Effect of Organic Matter on CO₂ Hydrate Phase Equilibrium in Phyllosilicate Suspensions

In this study, we examined various CO₂ hydrate phase equilibria under diverse, heterogeneous conditions, to provide basic knowledge for successful ocean CO₂ sequestration in offshore marine sediments. We investigated the effect of geochemical factors on CO₂ hydrate phase equilibrium. The three-phase (liquid–hydrate–vapor) equilibrium of CO₂ hydrate in the presence of (i) organic matter (glycine, glucose, and urea), (ii) phyllosilicates [illite, kaolinite, and Na-montmorillonite (Na-MMT)], and (iii) mixtures of them was measured in the ranges of 274.5–277.0 K and 14–22 bar. Organic matter inhibited the phase equilibrium of CO₂ hydrate by association with water molecules. The inhibition effect decreased in the order: urea < glycine < glucose. Illite and kaolinite (unexpandable clays) barely affected the CO₂ hydrate phase equilibrium, while Na-MMT (expandable clay) affected the phase equilibrium because of its interlayer cations. The CO₂ hydrate equilibrium conditions, in the illite and kaolinite suspensions with organic matter, were very similar to those in the aqueous organic matter solutions. However, the equilibrium condition in the Na-MMT suspension with organic matter changed because of reduction of its inhibition effect by intercalated organic matter associated with cations in the Na-MMT interlayer.