注CO2+N2混合气改造-开采CH4水合物储层

盖层不封闭且胶结弱是海域天然气水合物储层开采面临的挑战之一。直接降压开采所得气水比低，还可能引发储层失稳。对此，一种基于水合物原理的储层改造方法被提出，即向水合物储层上方注入CO₂形成人工CO₂水合物盖层，从而构造出一个相对封闭的开采环境。在前期工作基础上，本文研究了注入CO₂+N₂混合气改造-开采CH₄水合物储层的可行性。研究结果表明注入混合气体能够形成渗透性低、稳定性好的CO₂水合物盖层，可以有效降低降压开采过程中的产水量及提高CH₄采收率。当注入的混合气中N₂比例较高时，过量的N₂对CH₄水合物的分解存在促进作用，但N₂随甲烷采出增加了后续的分离难度。当注入的混合气中CO₂的比例较高时，人工盖层阻水效果更强，但CO₂产出量也随之增加，且限制了CH₄采收率的进一步提高。后续研究需要进一步优化注-采工艺条件来提高开采效率和降低气体分离能耗。

Exploitation of CH4 hydrate reservoir reformed by (CO2+N2) gas injection

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.