A model for threshold pressure gradient in hydrate-bearing sediments during creep

The flow behaviors of gas and water in hydrate-bearing sediments (HBS) are significantly affected by the threshold pressure gradient (TPG). During long-term natural gas hydrates (NGHs) mining, there exists creep deformation in HBS, which significantly alters pore structures, makes the flow path of fluid more complex, and leads to changes in TPG. Thus, clarifying the evolution of TPG in HBS during creep is essential for NGH production, but it also confronts enormous challenges. In this study, based on the nonlinear creep constitutive model, a novel theoretical TPG model of HBS during creep is proposed that considers pore structures and hydrate pore morphology. The established model is validated against experimental data, demonstrating its ability to capture the evolution of TPG and permeability in HBS during creep. Additionally, the relationship between initial hydrate saturation and TPG of HBS during creep is revealed by sensitivity analysis. The creep strain increases with the decrease in initial hydrate saturation, leading to a greater TPG and a lower permeability. The evolution of TPG at the stable creep stage and the accelerated creep stage is primarily controlled by the Kelvin element and visco-plastic element, respectively. This novel proposed model provides a mechanistic understanding of TPG evolution in HBS during creep, and it is of great significance to optimize the exploitation of NGHs.