Micro-pore structures determine the macro-mechanical behaviors of porous media, whereas their quantitative linkages remain ambiguous owing to the limitations of test techniques, especially for hydrate-bearing sediments. This study proposes an integrated approach that combines low-field nuclear magnetic resonance high-frequency detection with triaxial shearing, enabling the in-situ simultaneous monitoring of both the macro-mechanical parameters and micro-pore structures. The device, which consists of a high-pressure specimen vessel, a low-field nuclear magnetic resonance measurement module, a temperature and pressure control module, and a data acquisition module, allows the real-time acquisition of transverse relaxation time distribution and magnetic resonance images during triaxial loading, facilitating the detection of pore water distribution and crack development. Preliminary verification illustrates the high reliability of the device. Under relatively low strain, the signal intensity ratio of micropores rises with a transverse relaxation time of less than 10 ms, while that of macropores decreases gradually. Conversely, the signal intensity ratios for both micropores and macropores present the opposite tendency with the strain exceeding 5.1%. Besides, with the axial strain rising from 0 to 15%, there is an increase of about 16.9% in the peak area of macropores. Randomly distributed cracks observed under triaxial shearing correspond to the increasing peak area and signal intensity ratio of macropores, which is verified by comparing the magnetic resonance and computerized tomography images. This method provides a new possibility for characterizing the failure processes of hydrate-bearing sediments and establishing macro-to-micro equivalent relationships, enhancing the applications for porous media containing phase-reversible agents.

Natural gas hydrate is an alternative potential energy source that contributes to depressurizing the pressure of energy supply and environmental pollution in the future. Field hydrate production has a close association with geological risks. In this regard, accurate estimation of strength and deformation properties is crucial to risk prevention and control during hydrate development. However, the geomechanical properties of hydrate-bearing sediments and their applications remain unclear. Herein, this work provides a comprehensive summary of studies on the mechanical characteristics of hydrate-bearing sediments and their applications in field trials. It starts with the main research methods, including laboratory tests, constitutive modeling, and numerical simulations, followed by the effects of clay content, hydrate distribution, and morphology on mechanical properties. Besides, typical applications of geomechanical parameters are examined and discussed. Finally, the challenges and perspectives of mechanical studies on hydrate-bearing sediments are presented, which is favorable for the evaluation and control of geological risks during hydrate exploration and development.

The experimental testing and analysis of strength and deformation characteristics of hydrate reservoirs is an integral part of natural gas hydrate exploitation. However, studies so far have failed to deeply explore samples from the South China Sea. Especially, there is a lack of a simple and applicable method to estimate their mechanical behaviors. Thus, based on test data, an improved Duncan-Chang model is established in this paper to characterize the strength and deformation of reconstituted samples with various hydrate saturation and stress states from this area. This model can accurately describe the strain-hardening characteristics, and failure strength is estimated by the improved Drucker-Prager criterion with high fitting accuracy. The initial elastic modulus and failure ratio are given by the proposed empirical models, which are obtained from experimental data and fitting methods. Generally, this model has several advantages including simple structure, favorable performances, and a limited number of model parameters. Therefore, it could be widely used in strength and deformation analysis. This study can support the prevention and control of geological risks during natural gas hydrate exploitation in the South China Sea.

As a promising substitute for conventional fossil fuels with huge reserves, clayey-silt natural gas hydrate has been proved to be widely distributed in the continental margins of the marine environment. Characterization and development of this kind of natural gas hydrate reservoirs face unique challenges, compared with that of natural gas hydrate in marine sandy sediments. This review summarizes the basic methods for natural gas hydrate reservoir characterization and development, and discusses the applicability of these methods in marine clayey-silt natural gas hydrate reservoirs. Feasibilities of classical oil and gas reservoir characterization methods and models applied to hydrate-bearing strata remain elusive, let alone clayey-silt hydrate deposits. Current natural gas hydrate development methods are restricted by low gas productivity, potential geomechanical instability, and extremely high costs. Economically feasible technologies considering the influences of geotechnical issues are needed for the commercialization of natural gas hydrate contained in clayey-silt sediment.