关键词: 甲烷水合物, 二氧化碳, 氮气, 储层改造, 降压开采

Abstract:

It is a challenge for the exploitation of offshore gas hydrate reservoirs that the caprock is not closed and the cementation is weak. Direct depressurization always obtains high water yield and low gas to water ratios, which may also induce reservoir instability. Therefore, a reservoir reformation method based on hydrate principle was proposed, that is, CO₂ was injected above the hydrate reservoir to form an artificial CO₂ hydrate cap, so as to construct a relatively closed mining environment. Based on the previous works, this paper investigated the feasibility of injecting CO₂ + N₂ mixture to reform and exploit CH₄ hydrate reservoir. Results show that a CO₂ hydrate caprock with low permeability and good stability can form from the injected gas, which effectively reduces the water production and improves the recovery ratio of CH₄ during depressurization. When the proportion of N₂ in the injected is high, the presence of excess N₂ can promote the decomposition of CH₄ hydrate. However, N₂ always is produced with CH₄, increasing the difficulty of subsequent gas separation. When the proportion of CO₂ in the injected gas is high, the water blocking effect of the formed artificial caprock is enhanced, but CO₂ production also increases, and the further improvement of CH₄ recovery is limited. The follow-up research needs to further optimize the injection-production process conditions to improve the production efficiency and reduce the energy consumption of gas separation.

Key words: CH₄ hydrate, carbon dioxide, nitrogen, reservoir reformation, depressurization