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

Abstract

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.