Critical Parameters Influencing Mixed CH₄/CO₂ Hydrates Dissociation during Multistep Depressurization

Early studies show that multistep depressurization of CH₄/CO₂ mixed hydrates generates additional CH₄ while storing CO₂ in hydrate-bearing sediments. There are many critical factors that could affect the production and storage efficiency of this method. However, it is unclear how to achieve high efficiency in both CH₄ production and CO₂ storage by controlling these critical parameters. In this experimental work, we identified three critical parameters (CH₄/CO₂ ratio in mixed hydrates, residual water saturation (S_rw), and shut-in period) and investigated their effects on production parameters (CH₄ molar fraction in the gas phase (XCH₄), CH₄ recovery percentage (RCH₄), and CO₂ storage ratio (SCO₂)). Experiments were performed on sandstone cores using a high-pressure core flooding system equipped with pressure, temperature, and electrical resistivity measurements. Gas composition was analyzed by gas chromatography. The results showed that the optimal production parameters were determined at low S_rw of 43.7–47.4% and higher CH₄/CO₂ ratio of 1.76–2.06 in CH₄/CO₂ mixed hydrates. The optimized values were obtained at the equilibrium pressure of CH₄/CO₂ hydrate system at a specific reservoir temperature without water production during pressure release. In addition, the period between two pressure releases had a direct effect on the production and storage performances, and the most efficient production and storage was measured at 4-hour shut-in period of pressure release. The measured percent changes in normalized resistivity (ΔNR₂) were dependent on S_rw and shut-in period. Positive increase of ΔNR₂ indicated increased hydrate saturation or improved water gas distribution in the sediment during multistep depressurization. The results demonstrated the importance of three critical parameters in designing an effective production and storage scheme after CO₂ injection into CH₄ hydrates.