A Hydrate-Bearing Sediment Gas Replacement Mechanical Behavior Regulation Mechanism and Slope Stability Analysis

Deep-water shallow gas resources face challenges such as the absence of dense caprock and weak cementation, which lead to a narrow formation safety pressure window and poor long-term stability. Therefore, developing a CO2-based gas replacement strategy is essential to ensure the formation stability during oil and gas exploitation. CO2 replacement facilitates geological storage and hydrate exploitation simultaneously, reinforcing the slope stability while preserving cementation. In this study, a series of triaxial shear tests on sediments under varying replacement percentages, saturation levels, and effective confining pressures were conducted, simulating slope stability based on the derived mechanical characteristics. The results revealed the following: (1) the gas replacement process enhances the failure strength and stiffness of sediments. The strength criteria show that the internal friction angle is influenced solely by sand particle composition, while cohesion has an exponential relationship with saturation and a linear relationship with replacement percentage. (2) In terms of strain characteristics, within the critical state model, the slope M of the critical state line on the q–p′ plane increased as hydrate saturation rose. The Ψ value on the v-ln p′ plane is consistently negative, and gas replacement results in higher ln p′ values and lower v at the failure state point. As saturation increases, the Γ value of the critical state line decreases, while the λ value increases. (3) For slope simulations, increased hydrate saturation significantly raises the safety factor for gentler slopes, while the reinforcing effect of gas replacement is weaker for steeper slopes with higher saturation.