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.

The pilot test of infilling polymer-surfactant-preformed particle gel (PPG) flooding has been successfully implemented after polymer flooding in Ng3 block of Gudao Oilfield in China. However, the production characteristics and displacement mechanisms are still unclear, which restricts its further popularization and application. Aiming at this problem, this paper firstly analyzes the production performance of the pilot test and proposed four response types according to the change of water cut curves, including W-type, U-type, V-type response, and no response. Furthermore, the underlying reasons of these four types are analyzed from the aspects of seepage resistance and sweep efficiency. The overall sweep efficiency of gradual-rising W-type, gradual-decreasing W-type, and early V-type response increases from 0.81 to 0.93, 0.55 to 0.89, and 0.94 to 1, respectively. And the sum of seepage resistance along the connection line between production well and injection well for U-type and delayed V-type response increases from 0.0994 to 0.2425, and 0.0677 to 0.1654, respectively. Then, the remaining oil distribution after polymer flooding is summarized into four types on the basis of production and geological characteristics, namely disconnected remaining oil, streamline unswept remaining oil, rhythm remaining oil, and interlayer-controlled remaining oil. Furthermore, the main displacement mechanisms for each type are clarified based on the dimensionless seepage resistance and water absorption profile. Generally, improving connectivity by well pattern infilling is the most important for producing disconnected remaining oil. The synergistic effect of well pattern infilling and polymer-surfactant-PPG flooding increases the dimensionless seepage resistance of water channeling regions and forces the subsequent injected water to turn to regions with streamline unswept remaining oil. The improvement of the water absorption profile by polymer-surfactant-PPG flooding and separated layer water injection contributes to displacing rhythm remaining oil and interlayer-controlled remaining oil. Finally, the paper analyzes the relationships between the remaining oil distribution after polymer flooding and production characteristics of infilling polymer-surfactant-PPG flooding. The study helps to deepen the understanding of infilling polymer-surfactant-PPG flooding and has reference significance for more commercial implementations in the future.