Behavior Of Gas Hydrate-bearing Sediments Research Articles

Numerical simulation of direct shear tests on gas hydrate-bearing sediments using the discrete element method

Natural gas hydrate is a potential unconventional energy resource. Understanding the mechanical behavior of gas hydrate-bearing sediments (GHBS) is important to ensure the safe and efficient exploitation of natural gas hydrate and prevent geohazard problems. This study employs discrete element method to investigate the mechanical properties of GHBS with different hydrate saturations under various vertical stresses, and a series of direct shear tests, which requires less computational consumption than triaxial or biaxial tests. Comprehensive analyses of macroscopic and microscopic mechanical behaviors, including shear strength, internal friction angle, cohesion, shear band, average coordination number, and crack number evolution, are carried out, as an effort for understanding the shear strength breakpoint, which is recognized in laboratory experiments, probably caused by the transition in hydrate morphology in sediment pores. The results obtained reveal that: (1) the way of hydrate in strengthening GHBS is to enhance cohesion; (2) the increasing rate of the strength difference between the sediment matrix with and without hydrate remarkably increases when the hydrate saturation is over certain value at which the shear strength breakpoint is observed, and this rate decreases with the increase in median particle diameter (D50) of host sediments; (3) the transition in hydrate morphology in sediment pores is recognized when the hydrate saturation increases at microscale; and (4) the shear strength breakpoint in the curve of strength difference versus hydrate saturation is explained based on the micromechanical properties of GHBS, including shear bandwidth, average coordination number, average normal force, and crack number evolution.

A modified damage constitutive model of gas hydrate-bearing sediments based on discrete element method

Based on a series of discrete element method (DEM) simulations, a modified damage constitutive model that can predict the mechanical behavior of gas hydrate-bearing sediments (GHBS) has been developed. A new method for preparing the GHBS sample with different gas hydrate saturations is proposed. According to the triaxial compression simulation tests, the stress–strain curves, Mohr’s circles, and their envelopes of GHBS samples were obtained. Based on the results of simulation tests, material parameter equations for GHBS were established. Using the Drucker–Prager failure criterion, a modified damage constitutive model was constructed that can predict the mechanical behavior of the GHBS under high confining pressure more accurately than a typical damage constitutive model. The effects of confining pressure and gas hydrate saturation on the mechanical behavior of GHBS were investigated using the modified damage constitutive model. The model’s correctness is demonstrated by a comparison with simulation test results. This article can be used as a reference by researchers working on gas hydrate drilling design and analysis.

Physical Properties of Gas Hydrate-Bearing Pressure Core Sediments in the South China Sea

Gas hydrates are a potential future energy resource and are widely distributed in marine sediments and permafrost areas. The physical properties and mechanical behavior of gas hydrate-bearing sediments are of great significance to seafloor stability and platform safety. In 2013, a large number of pressure cores were recovered during China’s second gas hydrate drilling expedition in the South China Sea. In this study, we determined the gas hydrate distribution, saturation, physical properties, and mechanical behavior of the gas hydrate-bearing sediments by conducting Multi-Sensor Core Logger measurements and triaxial and permeability tests. Disseminated gas hydrates, gas hydrate veins, and gas hydrate slabs were observed in the sediments. The gas hydrate distribution and saturation are spatially heterogeneous, with gas hydrate saturations of 0%–55.3%. The peak deviatoric stress of the gas hydrate-bearing sediments is 0.14–1.62 MPa under a 0.15–2.3 MPa effective confining stress. The permeability is 0.006– 0.095 × 10 − 3 μ m 2 , and it decreases with increasing gas hydrate saturation and burial depth.

Dynamic coupling responses and sand production behavior of gas hydrate-bearing sediments during depressurization: An experimental study

Gas hydrates that occur in offshore sediments and onshore permafrost represent a vast source of natural gas. However, current gas production trials indicate that sand production, which refers to the migration of sand into boreholes is an obstacle for long-term safe and efficient gas production from gas hydrate reservoirs. Therefore, understanding sand production behaviors during hydrate mining is especially important. In this study, we conducted laboratory-scale tests to investigate the effect of hydrate saturation and overlying stress on the sand production behaviors of hydrate-bearing sediments with a sand screen before, during, and after hydrate dissociation by depressurization. The synthesized methane hydrate saturations and simulated overlying stresses were ~20%, ~30%, ~40%, ~50%, and ~60% and ~11, ~12, ~13, ~14, and ~15 MPa, respectively. The experimental results indicate that sand production before hydrate dissociation is relatively minor and primarily silty. The amount of sand produced increases and silt, fine sand, and medium sand were observed during and after hydrate dissociation. The mass of sand production increases as hydrate saturation increases under constant overlying stress; however, under constant hydrate saturation, it increases then decreases as overlying stress increases. Friction among particles may become the primary control factor once the overlying stress exceeds a critical value. We considered hydrate saturation and overlying stress to establish a sand production prediction model. Our findings help understand sand production behavior using sand screens during gas production from hydrate reservoirs and provide a reference for sand production control design in future hydrate mining.

Numerical modeling for the mechanical behavior of marine gas hydrate-bearing sediments during hydrate production by depressurization

Abstract Natural gas hydrate, which is called inflammable ice, has drawn global interest. Many countries have carried out the trial production of natural gas from marine gas hydrate formations. However, due to the failure of the bonded structure and the disappearance of the hydrate in the pores of formations, improper production might induce geological hazards such as submarine landslide, non-uniform subsidence and local large deformation of the formations. In this study, a constitutive model based on super-loading and sub-loading surface theory is proposed to investigate the mechanical behavior of gas hydrate-bearing sediments (GHBSs). The mechanical behavior of both GHBSs and the structured clay are modeled by introducing rotational hardening rule, sub-loading surface and super-loading surface. A fully coupled thermo-hydro-chemo-mechanical (THCM) model is applied to study the mutiphysical responses of GHBSs during hydrate production by depressurization. The simulation results of the two-layer model show that the mutiphysical responses are dependent on not only the different material properties of the two layers but also the difference between the hydrate dissociated and undissociated zones. Although depressurization is the driving factor of hydrate dissociation, the heat transfer from the clay layer is also a key factor in the hydrate dissociation. Hydrate dissociation results in the variation in the stiffness and strength of the GHBSs leading to stress and strain and strain redistribution. The yielding and structural failure of sediments mainly occur in the hydrate dissociated zone.

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.

A new strength criterion and constitutive model of gas hydrate-bearing sediments under high confining pressures

Mechanical properties of gas hydrate-bearing sediments are important for the safe production of natural gas from hydrate reservoirs, as well as conventional oil or gas beneath the gas hydrate bearing sediments. In order to determine the failure load and describe the deformation behavior of gas hydrate-bearing sediments in deepwater regions, a series of triaxial compression tests on synthetic gas hydrate-bearing sediment cores were conducted under high confining pressures and a temperature of −10 °C. The results verify that the deviator stress increases gradually with increasing axial strain and finally reaches a constant value without significant peak value. When confining pressure ≥5 MPa, the failure strength decreases with increasing confining pressure because of particle crushing and pressure melting of ice. A new strength criterion to well reflect the failure load of gas hydrate-bearing sediments under high confining pressures is proposed based on the experimental results. The Duncan–Chang model is used to describe the deformation behavior of gas hydrate-bearing sediments under various confining pressures, and it is found that the results predicted by the model agree well with the experimental data.