Multiscale investigation of hydrate pore habits and microscopic seepage mechanisms for optimizing CO2 storage in high-porosity sediments

Hydrate-based CO₂ storage (HBCS) has emerged as a promising approach within carbon capture and storage (CCS) technologies, offering a potential pathway for mitigating CO₂ emissions. The permeability of hydrate-bearing sediments (HBSs) is crucial for predicting the feasibility of HBCS, yet the seepage mechanisms become quite complex during the formation or dissociation of hydrates. The objective of this paper is to investigate the hydrate pore habits and the evolution of key physical properties at both macroscopic and microscopic scales, utilizing an integrated set of methodologies such as CT scan, nuclear magnetic resonance and ultrasonic pulse methods, and present a theoretical equation that correlates P-wave velocity with permeability for HBSs. The results indicate that the theoretical equation effectively explains the correlation between P-wave velocity and permeability. Formations with a lower initial water saturation and a higher proportion of type Ⅱ pores (ranging from 0.1 to 200 μm) were found to promote high-efficiency CO₂ storage. During CO₂ hydrate formation, the pore structure becomes progressively more complex, and hydrates are initially formed in the pore center at saturations around 0-10 %. The occurrence pattern transitions from a pore-filling pattern to a grain-coating pattern as hydrate saturation increases. The theoretical equation relating P-wave velocity to permeability is useful for assessing the permeability of HBSs. These findings provide valuable insights into HBCS in porous media and contribute to the selection of potential CO₂ storage sites.