Natural gas hydrate (NGH) is considered one of the most promising future energy sources because of its considerable reserves and high energy density, which could meet the growing global energy demand and improve the energy consumption structure. However, NGHs exist in the sediment mainly in the form of cementation or a part of the sediment skeleton, which may be responsible for the stability of native sediments. Therefore, during commercial NGH exploitation, the analysis of the mechanical characteristics of the hydrate-bearing sediment (HBS) is necessary to ensure safe exploitation. This review comprehensively summarizes recent mechanical studies on HBS; the influences of several important parameters (hydrate saturation, effective confining pressure, sediment composition, loading rate, temperature, and pore pressure) on the mechanical behavior of HBS are clarified, and the corresponding particle-level mechanisms are discussed. In addition, the influences of hydrate exploitation techniques (depressurization, thermal stimulation, and carbon dioxide replacement) on the strength and deformation behavior of HBS are investigated. The proposed relationships and mechanisms provide insightful guidance for understanding the mechanical behavior of HBS and developing models for predicting the structural evolution of HBS during NGH exploration and exploitation.

Approximately 90% of gas hydrates are buried in fine-grained sediments on earth, especially in the South China Sea. The potential instability of the fine-grained sediments induced by hydrate dissociation requires us to investigate the shear strength and pore pressure response of the sediments during the hydrate recovery. To date, most of studies focused on the undrained mechanical behavior of gas hydrate-bearing sand or gas hydrate-free clay, and few studies examined gas hydrate-bearing fine-grained sediments. According to the low-permeability and water-saturated characteristics of the sediments in the South China Sea, we performed a series of undrained triaxial shear tests on water-saturated methane hydrate-bearing clayey-silty sediments in this area. The experiment results show that the failure strength of methane hydrate-bearing sediments (MHBSs) increases with the increase in hydrate saturation and initial effective mean stress. The excess pore water pressure of MHBSs remains positive during shear. The cohesion in Mohr-Coulomb model increases with the increase in hydrate saturation, while the internal friction angle in Mohr-Coulomb model has little dependence on the hydrate saturation.

The macromechanical properties (strength, stiffness, stress-strain relationship, etc) of the hydrate-bearing sediment are often correlated with the hydrate cementation failure behavior. In this study, a consolidated drained triaxial shear test with X-ray computed tomography was conducted on a hydrate-bearing sediment with a hydrate saturation of 32.1% under 3 MPa effective confining pressure for revealing the cementation failure behavior. The hydrate occurrences were clearly identified to be cementing, grain-coating, prepatchy cluster and patchy cluster. The cementation failure behavior (morphology), deformation evolution (quantitative statistics), and localized shear deformation at different regions of stress-strain curve were observed and analyzed using computed tomography. In the linearity region, the hydrate-cemented clusters moved as a whole, while small hydrate particles would aggregate to the periphery of the clusters. The noncemented sand particles move disorderly during this process. Localized deformation occurred perfectly exhibit an antisymmetric bifurcation pattern. In the plasticity region, the specimen starts to deform plastically; an internal shear band occurs in the specimen. The hydrates begin to shed from the periphery of the cemented structure, and the grinding effect of hydrates begins to occur between sand particles. However, the shedded hydrates will not enter and fill the neighboring pores but hind the movement of sand particles structurally near the original position. In the yielding region, the hydrate-cemented cluster structure is completely damaged and crushed into small pieces; a shear band with a determined thickness and inclination angle was observed.

In this study, a microfocus X‐ray computed tomography‐based triaxial testing apparatus was used to observe and quantify the microstructure evolution of hydrate‐bearing sands during thermal dissociation of hydrate. Three triaxial shear tests with X‐ray computed tomography were conducted to study the influence of hydrate dissociation on the mechanical behavior of hydrate‐bearing sands. The results show that hydrate covering the sand particle surface dissociates first and then at the menisci between sand particles. The secondary hydrate formation mainly occurs on the menisci between sand particles and the surfaces of the hydrate shells where hydrates already exist. Hydrate dissociation could cause fabric changes in hydrate‐bearing sands, resulting in a more isotropic orientation distribution of sand particles. The failure behavior of the hydrate‐free sand specimen is similar to that of the specimen after hydrate dissociation, which shows an obvious drum‐shaped failure pattern with X‐shaped shear bands. However, the hydrate‐bearing sand specimen exhibits a shear band with a determined thickness and inclination angle. Hydrate dissociation could cause a significant loss of supporting cementation, resulting in a decline in the stiffness and failure strength. The failure strength of hydrate‐free sand specimen is slightly higher than that of the specimen after hydrate dissociation; this may due to that the homogeneous orientation frequency distribution can enhance the stability of the force chain between sand particles. The secondary hydrate formation causes sediment deformation resulting in a decrease in absolute permeability due to pore blockage.

Gas hydrate-bearing sediment shows complex mechanical characteristics. Its macroscopic deformation process involves many microstructural changes such as phase transformation, grain transport, and cementation failure. However, the conventional gas hydrate triaxial testing apparatus is not possible to obtain the microstructure in the samples. In this study, a novel, low-temperature (−35 to 20 °C), high-pressure (>16 MPa confining pressure and >95.4 MPa vertical stress) triaxial testing apparatus suitable for X-ray computed tomography scanning is developed. The new apparatus permits time-lapse imaging to capture the role of hydrate saturation, effective stress, strain rate, hydrate decomposition on hydrate-bearing sediment characteristic, and cementation failure behavior. The apparatus capabilities are demonstrated using in situ generation of hydrate on a xenon hydrate-bearing glass bead sample. In the mentioned case, a consolidated drained shear test was conducted, and the imaging reveals hydrate occurrence with a saturation of 37.3% as well as the evolution of localized strain (or shear band) and cementation failure along with axial strain.

Natural gas hydrate is considered to be a promising future energy resource; therefore, obtaining its physical properties is crucial for evaluating gas production efficiency and developing reasonable exploitation strategies. In this study, we present a novel pore-scale 3D morphological modeling algorithm considering various saturation and occurrences (cementing and pore-filling) of hydrate in sediments. To evaluate the performance of the presented algorithm, 14 hydrate-bearing sediment models were constructed based on X-ray computed tomographic images of a consolidated sandy specimen under an effective stress of 3 MPa. Morphologically, the new algorithm generated pore-scale hydrate occurrences coincide with published morphological behaviors of hydrate observed via computed tomography. The tortuosity and fractal dimension of pore spaces of these models were then characterized. The pore networks are also extracted, based on which the evolution of the pore characteristics including the distributions of pore radius, throat radius, throat length, and the coordination number were investigated. Using these generated models, simulations of permeability, thermal conductivity, and electrical conductivity were conducted to evaluate the influences of hydrate saturation and occurrence. These results are validated against published data, demonstrating that this algorithm could be an effective way to construct digital hydrate-bearing sediment models using a single set of computed tomographic images of a hydrate-free specimen. This new method can also be significant for the physical evaluation of natural cores in which hydrate has dissociated during core recovering and transferring.

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