectively.

Dynamic 3D imaging of gas hydrate kinetics using synchrotron computed tomography

The availability of natural gas hydrates and the continuing increase in energy demand, motivated researchers to consider gas hydrates as a future source of energy. Fundamental understanding of hydrate dissociation kinetics is essential to improve techniques of gas production from natural hydrates reservoirs. During hydrate dissociation, bonds between water (host molecules) and gas (guest molecules) break and free gas is released. This paper investigates the evolution of hydrate surface area, pore habit, and tortuosity using in-situ imaging of Xenon (Xe) hydrate formation and dissociation in porous media with dynamic three-dimensional synchrotron microcomputed tomography (SMT). Xe hydrate was formed inside a high- pressure, low-temperature cell and then dissociated by thermal stimulation. During formation and dissociation, full 3D SMT scans were acquired continuously and reconstructed into 3D volume images. Each scan took only 45 seconds to complete, and a total of 60 scans were acquired. Hydrate volume and surface area evolution were directly measured from the SMT scans. At low hydrate saturation, the predominant pore habit was surface coating, while the predominant pore habit at high hydrate saturation was pore filling. A second-degree polynomial can be used to predict variation of tortuosity with hydrate saturation with an R2 value of 0.997.

Hydrate growth in quartzitic sands and implication of pore fractal characteristics to hydraulic, mechanical, and electrical properties of hydrate-bearing sediments

Gas hydrate pore habits significantly impact the pore-scale structure within hydrate-bearing sediments, and thus, play a central role in the physical property evolution. Characterization and quantification of the effective pore space within hydrate-bearing sediments at different hydrate saturations have not been well offered. This study performs random simulations of hydrate nucleation and growth in quartzitic sands to understand effects of hydrate saturation and hydrate pore habits on fractal characteristics of the effective pore space. Normalized pore-size fractal dimension and normalized maximal pore diameter are characterized and found to decrease with increasing hydrate saturation. In order to predict hydrate saturation dependent pore fractal characteristics, theoretical and empirical models are proposed and further extended to give implications to hydraulic, mechanical, and electrical properties of hydrate-bearing sediments during hydrate dissociation. Implications include that hydrate dissociation facilitates the absolute permeability and the electrical conductivity, and enhances first and then reduces the saturation exponent of Archie's law; hydrate dissociation also lowers the capillary pressure, and this promotes relative permeability to gas but inhibits relative permeability to water even the water saturation remains as a constant; shear strength of unsaturated hydrate-bearing sediments drops down due to the decreasing capillary pressure as hydrate dissociation. These implications all meet with conclusions in previous literatures.