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.

A better understanding of the mobility of the CO2 phase and the sweep within the CH4 hydrate-bearing sediments is required for the success of CO2 storage and concurrent CH4 production. In this work, we investigate the injectivity of CO2 in CH4-hydrate-saturated porous media and the subsequent dissociation of CH4 / CO2 mixed hydrate at the pore level. A total of six pore-scale visualization experiments were performed using a high-pressure water-wetted silicon wafer-based micromodel whose pore network resembles a cross section of sandstone. Liquid CO2 was injected at a constant volumetric rate of 0.2-0.5 ml / hour at a high saturation of CH4 hydrate (SH = 0.81-0.99) at P = 59-69 bar and T = 3.3-4.5°C. The results confirmed the presence of two different hydrate arrangements at the end of the CO2 injection, such that the hydrate phase change and liquid distribution were influenced by the invasion behaviour of the liquid phase and the initial distribution of CH4 hydrate. Invasion of the CO2-rich liquid phase resulted in the formation of massive hydrates without residual liquid saturation. While the CH4 rich liquid phase invaded the hydrates and produced an excess of the liquid phase in the field of view. Later, massive hydrates were dissociated by stepwise depressurization, with multiple dissociation and reformation recorded between the thermodynamic stability pressure of pure CH4 hydrates and pure CO2 hydrates, supported by the presence of the liquid phase. Continuous mobilization of liquid phase and mixing of liquid and gas phases led to localized hydrate reforming below the CH4 hydrate stability pressure. This is the first pore-scale visualization of CO2 injection into CH4-hydrate saturated porous media and ManuscriptClick here to view linked References demonstrates the feasibility of combining CO2 Injection into CH4 hydrate with stepwise depressurization to produce CH4 gas.