Stability of submarine slopes during replacement of methane in natural gas hydrates with carbon dioxide

Replacement of methane (CH4) in natural gas hydrates (NGHs) with carbon dioxide (CO2) can sequestrate CO2 in the seabed while developing the natural gas, so it is an environmental-friendly energy production and carbon sequestration mode. A dynamic evolution model for the mechanical parameters of a NGH reservoir during CH4-CO2 replacement was established. Then, the finite element method was adopted to build a temperatureseepage-stress-saturation multi-field coupling model for CH4-CO2 replacement in the NGH reservoir to study the evolution of physical parameters of strata and the slope stability in the replacement process. Results demonstrate that mechanical parameters including the peak strength, elastic modulus, and cohesion of NGH and CO2 hydrate (CDH) bearing sediment samples are all positively correlated with the saturation. The generation of CDHs leads to a decrease in permeability of the reservoir during CH4-CO2 replacement, and the CO2 injection rate gradually slows down with the prolonging injection time of CO2. During CH4-CO2 replacement, the slope is more prone to failure with the increasing submarine slope angle, decomposition range of NGHs, reservoir thickness, and initial saturation; with the enlargement of the generation range of CDHs and the thickness of overlaying strata, the slope is more stable.