Kinetics of CO2 Hydrate Formation and Chemical-Potential-Driven Dissociation in Porous Media

Gas hydrates can form when water and a guest gas coexist within the hydrate stability field. Gas hydrates in porous rocks block flow fields. During CO2 storage in depleted reservoirs, pressure reduction and Joule–Thomson cooling may place near-well regions within the CO2 hydrate stability zone, raising concerns about injectivity loss. Hydrate stability is commonly inferred from the bulk pressure–temperature (P–T) phase diagram, which neglects flow and gas humidity transport under sustained flow. Using time-resolved optical microscopy in a “fish-bone” micromodel at fixed P and T, we find that injecting low-humidity CO2 produces a pore-scale cycle of hydrate formation and dissociation. The hydrate nucleates along gas–water–solid contact lines and grows by consuming CO2 dissolved in trapped pore water and then slows as CO2 supply becomes diffusion-limited through the hydrate layer. Despite constant bulk P and T, the low humidity of the injected CO2 extracts water as vapor, creating a water chemical potential deficit at the gas–hydrate interface and driving an inlet-to-outlet dissociation front. These results show that hydrate persistence under flow depends on pore-scale transport and gas-side water chemical potential and cannot be inferred from bulk P–T stability conditions alone.