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 (delta NR2) were dependent on S-rw and shut-in period. Positive increase of delta NR2 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.