Pore-scale formation of CO₂ hydrates in sandstone and global assessment of hydrate-based CO₂ storage potential in marine sediments

Hydrate-based geological storage of CO₂ is a solid-state CCUS technology characterized by high thermodynamic stability and long-term safety, and  is therefore regarded as a promising pathway for large-scale CO₂ sequestration. Sandstone formations widely distributed in marine sediments provide substantial pore volume and are considered favorable targets for CO₂ storage. In this study, the pore-scale formation behavior of CO₂ hydrates in sandstone was systematically investigated, and the global CO₂ storage potential in marine sandstones was further assessed.

In the first part of this work, sandstone pore structures were characterized using thin-section petrography, mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR) measurements. The NMR-derived pore size distribution was calibrated against the MIP results, showing excellent agreement (R² = 99.5%) and indicating that the pore sizes of the tested sandstone mainly ranged from 0.005 to 500 μm. Subsequently, in situ CO₂ hydrate formation experiments were conducted using an NMR-based hydrate formation and monitoring system at temperatures of 1–7 °C and pressures of 2–8 MPa, revealing both the kinetic and thermodynamic characteristics of CO₂ hydrate formation in micro- and nanopores. In the second part, global standard datasets were employed to estimate the volume of marine sedimentary sandstones suitable for hydrate-based CO₂ storage, and these results were combined with the water-to-hydrate conversion ratios obtained from laboratory experiments to quantify the total amount of CO₂ that could be stored in marine sediments.

The results indicate that due to the combined effects of pore confinement and the Kelvin effect, the equilibrium pressure of CO₂ hydrates at 7 °C in pores with a pore diameter of approximately 10 nm is elevated from about 2.87 MPa under bulk conditions to 6–8 MPa. When pore sizes exceed 0.1 μm, the influence of pore size on hydrate formation efficiency becomes negligible. Moreover, the large specific surface area provided by rock pores (2.186 m²/g for the samples used in this study) substantially reduces the nucleation energy barrier, leading to rapid hydrate formation kinetics, with all experimental groups reaching approximately 90% of the final conversion within 140 min. Under the investigated pressure–temperature conditions, the water-to-hydrate conversion ratio in pores larger than 0.1 μm ranges from 0.35 to 0.85.

Global-scale estimation suggests that the effective volume of marine sediments suitable for CO₂ hydrate formation within water depths shallower than 4000 m is approximately 3.48 × 10¹⁵ m³. Assuming a sandstone fraction of 1%, the corresponding theoretical CO₂ storage capacity reaches about 1324 Gt, which is close to half of the cumulative anthropogenic CO₂ emissions since the Industrial Revolution. This study provides strong scientific support for large-scale and safe geological sequestration of CO₂ and offers a potential technological pathway toward achieving global carbon neutrality.