Novel Pore-Scale Visualization during CO₂ Injection into CH₄ Hydrate-Saturated Porous Media

The current literature lacks visualization studies that could improve our understanding of fluid migration and hydrate rearrangement during CO₂ injection into a CH₄ hydrate. CO₂ injection into a CH₄ hydrate has been proposed as a novel technique to store CO₂ while generating CH₄. However, to improve this process, a better understanding of the pore-level processes is needed. This experimental study is the first to provide pore-level visualization (using a high-pressure micromodel) when liquid CO₂ (0.1–0.5 mL/h) is injected into gas-saturated CH₄ hydrates (S_H = 0.81–0.99, P = 59–69 bar, and T = 3.3–4.5 °C). CO₂ injection leads to invasion/displacement of the liquid phase within the gas-saturated CH₄ hydrate, resulting in thickening of the hydrate film and crystallization of the liquid phase. After injection, the crystalline hydrate phase with and without the liquid phase was observed and no residual gas phase was observed, indicating that the gas phase was consumed during hydrate formation. The gas-phase saturation and low injection rate controlled the hydrate formation mechanism, delayed liquid-phase crystallization to the hydrates, and contributed to a slower pressure buildup in the micromodel. The resulting CH₄/CO₂ mixed hydrates were dissociated by stepwise depressurization. During depressurization, several dissociation and reformation events were observed below the CH₄ hydrate stability pressure. The process was supported by continuous mobilization and mixing of the liquid and gas phases. The reforming process enhanced the CO₂ concentration in the hydrate phase. The entire hydrate system dissociated closer to the equilibrium pressure of pure CO₂ hydrates, suggesting the presence of hydrate mass below the stability pressure of the CH₄ hydrate. The results demonstrate the effects of the CO₂ injection scheme on the evolution of morphology in the CH₄ hydrate system, which is relevant to the proposed production method (CO₂ injection into CH₄ hydrate reservoirs is followed by a slow depressurization) to improve CH₄ gas recovery without the loss of hydrate mass.