Natural gas hydrate is a relatively realistic alternative energy source to conventional fossil fuels with considerable reserves. Natural gas hydrate sediments are widely distributed in marine sediment on continental margins. In this study, a numerical modeling method for sediment containing nodular gas hydrates is developed using the two-dimensional discrete element simulation software. The effects of saturation, confining pressure, and nodule radius on the mechanical properties of heterogeneous nodular gas-hydrate-bearing sediment were analyzed using the stress-strain, fracture development, and partial body strain curves, as well as force chain distribution. The results indicated that the mechanical strength of sediment containing round nodular gas hydrates was proportional to the gas hydrate saturation and simulated confining pressure. When hydrate saturation was low, the failure strength of the gas-hydrate-bearing sediment diminished as the nodule radius increased. The simulations showed that variations in sediment porosity influenced the development and evolution of the shear band, resulting in higher porosity around the shear band. These results were analyzed from the perspectives of saturation and confining pressure to determine the failure and deformation law of simple nodular gas hydrate-bearing sediment and provide theoretical support for the subsequent study of the exploitation method of shallow buried deep gas hydrates.

In order to study the synthesis and decomposition characteristics of natural gas hydrates, the exploitation performance of the reservoir, and the sand production and control during the mining process of natural gas hydrates, a set of multi-dimensional natural gas hydrate production simulation test system has been developed. The test system is mainly composed of gas injection system, constant pressure liquid supply system, steam or hot water injection system, 1D, 2D&3D model, back pressure control system, outlet metering system, data acquisition and control processing system and corresponding auxiliary systems. The main innovations of the experimental system are as follows: 1) The one-dimensional reactor wall made of high-pressure glass and the high-definition camera are combined to achieve visualization for monitoring the synthesis and decomposition of hydrate and the sand migration in the production process under medium and low pressure. 2) The self-developed double-cylinder constant speed and pressure pump is used to control the back pressure valve, so that the pressure in the reactor can be uniformly reduced at a decrement of higher precision during the entire depressurizing production process. Consequently, the gas hydrate reformation is greatly reduced, and the gas production rate becomes more uniform. Using quartz sand of different particle sizes, deionized water and methane gas with a purity of 99.9% as raw materials，the one-dimensional phase equilibrium test and two-dimensional depressurizing production test are conducted. The reliability of the test system is verified based on the primary application results from these tests.