Direct visualization of CH₄/CO₂ hydrate phase transitions in sandstone pores

This paper reports the formation and dissociation pattern of hydrate crystals with varying compositions of CH₄ and CO₂ in porous media. Direct visualization was carried out using a high-pressure, water-wet, silicon wafer-based micromodel with a pore network resembling sandstone rock. Hydrate crystals were formed under reservoir conditions (P = 45–65 bar and T = 1.7–3.5 °C) from either a two-phase system consisting of liquid water and a CH₄–CO₂ gas mixture or a three-phase system consisting of liquid water, CH₄-rich gas, and CO₂-rich liquid. A stepwise pressure reduction method was later applied to visualize multiple dissociation events occurring between the equilibrium pressures of pure CH₄ hydrates and pure CO₂ hydrates. The results showed that liberated gas from the initial dissociation became trapped and immobilized by surrounding undissociated hydrate crystals when the initial hydrate saturation was high. Mixing of liberated gas with liquid water led to rapid reformation of hydrates during the stepwise pressure reduction; the reformed hydrate crystals dissociated at a lower pressure close to the equilibrium pressure of pure CO₂ hydrates. The results demonstrate the possibility of producing gas liberated from local hydrate dissociation while simultaneously reforming hydrates in other parts of the sediments. This is relevant for the proposed production method where CO₂ injection in CH₄ hydrate reservoirs is followed by pressure depletion to enhance the CH₄ gas recovery.