Pore-scale visualization of hydrate phase transitions during CO₂ Injection into CH₄ hydrate saturatedporous media

In this work, we visualize pore-scale hydrate phase transitions during CO₂ injection into CH₄ hydrate-saturated porous media. A total of six visualization experiments were performed using a water-wet, high-pressure silicon wafer-based micromodel with a pore network resembling a cross-section of sandstone. Liquid CO₂ was injected at a constant volumetric rate of 0.2-0.5 mL/hour into the pores already saturated by CH₄ hydrates at P = 59-69 bar and T = 3.3-4.5°C. The initial saturation of CH₄ hydrates was high (SH = 0.81-0.99), while the rest of the pore space was saturated by liquid water and CH4 gas. The results showed that the hydrate phase changes were influenced by the initial liquid distribution and the amount of CO₂ injected. A low CO₂ injection rate formed massive hydrates surrounded by CO₂-rich fluid, while high CO₂ injection rates and ultimate injection volumes led to the formation of massive hydrates without residual fluid saturation. Later, stepwise depressurization was performed to visualize the dissociation and reformation patterns of the resulting hydrates. Multiple hydrate dissociations and reformations were observed between the stability pressure of pure CH₄ hydrates and pure CO₂ hydrates. Migration of pore water and mixing of fluids caused localized hydrate reforming at lower pressures. This is the first pore-scale visualization of CO₂ injection into CH₄ hydrate-saturated porous media and demonstrates the feasibility of combining CH₄/CO₂ exchange with pressure reduction to produce CH₄ gas.