Insights into multistep depressurization of CH₄/CO₂ mixed hydrates in unconsolidated sediments

Natural gas hydrates (NGH) are potential energy resource and distributed in unconsolidated marine sediments. CH₄–CO₂ swapping combined with depressurization is a promising method for simultaneously enhancing CH₄ recovery and CO₂ storage in NGH. This combined method was previously studied on CH₄/CO₂ mixed hydrates in consolidated sediments but no such studies are available for unconsolidated sediments. In this study, we investigated dissociation characteristics of CH₄/CO₂ hydrates in unconsolidated sediments using slow multistep depressurization. CH₄/CO₂ hydrates were formed in unconsolidated sandpack. The effects of CH₄/CO₂ molar ratio (CH₄-rich and CO₂-rich) in hydrate, residual water saturation (9.2% to 31.7%) and reservoir temperature (−3.3 °C to 2.2 °C) on production and storage efficiencies were mainly investigated, and the results were compared to those of bulk water and consolidated sediments in literatures. The results showed that CO₂ was preferably encaged in hydrate than CH₄ and this phenomenon was most apparent in unconsolidated sediments. CH₄-rich hydrates in unconsolidated sediments were recommended to be exploited by multistep depressurization. Higher residual water saturation was unbeneficial to CH₄ gas recovery, while a lower value was not conducive to CO₂ storage in hydrate. Temperature below or above freezing point mainly decided ice formation and affected CH₄ gas yield and CO₂ retention by mass transfer. CH₄ recovery was primarily dependent on degree of depressurization with higher CH₄ recovery percent at lower depletion pressure. The maximum increment of CH₄ concertation (6.5 mol%) and the highest CO₂ storage efficiency (88.2%) were achieved in CH₄-rich hydrates at optimum residual water saturation of 17.7% and temperature above 0 °C. In addition, sediments were the key factor controlling production and storage performances, and unconsolidated sediments provided a larger contact area among CO₂ gas, CH₄ hydrate and mobile water than consolidated sediments and bulk water. The results in this work could be useful for future NGH exploitation after CO₂ injection into CH₄ hydrates in unconsolidated sediments.