Capillarity-Driven Hydrate Film Formation in Geologic Carbon Storage

Abstract

Much of the continental margins in the world oceans provide the necessary thermodynamic conditions to store CO2 as ice-like hydrates (CO2·6 H2O). While resistant to buoyant migration and leakage, the fundamental growth mechanisms that control the injection, capacity, and security of CO2 hydrates stored in the seafloor remain unresolved. Extensive field and laboratory testing give rise to conflicting views on the kinetics and growth configurations of hydrates, where mechanistic models reconciling the formation of hydrates observed in nature remain missing. This work elucidates a fundamental pore-scale reactive transport mechanism that underpins the rate and morphology of hydrate formation. We reveal a previously unrecognized mode of hydrate formation in porous seafloor sediments, hydrate film growth via reaction-imbibition, where superhydrophilic hydrate crystallites (θ∼0∘) formed at water–CO2 interfaces create a secondary microporous medium (∼ 10 to 100 nm pores) within lithologic sediment pores (∼ 10 to 100 μm pores) to promote further hydrate growth. Unlike past diffusion-controlled models, we show that spontaneous water imbibition into the hydrate micropores establishes rapidly new water–CO2 interfaces (i.e., hydrate formation surfaces) via capillary-driven convection and is the dominant mechanism for supplying water to the hydrate formation interface.