Mechanical Properties Of Hydrate-bearing Sediments Research Articles

Effect of Temperature on the Mechanical Properties of Hydrate-Bearing Sand under Different Confining Pressures

Natural gas hydrate has always been the focus of scientists’ research as a kind of clean energy. There is a causal relationship between the thorough study of mechanical properties of hydrate-bearing sediments (HBSs) and the realization of the utilization of hydrate resources. We conducted a series of mechanical experiments on HBSs at different temperatures and effective confining pressures in this work. Experimental data showed that the samples’ mechanical properties have a remarkable correlation with temperature and effective confining pressure. The strength of HBSs is positively correlated to effective confining pressure and negatively correlated to temperature. Furthermore, the samples’ stress–strain behaviors shifted from hardening to softening with decreasing effective confining pressure and temperature. By assessment of the Mohr strength envelope, the cohesion linearly increased with decreasing temperature. A simple relationship among confining pressure, temperature, and strength was established. Moreover, the fitted failure point line was influenced by the temperature of the samples. The presence of hydrate solids in the sample pores weakened the samples’ compressibility. The samples’ dilatancy showed a negative correlation with temperature, and the maximum dilatancy and peak strength did not occur at the same strain level.

DEM analyses of cemented hydrate’s effect on the compression behavior of fine-grained sediments

The participation of gas hydrate constitutionally affects the mechanical properties of gas hydrate-bearing sediments. The hydrates contribute to the mechanical properties of hydrate-bearing sediments by pore-filling, bearing and bonding effects. A discrete element model of hydrate-bearing silt was established randomly filled by the hydrate particles into silt-sized granular skeleton reservoir. The cluster & bonding hydrate is generated in the pores of the numerical silt reservoir to study hydrate’s mechanical effects. One-dimensional loading was applied to the numerical sample to investigate its volume change, bond degradation and stress transmission. The main conclusions are as follows. The structural yield stress corresponds with the abrupt bond breakage. Particles and contacts will be redistributed after the widespread bond breakage. The breakage of hydrate-hydrate bonded contacts is more quickly than that of silt-hydrate contacts ascribe to a crushing process of hydrate clusters (especially for SMH=45%).

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.

Effect of Clay Content on the Mechanical Properties of Hydrate-Bearing Sediments during Hydrate Production via Depressurization

The effects of sediments with different clay contents on the mechanical properties of hydrate deposits were studied using a high-pressure, low-temperature triaxial apparatus with in-situ synthesis, as well as the mechanical properties of self-developed hydrate sediments. Through multi-stage loading, triaxial compression tests were conducted by adding quartz sand with different clay contents as the sediment skeleton, and the stress–strain relationship of the shearing process and the strength of sediments with different clay contents were determined. Volumetric changes were also observed during shearing. The results show that the strength of hydrate sediments decreases with the increasing clay content of sediments; in the processes of depressurization and shearing, the hydrate samples exhibited obvious shear shrinkage, regardless of the sediment particle size.

Pressure core based onshore laboratory analysis on mechanical properties of hydrate-bearing sediments recovered during India's National Gas Hydrate Program Expedition (NGHP) 02

Abstract A solid understanding of the mechanical properties of hydrate-bearing sediments is essential for the safe and economic development of methane hydrate as an energy resource. In 2015, 104 pressure cores were collected, recovering sediments from above and within concentrated hydrate reservoirs in the Krishna-Godavari Basin, as part of India's National Gas Hydrate Program Expedition 02 (NGHP-02). These cores provided minimally-disturbed sediment, retained at pressures and temperatures within the hydrate stability field, for the first-ever systematic triaxial test of dozens of subsections of hydrate-bearing pressure core sediments. Post-cruise testing in Japan evaluated multiple physical and hydro-mechanical properties. Consolidated drained and undrained triaxial compression tests, uniaxial (unconfined in effective stress) compression tests, multistage consolidation and compression tests, and alternating strain-rate compression tests were performed. Triaxial compression test results showed an increase in the strength and stiffness, as well as the positive dilatancy, with increasing hydrate saturation, supporting previous research on laboratory-formed and natural hydrate-bearing sediments. However, some strength results in this study were low compared to prior analyses of hydrate-bearing sediments. This low strength was likely caused by the host sediment's small particle size and loose packing, and the relatively slow applied compression strain rate. Results from uniaxial compression and multi-step compression tests confirmed that pore-space hydrates produce an apparent cohesion in hydrate-bearing sediment. More severe strength loss in sediments during the initial loading for multistage compression was also attributable to the presence of hydrates. The applicability of this multistage compression test for determining in situ properties was not confirmed, but results do provide bounds on the in situ values. Finally, from the variable strain-rate tests, it was revealed that strength in hydrate-bearing sediment has a large strain-rate dependence.

Effect of sediment particle size on the mechanical properties of CH4 hydrate-bearing sediments

Previous study proved that the methane hydrate reservoirs in clay permafrost regions have great potential for exploitation. However, the widespread gradation of sediment matrices in hydrate-bearing sediments (HBS) would seriously affect the stability of the hydrate reservoirs, causing uneven distribution of pressure at the wall of the exploitation wellbore, and leading to a severe engineering hazard. Therefore, the effects of the sediment particle size on the mechanical stability of hydrate reservoir need to be investigated before the large-scale commercial production of methane hydrate. In this paper, sediments matrices containing different proportions of clay, silt and sand were prepared and mechanical properties of HBS were investigated and compared with each other. The results show that a bigger proportion of large-particles in the sediment matrix could significantly enhance the failure strength of HBS under different confining pressures, temperatures and strain rates. A better graded size distribution would also boost the failures strength of HBS, it's also found that the failure strength of HBS firstly increases and then declines with the rise of confining pressure, and that material exhibits greater failure strength at lower temperatures and higher strain rates. Finally, an analysis using Mohr-Coulomb strength theory indicates that the internal friction angle plays a dominant role in the failure strength increase of HBS with a greater proportion of large-particles in the sediment matrix.

Mechanical Properties of Stratified Hydrate-bearing Sediments

Abstract Methane hydrate–bearing layers will be disturbed during the production of methane hydrates, which may induce the deformation or settlement of the layers, and the destruction of engineering structures, so it's important to study the mechanical properties of the hydrate-bearing sediments. In this paper, specimens were prepared in which the methane hydrates were in different location of the specimens by a special deposition method in which the methane hydrate was put in the mold separating with the kaolin, the volume of methane hydrate was 40% of the whole volume of the specimen. A series of triaxial shear tests were carried out under different confining pressures of 1.25 MPa, 2.5 MPa, 3.75 MPa and 5 MPa, conditions with temperature of -6 o C and strain rate of 1%/min. The results indicated that the maximum deviator stress of the sediments increased with the declining of the clay of hydrate and the failure strength achieved maximum when the hydrate clay was in the center of the sediments; the failure strength increased with the increasing of the confining pressure in the low confining pressure stage.

Dynamic strength characteristics of methane hydrate-bearing sediments under seismic load

The research on the mechanical properties of hydrate-bearing sediments has attracted great attention to ensure the safety production of methane from hydrates. To study the dynamic strength characteristics of methane hydrate-bearing sediments under seismic load, a series of dynamic triaxial tests was performed on artificial methane hydrate-bearing sediments under various conditions, with confining pressures at 2, 1 and 0.5 MPa; temperatures at −5, −3 and −1 °C; and porosities at 40% and 80%. The dynamic stress-strain behavior and curves of dynamic strain versus number of cycles were obtained and analyzed. The dynamic strength curves showed that the failure number of cycles was significantly affected by the dynamic stress amplitude. The dynamic strength of methane hydrate-bearing sediments increased with increasing confining pressure and decreased with increasing temperature and porosity. Based on the Mohr–Coulomb strength criterion, Mohr's circles and strength envelopes were drawn, and the mechanism of the influence of temperature and porosity on the dynamic strength of the sediments was revealed. The results indicated that increasing temperature would reduce the dynamic strength by reducing the soil cohesion and internal friction angle, while increasing porosity would reduce the dynamic strength mainly by reducing the soil internal friction angle.

Effects of Different Mining Methods on the Strength Behavior of Gas Hydrate-Bearing Sediments

The mining of methane hydrate from permafrost can cause the permafrost strength decreasing, softening and straining. It will be helpful to understand effects of different mining methods on the mechanical properties of hydrate-bearing sediments. In this paper, two kinds of hydrate dissociation methods (depressurization and heating) were conducted in triaxial pressure chamber. The mechanical properties of methane hydrate-bearing sediments after dissociation were measured by a low-temperature and high-pressure triaxial compression apparatus under exhaust and non-exhaust shear test. In addition, triaxial shear test of the CO2 and CH4 hydrate-bearing sediments prior to dissociation were also carried out in the identical conditions. The experimental results under the two kinds of dissociation methods showed that the strength of the methane hydrate-bearing sediments after dissociation is less than that before. The shear strength under the consolidated exhaust and consolidated non-exhaust shear tests was also quite different.

Advances in Study of Mechanical Properties of Gas Hydrate-Bearing Sediments

Gas hydrate (GH) is defined as the crystalline solid, or clathrate hydrate, which are formed by some kinds of low mass molecular gases, such as methane, carbon dioxide, and hydronitrogen, with water at relatively high pressure and low temperature conditions. Gas hydrate-bearing sediments (HBS) are some sand, clay and mixed sediment containing gas hydrates. Advances in the study on the mechanical properties of HBS are summarized mainly in aspects of the laboratory test, in-situ investigation, and theoretical model. Firstly, the main factors are discussed including the structure of GH, formation method and matrix characteristics of HBS; Secondly, progress on the laboratory tests and results are discussed, which mainly includes the tri-axial tests with the natural and synthesized HBS samples, the acoustic tests for measuring the elastic coefficients, the tests for investigating the effects of main factors such as the gas and water contents and soil types on the strength of HBS. Thirdly, for in-situ investigations, including the geophysical surveying, in-situ tests (such as downhole tests) and results are summarized; Fourthly, several theoretical models for estimating the mechanical properties of HBS are introduced; At last, the emphases and the tendency in the future study on the mechanical properties of HBS are discussed.

Mechanical properties of clathrate hydrates: status and perspectives

Knowledge of the mechanical properties of clathrate hydrates is central for studying the mechanical properties of hydrate-bearing sediments, their associated applications in wellbore stability, exploitation in stratum deformation, geological disaster prevention, and risk assessment of CO2 buried in oceans. However, because of the limited understanding of hydrate formation conditions and limited methods to investigate these, the understanding of the mechanical properties of hydrates is still poor and even controversial to some extent. This paper reviews current experimental and theoretical results on mechanical properties of hydrates, and discusses the typical difficulties faced in this area. On the experimental side, the most important problem is obtaining pure hydrate samples. Theoretically, the essential origin of the mechanical properties has not been explained in terms of molecular interactions. The hope is to resolve these issues by combining novel macroscopic experiments and microscopic methods. In order to avoid difficulties caused by impurities, it is proposed to use molecular dynamics simulations. This technique can be used to reveal the nature of the mechanical characteristics of hydrates at the molecular and nanometre scale. The goals of this paper are to establish a bridge between the micromechanical nature and the macromechanical properties of hydrates, and to lay a solid theoretical basis for the study of the mechanical properties of hydrate-bearing sediments. These goals are important for the future safe and efficient exploitation of natural hydrates, hydrate-induced seabed geological disaster prevention, the safety of CO2 geological burial, and the deployment of a reliable long-term seabed-borehole coupled hydrate observation system in the integrated Ocean Drilling Program.

Formation, growth and ageing of clathrate hydrate crystals in a porous medium

An experimental study was performed to visually observe the driving force dependence of hydrate growth in a porous medium filled with either liquid water and dissolved CO2 or liquid water and gaseous CO2. The given system subcooling, ΔT sub, i.e. the deficiency of the system temperature from the triple CO2−hydrate−water equilibrium temperature under a given pressure, ranged from 1.7 K to 7.3 K. The fine dendrites initially formed at ΔT sub = 7.3 K changed quickly into particulate crystals. For ΔT sub = 1.7 K, faceted hydrate crystals grew and the subsequent morphological change was hardly identified for an eight-day observation period. These results indicate that the physical bonding between hydrate crystals and skeletal materials becomes stronger with decreasing driving force, suggesting that the fluid dynamic and mechanical properties of hydrate-bearing sediments vary depending on the hydrate crystal growth process.