Combined replacement and depressurization methane hydrate recovery method

A new method for exploiting natural gas from methane hydrates, which combines carbon dioxide replacement and depressurization, was investigated in a 288-ml high-pressure vessel of 288 ml. Experimental results with and without depressurization were compared to reveal the effects of depressurization on the final replacement percentage. Results showed that the CH4–CO2 replacement reaction could be divided into two stages. The first stage lasts for about 4 h and can be described as the surface reaction stage; the reaction in this stage is rapid, and the results fit the Avrami model. In the second stage, the rate of the reaction is slow due to hydrates blocking the diffusion pathways. As the replacement reaction is restricted by the diffusion and transportation of CO2, partial melting of methane hydrates through depressurization enhances the second stage. This is because the melting of methane hydrates provides pathways for CO2 to penetrate into the inner layer of the hydrates. The combined method improves the replacement percentage. However, the effects of depressurization on the stability of the sediments remain to be explored.

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