Creep Response of Hydrate-Bearing Sediments during Gas Replacement: from Strain Deformation to Cementation Morphology Evolution

Understanding the deformation behavior, evolution of pore and cementation structures, and permeability changes of hydrate-bearing sediments (HBSs) during the gas replacement-stress coupling process is crucial for ensuring efficient gas replacement and maintaining structural stability. In this study, multistage creep tests, continuous in situ CT imaging combined with digital rock techniques, and pore network modeling were employed to systematically elucidate the evolution mechanisms of pore structure and fluid flow capacity, as well as hydrate morphology and cementation strengthening. Key findings include: (1) under gradually increasing axial load, HBSs initially exhibit stable deformation. However, once a critical strain threshold is exceeded, accelerated strain leads to strength failure. (2) Significant changes occur in pore structure and hydrate morphology throughout the process. Extensive crushing of hydrate particles at the early stage reduces both porosity and hydrate volume ratio, followed by an interplay of compaction, spalling, and recementation that increases hydrate volume. (3) Under the combined effects of gas replacement and stress, the hydrate cementation structure undergoes a distinct damage-repair process, resulting in a strengthened cementation mechanism. (4) Permeability simulations reveal that permeability dramatically decreases from approximately 95–110 D to about 0.015 D, indicating that enhanced cementation and irreversible deformation severely impede fluid transport. These findings provide a robust framework for improving gas replacement efficiency in hydrate reservoirs and ensuring their mechanical stability.