Study On The Permeability Characteristics Of Methane Hydrate Cap Reformed By CO₂/N₂ Injection

The overburden on the hydrate reservoir is mostly characterized by weak cementation and no diagenesis.If the conventional direct depressurization mining method is used,the gas production process will be accompanied by a large amount of water production,which leads to low mining efficiency.So,a new idea,injecting CO₂/N₂mixed gas to reform the hydrate reservoir,is proposed to improve the gas production rate and methane recovery ratio during the depressurization process.The specific contents are as follows:Several factors,such as sediment type,clay content,initial water saturation,and pressure,were investigated to understand their influence on the formation kinetics of CH₄hydrate in the sediments.Studies have shown that the smaller the particle size of quartz sand,the higher the hydrate saturation.By contrast,the saturation of the generated hydrate in sea mud is much lower than that of quartz sand sediments.At the same time,the high clay content,initial water saturation and formation pressure all contribute to the higher saturation of hydrate.In view of the permeability characteristics of the sediment containing CO₂ hydrate,constant flow method was applied in this article to measure and compare the experimental results with published models.When the CO₂ hydrate saturation is less than 10%,the permeability of the sediment containing CO₂ hydrate changes exponentially and is closer to the parallel capillary model where the hydrate occupies the center of the pore.When the hydrate saturation is greater than 10%,the change in permeability begins to slow,which is better consistent with the Masuda permeability decline model(n=12).The gas hydrate reservoir containing clay was reformed by CO₂/N₂ injection in the one-dimensional device.The results show that when the molar proportion of CO₂ in the mixed gas was increased from 50%to 75%,the water blocking effect of the generated carbon dioxide hydrate cap layer was improved,and the water production during the experiment was further reduced.However,due to the high proportion of CO₂ in the injected mixed gas,there is a risk of mass production of CO₂ in the middle of mining.At the same time,due to the small amount of infiltrating seawater in the middle of mining,the decomposition of methane hydrate is gradually controlled by the influence of mining pressure.The increase in the proportion of CO₂ in the mixed gas no longer improves the methane recovery rate.Besides,it was found that the increase of N₂ proportion in the mixed gas promotes the decomposition of methane hydrate...