Characteristics and morpholog y of CH₄/CO_2/N₂ mixed hydrates using multistep depressurization after hydrate swapping

Natural gas hydrates are a potential ly very large sourc e of energy in the near future. CH_4-CO₂ swapping is a kind of win win exploitation method to produce CH4 from hydrate reservoir s while simultaneously storing CO₂ To mitigate the problem of low efficiencies of CH₄ production and CO₂ storage, CH₄-CO₂ swapping combined with depressurization has been investigated intensively However, few studies focus on multistep depressurization for mixed CH₄/CO₂/N₂ hydrates after swapping. This work presents characteristics and morphology of well controlled multistep depressurization after CO₂/N₂/ or CO₂/air gas injection into CH₄ hydrates with a view to exploiting this large resource A high pressure cell was used to form CH₄ hydrate wi thin unconsolidated sand. Mixed gas of CO₂/N₂ or CO₂ air was then injected into the cell to perform the swapping step Subsequently pressure release was conducted in steps to produce gas mixtures Temperatures at the top and bottom as well as pressure in the cell were recorded. The gas compositions collected from produced gas at each multistep depressurization were analyzed via gas chromatography. The morphology of mixed hydrate dissociation was studied separately in a sapphire cell using same procedures with temperature, pressure, gas compositions and real time  photos recorded correspondingly The effects of CO₂ concentration, gas release position, air composition and L methionine on CH₄ recovery, CO₂ storage and mixed hydrate morphology were comprehensively investigated in the high pressure cell and the sapphire cell. Three evaluation parameters, i.e. CH₄ or CO₂ mole gas fraction X_CH4/X_CO2 CH₄ recovery percent (RCH₄ and CO₂ storage ratio (SCO₂₎ were calculated to quantitatively analyze the exploitation efficiency.